EFFECTS OF SYSTEMIC MORPHINE ON THE ACTIVITY OF CONVERGENT NEURONS OFSPINAL TRIGEMINAL NUCLEUS ORALIS IN THE RAT

The spinal trigeminal nucleus oralis has been shown to relay nocicepti ve inputs mainly from the oral and perioral regions. In this study, we examined the effects of intravenous administration of morphine on C-f iber-evoked activities of spinal trigeminal nucleus oralis convergent neurons in halothane-anesthetized rats. Morphine depressed the C-fiber -evoked responses of spinal trigeminal nucleus oralis convergent neuro ns in a dose-related (3-12 mg/kg range) and naloxone-reversible fashio n. The ED(50) was 6.1 mg/kg, a dose similar to that found in the spina l dorsal horn. The observed strong depressive action of morphine on no xious-evoked activities of spinal trigeminal nucleus oralis neurons is consistent with our previous statement, based on electrophysiological studies, that this region plays an important role in the transmission of trigeminal nociceptive information. The effect of morphine on the spinal trigeminal nucleus oralis neurons is discussed in relation to i ts possible site and mechanism of action.