Cocaine self-administration punished by i.v. histamine in rat models of high and low drug abuse vulnerability: effects of saccharin preference, impulsivity, and sex

Understanding sex differences and the translational value of models of persistent substance use despite negative consequences

Persistent substance use despite negative consequences is a key facet of substance use disorder. The last decade has seen the preclinical field adopt the use of punishment to model adverse consequences associated with substance use. This has largely involved the pairing of drug use with either electric foot shock or quinine, a bitter tastant. Whilst at face value, these punishers may model aspects of the physical and psychological consequences of substance use, such models are yet to assist the development of approved medications for treatment. This review discusses progress made with animal models of punishment to understand the behavioral consequences of persistent substance use despite negative consequences. We highlight the importance of examining sex differences, especially when the behavioral response to punishment changes following drug exposure. Finally, we critique the translational value these models provide for the substance use disorder field.

Adolescent exposure to sucrose increases cocaine-mediated behaviours in adulthood via Smad3

Adolescence, a critical period of developmental period, is marked by neurobiological changes influenced by environmental factors. Here, we show how exposure to sucrose, which is ubiquitously available in modern diets, results in changes in behavioural response to cocaine as an adult. Rats were given daily access to either 10% sucrose or water during the adolescent period (PND28-42). Following this period, rats are left undisturbed until they reach adulthood. In adulthood, rats were tested for (i) acquisition of a low dose of cocaine, (ii) progressive ratio (PR) test, and (iii) resistance to punished cocaine taking. Sucrose exposure resulted in significant alterations in all behavioural measures. To determine the neurobiological mechanisms leading to such behavioural adaptations, we find that adolescent sucrose exposure results in an upregulation of the transcription factor Smad3 in the nucleus accumbens (NAc) when compared with water-exposed controls. Transiently blocking the active form of this transcription factor (HSV-dnSmad3) during adolescence mitigated the enhanced cocaine vulnerability-like behaviours observed in adulthood. These findings suggest that prior exposure to sucrose during adolescence can heighten the reinforcing effects of cocaine. Furthermore, they identify the TGF-beta pathway and Smad3 as playing a key role in mediating enduring and long-lasting adaptations that contribute to sucrose-induced susceptibility to cocaine. Taken together, these results have important implications for development and suggest that adolescent sucrose exposure may persistently enhance the susceptibility to substance abuse.

Aversion-associated drug and alcohol seeking in females

Compulsive drug intake is characterized by the continuation of use regardless of negative consequences. This is modeled preclinically using procedures where a negative stimulus is delivered contingently with consumption of the reinforcer. In humans, women and men exhibit different drug taking behavior as it pertains to overall use, withdrawal symptoms, and rate of dependence. In substance use research, females have often been excluded from many studies due to concerns that circulating sex hormones may affect drug seeking behavior. However, the more recent inclusion of females in preclinical studies has identified interesting sex differences in aversion-resistant intake of drugs and alcohol. This review will serve to summarize key findings in aversion-related intake of alcohol, psychostimulants, and opioids in females by examining studies that have included female subjects. Further discussion will examine the effect of intake model, neuroanatomical pathways, and sex hormones in the expression of aversion-resistant drug and alcohol consumption.

Increased responsiveness to punishment of cocaine self-administration after experience with high punishment

One behavioral feature of drug addiction is continued drug use despite awareness that this causes negative consequences. Attempts to model this feature in animals typically involve punishing drug self-administration with electrical footshock to identify individuals whose drug use is differently suppressed by punishment. Here we sought to further study individual responsiveness of drug use to punishment in rats self-administering intravenous cocaine. Rats were first trained during several weeks to self-administer cocaine under a fixed-ratio 3 schedule of reinforcement. Then, their self-administration behavior was punished with increasing intensity of footshock (i.e., from 0.1 mA to 0.9 mA, every 30 min). With increasing intensity of punishment, rats first continued to self-administer cocaine before eventually stopping near completely. When retested, however, drug use became more responsive to punishment and was suppressed by a low and initially ineffective footshock intensity (i.e., 0.1 mA). This increase in responsiveness to punishment was seen in all individuals tested, albeit with varying degrees, and was acquired after one single experience with an intensity of punishment that near completely suppressed drug self-administration. Mere passive, non-contingent exposure to the same intensity, however, had no such effect. Once acquired, increased responsiveness to punishment persisted during at least one month when rats were tested every week, but not every day. Finally, increased responsiveness to punishment was not observed after exposure to a non-painful form of punishment (i.e., histamine). Overall, this study reveals that initial responsiveness of drug use to punishment can change rapidly and persistently with experience. We discuss several possible mechanisms that may account for this change in punishment responsiveness and also draw some of the implications and future perspectives for research on animal models of compulsion-like behavior.

