MORPHINE MICROINJECTED INTO THE NUCLEUS RAPHE MAGNUS DOES NOT BLOCK THE ACTIVITY OF SPINAL TRIGEMINAL NUCLEUS ORALIS CONVERGENT NEURONS IN THE RAT

This study investigated the effects of morphine microinjection into th e nucleus raphe magnus (RMg) on electrically evoked C-fiber activities of convergent neurons in the spinal trigeminal nucleus oralis (Sp5O), in halothane-anesthetized rats. Although the neurons could be depress ed by systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible fashi on, morphine microinjected into the RMg (2.5 mu g or 5 pg) neither dep ressed their C-fiber-evoked responses, nor the diffuse noxious inhibit ory controls acting on them. It is concluded that the RMg is not invol ved in reinforcing descending inhibitory controls that are tonic or tr iggered by noxious stimuli acting on Sp5O convergent neurons. (C) 1998 Elsevier Science B.V. All rights reserved.