Ability of periaqueductal gray subdivisions and adjacent loci to elicit analgesia and ability of naloxone to reverse analgesia.

This study is an investigation of the effects of stimulation of regions within and adjacent to the periaqueductal gray (PAG) matter. Eighty-five rats were implanted with 1 monopolar stimulating electrode into 1 of 5 loci. Potency of analgesia was evaluated by relative increases in tailflick latencies after brain stimulation, and threshold current intensity was used to elicit analgesia. The ability of naloxone to reverse the stimulation-induced analgesia was also evaluated. Results replicate the previous finding of differential naloxone reversibility of ventral vs. dorsal PAG sites, but they do not support a regional distinction in the potency of analgesia induced. The results suggest that dorsal PAG sites are involved in a separate nonopiate pain-inhibitory system, whereas ventral sites are involved in an opiate system. These systems, however, do not respect the cytoarchitectural boundaries of the PAG because sites adjacent to the PAG elicit similar effects with a corresponding dorsal-ventral distinction.