Experiments are being conducted to assess the different neuropharmacological and behavioral mechanisms underlying behavior controlled by drugs as discriminative stimuli in rats and monkeys and the ability of pharmacological or behavioral manipulations to modify such behavior. Currently, studies are focusing on a series of cannabinoids, including delta-9-tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana and an agonist at endogenous cannabinoid CB1 receptors, The CB1 receptor antagonists Rimonabant and AM251, the endogenous cannabinoid anandamide, the anandamide uptake inhibitor AM404, the fatty acid amide hydrolase (FAAH) inhibitor URB597. Studies are also being conducted on methamphetamine, cocaine, nicotine and heroin.? ? Although endogenous cannabinoid systems have been implicated in the modulation of the rewarding effects of abused drugs and food, little is known about direct effects of endogenous ligands for cannabinoid receptors on brain reward processes. In a new study (Solinas et al. 2006), we show for the first time that intravenous administration of anandamide, an endogenous ligand for cannabinoid receptors, and its longer-lasting synthetic analog methanandamide, increase extracellular dopamine levels in the nucleus accumbens shell of awake, freely-moving rats, an effect characteristic of most drugs abused by humans. Anandamide produced two distinctly different effects on dopamine levels: 1) a rapid, transient increase that was blocked by the cannabinoid CB1 receptor antagonist rimonabant but not by the vanilloid VR1 receptor antagonist capsazepine and was magnified and prolonged by the fatty acid amide hydrolase (FAAH) enzyme inhibitor, URB597, and 2) a smaller delayed and long-lasting increase, not sensitive to CB1, VR1, or FAAH blockade. Both effects were blocked by infusing either TTX or Calcium-free Ringer?s solution through the microdialysis probe, demonstrating that they were dependent on physiologic activation of dopaminergic neurotransmission. Thus, these results indicate that anandamide, through activation of the mesolimbic dopaminergic system, participates in the signaling of brain reward processes. Further, we have shown that FAAH enzyme inhibitor, URB597 does not exert rewarding effects in the conditioned place preference test or produce generalization to the discriminative effects of delta-9-tetrahydrocannabinol (THC) in rats (Gobbi et al. 2006). These findings support a role for anandamide in mood regulation and point to fatty-acid amide hydrolase as a previously uncharacterized target for antidepressant drugs.? ? It is known that strong functional interactions exist between endogenous cannabinoid and opioid systems. Previously we have shown that psychotropic effects of THC related to drug abuse liability are regulated by THC-induced elevations in extracellular beta-endorphin levels in brain areas involved in reward and reinforcement processes (Solinas et al. 2004). This novel mechanism may underlie many previous observations of opioid system modulation of cannabinoid effects. In a new series of experiments (Solinas and Goldberg 2005), we investigated whether cannabinoid?opioid interactions modulate motivational effects of food reinforcement. In rats responding for food under a progressive-ratio schedule, the maximal effort (break point) expended to obtain 45 mg pellets depended on the level of food deprivation, with free-feeding reducing break points and food-deprivation increasing break points. Delta-9-tetrahydrocannabinol and morphine dose-dependently increased break points for food reinforcement, while the cannabinoid CB1 receptor antagonist rimonabant (SR141716A) and the preferential mu-opioid receptor antagonist naloxone dose-dependently decreased break points. THC and morphine only increased break points when food was delivered during testing, suggesting that these treatments directly influenced reinforcing effects of food, rather than increasing behavior in a nonspecific manner. Effects of THC were blocked by rimonabant and effects of morphine were blocked by naloxone, demonstrating that THC?s effects depended on cannabinoid CB1 receptor activation and morphine?s effects depended on opioid-receptor activation. Furthermore, THC?s effects were blocked by naloxone and morphine?s effects were blocked by rimonabant, demonstrating that mu-opioid receptors were involved in the effects of THC and cannabinoid CB1 receptors were involved in the effects of morphine on food-reinforced behavior. Thus, activation of both endogenous cannabinoid and opioid systems appears to jointly facilitate motivational effects of food measured under progressive-ratio schedules of reinforcement and this facilitatory modulation appears to critically depend on interactions between these two systems. These findings support the proposed therapeutic utility of cannabinoid agonists and antagonists in eating disorders.? ? We also have conducted a series of experiments (Yasar et al. 2006) to better characterize the pharmacological effects of selegiline ((R)-(-)-deprenyl), which is a selective inhibitor of monoamine-oxidase B (MAO-B) used in the treatment of Parkinson?s disease and proposed as an antidepressant and an aid for cigarette-smoking cessation and treatment of psychostimulant abuse. Beneficial therapeutic effects of selegiline may also result from indirect actions. Brain levels of dopamine and ?-phenylethylamine (?-PEA), a behaviorally-active endogenous trace amine, increase after selegiline treatment due to MAO-B blockade and selegiline is metabolized to (R)-(-)-methamphetamine and (R)-(-)-amphetamine, suggesting that selegiline may have psychostimulant-like behavioral effects. Indeed, selegiline produces psychostimulant-like discriminative-stimulus effects in experimental animals. Here we tested the hypothesis that psychostimulant-like behavioral effects of selegiline are mainly mediated by its metabolites. Male Fisher F344 rats were trained to discriminate intra-peritoneal (i.p.) injection of 1.0 mg/kg (S)-(+)-methamphetamine or 10.0 mg/kg cocaine from injection of saline using two-lever choice schedules of food delivery or stimulus-shock termination. When selegiline was tested by substitution, it had (S)-(+)-methamphetamine- and cocaine-like discriminative-stimulus effects, but only at doses of 10-30 mg/kg, doses 10- to 20-times higher than those selective for MAO-B inhibition. Ro 16-1649, a selective inhibitor of MAO-B enzyme activity without psychoactive metabolites, had no psychostimulant-like discriminative effects. Also, blockade of selegiline?s metabolism with SKF 525A (50 mg/kg, i.p.) reduced or selegiline?s psychostimulant-like discriminative effects. When ?-PEA synthesis was blocked by NSD 1015 (30 mg/kg, i.p.), there was a modest reversal of selegiline?s psychostimulant-like discriminative effects under some conditions, indicating a facilitatory modulation of the psychostimulant-like discriminative effects of (R)-(-)-deprenyl metabolites by elevated levels of ?-PEA under certain conditions.
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