Localization of the antinociceptive action of morphine in primate brain

Microinjections of morphine sulfate (20-40 mu g) were made into various subcortical regions of the rhesus monkey brain. The effects of these injections were evaluated on the nociceptive threshold as defined by the shock titration technique. The results of this preliminary investigation indicate that the region of maximal antinociceptive sensitivity to morphine in the primate is the periventricular-periaqueductal gray matter. It is tentatively suggested that morphine lowers that affective tone or the aversive component of pain by its action on the midbrain central gray and periventricular areas--both important projection and integration areas of the extralemniscal somatosensory system.