Differential modulation of TMJ neurons in superficial laminae of trigeminal subnucleus caudalis/upper cervical cord junction region of male and cycling female rats by morphine

Sex differences in the cellular responses to morphine were examined in an animal model of temporomandibular joint (TMJ) pain. TMJ-responsive neurons were recorded in the superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C₂) junction region, the initial site of synaptic integration for TMJ afferents, in male and cycling female rats under barbiturate anesthesia. Unit activity was evoked by local injection of bradykinin into the TMJ capsule at 30 min intervals and the effects of morphine sulfate (0.03-3 mg/kg, i.v.) were assessed by a cumulative dose regimen. Morphine caused a dose-related inhibition of bradykinin-evoked unit activity in males and diestrous females in a naloxone-reversible manner, while evoked unit activity in proestrous females was not reduced. The apparent sex hormone-related aspect of morphine analgesia was selective for evoked unit activity, since the spontaneous activity of TMJ units was reduced similarly in all groups, while the convergent cutaneous receptive field area of TMJ units did not change in any group. These results were consistent with the hypothesis that sex hormone status interacts with pain control systems to modify neural activity at the level of the Vc/C₂ junction region relevant for TMD pain.