Individual differences in cocaine-induced locomotor activity in male Sprague-Dawley rats and their acquisition of and motivation to self-administer cocaine

Self-administration acquisition latency predicts locomotor sensitivity to cocaine in male rats

Individual differences in drug use emerge soon after initial exposure, and only a fraction of individuals who initiate drug use go on to develop a substance use disorder. Variability in vulnerability to establishing drug self-administration behavior is also evident in preclinical rodent models. Latent characteristics that underlie this variability and the relationship between early drug use patterns and later use remain unclear. Here, we attempt to determine whether propensity to establish cocaine self-administration is related to subsequent cocaine self-administration behavior in male Sprague-Dawley rats (n = 14). Prior to initiating training, we evaluated basal locomotor and anxiety-like behavior in a novel open field test. We then trained rats to self-administer cocaine in daily 3 h cocaine (0.75 mg/kg/infusion) self-administration sessions until acquisition criteria (≥30 active lever presses with ≥70 % responding on the active lever in one session) was met and divided rats into Early and Late groups by median-split analysis based on their latency to meet acquisition criteria. After each rat met acquisition criteria, we gave them 10 additional daily cocaine self-administration sessions. We then conducted a progressive ratio, cocaine-induced locomotor sensitivity test, and non-reinforced cocaine seeking test after two weeks of forced abstinence. Early Learners exhibited significantly less locomotion after an acute injection of cocaine, but the groups did not differ in any other behavioral parameter examined. These results indicate that cocaine self-administration acquisition latency is not predictive of subsequent drug-taking behavior, but may be linked to physiological factors like drug sensitivity that can predispose rats to learn the operant task.

Metformin Prevents Cocaine Sensitization: Involvement of Adenosine Monophosphate-Activated Protein Kinase Trafficking between Subcellular Compartments in the Corticostriatal Reward Circuit

Repeated cocaine exposure produces an enhanced locomotor response (sensitization) paralleled by biological adaptations in the brain. Previous studies demonstrated region-specific responsivity of adenosine monophosphate-activated protein kinase (AMPK) to repeated cocaine exposure. AMPK maintains cellular energy homeostasis at the organismal and cellular levels. Here, our objective was to quantify changes in phosphorylated (active) and total AMPK in the cytosol and synaptosome of the medial prefrontal cortex, nucleus accumbens, and dorsal striatum following acute or sensitizing cocaine injections. Brain region and cellular compartment selective changes in AMPK and pAMPK were found with some differences associated with acute withdrawal versus ongoing cocaine treatment. Our additional goal was to determine the behavioral and molecular effects of pretreatment with the indirect AMPK activator metformin. Metformin potentiated the locomotor activating effects of acute cocaine but blocked the development of sensitization. Sex differences largely obscured any protein-level treatment group effects, although pAMPK in the NAc shell cytosol was surprisingly reduced by metformin in rats receiving repeated cocaine. The rationale for these studies was to inform our understanding of AMPK activation dynamics in subcellular compartments and provide additional support for repurposing metformin for treating cocaine use disorder.

Individual differences in dopamine uptake in the dorsomedial striatum prior to cocaine exposure predict motivation for cocaine in male rats

A major theme of addiction research has focused on the neural substrates of individual differences in the risk for addiction; however, little is known about how vulnerable populations differ from those that are relatively protected. Here, we prospectively measured dopamine (DA) neurotransmission prior to cocaine exposure to predict the onset and course of cocaine use. Using in vivo voltammetry, we first generated baseline profiles of DA release and uptake in the dorsomedial striatum (DMS) and nucleus accumbens of drug-naïve male rats prior to exposing them to cocaine using conditioned place preference (CPP) or operant self-administration. We found that the innate rate of DA uptake in the DMS strongly predicted motivation for cocaine and drug-primed reinstatement, but not CPP, responding when "price" was low, or extinction. We then assessed the impact of baseline variations in DA uptake on cocaine potency in the DMS using ex vivo voltammetry in naïve rats and in rats with DA transporter (DAT) knockdown. DA uptake in the DMS of naïve rats predicted the neurochemical response to cocaine, such that rats with innately faster rates of DA uptake demonstrated higher cocaine potency at the DAT and rats with DAT knockdown displayed reduced potency compared to controls. Together, these data demonstrate that inherent variability in DA uptake in the DMS predicts the behavioral response to cocaine, potentially by altering the apparent potency of cocaine.

Evaluation of the rewarding properties of nicotine and caffeine by implementation of a five-choice conditioned place preference task in zebrafish

The rewarding properties of drugs in zebrafish can be studied using the conditioned place preference (CPP) paradigm. Most devices that have been used for CPP consist of two-half tanks with or without a central chamber. Here we evaluated the rewarding effects of nicotine and caffeine using a tank with five arms distributed radially from a central chamber that we have denoted Fish Tank Radial Maze (FTRM). Zebrafish were trained to associate nicotine or caffeine with a coloured arm. In testing sessions to assess CPP induction, between two and five different arms were available to explore. We found that when offering the two arms, one of them associated to the drug mediating conditioning for 14 days, zebrafish showed nicotine-induced CPP but not caffeine-induced CPP. When zebrafish had the option to explore drug-paired arms together with new coloured arms as putative distractors, the nicotine-CPP strength was maintained for at least three days. The presence of novel environments induced caffeine-CPP, which was still positive after three days of testing sessions. Complementary behavioural data supported these findings. Nicotine-CPP was prevented by the histone deacetylase inhibitor phenylbutyrate administered during conditioning; however, there were no effects on caffeine-CPP. The specific acetylation of lysine 9 in histone 3 (H3-K9) was increased in nicotine-conditioned zebrafish brains. This study suggests that novel environmental cues facilitate drug-environment associations, and hence, the use of drugs of abuse.

