Objective: To assess whether N-methyl-D-aspartate (NMDA) receptor (NR) or oestrogen receptor (OR) expression plays a role in the differences that temporalis muscle afferent fibres are less sensitive to peripheral receptor activation than masseter muscle afferent fibres and do not exhibit sex-related differences in NMDA-evoked discharge.
Methods: Immunohistochemical techniques were used to examine the expression of NR1, 2A, and 2B subunits of the NMDA receptor in male and female rats and the co-expression of NR2B subunits with ORs in female rats by trigeminal ganglion neurons that innervate the temporalis muscle. In vivo electrophysiological recording methods were employed to assess the response of afferent fibres to injection of NMDA into the temporalis muscle in female rats.
Results: Approximately 20% of temporalis ganglion neurons expressed NR1, NR2A and NR2B subunits, respectively, and there was no sex-related difference in the expression of these subunits. In female rats, both ORα and ORß receptors were identified in the trigeminal ganglion by Western blot. ORs were found on the majority (~80%) of temporalis ganglion neurons that expressed NR2B subunits. A significant positive correlation between blood oestrogen concentration and NMDA-evoked afferent discharge was identified.
Conclusion: The absence of sex-related differences in NMDA receptor expression may account for the lack of sex-related differences in NMDA-evoked temporalis afferent discharge. The association of elevated oestrogen concentration with increased afferent response to NMDA and the co-expression of NRs and ORs in temporalis ganglion neurons suggest that sensory input from the temporalis muscle may be modulated by oestrogenic tone.

Keywords: nociception, estrogen, NMDA, temporomandibular disorders, sex differences