Tolerance to cannabinoid-induced behaviors in mice treated chronically with ethanol
BACKGROUND The hippocampus is strongly implicated in memory processes and contains high concentrations of both cannabinoid receptors and their endogenous ligands. Chronic alcohol consumption impairs a variety of cognitive and performance tasks, including memory and learning. As the activation of cannabinoid receptors by their endogenous ligands modulates hippocampal neurotransmission, we hypothesized that the impaired memory and learning in alcoholism may be due to alterations in the hippocampal endocannabinoid system. METHODS We used the rat chronic intermittent ethanol (CIE) model for alcohol withdrawal and dependence which involves intermittent episodes of ethanol intoxication (60 doses) and withdrawal (approximating binge drinking episodes in humans). We measured the levels of cannabinoid 1 receptor (CB1R) protein (Western blot using a C-terminal-directed antibody), CB1R mRNA (real-time RT-PCR), CB1R localization (immunocytochemistry), tissue levels of the endocannabinoids N-arachidonoylethanolamine/anandamide (AEA) and 2-arachidonoylglycerol (2-AG), and function (patch-clamp recordings of depolarization-induced suppression of inhibition (DSI), as well as effects of CB1R agonist WIN 55,212-2 on inhibitory currents) in the hippocampus of CIE rats and their saline-treated controls. RESULTS Results were obtained in saline and CIE-treated rats after 2 and 40 days of withdrawal (DW) from their respective treatments. In 2 DW CIE rats, CB1R mRNA and protein levels were decreased by 27% (p<0.05) compared with saline controls. Surprisingly, in 40 DW CIE rats, CB1R mRNA increased by 100% and protein increased by 21%, confirmed by immunohistochemistry. Hippocampal [2-AG] increased in both 2 and 40 DW CIE rats; [AEA] increased only at 40 DW. Hippocampal DSI of CIE rats was significantly reduced at 2 DW but not at 40 DW. The CB1R agonist WIN 55,212-2 (0.5 microM) produced a significantly greater decrease in the frequency of spontaneous inhibitory currents from saline-treated rats compared with CIE rats at 2 DW, but not at 40 DW. CONCLUSIONS These data demonstrate that CIE treatment and withdrawal transiently down-regulates hippocampal CB1 Rs followed by a long-term up-regulation, including increased levels of endogenous cannabinoids. These findings are consistent with our hypothesis and suggest that long-term up-regulation of hippocampal CB1Rs may contribute to the long-term cognitive impairments in alcoholism. The data further suggest that the effectiveness of CB1R blockade in decreasing alcohol consumption may be greater after protracted abstinence from alcohol.