Spatiotemporal Dynamics of Re-Innervation and Hyperinnervation Patterns by Uninjured CGRP Fibers in the Rat Foot Sole Epidermis after Nerve Injury

Abstract

The epidermis is innervated by fine nerve endings that are important in mediating nociceptive stimuli. However, their precise role in neuropathic pain is still controversial. Here, we have studied the role of epidermal peptidergic nociceptive fibers that are located adjacent to injured fibers in a rat model of neuropathic pain. Using the Spared Nerve Injury (SNI) model, which involves complete transections of the tibial and common peroneal nerve while sparing the sural and saphenous branches, mechanical hypersensitivity was induced of the uninjured lateral (sural) and medial (saphenous) area of the foot sole. At different time points, a complete foot sole biopsy was taken from the injured paw and processed for Calcitonin Gene-Related Peptide (CGRP) immunohistochemistry. Subsequently, a novel 2D-reconstruction model depicting the density of CGRP fibers was made to evaluate the course of denervation and re-innervation by uninjured CGRP fibers. The results show an increased density of uninjured CGRP-IR epidermal fibers on the lateral and medial side after a SNI procedure at 5 and 10 weeks. Furthermore, although in control animals the density of epidermal CGRP-IR fibers in the footpads was lower compared to the surrounding skin of the foot, 10 weeks after the SNI procedure, the initially denervated footpads displayed a hyper-innervation. These data support the idea that uninjured fibers may play a considerable role in development and maintenance of neuropathic pain and that it is important to take larger biopsies to test the relationship between innervation of injured and uninjured nerve areas.

Inflammaging impairs peripheral nerve maintenance and regeneration

“…Following sciatic nerve crush injury, these showed a significant delay in recovery of sensory functions, indicated by the Semmes–Weinstein monofilament test (Figure 1b). Most sensory recovery may have arisen from collateral sprouting, as the saphenous nerve remained uninjured and might have hyperinnervated the paw region, leading to the observed hypersensitivity (Duraku et al, 2012). We investigated recovery of motoric functions—measuring the footbase angle of mice in single‐frame motion analysis (SFMA) (Figure 1c) as a highly reproducible marker for functional muscle reinnervation (Fey, Schachner, & Irintchev, 2010).…”

The CGRP receptor antagonist BIBN4096BS peripherally alleviates inflammatory pain in rats

“…This is confirmed by the low levels of the compound in the plasma circulation-values below the concentration needed to block the rat CGRP1 receptor [11]. There is evidence that CGRP is present and is mediating nociceptive stimuli in the skin [31,44,56] and the sole of the foot (glabrous skin) of rats [12]. Intraplantar injection of CGRP can induce hypersensitivity in mice [50], and CGRP antagonists can block capsaicin-induced hypersensitivity when applied to the skin [48].…”

Long-term follow-up of peptidergic and nonpeptidergic reinnervation of the epidermis following sciatic nerve reconstruction in rats

“…8 The sections were washed 5 times for 10 minutes in phosphate-buffered saline. Thereafter, the sections were pretreated with 30% hydrogen peroxide solution at room temperature for 10 minutes to ensure minimal background.…”

“…Duraku et al recently showed the importance of the distinction between subgroups of intra-epidermal nerve fibers (IENFs) by extensive collateral sprouting of peptidergic fibers from uninjured nerves, compared with the nonpeptidergic fibers following a spared nerve injury lesion. 8,9 However, regeneration of the different subgroups of IENFs has not yet been described following end-to-end reconstruction, which provides a better outcome compared with nerve graft repair. 45 It is also unclear if these fibers play a role in the pathogenesis of neuropathic pain similar to the spared nerve injury model.…”