Environmental, genetic and epigenetic contributions to cocaine addiction

Decades of research on cocaine has produced volumes of data that have answered many important questions about the nature of this highly addictive drug. Sadly, none of this information has translated into the development of effective therapies for the treatment of cocaine addiction. This review endeavors to assess the current state of cocaine research in an attempt to identify novel pathways for therapeutic development. For example, risk of cocaine addiction is highly heritable but genome-wide analyses comparing cocaine-dependent individuals to controls have not resulted in promising targets for drug development. Is this because the genetics of addiction is too complex or because the existing research methodologies are inadequate? Likewise, animal studies have revealed dozens of enduring changes in gene expression following prolonged exposure to cocaine, none of which have translated into therapeutics either because the resulting compounds were ineffective or produced intolerable side-effects. Recently, attention has focused on epigenetic modifications resulting from repeated cocaine intake, some of which appear to be heritable through changes in the germline. While epigenetic changes represent new vistas for therapeutic development, selective manipulation of epigenetic marks is currently challenging even in animals such that translational potential is a distant prospect. This review will reveal that despite the enormous progress made in understanding the molecular and physiological bases of cocaine addiction, there is much that remains a mystery. Continued advances in genetics and molecular biology hold potential for revealing multiple pathways toward the development of treatments for the continuing scourge of cocaine addiction.

Amphetamine, but not methylphenidate, increases ethanol intake in adolescent male, but not in female, rats

INTRODUCTION

There has been an increasing interest in analyzing the interactions between stimulants and ethanol during childhood and adolescence. Stimulants are used to treat attention-deficit hyperactivity disorder (ADHD) in these developmental stages, during which ethanol initiation and escalation often occur.

METHODS

This study assessed the effects of repeated d-amphetamine (AMPH) or methylphenidate (MPH) treatment during adolescence [male and female Wistar rats, between postnatal day (PD) 28 to PD34, approximately] on the initiation of ethanol intake during a later section of adolescence (PD35 to PD40).

RESULTS

Amphetamine and MPH exerted reliable acute motor stimulant effects, but there was no indication of sensitized motor or anxiety responses. MPH did not affect dopamine (DA) levels, whereas AMPH significantly reduced insular levels of DA in both sexes and norepinephrine levels in females only. Repeated treatment with AMPH, but not with MPH, enhanced ethanol intake during late adolescence in male, but not in female, rats.

CONCLUSION

A short treatment with AMPH during adolescence significantly altered DA levels in the insula, both in male and females, and significantly enhanced ethanol intake in males. The present results suggest that, in adolescent males, a very brief history of AMPH exposure can facilitate the initiation of ethanol intake.

HINT1 in Neuropsychiatric Diseases: A Potential Neuroplastic Mediator

Although many studies have investigated the functions of histidine triad nucleotide-binding protein 1 (HINT1), its roles in neurobiological processes remain to be fully elucidated. As a member of the histidine triad (HIT) enzyme superfamily, HINT1 is distributed in almost every organ and has both enzymatic and nonenzymatic activity. Accumulating clinical and preclinical evidence suggests that HINT1 may play an important role as a neuroplastic mediator in neuropsychiatric diseases, such as schizophrenia, inherited peripheral neuropathies, mood disorders, and drug addiction. Though our knowledge of HINT1 is limited, it is believed that further research on the neuropathological functions of HINT1 would eventually benefit patients with neuropsychiatric and even psychosomatic diseases.

Initial locomotor sensitivity to cocaine varies widely among inbred mouse strains

Initial sensitivity to psychostimulants can predict subsequent use and abuse in humans. Acute locomotor activation in response to psychostimulants is commonly used as an animal model of initial drug sensitivity and has been shown to have a substantial genetic component. Identifying the specific genetic differences that lead to phenotypic differences in initial drug sensitivity can advance our understanding of the processes that lead to addiction. Phenotyping inbred mouse strain panels are frequently used as a first step for studying the genetic architecture of complex traits. We assessed locomotor activation following a single, acute 20 mg/kg dose of cocaine (COC) in males from 45 inbred mouse strains and observed significant phenotypic variation across strains indicating a substantial genetic component. We also measured levels of COC, the active metabolite, norcocaine and the major inactive metabolite, benzoylecgonine, in plasma and brain in the same set of inbred strains. Pharmacokinetic (PK) and behavioral data were significantly correlated, but at a level that indicates that PK alone does not account for the behavioral differences observed across strains. Phenotypic data from this reference population of inbred strains can be utilized in studies aimed at examining the role of psychostimulant-induced locomotor activation on drug reward and reinforcement and to test theories about addiction processes. Moreover, these data serve as a starting point for identifying genes that alter sensitivity to the locomotor stimulatory effects of COC.

Differential peptidomics assessment of strain and age differences in mice in response to acute cocaine administration

Neurochemical differences in the hypothalamic-pituitary axis between individuals and between ages may contribute to differential susceptibility to cocaine abuse. This study measured peptide levels in the pituitary gland (Pit) and lateral hypothalamus (LH) in adolescent (age 30 days) and adult (age 65 days) mice from four standard inbred strains, FVB/NJ, DBA/2J, C57BL/6J, and BALB/cByJ, which have previously been characterized for acute locomotor responses to cocaine. Individual peptide profiles were analyzed using mass spectrometric profiling and principal component analysis. Sequences of assigned peptides were verified by tandem mass spectrometry. Principal component analysis classified all strains according to their distinct peptide profiles in Pit samples from adolescent mice, but not adults. Select pro-opiomelanocortin-derived peptides were significantly higher in adolescent BALB/cByJ and DBA/2J mice than in FVB/NJ or C57BL/6J mice. A subset of peptides in the LH, but not in the Pit, was altered by cocaine in adolescents. A 15 mg/kg dose of cocaine induced greater peptide alterations than a 30 mg/kg dose, particularly in FVB/NJ animals, with larger differences in adolescents than adults. Neuropeptides in the LH affected by acute cocaine administration included pro-opiomelanocortin-, myelin basic protein-, and glutamate transporter-derived peptides. The observed peptide differences could contribute to differential behavioral sensitivity to cocaine among strains and ages. Peptides were measured using mass spectrometry (MALDI-TOF) in individual lateral hypothalamus and pituitary samples from four strains and two ages of inbred mice in response to acute cocaine administration. Principal component analyses (PCA) classified the strains according to their peptide profiles from adolescent mice, and a subset of peptides in the lateral hypothalamus was altered by cocaine in adolescents.

