CALL FOR PAPERS Sex and Gender Differences in Pain and Inflammation Testosterone and estrogen have opposing actions on inflammation-induced plasma extravasation in the rat temporomandibular joint

Flake, Natasha M., Tracey O. Hermanstyne, and Michael S. Gold. Testosterone and estrogen have opposing actions on inflammation-induced plasma extravasation in the rat temporomandibular joint. Am J Physiol Regul Integr Comp Physiol 291: R343–R348, 2006. First published February 9, 2006; doi:10.1152/ajpregu.00835.2005.— The present study was designed to test the hypothesis that estrogen exacerbates inflammation of the temporomandibular joint (TMJ). Evans blue dye was used to quantify plasma extravasation (PE) around the rat TMJ. In an initial set of experiments, TMJ PE was compared in naı̈ve intact male and female rats, as well as in both groups after complete Freund’s adjuvant (CFA)-induced inflammation of the TMJ. In contrast to our hypothesis, TMJ PE was significantly greater in both naı̈ve and CFA-inflamed male rats than in females. To determine whether these differences were due to gonadal hormones, four additional groups of rats were studied: gonadectomized (Gx) males and females, Gx males with chronic testosterone (T) replacement, and Gx females with chronic estrogen (E) replacement. The sex difference in baseline TMJ PE appeared to reflect the actions of T. However, in the presence of TMJ inflammation, T augmented TMJ PE in males, while E attenuated TMJ PE in females. Changes in PE were also assessed in the contralateral TMJ. Results from this analysis indicated that there is a transient contralateral increase in TMJ PE in females but not males. Given that there is an inverse relationship between PE and joint damage, our results suggest that testosterone may mitigate, but estrogen may exacerbate, TMJ damage, particularly in the presence of overt inflammation. Importantly, our results may help explain both the higher prevalence and severity of temporomandibular disorder pain in females than males.