Serotonin (5-hydroxytryptamine, 5HT) is a neurotransmitter and proinflammatory mediator found largely in the peripheral nervous system where it can initiate pain signaling. 5HT binds a variety of 5HT receptors on sensory nerve endings specialized in detecting noxious stimuli, termed nociceptors. A subset of sensory neurons involved in pain signaling express the transient receptor potential vanilloid 1 ion channel (TRPV1), a pain generator. 5HT can both directly activate sensory neurons and sensitize TRPV1 leading to enhanced nociceptor sensitivity (peripheral sensitization). Previous studies in male rats reported that the 5HT receptor subtype 3A (5HT3A) and TRPV1 are co-expressed on sensory neurons, but it is unknown if 5HT3A and TRPV1 are co-expressed on female sensory neurons. Given that craniofacial pain disorders occur at a 2-3x greater prevalence in women, examining pain mechanisms in female trigeminal sensory neurons that innervate the craniofacial region is critical to advancing craniofacial pain management in women. Here we examined whether (1) 5HT acting via the 5HT3A receptor produces sexually dimorphic orofacial pain behaviors in rats and (2) whether 5HT3A receptor mRNA is expressed in trigeminal sensory neurons, including the TRPV1-expressing subpopulation, and increase pain signaling. We report that 5HT evokes pain behaviors in male and female rats, which was not significantly reduced by antagonism of 5HT3A. We performed in situ hybridization to label 5HT3A and TRPV1 mRNA in trigeminal sensory neurons and found distinct cell populations with either 5HT3A mRNA or TRPV1 mRNA in males and females. Further, 5HT3A antagonism failed to reduce pain signaling in cultured trigeminal sensory neurons. These data suggest that the 5HT3A subtype on trigeminal sensory neurons innervating the orofacial soft tissues does not play a significant role in sexually dimorphic craniofacial pain disorders. As previous studies have reported that granisetron reduces masseter muscle pain, 5HT3 may play a role in sex differences in myofascial pain disorders but not in other craniofacial pain disorders.
Keywords: 5HT3A receptor; RNAScope©; TRPV1 ion channel; pain; serotonin.