Yavin Shaham - US grants

High rates of relapse to drug use after prolonged drug-free periods characterize the behavior of experienced heroin and cocaine users. The behavioral and neurochemical events that contribute to these high rates, however, are not well understood. Relapse can be induced in human subjects and laboratory animals by reexposure to the drug previously used, reexposure to environmental cues paired with drug self-administration, and by exposure to stress. We are using an animal model of relapse, a reinstatement model, to study brain systems and neurotransmitters involved in relapse induced by environmental stressors, conditioned drug cues and drug reexposure in heroin- and cocaine-experienced rats after prolonged drug-free periods. During the last year, we have studied the relationship between the duration of the drug-free period and the propensity to relapse. The main finding of this line of research is that relapse to heroin and cocaine seeking is time-dependent and is not correlated with the severity of drug withdrawal. In other studies, we found a role for D1- and D2-like dopamine receptors in relapse to drug seeking induced by contextual drug cues. In another set of studies, we found that acute, 1-day food deprivation provokes relapse to heroin seeking, an effect attenuated by central injections of leptin, a hormone involved in energy balance and food intake. Finally, we have modified our reinstatement model to study relapse to cocaine seeking in mice. To this end, we found that exposure to cocaine, conditioned cocaine cues and food deprivation reinstate cocaine seeking.

Drugs that have abuse liability in humans typically serve as positive reinforcers to maintain and strengthen behavior leading to their administration in animals and provoke relapse to previously extinguished drug-seeking behavior in abstinent animals. Experiments are being conducted to assess neurobiological and behavioral mechanisms underlying drug self-administration and relapse behavior rats and monkeys, and the ability of pharmacological manipulations to modify such behavior. N-(4-hydroxyphenyl)-arachidonamide (AM404) is an anandamide transport inhibitor shown to reduce rewarding and relapse-inducing effects of nicotine in several animal models of tobacco dependence. However, the reinforcing/rewarding effects of AM404 are not clear. We investigated whether AM404 maintains self-administration behavior or reinstates extinguished drug seeking in squirrel monkeys. In monkeys with a history of anandamide or cocaine self-administration, we substituted injections of AM404 (1100 g/kg/injection). Using a 10-response, fixed-ratio schedule, self-administration behavior was maintained by AM404. Dose-response curves had inverted-U shapes, with peak response rates occurring at a dose of 10 g/kg/injection. In anandamide-experienced monkeys, we also demonstrated self-administration of another anandamide transport inhibitor VDM11. In addition to supporting self-administration, priming injections of AM404 (0.03-0.3 mg/kg) reinstated drug-seeking behavior previously reinforced by cannabinoids (THC or anandamide) or cocaine. Both, AM404 self-administration behavior and reinstatement of drug seeking by AM404 were reduced by treatment with the cannabinoid CB1 receptor antagonist/inverse agonist rimonabant (0.3 mg/kg). Moreover, the reinforcing effects of AM404 were potentiated by the treatment with the FAAH inhibitor URB597 (0.3 mg/kg) suggesting the major role of anandamide in these effects. Finally, AM404 (0.3 mg/kg) potentiated the reinforcing effects of anandamide, but not those of cocaine. In non-human primates, AM404 effectively reinforced self-administration behavior and induced reinstatement of drug-seeking behavior in abstinent monkeys. These effects appeared to be mediated by cannabinoid CB1 receptors. Therefore, compounds that promote actions of endocannabinoids throughout the brain by inhibiting their membrane transport may have a potential for abuse. Nicotine, the main psychoactive component of tobacco, and (-)-(9)-tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, play major roles in tobacco and marijuana dependence as reinforcers of drug-seeking and drug-taking behavior. Drugs that act as inverse agonists of cannabinoid CB1 receptors in the brain can attenuate the rewarding and abuse-related effects of nicotine and THC, but their clinical use is hindered by potentially serious side effects. The recently developed CB1-receptor neutral antagonists may provide an alternative therapeutic approach to nicotine and cannabinoid dependence. We compared the attenuation of nicotine and THC reinforcement and reinstatement in squirrel monkeys by the CB1-receptor inverse agonist rimonabant and by the recently developed CB1-receptor neutral antagonist AM4113. Both rimonabant and AM4113 reduced two effects of nicotine and THC that play major roles in tobacco and marijuana dependence: (1) maintenance of high rates of drug-taking behavior, and (2) priming- or cue-induced reinstatement of drug-seeking behavior in abstinent subjects (models of relapse). In contrast, neither rimonabant nor AM4113 modified cocaine-reinforced or food-reinforced operant behavior under similar experimental conditions. However, both rimonabant and AM4113 reduced cue-induced reinstatement in monkeys trained to self-administer cocaine, suggesting the involvement of a common cannabinoid-mediated mechanism in the cue-induced reinstatement for different drugs of abuse. These findings point to CB1-receptor neutral antagonists as a new class of medications for treatment of both tobacco dependence and cannabis dependence. The use cannabis is typically initiated during adolescence. The endocannabinoid system has an important role in formation of the nervous system, from very early development through adolescence. Cannabis exposure during this vulnerable period might lead to neurobiological changes that affect adult brain functions and increase the risk of cannabis use disorder. The aim of this study was to investigate whether exposure to (9)-tetrahydrocannabinol (THC) in adolescent rats might enhance reinforcing effects of cannabinoids in adulthood. Male adolescent rats were treated with increasing doses of THC (or its vehicle) twice/day for 11 consecutive days (PND 45-55). When the animals reached adulthood, they were tested by allowing them to intravenously self-administer the cannabinoid CB1-receptor agonist WIN55,212-2. In a separate set of animals given the same THC (or vehicle) treatment regimen, electrophysiological and neurochemical experiments were performed to assess possible modifications of the mesolimbic dopaminergic system, which is critically involved in cannabinoid-induced reward. Behavioral data showed that acquisition of WIN55,212-2 self-administration was enhanced in THC-exposed rats relative to vehicle-exposed controls. Neurophysiological data showed that THC-exposed rats displayed a reduced capacity for WIN55,212-2 to stimulate firing of dopamine neurons in the ventral tegmental area and to increase dopamine levels in the nucleus accumbens shell. These findings-that early, passive exposure to THC can produce lasting alterations of the reward system of the brain and subsequently increase cannabinoid self-administration in adulthood-suggest a mechanism by which adolescent cannabis exposure could increase the risk of subsequent cannabis dependence in humans.

