17β-Estradiol Mediates the Sex Difference in Capsaicin-Induced Nociception in Rats

We have previously shown that the male sex steroid testosterone inhibits slightly, but the female sex steroid 17 beta-estradiol (E-2) potentiates dramatically, the capsaicin receptor-mediated current in rat dorsal root ganglion (DRG) neurons. Here, we used pharmacological methods and the nociceptive behavioral test to determine whether there is a sex difference in capsaicin-induced acute pain in rats in vivo and what mechanism underlies this sex difference. Results revealed that intradermal injection of capsaicin induced a dose-dependent nocifensive response in males and females, with the dose required to produce a comparable level of nociception being approximately 3- to 4-fold higher in males than in females. In addition, females during the proestrus stage exhibited significantly greater capsaicin-induced nocifensive responses compared with the estrus stage. Moreover, the female's enhanced sensitivity to the capsaicin-induced nocifensive response was completely reversed by ovariectomy 6 weeks before capsaicin injection. It is noteworthy that intradermal coinjection of E-2 but not progesterone with capsaicin potentiated the capsaicin-induced nocifensive response in ovariectomized rats. Likewise, intradermal E-2 injection dose-dependently potentiated the capsaicin-induced nocifensive response in male rats. Furthermore, potentiation by E-2 of the capsaicin-induced nocifensive response in male rats was not significantly reduced by a selective protein kinase C (PKC) inhibitor or by a selective protein kinase A (PKA) inhibitor, indicating that neither PKC nor PKA was involved in the effect of E-2. These data demonstrate that E-2 mediates the female's enhanced sensitivity to capsaicin-induced acute pain, consistent with potentiation by E-2 of the capsaicin receptor-mediated current in rat DRG neurons.