Enhancement of anandamide formation in the limbic forebrain and reduction of endocannabinoid contents in the striatum of Delta(9)-tetrahydrocannabinol-tolerant rats

Recent studies have shown that the pharmacological tolerance observed after prolonged exposure to synthetic or plant-derived cannabinoids in adult rat s is accompanied by down-regulation/desensitization of brain cannabinoid re ceptors. However, no evidence exists on possible changes in the contents of the endogenous ligands of cannabinoid receptors in the brain of cannabinoi d-tolerant rats. The present study was designed to elucidate this possibili ty by measuring, by means of isotope dilution gas chromatography/mass spect rometry, the contents of both anandamide (arachidonoylethanolamide; AEA) an d its biosynthetic precursor, N-arachidonoylphosphatidylethanolamine (NArPE ), and 2-arachidonoylglycerol (2-AG) in several brain regions of adult male rats treated daily with Delta(9)-tetrahydrocannabinol (Delta(9)-THC) for a period of 8 days. The areas analyzed included cerebellum, striatum, limbic forebrain, hippocampus, cerebral cortex, and brainstem. The same regions w ere also analyzed for cannabinoid receptor binding and WIN-55,212-2-stimula ted guanylyl-5'-O-(gamma-[S-35]thio)-triphosphate ([S-35]GTP gamma S) bindi ng to test the development of the well known down-regulation/desensitizatio n phenomenon. Results were as follows: As expected, cannabinoid receptor bi nding and WIN-55,212-2-stimulated [S-35]GTP gamma S binding decreased in mo st of the brain areas of Delta(9)-THC-tolerant rats. The only region exhibi ting no changes in both parameters was the limbic forebrain. This same regi on exhibited a marked (almost fourfold) increase in the content of AEA afte r 8 days of Delta(9)-THC treatment. By contrast, the striatum exhibited a d ecrease in AEA contents, whereas no changes were found in the brainstem, hi ppocampus, cerebellum, or cerebral cortex. The increase in AEA contents obs erved in the limbic forebrain was accompanied by a tendency of NArPE levels to decrease, whereas in the striatum, no significant change in NArPE conte nts was found. The contents of 2-AG were unchanged in brain regions from De lta(9)-THC-tolerant rats, except for the striatum where they dropped signif icantly. In summary, the present results show that prolonged activation of cannabinoid receptors leads to decreased endocannabinoid contents and signa ling in the striatum and to increased AEA formation in the limbic forebrain . The pathophysiological implications of these findings are discussed in vi ew of the proposed roles of endocannabinoids in the control of motor behavi or and emotional states.