Individual differences in novelty- and cocaine-induced locomotor activity as predictors of food-reinforced operant behavior in two outbred rat strains

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.

Locomotor activity changes in female adolescent and adult rats during repeated treatment with a cannabinoid or club drug

Adolescents and young adults of both sexes are the primary consumers of "club" drugs; yet, most of the mechanistic preclinical research in this area has been performed in adult male rodents. The purpose of this study was to evaluate the acute and repeated effects of drugs that are commonly abused by adolescents in female adolescent and adult rats in a rodent model of behavioral sensitization. During two five-day periods separated by a two-day break, rats were injected daily with saline or with one of the following drugs: cocaine (7 or 15 mg/kg), ketamine (3 or 10 mg/kg), 3,4-methylenedioxymethamphetamine (MDMA) (3, 10, or 30 mg/kg), or Δ(9)-tetrahydrocannabinol (THC) (0.03, 0.1, 0.3 or 1 mg/kg) and their locomotor activity was measured. Cocaine increased activity across days in both age groups. Whereas ketamine produced progressive increases in activity with repeated administration in rats of both ages, MDMA increased, and then decreased, activity in the chronic dosing regimen in female adolescents only. Tolerance to the initial stimulatory effects of low doses of THC was observed at both ages. The results with THC are similar to those obtained for male rats tested under identical conditions in a previous study; however, in contrast with the present results in females, male adolescent rats in the previous study failed to develop behavioral sensitization to ketamine. Together, these results suggest that age and sex strongly influence the progressive adaptive changes that occur with repeated administration of some, but not all, of these commonly abused substances.

Influence of sex on reinstatement of cocaine-conditioned place preference

To explore sex differences in reinstatement of conditioned place preference, we subjected intact male and female Long Evans rats to an extended conditioned place preference (CPP) paradigm, which included observations of acquisition, extinction, and reinstatement of a preference to cocaine-paired stimuli. In a series of experiments, separate groups of animals were given six 30-min pairings of one chamber with cocaine (3, 5, 10, 15, 20, 25mg/kg) and six of the other with saline on alternate days. Overall, there were no sex differences in acquisition of cocaine CPP at any of the six doses tested (p>0.05). All animals established cocaine CPP at each of the six doses tested during the acquisition test, with the exception of the group of females conditioned with 5mg/kg. Preferences for the cocaine-paired chamber were successfully extinguished for both males and females after an extinction-training period. CPP reinstatement was achieved by the groups of males and females given training and priming doses of 10, 15, 20, and 25mg/kg (p<0.05). Overall, our reinstatement data demonstrate that reinstatement of cocaine CPP is greater for female versus male animals. Females showed a greater magnitude of reinstatement of cocaine CPP when trained and primed with 15 and 25mg/kg as compared to males (p<0.05). Further, at the three highest doses tested (15, 20, and 25mg/kg), females showed a greater magnitude of CPP in the reinstatement phase of CPP compared to that of the initial acquisition phase (p<0.05). The reinstatement data for the males show that the 20mg/kg dose resulted in the highest levels of reinstatement preference for male rats. These results indicate that sex differences in reinstatement to conditioned behavior maybe due, in part, to females forming a stronger association for the salience of the drug and the environment in which it was administered.

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.

Reduced sensitivity to the locomotor-stimulant effects of cocaine is associated with increased sensitivity to its discriminative stimulus properties

Outbred Long-Evans rats exhibit wide variation in their locomotor response to cocaine. Here, we investigated the relationship between these individual differences and interoceptive effects of cocaine in low cocaine responder (LCR) and high cocaine responder (HCR) phenotypes. Rats were trained to discriminate cocaine (10.0 mg/kg, intraperitoneally) from saline by repeated pairings of injections with one of two response levers. In subsequent tests for stimulus generalization to other cocaine doses (1.25-15.0 mg/kg), LCRs exhibited partial-to-full generalization at 1.85 and 2.5 mg/kg cocaine, respectively, whereas HCRs did not. When the selective 5-HT reuptake inhibitor fluoxetine (5.0 mg/kg) was coadministered with saline or different cocaine doses, we observed similar upward shifts in dose-response in both phenotypes. In contrast, coadministration of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 mg/kg) led to partial substitution of DOI for cocaine and enhancement of the stimulus properties of 1.25 mg/kg cocaine in LCRs only. Finally, a retest of cocaine-induced locomotion after discrimination testing revealed marked behavioral sensitization in LCRs and modest changes in behavior in HCRs. Taken together, these results suggest that initial sensitivity to the locomotor-stimulant effects of cocaine is inversely related to its interoceptive properties and that differences in 5-HT systems may contribute to the phenotypic differences observed.

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

RATIONALE

Factors that increase an individual's susceptibility to cocaine dependence remain largely unknown. We have previously shown that adult outbred male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity following the administration of a single dose of cocaine (10 mg/kg, i.p.). Furthermore, LCR/HCR classification predicts dopamine transporter function/inhibition, cocaine-induced locomotor sensitization, and cocaine-conditioned place preference.

OBJECTIVES

The present study assessed LCR/HCR classification and the development of locomotor sensitization on the latency to acquire cocaine self-administration and motivation to self-administer cocaine.

RESULTS

LCRs and HCRs did not differ in their latency to acquire low-dose cocaine self-administration (0.25 mg/kg/infusion over 12 s, fixed ratio 1 schedule of reinforcement). In a follow-up experiment, repeated experimenter-administered injections of cocaine (10 mg/kg, i.p.) resulted in locomotor sensitization for LCRs, but not HCRs; nonetheless, all rats exhibited decreased latency to acquire cocaine self-administration compared to the first experiment. Repeated cocaine preexposure and LCR/HCR classification predicted break point when rats responded for cocaine under a progressive ratio schedule of reinforcement (0.25, 0.5, and 1.0 mg/kg/infusion; multiple exposure>single exposure, LCR>HCR), but there was no interaction between these variables.

CONCLUSIONS

Although LCR/HCR classification did not predict the rate of acquisition of cocaine self-administration under these conditions, LCR rats demonstrated greater responding for cocaine after acquisition (PR). Thus, these findings demonstrate the relevance of using the LCR/HCR model when studying susceptibility to cocaine dependence.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Unreinforced responding during limited access to heroin self-administration

These studies were designed to explore a peculiar behavior displayed by rats during the acquisition of heroin self-administration (0.05 mg/kg/infusion) on a fixed-ratio 1 schedule of reinforcement in limited access conditions (i.e. 3 h/day). Rats trained under these conditions develop a tendency to emit extra lever presses during the time of heroin infusions (unreinforced responses). We found that a similar behavior develops in animals responding for sucrose pellets, but not for intravenous infusions of cocaine (0.5 mg/kg/infusion, 3 h/day). In sucrose trained rats, unreinforced responses emitted during the delivery of sucrose pellets was enhanced by food deprivation. In heroin trained rats, development of unreinforced responding was accompanied by an increase in responding for heroin on a progressive ratio schedule, and by a reduction of the depressant action of heroin (3 mg/kg, SC) on locomotor activity. On the basis of these findings, we concluded that unreinforced responding during heroin self-administration reflects a change in the motivation to obtain the drug, as well as a reduced sensitivity the motor impairing action of heroin. This suggests that acquisition of heroin self-administration is regulated by a balance between drug effects that promote and limit heroin intake.