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Neurochemical analysis of cocaine in adolescence and adulthood

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Title:
Neurochemical analysis of cocaine in adolescence and adulthood
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Book
Language:
English
Creator:
Stansfield, Kirstie Helen
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla.
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Subjects

Subjects / Keywords:
Development
Adolescent
Dopamine
Novelty-seeking
Impulsivity
Neurochemistry
Nucleus accumbens
Dissertations, Academic -- Psychology -- Masters -- USF   ( lcsh )
Genre:
government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: Adolescence is a time of high risk behavior and increased exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an increased risk to develop addiction and dependency in adulthood. Human adolescents are predisposed toward an increased likelihood of risk taking behaviors (Zuckerman M, 1986), including drug use or initiation. The purpose of this study was to examine differences in developmental risk taking behaviors and neurochemical responsivity to cocaine based on these behavioral characteristics. Adolescent and adult animals were exposed to a novel stimulus in a familiar environment to assess impulsivity, novelty preference and exploratory behaviors, subsequently, in vivo microdialysis was performed to assess dopaminergic responsivity to cocaine.
Thesis:
Thesis (M.A.)--University of South Florida, 2005.
Bibliography:
Includes bibliographical references.
System Details:
System requirements: World Wide Web browser and PDF reader.
System Details:
Mode of access: World Wide Web.
Statement of Responsibility:
by Kirstie Helen Stansfield.
General Note:
Title from PDF of title page.
General Note:
Document formatted into pages; contains 83 pages.

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aleph - 001680997
oclc - 62501121
usfldc doi - E14-SFE0001132
usfldc handle - e14.1132
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SFS0025453:00001


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ABSTRACT: Adolescence is a time of high risk behavior and increased exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an increased risk to develop addiction and dependency in adulthood. Human adolescents are predisposed toward an increased likelihood of risk taking behaviors (Zuckerman M, 1986), including drug use or initiation. The purpose of this study was to examine differences in developmental risk taking behaviors and neurochemical responsivity to cocaine based on these behavioral characteristics. Adolescent and adult animals were exposed to a novel stimulus in a familiar environment to assess impulsivity, novelty preference and exploratory behaviors, subsequently, in vivo microdialysis was performed to assess dopaminergic responsivity to cocaine.
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Neurochemical Analysis Of Cocai ne In Adolescence And Adulthood By Kirstie Helen Stansfield A thesis submitted in partial fullfillment of the requirements for the degree of Masters of Arts Department of Psychology College of Arts and Sciences University of South Florida Major Professor: Cheryl Kirstein, Ph.D Member: Cynthia Cimino, Ph.D Member: Toru Shimizu, Ph.D Date of Approval: March 22, 2005 Keywords: development, adolescent, dopamine, novelty-seeking, impulsivity, neurochemistry, nucleus accumbens Copyright 2005, Kirstie H. Stansfield

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i Table of Contents List of Figures iii Abstract iv Chapter One: Introduction 1 Theories of Addiction 3 Dopamine 6 Mesolimbic System and Relevant Brain Structures 12 Cocaine and the Mesolimbic System 14 Mesolimbic System and Beha vior during Adolescence 17 Chapter Two: Impulsivity and the Adolescent Rat 21 Abstract 21 Introduction 22 Methods 25 Animals 25 Procedure 25 Results 26 Novelty 26 Impulsive & Explorative behaviors 27 Discussion 27 Chapter Three: Neurochemical Effects of Cocaine in Adolescence Compared to Adulthood 30 Abstract 30 Introduction 31 Methods 36 Behavioral Testing 36 Surgery 37 In Vivo Microdialysis Apparatus 37 In Vivo Microdialysis 37 Neurochemical Analyses 38 Histology 39 Design and Analysis 39 Results 40 Discussion 41 Summary 46 References 48

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ii Appendices Appendix A: TDM Trail 1 Figure 61 Appendix B: TDM Across Trials Figure 62 Appendix C: TDM Test Figure 63 Appendix D: Novelty Preference Figure 64 Appendix E: Latency to Approach Figure 65 Appendix F: Frequency to Approach Novel Object Figure 66 Appendix G: Novel Environment Exploratory Behavior Figure 67 Appendix H: Novel Object Preference Figure 68 Appendix I: Novelty Induced Impulsivity Figure 69 Appendix J: Novelty-Induced Exploration Figure 70 Appendix K: Basal Dopamine Figure 71 Appendix L: Cocaine-Induced DAergic Activity Figure 72 Appendix M: Age-Related DOPAC/DA Turnover Figure 73 Appendix N: Novel Environmen t Exploratory Behavior & DAergic Response to Cocaine Figure 74 Appendix O: Novelty-Induced Impulsivity & DAergic Response to Cocaine Figure 75 Appendix P: Novel Object Preference & DAergic Response to Cocaine Figure 76 Appendix Q: Novelty-Induced Exploration & DAergic Response to Cocaine Figure 77

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iii List of Figures Figure 1. Total Distance Moved on Trial 1 61 Figure 2. Total Distance Moved Across Trials 62 Figure 3. Total Distance Moved on Test 63 Figure 4. Novelty Preference 64 Figure 5. Latency to Approach 65 Figure 6. Frequency to Approach 66 Figure 7. Novel Environment E xploratory Behavior 67 Figure 8. Novel Object Preference 68 Figure 9. Novelty-Induced Impulsivity 69 Figure 10. Novelty-Induced Exploration 70 Figure 11. Basal Dopamine 71 Figure 12. Cocaine-Induced DAergic Activity Across Age 72 Figure 13. Age-Related DOPAC/DA Turnover 73 Figure 14. Novel Environment Exploratory Behavior & DAergic Response to Cocaine 74 Figure 15. Novelty-Induced Impulsivity & DAergic Response to Cocaine 75 Figure 16. Novel Object Preference & DAergic Response to Cocaine 76 Figure 17. Novelty-Induced Exploration & DAergic Response to Cocaine 77

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iv Neurochemical Analysis Of Cocai ne In Adolescence And Adulthood Kirstie H. Stansfield ABSTRACT Adolescence is a time of high risk behavior and increas ed exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an increased risk to develop addiction and dependency in adulthood. Human adol escents are predisposed toward an increased likelihood of risk taking behaviors (Zuckerman M, 1986), including drug use or initiation. The purpose of this study was to examine differences in developmental risk taking behaviors and neurochemical responsivity to cocaine based on th ese behavioral characteristics. Adolescent and adult animals were exposed to a novel stimulus in a familiar environment to assess impulsivity, novelty preference and exploratory behavior s, subsequently, in vivo microdialysis was performed to assess dopaminergic responsivity to co caine. Adoles cent animals had greater novelty-induced locomotor activity, greater novelty preference, were more impulsive and showed higher exploratory behaviors compared to adult animals.

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v Furthermore, the results demonstrat e neurochemical differences between adolescent and adult animals in novel environment exploratory behavior, novel object preference, novelty-induced impulsiv ity and novelty-induced exploration. These data support the notion that adolescents may be predisposed toward sensation seeking and consequently are more likely to engage in risk taking behaviors, such as drug use initiation.

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1 Chapter One Introduction Adolescence is a time of high risk be havior and increased exploration. It is a period when the brain is undergoing many complex changes that can exert long-term influences on cognitive processes. Adolescence is marked by a greater probability to initiate drug use and initiation during this time is associated with an increased risk to develop addiction and dependency in adulthood. Specifically, Estroff (Estro ff TW, Schwartz RH & Hoffmann NG, 1989) has reported that most illicit drug use begins at approximately age 12, with peak periods of initiation between ages 15 and 19. In f act, initiation rates are so high that more than half (54%) of high school seniors have had at least one experience with an illicit compound (Johnston LD, 2000). During the 1990s, there was a steady rise in the fr equency of drug use in teenagers, by 2001, 4.3% of eighth graders, 5.7% of tenth graders, and 8.2% of high school seniors, reported a long-term use of cocaine (Johnston LD, 2000). The fact that initiation of cocaine use is so dramatic during the adolescent period is particularly disconcerti ng given that the escalati on of cocaine use appears more rapidly among teenagers than adult users, suggesting a greater addictive potential during adolescence than in adulthood (Estroff TW, Schwartz RH & Hoffmann NG, 1989).

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2 Generally, adults who initiated dr ug use during adolescence are more likely to have higher lifetime rates of drug use and progress to dependency more rapidly than those who began drug use in adulthood (Clark DB, Kirisci L Tarter RE, 1998). Development of the central nervous syst em (CNS) during adolescence may play a key ro le in the increased likelihood to ini tiate drug use. Moreover, disruption of the development of the CNS may resu lt in subsequent long term increases in the probability of drug use and dependence. During adolescence, critical structures invo lved in substance abuse are regulated primarily by the limbic system which is associated with emotional and impulsive behaviors. However, adoles cence is a period of transition from a more emotional regulation of critical structures that mediate substance abuse to a more mature cortical regulatory mechanism (Spear LP, 2000). During adolescence, the primary dopaminergic (DAergic) projections to the nucleus accumbens septi (NAcc) extend from th e ventral tegmental area (VTA), and are predominately modulated by the amygdala. However by adulthood, these previously amygdala modulated regulat ory actions are replaced by those projecting from the medial prefront al cortex (mPFC) indicating some developmental transition in the func tional nature of the system. The development of this system allows for a transition from more emotionally directed beha vior to more contextua lly regulated behavior. Because adolescents lack sufficient cort ical regulation provided by the mPFC, their behavior tends to be more impul sive and guided by emotion than adults, increasing the chances of initiating drug use. Chronic administration of drugs

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3 (e.g. cocaine) during this period may cause a functional change in accumbal DA efflux by altering amygdalar modulati on of accumbal DA release and/or altering the functional role of the mPFC input; consequently, leading to an increased risk of dependency during adu lthood. Together, th ese implications make a powerful argument for treating adolescence as a key time period for researching the developmen t of drug addiction. Theories of Addiction Anhedonia Hypothesis : Over the years, many different theories have been proposed to explain the myst eries of drug addiction. On e of the initial beliefs about addiction is that early in the process, drug use is maintained due to subjective euphoric effects and with subsequent repeated exposure, homeostatic neuroadaptations lead to tolerance and dependency. Further, following these compensatory changes, withdrawal becomes extremely unpleasant, and often the individual will r eestablish drug use again to avoid the negative symptoms associated with withdrawal. This theory has been known by a variety of names such as: pleasurepain, hedonic homeostasis, hedonic dysregulation, positive-negative reinforcement and reward allostasis (Koob GF & Le Moal M, 1997; Koob GF & Le Moal M, 2001; Koob GF, Caine SB Parsons L Markou A & Weiss F, 1997; Solomon RL, 1977). The basic principle of this theory is that a drug user initiates drug use to get the positive hi ghs and after the neuroadaptations, to avoid the negative lows associated w ith withdrawal. The dependence on the drug to feel normal is presumed to su stain regular and addictive use. This

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4 theory has limitations in that it fail s to explain prolonged drug relapse. Drug addicts often relapse into drug-taking again, even after they have been abstinent and free from the effects of withdrawal. Also, the absence of withdrawal symptoms does not protect against future relapse, as so many drug rehabilitation survivors can confirm. To summarize, conditioned feelings of withdrawal do not seem to be sufficiently strong enough or reliable enough to serve as the principle explanation of relapse (Robinson TE & Berridge KC, 1993). Aberrant Learning Theory: Another more recent theo ry of addiction that has gained a considerable amount of attenti on investigates the role of learning in the transition to addiction. For example, cues that predict the availability of rewards can powerfully activate brain re ward circuitry e.g. (NAcc) in both animals (Schultz W, Dayan P & Montague PR, 1997a) and humans (Knutson B, Adams CM Fong GW & Hommer D, 2001), sometimes even better than the reward itself (Schultz W, 1998). Anim als that are trained in the conditioned place preference paradigm (CPP) will spend more time in the environment which was previously paired with th e drug (Tzschentke TM, 2000) and less time in the unpaired chamber. Also, rats that were differe ntially trained to lever press for either cocaine and an a uditory stimulus or water and a different auditory stimulus, showed discrete populations of NAcc neurons that were selectively activated by cocaine-associat ed stimuli but not water-associated stimuli (Carelli RM, Ijames SG, 2001). Rats were able to discriminate between the auditory stimuli cues for cocaine and water and therefore were

