Alterations in central preproenkephalin mRNA expression after chronic free-choice ethanol consumption by fawn-hooded rats

Background: Neurotransmission mediated via opioid and dopamine receptors is believed to be involved in the reinforcing and/or rewarding effects of ethanol consumption. We previously examined the effect of ethanol consumption (and naltrexone treatment, used clinically to treat alcoholism) on A-opioid receptor density. We describe here the effect of free-choice ethanol consumption and naltrexone treatment on preproenkephalin, preprodynorphin, and dopamine D-1 and D-2 receptor mRNA expression in the central nervous system. Methods: Fawn-hooded rats were given continual free-choice access to a 5% ethanol solution or water (4 weeks) followed by 2 weeks of water alone. At the end of this abstinence period, osmotic minipumps were implanted subcutaneously to deliver saline (n = 4) or naltrexone (n = 4; 8.4 mg/kg/day for 4 weeks). After recovery from surgery, the rats again were given access to 5% ethanol under the same free-choice conditions (4 weeks). A third group of age-matched controls drank only water during the behavioral trial. At the end of the behavioral trial, the rats were decapitated, and a quantitative examination of peptide precursor mRNAs was made by using in situ hybridization histochemistry. Results: Naltrexone treatment significantly decreased preprodynorphin expression in the nucleus accumbens, but neither naltrexone treatment nor ethanol consumption significantly affected dopamine D-1 and D-2 receptor mRNA expression. In contrast, ethanol consumption increased preproenkephalin mRNA in the central and intercalated nuclei of the amygdala but decreased preproenkephalin mRNA in the nucleus accumbens and olfactory tubercle. The decreased level of preproenkephalin mRNA in the nucleus accumbens may reflect a neuroadaptive response to increased release of dopamine, whereas the increased level of preproenkephaLin mRNA in the central nucleus of the amygdala may be associated with an anxiolytic effect of ethanol consumption. Conclusions: The data support the putative role of opioid peptides in the effects of ethanol and suggest that the nucleus accumbens and central nucleus of the amygdala are loci for the reinforcing effects of ethanol.