Continuous exposure to dizocilpine facilitates the acquisition and escalation of cocaine consumption in male Sprague-Dawley rats

BACKGROUND

Blocking N-methyl-d-aspartate (NMDA) glutamate receptors (NMDARs) prevents cocaine locomotor sensitization, but facilitates escalation of cocaine self-administration and produces ambiguous effects on acquisition of cocaine self-administration. This study used a recently described model of acquisition and escalation to test the hypothesis that continuous NMDAR antagonism functionally increases the effects of a given dose of cocaine.

METHODS

We assessed acquisition of cocaine self-administration (0.6 mg/kg/infusion) in rats treated continuously with either vehicle or the NMDAR antagonist dizocilpine (0.4 mg/kg/day) for 14 consecutive 2h fixed ratio 1 (FR1) sessions. In a separate experiment that assessed the effect of dizocilpine treatment on escalation of cocaine self-administration, rats acquired cocaine self-administration (0.6 mg/kg/infusion) prior to vehicle or dizocilpine treatment. Then, immediately post-acquisition, rats were treated continuously with either vehicle or dizocilpine and allowed to self-administer either 0.6 or 1.2mg/kg/infusion cocaine for an additional seven consecutive 2h FR1 sessions.

RESULTS

Relative to vehicle-treated rats, a significantly greater percentage of dizocilpine-treated rats acquired cocaine self-administration. During the escalation experiment, both vehicle- and dizocilpine-treated rats escalated intake of 1.2mg/kg/infusion cocaine. Whereas vehicle-treated rats exhibited stable intake of 0.6 mg/kg/infusion cocaine, dizocilpine-treated rats escalated intake of this moderate cocaine dose to levels indistinguishable from intake levels produced by self-administration of the high cocaine dose (i.e., 1.2mg/kg/infusion).

CONCLUSIONS

These findings suggest that chronic NMDAR blockade potentiates, rather than attenuates, cocaine's effects and argue for reconsideration of the role of NMDARs in cocaine "addiction-like" behavior.

Basal activity level in mice predicts the initial and sensitized locomotor response to nicotine only in high responders

Not all humans become addicted to drugs of abuse following casual use. Thus, it is important to identify factors that may contribute to subsequent drug responding. Previous studies have identified characteristics such as novelty-seeking, impulsivity, and anxiety as factors involved in the progression to drug dependence. The current experiment investigated basal locomotor activity in C57Bl/6N mice as a potential predictor of subsequent nicotine responses. We examined the ability of differences in basal locomotor activity to predict the acute and sensitized response to nicotine, as well as nicotine conditioned reinforcement. A median split was used to distinguish between low and high responders with regard to basal locomotor activity in mice. We then measured the acute response to nicotine (0.5mg/kg IP) in these mice, followed by measures of conditioned place preference (CPP; 0.5mg/kg IP) and locomotor sensitization (0.5mg/kg IP), to determine whether basal locomotion is predictive of subsequent responding to nicotine. High, but not low, basal activity was found to be a predictor of both the acute and sensitized response to nicotine. Interestingly, only mice classified as having low basal activity demonstrated a significant CPP, suggesting that pre-exposure to nicotine differentially affects conditioned reinforcement on the basis of initial activity level. Basal locomotor activity may be an efficient measure of subsequent locomotor responding to nicotine, but only in animals classified as having high basal activity. However, animals with low basal locomotor activity may be more susceptible to the reinforcing properties of nicotine.

Initial d2 dopamine receptor sensitivity predicts cocaine sensitivity and reward in rats

The activation of dopamine receptors within the mesolimbic dopamine system is known to be involved in the initiation and maintenance of cocaine use. Expression of the D₂ dopamine receptor subtype has been implicated as both a predisposing factor and consequence of chronic cocaine use. It is unclear whether there is a predictive relationship between D₂ dopamine receptor function and cocaine sensitivity that would enable cocaine abuse. Therefore, we exploited individual differences in behavioral responses to D₂ dopamine receptor stimulation to test its relationship with cocaine-mediated behaviors. Outbred, male Sprague-Dawley rats were initially characterized by their locomotor responsiveness to the D₂ dopamine receptor agonist, quinpirole, in a within-session ascending dose-response regimen (0, 0.1, 0.3 & 1.0 mg/kg, sc). Rats were classified as high or low quinpirole responders (HD₂ and LD₂, respectively) by a median split of their quinpirole-induced locomotor activity. Rats were subsequently tested for differences in the psychostimulant effects of cocaine by measuring changes in cocaine-induced locomotor activity (5 and 15 mg/kg, ip). Rats were also tested for differences in the development of conditioned place preference to a low dose of cocaine (7.5 mg/kg, ip) that does not reliably produce a cocaine conditioned place preference. Finally, rats were tested for acquisition of cocaine self-administration and maintenance responding on fixed ratio 1 and 5 schedules of reinforcement, respectively. Results demonstrate that HD₂ rats have enhanced sensitivity to the locomotor stimulating properties of cocaine, display greater cocaine conditioned place preference, and self-administer more cocaine compared to LD₂ animals. These findings suggest that individual differences in D₂ dopamine receptor sensitivity may be predictive of cocaine sensitivity and reward.

