title: Low density of sodium channels supports action potential conduction in axons of neonatal rat optic nerve.
abstract: 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 approximately 2/microns 2 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.
authors.affiliation: Department of Neurology, Yale University School of Medicine, New Haven, CT 06510.
authors.forename: S G
authors.initials: SG
authors.lastname: Waxman
authors.forename: J A
authors.initials: JA
authors.lastname: Black
authors.forename: J D
authors.initials: JD
authors.lastname: Kocsis
authors.forename: J M
authors.initials: JM
authors.lastname: Ritchie
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D029845:Amphibian Proteins; D000818:Animals; D000831:Animals, Newborn; D001369:Axons / Q000502:physiology* / Q000648:ultrastructure; D002352:Carrier Proteins / Q000378:metabolism; D008854:Microscopy, Electron; D009900:Optic Nerve / Q000502:physiology* / Q000648:ultrastructure; D051381:Rats; D012530:Saxitoxin / Q000378:metabolism / Q000494:pharmacology; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*
chemical_list: D029845:Amphibian Proteins; D002352:Carrier Proteins; D015222:Sodium Channels; C027525:saxitoxin-binding protein, Rana catesbeiana; D012530:Saxitoxin