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5 anticipating and/or expecting the rewar d, as evidenced by the activation of neurons in the NAcc. This learning theory ascertains that the change from recreational use to addiction involves a transition from behavior originally controlled by explicit and cognitively guided expectations produced by the memory of drug pleasure to compulsive drug use. However, this fails to explain why drug cues become overpowering. Humans exhibit many habits in every day life, but there is a noticeable difference in this type of behavior as compared to the compulsive actions of drug addicts. This is a very insightful theory; however it fails to explain why compulsive behaviors become dominant over everyday activities, which leads us to the next theory of addiction. Incentive-Sensitization Theory : One contemporary theory of addition, labeled incentive-sensitization, focuses on how dr ug cues trigger excessive incentive motivation for drugs, leading to com pulsive drug seeking, drug taking and relapse (Robinson TE & Berridge KC, 1993). The main idea being that drugs of abuse change specific c onnections and circuits in brain systems, specifically accumbal-related areas, that mediate motiv ational functioning and learning, the emphasis of incentive salience. As a consequence, these neural circuits may become enduringly hypersensitiv e (or sensitized) to specific drug effects and to drug-associated stimuli (Schultz W, Dayan P & Montague PR, 1997c). This drug-induced change is called neural sensitization (Berridge KC & Robinson TE, 1998). Robinson and Berridge (Be rridge KC & Robinson TE, 1998) have proposed that this sensitized system leads psychologically to excessive

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6 attribution of incentive salience to drug-cues causing craving for drugs. The incentive-sensitization view suggests that addiction is a disorder of incentive motivation due to drug-induced sensitizat ion of neural systems that mediate stimulus salience; therefore drug crav ing and use can be triggered by the presence of drug cues whose enhanced salience increases the likelihood of addictive behaviors (Robinson TE & Be rridge KC, 1993). This theory is appropriate for explaining the occurrence of findings such as the effects of novel and aversive stimuli on accumbal dopamine (DA) levels (see DA and salience section below). In summary, all three of these theori es contribute much insight to aid in the understanding of drug addiction. Howeve r, just one theory cannot seem to explain addiction in its en tirety, but possibly a combination of them can give us a more accurate representation of what is occurring along the complex path to addiction. Dopamine There are several neurotransmitters that have a considerable effect on brain activity. One that seems to be of major interest in regards to the effects of drugs of abuse including cocaine is DA. DA is synthesized from tyrosine and is broken down into 3,4-dih ydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) (Lindvall O & Bjorklund A, 1974). Many researchers have concluded that DA play s an important role in mediating the reward value of food, drink, sex, drugs of abuse, and brain stimulation (Bardo MT, 1998).

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7 Natural Reinforcers: Early in the 1970s, intracranial self-stimulation (ICSS) was studied extensively in relation to its effect on catecholamines, including DA. Historically, Olds and Milner have shown that an animal will lever-press for ICSS (Olds J & Milner P, 1954), a nd several studies indicated that DA systems are critically involved in this process (Crow TJ, 1972; German DC & Bowden DM, 1974). Microdialysis and voltammetry studies in rats have shown significant increases in DA in the NAcc during drinking, feeding and sexual behaviors (Di Chiara G, 1995; Wilson C, Nomikos GG Collu M & Fibiger HC, 1995). Additionally, in opera nt responding for juice reinforcement in monkeys electrophysiology techniques ha ve shown activation of neurons in the NAcc (Bowman EM, Aigner AT Richmond ABJ, 1996). Not only will animals respond for these natural reinfo rcers, there is al so evidence for increased neuronal firing in the VTA. Studies have also shown that drinking (induced by restricted access); salt in take (induced by sodi um depletion); or eating (induced by food deprivation) will trigger the release of DA in the NAcc (Blander DS, Mark GP Hernandez L a nd Hoebel BG, 1988; Chang VC, Mark GP Hernandex L & Hoebel BG, 1988). Sexual behavior, additionally, causes the release of DA in the NAcc (Damsm a G, Pfaus JG Wenkstern D Phillps AG & Fibiger HC, 1992) (Becker JB, R udick CN Jenkins WJ, 2001)whereby sexual contact with a rat of the opposite sex triggers an increase in DA levels. Laboratory animals will also se lf-administer DA reuptake blockers such as buproprion, mazindol, and nomi fensine (Corwin RL, Woolverton WL Schuster CR & Johanson CZE, 1987; Wilson MC & Schuster CR, 1976;

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8 Winger G & Woods JH, 1985) as well as piperazine, a highly selective DA reuptake blocker (Van Der Zee P, K oger HS Gootjes J & Hespe WZ, 1980). Along the same lines, animals will also se lf-administer direct DA agonists such as apomorphine and piribedil (Yokel RA & Wise RA, 1978). Moreover, DA blockade decreases responding for reinforcers. Animals trained to selfadminister a saccharin solution, decreased their appetitive responding after a DA antagonist, haloperidol, was admini stered (Royalle DR & Klemm WR, 1981). Even responding for naturally reinforcing stimuli such as food, water and sex can be altered by the administration of either a DA agonist or antagonist, demonstrating that the DA system is critically involved. As seen from previous research, natural re inforcers have a profound influence on reward behavior and these types of re inforcers also generate an increase activity in the mesolimbic DA pathway and in accumbal DA levels. Drug Use : Drugs of abuse also have a profound effect on the mesolimbic DA system. It has been shown that opiat es (Esposito RU & Kornetsky C, 1978), amphetamines (Olds ME, 1978), marijuan a (Gardner EL, Paredes W Smith D Donner A Milling C Cohen D & Morris on D, 1988), dissociate anesthetics, barbiturates, benzodiazepi nes and alcohol (Wise RA 1980) all increase DA in the NAcc. A number of laboratories ha ve shown that cocaine produces a strong enhancement of extr acellular DA in the neostr iatal and NAcc terminal projection areas of this reward-related DA system (Di Chiara G, Imperato A, 1988; Hernandez L & Hoebel BTG, 1988; Hurd YL, Weiss F Koob G & Ungerstedt U, 1989). As of today, many researchers have found this

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9 phenomenon using in vivo microdialysis which allows sampling from the brain of freely moving animals. Not only doe s cocaine administration increase DA levels, but DA antagonists block the rewa rding efficacy of cocaine (Koob GF, Caine SB Parsons L Markou A & Weiss F, 1997). Given that all these drugs have an impact on DA levels; it is im portant to consider how and by what mechanisms DA plays a role in mediating reward. Aversive Stimuli: Not only do natural reinforcers have an influence on accumbal DA, but stimulus salience, (e.g. novel and/or aversive stimuli) also raise fundamental questions. Stressors su ch as footshock and restraint have been shown to activate the mesolimbic DA system. Previous research has shown that 15 minutes of restraint stress increases the content of DA metabolites in the shell but not the core of the NAcc (Deutch AY, Bourdelais AJ & Zahm DS, 1993). Also, Kalivas & Duffy (Kalivas PW & Duffy P, 1995) confirmed that mild stress induced eleva tions of extracellular DA for a period of at least 20 minutes in the shell of the NAcc. An imals exposed to aversive (shock) conditioning exhibited elevated DA activity in the NAcc, VTA and mPFC (Morrow BA, Taylor JR & Roth RH, 1995). The fact that aversive stimuli increase DA levels has implications in favor of the incentive-sa lience theory. Specificall y, not only positive hedonic stimuli can activate the mesolimbic DA system, but negative stimuli also have an effect on this system, therefore the attribution of incentive salience. Latent Inhibition (LI): DA is not only implicated in reward, but appears to play an important role in attentional pr ocesses. LI is a procedure designed to

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10 measure attention by exposing subjects to a stimulus repeatedly without consequence (preexposure) and later using this stimulus as a conditioned stimulus (CS) in a classical conditioning paradigm (Killcross AS & Robbins TW, 1993). This procedure results in a delayed attainment of conditioned responding compared to subjects that had no preexposure to the stimulus. The preexposure to a stimulus interferes with the ability to later learn an association. Accumbal DA plays an im portant role in LI, and attentional processes. Administration of DAergic agonists prevent LI, meaning that subjects learn the association in the conditioning paradigm even when that stimulus has been preexposed (Solom on PR & Staton DM, 1982). Therefore, facilitating attention to the stimulus and not allowing for generalization. However, administration of DAergic an tagonists aid LI, for example, subjects take longer to learn the associations in the contingency (Weiner I & Gal G, 1996), due to decreased attention to that stimulus. DA and its relationship to LI provides insight that the NAcc plays no t just a role in reward but in the regulation of attentional processes, and subsequently, a role in drug use maintenance through increased attention to environmental factors and/or cues that surround drug use. Novelty: The personality trait, sensation seeking, has long been associated with the increased risk of drug abus e in humans (Fulker D, Eysenck SBG & Zuckerman M, 1980). In rodent models novelty preference is used as an indicator of sensation seeking given th at rats are inherently neophobic. Animals are considered high responde rs (HR) to novelty (or novelty

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11 preference) when they spend more time in a novel environment compared to the time they spend in a familiar environment. Animals are considered to be low responders (LR) to novelty (or novelty aversion) when they spend more time in the familiar environment compared to the new or novel environment (Dellu F, Piazza PV Mayo W Le Moal M &Simon H, 1996). When exposed to a novel environment, HR rats have high rates of locomotor activity whereas the LR rats show low rates of activity. Ra ts placed in a novel environment express a surge in catecholamine activity in the NAcc (shell) and mPFC (Rebec GV, Grabner CP Johnson M Pierce RC & Bardo MT, 1997). Novelty HRs show greater increases in extra cellular DA in the NAcc than LR when exposed to an environmental (tail pinch) or a pharm acological (cocaine) challenge (Hooks MS, Colvin AC Juncos JL & Justice JB Jr, 1992; Rouge-Pont F, Piazza PV Kharouby M Le Moal M & Simon H, 1993) Typically, there is a robust sensitization that occurs with repeated cocaine administration (Kalivas PW & Duffy P, 1995). A less robust sensit ization occurs when the drug is administered to animals in a novel test environment compared to those given the same dose in the home cage (Bad iani A, Browman KE Robinson TE, 1995). HR rats show higher rates of amphetamine and cocaine-induced locomotor activity and will self-administe r these drugs more readily than LR rats (Hooks MS, Jones GH Smith AD Neill DB & Justice JB Jr., 1991). Moreover, HR rats seem to participate in far greater risk taking behaviors and show much higher behavior al and neurochemical responses in reaction to environmental stressors or pharmacologi cal challenges than LR rats (Bevins

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12 RA, Klebaur JE & Bardo MT., 1997). This could be comparable to humans in so much that people labeled as high se nsation-seekers, may be more likely to become involved in risky behaviors such as reckless driving, sky diving or drug use (Zuckerman M, 1990). Mesolimbic DA System and Brain Structures Ventral Tegmental Area (VTA ): The mesolimbic system begins in the VTA and projects through the medial fore brain bundle to the amygdala, lateral septum, bed nucleus of the stria terminalis, hippocampus, and the NAcc (Oades RD & Halliday GM, 1987). DA neurons in the VTA are the cells of origin of the mesolimbic/mesocortic al DA pathway and provide DAergic innervations of the NAcc (Oades RD & Halliday GM, 1987). Electrical selfstimulation of this area has generally shown an increase in DA release and metabolism in the NAcc and medial pref rontal cortex (mPFC) (Fiorino DF, Coury A Fibiger HC & Phillips AG, 1993) You et. al (You ZB, Chen YG & Wise RA, 2001), have shown that lateral hypothalamic self-stimulation increases dendritic release of DA a nd accumulation of its metabolites in the VTA. Different drugs of abuse ha ve effects on DA along the mesolimbic pathway. However, not all drugs have the same effect on different regions. For example, animals will self administ er ethanol directly into the VTA (Rodd ZA, Mckinzie DL Dagon CL Murphy JM & McBride WJ, 1998) but interestingly enough, animals will self-a dminister cocaine into the nucleus accumbens (McBride WJ, Murphy JM & Ikemoto S, 1999), but not the VTA (De La Garza R, Callahan PM & Cunni ngham KA, 1998). This showing that