Experiments are being conducted to assess the direct effects of abused drugs, or drugs proposed as treatments for drug abuse, on on-going behavior, on brain neurochemistry, as discriminative stimuli and on physiological function. Currently, studies are focusing on cannabinoids, nicotine, the constituents of psychoactive bath salts products (cathinones), cocaine and methamphetamine. Manipulations of the endocannabinoid system could potentially produce therapeutic effects with minimal risk of adverse cannabis-like side effects. Inhibitors of fatty acid amide hydrolase (FAAH) increase endogenous levels of the cannabinoid-receptor agonist, anandamide, and show promise for treating a wide range of disorders. However, their effects on learning and memory have not been fully characterized. We determined the effects of five structurally different FAAH inhibitors in an animal model of working memory known to be sensitive to impairment by delta-9 tetrahydrocannabinol (THC). A delayed nonmatching-to-position procedure was used in rats. Illuminated nosepoke holes were used to provide sample cues (left versus right) and record responses (correct versus incorrect) after delays ranging from 0 to 28 s. Various test drugs were given acutely up to two times per week before daily sessions. One FAAH inhibitor, AM3506 (3 mg/kg), decreased accuracy in the memory task. Four other FAAH inhibitors (URB597, URB694, PF-04457845, and ARN14633) and a monoacylglycerol lipase inhibitor (JZL184, which blocks the degradation of the endocannabinoid 2-arachidonoylglycerol) had no effect. Testing of AM3506 in combination with antagonists for receptors known to be affected by anandamide and other fatty acid amides indicated that the impairment induced by AM3506 was mediated by cannabinoid CB1 receptors, and not by alpha-type peroxisome proliferator-activated receptors (PPAR-alpha) or vanilloid transient receptor potential cation channels (TRPV1). FAAH inhibitors differ with respect to their potential for memory impairment, abuse liability, and probably other cannabis-like effects, and they should be evaluated individually for specific therapeutic and adverse effects. Methamphetamine abuse is linked with brain abnormalities, but its peripheral effects constitute an integral aspect of long-term methamphetamine use. Eight male rhesus monkeys with long histories of intravenous methamphetamine self-administration were evaluated 1 day, and 1, 4, 12, 26, and 52 weeks after their last methamphetamine self-administration session. On test days, isoflurane-anesthetized animals received a 0.35 mg/kg IV methamphetamine challenge. A control group consisted of 10 age and gender matched drug nave monkeys. Cardiovascular responses to methamphetamine were followed for 2.5 h. Echocardiograms were acquired at 3 and 12 months of abstinence and in the control animals. No pre-methamphetamine baseline differences existed among 7 physiological measures across all conditions and controls. As expected, methamphetamine increased heart rate and blood pressure in controls. However, immediately following the self-administration period, the blood pressure response to methamphetamine challenge was reduced when compared to control monkeys. The peak and 150-min average heart rate increases, as well as peak blood pressure increases following methamphetamine were significantly elevated between weeks 12 to 26 of abstinence. These data indicate the development of tolerance followed by sensitization to methamphetamine cardiovascular effects. Echocardiography demonstrated decreased left ventricular ejection fraction and cardiac output at 3 months of abstinence. Importantly, both cardiovascular sensitization and cardiotoxicity appeared to be reversible as they returned toward control group levels after 1 year of abstinence. Enhanced cardiovascular effects may occur after prolonged abstinence in addicts relapsing to methamphetamine and may underlie clinically reported acute cardiotoxic events.

High rates of relapse to drug use after prolonged drug-free periods characterize the behavior of people addicted to opioid and psychostimulant drugs. The behavioral and neurochemical events that contribute to these high relapse rates, however, are not well understood. Relapse can be induced in human subjects and laboratory animals by re-exposure to the drug previously used or re-exposure to environmental cues paired with drug self-administration. We are using different animal models of relapse to study brain systems and neurotransmitters involved in relapse induced by these stimuli in rats with a history of opioid (heroin, oxycodone) and psychostimulant (cocaine, methamphetamine) self-administration.