Experimental and computational evidence for an essential role of NaV1.6 in spike initiation at stretch-sensitive colorectal afferent endings

Stretch-sensitive afferents comprise similar to 33% of the pelvic nerve innervation of mouse colorectum, which are activated by colorectal distension and encode visceral nociception. Stretch-sensitive colorectal afferent endings respond tonically to stepped or ramped colorectal stretch, whereas dissociated colorectal dorsal root ganglion neurons generally fail to spike repetitively upon stepped current stimulation. The present study investigated this difference in the neural encoding characteristics between the soma and afferent ending using pharmacological approaches in an in vitro mouse colon-nerve preparation and complementary computational simulations. Immunohistological staining and Western blots revealed the presence of voltage-gated sodium channel (Na-V) 1.6 and Na(V)1.7 at sensory neuronal endings in mouse colorectal tissue. Responses of stretch-sensitive colorectal afferent endings were significantly reduced by targeting Na(V)1.6 using selective antagonists (mu-conotoxin GIIIa and mu-conotoxin PIIIa) or tetrodotoxin. In contrast, neither selective Na(V)1.8 (A803467) nor Na(V)1.7 (ProTX-II) antagonists attenuated afferent responses to stretch. Computational simulation of a colorectal afferent ending that incorporated independent Markov models for Na(V)1.6 and Na(V)1.7, respectively, recapitulated the experimental findings, suggesting a necessary role for Na(V)1.6 in encoding tonic spiking by stretch-sensitive afferents. In addition, computational simulation of a dorsal root ganglion soma showed that, by adding a Na(V)1.6 conductance, a single-spiking neuron was converted into a tonic spiking one. These results suggest a mechanism/channel to explain the difference in neural encoding characteristics between afferent somata and sensory endings, likely caused by differential expression of ion channels (e.g., Na(V)1.6) at different parts of the neuron.