ANTINOCICEPTIVE EFFECT OF MORPHINE MICROINJECTIONS INTO THE SPINAL TRIGEMINAL SUBNUCLEUS ORALIS

WE analysed the effects of morphine microinjections (0.2 mu l) into th e subnucleus oralis (SNO), i.e. the rostral division of the spinal tri geminal nucleus, on the formalin test adapted for orofacial pain. Dura tion of rubbing following an injection (50 mu l) of formalin solution (1.5%) in the right upper lip of the rat was measured. Compared to mic roinjections of saline (NaCl, 0.9%) used as control, 90 and 180 nmol m u l(-1) of morphine solution in the SNO significantly curtailed rubbin g. This effect was dose-dependent, reversed by naloxone, and also site -specific to the SNO since no effect was observed after microinjection of morphine into areas adjacent to the SNO. These results are further evidence for the involvement of the SNO in perioral nociceptive mecha nisms.