Females develop features of an addiction-like phenotype sooner during withdrawal than males

RATIONALE

Women meet criteria for substance use disorder after fewer years of drug use than men; this accelerated time course, or telescoping effect, has been observed for multiple drugs, including cocaine. Preclinical findings similarly indicate an enhanced vulnerability in females to developing an addiction-like phenotype; however, it is not yet known if this phenotype develops faster in females versus males.

OBJECTIVES

The goal of this study was to determine using a rat model whether two key features of addiction in humans, an enhanced motivation for cocaine and compulsive use, emerge sooner during withdrawal from extended access cocaine self-administration in females versus males.

METHODS

Motivation for cocaine, as assessed under a progressive-ratio reinforcement schedule, was determined prior to and following extended access cocaine self-administration (24 h/day, 96 infusions/day, 10 days) and after 7, 14, or 60 days of withdrawal. Compulsive use, or use despite punishment, was evaluated once progressive-ratio responding stabilized by adding histamine, an aversive stimulus, to the cocaine solutions.

RESULTS

Motivation for cocaine increased from baseline sooner during withdrawal in females than males (at 7 versus 14 days); motivation was also highest in the 60-day group. Histamine decreased progressive-ratio responding for cocaine in both sexes, although effects were greatest in males in the 7-day withdrawal group; males reached the female-level of resistance to histamine punishment by 14 days of withdrawal.

CONCLUSIONS

Female rats developed addition-like features sooner during withdrawal than male rats indicating that the telescoping effect observed in humans is biologically based. Additionally, like drug-seeking/craving, motivation for cocaine and measures of compulsive use incubate over withdrawal.

Resurgence of alcohol seeking following abstinence induced by punishment in male and female rats

To better approximate the human condition, animal models of relapse to drug and alcohol seeking have increasingly employed negative consequences to generate abstinence. Here we report the first demonstration of relapse to punishment-suppressed alcohol seeking induced by loss of non-drug reward (i.e., resurgence). We also report the first examination of potential sex differences in any form of relapse to alcohol seeking following suppression by punishment. Male and female rats first pressed a lever for 20 % oral alcohol. Next, lever pressing for one group continued to produce alcohol, but also produced occasional footshock. For another group, lever pressing similarly produced alcohol and occasional footshock, and a nose-poke response produced alternative non-drug reward (i.e., food). Males showed similar suppression of alcohol seeking by punishment alone and punishment + alternative non-drug reward, whereas females showed less suppression by punishment alone. Finally, when alternative reinforcement and punishment were suspended, resurgence occurred for both sexes in the group that previously had access to non-drug reward. Exposure to and then removal of punishment alone did not produce relapse for males, but it did for females. These results suggest that loss of alternative non-drug reward can generate relapse to alcohol seeking following abstinence induced by negative consequences. Future research should further examine the role of potential sex differences in sensitivity to punishment and how such differences may contribute to relapse more broadly.

Aversion-resistant fentanyl self-administration in mice

RATIONALE

Animal models of compulsive drug use that continues despite negative consequences can be used to investigate the neural mechanisms of addiction. However, models of punished or aversion-resistant opioid self-administration are notably lacking.

OBJECTIVES

We sought to develop an aversion-resistant, oral fentanyl self-administration paradigm.

