Sodium channel expression and assembly during development of retinal ganglion cells.

Acquisition of functional Na+ channels is a critical event in the development of a neuron because it allows the generation of conducted action potentials. alpha subunit mRNA is first detected in developing rat retina at 1% of its maximum level on embryonic day 15, 4 days after the first ganglion cells are formed. alpha subunit protein is detected in the axons of the ganglion cells at this time, but beta 1 subunits, beta 2 subunits, and high affinity saxitoxin binding sites are not detected until after birth. There is an approximately coordinate increase in alpha subunit mRNA, alpha, beta 1, and beta 2 subunit protein, assembled complexes of alpha, beta 1, and beta 2 subunits, and high affinity saxitoxin binding sites between postnatal days 7 and 21. Expression of alpha subunit genes is an early event in ganglion cell differentiation, and both gene transcription and posttranslational assembly are separate, rate-limiting steps in development of Na+ channels.