Differential modulation of voltage-gated sodium channels by nerve growth factor in three major subsets of TrkA-expressing nociceptors

Abstract

Nerve growth factor is an inflammatory mediator that induces long-lasting hyperalgesia, which can partially be attributed to nerve growth factor-induced sensitization of primary afferent nociceptors. It was shown that nerve growth factor increases the excitability of polymodal C-fibre nociceptors by modulating tetrodotoxin-sensitive and tetrodotoxin-resistant voltage-gated sodium channels, but hitherto only little is known about the effects of nerve growth factor on sodium currents in other nociceptor subtypes that express the nerve growth factor receptor TrkA. We previously characterized two reporter mouse lines that allow the unequivocal identification of two important subclasses of TrkA-expressing nociceptors – i.e. neuropeptide Y receptor type 2 (NPY2R+ ) Aδ-fibre nociceptors that mediate pinprick pain and nicotinic acetylcholine receptor alpha-3 subunit (CHRNA3+ ) silent nociceptors, which are the most abundant TrkA+ nociceptors in visceral organs and deep somatic tissues. Here, we utilized these mouse lines to investigate the expression patterns and the possible nerve growth factor-dependent modulation of sodium channels in these neurons using whole-cell patch-clamp recordings and quantitative real-time polymerase chain reaction. We demonstrate that NPY2R+ nociceptors, CHRNA3+ ‘silent’ nociceptors and polymodal C-fibre nociceptors express different combinations of sodium channel α- and β-subunits and accordingly exhibit functionally different sodium currents. Moreover, we demonstrate that nerve growth factor produces robust hyperpolarizing shifts in the half-activation voltage of tetrodotoxin-resistant currents in NPY2R+ nociceptors and polymodal C-fibre nociceptors and also shifts the half-activation of tetrodotoxin-sensitive currents in polymodal C-fibre nociceptors. In silent nociceptors, however, nerve growth factor solely increases the current density of the tetrodotoxin-resistant current but does not alter other sodium channel properties. Considering the different peripheral target tissues and the previously reported roles in different forms of pain of the nociceptor subpopulations that were examined here, our results suggest that nerve growth factor differentially contributes to the development visceral and cutaneous pain hypersensitivity and highlights the importance of developing different therapeutic strategies for different forms of pain.

Maximum axonal following frequency separates classes of cutaneous unmyelinated nociceptors in the pig

“…2000; Schaefer et al . 2018), which reduces activation thresholds; higher expression of both channels is expected to increase peak following frequency. However, is important to note that pain intensity is not simply encoded by frequency, and that spike timing and temporal pattern are also of crucial importance (Cho et al .…”

“…The protracted time course of hyperalgesia, with a maximum at 3 weeks after intracutaneous injection of NGF in humans (Rukwied et al 2010), suggests protein biosynthesis or structural changes as underlying mechanisms. Among the NGF targets for upregulation are nociceptor-specific ion channels, including the fast repriming NaV1.8 (Bennett et al 2019) and possibly NaV1.7 (Gould et al 2000;Schaefer et al 2018), which reduces activation thresholds; higher expression of both channels is expected to increase peak following frequency. However, is important to note that pain intensity is not simply encoded by frequency, and that spike timing and temporal pattern are also of crucial importance (Cho et al 2016;Barkai et al 2020).…”

TTX-Resistant Sodium Channels Functionally Separate Silent From Polymodal C-nociceptors

“…High levels of Na V 1.8 were found in CHRNA3+ neurons, with their TTX-s currents being lower and the TTX-r currents higher than in presumed polymodal (IB4-/capsaicin+) nociceptors (Schaefer et al, 2018). In addition, TTX-r currents in the silent nociceptors were activated at less depolarized membrane potentials (Schaefer et al, 2018). In support of our data, higher SCNA10 and CHRNA3 expression was found in a population of isolectin B4 negative vs. IB4-positive sensory neurons among presumed nociceptors (SNS-Cre/TdTomato reporter mice; Chiu et al, 2014).…”

“…However, it needs to be pointed out that murine silent nociceptors are mainly innervating viscera and deep somatic tissue, but not skin (Prato et al, 2017) precluding direct translation. High levels of Na V 1.8 were found in CHRNA3+ neurons, with their TTX-s currents being lower and the TTX-r currents higher than in presumed polymodal (IB4-/capsaicin+) nociceptors (Schaefer et al, 2018). In addition, TTX-r currents in the silent nociceptors were activated at less depolarized membrane potentials (Schaefer et al, 2018).…”

“…To compare the properties of action potentials of mechanoinsensitive nociceptors and polymodal C-fiber nociceptors in mice, we utilized a reporter mouse line in which mechanoinsensitive nociceptors that can readily be identified in primary DRG cultures by means of EGFP expression (Prato et al, 2017;Schaefer et al, 2018). Polymodal C-fiber nociceptors were identified as small-diameter neurons that are not labeled by the isolectin-B4 (IB4) and respond to the TRPV1 agonist capsaicin (applied to the cells after recording action potentials).…”