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Central morphine analgesia is significantly greater in male than in female rats. Since mu and delta opioid receptor subtypes have been implicated in supraspinal analgesia, the present study evaluated whether gender or adult gonadectomy altered (a) analgesia on the tail-flick and jump tests following central administration of the mu-selective agonist, [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAMGO) and the delta-selective agonist, [D-Ser2,Leu5] enkephalin-Thr6 (DSLET) and (b) mu1, mu2 and delta opioid receptor binding. Sham-operated male rats displayed significantly greater magnitudes of peak and total analgesia than sham-operated females on the tail-flick test following DAMGO, but not DSLET. Gender differences were not observed for DAMGO and DSLET analgesia on the jump test. Gonadectomy failed to consistently affect either DAMGO or DSLET analgesia. Regression analyses failed to reflect significant shifts in the dose-response functions for either agonist on either measure. Gender differences were not observed for mu1, mu2, or delta binding in hypothalamus or cortex. These data are compared with analgesic responses sensitive to gender differences.

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