Analgesia for orofacial nociception produced by morphine microinjection into the spinal trigeminal complex.

Morphine sulfate (0.75 micrograms) was microinjected into (rat) rostral and caudal trigeminal areas singly and simultaneously, using cannula-electrode combinations. Also, 0.5 micrograms or 1.0 micrograms of morphine was injected into nucleus reticularis paragigantocellularis (PGC). Both single trigeminal nuclear microinjections significantly elevated the latency to a defensive face-rub reaction to noxious facial heat, bilaterally. There was no summation effect with the conjoint injection of 1.5 micrograms total to rostral and caudal nuclear areas. The 0.5 micrograms injection in PGC had a significantly larger effect than did the 0.75 microgram trigeminal injections. The caudal but not the rostral trigeminal injections did elevate the threshold for aversive reaction to caudal trigeminal nuclear stimulation of the injected tissue. This finding contrasts with the previously reported fact that as much as 1.0 microgram of morphine in PGC has no effect on this aversive reaction threshold to brain stimulation.