MORPHINE ADMINISTERED IN THE SUBSTANTIA-GELATINOSA OF THE SPINAL TRIGEMINAL NUCLEUS CAUDALIS INHIBITS NOCICEPTIVE ACTIVITIES IN THE SPINAL TRIGEMINAL NUCLEUS ORALIS

The present study investigates the effects of morphine microinjection into the spinal trigeminal nucleus caudalis (SpSC) or the spinal trige minal nucleus oralis (Sp5O) on C-fiber-evoked activities of Sp5O conve rgent neurons, after supramaximal per cutaneous electrical stimulation in halothane-anesthetized rats. When it was microinjected into the Sp 5O, morphine (2.5 mu g in 0.25 mu l) never depressed the C-fiber-evoke d responses of Sp5O convergent neurons (n = 13), whereas these neurons were responsive to the inhibitory effects of systemic morphine (6 mg/ kg, i.v.) in a naloxone-reversible manner. On the contrary, morphine m icroinjected into the Sp5C produced a naloxone-reversible inhibition o f the C-fiber-evoked responses of Sp5O neurons (n = 14). The magnitude and the time course of this effect varied according to the location o f the injection sites. After microinjection into the superficial lamin ae (n = 7), a strong depressive effect of morphine (7 +/- 5% of contro l) on the C-fiber-evoked responses was apparent as soon as 5 min after the injection and could always be reversed by naloxone, administered either intravenously (0.4 mg/kg) or locally (2.5 mu g in 0.6 mu l) at the same site as morphine. After microinjection into deeper laminae (V -VI), a significant depressive effect (34 +/- 5% of control) of morphi ne could be detected only 20 min after the injection and was reversed only by intravenous administration of naloxone. These results suggest that morphine exerts its antinociceptive action on Sp5O convergent neu rons by blocking the C-fiber inputs that relay in the Sp5C substantia gelatinosa. The mechanisms that underlie the activation of Sp5O conver gent neurons by C-fibers and the inhibition of C-fiber-evoked response s of Sp5O convergent neurons by morphine microinjected into the SpSC a re discussed.