Identification of perineal sensory neurons activated by innocuous heat.

C-fiber sensory neurons comprise nociceptors and smaller populations of cells detecting innocuous thermal and light tactile stimuli. Markers identify subpopulations of these cells, aiding our understanding of their physiological roles. The transient receptor potential vanilloid 1 (TRPV1) cation channel is characteristic of polymodal C-fiber nociceptors and is sensitive to noxious heat, irritant vanilloids, and protons. By using immunohistochemistry, in situ hybridization, and retrograde tracing, we anatomically characterize a small subpopulation of C-fiber cells that express high levels of TRPV1 (HE TRPV1 cells). These cells do not express molecular markers normally associated with C-fiber nociceptors. Furthermore, they express a unique complement of neurotrophic factor receptors, namely, the trkC receptor for neurotrophin 3, as well as receptors for neurturin and glial cell line-derived neurotrophic factor. HE TRPV1 cells are distributed in sensory ganglia throughout the neuraxis, with higher numbers noted in the sixth lumbar ganglion. In this ganglion and others of the lumbar and sacral regions, 75% or more of such HE TRPV1 cells express estrogen receptor alpha, suggestive of their regulation by estrogen and a role in afferent sensation related to reproduction. Afferents from these cells provide innervation to the hairy skin of the perineal region and can be activated by thermal stimuli from 38 degrees C, with a maximal response at 42 degrees C, as indicated by induction of extracellular signal-regulated kinase phosphorylation. We hypothesize that apart from participating in normal thermal sensation relevant to thermoregulation and reproductive functions, HE TRPV1 cells may mediate burning pain in chronic pain syndromes with perineal localization.