METHODS

In Experiment 1, C57BL/6J male and female, adult mice consumed fentanyl (10 μg/mL) in a two-bottle drinking in the dark task and escalating concentrations of quinine were added to the bottles. In Experiment 2, mice were trained to administer oral fentanyl (10 μg/mL) in an operant response task. Quinine was next added to the fentanyl solution in escalating concentrations. In Experiment 3, mice were trained to respond for oral fentanyl or fentanyl adulterated with 500 μM quinine on every session. In Experiment 4, mice were trained to respond for a 1% sucrose solution before introduction of quinine.

RESULTS

Quinine reduced two-bottle choice consumption in males but not in females. Both sexes demonstrated the ability to detect the selected concentrations of quinine in fentanyl. In the operant chamber, mice responded robustly for oral fentanyl but introduction of quinine at any stage of training was insufficient to reduce responding. In contrast, quinine reduced responding for sucrose at concentrations above 250 μM.

CONCLUSIONS

Mice will respond for and consume oral fentanyl in both a two-bottle choice and an operant response task. Quinine is detectable in fentanyl but mice will continue to respond for and consume fentanyl with quinine in both paradigms. These data support the use of these models in behavioral studies of compulsive-like opioid use.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Compulsive sucrose- and cocaine-seeking behaviors in male and female Wistar rats

RATIONALE

Compulsive cocaine use is a key feature of cocaine addiction and understanding the factors that promote the development of such a behavior will provide important insights into the mechanism of cocaine addiction and is essential for the clinical management of the disorder.

OBJECTIVES

This study aimed to determine how the preexisting compulsive reward-seeking behavior is related to the development of compulsive cocaine-seeking behavior in male and female rats and the potential impact of the reward value and estrous cycle on such behaviors.

METHODS

Adult male and female Wistar rats were first trained to self-administer sucrose pellets under a chained schedule, and then, the intensity-response effects of footshock punishment on sucrose SA reinforced by different values of sucrose were measured. Subsequently, the same rats went on to self-administer intravenous cocaine and the punishment intensity-response effects on cocaine SA reinforced by different doses of cocaine were similarly determined. For the female rats, the measurements were made during different phases of the estrous cycle.

RESULTS

The rats showed a wide range of levels of the compulsive behaviors despite the similar training history. Surprisingly, the compulsive sucrose-seeking behavior did not predict the compulsive cocaine-seeking behavior in either sex. Increasing cocaine dose significantly increased the compulsive cocaine-seeking behavior in the female but not male rats. Estrous cycle did not have impact on the compulsive behaviors.

CONCLUSION

Preexisting differences in compulsive sucrose-seeking behavior do not predict compulsive cocaine-seeking behavior. Compulsive cocaine-seeking behavior is influenced by cocaine dose but not estrous cycle in the female rats.

Decrease of cocaine, but not heroin, self-administration and relapse by the tyrosine kinase inhibitor masitinib in male Sprague Dawley rats

RATIONALE

Accumulating evidence shows that cocaine, and also heroin, influence several tyrosine kinases, expressed in neurons and in non-neuronal populations such as microglia, astrocytes and mast-cells. Drug-induced activation of mast cells both triggers inflammatory processes in the brain mediated by the glial cells they activate, and facilitates histamine release which may directly influence the dopamine system. Thus, by triggering the activation and degranulation of mast cells dependent on the tyrosine kinase c-kit and Fyn, the latter being also involved in NMDA-dependent synaptic plasticity, cocaine and heroin may indirectly influence the neural mechanisms that mediate their reinforcing properties. Masitinib, a novel tyrosine kinase inhibitor with high selectivity for c-Kit, Fyn and Lyn, may alter the aberrant consequences of the activation of these tyrosine kinases by cocaine and heroin.

OBJECTIVE

We investigated in rats the effect of a chronic oral treatment with masitinib (20 mg/kg) on the reinforcing and motivational properties of self-administered cocaine (250 μg/infusion) and heroin (40 μg/infusion).

METHODS

Three different cohorts of rats were trained instrumentally to respond for cocaine, heroin or food under continuous reinforcement. In each group, we assessed the influence of chronic daily treatment with masitinib on the maintenance of instrumental responding and intake and the motivation for the reinforcer. Thus, masitinib and vehicle-treated rats were challenged to adapt to high behavioural demand, to respond under a progressive ratio schedule of reinforcement and to reinstate instrumental responding after extinction and/or abstinence.

