INHIBITION BY INTRATHECAL PRAZOSIN BUT NOT YOHIMBINE OF FENTANYL-INDUCED MUSCULAR RIGIDITY IN THE RAT

We evaluated the effect of intrathecally administered prazosin, alpha( 1)-adrenoceptor antagonist, or yohimbine, alpha(2)-adrenoceptor antago nist, on fentanyl-induced muscular rigidity. Adult, male Sprague-Dawle y rats were anesthetized with ketamine and were under mechanical venti lation. Fentanyl given intravenously (100 mu g/kg) or microinjected in to the bilateral locus coeruleus (LC) (2.5 mu g/50 nl) consistently ev oked a significant increase in the electromyographic activity recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscu lar rigidity was appreciably antagonized by prior intrathecal (10 mu l ) administration of prazosin (5 or 10 nmol), but not equimolar dose of yohimbine. These results suggest that the spinal alpha(1)-adrenocepto rs in the coerulospinal noradrenergic pathway play a key role in fenta nyl-induced muscular rigidity.