Abrahamsen et al. (2008) crossed Nav1.8 knock-in heterozygous Cre-expressing mice with mice homozygous for the floxed stop codon upstream of a diphtheria toxin A (DTA)-subunit gene. DTA thus killed all Nav1.8-expressing sensory neurons (Nav1.8DTA). Staining of neurons with an antibody to peripherin revealed that more than 85% of peripherin-expressing DRG neurons were killed. Nav1.
8DTA mice responded normally to acute noxious heat stimuli, but were insensitive to noxious mechanical or cold stimuli. These findings appear to highlight that neurons that do not express Nav1.8 channels form neural pathways specific for the transduction of noxious heat stimuli, whereas Nav1.8-containing neurons transduce noxious cold and mechanical stimuli.
Further support for the concept of modality-specific pain pathways has been offered by the study of Emery et al. (2016). They used C57BL/6 mice that expressed the fluorescent Ca2+ indicator GCaMP in their DRG neurons, in vivo. Using this technique, they visualized different sets of DRG sensory neurons that become active upon application of noxious mechanical, cold and heat stimuli. Deletion of Nav1.8 channels prevented the activation of mechanical-sensing neurons, but not that of heat-sensing neurons. It also resulted in loss of inflammatory pain, but not loss of neuropathic pain.
This not only shows that different pain modalities, e.g. heat, cold, mechanical pressure, are associated with the activation of specific sets of sensory neurons and special pain pathways, but also that voltage-gated Na+ channels, such as Nav1.8 channels, may be playing a critical role in the transmission of signals within these modality-specific sets of nociceptors.The roles of voltage-gated Na+channels in cold-induced pain are summarized by Foulkes and Wood (2007). They suggest that noxious cold temperatures alter neuronal membrane properties, with the passive conductance of K+ ‘leak’ channels and the activity of the Na+/K+ ATPase being reduced.
This effectively increases the membrane resistance, rendering depolarizing currents more effective at depolarizing the membrane and reaching the threshold for activation of Nav1.8 channels. Furthermore, they suggest that low temperatures could also act to directly shift the activation threshold for Nav1.8 channels to more negative membrane potentials. A third mechanism that is suggested implicates cold-induced upregulation of p38 MAPK (see Mizushima et al.
, 2006), which then augments Nav1.8 current density, specifically via phosphorylation at S551 and S556 (Hudmon et al., 2008). However, according to Foulkes and Wood (2007), what is unique about the role of Nav1.8 channels is that they are the only voltage-gated Na+channels that do not inactivate at low temperatures. This, along with tissue specific expression of these channels, is proposed to give rise to modality-specific activation of nociceptors by noxious cold, as shown by the studies of Abrahamsen et al. (2008) and Emery et al. (2016).
Evidence for the effect of noxious cold on the membrane properties, Nav1.8 activation threshold and inactivation of Nav1.8 channels has been offered by Zimmermann et al.(2007). Throughout their study, they made use of the Na+ channel blocker tetrodotoxin (TTX) to isolate Nav1.8 currents, as Nav1.8 are resistant to it (see Table 1).
They first blocked rat mechanocold-sensitive C fibre nociceptors in vitro at 30°C with 1 ?M TTX, rendering the cells unexcitable. Upon cooling, the nociceptors became excitable. Cooling also resulted in slower inactivation kinetics of TTX-sensitive (TTXs) and TTX-resistant (TTXr) Na+ currents in DRG neurons.
However, this effect was more pronounced in TTXs currents at a holding potential of ?80 mV, whereas any differences were abolished at ?120 mV. This shows that the slow inactivation threshold was lowered for TTXs upon cooling, that TTXr currents were mainly resistant to shifts in their slow inactivation threshold, whereas cooling led to a small shift in activation threshold towards hyperpolarized potentials for both TTXs and TTXr currents. Given that Nav1.9 channels are also TTXr, Zimmermann et al. (2007) also studied the effect of cooling on Nav1.8-deficient neurons. In the presence of TTX and at 10°C, Nav1.
8-deficient neurons failed to generate action potentials in response to current injections and mechanical stimuli, whereas wild-type (WT) neurons remained excitable. They confirmed that Nav1.8 knockout results in loss of behavioural responses, e.g. foot-lifting, to noxious cold in the cold-plate test.
Finally, whereas TTX and cooling each increased the rheobase current and the chronaxy at all current durations tested in rat skin-nerve terminal preparations, cooling in the presence of TTX decreased the rheobase current regardless of stimulus duration, highlighting an increase in membrane resistance. This, in turn, increases the voltage response of Nav1.8 in response to depolarizing currents.
Despite that Nav1.9 channels were not deemed to be essential in the study by Zimmermann et al. (2007), Lolignier et al. (2015) observed that Nav1.9 function is upregulated in a set of nociceptors responding to cooling, whereas preventing Nav1.9 expression in rodents impairs cold-induced neuronal firing, increases pain threshold to cold, and reduces oxaliplatin-induced cold-pain hypersensitivity. However, in contrast to Nav1.8 channels and what was found by Zimmermann et al.
(2007), they observed that cooling-induced slowing of activation and inactivation was voltage-independent. They concluded that Nav1.9 channels act as subthreshold amplifiers in cold modality-specific nociceptors, important in the integration of impulses. In addition, it has been supported that, in Nav1.
8 and Nav1.9 knockout mice, Nav1.9 channels play a greater modulatory role than Nav1.8 channels in the development of cold allodynia in neuropathic pain, whereas the involvement of both Nav1.
8 and Nav1.9 channels in inflammatory pain hypersensitivity may be limited (Leo, D’Hooge, Meert, 2010). A missense mutation (p.V1184A) in SCN11A, the gene encoding Nav1.9 channels, increases the open probability of the mutant channels and the hyperexcitabillity of DRG neurons, the latter effect being less attenuated at low temperatures compared to WT channels, and has been associated with cold-aggravated pain (Leipold et al.
, 2015). Therefore, both Nav1.8 and Nav1.
9 channels are deemed to be electrophysiologically suitable for the transmission of noxious cold stimuli (Lolignier et al., 2016). Both are resistant to cold-induced inactivation, while channels in other afferents are inactivated by low temperatures, resulting in loss of other sensory modalities (Lolignier et al., 2016).