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 discrimination, as well as self-administration, of THC or nicotine, to disrupt ongoing food-maintained behavior to alter emotional responses such as anxiety or to modulate attention learning and memory processes. Currently, studies are focusing on nicotine, caffeine and a series of cannabinoids, including delta-9-tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, the cannabinoid CB1 receptor antagonists rimonabant and AM251, and AM4113, the endogenous cannabinoids anandamide and 2AG, the non-cannabinoid fatty acid amides OEA and PEA, the anandamide uptake inhibitor AM404, and the fatty acid amide hydrolase (FAAH) inhibitor URB597. Studies are also being conducted on methamphetamine, cocaine and heroin. Endocannabinoids are involved in a variety of behavioral and physiological processes that are just beginning to be understood. Inhibitors of fatty acid amide hydrolase (FAAH) increase endogenous levels of anandamide (a cannabinoid CB1-receptor ligand) and oleoylethanolamide and palmitoylethanolamide (OEA and PEA, ligands for a-type peroxisome proliferatoractivated nuclear receptors, PPAR- alpha) when and where they are naturally released in the brain. Using a passive-avoidance task in rats, we previously found that memory acquisition was enhanced by the FAAH inhibitor URB597 or by the PPAR-alpha agonist WY14643, and these enhancements were blocked by the PPAR- alpha antagonist MK886. These findings demonstrate novel mechanisms for memory enhancement by activation of PPAR- alpha, either directly by administering a PPAR- alpha agonist or indirectly by administering a FAAH inhibitor.Experiments are continuing using transgenic Alzheimer's disease rodent models. Cannabis and caffeine are two of the most widely used psychoactive substances. THC, the main psychoactive constituent of cannabis, is known to produce deficits in short term memory. Caffeine, a non-selective adenosine receptor antagonist, attenuates some kinds of memory deficits, but there have been few studies addressing the effects of caffeine and THC in combination. Rats were given THC along with either caffeine or the selective adenosine A1-receptor antagonist CPT and tested with a delayed nonmatching-to-position procedure in which behavior during the delay is automatically recorded as a model of memory rehearsal. THC alone produced dose-dependent memory deficits. The initial exposure to caffeine disrupted the established pattern of rehearsal-like behavior, but tolerance developed rapidly to this effect. Caffeine and CPT alone had no significant effects on rehearsal or memory in subsequent testing. When a subthreshold dose of THC was combined with caffeine or CPT, memory performance was significantly decreased, even though performance of the rehearsal-like pattern was not significantly altered. Caffeine and CPT did not counteract memory deficits induced by THC, but in fact exacerbated them. These results are consistent with recent findings that adenosine receptors modulate cannabinoid signaling in the hippocampus. In other experiments using a five-choice serial reaction time task,the effects of the endocannabinoid anandamide were studied. Since anandamide is a ligand for not only cannabinoid receptors but also transient receptor potential vanilloid 1 (TRPV1) receptors, and as recently suggested, peroxisome proliferator-activated nuclear receptor- (PPAR), we also determined whether anandamide's effects in this task were mediated by each of these receptors. Anandamide increased omission errors and decreased responding during inter-trial intervals. These effects were blocked by the TRPV1 antagonist capsazepine, but not by the cannabinoid-receptor antagonist rimonabant or the PPAR- alpha antagonist MK886. Testing with open-field activity and food-consumption procedures in the same rats suggested that the disruption of operant responding observed in the attention task was not due to motor depression, anxiety, decreased appetite, or an inability to find and consume food pellets. Thus, the vanilloid-dependent behavioral disruption induced by anandamide was specific to the operant attention task. These effects of anandamide resemble effects of systemically administered dopamine antagonists and might reflect changes in vanilloid-mediated dopamine transmission. A common molecular mechanism contributing to the development of addiction that is shared by drugs of abuse(including cannabinoids) is their ability to increase levels of extracellular dopamine in the shell of the nucleus accumbens, but the underlying mechanisms have not yet been well established. Although cannabinoid CB1 receptors in the ventral tegmental area (VTA) have been suggested to be involved, in vivo administration of THC directly into the VTA does not induce dopamine release in the nucleus accumbens shell, while direct infusion of THC in the nucleus accumbens shell does. It has been suggested that presynaptic CB1 receptors that control striatal glutamate release are main targets for the dopamine-releasing effects of cannabinoids by decreasing excitability of striatal GABAergic dynorphinergic neurons that project to the mesencephalon and tonically inhibiting dopaminergic cells in the VTA. Presynaptic A2A receptors localized in striatal glutamatergic terminals could be responsible for the counteracting effects of the A2A receptor antagonist on the reinforcing effects of cannabinoids we found in parallel studies of intravenous THC self administration behavior. In that case, A2A receptor antagonists should also counteract THC-induced, but not cocaine-induced, dopamine release in the nucleus accumbens shell. Using in vivo microdialysis in freely moving rats, we found that a behaviorally active dose of MSX-3 significantly counteracted THC-induced increases in extracellular dopamine levels in the nucleus accumbens shell induced by THC. These effects of MSX-3 were selective, occurring with cannabinoids but not with cocaine.Further findings that the A2A receptor antagonist was able to reduce cannabinoid-induced self administration as well as dopamine release yet failed to attenuate cannabinoid-induced reinstatement of THC-seeking behavior in abstinent subjects reinforces the hypothesis that different mechanisms and brain circuits are involved in these phenomena;that dopamine is involved in cannabinoid self-administration, but its involvement in the reinstatement of cannabinoid drug-seeking may not be as pronounced.
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