We have shown that the endocannabinoids AEA (and its metabolically stable analogue methAEA) and 2AG share with drugs abused by humans the ability to acutely stimulate DA levels in the rat NAC shell, suggesting endocannabinoids may possess reinforcing effects. Indeed, AEA promotes and maintains intravenous self-administration behavior in monkeys. These effects are blunted by pretreatment with a selective CB1 antagonist, indicating a direct CB1 receptor involvement in elevation of DA and reinforcing effects. Compounds known to increase brain endocannabinoid levels may also elicit DA release. URB597 inhibits the enzyme Fatty Acid Amide Hydrolase (FAAH) that metabolizes AEA. This drug alone (at doses that fully block FAAH) did not produce DA stimulation, suggesting that, in animals at rest, levels of AEA are not high enough to activate CB1 receptors, even after FAAH blockade. Alternatively, FAAH might not be sufficiently expressed in areas related to DA transmission. However, URB597 enhanced the effects of exogenous AEA on DA levels, supporting the hypothesis of low levels of AEA in animals at rest, and suggesting that URB597 can block AEA metabolism resulting in increased levels of AEA in areas related to DA transmission. URB597 alone does not elevate DA levels, and its degree of abuse liability appears low, in agreement with negative findings of self-administration in monkeys, and place preference in rats. AM404, another endocannabinoid enhancer, produces cannabinoid-like behaviors in rodents, but in our experiments it did not alter DA levels, and surprisingly, did not enhance the effect of AEA on NAC shell DA. In agreement, in rats trained to discriminate THC from saline (a behavior selectively mediated by CB1 receptors), AEA alone did not produce full THC-like effects (likely due to its rapid metabolism). In contrast, AEA produced full THC-like effects after pretreatment with URB597, but not AM404. DA D2 receptor activation stimulates AEA levels, thus, drugs that increase DA levels in the brain, like abused drugs, activate DA D2 receptors, and in turn, increase AEA levels. We have tested the effects of abused drugs in producing CB1-receptor mediated generalization in THC-discrimination tests. Cocaine, and amphetamine injected alone did not produce effects significantly different from vehicle, but potentiated the THC-like effects of THC. Nicotine and the D2/3 DA receptor agonist quinpirole alone did not generalize to the THC cue, but both drugs did so in animals pretreated with the inhibitor of FAAH that metabolizes AEA. Nicotine and quinpirole also potentiated the effects of THC. We have suggested that AEA is released by these drugs by a D2 receptor mediated mechanism. So, administered alone these drugs do not stimulate AEA levels sufficiently to provide CB1-mediated THC-like effects, but potentiate ineffective small doses of THC. However, when the same drugs are administered in combination with URB-597, AEA levels are magnified by blockade of its metabolism, and its concentration could thus activate CB1 receptors producing THC discriminative effects. Recently it has also been shown that effects of anandamide and blockers of its metabolism (e.g. URB-597) might be mediated not only by the endocannabinoid system, but also by PPAR-alpha receptors. It has also been demonstrated that blockade of anandamide metabolism through fatty acid amide hydrolase enzymes by URB-597 might lead to increased levels of oleoylamide (OEA) or palmytoilamide (PEA), as well as anandamide. While anadamide has both CB1 and PPAR-alpha receptor activities, OEA and PEA are selective ligands for PPAR receptors, with negligible activity at CB1 receptors. We showed that drugs acting specifically at brain PPAR-alpha receptors can block the addictive actions of nicotine in rats and monkeys. Psychostimulant sensitization might play a role in the path to abuse and addiction, and even a single exposure to psychostimulants could produce long-term sensitization by increasing strength of excitatory synapses in midbrain dopaminergic regions. Sensitization can be viewed as a type of synaptic plasticity, which is also related to alterations in the cannabinoid system. We hypothesized that development of psychostimulant sensitization might involve stimulation of brain endocannabinoid levels that can bind to and activate CB1 receptors. We started this project studying cocaine sensitization in mice, measured as increased stimulation of behavioral activities before and after sensitizing doses of cocaine. We are testing the hypothesis that the development of cocaine sensitization requires release of endocannabinoids, and can be reversed by CB1 receptor blockade. Based on our original hypothesis, low doses of cocaine that do not induce behavioral sensitization might become effective when animals are pretreated with enhancers of endocannabinoid levels. DA transmission, believed to mediate behavioral and reinforcing effects of cocaine, will also be measured before and after cocaine sensitization. Our results have confirmed that a single exposure to cocaine induces behavioral sensitization in mice. Rimonabant, a CB1 antagonist, injected before the sensitizing dose of cocaine, reduced the sensitization produced by cocaine. In addition, cocaine-induced sensitization was paralleled by a sensitized, larger stimulation of DA levels, compared to saline treated animals in the nucleus accumbens core, but not in the NAC shell. Our results suggest also that blockade of endocannabinoid metabolism (obtained by pretreatment with URB-597 in mice) enhances the extracellular levels of endocannabinoids released by cocaine, and this enhancement could be related to the induction of behavioral sensitization by doses of cocaine otherwise not effective in inducing behavioral or neurochemical sensitization. We have also found that the enhancement of cocaine-induced anandamide levels in the brain will also result in a specific related neurochemical sensitization of DA stimulation in the core but not in the shell of the nucleus accumbens. It has been recently suggested that selected blockers of the dopamine transporter might possess the ability to negatively interact with an allosteric site of the cannabinoid CB1 receptors. It has also been suggested that this negative allosteric modulation of CB1 receptors might be important in these DAT blockers for expressing antagonism to the behavioral/reinforcing effects of cocaine. To test the validity of these suggestions, we tested antagonism of cocaine by JHW007, a DAT blocker that also show allosteric CB1 activity. Antagonism of cocaine was obtained in both CB1 knockout and wild-type mice suggesting that allosteric modulation of CB1 receptor actions has a minimal role in the cocaine-antagonist effects of JHW 007. In human subjects a D4-7 variant of the D4 dopamine receptor has been associated with neurologic disorders, such as attention-deficit and hyperactivity disorder and drug abuse. We now have mice that are genetically modified to express this variant of the human D4 DA receptor protein. Because of the potential involvement of this gene in the reinforcing effects of drugs like marijuana and cocaine, we will examine locomotor activity, DA neurotransmission and possibly effects on acquisition of the behavior in the five holes serial reaction time task, in these mice and their wild-type controls. Because this genetic variant has been associated with ADHD in humans, it will be important to test the effects of this genotype on acquisition of behavior and performances in an animal models which measure impulsivity.

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Tanda, Gianluigi (2016) Preclinical studies on the reinforcing effects of cannabinoids. A tribute to the scientific research of Dr. Steve Goldberg. Psychopharmacology (Berl) 233:1845-66
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