LAUSR

Female reproductive tract pain encompasses a variety of common and debilitating conditions such as vulvodynia, vaginal hyperalgesia secondary to endometriosis, pudendal neuralgia, and many others. These conditions lack effective pharmacotherapies due to a poor understanding of the mechanisms underlying sensory transmission from female reproductive organs. Studies investigating bladder and colon nociception have shown that different voltage-gated sodium (NaV) channels play a critical and complex role in pain transmission, suggesting selective sodium channel blockade may be a more potent therapeutic strategy for pelvic pain. However, no study has specifically investigated the role of NaV channels in pain signaling from the reproductive tract. In the present issue of PAIN, Castro et al. address this gap in knowledge. Equipped with a novel ex vivo vaginal preparation, the authors investigated the mechanosensory properties and receptive fields of pelvic nerve afferents innervating the mouse vagina (Figs. 1A and B). Using this approach, these afferents were found to all be polymodal, with punctate receptive fields distributed along the entire length of the vagina. Interestingly, these mechanosensory responses were globally augmented through the pan-NaV channel activator veratridine but only partly attenuated by the partially selective NaV channel blocker tetrodotoxin (TTX), suggesting that TTX-resistant NaV channels contribute significantly to afferent transmission from the vagina. The authors complement these results by performing single-cell polymerase chain reaction (PCR) on dorsal root ganglion (DRG) neurons retrogradely traced from the vagina and found abundant expression of NaV channels, with distinctive expression patterns across each subtype (Fig. 1C). Finally, the authors measured vaginal pain sensitivity by performing vaginal balloon distention and measuring visceromotor responses in live mice. They found that TTX attenuated and veratridine enhanced pain responses in vivo and similarly altered neuronal activity in the spinal cord in vitro (Fig. 1D). An important premise of the authors’ experimental design is in the differential effects of TTX on NaV channels. There are no selective pharmacological modulators of NaV channels; however, NaV channels can be distinguished by their sensitivity to TTX, with both TTX-resistant (TTX-R; NaV1.5, NaV1.8, and NaV1.9) and TTX-sensitive (TTX-S; NaV1.1-1.4, NaV1.6, and NaV1.7) subtypes. The authors showed that TTX partially attenuated vaginal afferent excitability and concluded that TTX-R NaV channels contribute appreciably to afferent transmission from the mouse vagina. Although this pharmacological manipulation does not within itself identify specific NaV channel subtypes integral to vaginal pain transmission, the combination of these experiments with PCR allows for the identification of a few candidates, namely the abundantly expressed TTX-RNaV1.8 and NaV1.9 and TTX-S NaV1.1, NaV1.2, NaV1.6, and NaV1.7 (Fig. 1C). By contrast, bladder afferent excitability is almost completely dependent on TTX-S NaV channels 8 and colon nociception is largely independent of NaV1.7. 9 These findings highlight the vagina’s unique neurophysiology in comparison with other pelvic organs and point to potential targets which may be specific to the treatment of vaginal pain. The major technical advance presented in this article is the novel ex vivo single-unit extracellular vaginal afferent recording preparation (Fig. 1B). With their flat sheet approach, they identified mechanosensory properties of the mouse vagina, which agree with those identified by the in vivo rat studies by Berkley’s group. Furthermore, although this study solely involved pelvic nerve recordings, this preparation can also be used to record from the pudendal nerve, allowing for the study of NaV channels in vulvar and perineal pain. Considering the novel genetic, physiological, and behavioral findings in this study, it is likely that vaginal afferents make up a genetically distinct population or populations. Single-cell transcriptomics have revealed the genetic diversity of sensory neurons arising from a variety of sites. Unfortunately, most of these studies either do not treat spinal cord level as a variable or exclude the sacral DRG entirely, where a proportion of afferents from the vulva and vagina originate. New transcriptomic evidence shows that a collection of genes within nociceptors are differentially expressed in sacral DRG compared with lumbar DRG; however, no study has applied high-throughput gene expression approaches to the study of sensory neurons arising from the female reproductive tract. By performing single-cell PCR on DRG neurons retrogradely traced from the mouse vagina, the authors took a critical first step in the genetic phenotyping of vaginal sensory neurons. They characterize a highly overlapping expression profile of all NaV channel subtypes within vaginainnervating neurons and observed significant heterogeneity between the cells they sampled. Whether or not NaV channels themselves prove to be viable targets in vaginal pain, the NaV channel expression patterns identified in this formative study could potentially be correlated with expression of other cell type–specific genes. Single-cell RNA sequencing analyses from neurons retrogradely traced from the vagina, as has been similarly Sponsorships or competing interests that may be relevant to content are disclosed at the end of this article.