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13 although the rewarding effects of certain drugs are mediated by the mesolimbic pathway, their primary action occurs at different points of the pathway, and possibly by different mechanisms/pat hways (e.g. reuptake inhibition vs. stimulation of preor postsynaptic receptors). Nucleus Accumbens (NAcc): The NAcc is located in the basal forebrain, rostral to the preoptic area and immedi ately adjacent to the septum. It is innervated by DA-secreting terminal boutons from neurons of the VTA (Skagerberg G, Lindvall O & Bjorklund A, 1984). This is an area that seems to play a very important ro le in the physiology of re ward and reinforcement in relation to drugs of abuse, including cocaine. Stimu lation of the DA receptors in the NAcc will reinforce behavior [e.g. animals will lever press for electrical stimulation of the NAcc (Olds ME & Fobes JK, 1981)]. Animals will also lever press for direct infusions of DA and amphetamines directly into the NAcc (Hobel BG, Monaco AP Hernandez L Ausili EF Stanley BG & Lenard L, 1983). As mentioned previously, DA levels in the NAcc can be measured by in vivo microdialysis, which samples extracellular cerebral spinal fluid (CSF). Many studies have found that ad ministration (either self-administration or experimenter administration) of cocaine and amphetamine increase the levels of extracellular DA in the NAcc (Hoebel BG & Hernandez L, 1989). As mentioned earlier, the NAcc not only mediates reward, but other salient (e.g. aversion) stimuli as well (Salamone JD, 1992). From the multitude of research performed in the NAcc, it is evident that there are complex mechanisms regulating not only reward, but other aversive and attentional stimuli in

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14 relation to DA levels, suggesting the possi bility that use of drugs of abuse may not be maintained just because they are rewarding, but because they are salient or conditioned. Cocaine and the Mesolimbic DA System When cocaine is administered, it reaches all areas of the brain, but readily binds to specific areas within the reward pathway (i.e., NAcc and VTA). As previously discussed, th e NAcc and VTA consist of DA synapses. In a normally functioning individual, DA is released from the presynaptic cell into the synaptic cleft where it either binds to the postsynapt ic cell or reuptaken into the presynaptic cell by a dopamine transporter protein (DAT). When cocaine is administered, it binds with high-affinity to the DAT which in turn, inhibits reuptake into the presynaptic cell, therefore increasing the amount of DA present in the synaptic cleft. Acute doses of cocaine have been shown to increase accumbal DA levels from 200-1170% for 80 to 100 minutes depending upon dose (Camp DM, Brow man KE & Robinson TE, 1994; Kuczenski R, Segal DS & Aizenstein ML 1991; Reith ME, Li MY & Yan QS, 1997; Strecker RE, Eberle WF & Ashby CR Jr, 1995). As shown from previous research, acute administrati on of cocaine, regard less of dose but following a dose response curve, pr oduces significant and long lasting increases in extracellular levels of DA in the mesolimbic DA system. Similar findings have been shown in preadolescent animals (Philpot RM & Kirstein CL, 1998).

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15 Repeated administratio n of psychostimulants results in behavioral sensitization or reverse tolerance in an enhanced behavi oral response to a subsequent drug challenge (Vanders churen LJ & Kalivas PW, 2000). Consequently, rats who have repeatedly administered cocaine over at least 7 days, will show an elevated locomotor reaction in response to the drug which prevails up to seven days after cessati on of the drug (Cass WA & Zahniser NR, 1993). Sensitization not only occurs be haviorally, but neurochemically. Repeated drug exposure produces changes and adaptations at a cellular level which in turn alters the functioning of the entire pathway in which those neurons work (Kleven M, Woolverton W Schuster C & Seiden, 1988). These changes lead to the complex processes of tolerance, dependence and of course, sensitization (Koob GF & Le Moal M, 1997; Wise RA, 1980). Sensitization is characteristic of repeated intermittent cocaine administration, where in tolerance (defined as a smaller effect from a given dose of drug after previous exposure to that drug) occurs after c ontinuous infusion of cocaine (Post RM, 1980). Rats injected once a day with co caine show enhanced inhibition of DA uptake (Izenwasser S & Cox BM, 1992), whereas rats getting a continuous infusion of cocaine show attenuated inhibition of DA uptake by cocaine (Izenwasser S & Cox BM, 1992). Also, ther e seems to be different degrees of sensitization, such that longer times between cocaine injections produce greater sensitization (Post RM, 1980). Sensitization, tolerance and dependence also result in functional adaptations such as increased cAMP pathway activity, increased calcium regulatory elemen t binding protein (CREB) and also

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16 increased changes in immediate early ge nes (FosB) (Nestler EJ & Aghajanian GK, 1997). Repeated administration of cocaine also produces significant changes in DA during withdrawal. In vivo microdialysis studies in the NAcc have shown that once self-administration of cocaine has ended, basal DA levels decrease significantly during this wit hdrawal period (Parsons LH, Smith AD & Justice JB Jr, 1991). Taken together, these studies in adult animals show that repeated cocaine administration results in complicated changes in the DA mesolimbic pathway that continues long after drug use has stopped, and may be implicated in craving and relapse. Expectancy : Another puzzling aspect of drug use deals with the issue of drug expectancy-induced changes. Cues that were previously paired with a reward initiate neurochemical and behavioral responses like those present during the actual reward. CPP studies have shown that an animal will spend more time in the chamber in which it expects to r eceive a reward than the one it never received a reward in previously, sugge sting an anticipatory or expectancy effect. In addition to expectancy-indu ced behavioral changes there are also expectancy-induced neurochemical changes. Cocaine and alcohol in vivo microdialysis studies have shown an e xpectancy effect with accumbal DA levels increasing significantly when the animal expect to re ceive an injection of the drug, but actually receives a saline injection(Philpot RM & Kirstein CL, 1998). DA neurons and subsequent behaviors seem to be activated by conditioned, reward-predict ing stimuli that enable the animal to learn and

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17 eventually expect a reward based on pr evious performance. Expectancies may be an evolutionary adaptation that allows an animal to predict future events, allowing extra time for preparatory be haviors and possibly increasing the likelihood of escape from dangerous situations. Drug expectancies may play an important role in human drug addiction, since stimuli associated w ith drug taking behavior in humans have the ability to elicit strong drug craving feelings which repeatedly leads to drug relapse (O'Brien CP, Childress AR McLellan AT & Ehrman R, 1992). Non-human primate studies have shown via physiol ogical recordings, activation of VTA, NAcc and ventral striatum neurons in response to anticipation of rewarding stimuli such as water or fruit juice (Schultz W, Dayan P & Montague PR, 1997b). Recent studies have been able to replicate these non-human primate studies of reward prediction to human brain reward activation. Berns et al. (Berns GS, McClure SM Pagnoni G & Montague PR, 2001) have shown activation of brain reward regions in re sponse to temporal predictability of rewards such as water and juice. Mesolimbic System and Behavior during Adolescence Adolescence is an important developm ental period. It is also the period of initiation and maintenance of drug use and potentially drug addiction. Sexual maturation in the male rat encompasses postnatal days (PND) 30 through 55; this is the in dicator to denote adoles cence (Odell WD, 1990) and the reason for selecting these ages to investigate. Very few models of adolescent drug addiction in animals have been developed to examine the

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18 remarkable differences between adolescen ts and adults. Many neurobehavioral alterations that are age-specific seen in human adolescents are observed in adolescent rats from PND 30 to P ND 42, making adolescent animal models very useful in their ability to evalua te neurochemical and behavioral changes due to drug use during this impor tant stage of development. Novelty-seeking and high risk behavior s seem to be highly associated with adolescence. Along th is unique stage of development, distinct social, behavioral and neurochemical changes emer ge, to assist with the important life events that will occur. For example, learning and acquiring skills necessary to permit survival away from parental caretakers (Spear LP, 2000). This phenomenon being evolutionary adaptiv e as a means to avoid inbreeding (Schlegel A & Barry III H, 1991). Human Social Interaction : In order for a successful transition from childhood to adulthood, an important aspect to gaining independe nce is when adolescents shift their social orientations from a dults to peers (Steinberg L, 1989) and typically spend a significant amount of time interacting with their peers as opposed to adults. Human adolescents as a group exhibit a disproportional amount of reckless behavior, sensationseeking and risk taking (Trimpop RM, Kerr JH & Kirkcaldy B, 1999). Risk taking in adolescents poses some negative consequences such as suicides accidents, AIDS, pregnancy and drug dependence (Irwin Jr.CE, 1989). Alt hough risk taking may be hazardous, it can also be beneficial. Risk taking and exploratory type behaviors allow an individual to explore adult behavior and may also serve (as mentioned above)

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19 as a protective evolutionary factor. A dolescents increase in risk taking and novelty-seeking may trigger adolescent de parture from the parental units by giving incentive to explore novel areas aw ay from home (Schlegel A & Barry III H, 1991). Similar to humans, periadoles cent rats are behaviorally and pharmacologically different from younger a nd adult rats. Periadolescent rats have been reported to be more hyperact ive and inattentive (Spear LP & Brake SC, 1983) and have reduced responsiveness to some of the effects of alcohol (Silveri MM & Spear LP, 1998), amphetamine (Bolanos CA, Glatts J and Jackson D, 1998), and cocaine (Lavio la G, Wood RD Kuhn C Francis R & Spear LP, 1995). In the CPP paradigm, adolescent rats show a preference for nicotine, whereas the adult rats did not (Vastola BJ, Douglas LA Vaarlinskaya EI & Spear LP, 2002). Also, Philpot et al (Philpot RM, Badanich KA & Kirstein CL, 2003) demonstrated that adol escent rats showed a preference for moderate doses of alcohol, whereas the adults had no preference at that dose. Neurochemical Changes: There are also dramatic changes in the adolescent brain, both circuitry and neurochemistry. The mesolimbic and mesocortical brain regions and their DA projections undergo subs tantial remodeling during the adolescent period, for review see (Spear LP, 2000). Rosenberg & Lewis (Rosenberg DR & Lewis DA, 1995) were among those researchers who saw a common developmental pattern in the ove rproduction and subsequent pruning of synaptic connections during the period preceding adulthood. The D1 and D2 receptors have been of major focus for years in regards to overproduction

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20 and pruning. D1 and D2 receptors increase in density in the first few weeks of life (Hartley EJ & Seeman P, 1983). S ubsequently, Teicher et al (Andersen SL, Thompson AT Rutstein M Hostetter JC & Teicher MH, 2000; Teicher MH, Andersen SL & Hostetter JC Jr ., 1995) have demons trated receptor overproduction and elimination in both the striatum and prefrontal cortex, but have failed to show evidence that the NAcc follows the same overproduction and pruning construct (Andersen SL, Th ompson AT Rutstein M Hostetter JC & Teicher MH, 2000). In addition, alterations in receptor binding and sensitivity in various neurotransmitter systems have been reported during adolescence (Trauth JA, Seidler FJ McCook EC & Slotkin TA, 1999) along with changes in myelination of neurons (Hamano K, Iwasaki N Takeya T & Takita H, 1996). Adolescents, whether human or non-human animals, exhibit many behavioral, social and neurochemical adap tations that enable them to develop successfully, however, these adaptations can have negative implications when these normal developmental behaviors resu lt in persistent deviant actions such as drug abuse. The present studies are designed to look at the relationship between novelty preference and DA re sponsiveness to cocaine among adolescent and adult animals.

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21 Chapter Two Experiment One Impulsivity in the Adolescent Rat Adolescence is a time of high risk be havior and increased exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an incr eased risk to develop addiction and dependency in adulthood. Human adol escents are predisposed toward an increased likelihood of risk taking be haviors (Zuckerman M, 1986), including drug use or initiation. The purpose of th is study was to examine differences in developmental risk taking behaviors. Adolescent and adult animals were exposed to a novel stimulus in a familia r environment to assess impulsivity, novelty preference and exploratory behavi ors. Adolescent an imals had greater novelty-induced locomotor activity, greater novelty pr eference, were more impulsive and showed higher explorat ory behaviors compared to adult animals. These data support the notion that adolescents may be predisposed toward sensation seeking and consequently are more likely to engage in risk taking behaviors, such as drug use initiation.

