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

Abstract

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.

Cellular basis for the effects of substance P in the periaqueductal gray and dorsal raphe nucleus

“…Consistently with this, SP produces an enkephalin release in a PAG slice preparation (Del Rio et al, 1983). Opioid-dependent antinociception is preferentially associated with the ventral PAG or the DRN, whereas antinociception elicited from the lateral or dorsolateral PAG is opioid independent (Cannon et al, 1982;Thorn et al, 1989). This suggests that NK1 receptor localized to the ventrolateral PAG at caudal levels or that the DRN could be responsible for the antinociceptive effects; however, NK1 did not appear to colocalize with enkephalin in these areas.…”

“…This indicates that to some extent SP may act directly on enkephalin-containing neurons in the PAG. Even though antinociception in this area is not typically opioid dependent, occasionally stimulation sites mapped close to this area can produced a naloxone-attenuated antinociception (Thorn et al, 1989). Therefore, the possibility remains that SP acts directly on enkephalin-containing neurons to produce antinociception.…”

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

“…It is interesting in this context to note that within the UCC, a population of cells located within the nucleus of the DLF also project back down the spinal cord (Burton and Loewy, 1976;Matsushita et al, 1979;Svensson et al, 1985;Flink and Svensson, 1986;Flink and Westman 1986). This descending pathway mediates a powerful antinociception; it would therefore be interesting to determine whether the UCC neurons that project to the vlPAG, a region that itself generates an opioid-mediated antinociception (Millan et al, 1986(Millan et al, , 1987Morozova and Zvartua, 1986;Nicols et al, 1989;Thorn et al, 1989), also project back down the cord.…”