Acute ethanol administration differentially modulates mu opioid receptors in the rat meso-accumbens and mesocortical pathways

Biochemical and pharmacological evidence suggest that the dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of alcohol and other drugs of abuse. Alcohol reinforcement and high alcohol drinking behavior have been postulated to be partially mediated by a neurobiological mechanism involving the alcohol-induced activation of the endogenous opioid system. The aim of this work was to study the effect of the in vivo acute administration of ethanol on mu (mu) opioid receptors in the rat dopaminergic meso-accumbens and mesocortical pathways by quantitative receptor autoradiography. [(3)H]DAMGO binding was significantly decreased in the ventral tegmental area (VTA) 30 min after ethanol administration. A small ethanol-induced reduction was observed in the shell region of the nucleus accumbens 1 h after exposure. In contrast, 2 h after ethanol administration, [(3)H]DAMGO binding was significantly increased in the frontal and prefrontal cortices. The observed changes correlated well with high ethanol plasma levels. Our results suggest that the reinforcing properties of ethanol may be partially mediated by mechanisms involving the ethanol-induced down- and up-regulation of mu receptors in the dopaminergic mesolimbic system. Mu receptors in the VTA and the frontal and prefrontal cortices may be involved in the in vivo acute responses to ethanol and could play a key role in modulating the dopaminergic activity of the mesocortical pathway in response to the drug. In contrast, the contribution of both mu and delta receptors in the nucleus accumbens might be relevant in these processes.