Inhibition by neuropeptide Y of fentanyl-induced muscular rigidity at the locus coeruleus in rats

The aim of this study was to evaluate the effects of centrally administered neuropeptide Y (NPY) on muscular rigidity induced by fentanyl in Sprague-D awley rats. They were anesthetized with ketamine (120 mg/kg, i.p.) and thei r lungs were mechanically ventilated. Intravenous administration of fentany l (100 mu g/kg) consistently evoked a significant increase in the electromy ographic activity (EMG) recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscular rigidity was appreciably attenuated by intra cerebroventricular administration of NPY (4 nmol/2.5 mu l) Microinjection o f NPY (40 or 160 pmol/50 nl) into the bilateral locus coeruleus (LC) elicit ed an inhibition of the EMG activation induced by fentanyl in a dose-depend ent manner. Microinjection of 160 pmol NPY plus antiserum against NPY (NPYA b, 1:20) into the LC failed to suppress fentanyl-induced muscular rigidity. However, this implied muscular rigidity was not affected by NPY plus norma l rabbit serum (NRS, 1:20). In addition, NPYAb Or NRS per se had no substan tial effect on the rigidity. Microinjection of NPY (160 pmol) into the area s adjacent to the LC did not attenuate the rigidity. Our results suggest th at the centrally administered NPY attenuated fentanyl-induced muscular rigi dity by acting at the LC, but endogenous NPY may not be involved in this pr ocess. (C) 2000 Elsevier Science ireland Ltd. All Fights reserved.