• Energy Research

AbstractAs the contribution of cannabinoid (CB1) receptors in the neuroadaptations following chronic alcohol exposure is unknown, we investigated the neuroadaptations induced by chronic alcohol exposure on both NMDA and GABAA receptors in CB1−/− mice. Our results show that basal levels of hippocampal [3H]MK‐801 ((1)‐5‐methyl‐10,11‐dihydro‐5Hdibenzo[a,d]cyclohepten‐5,10‐imine) binding sites were decreased in CB1−/− mice and that these mice were also less sensitive to the locomotor effects of MK‐801. Basal level of both hippocampal and cerebellar [3H]muscimol binding was lower and sensitivity to the hypothermic effects of diazepam and pentobarbital was increased in CB1−/− mice. GABAAα1, β2, and γ2 and NMDA receptor (NR) 1 and 2B subunit mRNA levels were altered in striatum of CB1−/− mice. Our results also showed that [3H]MK‐801 binding sites were increased in cerebral cortex and hippocampus after chronic ethanol ingestion only in wild‐type mice. Chronic ethanol ingestion did not modify the sensitivity to the locomotor effects of MK‐801 in both genotypes. Similarly, chronic ethanol ingestion reduced the number of [3H]muscimol binding sites in cerebral cortex, but not in cerebellum, only in CB1+/+ mice. We conclude that lifelong deletion of CB1 receptors impairs neuroadaptations of both NMDA and GABAA receptors after chronic ethanol exposure and that the endocannabinoid/CB1 receptor system is involved in alcohol dependence.

Recent advances in the understanding of the neurobiological basis of alcohol dependence suggest that the endocannabinoid system may play a key role in the reinforcing effects of ethanol. In the present study, disruption of CB1 receptors in mice generated on a CD1 background decreased both ethanol consumption and preference. This decreased ethanol self-administration was associated with increased sensitivity to the acute intoxicating effects of ethanol. Mutant mice were more sensitive to the hypothermic and sedative/hypnotic effects of acute ethanol administration (1.5-4.0 g/kg), although plasma ethanol concentrations did not differ from those of controls. Moreover, wild-type mice exhibited normal locomotor activation caused by 1.0-2.5 g/kg injection of ethanol, whereas mutant mice displayed sedation in response to the injection of the same ethanol doses. The severity of alcohol withdrawal-induced convulsions was also increased in CB1(-/-) mice. Our results suggest that CB1 receptors participate in the regulation of ethanol drinking and demonstrate that their disruption lead to increased ethanol sensitivity and withdrawal severity.

We have shown previously that mice lacking the adenosine A2A receptor (A2AR) generated on a CD1 background self‐administer more ethanol and exhibit hyposensitivity to acute ethanol. We aimed to investigate if the increased propensity of A2A−/− mice to consume ethanol is associated with an altered sensitivity in the motivational properties of ethanol in the conditioned place preference (CPP) and conditioned taste aversion (CTA) paradigms and with an altered development of sensitization to the locomotor effects of ethanol. We also tested their sensitivity to the anxiolytic effects of ethanol. Our results show that A2A−/− mice produced on a CD1 background displayed a reduced ethanol‐induced CPP and an increased sensitivity to the anxiolytic and locomotor‐stimulant effects of ethanol, but they did not show alteration in ethanol‐induced CTA and locomotor sensitization. Ethanol‐induced CPP, ethanol consumption and the locomotor effects of ethanol were also tested in A2A−/− mice produced on a C57BL/6J background. Our results emphasized the importance of the genetic background because alteration in ethanol consumption and preference, ethanol‐induced CPP and locomotor‐stimulant effects were not found in knockout mice produced on the alcohol‐preferring C57BL/6J genetic background. Finally, the A2AR agonist, 2‐p‐(2‐carboxyethyl)‐phenylethylamino‐5′‐N‐ethylcarboxamidoadenosine hydrochloride (CGS 21680), reduced ethanol consumption and preference in C57BL/6J mice. In conclusion, A2AR deficiency in mice generated on a CD1 background leads to high ethanol consumption that is associated with an increased sensitivity to the locomotor‐stimulant/anxiolytic effects of ethanol and a decrease in ethanol‐induced CPP.

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.

Cannabinoids and ethanol activate the same reward pathways, and recent advances in the understanding of the neurobiological basis of alcoholism suggest that the CB1 receptor system may play a key role in the reinforcing effects of ethanol and in modulating ethanol intake. In the present study, male CB1 receptors knockout mice generated on a CD1 background displayed decreased ethanol-induced conditioned place preference (CPP) compared to wild-type (CB1(+/+)) mice. Ethanol (0.5, 1.0, 1.5, and 2.0 g/kg) induced significant CPP in CB1(+/+) mice at all doses tested, whereas it induced significant CPP only at the highest dose of ethanol (2.0 g/kg) in CB1(-/-) 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 CB1(-/-) mice was correlated with an increase in D2/D3 receptors, as determined by [3H]raclopride binding, whereas there was no difference in D1-like receptors, as determined by [3H]SCH23390 binding, measured in the striatum from drug-naive mice. This increase in D2/D3 binding sites observed in CB1 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 CB1 receptors reduced ethanol-induced CPP and that these reduced rewarding effects of ethanol are correlated to an overexpression of striatal dopamine D2 receptors.