Estradiol replacement modifies c-fos expression at the spinomedullary junction evoked by temporomandibular joint stimulation in ovariectomized female rats

The influence of estradiol (E2) treatment on temporomandibular joint (TMJ) nociceptive processing in the caudal trigeminal sensory brain stem complex was assessed in ovariectomized female rats by quantitative Fos-immunoreactivity (Fos-LI). After 2 days of daily injections of high (HE2) or low (LE2) dose E2 rats were anesthetized and the small fiber excitant, mustard oil (MO, 0–20%), was injected into the TMJ and after 2 h brains were processed for Fos-LI. TMJ-evoked Fos-LI in laminae I–II at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) junction and the dorsal paratrigeminal region (dPa5) was significantly greater in HE2 than LE2 rats, while Fos-LI produced at the ventral trigeminal interpolaris/caudalis transition region (Vi/Vcvl) was similar. E2 treatment also modified the influence of N-methyl-d-aspartate (NMDA) and AMPA receptor antagonists on TMJ-evoked Fos-LI. The NMDA antagonist, MK-801, dose-dependently reduced the Fos-LI response at the Vc/C1-2 junction in HE2 rats, while only high dose MK-801 was effective in LE2 rats. MK801 reduced equally the Fos-LI response at the Vi/Vc transition in both groups, while only minor effects were seen at the dPa5 region. The AMPA receptor antagonist, NBQX, reduced Fos-LI at the Vc/C1-2 and Vi/Vcvl regions in HE2 rats, while only high dose NBQX was effective in LE2 rats. NBQX did not reduce Fos-LI at the dPa5 region in either group. These results suggest that estrogen status plays a significant role in TMJ nociceptive processing at the Vc/C1-2 junction mediated, in part, through ionotropic glutamate receptor-dependent mechanisms.