Influences of the female reproductive cycle on inflammatory induced pain responses

Clinical and preclinical studies have demonstrated significant sex differences in the perception of inflammatory pain; females display higher nociceptive responses to inflammatory stimuli than male rats. Additionally, the complex endocrinological profile of females has been shown to impact their nociceptive responses. For example, estradiol reduces Phase II behavioral-nociceptive responses after formalin administration. However, little is known about the specific biological pathway(s) and/or mechanisms in which cycling endogenous female sex hormones affect inflammatory pain responses. Current literature has established that cyclooxygenases and prostanoids are major pro-inflammatory mediators directly linked to inflammatory responses. Additionally, glucocorticoids, (i.e. corticosterone) negatively regulate inflammatory induced COX-2, resulting in attenuation of inflammatory responses. The objective of this study was to further understand how fluctuations of endogenous female sex hormones alter inflammatory-induced responses by examining two physiological pathways (i.e. NO/COX-2 regulation of the prostanoid biosynthetic pathway and corticosterone regulation of the NO/COX pathway) which may in part be responsible for these effects. Endogenous peaks of estrogen and progesterone during proestrus, were shown to significantly attenuate behavioral responses after formalin administration. This attenuation of behavioral responding was accompanied by a significant increase in PGD2 serum levels. Cortiscosterone serum levels were unaffected after formalin administration suggesting that regulation of behavioral responses by endogenous hormones may be occurring through a pathway independent of the corticosterone biosynthetic pathway. COX-2 and nNOS levels in the spinal cord were not significantly affected by the estrous cycle, suggesting that regulation of behavioral responses by endogenous hormonal fluctuations may be occurring through a pathway independent of the NO/COX biosynthetic pathway.;Furthermore, although no estrous cycle effects were seen in paw withdrawal latency after carrageenan administration, we observed estrous cycle effects in the contralateral paw at baseline and one hour post-injection. Rats in proestrus showed a significant reduction in thermal-induced hyperalgesia as measured by increased paw withdrawal latency. Although no significant differences were seen in PGD2 serum levels, rats in estrus had significantly higher PGE2 serum levels after carrageenan administration. A significant decrease in PWL was observed in rats during estrus, a time of the lowest levels of fluctuating hormones. These results suggest that hormonal troughs during the cycle may affect inflammation through the PG biosynthetic pathway. Finally, during ages when animals are considered "middle aged" attenuation in inflammatory induced behavior was observed. This finding was accompanied by significant decreases in PGE2 and PGD2 levels and a significant increase in corticosterone serum levels. Taken together these results suggest a relationship between endogenous hormonal fluctuations, corticosterone release and PG activity. In summary, our results suggest that endogenous hormonal peaks and troughs effects on inflammation may be mediated through the regulation of the NO/COX-2/prostanoid biosynthetic pathway.