Lesions in nucleus reticularis gigantocellularis: effect on the antinociception produced by micro-injection of morphine and focal electrical stimulation in the periaqueductal gray matter.

The effect of bilateral electrolytic lesions in the nucleus reticularis giganto-cellularis (NGC) on the antinociceptive efficacy of morphine and electrical stimulation applied in the periaqueductal central gray matter (PAG) was investigated. Antinociception, evaluated by standard hot plate and tail-flick analgesiometric tests, was reliably produced by morphine (5 microgram) and focal electrical stimulation (40-200 micro A) administered in the PAG of rats via chronic indwelling cannula/electrode assemblies. Subsequent to the initial antinociceptive testing, bilateral electrolytic lesions were introduced in the NGC and the antinociceptive efficacy of morphine and stimulation in the PAG was again evaluated. Lesions in the NGC prevented the expression of the antinociception produced by the microinjection of morphine in the PAG whereas the antinociception resulting from electrical stimulation in the PAG was unaffected. Further, lesions in the NGC did not alter baseline (control) nociceptive thresholds in either analgesiometric test. These results provide additional support for involvement of the NGC in morphine-induced antinociception and, in addition, suggest that the NGC is not essential to a tonically-active inhibitory system or to the antinociception produced by focal electrical stimulation in the PAG.