Expression and biophysical characterization of voltage-gated sodium channels in axons and growth cones of the regenerating optic nerve

Abstract

Purpose: Successful regeneration and re-establishment of synaptic connections in the adult central nervous system is a complex process determined by both the exterior environment and the endogenous neural activity of the regenerating growth cones. The purpose of this study was to determine the expression and properties of voltage-gated sodium channels (Na(v)) expressed by regenerating growth cones.

Methods: Na(v) channels were studied in an organotypic explant culture of the adult rat retina by immunocytochemistry and whole-cell, patch-clamp recordings.

Results: Regenerating axons and growth cones, but not glial processes, expressed Na(v) channels. Whole-cell, current-clamp recordings from growth cones displayed a high input resistance of 1.29 GOmega and a resting membrane potential of -69.0 mV. All growth cones responded to depolarizing voltage steps with fast, transient, inward currents mediated by Na(+) ions, followed by slow, sustained outward K(+) currents. Half-maximum activation clustered in two groups, suggesting the presence of at least two Na(v) channel isoforms. Steady state inactivation and recovery from fast inactivation were characterized by a half-maximum value of -69.7 mV and by a time constant of 3.64 ms, respectively. Injection of depolarizing current steps larger than threshold (-29.3 mV) consistently induced a single action potential, whereas ganglion cell bodies responded to above-threshold stimulation with a series of fast, all-or-none action potentials.

Conclusions: These experiments describe for the first time the biophysical properties of Na(v) channels recorded from the growth cones of regenerating retinal ganglion cells and contrast their properties with those of adult retinal ganglion cells.