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22 Introduction Adolescence is a time of high risk be havior and increased exploration. It is also a period when the brain is undergoing many complex changes that can exert long-term influences on decision making and cognitive processes (Spear LP, 2000). Adolescence is also marked by a greater probability to initiate drug use and is associated with an increased risk to develop addiction and dependency in adulthood. Estroff (Estroff TW, Schwartz RH & Hoffmann NG, 1989) has reported that illicit drug use can begin at approximately age 12, with peak periods of initia tion between ages 15 and 19. In fact, more than half (54%) of high school seniors have had at least one experience with an illicit compound (Wallace JM Jr., 2003) The fact that illicit drug use is so dramatic during the adolescent period is of partic ular concern given that the escalation of use appears more rapidly among teen agers than adult us ers, suggesting a greater abuse poten tial during adolescence than in adulthood (Estroff TW, Schwartz RH & Hoffmann NG, 1989). I ndividuals who initiate use prior to ages 11-14 are more likely to progress to addiction as adults (DeWit DJ, Adlaf EM Offord DR Ogborne AC, 2000). Several researchers (Trimpop RM, Kerr JH & Kirkcaldy B, 1999) (Arnett, JJ., 1999) have shown a relative predisposition toward sensation-

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23 seeking in human adolescents, a factor that Zuckerman associates with increased likelihood of risk taki ng behaviors (Zuckerman M, 1984) (Zuckerman M, 1986), including drug us e or initiation (Bardo MT, Donohew RL Harrington NG, 1996; Bates ME, La bouvie EW White HR, 1986; Forsyth G, Hunleby JD, 1987). Measures of se nsation-seeking are highly associated with impulsivity (Eysenck SB & Eysenc k HJ, 1977; Shedler J, Block J, 1990), indicating this as a valid measure of risk taking behavior probability, specifically, drug use initiation (Hanse ll S and White HR, 1991). Similar to humans, adolescent rats have been s hown to exhibit greater responding to novelty compared to adult rats (Douglas L, Varlinskaya E Spear L, 2003). Furthermore, numerous studies have indica ted that there is a strong correlation between novelty preference and impulsive reactivity with both the rewarding efficacy of psychomotor stimulants and self-administration rates in animals (Hooks MS, Colvin AC Juncos JL & Just ice JB Jr, 1992) (Klebaur JE, Bevins RA Segar TM Bardo MT, 2001). High se nsation-seeking (H S) rats show higher rates of amphetamine and cocaine -induced locomotor activity and will self-administer these drugs more readily than low sensation-seeking (LS) rats (Hooks MS, Jones GH Smith AD Neill DB & Justice JB Jr., 1991). Moreover, HS rats seem to participate in far great er risk taking behaviors and show much higher behavioral and neurochemical re sponses in reaction to environmental stressors or pharmacological challenges th an LS rats (Bevins RA, Klebaur JE & Bardo MT., 1997) (Klebaur JE, Bevi ns RA Segar TM Bardo MT, 2001). Interestingly, adolescents who have been diagnosed with attention-

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24 deficit/hyperactivity disorder (ADHD) are at a greater risk for substance use than an adolescent not suffering from th is disorder (Biederman J, Wilens TE Mick E Faraone SV Spencer TJ, 1998; Mo lina B, Pelham W, 2003). This is important as one of the key features of ADHD is im pulsivity, suggesting this trait may play a role in individuals w ith ADHD being predisposed to substance abuse. Taken together, these data suggest a strong re lationship between sensation-seeking and novelty -seeking/impulsivity, making it more likely that adolescents will become involved in risky behaviors including drug use and initiation. Several approaches have been used to divide animals into high or low drug abuse profiles. Some research ers have used exposure to a novel environment to induce locomotor increa ses as a predicto r of drug abuse liability (Kabbaj M, Devine DP Savage VR and Akil H., 2000). Recent work has shown that novelty preference is a reliable measure that can be used to divide animals into high responders (H R) and low responders (LR) (Stansfield KH, Philpot RM & Kirstein CL, 2004) To examine differences between adolescent and young adult animals, the present study examined behavioral responses to a novel context or novel object in a familiar environment. The purpose of this study was to determine an effective procedure for characterization of individual and de velopmental differences in novelty induced locomotion and impulsivity (i.e., decreased latency to approach a novel object).

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25 Methods Animals Fifty Sprague-Dawley (Harlan Labora tories) rats postnatal day (PND) 34 (=134 g) and PND 59 (=293 g) at the time of testing were used as subjects in these experiments. No more than one male per litter per age was used in a given condition. Pups were se xed and culled to 10 pups per litter on PND 1. Pups remained housed with thei r respective dams in a temperature and humidity-controlled vivarium on a 12:12 h light:dark cycle (07:00 h/19:00 h) until PND 21, following which pups were weaned and group housed. The care and use of animals was in accordance with local standards set by the Institutional Animal Care and Use Co mmittee and the NIH Guide for the Care and Use of Laboratory Animals (Na tional Institutes of Health, 1986). Procedure Animals were tested on a black plastic circular platform (216 cm in diameter) standing 70 cm from the ground, with a white plastic barrier enclosing the arena (216 cm). A vide o camera was suspended directly over the table and recorded the animal's behavior using a Noldus Be havioral Tracking System. Over a period of four consecutive days, each rat (PND 31-34 and 5659) was placed on the open field in one of four randomly selected zones and

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26 allowed to freely explore the novel environment for five minutes. This procedure was performed twice a day fo r a total of 8 habituation trials. Immediately following the 8 th trial, animals were removed for 1 minute while a single novel object (approximately 6.5 in. high) was attached to the center of the table. Rats were placed in a random zone and allowed to explore the familiar environment for five minutes. Both time spent in proximity of the novel object, and activity i nduced by the presence of the novel object were measured. Novelty preference was defi ned as time spent within 10.16 cm of the object on test. Novelty-induced locomotion and total distance moved (TDM) were measured on all trials. Impulsivity was operationalized as latency to approach the novel object. RESULTS Novelty The present findings demonstrate th at adolescent animals exhibited significantly greater TDM on the first tr ial as compared to adult animals, t(1,46)=2.100, p<0.05 (appendix A). Both adolescent and adult animals exhibited a significant reduc tion in TDM from trial 1 to trial 8, t(1,42)= 3.533, p< 0.001, t(1,49)= 3.006, p<0.05, respectively (appendix B). Importantly, activity in the presence of the novel obj ect on test did not differ across age, t(1,48)=0.3005, p>0.05 (appendix C). Additionally, adolescent animals spent more time with the novel object, t(1,43)= 2.082, p<0.05 as compared with young adult animals (appendix D).

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27 Impulsive & explorative behavior There was a significant effect of age on latency to approach the novel object, t(1,44)=2.449, p<0.05 (appendix E), and frequency approaching the novel object, t(1,43)=2.370, p<0.05 (appendix F). Adolescents approached the object more rapidly and returned to the object more frequently on test. Discussion The present study utilized a novel paradigm to assess impulsivity and novelty preference in adoles cent and adult animals. The results indicate a developmental difference between impul sive and novelty preference behaviors in adolescent versus adult animals. Th e present results replicate and extend present findings of enhanced novelty responding in adolescent animals using a Conditioned Place Preference paradigm (Douglas L, Varlinskaya E Spear L, 2003). Adolescent animals are more act ive in a novel context than adult counterparts, while activity induced by a localized novel stimulus was similar across age. Importantly, the present study showed from trial 1 to trial 8, there was a significant re duction in total dist ance moved in both age groups, with no differences detected between ages on trial 8. Notably, adolescent rats habituated significantly faster to the novel environment than did adult animals. TDM in younger animals was significantly higher on the first trial compared to all other trials, however, with adult rats, only trial 1 differed from the last trial in TDM. Also, adolescent rats spent more than twice as much time interacting with a novel object placed in a familia rized environment compared to older rats (i.e., novelty preferen ce) which supports previously published data using a

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28 different paradigm (Douglas L, Varlinsk aya E Spear L, 2003). In the present study, the behavior was recorded by a computer ized tracking system, suggesting that this effect is robust. Together, these re sults indicate that adolescent animals are highly reactive to a novel environment, stressing the importance of habituating animals when performing developmental research. The second part of the study examined impulsive and exploratory-type behaviors across adolescent and adult an imals. Adolescent animals exhibited a significantly lower latency to approach the novel object when placed in the habituated environment than did the adul t animals. This would suggest that adolescents engage in more risk-taking behaviors more frequently than older animals, because the shorter the latenc y to approach, the less time an animal has to evaluate whether the novel object is a threat, a behavi or that would be considered risky. Not only were adol escent animals more im pulsive, they also approached the novel object more frequently, suggesting they are more likely to explore something unfamiliar in their environment and subsequently spent more time, on average, with the novel obj ect after approach. Taken together, these data reveal that adolescent animals express greater novelty induced reactivity along with a greater preference for novelty. Interestingly, adolescent animals exhibited a significant reduction in the number of approaches to the novel object on test from minutes 1 to 2, whereas adult animals number of approaches remained relatively constant over the entire trial, suggesting that adults do not habituate. This observa tion suggests that, lik e TDM across trials, adolescent animals habituate faster within a given trial than do adult animals.

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29 Furthermore, adolescents are more impul sive and engage in more exploratory behaviors. These data s upport the notion that adoles cents may be predisposed toward sensation seeking (Arnett, JJ., 1999) and consequently are more likely to engage in risk taking behaviors (Zuckerman M, 1986), such as drug use initiation.

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30 Chapter Three Experiment Two Neurochemical Effects of Cocaine in Adolescence Compared to Adulthood Adolescence is a time of high risk be havior and increased exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an incr eased risk to develop addiction and dependency in adulthood. Human adol escents are predisposed toward an increased likelihood of risk taking be haviors (Zuckerman M, 1986), including drug use or initiation. The purpose of th is study was to characterize adolescent versus adult developmental differences classified as high-responding or lowresponding based on several behavioral m easures and subsequently to examine the neurochemical responsivity to a systemic challenge of cocaine. The results demonstrate neurochemical differences between adolescent and adult animals in novel environment exploratory behavi or, novel object pr eference, noveltyinduced impulsivity and nove lty-induced exploration. The data demonstrate that adolescent animals exhibit a greater behavioral activation compared to their adult counterparts, in addition, the paradigm shows the simplicity of separation based on individual variability within each behavioral measure. These results illustrate that a res ponse to a pharmacological challenge of cocaine exhibits a complex interaction with age and behavioral characteristics.

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31 Introduction Adolescence is a time of high risk be havior and increased exploration. It is a period when the brain is undergoing many complex changes that can exert long-term influences on decisi on making and cognitive processes. Adolescence is also marked by a greater probability to initiate drug use and initiation during this time is associated with an increased risk to develop addiction and dependency in adulthood. Specifically, Estroff (Estroff TW, Schwartz RH & Hoffmann NG, 1989) has reported that most illicit drug use begins at approximately age 12, with p eak periods of init iation between ages 15 and 19. In fact, initiati on rates are so high that mo re than half (54%) of high school seniors have had at least one experience with an illicit compound (Johnston LD, 2000). During the 1990s, there was a steady rise in the frequency of drug use in teenagers, by 2003, 4.3% of eighth graders, 5.7% of tenth graders, and 8.2% of high school se niors, reported a long-term use of cocaine (Johnston LD, 2000). The fact that initiation of cocaine use is so dramatic during the adolescent period is particularly disconcerting given that the escalation of cocaine use appears more rapidly among teenagers than adult users, suggesting a greater addictive potential during adolescence than in adulthood (Estroff TW, Schwartz RH & Hoffmann NG, 1989). Generally, adults who initiate drug use during adolescence are more likely to have higher lifetime rates of drug use and progress to dependency

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32 more rapidly than those who began drug use in adulthood (Clark DB, Kirisci L Tarter RE, 1998). Development of the cen tral nervous system (CNS) during adolescence may play a key ro le in the increased likelihood to ini tiate drug use. Moreover, disruption of the development of the CNS may resu lt in subsequent long term increases in the probability of drug use and dependence. During adolescence, critical structures invo lved in substance abuse are regulated primarily by the limbic system which is associated with emotional and impulsive behaviors. However, adoles cence is a critical period of transition from a more emotional regulation of th e structures that mediate substance abuse to a more mature cortical regulatory mechanism (Spear LP, 2000). During adolescence, the primary dopamine rgic (DAergic) projections to the nucleus accumbens septi (NAcc) extend from the ventral tegmental area (VTA), and are predominately modulat ed by the amygdala (Oades RD & Halliday GM, 1987). However by adulthood, these previously amygdalamodulated regulatory actions are repl aced by projections from the medial prefrontal cortex (mPFC) indicating some developmental transition in the functional nature of the system. The de velopment of this system allows for a transition from more emoti onally directed behavior to more contextually regulated behavior. Because adolescents lack sufficient cortical regulation provided by the mPFC, their behavior te nds to be more impulsive and guided by emotion than adults, increasing the chances of risky behaviors (e.g. initiating drug use). Additionally, chronic administration of an agonist (e.g. cocaine) during this period may cause a functional change in accumbal

