Microinjection of morphine into nucleus reticularis paragigantocellularis of the rat: suppression of noxious-evoked activity of nucleus raphe magnus neurons.

Microinjection of 1 microgram of morphine into nucleus reticularis paragigantocellularis (Pgc) of anesthetized rats depressed both noxious-evoked and spontaneous activity of nociresponsive neurons in the nucleus raphe magnus (NRM). This effect was naloxone-reversible, and was not observed after control injections dorsal to Pgc. The percent change in spontaneous firing was significantly greater than the percent change in pinch-evoked firing. This reduction in NRM neuronal discharge may contribute to the antinociceptive effects produced by microinjection of morphine into Pgc.