Genetic engineering of mammalian somatosensory circuits

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Our group investigates pain mechanisms and we pioneered transgenic strategies to activate selectively and non-invasively primary C-fiber afferents mediating pain and somatosensation in mice (Daou 2013, 2016; Beaudry 2017; Sharif 2019). C-fibers are located in the dorsal root (DRG), trigeminal (TG), and nodose (vagal) ganglia and their peripheral terminals transduce noxious, itch, thermal or tactile stimuli. Their activation generates the relevant signals propagated to the CNS where the somatosensory percepts are produced. We propose to focus this research program on transgenic mouse models for the selective activation of C-afferents mediating light pleasant touch and neurogenic inflammation. Our two main objectives are: 1) To assess the dual role of C-tactile afferents in pleasant light touch and mechanical allodynia Increasing evidence from human studies as well as rodent preclinical experiments point to a dual role of C-tactile afferents: they mediate light innocuous touch in normal conditions and aversive mechanical allodynia in chronic pain conditions. We propose to functionally characterize this population of Nav1.8+ Th+ C low-threshold mechanoreceptors (C-LTMRs) in mice. We will use intersectional genetic strategies to target selectively these neurons for optogenetic stimulation and chemogenetic sensitization or silencing in order to interrogate their contribution to affective pleasant touch and painful tactile hypersensitivity. In normal as well as chronic pain conditions (CFA and SNI), we will document behavioral responses and thresholds to tactile stimuli before, during and after opto/chemogenetic interventions. Analgesic and affective (pleasant?) effects of interventions on C-LTMRs activity will be tested using place preference assays, while mechanical and thermal sensitivity assays will be performed. 2) To investigate the cellular basis of neurogenic inflammation Peripheral sensitization induces burst firing of C-fiber nociceptors. Pathological sensitization leads to neurogenic inflammation at the source of chronic pain and migraine. We have shown that Gq-coupled pathways linked to endogenous or heterologous metabotropic receptors recruit TRPC and Orai1 channels to induce peripheral sensitization (Alkhani 2014). We observed distinct behavioral responses evoked by optical vs. metabotropic stimulation of MrgprA3+ C-afferents that encode both pain and itch (Sharif 2019). We make the hypothesis that the mode of activation of C-fibers determines their contribution to neurogenic inflammation, characterized by the release of the vasoactive neuropeptide CGRP and the infiltration of immune cells. We will compare in vitro and in vivo inflammatory responses induced by metabotropic vs. optical stimulation of thermosensitive TRPV1+ and mechanosensitive MrgprD+ nociceptors. This work will allow us to explore the modal diversity of C-afferents by probing their contribution to touch, pain and inflammation in normal and diseased states.