Mrgprd-Expressing Polymodal Nociceptive Neurons Innervate Most Known Classes of Substantia Gelatinosa Neurons

Abstract

TheMas-related G-protein-coupled receptor D(Mrgprd) marks a distinct subset of sensory neurons that transmit polymodal nociceptive information from the skin epidermis to the substantia gelatinosa (SG, lamina II) of the spinal cord. Moreover,Mrgprd-expressing (Mrgprd⁺) neurons are required for the full expression of mechanical but not thermal nociception. While such anatomical and functional specificity suggestsMrgprd⁺neurons might synapse with specific postsynaptic targets in the SG, precisely howMrgprd⁺neurons interface with spinal circuits is currently unknown. To study circuit connectivity, we genetically targeted the light-activated ion channel Channelrhodopsin-2-Venus (ChR2-Venus) to theMrgprdlocus. In these knock-in mice, ChR2-Venus was localized to nonpeptidergicMrgprd⁺neurons and axons, while peptidergic CGRP⁺neurons were not significantly labeled. DissociatedMrgprd⁺DRG neurons from mice expressing one or two copies of ChR2-Venus could be activatedin vitroas evidenced by light-evoked currents and action potentials. In addition, illumination ofMrgprd-ChR2-Venus⁺axon terminals in spinal cord slices evoked EPSCs in half of all SG neurons. Within this subset,Mrgprd⁺neurons were monosynaptically connected to most known classes of SG neurons, including radial, tonic central, transient central, vertical, and antenna cells. This cellular diversity ruled out the possibility thatMrgprd⁺neurons innervate a dedicated class of SG neuron. Our findings set broad constraints on the types of spinal neurons that process afferent input fromMrgprd⁺polymodal nociceptors.