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 nece ssitates comprehensive understanding of the murine somatosensory system. Us ing an in vivo mouse preparation, we studied response properties of tibial nerve afferent fibers innervating glabrous skin. Recordings were obtained f rom 225 fibers identified by mechanical stimulation of the skin. Of these, 106 were classed as A beta mechanoreceptors, 51 as A delta fibers, and 68 a s C fibers. A beta 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 m N (0.4-56.6 mN) and were subclassed as rapidly adapting (RA, n = 75) or slo wly adapting (SA, n = 31) based on responses to constant force mechanical s timuli. Conduction velocities ranged from 1.4 to 13.6 m/s (mean 7.1 +/- 0.6 m/s) for A delta 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 A delta and C fibe rs, respectively. Responses of A delta and C fibers evoked by heat (35-51 d egreesC) and by cold (28 to -12 degreesC) stimuli were determined. Mean res ponse thresholds of A delta fibers were 42.0 +/- 3.1 degreesC for heat and 7.6 +/- 3.8 degreesC for cold, whereas mean response thresholds of C fibers were 40.3 +/- 0.4 degreesC for heat and 10.1 +/- 1.9 degreesC for cold. Re sponses evoked by heat and cold stimuli increased monotonically with stimul us intensity. Although only 12% of tested A delta fibers were heat sensitiv e, 50% responded to cold. Only one A delta nociceptor responded to both hea t and cold stimuli. In addition, 40% of A delta fibers were only mechanosen sitive since they responded neither to heat nor to cold stimuli. Thermal st imuli evoked responses from the majority of C fibers: 82% were heat sensiti ve, while 77% of C fibers were excited by cold, and 68% were excited by bot h 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 n oxious thermal stimuli within the peripheral somatosensory system of the mo use. These results will be useful for future comparative studies with trans genic mice.