Response properties of mechanoreceptors and nociceptors in mouse glabrous skin: An in vivo study

The increasing use of transgenic mice for the study of pain mechanisms necessitates comprehensive understanding of the murine somatosensory system. Using an in vivo mouse preparation, we studied response properties of tibial nerve afferent fibers innervating glabrous skin. Recordings were obtained from 225 fibers identified by mechanical stimulation of the skin. Of these, 106 were classed as Aβ mechanoreceptors, 51 as Aδ fibers, and 68 as C fibers. Aβ mechanoreceptors had a mean conduction velocity of 22.2 ± 0.7 (SE) m/s (13.8-40.0 m/s) and a median mechanical threshold of 2.1 mN (0.4-56.6 mN) and were subclassed as rapidly adapting (RA, n = 75) or slowly adapting (SA, n = 31) based on responses to constant force mechanical stimuli. Conduction velocities ranged from 1.4 to 13.6 m/s (mean 7.1 ± 0.6 m/s) for Aδ fibers and 0.21 to 1.3 m/s (0.7 ± 0.1 m/s) for C fibers. Median mechanical thresholds were 10.4 and 24.4 mN for Aδ and C fibers, respectively. Responses of Aδ and C fibers evoked by heat (35-51°C) and by cold (28 to -12°C) stimuli were determined. Mean response thresholds of Aδ fibers were 42.0 ± 3.1°C for heat and 7.6 ± 3.8°C for cold, whereas mean response thresholds of C fibers were 40.3 ± 0.4°C for heat and 10.1 ± 1.9°C for cold. Responses evoked by heat and cold stimuli increased monotonically with stimulus intensity. Although only 12% of tested Aδ fibers were heat sensitive, 50% responded to cold. Only one Aδ nociceptor responded to both heat and cold stimuli. In addition, 40% of Aδ fibers were only mechanosensitive since they responded neither to heat nor to cold stimuli. Thermal stimuli evoked responses from the majority of C fibers: 82% were heat sensitive, while 77% of C fibers were excited by cold, and 68% were excited by both heat and cold stimuli. Only 11% of C fibers were insensitive to heat and/or cold. This in vivo study provides an analysis of mouse primary afferent fibers innervating glabrous skin including new information on encoding of noxious thermal stimuli within the peripheral somatosensory system of the mouse. These results will be useful for future comparative studies with transgenic mice.