Inflammation-induced mast cell-derived nerve growth factor: a key player in chronic vulvar pain?

Provoked vulvodynia (PV) is characterized by localized chronic vulvar pain. It is associated with a history of recurrent inflammation, mast cell (MC) accumulation and neuronal sprouting in the vulva. However, the mechanism of how vulvar-inflammation promotes neuronal sprouting and gene-expression adaptation in the spinal cord, leading to hypersensitivity and painful sensations, is unknown. Here, we found that vulvar tissue from women with PV (n = 8) is characterized by MC accumulation and neuronal sprouting compared to women without PV (n = 4). In addition, we observed these changes in an animal study of PV. Thus, we found that repeated vulvar zymosan-inflammation challenges lead to long-lasting mechanical and thermal vulvar hypersensitivity, which is mediated by MC accumulation, neuronal sprouting, overexpression of the pain channels (TRPV1 and TRPA1) in vulvar neurons, as well as a long-term increase of gene expression related to neuroplasticity, neuroinflammation and nerve growth factor (NGF) in the spinal cord/dorsal root ganglia (DRG) (L6-S3). However, regulation of the NGF pathway by stabilization of MC activity with ketotifen fumarate (KF) during vulvar inflammation attenuates the local increase of NGF and histamine, as well as the elevated transcription of pro-inflammatory cytokines and NGF pathway in the spinal cord. Additionally, KF treatment during inflammation modulates MC accumulation, neuronal hyperinnervation and overexpression of the TRPV1 and TRPA1 channels in the vulvar neurons, consequently preventing the development of vulvar pain. A thorough examination of the NGF pathway during inflammation revealed that blocking NGF activity by using an NGF-non-peptide-inhibitor (Ro08-2750) regulates the upregulation of genes related to neuroplasticity and the NGF pathway in the spinal cord, as well as modulating neuronal sprouting and overexpression of the pain channels, resulting in a reduced level of vulvar hypersensitivity. On the other hand, stimulation of the NGF pathway in the vulvar promotes neuronal sprouting, overexpression of pain channels and increase of gene expression related to neuroplasticity, neuroinflammation and NGF in the spinal cord, resulting in long-lasting vulvar hypersensitivity. In conclusion, our findings suggest that vulvar allodynia induced by inflammation is mediated by MC accumulation, neuronal sprouting and neuromodulation in the vulvar. Additionally, chronic vulvar pain may involve a long-term adaptation in gene expression in the spinal cord, which probably plays a critical role in central sensitization and pain maintenance. Strikingly, regulating the NGF pathway during the critical period of inflammation prevents vulvar pain development via modulating the neuronal changes in the vestibule and spinal cord, suggesting a fundamental role for the NGF pathway in PV development.