Low Density of Sodium Channels Supports Action Potential Conduction in Axons of Neonatal Rat Optic Nerve

The density of sodium channels in premyelinated axons was estimated from measurements of the binding of [3H]saxitoxin to neonatal rat optic nerve. The maximum saturable binding capacity of the nerve was 16.2 ± 1.2 fmol/mg of wet weight, with an equilibrium dissociation constant of 0.88 ± 0.18 nM (mean ± SEM). These values correspond to a high-affinity saxitoxin-binding site density of ≈ 2/μ m2 within premyelinated axon membrane. Action potential propagation in neonatal rat optic nerve is completely blocked by 5 nM saxitoxin, indicating that action potential electrogenesis is mediated by channels that correspond to high-affinity saxitoxin-binding sites. These results demonstrate that action potential conduction is supported by a low density of sodium channels in this system. Since the internodal axon membrane of myelinated fibers may contain a low density of sodium channels, it is possible that restoration of conduction in some demyelinated fibers may not require additional sodium channel incorporation into the demyelinated axon membrane.