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33 dopamine (DA) efflux by altering amygdalar modulation of accumbal DA release and/or altering the functional ro le of the mPFC input; consequently, leading to an increased risk of dependency during adulthood. Together, these implications make a powerful argumen t for treating adolescence as a key time period for researching the de velopment of drug addiction. It is clear that adolescence is an important developmental period. Despite this, very few models of adol escent drug addiction in animals have been developed to examine the remarkable differences between adolescents and adults. Many neurobehavioral age-sp ecific alterations that are seen in human adolescents are also observed in adolescent rats from around postnatal days (PND) 30 to PND 42 (Odell WD, 1990) Adolescent animal models need to evaluate neurochemical and behavioral changes due to drug use during this important stage of development. Novelty seeking and high risk behaviors seem to be highly associated with adolescence (Douglas L, Varlinskaya E Spear L, 2003; Stansfield KH, Philpot RM & Kirstein CL, 2004) (Ful ker D, Eysenck SBG & Zuckerman M, 1980). Along this unique stage of developm ent, distinct soci al, behavioral and neurochemical changes emerge, to assist with the important life events that will occur. For example, learning and acquiring skills necessary to permit survival away from parental careta kers(Spear LP, 2000). This phenomenon being evolutionary adaptive as a mean s to avoid inbreeding (Schlegel A & Barry III H, 1991). In order for a successful transition from childhood to adulthood, an important aspect to gain ing independence is when adolescents

PAGE 40

34 shift their social orientations from a dults to peers (Steinberg L, 1989) and typically spend a significant amount of ti me interacting with their peers as opposed to adults. Adolescence is also marked by high levels of risk taking behavior relative to indivi duals of other ages. Hu man adolescents as a group exhibit a disproportional amount of reck less behavior, sensation seeking and risk taking (Trimpop RM, Kerr JH & Kirkcaldy B, 1999). Risk taking in adolescents poses some nega tive consequences such as suicides, accidents, AIDS, pregnancy and drug dependence(Irwin Jr.CE, 1989). Although risk taking may be hazardous, it can also be beneficial. Similar to humans, adolescent rats are behaviorally and pharmacologically different from younger a nd older adult rats. Adolescent rats have been reported to be more hyper active and inattentive (Maldonado AM & Kirstein CL, 2005) (Spear LP & Brake SC, 1983) and have reduced responsiveness to some of the sedating effects of al cohol (Silveri MM & Spear LP, 1998), amphetamine (Bolanos CA, Glatts J and Jackson D, 1998), and cocaine (Laviola G, Wood RD Kuhn C Francis R & Spear LP, 1995). In the conditioned place prefer ence (CPP) paradigm, adolescent rats show a preference for nicotine, wher eas the adult rats did not (Vastola BJ, Douglas LA Vaarlinskaya EI & Spear LP, 2002). Al so, Philpot et al (Philpot RM, Badanich KA & Kirstein CL, 2003) demonstrated that adolescent rats showed a preference for moderate doses of alcohol, whereas the adults had a conditioned place aversion.

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35 There are also dramatic changes in the adolescent brain, both circuitry and neurochemistry. The mesolimbic and mesocortical brain regions and their DA projections undergo substantial rem odeling during the adolescent period (Spear LP, 2000). Rosenberg & Lewis (Rosenberg DR & Lewis DA, 1995) were among those researchers who saw a common developmental pattern in the overproduction and subsequent pruning of synaptic connections during the period preceding adulthood. The D1 and D2 receptors have been of major focus for years in regards to overproduc tion and pruning. D1 and D2 receptors increase in density in the first few w eeks of life (Hartley EJ & Seeman P, 1983). Subsequently, Teicher et al (Tei cher MH, Andersen SL & Hostetter JC Jr., 1995) have demonstrated receptor overproduction and elimination in both the striatum and prefrontal cortex, but have failed to show evidence that the NAcc follows the same overproduction and pruning construct (Andersen SL, Thompson AT Rutstein M Hostetter JC & Teicher MH, 20 00). In addition, alterations in receptor binding and sens itivity in various neurotransmitter systems have been reported during adolescence (Trauth JA, Seidler FJ McCook EC & Slotkin TA, 1999) along with changes in myelination of neurons (Hamano K, Iwasaki N Takeya T & Takita H, 1996). Adolescent animals, both huma n and non-human, exhibit many behavioral, social and neurochemical adap tations that enable them to develop successfully. However, these adaptations can have negative implications when these normal developmental behaviors emerge as persistent deviant actions that result in drug abuse. The present study examined the relationship between

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36 novel environment exploratory behavior novel object pref erence, noveltyinduced exploration and novelty-induced impulsivity in relation to DA responsiveness to cocaine among adolescent and adult animals. Methods Behavioral testing To isolate high responding (HR) vers us low responding (LR) rats based on several measures of behavioral sensitivity, fifty Sprague-Dawley (Harlan) rats postnatal day (PND) 34 (=134g) and PND 59 (=293.13g) at the time of testing were used as subjects in these e xperiments. No more than one male per litter per age was used in a given conditi on. Pups were sexed and culled to 10 pups per litter on PND 1. Pups remained housed with their respective dams in a temperature and humidity-controlled vi varium on a 12:12 h light:dark cycle (07:00 h/19:00 h) until PND 21, following which pups were weaned and group housed. Animals were tested in a dimly lit room on a black plastic circular platform (216 cm in diameter) standi ng 70 cm from the gr ound, with a white plastic barrier enclosing the arena ( 216 cm). A video camera was suspended directly over the table and recorded th e animal's behavior using a Noldus Behavioral Tracking System (See experiment one). In all respects, maintenance and treatment of the animals were in accordance with the guidelines estab lished by the NIH (NIH, Guide for the care and use of laboratory animals, 2005).

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37 Surgery Animals were anesthetized on either PND 34 or 59 using a ketamine/ xylazine cocktail (1.0 and 0.15 mg/kg/ip re spectively). An incision was made over the skull and the rat wa s mounted on a stereotaxic in strument for surgery. Three holes were drilled in the skull (two for skull screws and one for the guide cannula). The guide cannula was lowered to the NAcc shell (Philpot RM, McQuown S & Kirstein CL, 2001) and affixed to the skull with cranioplast. Probes were immediatel y lowered following surgery into the anesthetized rat aimed at the NAcc. In vivo microdialysis Apparatus Animals were singly housed with ad lib food and water in a BAS Raturn System bowl for recovery overnigh t. The Raturn system consisted of a large round bottom bowl (14 by 16). The animals were tethered via a locking collar clamp and a counter bala nced arm through which dialysis tubing was threaded. An optical switch mechanism signaled rotation of the bowl in the opposite direction of the animals m ovement enabling the animal to move about freely. In Vivo Microdialysis The probe inlet tubing was att ached to a 2 ml Hamilton syringe mounted on a BAS syringe pump set to a flow rate of 0.5 l/min. In vivo microdialysis probes with 2 mm membrane tips (BAS) were perfused continuously with artificial cerebrospi nal fluid (145 mM NaCl, 2.4 mM KCL, 1.0 mM MgCl, 0.2 mM ascorbate, pH=7.4) for twelve hours prior to the start

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38 of sampling. On either PND 35 or 60, dial ysates were collecte d at a flow rate of 0.5 ul/min at ten-minute interval s from the probe outlet tubing into refrigerated microcentrifuge tubes c ontaining 2.0 l of 0.25M hydrochloric acid (HCl). Following th e collection of six baseline samples, animals received an injection of 0.9% saline mg/kg/ip. After the control in jections, sampling continued at ten-minute intervals for 120 minutes after which an injection of cocaine (cocaine HCL was obtained from Sigma and di ssolved in 0.9% saline) was administered (20 mg/kg/ip). Samp ling continued at ten-minute intervals for an additional 120 minutes. Dialysat e samples (12.5 l) were either run immediately on an HPLC-EC or stored at -80C until analyzed at a later date. Neurochemical Analyses Analysis of dialysate samples was performed with a reverse phase high performance liquid chromatogra phy system (BAS) coupled with electrochemical detection (HPLC-EC) set to oxidize catecholamines (650 mV). An amperometric detector with a LC-4 C carbon working electrode referenced to an Ag/AgCl electrode was used to identify chemicals. Neurochemical analyses included the detection of DA and its major metabolite 3,4dihydroxyphenylacetic acid (DOPAC). Th e mobile phase consisted of 0.04 M sodium acetate, 0.01 M citric acid, 0.05 mM sodium octyl sulfate, 20.911 M disodium EDTA, 0.013 M NaCl and 10% v/ v methanol (pH 4.5) set at a flow rate of 60 l/min. Samples (6l) were injected onto a C-18 microbore column for peak separation. Data were record ed and quantified by Rainin Dynamax Software on a Power Macintosh 7500/100.

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39 Histology Following probe removals, rats were euthanized via CO inhalation. Brains were rapidly rem oved and frozen in 2 met hylbutane and stored at 80C. Brains were sliced into 40m sections, which were mounted on slides and stained with Cresyl Vi olet. Probe placements we re verified for placement in the NAcc shell. Any animals whose probes were not verified in the NAcc shell were examined but excluded from statistical analysis. Design and Analysis Basal DA values were converted to Area Under the Curve (AUC) to determine DA levels after the saline and cocaine injections. Cocaine induced DA levels were divided from the control levels (saline) in order to determine an individual animals responsivity to cocai ne. Behavioral da ta was then used to separate animals into HR or LR ba sed on the mean split of all animals in the experiment. Subsequent Students t-tests were performed to isolate differences between groups. In addition, DA turnover was assessed by a DOPAC/DA ratio and performing the same statisti cal analyses as described above. Results Animals were separated based on several behavioral measures including novel environm ent exploratory behavior novel object preference, novelty-induced exploration and novelty-induced impulsivity. As seen in appendices G-J, behavior was clearly de fined and easily differentiated as to being classified as a high re sponding or a low responding animal.

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40 Interestingly, basal DA was significantly lower in adolescent animals compared to adult animals [t(1,17 )=2.057, p<0.05, appendix K]. Overall, no cocaine induced dopaminergic differen ces were detected when collapsing across age [t(1,17)=0.2403, p>0.05, appendix L] and an analysis of DA turnover (DOPAC/DA AUC) revealed no differences in cocaine induced dopaminergic activity in adolescent an imals compared to adult animals, [t(1,8)= 0.04, p>0.05, appendix M], for this reason an AUC stat istical analysis was performed on each animals basal levels of DA. The present findings indicate that ad ult animals who exhibited a greater novel environment exploratory behavior (total distance moved on trial 1) had greater DAergic responsivity to cocai ne that the LR counterparts [t(1,9)= 2.347, p<0.05, appendix N]. No differences were detected between low and high responding adolescent animals. The present findings indicate that mo re impulsive adolescent animals have a greater cocaine induced DAergic response compared to less impulsive animals [t(1,7)= 3.581, p<0.05, appendix O]. No differences in DAergic activity in adult animals was detected based on the impulsivity measure [t(1,9)= 0.178, p>0.05, appendix O]. Interestingly, no differences were de tected between adolescent animals DA responsivity in relationship to the time spent with the novel object. However, adult animals who spent less time with the novel object on test exhibited greater DAergic responsivity to a cocaine challenge [t(1,9)= 2.444, p<0.05, appendix P]. Nevertheless, adol escent animals who approached the

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41 novel object more frequently exhibited a greater dopaminergic responsivity to a cocaine challenge versus adolescents who approached the novel stimuli less during the test, [t(1,7)= 3.581, p<0.05, appendix Q]. In comparison, adult animals who approached the novel stimuli more frequently exhibited a smaller dopaminergic responsivity to cocain e, [t(1,9)= 2.734, p<0.05, appendix Q]. Discussion Previous work in adult animals ha s shown that a preference for novelty is indicative of a facili tated acquisition of drug abuse (Klebaur JE, Bevins RA Segar TM Bardo MT, 2001). Research l ooking at adolescents has found that adolescent animals and humans who prefer novelty are more likely to use/abuse drugs (Spear LP, 2000; Zuck erman M, 1986). The present studys goal was to determine if differences existed in neurochemical activity in relationship to the noveltyseeking profiles of adolescen t and adult animals. Several researchers have shown that a dult animals have higher basal DA in response to a novel environment (TDM on trial 1) and also a greater DAergic response to cocaine and amphetamine th at exhibited high novel environment exploratory behavior (Bradberry CW, Gruen RJ Berridge CW & Roth RH, 1991; Hooks MS, Colvin AC Juncos JL & Justice JB Jr 1992; Hooks MS, Jones GH Smith AD Neill DB & Justice JB Jr., 1991; Rouge-Pont F, Piazza PV Kharouby M Le Moal M & Simon H, 1993). These studies show that HR adult rats classified by novel envir onment exploratory be havior exhibit a greater DAergic response to a pharmacol ogical challenge of cocaine compared to their LR counterparts.

