Role of TTX-sensitive and TTX-resistant sodium channels in Adelta- and C-fiber conduction and synaptic transmission.

Thin afferent axons conduct nociceptive signals from the periphery to the spinal cord. Their somata express two classes of Na+ channels, TTX-sensitive (TTX-S) and TTX-resistant (TTX-R), but their relative contribution to axonal conduction and synaptic transmission is not well understood. We studied this contribution by comparing effects of nanomolar TTX concentrations on currents associated with compound action potentials in the peripheral and central branches of Adelta- and C-fiber axons as well as on the Adelta- and C-fiber-mediated excitatory postsynaptic currents (EPSCs) in spinal dorsal horn neurons of rat. At room temperature, TTX completely blocked Adelta-fibers (IC50, 5-7 nM) in dorsal roots (central branch) and spinal, sciatic, and sural nerves (peripheral branch). The C-fiber responses were blocked by 85-89% in the peripheral branch and by 65-66% in dorsal roots (IC50, 14-33 nM) with simultaneous threefold reduction in their conduction velocity. At physiological temperature, the degree of TTX block in dorsal roots increased to 93%. The Adelta- and C-fiber-mediated EPSCs in dorsal horn neurons were also sensitive to TTX. At room temperature, 30 nM blocked completely Adelta-input and 84% of the C-fiber input, which was completely suppressed at 300 nM TTX. We conclude that in mammals, the TTX-S Na+ channels dominate conduction in all thin primary afferents. It is the only type of functional Na+ channel in Adelta-fibers. In C-fibers, the TTX-S Na+ channels determine the physiological conduction velocity and control synaptic transmission. TTX-R Na+ channels could not provide propagation of full-amplitude spikes able to trigger synaptic release in the spinal cord.