Genetic identification of C-fibers that detect massage-like stroking of hairy skin in vivo

Stroking of the skin produces pleasant sensations that can occur during social interactions with conspecifics, such as grooming(1). Despite numerous physiological studies (reviewed in ref. (2)), molecularly defined sensory neurons that detect pleasant stroking of hairy skin(3,4) in vivo have not been reported. Previously, we identified a rare population of unmyelinated sensory neurons that express the G protein-coupled receptor (GPCR) MrgprB4(5,6). These neurons exclusively innervate hairy skin with large terminal arborizations(7) that resemble the receptive fields of C-tactile (CT) afferents in humans(8). Unlike other molecularly defined mechanosensory C-fiber subtypes(9,10), MrgprB4(+) neurons could not be detectably activated by sensory stimulation of the skin ex vivo. Therefore, we developed a preparation for calcium imaging in their spinal projections during stimulation of the periphery in intact animals. MrgprB4(+) neurons were activated by massage-like stroking of hairy skin, but not by noxious punctate mechanical stimulation. By contrast, a different population of C-fibers expressing MrgprD(11) was activated by pinching but not by stroking, consistent with previous physiological and behavioral data(10,12). Pharmacogenetic activation of MrgprB4- expressing neurons in freely behaving animals promoted conditioned place preference(13), suggesting that such activation is positively reinforcing and/or anxiolytic. These data open the way to understanding the function of MrgprB4 neurons during natural behaviors, and provide a general approach to functionally characterizing genetically identified subsets of somatosensory neurons in vivo.