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

Stretch-sensitive afferents comprise ∼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 (NaV) 1.6 and NaV1.7 at sensory neuronal endings in mouse colorectal tissue. Responses of stretch-sensitive colorectal afferent endings were significantly reduced by targeting NaV1.6 using selective antagonists (μ-conotoxin GIIIa and μ-conotoxin PIIIa) or tetrodotoxin. In contrast, neither selective NaV1.8 (A803467) nor NaV1.7 (ProTX-II) antagonists attenuated afferent responses to stretch. Computational simulation of a colorectal afferent ending that incorporated independent Markov models for NaV1.6 and NaV1.7, respectively, recapitulated the experimental findings, suggesting a necessary role for NaV1.6 in encoding tonic spiking by stretch-sensitive afferents. In addition, computational simulation of a dorsal root ganglion soma showed that, by adding a NaV1.6 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., NaV1.6) at different parts of the neuron.

Stretch-sensitive afferent neurons in cat knee joint capsule: sensitivity to axial and compression stresses and strains

1. Experiments were performed to determine whether the response of stretch-sensitive mechanoreceptors to tissue deformation is caused by the axial stretching of the tissue or by the associated transverse compression of the tissue caused by the Poisson effect. 2. Single, stretch-sensitive mechanoreceptors were recorded in vitro in a preparation of innervated, isolated capsule from the cat knee. Afferents were isolated in a ligamentous capsule thickening that has a uniform geometry and parallel collagen fibers. The tissue was loaded axially while simultaneously stretching it around the surface of a cylinder to produce compression stresses and strains. Axial stresses and strains were measured or estimated. 3. By altering the diameter of the cylinder, given axial stresses and strains produced different levels of compression stresses and strains. It was possible to compare the neuronal response to pure uniaxial tension with the response when both axial stretching and transverse compression was applied. 4. In 8 of 11 experiments, transverse compression did not significantly change the response (P > 0.05). In one experiment, the response was decreased by compression. In the other two experiments, the response was increased but was not a function of the magnitude of the compressive stress. 5. Compressive strain was not significantly correlated with neuron response in any experiment. 6. The data do not sustain the model that the responses of stretch-sensitive neurons are due to local compression of the afferent ending.

Relation between spinal level and peripheral location of afferents in calf muscles of the cat

The relationship between the location of sensory endings in the medial gastrocnemius or soleus muscles of the cat and the level of entrance into the cord of their afferents has been studied. The hind legs were denervated except for the medial gastrocnemius or soleus muscle and the lumbosacral cord bathed in an oil pool. Afferent rootlets in the dorsal roots of L7 and S1 were examined in cranio-caudal sequence in search of filaments coming from the muscle under study. Indirect stimulation of the muscle permitted identification of the afferent as a muscle spindle or tendon organ, and distinction between annulospiral and flower-spray afferents was made on the basis of conduction rates. The position of the afferent ending in the muscle was found by probing the surface lightly with a blunt instrument. Results indicate that: a) since only one sensitive focus in the muscle is detectable for an isolated dorsal root filament, only one spindle or Golgi organ is innervated by an afferent fiber; b) the regions of inflow of annulospiral, flower-spray and Golgi afferents coincide; c) the relation between position of the afferent within the muscle and exact level of entrance in the cord is largely a random one.