Electrophysiological evidence for the existence of a rare population of C-fiber low threshold mechanoreceptive (C-LTM) neurons in glabrous skin of the rat hindpaw

• C-LTMs (C-mechanoreceptors) are thought to exist only in the hairy skin.
• Direct evidence that C-LTMs exist in glabrous skin of rat hindpaw is provided.
• They constituted >6% of the C-fiber neurons with receptive fields in glabrous skin.
• Their mechanical and electrical thresholds are much lower than those of C-nociceptors.
• They have faster action potential kinetics than C-nociceptors.

The mammalian skin in innervated by distinct classes of low-threshold mechanoreceptive (LTM) primary afferent neurons that are classified as Aβ-, Aδ- or C-LTMs according to their axonal conduction velocities (CVs). C-LTMs are thought to signal pleasant and erotic touch sensations in humans, and to exist only in the hairy skin of primates and other species. Using intracellular recordings from rat L4/L5 dorsal root ganglion (DRG) neurons that were classified in vivo as C-nociceptors or C-LTMs, according to their dorsal root CVs and their responses to mechanical and thermal stimuli, the present study provides the first electrophysiological evidence that C-LTMs exist in the glabrous skin of the rat’s hindpaw. Indeed 6.4% (5/78) of the total sample of lumbar C-fiber DRG neurons with receptive fields in the glabrous skin of the rat hindpaw were C-LTMs. The electrophysiological properties of this rare subpopulation of C-fiber neurons (mean CV = 0.48 ± 0.06 m/s) are distinct from those of C-fiber high threshold mechanoreceptors (HTMs). Indeed, their mean mechanical (1.7 ± 1.1 mN) and electrical (4.0 ± 0.4 V) thresholds was significantly different from that of C-HTMs. They also exhibited faster action potential and afterhyperpolarization kinetics than C-HTMs. The present study lends support to previous studies that have provided indirect evidence for the presence of C-LTMs in glabrous skin. If C-LTMs are present in human glabrous skin, they may, in this type of skin, represent a novel peripheral neuronal substrate for the pleasant/social touch sensation, and account for or contribute to touch hypersensitivity after injury.