Morphine administered in the substantia gelatinosa of the spinal trigeminal nucleus caudalis inhibits nociceptive activities in the spinal trigeminal nucleus oralis

Abstract

The present study investigates the effects of morphine microinjection into the spinal trigeminal nucleus caudalis (Sp5C) or the spinal trigeminal nucleus oralis (Sp50) on C-fiber-evoked activities of Sp50 convergent neurons, after supramaximal percutaneous electrical stimulation in halothane- anesthetized rats. When it was microinjected into the Sp50, morphine (2.5 μg in 0.25 μl) never depressed the C-fiber-evoked responses of Sp50 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 microinjected into the Sp5C produced a naloxone- reversible inhibition of the C-fiber-evoked responses of Sp50 neurons (n = 14). The magnitude and the time course of this effect varied according to the location of the injection sites. After microinjection into the superficial laminae (n = 7), a strong depressive effect of morphine (7 ± 5% of control) on the C-fiber-evoked responses was apparent as soon as 5 rain after the injection and could always be reversed by naloxone, administered either intravenously (0.4 mg/kg) or locally (2.5 μg in 0.6 μl) at the same site as morphine. After microinjection into deeper laminae (V-Vl), a significant depressive effect (34 ± 5% of control) of morphine 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 Sp50 convergent neurons by blocking the C-fiber inputs that relay in the Sp5C substantia gelatinosa. The mechanisms that underlie the activation of Sp50 convergent neurons by C-fibers and the inhibition of C- fiber-evoked responses of Sp50 convergent neurons by morphine microinjected into the Sp5C are discussed.