Gonadal hormones do not account for sexual dimorphism in vagal modulation of nociception in the rat.

Subdiaphragmatic vagotomy produces a decrease in mechanical nociceptive threshold that is greater in male rats and an enhancement of bradykinin hyperalgesia that is greater in female rats. To examine the role of gonadal hormones in these sex differences, we evaluated the effect of gonadectomy, with or without gonadal hormone replacement, on vagal modulation of nociceptive threshold and bradykinin hyperalgesia by using the Randall-Selitto paw withdrawal test. Gonadectomy (before sexual maturation) plus vagotomy decreased nociceptive threshold in male rats more than either lesion alone, whereas neither lesion nor in combination had an effect on nociceptive threshold in female rats. Testosterone or dihydrotestosterone replacement in gonadectomized plus vagotomized males and 17 beta-estradiol in females did not significantly alter nociceptive threshold compared to vagotomy plus gonadectomy, respectively. Combined vagotomy and gonadectomy unexpectedly almost completely abolished bradykinin hyperalgesia, whereas gonadectomy alone had no effect on bradykinin hyperalgesia in both sexes. Testosterone replacement in vagotomized males and 17 beta-estradiol in vagotomized females reversed the effect of gonadectomy. Dihydrotestosterone replacement in vagotomized males also reversed the effect of gonadectomy on bradykinin hyperalgesia, although to a lesser degree than testosterone. We conclude that although gonadal hormones and other gonadal-dependent mechanisms influence nociception, they do not account for sexual dimorphism in vagal modulation of mechanical nociceptive threshold or bradykinin hyperalgesia.