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

Abstract

Pronounced activity-dependent slowing of conduction has been used to characterize mechano-insensitive, "silent" nociceptors and might be due to high expression of Na V 1.8 and could, therefore, be characterized by their tetrodotoxin-resistance (TTX-r). Nociceptor-class specific differences in action potential characteristics were studied by: (i) in vitro calcium imaging in single porcine nerve growth factor (NGF)-responsive neurites; (ii) in vivo extracellular recordings in functionally identified porcine silent nociceptors; and (iii) in vitro patch-clamp recordings from murine silent nociceptors, genetically defined by nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) expression. Porcine TTX-r neurites (n = 26) in vitro had more than twice as high calcium transients per action potential as compared to TTX-s neurites (n = 18). In pig skin, silent nociceptors (n = 14) characterized by pronounced activity-dependent slowing of conduction were found to be TTX-r, whereas polymodal nociceptors were TTX-s (n = 12) and had only moderate slowing. Mechano-insensitive cold nociceptors were also TTX-r but showed less activity-dependent slowing than polymodal nociceptors. Action potentials in murine silent nociceptors differed from putative polymodal nociceptors by longer duration and higher peak amplitudes. Longer duration AP in silent murine nociceptors linked to increased sodium load would be compatible with a pronounced activity-dependent slowing in pig silent nociceptors and longer AP durations could be in line with increased calcium transients per action potential observed in vitro in TTX-resistant NGF responsive porcine neurites. Even though there is no direct link between slowing and TTX-resistant channels, the results indicate that axons of silent nociceptors not only differ in their receptive but also in their axonal properties.

Mechanical sensitization, increased axonal excitability, and spontaneous activity in C-nociceptors after ultraviolet B irradiation in pig skin

“…Unfortunately, little is known about the mechanisms that lead to ongoing nociceptor discharge. The finding that Nav 1.8 is required for spontaneous activity in primary nociceptive afferents 34 might be in-line with high prevalence of ongoing activity in silent nociceptors that have been shown to be TTX-resistant in pig, 18 but our data do not provide further mechanistic clues.…”

“…43 Such an expression pattern would be in-line with the TTX-resistant silent nociceptors in the pig. 18 Thus, axonal excitability as assessed by slowing of conduction is highly useful for classification of C-nociceptor classes. Albeit indicating axonal hyperexcitability, its particular functional relevance for pain and hyperalgesia is unclear.…”

“…Early postspike changes in the conductive properties of an axon were investigated by twin pulse stimulation at 20 Hz (50 ms). 18 Late postspike changes in the conductive properties of an axon were assessed by the activity-dependent slowing (ADS) of CV and recovery kinetics. We used continuous stimulation at 2 Hz for 3 minutes (ie, 360 pulses), followed by 25 pulses at 0.25 Hz 20,22 and assessed ADS after 10 pulses (ie, 5 seconds; ADS 10 ), after 50 pulses (ie, 25 seconds; rS50), and at the end of 2 Hz stimulation after 360 pulses (ADS 360 ).…”

Thermal hyperalgesia and dynamic weight bearing share similar recovery dynamics in a sciatic nerve entrapment injury model

“…As we consider the non-evoked nature of the dynamic weight bearing tool and the parallel recoverability phenotype it shares with thermal sensitivity, it is possible that the fibers mediating thermal sensation are those that generate the ongoing pain experienced in neuropathic models ( He et al, 2012 , Huang et al, 2019 ) and this ongoing pain leads to the non-evoked weight bearing changes observed in this model. Alternatively, it is possible that sciatic nerve cuff activates silent nociceptors ( Jonas et al, 2020 , Michaelis et al, 1996 ) which become mechanically sensitive following injury. Silent nociceptors have been shown to be sensitive to noxious thermal stimuli and insensitive to mechanical stimuli.…”

Multitarget nociceptor sensitization by a promiscuous peptide from the venom of the King Baboon spider

“…The effects of sPm1a on each nociceptive neuron would rely predominantly on the relative expression of Na V 1.8 and the abundance of (sensitive) delayed rectifier K channels, but also on other targets such as TTX-S Na V channels, including Na V 1.7. High levels of Na V 1.8 expression have been reported in mechano-insensitive CHRNA3 + murine nociceptors and in functionally silent porcine nociceptors ( 53 ). Unsilencing by exposure to δ/κ-TRTX-Pm1a/sPm1a upon envenomation would recruit this population of C-fibers to contribute to hyperalgesia.…”