Differences in the content of proenkepephalin and prodynorphin mRNAan opioid receptor density in the brains of alcohol preferring AA and alcohol avoiding ANA

The present studies examined differences in proenkephalin and prodynorphin peptide mRNA content, using in situ hybridization, and in opioid receptor density, using receptor autoradiography, between rats bred selectively for their high (AA) and low (ANA) alcohol consumption. Results indicated that AA rats had a significantly greater content of proenkephalin mRNA in the, arcuate nucleus (p ≤ .01) and caudate putamen ( p ≤ .005) as well as a greater overall content of proenkephalin mRNA, compared to ANA rats (p ≤ .01). The content of prodynorphin mRNA was significantly higher in the arcuate nucleus (p ≤ .01) of the AA than ANA rats. Receptor autoradiography results indicated that AA rats had a significantly greater density of mu opioid receptors in the shell region of the nucleus accumbens and the prefrontal cortex ( p ≤ .01), and a greater overall binding for the mu ( p ≤ .05), but not for the delta or kappa opioid receptors, compared to ANA rats. The presence of inherited differences in the activity of distinct components of the endogenous opioid system in brain regions associated with the processes of reward and reinforcement are in line with previous findings and suggest that endogenous opioids may play a role in determining the difference in preference for ethanol between the AA and ANA rats