Opiate and serotonergic mechanisms of stimulation-produced analgesia within the periaqueductal gray.

These studies investigated the distribution of analgesia-producing sites within the periaqueductal gray (PAG), and their potential reversal by naloxone and methysergide. The PAG is not differentiable in its ability to elicit stimulation-produced analgesia (SPA) until the point of stimulation is caudal to the dorsal raphe nucleus, where analgesia was not obtained. Naloxone, however, was found to have a differential effect at specific loci, significantly reducing SPA from ventral but not dorsal sites. In contrast, methysergide was effective in reversing analgesia both at ventral and dorsal sites. The site of stimulation was critical to whether motor effects were elicited: Motor effects accompanied by analgesia were most often produced rostrally, while motor effects without analgesia were most frequently produced in the middle PAG. Null effects for both motor activity and analgesia were obtained from caudal PAG sites.