Rats classified as low or high cocaine locomotor responders: a unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.

Low- and high-cocaine locomotor responding rats differ in reinstatement of cocaine seeking and striatal mGluR5 protein expression

Behavioral responsiveness to initial cocaine use varies among individuals and may contribute to differential vulnerability to cocaine addiction. Rats also exhibit individual differences in cocaine's effects and can be classified as low or high cocaine responders (LCRs or HCRs, respectively), based on their initial cocaine-induced locomotor activity (10 mg/kg, i.p.). Here, we used the extinction/reinstatement model to address whether or not LCRs and HCRs differ in (i) extinction/reinstatement of cocaine self-administration behavior and (ii) levels of metabotropic glutamate receptors (mGluRs) following these behaviors. During the earliest acquisition sessions, LCRs exhibited significantly greater cocaine intake (0.8 mg/kg/infusion) and cocaine-paired lever responding than HCRs, but intake and lever responding converged by the end of the cocaine self-administration portion of the study. LCRs and HCRs did not differ in cocaine seeking during the first extinction session and extinguished cocaine seeking similarly. HCRs exhibited greater reinstatement than LCRs to lower (2.5 and 5 mg/kg), but not higher (10 mg/kg), i.p. priming doses of cocaine. The effect of drug-paired cues on reinstatement following extinction was complex, with HCRs and LCRs showing the greater effect of cue depending on the order in which cue- and drug-primed tests were given. Western blot analysis revealed that mGluR5 heteromers were significantly higher in the dorsal striatum of HCRs than LCRs following reinstatement testing. Although our previous findings with the LCR/HCR model have uniformly supported the idea that lower initial cocaine-induced activation predicts more ready development of cocaine addiction-like behaviors, here, we show a more complex relationship with cocaine reinstatement.

Mice expressing markedly reduced striatal dopamine transporters exhibit increased locomotor activity, dopamine uptake turnover rate, and cocaine responsiveness

Variations in the expression levels of the dopamine transporter (DAT) can influence responsiveness to psychostimulant drugs like cocaine. To better understand this relationship, we studied a new DAT-low expresser (DAT-LE) mouse model and performed behavioral and biochemical studies with it. Immunoblotting and [(3) H]WIN 35,428 binding analyses revealed that these mice express ∼35% of wildtype (WT) mouse striatal DAT levels. Compared to WT mice, DAT-LE mice were hyperactive in a novel open-field environment. Despite their higher basal locomotor activity, cocaine (10 or 20 mg/kg, i.p.) induced greater locomotor activation in DAT-LE mice than in WT mice. The maximal velocity (Vmax ) of DAT-mediated [(3) H]DA uptake into striatal synaptosomes was reduced by 46% in DAT-LE mice, as compared to WT. Overall, considering the reduced number of DAT binding sites (Bmax ) along with the reduced Vmax in DAT-LE mice, a 2-fold increase in DA uptake turnover rate (Vmax /Bmax ) was found, relative to WT mice. This suggests that neuroadaptive changes have occurred in the DAT-LE mice that would help to compensate for their low DAT numbers. Interestingly, these changes do not include a reduction in tyrosine hydroxylase levels, as was previously reported in DAT knockout homozygous and heterozygous animals. Further, these changes are not sufficient to prevent elevated novelty- and cocaine-induced locomotor activity. Hence, these mice represent a unique model for studying changes of in vivo DAT function and regulation that result from markedly reduced levels of DAT expression.

Estradiol replacement enhances cocaine-stimulated locomotion in female C57BL/6 mice through estrogen receptor alpha

Psychostimulant effects are enhanced by ovarian hormones in women and female rodents. Estradiol increases behavioral responses to psychostimulants in women and female rats, although the underlying mechanism is unknown. This study utilized mice to investigate the time frame and receptor mediation of estradiol's enhancement of cocaine-induced behavior as mice enable parallel use of genetic, surgical and pharmacological methods. The spontaneous behavior of Sham and Ovariectomized (Ovx) female wildtype (WT) mice was determined during habituation to a novel environment and after cocaine administration. Ovx mice were replaced with vehicle (sesame oil) or 17β-estradiol (E2) for 2 days or 30 min prior to a cocaine challenge to investigate the time course of E2's effects. To examine receptor mediation of estradiol effects, Ovx mice replaced for 2 days with either the ERα-selective agonist PPT or the ERβ-selective agonist DPN were compared to Sham mice, and mice lacking either ERα (αERKO) or ERβ (βERKO) were compared to WT littermates. Ovx mice exhibited fewer ambulations during habituation than Sham females. Cocaine-induced increases in behavioral ratings were greater in Sham than in Ovx mice. Two days but not 30 min of E2 replacement in Ovx mice increased cocaine responses to Sham levels. PPT replacement also increased the cocaine response relative to vehicle- or DPN- treated Ovx mice. αERKO mice displayed modestly attenuated behavioral responses to novelty and cocaine compared to αWT littermates, but no behavioral differences were found between βERKO and βWT mice. These results suggest that E2 enhances cocaine-stimulated locomotion in mice predominantly through ERα.

The effect of previous exposure to nicotine on nicotine place preference

RATIONALE

Prior exposure to drugs of abuse may increase or decrease the reinforcing effects of the drug in later consumptions. Based on the initial locomotor activity (LA) response to an acute drug administration or to novelty in an open-field arena, animals can be classified as low or high LA responders (LR or HR). Few studies have used this classification with nicotine, and the results are controversial. Some authors suggested that nicotine can induce conditioned-place preference (CPP) following prior nicotine exposure, whereas others suggested that previous nicotine exposure extinguishes nicotine-CPP.

