Glycyl-L-glutamine injected centrally suppresses alcohol drinking in P rats

Recent reports show that central beta-endorphin (1-31) injection augments the volitional intake of alcohol. Correspondingly, alcohol drinking stimulates beta-endorphin (1-31) release from the hypothalamus of the rat. Glycyl-l-glutamine (Gly-Gln) is produced in beta-endorphin-containing neurons and is co-released with beta-endorphin(1-31) and other processing products. Because Gly-Gln is apparently an endogenous antagonist of beta-endorphin(1-31) in several systems, the present study was designed to investigate the hypothesis that Gly-Gln injected i.c.v. would alter voluntary alcohol drinking in the genetic, high-alcohol-preferring P rat. After a guide tube was implanted stereotaxically above the lateral cerebral ventricle, the rats were offered 3-30% alcohol over 10 days, and then given their maximally preferred concentration of alcohol in the presence of water for the remainder of the experiment. Gly-Gln or artificial cerebrospinal fluid (CSF) vehicle then was injected i.c.v. in a dose of 10 or 100 nmol for 3 consecutive days, which was followed by a 7-day postinjection interval. Gly-Gln suppressed significantly the intakes of alcohol in terms of both g/kg and proportion to total fluid. During the postinjection days, alcohol drinking continued to be suppressed, whereas neither the daily intakes of food or water nor the body weights of the rats were changed. The present results are consistent with the concept of a functional antagonism by Gly-Gln of the role of beta-endorphin(1-31) in mediating certain central functions. These results demonstrate that alcohol consumption is suppressed by the direct intracerebral application of this unique peptide.