RESULTS

Masitinib selectively decreased cocaine intake, the motivation for cocaine and the subsequent propensity to respond for cocaine under extinction, while having no effect on instrumental responding for heroin or food.

CONCLUSION

The present findings suggest masitinib, a drug with proven efficacy in CNS disorders, could represent a novel treatment for cocaine addiction provided its influence on the reinforcing and incentive properties of the drug is confirmed.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

How to study sex differences in addiction using animal models

The importance of studying sex as a biological variable in biomedical research is becoming increasingly apparent. There is a particular need in preclinical studies of addiction to include both sexes, as female animals are often excluded from studies, leaving large gaps in our knowledge of not only sex differences and potential prevention and treatment strategies but also with regard to the basic neurobiology of addiction. This review focuses on methodology that has been developed in preclinical studies to examine sex differences in the behavioral aspects and neurobiological mechanisms related to addiction across the full range of the addiction process, including initiation (acquisition), maintenance, escalation, withdrawal, relapse to drug seeking and treatment. This review also discusses strategic and technical issues that need to be considered when comparing females and males, including the role of ovarian hormones and how sex differences interact with other major vulnerability factors in addiction, such as impulsivity, compulsivity and age (adolescent versus adult). Novel treatments for addiction are also discussed, such as competing non-drug rewards, repurposed medications such as progesterone and treatment combinations. Practical aspects of conducting research comparing female and male animals are also considered. Making sex differences a point of examination requires additional effort and consideration; however, such studies are necessary given mounting evidence demonstrating that the addiction process occurs differently in males and females. These studies should lead to a better understanding of individual differences in the development of addiction and effective treatments for males and females.

Histamine H3 receptor as a potential target for cognitive symptoms in neuropsychiatric diseases

The potential contributions of the brain histaminergic system in neurodegenerative diseases, and the possiblity of histamine-targeting treatments is attracting considerable interests. The histamine H3 receptor (H3R) is expressed mainly in the central nervous system, and is, consequently, an attractive pharmacological target. Although recently described clinical trials have been disappointing in attention deficit hyperactivity disorder (ADHD) and schizophrenia (SCH), numerous H3R antagonists, including pitolisant, demonstrate potential in the treatment of narcolepsy, excessive daytime sleepiness associated with cognitive impairment, epilepsy, and Alzheimer's disease (AD). This review focuses on the recent preclinical as well as clinical results that support the relevance of H3R antagonists for the treatment of cognitive symptoms in neuropsychiatric diseases, namely AD, epilepsy and SCH. The review summarizes the role of histaminergic neurotransmission with focus on these brain disorders, as well as the effects of numerous H3R antagonists on animal models and humans.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

Cocaine self-administration punished by intravenous histamine in adolescent and adult rats

Adolescence is a transitional phase marked by a heightened vulnerability to substances of abuse. It has been hypothesized that both increased sensitivity to reward and decreased sensitivity to aversive events may drive drug-use liability during this phase. To investigate possible age-related differences in sensitivity to the aversive consequences of drug use, adolescent and adult rats were compared on self-administration of cocaine before, during, and after a 10-day period in which an aversive agent, histamine, was added to the cocaine solution. Adult and adolescent female rats were trained to self-administer intravenous cocaine (0.4 mg/kg/infusion) over 10 sessions (2 h/session; 2 sessions/day). Histamine (4 mg/kg/infusion) was then added directly into the cocaine solution for the next 10 sessions. Finally, the cocaine/histamine solution was replaced with a cocaine-only solution, and rats continued to self-administer cocaine (0.4 mg/kg) for 20 sessions. Compared with adolescent rats, adult rats showed a greater decrease in cocaine self-administration when it was punished with intravenous histamine compared with their baseline cocaine self-administration rates. These results suggest that differences in the sensitivity to negative consequences of drug use may partially explain developmental differences in drug use vulnerability.

Intracranial self-stimulation reward thresholds during morphine withdrawal in rats bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Sweet preference is a marker of vulnerability to substance use disorders, and rats selectively bred for high (HiS) vs. low saccharin (LoS) intake display potentiated drug-seeking behaviors. Recent work indicated that LoS rats were more responsive to the negative effects of drugs in several assays.

