RECEPTIVE PROPERTIES OF MOUSE SENSORY NEURONS INNERVATING HAIRY SKIN

Using an in vitro nerve skin preparation and controlled mechanical or thermal stimuli, we analyzed the receptive properties of 277 mechanose nsitive single primary afferents with myelinated (n = 251) or unmyelin ated (n = 26) axons innervating the hairy skin in adult or 2-wk-old mi ce. Afferents were recorded from small filaments of either sural or sa phenous nerves in an outbred mice strain or in the inbred Balb/c strai n. On the basis of their receptive properties and conduction velocity, several receptor types could be distinguished. In adult animals (>6 w k old), 54% of the large myelinated fibers (A beta, n = 83) showed rap idly adapting (RA) discharges a to constant force stimuli and probably innervated hair follicles, whereas 46% displayed a slowly adapting (S A) response and probably innervated Merkel cells in touch domes. Among thin myelinated fibers (A delta, n = 91), 34% were sensitive D hair r eceptors and 66% were high-threshold mechanoreceptors (AM fibers). Unm yelinated fibers had high mechanical thresholds and nociceptive functi ons. All receptor types had characteristic stimulus-response functions to suprathreshold force stimuli. Noxious heat stimuli (15-s ramp from 32 to 47 degrees C measured at the cerium side of the skin) excited 2 6% (5 of 19) of AM fibers with a threshold of 42.5 +/- 1.4 degrees C ( mean +/- SE) and an average discharge of 15.8 +/- 9.7 action potential s and 41% (7 of 17) C fibers with a mean threshold of 37.6 +/- 1.9 deg rees C and an average discharge of 22.0 +/- 6.0 action potentials. Nox ious cold stimuli activated 1 of 10 AM fibers and 3 of 10 C fibers. On e of 10 C units responded to both heat and cold stimuli. All types of afferent fibers present in adult mice could readily be recognized in m ice at postnatal day 14. However, fibers had reduced conduction veloci ties and the stimulus-response function to mechanical stimuli was more shallow in all fibers except for the D hairs. In juvenile mice, 22% o f RA units also displayed an SA response at high stimulus intensities; these units were termed RA/SA units. We conclude that all types of cu taneous afferent fibers are already committed to their phenotype 2 wk after birth but undergo some maturation over the following weeks. This preparation has great potential for the study of transgenic mice with targeted mutations of genes that code factors that are involved in th e specification of sensory neuron phenotypes.