Cross tolerance between morphine and the long-term analgesic reaction to inescapable shock.

Animals exposed to a variety of stressors display a temporary analgesic reaction. This short-term analgesia has been shown to be reversible by opiate antagonists and cross-tolerant with morphine following some stress conditions, but not following others. It has recently been shown that inescapable shock parameters which produce behavioral '"learned helplessness" effects also produce a short-term analgesic reaction, and that this reaction can be re-aroused by a brief exposure to shock 24 hours later. Further, both the immediate and long-term antinociceptive reaction which follow shocks of this type have been shown to be reversible by opiate antagonists. Here it is shown that the long-term analgesic reaction is completely cross tolerant with morphine. Implications of these results for opioid mediation of learned helplessness and opioid versus nonopioid mediation of stress-induced analgesia are discussed.