OBJECTIVE

The current study used the intracranial self-stimulation (ICSS) procedure to investigate the anhedonic component of morphine withdrawal in male HiS and LoS rats.

METHODS

Rats were administered morphine (10mg/kg) or saline for 8 days. To evaluate withdrawal effects, reward thresholds were measured 24 and 28h following the 8th morphine injection (spontaneous withdrawal) and again for 4 days following daily acute morphine and naloxone (1mg/kg) administration (precipitated withdrawal).

RESULTS

24h following the final morphine injection, reward thresholds in LoS rats were significantly elevated compared to reward thresholds in LoS controls, indicating spontaneous withdrawal. This effect was not observed in HiS rats. LoS rats also showed greater elevations of reward thresholds on several days during naloxone-precipitated withdrawal compared to their HiS counterparts.

CONCLUSIONS

LoS rats were more sensitive to morphine withdrawal-mediated elevations in ICSS thresholds than HiS rats. While these differences were generally modest, our data suggest that severity of the negative affective component of opiate withdrawal may be influenced by genotypes related to addiction vulnerability.

Prolonged withdrawal following cocaine self-administration increases resistance to punishment in a cocaine binge

Drug addiction is characterized by compulsive drug-taking behaviors and a high propensity to relapse following drug cessation. Drug craving and seeking can increase during a period of abstinence, but this phenomenon is not observed in drug-induced reinstatement models. To investigate the effect of withdrawal on cocaine relapse, rats were exposed to extended-access cocaine self-administration and subjected to either 1 or 30 d of withdrawal. When tested during 12 h unlimited access to cocaine (binge), the duration of the withdrawal did not influence cocaine intake. However, using a histamine punishment procedure that greatly suppresses drug-taking behavior, we demonstrate that longer periods of abstinence from cocaine induce a greater persistence in responding for drug in the face of negative consequences.

Cocaine-, caffeine-, and stress-evoked cocaine reinstatement in high vs. low impulsive rats: treatment with allopregnanolone

BACKGROUND

Previous research indicates that individual differences in traits such as impulsivity, avidity for sweets, and novelty reactivity are predictors of several aspects of drug addiction. Specifically, rats that rank high on these behavioral measures are more likely than their low drug-seeking counterparts to exhibit several characteristics of drug-seeking behavior. In contrast, initial work suggests that the low drug-seeking animals are more reactive to negative events (e.g., punishment and anxiogenic stimuli). The goal of this study was to compare high and low impulsive rats on reinstatement of cocaine-seeking behavior elicited by cocaine (COC) and by negative stimuli such as the stress-inducing agent yohimbine (YOH) or a high dose of caffeine (CAFF). An additional goal was to determine whether treatment with allopregnanolone (ALLO) would reduce reinstatement (or relapse) of cocaine-seeking behavior under these priming conditions.

METHODS

Female rats were selected as high (HiI) or low (LoI) impulsive using a delay-discounting task. After selection, they were allowed to self-administer cocaine for 12 days. Cocaine was then replaced with saline, and rats extinguished lever responding over 16 days. Subsequently, rats were pretreated with either vehicle control or ALLO, and cocaine seeking was reinstated by injections of COC, CAFF, or YOH.

RESULTS

While there were no phenotype differences in maintenance and extinction of cocaine self-administration or reinstatement under control treatment conditions, ALLO attenuated COC- and CAFF-primed reinstatement in LoI but not HiI rats.

CONCLUSIONS

Overall, the present findings suggest that individual differences in impulsive behavior may influence efficacy of interventions aimed to reduce drug-seeking behavior.

Assessment of the kappa opioid agonist, salvinorin A, as a punisher of drug self-administration in monkeys

RATIONALE

Drugs can function as punishers. However, work on the study of drugs as punishers is limited, as is the range of compounds known to function as punishers. Kappa opioid agonists, which have received much experimental attention as potential therapeutics for drug abuse, reportedly produce aversive effects. However, kappa agonists have yet to be tested as punishers of behavior.

OBJECTIVE

The goal of the current study was to determine if a kappa agonist could function as a punisher of drug self-administration.

