Role of adrenal glucocorticoids in the blockade of the development of analgesic tolerance to morphine by footshock stress exposure in mice.

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Abstract
To elucidate the mechanism for the suppression by concurrent footshock (FS) exposure of the development of morphine tolerance, the effect of adrenalectomy and a possible participation of glucocorticoids in the mechanism were examined. The analgesic effect of morphine was potentiated in adrenalectomized (ADX) mice, and further enhancement of the effect was shown by the simultaneous exposure to FS (2 mA, 0.2 Hz, 1 sec duration for 15 min) stress, while no such effects were observed in sham-operated (Sham) animals. Daily morphine treatment developed tolerance in Sham and ADX mice. The combined treatment with FS stress suppressed the development of morphine tolerance in Sham mice, whereas such suppression was abolished by adrenalectomy. The suppression of tolerance development was restored in ADX mice by supplement of prednisolone. In contrast to FS stress which produces analgesia through an opioid receptor, forced swimming stress which exerts analgesia through a non-opioid mechanism did not affect the development of morphine tolerance in both Sham and ADX mice. Thus, an opioid mediated stress, FS, could prevent the development of morphine tolerance, and adrenal glucocorticoids play an essential role in the mechanism.