Role of mu and delta opioid receptors in alcohol drinking behaviour.

The dopaminergic mesolimbic system plays a key role in the mechanisms of reinforcement elicited by alcohol (ethanol) and other drugs of abuse. Numerous lines of evidence indicate that ethanol reinforcement mechanisms involve, at least partially, the ethanol-induced activation of the endogenous opioid system. Ethanol may alter opioidergic transmission at different levels, including the biosynthesis, release, and degradation of opioid peptides, as well as binding of endogenous ligands to opioid receptors. Several studies suggest that mu and delta opioid receptors play a major role in ethanol reinforcement and dependence. These studies implicate enkephalins and beta-endorphin as physiological mediators of ethanol's actions in the brain. In this review we describe the pharmacological characteristics of opioid receptors and their distribution in brain, as well as the major functions of their endogenous ligands. Thereafter, we present evidence supporting the participation of mu and delta opioid receptors in ethanol reinforcement mechanisms and high alcohol drinking behaviour. The use of opioid receptor agonists and antagonists, as well as ethanol-preferring selected rodents and knockout mice, has contributed to understand the role of mu and delta receptors in these processes. The effects of ethanol on binding of selective ligands to opioid receptors in different experimental models are also reviewed. The relevance of opioid receptors in human alcoholism is further evidenced by the association of mu receptor polymorphisms with ethanol dependence. The clinical implication of these findings is discussed regarding the differential responses observed in some alcoholic patients to treatment with opioid receptor antagonists such as naltrexone.