Expression of sodium channels Nav1.2 and Nav1.6 during postnatal development of the retina.

During the second and third postnatal weeks, there is a developmental switch from sodium channel isoform Na(v)1.2 to isoform Na(v)1.6 at initial segments and nodes of Ranvier in rat retinal ganglion cells. We used quantitative, real-time PCR to determine if the developmental appearance of Na(v)1.6 channels is accompanied by an increase in steady-state level of Na(v)1.6 mRNA in the retina. Between postnatal day 2 (P2) and P10, Na(v)1.6 levels did not change, but between P10 and P19, there was an approximately three-fold increase in Na(v)1.6 transcript levels. This coincides with the appearance of Na(v)1.6 channels in the retina and optic nerve. The steady-state level of Na(v)1.2 mRNA also increased during this same period, which suggests that the rise in Na(v)1.6 may be part of a general increase in sodium channel transcripts at about the time of eye opening at P14. The results are consistent with a developmental increase in steady-state transcripts giving rise to a corresponding increase in sodium channel protein expression.