Cross Tolerance Between Morphine and the Long-Term Analgesic Reaction to Inescapable Shock

ROBERT C. DRUGAN, JAMES W. GRAU, AND STEVEN F. MAIER Department of Psychology, University of Colorado, Boulder Co

AND JOHN MADDEN IV AND JACK D. BARCHAS Nancy Pritzker Laboratory of Behavioral Neurochemistry, Department of Psychiatry and Behavioral Sciences Stanford University School of Medicine, Stanford, CA 94305

Received 1 November 1980

DRUGAN, R. C., J. W. GRAU, S. F. MAIER, J. MADDEN, IV AND J. D. BARCHAS. Cross-tolerance between morphine and the long-term analgesic reaction to inescapable shock. PHARMAC. BIOCHEM. BEHAV. 14(5) 677-682, 1981. 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.

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