Estrogen modulation of macrophages: A potential mechanism underlying sex differences in orofacial pain

Cranio-orofacial pain disorders are 3-4x more common in women and include migraine, headache, trigeminal neuralgia, burning mouth syndrome, and temporomandibular joint disorder; all of which are exacerbated by stress to a greater degree in women. The presentation of cranio-orofacial pain changes across menarche, the menstrual cycle, and menopause, implicating a modulatory role of gonadal hormones. While progesterone and testosterone are protective against pain, 17-estradiol (E2) has a perplexing dichotomic effect. Our research is focused on the role of E2 on trigeminal nociceptors innervating the orofacial tissues. We find that E2 enhances the pronociceptive effect of the neurotransmitter serotonin (5HT) on trigeminal sensory neurons and that 5HT can be released from macrophages to contribute to pain. Our overarching hypothesis is that E2 modulates the neuroimmune communication between macrophages and trigeminal nociceptors to promote a heightened sex difference in orofacial pain. In a rat model of inflammatory orofacial pain, we report estrogen receptor alpha agonism increased 5HT-evoked pain behavior, which was decreased by estrogen receptor beta antagonism. Stress induced more pain behaviors in females, that correlated with reduced serotonergic neurotransmission and attenuated by targeting 5HT receptors. We next investigated whether E2 alters release of inflammatory mediators, including 5HT, from macrophage cell lines. E2 decreased 5HT release from macrophages via activity at ERβ, but not ER, and the coincident activity of E2 and 5HT increased the release of key proinflammatory mediators known to enhance pain. Lastly, we induced a unilateral anterior crossbite in rats and characterized the development of pain behaviors as a clinically relevant model of temporomandibular joint disorder. We report sex differences in the development of orofacial pain and that exposure to stress evokes greater widespread pain behaviors in females compared to males. Overall, we provide evidence of a pronociceptive effect of E2 on 5HT-evoked orofacial pain via ER receptors in the trigeminal sensory system, and that E2 can act coincident with 5HT at ERs on macrophages to release key proinflammatory mediators known to target neurons and contribute to pain. We provide evidence of a novel neuroimmune mechanism that may contribute to the greater prevalence of cranio-orofacial pain disorders in women.