Properties of Nav1.8ChR2-positive and Nav1.8ChR2-negative afferent mechanoreceptors in the hindpaw glabrous skin of mice

Nav1.8-positive afferent fibers are mostly nociceptors playing a role in mediating thermal and mechanical pain, but mechanoreceptors within these afferents have not been fully investigated. In this study, we generated mice expressing channel rhodopsin 2 (ChR2) in Nav1.8-positive afferents (Nav1.8(ChR2)), which showed avoidance responses to mechanical stimulation and nocifensive responses to blue light stimulation applied to hindpaws. Using ex vivo hindpaw skin-tibial nerve preparations made from these mice, we characterized properties of mechanoreceptors on Nav1.8(ChR2)-positive and Nav1.8(ChR2)-negative afferent fibers that innervate the hindpaw glabrous skin. Of all A beta-fiber mechanoreceptors, small portion was Nav1.8(ChR2)-positive. Of all A delta-fiber mechanoreceptors, more than half was Nav1.8(ChR2)-positive. Of all C-fiber mechanoreceptors, almost all were Nav1.8(ChR2)-positive. Most Nav1.8(ChR2)-positive A beta-, A delta-, and C-fiber mechanoreceptors displayed slowly adapting (SA) impulses in response to sustained mechanical stimulation, and their mechanical thresholds were high in the range of high threshold mechanoreceptors (HTMRs). In contrast, sustained mechanical stimulation applied to Nav1.8(ChR2)-negative A beta- and A delta-fiber mechanoreceptors evoked both SA and rapidly adapting (RA) impulses, and their mechanical thresholds were in the range of low threshold mechanoreceptors (LTMRs). Our results provide direct evidence that in the mouse glabrous skin, most Nav1.8(ChR2)-negative A beta-, A delta-fiber mechanoreceptors are LTMRs involving in the sense of touch, whereas Nav1.8(ChR2)-positive A beta-, A delta-, and C-fiber mechanoreceptors are mainly HTMRs involving in mechanical pain.