Genetic identification of C fibres 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. Despite numerous physiological studies (reviewed in ref. 2), molecularly defined sensory neurons that detect pleasant stroking of hairy skin in vivo have not been reported. Previously, we identified a rare population of unmyelinated sensory neurons in mice that express the G-protein-coupled receptor MRGPRB4. These neurons exclusively innervate hairy skin with large terminal arborizations that resemble the receptive fields of C-tactile (CT) afferents in humans. Unlike other molecularly defined mechanosensory C-fibre subtypes, MRGPRB4^+ neurons could not be detectably activated by sensory stimulation of the skin ex vivo. Therefore, we developed a preparation for calcium imaging in the spinal projections of these neurons during stimulation of the periphery in intact mice. Here we show that MRGPRB4^+ neurons are activated by massage-like stroking of hairy skin, but not by noxious punctate mechanical stimulation. By contrast, a different population of C fibres expressing MRGPRD was activated by pinching but not by stroking, consistent with previous physiological and behavioural data. Pharmacogenetic activation of Mrgprb4-expressing neurons in freely behaving mice promoted conditioned place preference, indicating that such activation is positively reinforcing and/or anxiolytic. These data open the way to understanding the function of MRGPRB4 neurons during natural behaviours, and provide a general approach to the functional characterization of genetically identified subsets of somatosensory neurons in vivo.
Additional Information© 2013 Macmillan Publishers Limited. Received 17 June; accepted 20 November 2012. We thank R. Robertson for programming and imaging data analysis; M. Walsh and T. Heitzman for the stimulus delivery system and associated electronics; M. Zelikowsky for help with statistical analysis of behavioural data; H. Inagaki for experimental advice and comments on the manuscript; S. Pease for help with generation of knock-in mice; N. Verduzco, K. Lee and R. Sauza for mice colony maintenance; M. Visel and J. Flannery for training in AAV8 preparation; A. Anderson and C. Pagan for initial experiments using their stereotaxic apparatus; J. Zhang and A. Basbaum for teaching the dorsal laminectomy and for comments on the manuscript; D. Davalos, K. Akassoglou and H. Johanssen for help with the in vivo imaging preparation; L. Lagnado and B.Odermatt for SypHyand SyGCamp2plasmids; C. Shea and M. Martinez for technical assistance; H. Oates-Barker for laboratory management and G. Mancuso for administrative assistance. This work was supported by NIH grants 5PO1NS-48499 and 5R01 NS023476, and by fellowships from EMBO and the Human Frontiers Science Program (S.V.) and the Helen Hay Whitney Foundation (A.M.W.). D.J.A. is an Investigator of the Howard Hughes Medical Institute. Author Contributions: S.V. carried out all imaging and behavioural experiments; A.M.W. helped to configure the two-photon imaging system and developed the light grid method; K.K.R. and H.R.K. carried out electrophysiological recordings in isolated skin-nerve preparations; D.J.A. participated in experimental design and data interpretation and wrote the manuscript together with S.V.
Accepted Version - nihms-423047.pdf
Supplemental Material - nature11810-s1.pdf