There is a school of thought that alcoholism is an appetitive disorder, based on evidence that the same group of neuropeptides and classical transmitters appear to be involved both in the control of appetite and of ethanol preference. Following our previous findings that implicated endocannabinoids and CB1 receptors in the control of food intake (Nature 410:822-825, 2001), we examined their role in the regulation of alcohol drinking behavior, using wild-type and CB1 receptor deficient, C57BL/6J mice. The high ethanol preference of young (6-10 weeks of age) mice was reduced by the cannabinoid CB1 receptor antagonist SR141716 to levels observed in their CB1 receptor knockout littermates or in older (26-48 weeks) wild-type mice, in both of which ethanol preference is unaffected by SR141716. Similarly, SR141716 inhibited food intake in food-restricted young, but not older, wild-type mice. There were no age-dependent differences in the tissue levels of the endocannabinoids anandamide and 2-arachidonoyl glycerol or the in the density of CB1 receptors in the hypothalamus, limbic forebrain, amygdala or cerebellum. CB1 receptor-stimulated GTPgammaS binding was selectively reduced in the limbic forebrain of older compared to young wild-type mice. There was no age-dependent difference in Gi or Go subunit protein expression in the limbic forebrain, and the selective reduction in GTPgammaS labeling in tissue from older mice was maintained in a receptor/G protein reconstitution assay using functional bovine G protein. These findings suggest that endocannabinoids acting at CB1 receptors contribute to ethanol preference, and decreased coupling of CB1 to G proteins in the limbic forebrain by mechanisms other that altered receptor of G protein levels may be involved in the age-dependent decline in the appetite for both ethanol and food. In a related project, we have begun to use rat strains with high and low preference to ethanol (P and NP rats) to further analyze the role of the endocannabinoid system in ethanol preference, and to determine the brain site(s) where such interactions occur. In preliminary experiments we found that the very high ethanol preference of P rats (90-95% of fluid intake from the ethanol containing bottle in a two-bottle, free-choice paradigm) is dramatically reduced (to 30-40%) by systemic treatment with the CB1 receptor antagonist SR141716 (3 mg/kg i.p.). We are now doing reverse microdialysis studies where SR141716 is delivered bilaterally into discreet regions of the limbic forebrain or hypothalamus, using osmotic minipumps.
The aim i s to determine the site(s) where local microinfusion of SR141716 will result in reduced ethanol preference and intake.
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