Antinociceptive action following microinjection of methionine-enkephalin in the nucleus raphe magnus of the rat.

The analgesic effect of the microinjection of low doses of methione-enkephalin (20 micrograms in 0.5 microliter) into the caudal brain stem of the unrestrained rat was investigated by means of one vocalisation test. Immediate short duration significant increases in threshold (21%) were seen from sites in the nucleus raphé magnus (NRM). Delayed effects were seen from sites immediately adjacent to this nucleus; sites more lateral produced no significant effect. These results lend further support to the postulated role of NRM in antinociception.