DA D2R activation stimulates AEA levels in specific brain areas. Drugs of abuse increase brain DA levels activating DA D2Rs thus increasing AEA levels. We have tested the effects of abused drugs in producing CB1R mediated generalization in THC-discrimination tests in rats. Cocaine, and amphetamine injected alone did not produce significant effects, but potentiated THC effects. Nicotine and the D2/3 DAR 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, suggesting that AEA is released by drugs through activation of D2R mediated mechanism. So, administered alone these drugs do not stimulate AEA levels, 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, producing THC discriminative effects. The effects of AEA and blockers of its metabolism (e.g. URB-597) might be mediated also by PPAR-alpha. Indeed, blockade of AEA metabolism by URB-597 might increase levels of oleoylamide (OEA) or palmytoilamide (PEA), selective ligands for PPAR, with negligible activity at CB1R. We showed that drugs acting specifically at brain PPAR-alpha can block the addictive actions of nicotine in rats and monkeys. Psychostimulant sensitization might play a role in the path to abuse and addiction. We hypothesized that development of psychostimulant sensitization might involve stimulation of brain endocannabinoid levels that can bind to and activate CB1Rs. We hypothesized that development of cocaine sensitization requires release of endocannabinoids, and is reversed by CB1R blockade. Thus, low doses of cocaine that do not induce behavioral sensitization might become effective when animals are pretreated with enhancers of endocannabinoid levels. Our results have confirmed that a single exposure to cocaine induces behavioral sensitization in mice. Rimonabant, CB1 antagonist, injected before cocaine, reduced the behavioral sensitization. Cocaine- sensitization was paralleled by a larger stimulation of DA levels, in the NAC core, but not in the NAC shell. Blockade of endocannabinoid metabolism, by pretreatment with URB-597, enhanced the extracellular levels of endocannabinoids released by cocaine, and this enhancement was related to the sensitization by doses of cocaine otherwise not effective. THC, the main psychoactive ingredient in marijuana, activates brain pathways mediating its reinforcing effects by enhancing the firing of DA neurons in the ventral tegmental area (VTA), resulting in increased release of DA from nerve terminals in the NAC shell. Developing medications that modulate these effects of THC as a reinforcer might provide a therapeutic approach for the treatment of marijuana dependence. For example, previously we found that reward-related behavioral and neurochemical effects of THC could be blocked by methyllycaconitine (MLA), a selective antagonist of alpha7-nAChRs that are present in both the VTA and the NAc shell on glutamatergic nerve terminals. Their activation elicits GLU release, which in turn acts at ionotropic GLUR on DAergic terminals to stimulate DA release. Unfortunately, systemic use of direct antagonists of alpha7-nAChRs is associated with side effects that limit their therapeutic utility. To avoid these unwanted effects, we tested a compound 3,4-dimethoxy-N-4-(3-nitrophenyl)thiazol-2-ylbenzenesulfonamide (Ro 61-8048), a potent, selective, peripherally acting kynurenine 3-monooxygenase (KMO) inhibitor, to indirectly increase brain KYNA, an endogenous negative allosteric modulators of alfa7nAChRs that might be better tolerated than directly acting cholinergic antagonists. Indeed, allosteric modulators change Rs conformations in the presence of orthosteric ligands and often have no effect on their own, acting only when physiologicalRs are activated. Newly formed KYNA is promptly released into the extracellular compartment. Notably, no reuptake processes exist for KYNA, and extracellular KYNA is not degraded enzymatically, but is slowly eliminated from the brain by a non-specific acid transporter. We found that administration of the kynurenine 3-monooxygenase (KMO) inhibitor Ro 61-8048 increases brain KYNA levels and attenuates THC-induced stimulation of DA levels in reward-related brain areas. Administration of Ro 61-8048 also reduced the reinforcing effects of THC measured under self-administration behavioral procedures, also preventing relapse to drug-seeking induced by re-exposure to cannabinoids or cannabinoid-associated cues. The involvement of alpha7-nAChRs was confirmed by administration of positive allosteric modulators of alpha7-nAChRs. These results suggest a therapeutic strategy for treatment of marijuana dependence. We are now studying the effects of these drugs on related GLU levels in the VTA and NAC shell. We found that THC increases GLU levels and decreases KYNA levels in the rat NAc shell via CB1R-dependent mechanism. We are now testing the effects of KMO blockade on THC effects. We have discovered that endogenous cannabinoids possess reinforcing effects. Also, in agreement with positive self-administration behavior, systemic injection of endogenous cannabinoid agonists would increase extracellular levels of DA. One of these endocannabinoids, 2AG, produces a small, transient increase in DA levels in the NAC shell. This is in concordance with reports showing that 2AG is usually very rapidly metabolized in vivo by a specific enzyme, mono-acyl-glyceryl-lipase (MAGL). We have now available a drug, AM-4301, that would selectively block the activity of the MAGL enzyme. By blocking this enzyme we should be able to potentiate the behavioral and neurochemical effects of 2AG. Since we do not know if circulating levels of 2AG can be enhanced to a level sufficient to induce cannabinoid-like behavioral and neurochemical effects, AM-4301 will be tested alone, and then will be tested in combination with 2AG. When tested alone, AM-4301 slightly decreased levels of DA in the shell in rats. When administered in combination with 2AG the results show a larger increase in DA than with 2AG alone. It is interesting to note that our preliminary tests show that AM-4301 might be interacting with the MAGL enzyme with a delayed onset and longer-lasting action. Indeed we found a larger increase in DA after AM-4301 pretreatments in animals injected with 2-AG, when the pretreatment time was 24 hours as compared to 40 or 60 minutes. In a recent study, mice genetically modified to include the human D4.7 variant in the brain D4 receptor have been tested with cocaine and methamphetamine. Results on changes in dopaminergic responses in reward related areas have been evaluated in comparison with wild type mice. Results about stimulation of dopamine levels in the accumbens suggest a possible involvement of glutamate release under control of the D4 receptors. Ongoing studies would test differences in the effects of THC in D4-D4.7 mice compared to wild type. In a new project, we have started preclinical studies on EEG in rats treated acutely or chronically with drugs abused by humans. We are characterizing the EEG during drug treatment or withdrawal, with and without THC treatment. This might help find cannabinoid drugs that could alleviate/attenuate the strong symptoms of withdrawal in subjects dependent on opioids.