OBJECTIVE

To explore if the administration of nicotine in a novel environment without explicit behavioral consequences to classify animals in low and high nicotine responders (LNR and HNR) could affect the establishment of nicotine CPP in male Sprague-Dawley rats.

RESULTS

Prior exposure to a single dose of nicotine (0.4 mg/kg, subcutaneously) induced CPP in LNR rats after 14 days of conditioning (seven-trial) but not after two or eight conditioning days. In contrast, HNR rats did not show CPP under any condition. In addition, our results indicated that previous exposure to nicotine decreased its rewarding effects in eight conditioning days CPP (four-trial), which can be regularly established without prior exposure to nicotine.

CONCLUSION

The results suggested that response to a single exposure to nicotine predicts the acquisition of nicotine preference in a 14-day conditioning protocol only for LNR rats. Thus, our findings demonstrated the relevance of using LNR and HNR classification when the individual susceptibility to nicotine preference is studied.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.

Operant psychostimulant self-administration in a rat model of depression

Depression and psychostimulant addiction are co-morbid conditions; depression is a significant risk factor for psychostimulant abuse, and the rate of depression in drug addicts is higher than in the general population. Despite the prevalence of this comorbidity, there are few animal models examining psychostimulant abuse behaviors in depression. We have shown previously that while rats selectively bred for depression-like phenotypes (SwLo) have blunted mesolimbic dopamine (DA) signaling and locomotor responses to dopaminergic drugs, they voluntarily administer excessive amounts of psychostimulants compared to normal or depression-resistant (SwHi) rats in oral consumption paradigms. To determine whether this increased drug intake by depression-sensitive rats extends to operant self-administration, we assessed fixed ratio-1, progressive ratio, extinction, and reinstatement responding for cocaine and amphetamine in SwLo and SwHi rats. Contrary to the oral consumption results, we found that the SwHi rats generally responded more for both cocaine and amphetamine than the SwLo rats in several instances, most notably in the progressive ratio and reinstatement tests. Food-primed reinstatement of food seeking was also elevated in SwHi rats. These results provide further insight into the neurobiology of depression and addiction comorbidity and caution that oral and operant psychostimulant self-administration paradigms can yield different, and this case, opposite results.

Cocaine dose and self-administration history, but not initial cocaine locomotor responsiveness, affects sensitization to the motivational effects of cocaine in rats

Cocaine addiction is a significant and complex disease. Part of this complexity is caused by the variability of the drug experience early in drug use (initial responsiveness, amount of use, etc.). In rats, individual differences in initial cocaine responsiveness and cocaine self-administration history both predict the development of cocaine sensitization, a putative mechanism contributing to the development of cocaine addiction. Here, we sought to determine the role of these factors and cocaine dose on the development of sensitization to cocaine's motivational effects during the earliest stages of self-administration. Rats were classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced locomotor activity (10 mg/kg i.p.) before learning to self-administer cocaine (0.6 mg/kg/infusion i.v.) under a fixed ratio 1 (FR1) schedule of reinforcement. After acquisition, rats self-administered cocaine (0.6 or 1.2 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement either immediately or after an additional five FR1 sessions (0.6 or 1.2 mg/kg/infusion). No LCR/HCR differences in sensitization were observed. However, regardless of LCR/HCR classification, exposure to the higher dose of cocaine produced sensitization to cocaine's motivational effects on the PR schedule (i.e., increased break points) and an escalation of consumption on the FR schedule. Thus, our results reveal a novel model for studying escalation and sensitization very early after acquisition and suggest that sensitization may be important in the earliest stages of the cocaine addiction process.

Do initial responses to drugs predict future use or abuse?

Individuals vary in their initial reactions to drugs of abuse in ways that may contribute to the likelihood of subsequent drug use. In humans, most drugs of abuse produce positive subjective states such as euphoria and feelings of well-being, which may facilitate repeated use. In nonhumans, many drugs initially increase locomotor activity and produce discriminative stimulus effects, both of which have been considered to be models of human stimulant and subjective states. Both humans and nonhumans vary in their sensitivity to early acute drug effects in ways that may predict future use or self-administration, and some of these variations appear to be genetic in origin. However, it is not known exactly how the initial responses to drugs in either humans or nonhumans relate to subsequent use or abuse. In humans, positive effects of drugs facilitate continued use of a drug while negative effects discourage use, and in nonhumans, greater genetic risk for drug intake is predicted by reduced sensitivity to drug aversive effects; but whether these initial responses affect escalation of drug use, and the development of dependence is currently unknown. Although early use of a drug is a necessary step in the progression to abuse and dependence, other variables may be of greater importance in the transition from use to abuse. Alternatively, the same variables that predict initial acute drug effects and early use may significantly contribute to continued use, escalation and dependence. Here we review the existing evidence for relations between initial direct drug effects, early use, and continued use. Ultimately, these relations can only be determined from systematic longitudinal studies with comprehensive assessments from early drug responses to progression of problem drug use. In parallel, additional investigation of initial responses in animal models as predictors of drug use will shed light on the underlying mechanisms.

Post-conditioning propranolol disrupts cocaine sensitization

Antagonists of β-adrenergic receptors (β-ARs) have previously been demonstrated to impair long-term memory in a variety of animal behavioral paradigms. Surprisingly little is known about the ability of β-ARs to modulate initial memory formation in drug conditioning paradigms. The current study examined whether the post-training administration of the β-AR antagonist, propranolol, would disrupt single-trial cocaine-induced sensitization. Rats received 10 mg/kg propranolol immediately following a 30-min cocaine or vehicle exposure in a conditioning context and were later tested for their locomotor response to a cocaine challenge. Rats that received propranolol following cocaine conditioning showed an impairment of locomotor sensitization during testing. However, this effect was only seen in animals with an initial sensitivity to the locomotor effects of cocaine. Rats that failed to show a locomotor response to cocaine during conditioning were not affected by propranolol. We discuss the implications of these findings as they relate to drug conditioning and memory consolidation studies.

