Estradiol upregulates voltage-gated sodium channel 1.7 in trigeminal ganglion contributing to hyperalgesia of inflamed TMJ

Background Temporomandibular disorders (TMDs) have the highest prevalence in women of reproductive age. The role of estrogen in TMDs and especially in TMDs related pain is not fully elucidated. Voltage-gated sodium channel 1.7 (Nav1.7) plays a prominent role in pain perception and Nav1.7 in trigeminal ganglion (TG) is involved in the hyperalgesia of inflamed Temporomandibular joint (TMJ). Whether estrogen could upregulate trigeminal ganglionic Nav1.7 expression to enhance hyperalgesia of inflamed TMJ remains to be explored. Methods Estrous cycle and plasma levels of 17 beta estradiol in female rats were evaluated with vaginal smear and enzyme linked immunosorbent assay, respectively. Female rats were ovariectomized and treated with 17 beta-estradiol at 0 mu g, 20 mu g and 80 mu g, respectively, for 10 days. TMJ inflammation was induced using complete Freund's adjuvant. Head withdrawal thresholds and food intake were measured to evaluate the TMJ nociceptive responses. The expression of Nav1.7 in TG was examined using real-time PCR and western blot. The activity of Nav1.7 promoter was examined using luciferase reporter assay. The locations of estrogen receptors (ER alpha and ER beta), the G protein coupled estrogen receptor (GPR30), and Nav1.7 in TG were examined using immunohistofluorescence. Results Upregulation of Nav1.7 in TG and decrease in head withdrawal threshold were observed with the highest plasma 17 beta-estradiol in the proestrus of female rats. Ovariectomized rats treated with 80 mu g 17 beta-estradiol showed upregulation of Nav1.7 in TG and decrease in head withdrawal threshold as compared with that of the control or ovariectomized rats treated with 0 mu g or 20 mu g. Moreover, 17 beta-estradiol dose-dependently potentiated TMJ inflammation-induced upregulation of Nav1.7 in TG and also enhanced TMJ inflammationinduced decrease of head withdrawal threshold in ovariectomized rats. In addition, the estrogen receptor antagonist, ICI 182,780, partially blocked the 17 beta-estradiol effect on Nav1.7 expression and head withdrawal threshold in ovariectomized rats. ER alpha and ER beta, but not GPR30, were mostly co-localized with Nav1.7 in neurons in TG. In the nerve growth factor-induced and ERa-transfected PC12 cells, 17 beta-estradiol dose-dependently enhanced Nav1.7 promoter activity, whereas mutations of the estrogen response element at -1269/1282 and -1214/-1227 in the promoter completely abolished its effect on the promoter activity. Conclusion Estradiol could upregulate trigeminal ganglionic Nav1.7 expression to contribute to hyperalgesia of inflamed TMJ.