Endogenous opioids support the spinal inhibitory action of an α2-adrenoceptor agonist in the decerebrated, spinalised rabbit

The present study examined the possible contribution of endogenous opioids to inhibition of spinal reflexes by an alpha(2)-adrenoceptor agonist. In rabbits decerebrated and spinalised under halothane/nitrous oxide anaesthesia, the selective alpha(2)-adrenoceptor agonist dexmedetomidine (3-30 mug intrathecal) induced significant decreases in short- and long-latency reflex responses evoked in medial gastrocnemius (MG) motoneurones by stimulation of the sural nerve. After recovery from dexmedetomidine, the mu-opioid receptor antagonist P-funaltrexamine (beta-FNA; 100 mug intrathecal) significantly enhanced short-latency but not long-latency MG reflex responses. After beta-FNA, inhibition of all reflexes by dexmedetomidine was significantly weaker than in the control state, whereas the cardiovascular actions of dexmedetomidine were unaffected. These data confirm that activation of spinal alpha(2)-adrenoceptors depresses MG reflexes evoked by all groups of sural nerve afferent fibres, and shows that endogenous opioid tone supports the inhibitory action Of alpha(2) agonists, possibly by a synergistic interaction in the spinal cord. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.