Pharmacological characterization of the nociceptin receptor which mediates reduction of alcohol drinking in rats

Chronic intracerebroventricular (ICV) treatment with nociceptin/orphanin FQ (NC), the endogenous ligand for the opioid receptor-like 1 (ORL1) receptor, reduces ethanol intake in alcohol-preferring rats and abolishes the rewarding properties of ethanol in the place conditioning paradigm. To pharmacologically characterize the receptor involved, the present study evaluated the effect on ethanol drinking in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats of ICV injections for 8 days of NC or of the NC analogs NC(1–17)NH2, NC(1–13)NH2, NC(1–12)NH2 and [Nphe1]NC(1–13)NH2. In vitro studies indicate that NC, NC(1–17)NH2, NC(1–13)NH2 and NC(1–12)NH2 are agonists, while [Nphe1]NC(1–13)NH2 is a selective antagonist at the ORL1 receptor. Freely feeding and drinking rats were offered 10% ethanol 30 min/day at the beginning of the dark phase of the light cycle. NC significantly attenuated ethanol intake at 500 or 1000 ng/rat (210 or 420 pmol/rat). NC(1–17)NH2, markedly reduced ethanol intake, but its effect was statistically significant at 1000 (420 pmol/rat), not at 500 ng/rat (210 pmol/rat). After the end of treatment ethanol drinking promptly came back to baseline level. Ethanol consumption was also reduced by NC(1–13)NH2; however, its effect was less potent and pronounced. NC(1–12)NH2 did not modify ethanol intake at doses up to 4000 ng/rat (2339 pmol/rat). Water and food consumption were not modified. Treatment with [Nphe1]NC(1– 13)NH2, 66 or 99 g/rat, did not modify ethanol intake; however, [Nphe1]NC(1–13)NH2, 66 g/rat, given just before 1000 ng/rat of NC(1–17)NH2, abolished the effect of the agonist. The present results show that the 13 aminoacid N-terminal sequence of NC is essential for the effect on ethanol intake and indicate that [Nphe1]NC(1–13)NH2 acts as an antagonist to block the effect of NC. These findings provide further evidence that selective agonists at the ORL-1 receptor attenuate ethanol intake in alcohol-preferring rats and suggest that the NC/ORL1 system may represent an interesting target for treatment of alcohol abuse