The levels of the endogenous cannabinoid agonist anandamide (AEA) can be stimulated by activation of DA D2R in specific brain areas. For example, drugs of abuse increase extracellular levels of dopamine (DA) leading to activation of DA D2Rs, which in turn may increase AEA levels and activate CB1R. These effects may explain why cocaine, amphetamine, nicotine and the D2/3 DAR agonist quinpirole potentiate the effects of the main psychoactive ingredient in cannabis, THC, in THC-discrimination tests in rats, but they do not produce significant effects when injected alone. Nicotine and the D2/3 DAR agonist quinpirole generalize to the THC cue in animals pretreated with the inhibitor of FAAH that metabolizes AEA. Nicotine and quinpirole also potentiate the discriminative effects of THC. Taken together the results of these experiments suggest that AEA is released by drugs of abuse through activation of a D2R-mediated mechanism. 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 oleamide (OEA) or palmitoylethanolamide (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. We have shown that a single exposure to cocaine induces behavioral sensitization in mice and that a CB1 antagonist can reduce cocaine-induced behavioral sensitization. The behavioral sensitization was paralleled by a neurochemical sensitization with a larger stimulation of DA levels in the nucleus accumbens (NAC) core compared to 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. We have discovered that endogenous cannabinoids possess reinforcing effects assessed by self-administration procedures. Also, systemic administration 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. 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. 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 cannabinoid drug treatments to find pharmacologic therapies that could alleviate/attenuate the strong symptoms of withdrawal, for example in subjects dependent on opioids. The reinforcing effects of THC, the main psychoactive ingredient in cannabis, are likely mediated by activation of specific brain pathways. THC enhances the firing of DA neurons in the ventral tegmental area (VTA), which leads to stimulation of DA release from nerve terminals in the NAC shell. Developing medications that modulate the neurotransmission involved in the reinforcing actions of THC might provide a therapeutic approach for the treatment of cannabis use disorder. Reward-related behavioral and neurochemical effects of THC could be blocked by methyl-lycaconitine (MLA), a selective antagonist of alpha7-nAChRs, expressed by glutamatergic nerve terminals in both the VTA and the NAc shell. Their activation elicits GLU release activating 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. Unwanted effects might be avoided by using endogenous negative allosteric modulators of alfa7nAChRs, like KYNA, which might be better tolerated than directly acting cholinergic antagonists. Allosteric modulators change Rs conformations in the presence of orthosteric ligands and often have no effect on their own. To this end we tested Ro 61-8048, an inhibitor of the KMO enzyme that indirectly increase the endogenous levels of KYNA. Newly formed KYNA, is synthesized by astroglia 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. The reinforcing effects of THC might also be mediated by GLU neurotransmission in the VTA and NAC shell. Acute systemic administration of THC increased extracellular levels of glutamate in the NAC shell, VTA and medial prefrontal cortex (mPFC). THC also reduced extracellular levels of KYNA in the shell. These effects were prevented by administration of Ro 61-8048 or the CB1R antagonist, rimonabant. THC increased the firing activity of glutamatergic pyramidal neurons projecting from the mPFC to the NAc or to the VTA in vivo. These effects were prevented by pretreatment with Ro 61-8048. In vitro, THC elicited glutamate release from cortical astrocytes (on which we demonstrated co-localization of the CB1Rs and 7nAChRs mRNAs), and this effect was prevented by KYNA and rimonabant. These results suggest a key role of astrocytes in interactions between the endocannabinoid system, kynurenine pathway and glutamatergic neurotransmission, with ramifications for the pathophysiology and treatment of psychiatric and neurodegenerative diseases.
