Central anatomy of individual rapidly adapting low‐threshold mechanoreceptors innervating the “hairy” skin of newborn mice: Early maturation of hair follicle afferents

Abstract

Adult skin sensory neurons exhibit characteristic projection patterns in the dorsal horn of the spinal gray matter that are tightly correlated with modality. However, little is known about how these patterns come about during the ontogeny of the distinct subclasses of skin sensory neurons. To this end, we have developed an intact ex vivo somatosensory system preparation in neonatal mice, allowing single, physiologically identified cutaneous afferents to be iontophoretically injected with Neurobiotin for subsequent histological analyses. The present report, centered on rapidly adapting mechanoreceptors, represents the first study of the central projections of identified skin sensory neurons in neonatal animals. Cutaneous afferents exhibiting rapidly adapting responses to sustained natural stimuli were encountered as early as recordings were made. Well-stained representatives of coarse (tylotrich and guard) and fine-diameter (down) hair follicle afferents, along with a putative Pacinian corpuscle afferent, were recovered from 2-7-day-old neonates. All were characterized by narrow, uninflected somal action potentials and generally low mechanical thresholds, and many could be activated via deflection of recently erupted hairs. The central collaterals of hair follicle afferents formed recurrent, flame-shaped arbors that were essentially miniaturized replicas of their adult counterparts, with identical laminar terminations. The terminal arbors of down hair afferents, previously undescribed in rodents, were distinct and consistently occupied a more superficial position than tylotrich and guard hair afferents. Nevertheless, the former extended no higher than the middle of the incipient substantia gelatinosa, leaving a clear gap more dorsally. In all major respects, therefore, hair follicle afferents display the same laminar specificity in neonates as they do in adults. The widely held misperception that their collaterals extend exuberant projections into pain-specific regions of the dorsal horn during early postnatal life is shown to have multiple, deep-rooted underpinnings.

Myelinated Skin Sensory Neurons Project Extensively throughout Adult Mouse Substantia Gelatinosa

“…In accord with seminal in vivo studies in guinea pig (Sugiura et al, 1986) and in vitro studies in mice (Woodbury et al, 2001(Woodbury et al, , 2004Albers et al, 2006), all labeled C-nociceptors terminated in the SG and, to a lesser extent, marginal zone (lamina I). Most (12/13) were C-polymodal nociceptors that responded to moderately firm stroking of the RF with a fine paintbrush (mechanical threshold, 0.63 Ϯ 0.29 mN; range, 0.16 -1.24 mN) as well as heat (threshold, 42.2 Ϯ 3.2°C; range, 38 -46°C) and cold (threshold, 16.3 Ϯ 4.5°C; range, 10 -21°C).…”

“…Arbors gave rise to a longitudinal column of dense terminal neuropil that was well circumscribed in the transverse plane and extended uninterrupted over many sections (400 -550 m) in the rostrocaudal plane. As seen in Figure 1, terminal arbors were centered in the outer SG (i.e., lamina IIo), with only minimal overlap into lamina I; occasional processes extended ventrally into laminas IIi and III as noted previously (Sugiura et al, 1986;Sugiura, 1996;Woodbury et al, 2001). …”

“…Less widely appreciated, however, is the fact that the SG also receives direct inputs from myelinated skin sensory neurons. Indeed, down hair afferents have been shown to project into the ventral SG , and, using a semi-intact ex vivo preparation from neonatal mice, this adult organization was found to develop shortly after birth (Woodbury et al, 2001). Additional studies using this in vitro preparation have also shown that myelinated nociceptors project extensively throughout the SG in neonates (Woodbury and Koerber, 2003).…”

“…If true, this would predict that the growth of myelinated nociceptor central projections is initially exuberant and these arbors subsequently undergo extensive pruning during early postnatal life or, alternatively, that the cells responsible for these widespread SG projections represent a transient population that is lost through cell death postnatally. However, uncertainty surrounds the identities of afferents that transport cholera toxin (Tong et al, 1999;Woodbury et al, 2001;Sántha and Jancsó , 2003), and bulklabeling studies may lack sufficient power to resolve this important question regarding SG functional organization (Woodbury et al, 2000(Woodbury et al, , 2001Woodbury and Koerber, 2003).…”

Whisker plucking alters responses of rat trigeminal ganglion neurons

“…Whisker plucking also likely preserves the physiologic identity of nerve endings and their parent cell bodies as the proportions of SA and RA neurons in the NVg population are equivalent between the plucked and the control groups. The SA vs the RA nature of neuronal response to sustained stimuli is determined early in life by genetic and/or molecular factors intrinsic to the cell (Woodbury et al, 2001;Woodbury and Koerber, 2003). Lastly, plucking-induced alterations in responses of NVg neurons are probably not due to long-lasting changes in the myelination status of primary afferent axons as response latencies also remain unaffected by plucking (see also Li et al, 1995).…”