Cannabinoids and ethanol activate the same reward pathways, and recent advances in the understanding of the neurobiological basis of alcoholism suggest that the CB 1 receptor system may play a key role in the reinforcing effects of ethanol and in modulating ethanol intake. In the present study, male CB 1 receptors knockout mice generated on a CD1 background displayed decreased ethanol-induced conditioned place preference (CPP) compared to wild-type (CB 1 þ / þ ) mice. Ethanol (0.5, 1.0, 1.5, and 2.0 g/kg) induced significant CPP in CB 1 þ / þ mice at all doses tested, whereas it induced significant CPP only at the highest dose of ethanol (2.0 g/kg) in CB 1 À/À mice.However, there was no genotypic difference in cocaine (20 mg/kg)-induced CPP. There was also no genotypic difference, neither in cocaine (10-50 mg/kg) nor in D-amphetamine (1.2-5 mg/kg)-induced locomotor effects. In addition, mutant and wild-type mice did not differ in sensitivity to the anxiolytic effects of ethanol (1.5 g/kg) when tested using the elevated plus maze. Interestingly, this decrease in ethanol efficacy to induce CPP in CB 1 À/À mice was correlated with an increase in D2/D3 receptors, as determined by [ 3 H]raclopride binding, whereas there was no difference in D1-like receptors, as determined by [ 3 H]SCH23390 binding, measured in the striatum from drug-naïve mice. This increase in D2/D3 binding sites observed in CB 1 knockout mice was associated with an altered locomotor response to the D2/D3 agonist quinpirole (low doses 0.02-0.1 mg/kg) but not to an alteration of quinpirole (0.1-1.0 mg/kg)-induced CPP compared to wild-type mice. Altogether, the present results indicate that lifelong deletion of CB 1 receptors reduced ethanol-induced CPP and that these reduced rewarding effects of ethanol are correlated to an overexpression of striatal dopamine D2 receptors.

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We have shown previously that the severity of handling-induced convulsions during ethanol withdrawal was reduced in A2A receptor knock-out (A2AR-/-) mice. In the present report, we further characterize the role of adenosine A(2A) receptors in ethanol consumption and neurobiological responses to this drug of abuse. Male A2AR-/- mice showed increased consumption of solutions containing 6 and 20% (v/v) ethanol compared with wild-type (A2AR+/+) control mice; female A2AR-/- mice showed increased consumption of solutions containing 6 and 10% ethanol. This slightly higher ethanol consumption was also related to increased ethanol preference. In contrast, A2AR-/- mice showed normal consumption of solutions containing either sucrose or quinine. Relative to A2AR+/+ mice, A2AR-/- mice were found to be less sensitive to the sedative effect of 3.0 gm/kg ethanol, as measured by more rapid recovery from ethanol-induced loss of righting reflex, and to the hypothermic effects of 1.5, 3.0, and 4.0 gm/kg ethanol, although plasma ethanol levels did not differ significantly between the two genotypes. The selective adenosine A2A receptor antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol) (10-30 mg/kg) significantly attenuated ethanol-induced (4.0 gm/kg) hypothermia in CD1 mice. To assess whether ethanol administration would induce differential tolerance in A2AR-/- and wild-type mice, we administered ethanol (3.0 gm/kg) over 4 consecutive days and found no difference in the development of tolerance; however, female A2AR-/- mice showed a lower tolerance-acquisition rate. These data suggest that activating the A2A receptors may play a role in suppressing alcohol-drinking behavior and is associated with the sensitivity to the intoxicating effects of acute ethanol administration.

A better understanding of the neural circuits affected by ethanol and their adaptations during the development of ethanol addiction will provide new opportunities for developing appropriate therapies.