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42 The present data support and extend these previous findings. Interestingly, adolescent animals dem onstrated a different behavioral and neurochemical pattern than adult an imals. HR adolescent animals who approached the novel object faster during test exhibited a si gnificantly greater DAergic response to a subsequent cocai ne challenge compar ed to their LR counterparts. This is interesting as HR adolescent animals DA levels were significantly higher over LR while adult animals LR and HR had comparable cocaine-induced increases in DA. A dolescent LR and HR did exhibit equal cocaine-induced DA when divided base d on time spent with novel object. However, on this behavioral measure, LR adults (i.e., adult animals that spent less time with the object ) had significantly increased cocaine-induced DA when compared to the adult animals th at spent more time with the object, perhaps indicative of a more responsive mesolimbic pathway (i.e. attention due to neophobia) that was not apparent in a dolescent animals. Similarly, adults which approached the object less during test (i.e., LR) had greater cocaineinduced DA during challenge compared to HR adults while the opposite was true for adolescent animals (i.e., LR had a significantly increased cocaineinduced DA than HR adolescent animals). During adolescence, behaviors associated with higher nove lty-induced exploration and impulsivity is related to a greater DAergic response to cocaine whereas adult animals that exhibit the same behavioral profile demonstrat e a reduced DAergic response compared to their LR counterparts This suggests that DA is involved with sensory gaiting which may explain why initiati on of drug use during adolescence may

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43 lead to enhanced incentive salience (i.e. attention) of environmental cues surrounding use. A pharmacological challenge of cocaine acts as an indirect agonist blocking reuptake of DA via the dopami ne transporter (DAT) and does not increase the amount of DA being synthesized, therefore, the extracellular DA in response to cocaine demonstrates a measure of basal DA tone. However, it is known that cocaine decr eases firing of VTA neurons, potentially due to actions at the D2 autoreceptors. Therefore, this increase DA in adolescence may represent decreased sensitivity or number of autoreceptors (Chen YC, Choi JK Andersen SL Rosen BR Jenkins BG, 2004) or perhaps an immaturity of other feedback regulatory systems (Jones EA, Want JQ McGinty JF, 2001). These findings further suggest that adolescent animals who exhibit greater novelty-induced exploration (i.e. fre quency of approaches) and noveltyinduced impulsivity (i.e. latency to appr oach) have an elevated DAergic tone. In contrast, adult animals who have greater novelty-in duced exploration (frequency of approaches), novel object preference (time spent with object) and behavioral activation in response to a novel object (TDM on test) have a lower DAergic tone supporting the matura tion of inhibitory control in the regulation of DA in the NAcc. The tr ansition from adolescence to adulthood involves several critical developmental changes in brain pathways involving attention, decision making, emotional regu lation and behavioral activation and inhibition. Specifically, the corticolimbi c circuitry consisting of the PFC and the subsequent in teraction with the amygda la (AMY) and NAcc with

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44 innervations mediated via DAergic and glutamatergic (GLUer gic) projections. Since the AMY is involved with contextual conditioning and emotional regulation, it can be viewed as an activ ational system as opposed to the PFC, which is involved with behaviors i nvolved in the cognitive processes of decision making, planning, impulse cont rol, self-monitoring and forward thinking, it can be characterized as a behavioral inhibitory system. Cunningham (Cunningham MG, Bhattacharyya S & Benes FM, 2002) demonstrated an increase in amygda lo-prefrontal fiber innervation during adolescence, suggesting that the conn ectivity between em otional learning (AMY), and executive decision making (PFC ) regulating regions is still being developed. Campbell (Campbell BA, Lytle LD & Fibiger HC, 1969) has demonstrated that the activational system develops before the inhibitory system matures, which subsequently leads to a period during adolescence characterized by high novelty-seekin g and risk taking behaviors. As the present data demonstrate, adolescent animals exhibit a greater behavioral activation compar ed to their adult counterparts, in addition, this newly established paradigm is an e ffective means by which separation based on individual variability within each behavioral measure can be achieved. Additionally, these re sults illustrate that a response to a pharmacological challenge of cocaine exhibits a comple x interaction with age and behavioral measures such as impulsivity and novelty preference. The transition from adolescence to adulthood is a criti cal developmental period involving the maturation of the corticolimbic circ uitry, where the development of the

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45 activational system (with li ttle inhibitory control) produces increased noveltyseeking, and risk taking behaviors corresponding to changes in DA levels in the NAcc. The transi tion into adulthood is associated with the development of an inhibitory behavioral system that competes with the activational system and is manifested by a reduction in risk taking behaviors and a subsequent alteration of DA production in the NAcc. The present findings support the no tion that adolescen t animals show different behavioral and ne urochemical profiles than adults that may possibly be driven by the initial development of the AMY regulated activational system and the delay in the development of the PFC inhibitory system. The interval between the development of the activational and inhibitory system may account for the novelty-seeking and novelty-induced impulsivity that distinguishes the adolescen t period which is marked by increased risk-taking behaviors such as drug use initiation.

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46 Summary Taken together, there is a complex interaction between adolescent and adult behavioral and neurochemical char acteristics that must be considered when designing developmental resear ch experiments modeled on adult paradigms. Adolescent animals exhib it greater novel environment exploratory behavior, novel object explorati on, novelty-induced exploration and impulsivity in relationship to adult animals. This is a clear developmental difference between adolescence and adult hood that is behaviorally observable with the purpose of isolating novelty-seek ing and risk-taking behaviors. Using the present paradigm, not only are thes e behaviors between adolescent and adult animals reliably iden tified, this approach also serves as a good means to distinguish between high responding and low res ponding animals based on these behavioral categories. In addition, adolescent and adult animals demonstrate unique neurochemical prof iles possibly due to the continuing development of certain brai n structures. The robust findings demonstrated in the area examined in the present study (i e. the mesolimbic projection area, the NAcc) are regulated by both the AMY and the PFC. The AMY is associated with emotional learning and is developed during adolescence, however, a region critically involved with executiv e decision making (PFC) is still being developed, which suggests that while adol escents have an ac tivational system,

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47 they lack inhibitory control therefore are more like ly to engage in risky behaviors such as drug use initiati on. Future studies should examine the dynamic interaction and ontoge ny of these structures and their role in the development of addiction.

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48 REFERENCES 1. Andersen SL, Thompson AT Rutstein M Hostetter JC & Teicher MH. (2000). Dopamine receptor pruning in prefrontal cortex during the periadolescent period in rats. Synapse, 37(2), 167-169. 2. Arnett, JJ. (1999). Adolescent storm and stress, reconsidered. Am.Psychol, 54(5), 317-326. 3. Badiani A, Browman KE Robi nson TE. (1995). Influence of novel versus home environment on sensitization to the psychomotor stimulant effects of cocaine and amphetamine. brain research 674:291-298. 4. Bardo MT. (1998). Neuropharmacological mechanisms of drug reward: beyond dopamine in the nucleus accumbens. Crit Rev.Neurobiol, 12:37-67, 37-67. 5. Bardo MT, Donohew RL Harrington NG. (1996). Psychobiology of novelty seeking and drug seeking behavior. Behav Brain Res, 77, 23-43. 6. Bates ME, Labouvie EW White HR. (1986). The effects of sensation seeking needs on alcohol and marijuana use in adolescence. Bull Soc Psychol Addict Behav, 5, 29-36. 7. Becker JB, Rudick CN Jenkins WJ (2001). The role of dopamine in the nucleus accumbens and striatum during sexual behavior in the female rat. J.Neurosci, 21(9), 3236-3241. 8. Berns GS, McClure SM Pa gnoni G & Montague PR. (2001). Predictability modulates human br ain response to reward. The Journal of Neuroscience, 21(8), 2793-2798. 9. Berridge KC & Robinson TE. (1998). What is the role of dopamine in reward, hedonic impact, reward l earning or incentive salience? Brain research, 28, 309-369.

PAGE 55

49 10. Bevins RA, Klebaur JE & Bardo MT. (1997). Individual differences in response to novelty, amphetamine-induced activity and drug discrimination in rats. Beha v Pharmacol., 8(2-3), 113-123. 11. Biederman J, Wilens TE Mick E Faraone SV Spencer TJ. (1998). Does attention deficit hyperactivity disorder impact the developmental course of drug and alcohol abuse and dependence? Biol Psychiatry, 269-273. 12. Blander DS, Mark GP Hern andez L and Hoebel BG. (1988). Angiotensin and drinking indu ce dopamine release in the nucleus accumbens. Society for neuroscience abstracts, 14, 527. 13. Bolanos CA, Glatts J and Jack son D. (1998). Subsensitivity to dopaminergic drugs in periadoles cent rats: a behavioral and neurochemical analysis. Dev.Brain Res, 111, 25-33. 14. Bowman EM, Aigner AT Richmond ABJ. (1996). Neural signals in the monkey ventral striatum relate d to motivation for juice and cocaine rewards. Society for neuroscience abstracts, 75, 10611073. 15. Bradberry CW, Gruen RJ Berridge CW & Roth RH. (1991). Individual differences in behavioral measur es: correlations with nucleus accumbens dopamine measured by microdialysis. Pharmacol Biochem Behavior, 39(4), 877-882. 16. Camp DM, Browman KE & Robinson TE. (1994). The effects of methamphetamine and cocaine on motor behavior and extracellular dopamine in the vent ral striatum of lewis versus fischer 344 rats. Brain Res., 1-2, 180-193. 17. Campbell BA, Lytle LD & Fibige r HC. (1969). Ontogeny of adrenergic arousal and cholinergic inhibito ry mechanisms in the rat. Science, 166(905), 635-637. 18. Carelli RM, Ijames SG. (2001) Selective activation of accumbens neurons by cocaine-associated s timuli during a water/cocaine multiple schedule. Brain research, 907, 156-161. 19. Cass WA & Zahniser NR. (1993). Cocaine levels in striatum and nucleus accumbens: augmentation following challenge injection in rats withdrawn from repeated cocaine administration. Neurosci.Lett, 152, 177-180.

PAGE 56

50 20. Chang VC, Mark GP Hernandex L & Hoebel BG. (1988). Extracellular dopamine increases in the nucleus accumbens following rehydration or sodium repletion. Society for neuroscience abstracts, 14, 527. 21. Chen YC, Choi JK Andersen SL Rosen BR Jenkins BG. (2004). Mapping dopamine d2/d3 receptor function using pharmacological magnetic resonance imaging. Psychopharmaclogy (Berl), epub. 22. Clark DB, Kirisci L Tarter RE. (1998). Adolescent versus adult onset and the development of substance use disorders in males. Drug Alcohol Depend., 49, 115-121. 23. Corwin RL, Woolverton WL Schuster CR & Johanson CZE. (1987). Anorectics: effects on food inta ke and self-administration in rhesus monkeys. Alcohol drug res. 7, 351-361. 24. Crow TJ. (1972). Catecholamine-c ontaining neurons and electrical selfstimulation: a review of some data. Psychol.Med, 2, 414-421. 25. Cunningham MG, Bhattacharyya S & Benes FM. (2002). Amygdalocortical sprouting con tinues into early adulthood: implications for the development of normal and abnormal function during adolescence. J.Comp.Neurol. 453(2), 116-130. 26. Damsma G, Pfaus JG Wenkste rn D Phillps AG & Fibiger HC. (1992). Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: comparison with novelty and locomotion. Behavioral neuroscience, 106, 181-191. 27. De La Garza R, Callahan PM & Cunningham KA. (1998). The discriminative stimulus properties of cocaine: effects of microinfusion of cocaine, a 5-ht1a agonist or antagonist, into the ventral tegmental area. Psychopharmaclogy (Berl), 137, 1-6. 28. Dellu F, Piazza PV Mayo W Le Moal M &Simon H. (1996). Noveltyseeking in ratsbi obehavioral character istics and possible relationship with the sensation-seeking trait in man. Neuropsychobiology, 34, 136-145. 29. Deutch AY, Bourdelais AJ & Zahm DS. (1993). The nucleus accumbens core and shell: accumbal compartments and their functional attributes. Limbic Motor Circuits and Neuropsychiatry, 45-88.

