Studies on the antagonism by raphe lesions of the antinociceptive action of systemic morphine.

In rats, lesions were placed in the dorsal/median raphe (DMR), in the ventral raphe (VR: raphe magnus), in both the dorsal/median and ventral raphe (DMVR) or in the reticular formation (RF). The effect of the lesions on the antinociception and catalepsy produced by 3 doses of morphine (3, 10 and 30 mg/kg) was examined. The lesions had no significant effect on the catalepsy produced by any of the doses of morphine tested. DMR lesions produced a partial attentuation of the antinociceptive action of both the 3 and 10 mg doses. VR lesions produced a complete blockade of the 3 mg and only a partial attenuation of the 10 mg dose. In contrast, the combined (DMVR) lesions yielded virtually a total blockade of the 3 and 10 mg. Yet, as with the DMR and VR groups, the DMVR lesions failed to produce a significant antagonism on either of the nociceptive tests at the 30 mg dose. These findings suggest that the ascending and descending fiber systems emanating from the dorsal/median and ventral raphe, respectively, facilitate the expression of morphine-induced analgesia but that neither system alone can be regarded as essential for the manifestation of the antinociceptive effects of systematically administered morphine.