The resistance of cutaneous feline C-fiber mechano-heat-sensitive unit termination to tetrodotoxin and its possible relation to tetrodotoxin-resistant sodium channels

To study the features of ionic conductance system in termination of sensory units the responses of feline cutaneous A-β mechano-sensitive and C-fiber mechano-heat-sensitive (CMH) units to mechanical and heat stimuli were recorded in intact skin and under the action of subcutaneously applied tetrodotoxin (TTX). Both mechanical and thermal sensitivity of CMH units were not inhibited by 30 μM and in some units by 300 μM TTX, while the responses of A-β units to mechanical stimulation was eliminated by 3 μM TTX. Taking into account the data on low-frequency use-dependent inhibition of CMH unit termination by amine local anesthetics, the availability of TTX-resistant sodium channels in the somatic membrane of C-neurons liable to low-frequency use-dependent inhibition and the slow inactivation of these channels which corresponds to the requirements of mathematical simulation of spike initiation in C-fibers, our results are thought to indicate the presence of TTX-resistant sodium channels in the regenerative region of cutaneous afferent C-fibers.