PAGE 57

51 30. DeWit DJ, Adlaf EM Offord DR Ogborne AC. (2000). Age at first alcohol use: a risk factor for the development of alcohol disorders. Am.J.Psychiatry, 157(5), 745-750. 31. Di Chiara G. (1995). The role of dopamine in drug abuse viewed from the perspective of its role in motivation. Drug Alcohol Depend, 38, 95-137. 32. Di Chiara G, Imperato A. (1988). Drugs abused by humans preferentially increase synaptic dopamine concentrations in the mesolimbic system of freely moving rats. Proc.Natl.Acad.Sci, 85, 5274-5278. 33. Douglas L, Varlinskaya E Sp ear L. (2003). Novel-object place conditioning in adolescent and adult male and female rats: effects of social isolation. Physiology & Behavior, 80, 317325. 34. Esposito RU & Kornetsky C. (1978). Opioids and rewarding brain stimulation. Neurosci.Biobehav.Rev, 2, 155. 35. Estroff TW, Schwartz RH & Hoffmann NG. (1989). Adolescent cocaine abuse. addictive potenti al, behavioral and psychiatric effects. Clin.Pediatr, 28, 550-555. 36. Eysenck SB & Eysenck HJ. ( 1977). The place of impulsiveness in dimensional system of personal ity description. The British Journal of Social & Clinical Psychology, 16, 57-68. 37. Fiorino DF, Coury A Fibiger HC & Phillips AG. (1993). Electrical stimulation of reward sites in the ventral tegmental area increases dopamine transmission in the nucleus accumbens of the rat. Behav.Brain Res., 55, 131-141. 38. Forsyth G, Hunleby JD. (1987). Personality and situation as determinants of desire to drink in young adults. Int. J Addict, 22, 654-659. 39. Fulker D, Eysenck SBG & Zuckerman M. (1980). A genetic and environmental analysis of sensation seeking. J. Res.Pers., 14, 261-281. 40. Gardner EL, Paredes W Smith D Donner A Milling C Cohen D & Morrison D. (1988). Facilitation of brain stimulation reward by (delta) 9-tetraahydrocannabinol. Psychopharmacology, 96, 142-144.

PAGE 58

52 41. German DC & Bowden DM. (1974) Catecholamine systems as the neural substrate for intracranial self-stimulation: a hypothesis. Brain research, 73, 381-419. 42. Hamano K, Iwasaki N Takeya T & Takita H. (1996). A quantitative analysis of rat central nervous system myelination using the immunohistochemical method for mbp. Dev.Brain Res., 93, 18-22. 43. Hansell S and White HR. (1991). Adolescent drug use, psychological distress, and physical symptoms J.Hlth Social Behav, 32, 2881-301. 44. Hartley EJ & Seeman P. (1983). Development of receptors for dopamine and noradrenaline in rat brain. Eur.J.Pharmacol., 91(4), 391-397. 45. Hernandez L & Hoebel BTG. (1988). Food reward and cocaine increase extracellular dopamine in the nucleus accumbens as measured by microdialysis. Life Sci., 42, 1705-1712. 46. Hobel BG, Monaco AP Hernand ez L Ausili EF Stanley BG & Lenard L. (1983). Self-injection of ampheta mine directly into the brain. Psychopharmacology, 81, 158-163. 47. Hoebel BG & Hernandez L. (1989). Microdialysis studies of psychostimulants. NIDA Res Monogr, 343-344. 48. Hooks MS, Colvin AC Juncos JL & Justice JB Jr. (1992). Individual differences in basal and cocai ne-stimulated extracellular dopamine in the nucleus accumbens using quantitative microdialysis. Brain research, 587, 306-312. 49. Hooks MS, Jones GH Smith AD Neill DB & Justice JB Jr. (1991). Response to novelty predicts the locomotor and nucleus accumbens dopamine response to cocaine. Synapse, 9, 121128. 50. Hurd YL, Weiss F Koob G & Ungerstedt U. (1989). Cocaine reinforcement and extracellular dopamine overflow in rat nucleus accumbens: an in vivo mi crodialysis study. Brain Res, 498, 199-203. 51. Irwin Jr.CE. (1989). Risk taking be haviors in the adol escent patient: are they impulsive? Pediatric Annals, 18, 122-133.

PAGE 59

53 52. Izenwasser S & Cox BM. (1992). Inhibition of dopamine uptake by cocaine and nicotine: tolerance to chronic treatment. Brain Res., 573, 119-125. 53. Johnston LD. (2000). Monitoring the future: national survey results on drug use, 1975-1999. 54. Jones EA, Want JQ McGinty JF. (2001). Intrastriatal GABA(A) receptor blockade does not alter dopamine d1/d2 receptor interactions in the intact rat striatum. Neuroscience, 102(2), 381-389. 55. Kabbaj M, Devine DP Savage VR and Akil H. (2000). Neurobiological correlates of individual differe nces in novelty-seeking behavior in the rat: differential expression of stress-related molecules. The Journal of Neuroscience, 20(18), 6983-6988. 56. Kalivas PW & Duffy P. (1995). Selective activation of dopamine transmission in the shell of the nucleus accumbens by stress. Brain research, 675, 325-328. 57. Killcross AS & Robbins TW. (1993). Differential effects of intraaccumbens and systemic amphe tamine on latent inhibition using an on-baseline, within -subject conditioned suppression paradigm. Psychopharmacology, 110, 479-489. 58. Klebaur JE, Bevins RA Segar TM Bardo MT. (2001). Individual differences in behavioral res ponses to novelty and amphetamine self-administration in male and female rats. Behav.pharmacol., 12(4), 267-275. 59. Kleven M, Woolverton W Schus ter C & Seiden. (1988). Behavioral and neurochemical effects of re peated or continuous exposure to cocaine. NIDA Res Monogr, 81, 86-93. 60. Knutson B, Adams CM Fong GW & Hommer D. (2001). Anticipation of increasing momentary reward selectively recruits nacc. J.Neurosci., 21, 159. 61. Koob GF & Le Moal M. (1997). Drug abuse: hedonic homeostatic dysregulation. Science, 278, 52-58. 62. Koob GF & Le Moal M. (2001) Drug addiction, dysregulation of reward, and allostasis. Ne uropsychopharmacology, 24(2), 97129.

PAGE 60

54 63. Koob GF, Caine SB Parsons L Markou A & Weiss F. (1997). Opponent process model and psychostimulant addiction. Pharmacol.Biochem.Behav., 57, 513-521. 64. Kuczenski R, Segal DS & Aizenstein ML. (1991). Amphetamine, cocaine, and fencamfamine: re lationship between locomotor and stereotypy response profiles and caudate and accumbens dopamine dynamics. J.Neurosci., 11(9), 2703-2712. 65. Laviola G, Wood RD Kuhn C Francis R & Spear LP. (1995). Cocaine sensitization in periadolescent and adult rats. J.Pharmacol.Exp.Ther., 275, 345-357. 66. Lindvall O & Bjorklund A. (1974) The organization of the ascending catecholamine neuron systems in the rat brain as revealed by the glyoxylic acid fluorescence method. Acta Physiol Scand Suppl., 412, 1-48. 67. Maldonado AM & Kirstein CL. (2005). Handling alters cocaineinduced activity in adolescen t but not adult male rats. Physiology & Behavior, 84(2), 321-326. 68. McBride WJ, Murphy JM & Ikemot o S. (1999). Localization of brain reinforcement mechanisms: intracranial self-administration and intracranial place-conditioning stud ies. Behav.Brain Res., 101, 129-152. 69. Molina B, Pelham W. (2003). Ch ildhood predictors of adolescent substance use in a longitudinal study of children with ADHD. Journal of abnormal psychology, 112,3, 497-507. 70. Morrow BA, Taylor JR & Roth RH. (1995). Prior exposure to cocaine diminishes behavioral and bioc hemical responses to aversive conditioning: reversal by gl ycine/n-methyl-d-aspartate antagonist co-treatment. Neuroscience, 69, 233-240. 71. National Institutes of Health. (1986). Guide for the care and use of laboratory animals. (DHEW Publication No.8623).Washington, DC: U.S.Gove rnment Printing Office, 72. Nestler EJ & Aghajanian GK. ( 1997). Molecular and cellular basis of addiction. Review. Sc ience, 278(5335), 58-63. 73. NIH, Guide for the care and us e of laboratory animals. (2005).

PAGE 61

55 74. O'Brien CP, Childress AR McLellan AT & Ehrman R. (1992). Classical conditioning in drug-dependent humans. Ann.NY Acad.Sci, 654, 400-415. 75. Oades RD & Halliday GM. (1987). Ventral tegmental (a10) system: neurobiology. 1. anatomy and c onnectivity. Brain Res, 434, 117-165. 76. Odell WD. (1990). Sexual maturation in the rat. 183-210. 77. Olds J & Milner P. (1954). Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J Comp.Physiol.Psychol., 47, 419-427. 78. Olds ME. (1978). Comparative effects of amphetamine, scopolamine, chlordiazepoxide and diphenylhydantoin on operant and extinction behaviour with brain stimulation and food reward. Neuropharmacology, 9, 519-532. 79. Olds ME & Fobes JK. (1981). Th e central basis of motivation: intracranial self-stimulation studies. Annual Review of Psychology, 32, 523-574. 80. Parsons LH, Smith AD & Justice JB Jr. (1991). Basal extracellular dopamine is decreased in the rat nucleus accumbens during abstinence from chronic cocaine. Synapse, 9(1), 60-65. 81. Philpot RM & Kirstein CL. (1998) The effects of repeated alcohol exposure on the neurochemistry of the periadolescent nucleus accumbens septi. Neuroreport, 9, 1359-1363. 82. Philpot RM, Badanich KA & Kirstein CL. (2003). Place conditioning: age-related changes in the rewa rding and aversive effects of alcohol. Alcoholism: Clinical and Experimental Research, 27(4), 593-599. 83. Philpot RM, McQuown S & Ki rstein CL. (2001). Stereotaxic localization of the developing nuc leus accumbens septi. Brain Res Dev Brain Res., 130(1), 149-153. 84. Post RM. (1980). Intermittent versus continuous stimulation: effect of time interval on the development of sensitization or tolerance. Life Sci, 26, 1275-1282.

PAGE 62

56 85. Rebec GV, Grabner CP Johns on M Pierce RC & Bardo MT. (1997). Transient increases in catecholaminergic activity in medial prefrontal cortex and nucleus accumbens shell during novelty. Neuroscience, 76, 707-714. 86. Reith ME, Li MY & Yan QS. (1997). Extracellular dopamine, norepinephrine, and serotonin in the ventral tegmental area and nucleus accumbens of freely m oving rats during intracerebral dialysis following systemic administration of cocaine and other uptake blockers. Psychopha rmacology (Berl), 134(3), 309-317. 87. Robinson TE & Berridge KC. (1993). Th e neural basis of drug craving: an incentive-sensitization theory of addiction. Brain Research Rev, 18, 247-291. 88. Rodd ZA, Mckinzie DL Da gon CL Murphy JM & McBride WJ. (1998). Intracranial self-adminis tration of ethanol into the posterior vta by wist ar rats. Soc.Neurosci.Abst, 24, 1479. 89. Rosenberg DR & Lewis DA. (1995). Postnatal maturation of dopaminergic innervation of monkey prefrontal and motor cortices: a tyrosine hydroxilase immunohistochemical analysis. J.Comp.Neurol., 358, 383-400. 90. Rouge-Pont F, Piazza PV Kharouby M Le Moal M & Simon H. (1993). Higher and longer stress-indu ced increase in dopamine concentrations in the nucleus accumbens of animals predisposed to amphetamine self-administration. a microdialysis study. Brain research, 602, 169-174. 91. Royalle DR & Klemm WR. ( 1981). Dopaminergic mediation of reward: evidence gained using a natural reinforcer in a behavioral contrast paradigm Neurosci.Lett, 21, 223-229. 92. Salamone JD. (1992). Complex mo tor and sensorimotor function of striatal and accumbens dopamine: involvement in instrumental behavior processes. Psychopharmacology, 107, 160-174. 93. Schlegel A & Barry III H. (1991) Adolescence: an anthropological inquiry. 94. Schultz W. (1998). Predictive reward signal of dopamine neurons. J.Neurophysiol, 80(1), 1-27. 95. Schultz W, Dayan P & Montague PR. (3-14-1997b). A neural substrate of prediction and reward. Science, 275, 1593-1599.

