Pain mechanismSex-related differences in NMDA-evoked rat masseter muscle afferent discharge result from estrogen-mediated modulation of peripheral NMDA receptor activity

In the present study, the hypothesis that sex-related differences in glutamate-evoked rat masseter muscle afferent discharge may result from estrogen-related modulation of peripheral N-methyl-d-aspartate (NMDA) receptor activity and/or expression was tested by examining afferent fiber discharge in response to masseter injection of NMDA and the expression of NR2A/B subunits by masseter ganglion neurons in male and female rats. The results showed that injection of NMDA into the masseter muscle evoked discharges in putative mechanonociceptive afferent fibers and increased blood pressure that was concentration-dependent, however, a systemic action of NMDA appeared responsible for increased blood pressure. NMDA-evoked afferent discharge was significantly greater in female than in male rats, was positively correlated with plasma estrogen levels in females and was significantly greater in ovariectomized female rats treated with a high dose (5 μg/day) compared with a low dose (0.5 μg/day) of estrogen. Pre-treatment of high dose estrogen-treated-ovariectomized female rats with the Src tyrosine kinase inhibitor PP2 did not affect NMDA-evoked afferent discharge. NMDA-evoked afferent discharge was attenuated by the antagonists ketamine and ifenprodil, which is selective for NR2B containing NMDA receptors. Fewer masseter ganglion neurons expressed the NR2A (16%) subunit as compared with the NR2B subunit (38%), which was expressed at higher frequencies in intact female (46%) and high dose estrogen-treated ovariectomized female (60%) rats than in male (31%) rats. Taken together, these results suggest that sex-related differences in NMDA-evoked masseter afferent discharge are due, at least in part, to an estrogen-mediated increase in expression of peripheral NMDA receptors by masseter ganglion neurons in female rats.