Effects of midbrain, bulbar and combined morphine microinjections and systemic injections on orofacial nociception and rostral trigeminal stimulation: independent midbrain and bulbar opiate analgesia systems?

Microinjections of 0.35 microgram and 0.7 microgram morphine were made into rat nucleus reticularis paragigantocellularis and ventral central gray respectively. When injected simultaneously, the analgesic effect on orofacial thermal pain was significantly greater than the effect of either injection alone, suggesting a summation mechanism for the two sites. No microinjection affected the threshold for aversive response to main sensory trigeminal nuclear stimulation. This threshold was, however, profoundly elevated by 10 mg/kg systemic injection.