Differences in rat dorsal striatal NMDA and AMPA receptors following acute and repeated cocaine-induced locomotor activation

Sprague-Dawley rats can be classified as low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity induced by an acute low dose of cocaine. Upon repeated cocaine exposure, LCRs display greater locomotor sensitization, reward, and reinforcement than HCRs. Altered glutamate receptor expression in the brain reward pathway has been linked to locomotor sensitization and addiction. To determine if such changes contribute to the differential development of locomotor sensitization, we examined protein levels of total, phosphorylated, and cell surface glutamate N-methyl D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (Rs) following acute or repeated cocaine (10 mg/kg, i.p.) in LCRs, HCRs and saline controls. Three areas involved in the development and expression of locomotor sensitization were investigated: the ventral tegmental area (VTA), nucleus accumbens (NAc) and dorsal striatum (dSTR). Our results revealed differences only in the dSTR, where we found that after acute cocaine, GluN2B(Tyr-1472) phosphorylation was significantly greater in LCRs, compared to HCRs and controls. Additionally in dSTR, after repeated cocaine, we observed significant increases in total GluA1, phosphorylated GluA1(Ser-845), and cell surface GluA1 in all cocaine-treated animals vs. controls. The acute cocaine-induced increases in NMDARs in dSTR of LCRs may help to explain the more ready development of locomotor sensitization and susceptibility to addiction-like behaviors in rats that initially exhibit little or no cocaine-induced activation, whereas the AMPAR increases after repeated cocaine may relate to recruitment of more dorsal striatal circuits and maintenance of the marked cocaine-induced locomotor activation observed in all of the rats.

Acquisition of cocaine self-administration in male Sprague-Dawley rats: effects of cocaine dose but not initial locomotor response to cocaine

RATIONALE

We have previously described a model in which adult outbred male Sprague-Dawley rats are classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced open-field activation. This model revealed important individual differences in cocaine's effects, including that LCRs exhibited greater responding than HCRs on a progressive ratio schedule of cocaine reinforcement. However, no LCR/HCR differences in acquisition of cocaine self-administration (0.25 mg/kg/12 s infusion) were observed under these conditions.

OBJECTIVES

To determine if LCRs and HCRs differ in the effectiveness of cocaine to function as a reinforcer under a broader range of conditions, the present study assessed the acquisition of cocaine self-administration (fixed ratio 1 schedule of reinforcement) as a function of i.v. cocaine dose (0.1875, 0.375, 0.5, 1, or 1.5 mg/kg/6 s infusion).

RESULTS

LCRs and HCRs did not differ significantly on any measure of acquisition examined, including the day to meet acquisition criterion, percent acquired, and cocaine intake. The effect of dose on percent acquired and rate of acquisition peaked at the 1-mg/kg/infusion dose of cocaine. In contrast, the effect of dose on cocaine intake was linear, with the highest rate of intake occurring at the 1.5-mg/kg/infusion dose of cocaine.

CONCLUSIONS

LCRs and HCRs do not appear to differ in their acquisition of cocaine-reinforced operant responding across a range of cocaine doses, including conditions that lead to high levels of cocaine intake.

Juvenile exposure to methamphetamine attenuates behavioral and neurochemical responses to methamphetamine in adult rats

Previous research has shown that children living in clandestine methamphetamine (MA) labs are passively exposed to the drug [1]. The long-term effects of this early exposure on the dopaminergic systems are unknown, but may be important for adult behaviors mediated by dopamine, such as drug addiction. The current study sought to determine if juvenile exposure to low doses of MA would lead to altered responsiveness to the stimulant in adulthood. Young male and female rats (PD20-34) were injected daily with 0 or 2 mg/kg MA or left undisturbed and then tested at PD90. In the open field, adult rats exposed to MA during preadolescence had reduced locomotor activity compared to control non-exposed rats following an acute injection of MA (2 mg/kg). Likewise, methamphetamine-induced dopamine increases in the dorsal striatum were attenuated in male and female rats that had been exposed to MA as juveniles, although there were no changes in basal in vivo or ex vivo dopamine levels. These findings suggest that exposure of juveniles to MA leads to persistent changes in the behavioral and neurochemical responses to stimulants in adulthood.

Measuring the incentive value of escalating doses of heroin in heroin-dependent Fischer rats during acute spontaneous withdrawal

RATIONALE/OBJECTIVES

Although continued heroin use and relapse are thought to be motivated, in part, by the positive incentive-motivational value attributed to heroin, little is understood about heroin's incentive value during the relapse-prone state of withdrawal. This study uses place preference to measure the incentive value attributed to escalating-dose heroin in the context of heroin dependence.

METHODS

Male Fischer rats were exposed chronically to escalating doses of heroin in the homecage and during place preference conditioning sessions. Conditioned preference for the context paired with escalating-dose heroin was tested after homecage exposure was discontinued and rats entered acute spontaneous withdrawal. Individuals' behavioral and locomotor responses to heroin and somatic withdrawal signs were recorded.

RESULTS

Conditioned preference for the heroin-paired context was strong in rats that received chronic homecage exposure to escalating-dose heroin and were tested in acute withdrawal. Behavioral responses to heroin (e.g., stereotypy) varied widely across individuals, with rats that expressed stronger heroin preference also expressing stronger behavioral activation in response to heroin. Individual differences in preference were also related to locomotor responses to heroin but not to overt somatic withdrawal signs.

