Opioid and nonopioid forms of stress-induced analgesia: some environmental determinants and characteristics

Exposure to a series of inescapable shocks produces both an early nonopioid analgesia and a late-appearing opioid analgesia, and the environmental conditions necessary to produce and maintain the two forms of analgesia were examined.

Abstract

Grau, Hyson, Maier, Madden, and Barchas (Science, 213, 1409, 1411) found that exposure to a series of inescapable shocks produces both an early nonopioid analgesia and a late-appearing opioid analgesia. The nonopioid analgesia was observed following 20 shocks, while the opioid analgesia was aroused after 60 and 80 shocks. Here in three experiments the environmental conditions necessary to produce and maintain the two forms of analgesia were examined. It was revealed in Experiment 1 that an extended number of continued shocks was necessary to arouse the late-appearing opioid analgesia. Neither remaining in the stress situation following an initial series of shocks nor receiving fewer shocks spread across the entire session was sufficient to produce the opioid analgesia. In Experiment 2 it was found that only five 5-sec shocks were sufficient to produce the nonopiate analgesia. In Experiment 3 the dissipation of the nonopioid analgesia aroused by 5 shocks was examined. The analgesia decayed to control levels within 10 min of shock termination. However, if the shocks were continued, the analgesia was still well above control levels 20 min following initial arousal. A large dose of an opiate antagonist failed to attenuate this analgesia.