METHOD

In separate experiments, monkeys were allowed to choose in a two-lever choice design between intravenous injections of equal doses of either cocaine (0.1 mg/kg/injection on each lever) or remifentanil (0.1 μg/kg/injection on each lever) when one of the two options was mixed with various doses of the kappa agonist, salvinorin A.

RESULTS

Choice for the cocaine and remifentanil options that were combined with salvinorin A decreased as a function of salvinorin A dose in all monkeys. However, operant response rates were not systematically affected by salvinorin A administration.

CONCLUSION

The present findings demonstrate that the kappa agonist, salvinorin A, can punish self-administration of a psychotimulant, cocaine, and a mu opioid, remifentanil. In consideration of these findings, it may be possible to curtail the abuse of some drugs by contingently delivering kappa agonists (e.g., as combination formularies for prescription medications).

Assessment of the effects of contingent histamine injections on the reinforcing effectiveness of cocaine using behavioral economic and progressive-ratio designs

RATIONALE

Recent research has demonstrated that the drug, histamine, can function as a punisher of cocaine self-administration. However, little is known about how drug punishers affect the maximum reinforcing effectiveness of drugs as reinforcers.

OBJECTIVE

The goal of the present study was to determine if histamine, when self-administered as a mixture with cocaine, could reduce cocaine's maximum reinforcing effectiveness using two procedures designed for measuring reinforcing effectiveness.

METHODS

In the first experiment, rhesus monkeys were allowed to self-administer cocaine (0.1 mg/kg/inj) alone or as a mixture with histamine (0.012-0.05 mg/kg/inj) in a behavioral economic design. In the second experiment, monkeys were allowed to self-administer cocaine alone (0.006-0.56 mg/kg/inj) or as a mixture with histamine (0.025-0.1 mg/kg/inj) under a progressive-ratio schedule of reinforcement.

RESULTS

In Experiment 1, histamine decreased the reinforcing effectiveness of cocaine in a dose-dependent manner as evidenced by increases in cocaine's demand elasticity with increases in histamine dose. In Experiment 2, histamine decreased cocaine's potency and effectiveness as a reinforcer in a dose-dependent manner as indicated by rightward and downward shifts, respectively, in the dose-response functions.

CONCLUSION

The reinforcing effectiveness of cocaine can be reduced by contingent self-administration of histamine. These results indicate that combining drug punishers with drug reinforcers reduces the maximum reinforcing effect of the drug reinforcer, which suggests a use for drug punishers as a deterrent to drug abuse (e.g., as mixtures with prescription medications with abuse potential).

Cocaine-induced c-Fos expression in rats selectively bred for high or low saccharin intake and in rats selected for high or low impulsivity

Sweet preference and impulsivity are predictors of cocaine self-administration; however, no research has been conducted to investigate neuronal activation in key brain reward areas after first time exposure to cocaine in rats that differ in their propensity for cocaine-seeking and -taking behavior. In this study we used rats that had been selectively bred for high vs. low saccharin intake and rats selected for high vs. low impulsivity for food. The goal of this study was to investigate whether there are differences of c-Fos reactivity between high and low phenotypes and determine whether these differences are similar between the two animal models. A group of rats was bred for high or low saccharin intake. Another group of rats was selected as high or low impulsive based on performance in a delay-discounting task. Subsequently, rats were given an acute injection of cocaine or saline and then c-Fos expression was observed and analyzed in several brain regions. The low reward-seeking phenotypes showed higher cocaine-induced c-Fos expression in several of these regions. Low saccharin preferring rats showed higher cocaine-induced c-Fos expression in the nucleus accumbens shell, and low impulsive rats showed higher cocaine-induced c-Fos expression in the orbitofrontal cortex and cingulate gyrus 1 area. In addition, both low impulsive and low saccharin rats had higher cocaine-induced c-Fos in the dorsal medial and dorsal lateral caudate putamen. The results indicate that individual differences in neuronal reactivity exist prior to chronic exposure to drugs of abuse. Furthermore, similar differences between the two animal models may be indicative of a common mechanism underlying vulnerability to drugs of abuse.