CONCLUSIONS

Escalating doses of heroin evoked place preference in rats, suggesting that positive incentive-motivational value is attributed to this clinically relevant pattern of drug exposure. This study offers an improved preclinical model for studying dependence and withdrawal and provides insight into individual vulnerabilities to addiction-like behavior.

Role of individual and developmental differences in voluntary cocaine intake in rats

RATIONALE

Early-onset drug taking is associated with increased likelihood of addiction, but it is unclear whether early onset is causal in development of addiction. Many other factors are associated with increased risk of addiction and also promote early intake. Here, a rodent model is used to explore the causality of early onset in development of self-administration and addiction-like behavior and to examine factors that promote self-administration.

METHODS

We used cocaine self-administration to examine drug taking and addiction-like behavior in adolescent and adult rats a priori characterized for their locomotor responses to novelty and cocaine and behavior in the light-dark task.

RESULTS

Adolescent animals initially sought more cocaine than adults. However, as the adolescents matured, their intake fell and they did not differ from adults in terms of unreinforced lever-pressing, extinction or reinstatement behavior. For both age groups, self-administration was positively correlated with the locomotor response to novelty, the locomotor response to cocaine, and with time in light in the light-dark task. The rats that were insensitive to cocaine's locomotor effects and that spent the least time in light in the light-dark task sought the least cocaine, appearing to be "protected" from the reinforcing effects of cocaine. There was no difference between the two age groups in appearance of this "protected" phenotype.

CONCLUSIONS

These results suggest that early onset of drug taking may promote increased use, but does not promote progression to addiction-like behavior. Furthermore, protective factors, such as innate anxiety and insensitivity to cocaine's pharmacological effects, function across developmental stages.

Early methylphenidate exposure enhances cocaine self-administration but not cocaine-induced conditioned place preference in young adult rats

RATIONALE

Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration.

OBJECTIVE

The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms.

METHODS

Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal days (PDs) 11 to 20, and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included 8 days of acquisition training, 8 days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10, or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) one schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for 5 days.

RESULTS

Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats.

CONCLUSIONS

These data suggest that in males, methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.

Gene expression of conditioned locomotion and context-specific locomotor sensitization controlled by morphine-associated environment

The nucleus accumbens (NAc) is involved in contextual drug associations, which might be particularly important for environmental cue-induced relapse to drug seeking. In the present study, rats were first administered repeated morphine for 5 days (5 mg/kg, i.p.) in a contextually paired and unpaired design. After reexposure to the morphine-associated environment, which induced conditioned locomotor activity in the morphine-paired group, we performed a rat 27k 70-mer oligo array to profile gene expression in the NAc. One hundred fifty-five upregulated and 88 downregulated genes were found in the paired group compared with the unpaired group. Eight gene transcripts were then selected to confirm their alterations by quantitative real-time polymerase chain reaction (qRT-PCR). The identified genes generally play important roles in neuroactive receptor-ligand interactions, synapse plasticity, ion transport, and protein phosphorylation. Furthermore, the expression of the eight selected genes that were identified and confirmed to show significant fold changes in the first microarray experiment were again measured with qRT-PCR after morphine challenge (2 mg/kg, i.p.). As expected, 2 mg/kg morphine-induced context-specific sensitization. Meanwhile, mRNA expression of the selected genes showed marked upregulation in the morphine-paired group compared with the unpaired and acute groups. These results suggest that alterations in the expression of the identified genes in the NAc may contribute to the neuroplasticity underlying contextual cue-induced relapse to drug use.

Mass spectrometry screening reveals peptides modulated differentially in the medial prefrontal cortex of rats with disparate initial sensitivity to cocaine

To better understand why certain individuals are more vulnerable to cocaine abuse and addiction, we identify peptide markers associated with individual variation in sensitivity to the behavioral effects of cocaine. Previous studies in rats show that low, compared to high, cocaine responders are more sensitive to cocaine-induced behavioral plasticity (sensitization), exhibit enhanced conditioning to cocaine's rewarding effects, and are more motivated to self administer cocaine. In the current study, we combine matrix-assisted laser desorption/ionization mass spectrometry with multivariate statistical methods to analyze tissue extracts from rat dorsal striatum, nucleus accumbens, and medial prefrontal cortex (mPFC) to examine trends in peptide changes that coincide with behavioral phenotype. Peptide profiles of these three regions from individual animals were characterized via mass spectrometry. Resulting mass peaks that were statistically different between these groups were identified using principal component analysis. The mass peaks were then identified in pooled samples via multistage liquid chromatography mass spectrometry. A total of 74 peptides from 28 proteins were sequenced from defined brain regions. Statistically significant changes in peak intensities for seven peptides were found in the mPFC of rats given a single injection of 10 mg/kg cocaine, with low cocaine responders showing approximately 2-fold increase in peak intensities for the acetylated N terminus peptides of stathmin and Hint 1, as well as truncated ATP synthase. These results suggest that distinct peptide profiles in the mPFC are associated with individuals that exhibit reduced sensitivity to the behavioral effects of cocaine.