PAGE 63

57 96. Schultz W, Dayan P & Montague PR. (3-14-1997c). A neural substrate of prediction and reward. Science, 275, 1593-1599. 97. Schultz W, Dayan P & Montague PR. (3-14-1997a). a neural substrate of prediction and reward. Science, 275, 1593-1599. 98. Shedler J, Block J. (1990). Adolescent drug use and psychological health. Am.Psychol, 612-630. 99. Silveri MM & Spear LP. (1998). De creased sensitivity to the hypnotic effects of ethanol early in ontogeny. Alcohol, Clin.Exp.Res., 22, 670-676. 100. Skagerberg G, Lindvall O & Bjor klund A. (1984). Origin, course and termination of the mesohabenular dopamine pathway in the rat. Brain Res, 307(1-2), 99-108. 101. Solomon PR & Staton DM. (1982). Differential effects of microinjections of d-amphetamine into the nucleus accumbens or the caudate putamen on the rat's ability to ignore an irrelevant stimulus. Biol Psychiatry, 17(6), 743-756. 102. Solomon RL. (1977). Addiction: an opponent-process theory of acquired motivation: the affectiv e dynamics of addiction. 66103. 103. Spear LP. (2000). The adolescent brain and age-related behavioral manifestations. Neurosci Biobehav Rev, 24(4), 417-463. 104. Spear LP & Brake SC. (1983). Periadolescent: age development behavior and psychopharmacologi cal responsivity in rats. Dev.Psychobiol., 16, 83-109. 105. Stansfield KH, Philpot RM & Kirstein CL. (2004). An animal model of sensation-seeking: the adolescen t rat. New York Academy of Sciences, 1021, 453-458. 106. Steinberg L. (1989). Pubertal maturation and parent adolescent distance: an evolutionary perspective. 71-97. 107. Strecker RE, Eberle WF & Ashby CR Jr. (1995). Extracellular dopamine and its metabolites in the nucleus accumbens of fischer and lewis rats: basa l levels and cocaine-induced changes. Life Sci., 56(6), 135-141.

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58 108. Teicher MH, Andersen SL & Ho stetter JC Jr. (1995). Evidence for dopamine receptor pruning between adolescence and adulthood in striatum but not nucleus accumbens. Brain Res Dev Brain Res., 89(2), 167-172. 109. Trauth JA, Seidler FJ McCook EC & Slotkin TA. (1999). Adolescent nicotine exposure causes persis tent upregulation of nicotinic cholinergic receptors in rat br ain regions. Brain Res, 851, 9-19. 110. Trimpop RM, Kerr JH & Kirkcaldy B. (1999). Comparing personality constructs of risk-taking behavi or. Personality and individual differences, 26, 237-254. 111. Tzschentke TM. (2000). The medial prefrontal cortex as a part of the brain reward system. Amino Acids, 19(1), 211-219. 112. Van Der Zee P, Koger HS Gootjes J & Hespe WZ. (1980). Aryl 1,4dialk(en)yl-piperazines as selectiv e and very potent inhibitors of dopamine uptake. Eur.J Med.Chem, 15, 363-370. 113. Vanderschuren LJ & Kalivas PW. (2000). Alterations in dopaminergic and glutamatergic transmission in the induction and expression of behavioral sensitization: a critical review of preclinical studies. Psychopharmacology (Berl), 151, 99-120. 114. Vastola BJ, Douglas LA V aarlinskaya EI & Spear LP. (2002). Nicotine-induced conditioned place preference in adolescent and adult rats. Physiology & Behavior, 77, 104-114. 115. Wallace JM Jr. (2003). Gender a nd ethnic differences in smoking, drinking and illicit drug use among American 8th, 10th and 12th grade students. Addiction, 98.2, 225-233. 116. Weiner I & Gal G. (1996). Differe ntial involvement of the shell and core subterritories of the nucleus accumbens in latent inhibition and amphetamine-induced activity. 81 (1-2) 123-33. Behav Brain Res, 81(1-2), 123-133. 117. Wilson C, Nomikos GG Collu M & Fibiger HC. (1995). Dopaminergic correlates of motivated behavi or: importance of drive. J Neurosci, 15, 5169-5178. 118. Wilson MC & Schuster CR. (1976). Mazindol self-administration in the rhesus monkey. Pharmacol.Biochem.Behav. 4, 207-210.

PAGE 65

59 119. Winger G & Woods JH. (1985). Comparison of fixed-ratio and progressive-ratio schedules of maintenance of stimulant drugreinforced responding. Dr ug Alcohol Depend., 15, 123-130. 120. Wise RA. (1980). Action of drugs of abuse on brain reward systems. Pharmacol.Biochem.Behav. 13(suppl 1), 213-223. 121. Yokel RA & Wise RA. (1978). Amphetamine-type reinforcement by dopaminergic agonists in the rat. Psychopharmacology, 58, 289-296. 122. You ZB, Chen YG & Wise RA (2001). Dopamine and glutamate release in the nucleus accumbens and ventral tegmental area of rat following lateral hypothalamic self-stimulation. Neuroscience, 107, 629-639. 123. Zuckerman M. (1984). Sensation-seeking: a common approach to a human trait. Behavioral and brain sciences, 7, 413-471. 124. Zuckerman M. (1986). Sensati on-seeking and the endogenous deficit theory of drug abuse. NIDA Res Monogr, 74, 59-70. 125. Zuckerman M. (1990). The psyc hophysiology of sensation seeking. J Pers., 58(1), 313-345.

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60 Appendices

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Appendix A: Total Distance Moved on Trial 1 Total Distance Moved Trial 1 PND 35 PND 60 0 1000 2000 3000 4000AGETDM (CM) Adolescent animals (white bar) moved significantly more during the first exposure to the novel environment than did adult animals (black lines). 61

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Appendix B: Total Distance Moved Across Trials Total Distance Moved Across Trials20002200240026002800300032003400360038004000123456789Trial NumberTDM (CM) PND 35 PND 60 Adolescent animals (black triangles) habituated to the novel environment after 4 trials while activity levels in adults (grey squares) remain relatively stable across trials. 62

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Appendix C: Total Distance Moved on Test Total Distance Moved Test PND 35 PND 60 0 500 1000 1500 2000 2500 3000AGETDM(CM) During testing with the novel object, adolescent (white bar) and adult (black lines) animals traveled similar amounts. 63

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Appendix D: Novelty Preference NOVELTY PREFERENCE PND 35 PND 60 0 5 10 15 20 25 30 35 40 45AGETIME (SEC) During test, adolescent animals (white bar) spent significantly more time interacting with the novel object than did adults (black lines). 64

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Appendix E: Latency to Approach Latency to approach PND 35PND 60 0 10 20 30 40 50 60 70 80 90 100 110 120AGETIME (SEC) During test, adolescent animals (white bar) approached the novel object significantly faster than did adults (black lines). 65

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Appendix F: Frequency to Approach Frequency to approach PND 35 PND 60 0 1 2 3 4 5 6 7 8 9 10 11 12AGE# APPROACHES Adolescent animals (white bar) approached the novel object significantly more times than did the adults (black lines). 66

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Appendix G: Novel Environment Exploratory Behavior Novel Environment Exploratory Behavior PND 35 PND 60 0 1000 2000 3000 4000 5000 6000AGETDM (CM) During exposure to a novel environment, adolescent (black triangles) and adult animals (grey squares) activity levels revealed a good distribution of scores allowing for reliable separation into high and low responders. 67

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Appendix H: Novel Object Preference Novel Object Preference PND 35 PND 60 0 10 20 30 40 50 60 70 80AGETime (sec) Spent w/ Object During test, adolescent (black triangles) and adult animals (grey squares) were reliably separated into high and low responders based on time spent interacting with the novel object. 68

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Appendix I: Novelty-Induced Impulsivity Novelty-Induced Impulsivity PND 35 PND 60 0 100 200 300AGELatency to Approach Novel Object(sec) A measure of novelty-induced impulsivity (latency to approach the novel object) was used with the purpose of separating adolescent (black triangles) and adult (grey squares) animals into high and low responders. 69

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Appendix J: Novelty-Induced Exploration Novelty-Induced Exploration PND 35 PND 60 0 10 20 30 40 50AGEFrequency of Approaches Animals were separated into high and low responding adolescent (black triangles) and adult (grey squares) animals based on the frequency to approach the novel object on test. 70

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Appendix K: Basal Dopamine Basal Dopamine PND 35 PND 60 0 1 2 3 4AGEDA (nM) Adolescent animals (white bar) had significantly lower basal DA levels when compared to young adult animals (black lines) (note: values not corrected for probe recovery). 71

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Appendix L: Cocaine-Induced DAergic Activity Across Age Cocaine-Induced DAergic Activity Across Age PND 35 PND 60 0 100 200 300 400 500 600 700 800 900 1000 1100AGECocaine AUC/ Saline AUC Cocaine-induced increases in DA were comparable across age. 72

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Appendix M: Age-Related DOPAC/DA Age-related DOPAC/DA Turnover PND 35 PND 60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8AGEDOPAC/DA (AUC) Turnover rates of DA (DOPAC to DA ratio) were comparable across age. 73

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Appendix N: Novel Environment Behavior and the DAergic Response to Cocaine Novel Environment Exploratory Behavior & DAergic Response to Cocaine LRHR 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900PND 35 PND 60 Total Distance Moved on Trial 1Cocaine(AUC)/ Saline (AUC) Adult animals (black lines) that exhibited greater novelty-induced locomotor activity (i.e., introduction to the novel environment on trial 1) had significantly higher cocaine-induced increases in DA compared to adults who scored lower on this behavioral measure. LR and HR adolescent animals (white bar) exhibited equal amounts of cocaine-induced DA efflux regardless of activity on trial one. 74

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Appendix O: Novelty-Induced Impulsivity and the DAergic Response to Cocaine Novelty-Induced Impulsivity & DAergic Response to Cocaine LRHR 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700PND 35 PND 60 Latency to Approach Novel ObjectCocaine(AUC)/ Saline(AUC) Less impulsive adolescent animals (white bar) had a significantly lower DAergic response to a challenge of cocaine compared to adolescents who were more impulsive. Both LR and HR young adults had comparable cocaine-induced increases in DA. (black lines). 75

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Appendix P: Novel Object Preference and the DAergic Response to Cocaine Novel Object Preference & DAergic Response to Cocaine LRHR 0 250 500 750 1000 1250 1500 1750 2000PND 35 PND 60 Time (sec) Spent with ObjectCocaine(AUC)/ Saline(AUC) Adult animals (black lines) who had a greater novel object preference (i.e. spent more time with the object) had a significantly lower DAergic response to a challenge of cocaine. Both LR and HR adolescent animals exhibited comparable cocaine-induced increases in DA (white bar). 76

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Appendix Q: Novelty-Induced Exploration and the DAergic Response to Cocaine Novelty-Induced Exploration & DAergic Response to Cocaine LRHR 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900PND 35 PND 60 Frequency of ApproachesCocaine(AUC)/ Saline(AUC) Adolescent animals (white bar) that had greater novelty-induced exploration scores (i.e. frequency of approaches) had a greater DAergic response to a challenge of cocaine compared to adolescent animals that approached the novel object less. Conversely, adult animals (black lines) that had greater novelty-induced exploration demonstrated lower DAergic responsivity to cocaine when compared to the LR adult animals. 77