Ethanol desensitizes cannabinoid CB1 receptors modulating monoamine synthesis in the rat brain in vivo

The endocannabinoid system and the cannabinoid CB1 receptors are involved in the development of ethanol tolerance and dependence. This study aimed to investigate the in vivo sensitivity of a CB1 receptor agonist (WIN 55,212-2) modulating the synthesis of 3,4-dihydroxy-phenylalanine/dopamine/noradrenaline (DOPA/DA/NA) and that of 5-hydroxy-tryptophan/serotonin (5-HTP/5-HT) in rat brain after ethanol treatment and withdrawal. In control rats, WIN 55,212-2 (4 mg/kg, i.p., for 1 h), through a mechanism sensible to the CB1 antagonist SR 141716A, increased the synthesis of DOPA/NA in a slice of brainstem containing the locus ceruleus (250%) and in the hippocampus (64%), and it reduced DOPA/DA synthesis in the striatum (47%). WIN 55,212-2 also decreased the synthesis of 5-HTP/5-HT in the locus ceruleus (43%), hippocampus (35%) and striatum (35%). In the locus ceruleus of ethanol-treated rats, the stimulatory effect of WIN 55,212-2 on DOPA/NA synthesis was abolished (acute treatment) or markedly attenuated (53–55%, chronic treatment and withdrawal), whereas in the hippocampus this effect was reduced only in chronic ethanol-withdrawn rats (33%). In the striatum of ethanol-treated rats (acute, chronic and withdrawal), the inhibitory effect of WIN 55,212-2 on DOPA/DA synthesis was completely blunted or markedly reduced. Similarly, the inhibitory effect of WIN 55,212-2 on 5-HTP/5-HT synthesis was reduced or abolished in the three brain regions after chronic ethanol and during withdrawal. These results indicate that treatment with ethanol in rats induces a functional desensitization of CB1 receptors modulating the synthesis of brain monoamines.