Individual differences in cocaine-induced locomotor activity of male Sprague-Dawley rats are not explained by plasma corticosterone levels

Humans differ in their initial response to, and subsequent abuse of, addictive drugs like cocaine. Rodents also exhibit marked individual differences in responsiveness to cocaine. Previously, we classified male Sprague-Dawley rats as either low or high cocaine responders (LCRs or HCRs, respectively), based on their acute low-dose cocaine-induced locomotor activity, and found that with repeated drug exposure LCRs exhibit greater cocaine locomotor sensitization, reward and reinforcement than HCRs. Differential cocaine-induced increases in striatal dopamine help to explain the LCR/HCR phenotypes. Differential levels of stress and/or anxiety could also contribute but have not been explored. Here we measured open-field activity and plasma corticosterone levels both pre- and post-cocaine treatment in LCRs, HCRs, and saline-treated controls. The three groups did not differ in baseline locomotor activity or corticosterone levels. Importantly, LCR/HCR differences in corticosterone levels were also not observed following acute cocaine (10mg/kg, i.p.), when cocaine induced approximately 3.5-fold greater locomotor activity in HCRs than LCRs. Additionally, there were no LCR/HCR differences in plasma corticosterone levels following 5 days of once-daily cocaine, during which time LCRs developed locomotor sensitization such that their cocaine-induced locomotor activity no longer differed from that of HCRs. Likewise, there were no group activity differences in any of four concentric zones within the open-field chamber. In summary, neither plasma corticosterone levels nor thigmotaxis-type anxiety appears to be a factor that contributes to the observed cocaine-induced LCR/HCR behavioral differences.

Low and high cocaine locomotor responding male Sprague-Dawley rats differ in rapid cocaine-induced regulation of striatal dopamine transporter function

Adult outbred Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively). Importantly, LCRs and HCRs are distinguished by their differential responsiveness to acute cocaine-induced (but not baseline) locomotor activity, inhibition of the dopamine transporter (DAT) and resulting extracellular DA (HCR > LCR), as well as by repeated cocaine-induced locomotor sensitization and measures of cocaine's rewarding and reinforcing effects (LCR > HCR). Curiously, 30 min after acute cocaine HCRs exhibit greater DAT-mediated [(3)H]DA uptake into striatal synaptosomes than LCRs. To investigate this finding further, we measured locomotor activity, striatal [(3)H]DA uptake kinetics and DAT cell surface expression in LCRs and HCRs over an extended period (25-180 min) after a single relatively low-dose of cocaine (10 mg/kg, i.p.). HCRs exhibited the "predicted" locomotor response: a marked initial activation that returned to baseline by 120 min post-injection. While LCRs exhibited a >50% lower maximal locomotor response, this increase was sustained, lasting approximately 33% longer than in HCRs. At 25 min post-cocaine, maximal velocity (V(max)) of [(3)H]DA uptake was significantly higher by 25% in HCRs than LCRs, with no difference in affinity (K(m)). Despite the DAT V(max) difference, however, DAT surface expression did not differ between LCRs and HCRs. There was a similar trend (HCR > LCR) for DAT V(max) at 40 min, but not at 150 or 180 min. These findings suggest that, compared to LCRs, HCRs have an enhanced ability to rapidly up-regulate DAT function in response to acute cocaine, which may contribute to their more "normal" cocaine-induced locomotor activation.

Low or high cocaine responding rats differ in striatal extracellular dopamine levels and dopamine transporter number

Both humans and animals exhibit marked individual differences in cocaine responsiveness. By using the median split of cocaine-induced locomotor activity, we have classified outbred male Sprague-Dawley rats as either low or high cocaine responders (LCRs or HCRs, respectively). LCR/HCR classification predicts differences in cocaine inhibition of striatal dopamine (DA) transporters (DATs), cocaine-induced locomotor sensitization, cocaine-conditioned place preference, and motivation to self-administer cocaine. In this study, we used in vivo microdialysis to investigate whether the differential cocaine inhibition of DATs in LCRs and HCRs is translated into differential extracellular DA levels. Paralleling their locomotor profiles, LCRs and HCRS had similar basal extracellular DA levels in dorsal striatum (dSTR) and nucleus accumbens (NAc); after acute cocaine injection (10 mg/kg i.p.), HCRs showed greater cocaine-induced increases in DA than LCRs, with more pronounced differences in NAc. After repeated cocaine injection, LCRs and HCRs no longer differed in cocaine-induced locomotor activity or extracellular DA. To further explore the differential susceptibility of LCR/HCR DATs to cocaine, we used in vitro [(3)H]2-carbomethoxy-3-(4-fluorophenyl)tropane ([(3)H]WIN 35,428) binding and quantitative autoradiography to measure the number of DAT binding sites and cocaine's affinity for them. After acute cocaine administration, HCRs had fewer DAT binding sites in dSTR and NAc shell, compared to LCRs. No LCR/HCR differences were observed in DAT number after repeated cocaine injection or in cocaine's affinity. Our findings suggest that levels of striatal extracellular DA and DATs both make important contributions to initial differences in cocaine activation, which in LCRs/HCRs predict differential cocaine reward and reinforcement.

Individual differences in initial low-dose cocaine-induced locomotor activity and locomotor sensitization in adult outbred female Sprague-Dawley rats

Sex and individual differences are important considerations when studying cocaine responsiveness. We have previously shown that male Sprague-Dawley (S-D) rats can be classified as low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity following a single dose of cocaine (10 mg/kg, i.p.). Further, this distinction was found to predict dopamine transporter function, cocaine-induced locomotor sensitization, cocaine conditioned place preference and motivation to self-administer cocaine. Here we investigated whether or not individual differences in cocaine-induced locomotor activity and locomotor sensitization exist in female S-D rats. Female rats exhibited a broad range of locomotor activation following either a 5 or 10 mg/kg cocaine injection, allowing for classification as LCRs or HCRs. When administered over 7 days, both doses induced locomotor sensitization in female LCRs/HCRs. However, the magnitude of effects produced by 5 mg/kg cocaine in female LCRs/HCRs was more comparable to that produced by 10 mg/kg in male LCRs/HCRs, both of which, interestingly, developed sensitization in this study. These findings suggest that female S-D rats, like male S-D rats, can be classified as LCRs/HCRs and highlight the importance of accounting for dose when studying sex and individual differences to the effects of cocaine.