REGULATION OF MU-OPIOID RECEPTOR MESSENGER-RNA IN RAT GLOBUS-PALLIDUS- EFFECTS OF ENKEPHALIN INCREASES INDUCED BY SHORT-TERM AND LONG-TERMHALOPERIDOL ADMINISTRATION

The mRNA encoding mu-opioid receptors is expressed in neurons of the g lobus pallidus, a region of the basal ganglia that receives a dense en kephalinergic innervation from the striatum. The regulation of the mRN As encoding the opioid peptide enkephalin in the striatum and the mu-o pioid receptor in the globus pallidus was examined with in situ hybrid ization histochemistry following short- or long-term haloperidol treat ments, which alter striatal enkephalin mRNA levels. Animals were admin istered haloperidol daily for 3 or 7 days (1 mg/kg, s.c.) or continuou sly for 8 months(1 mg/kg, depot followed by oral). Enkephalin and mu-o pioid receptor mRNA levels were unchanged after 3 days of haloperidol treatment. In contrast, the enkephalin mRNA level was increased in the striatum, and mu-opioid receptor mRNA levels were markedly decreased in the globus pallidus after 7 days of haloperidol administration. Sim ilar effects were observed in rats treated with haloperidol for 8 mont hs. The results provide the first evidence of regulation of mu-opioid receptor mRNA in vivo.