Purpose: Recent studies indicate the presence of functional voltage-gated sodium channels (Na(v) channels) in the distal retina in several species. This study examined the distribution of Na(v) channels in the outer plexiform layer (OPL) of rat, mouse, and rabbit retinas.
Methods: Immunohistochemical and electroretinographic approaches were used.
Results: Antibodies specific for Na(v)1 alpha-subunits appropriately labeled retinal ganglion cells, their axons, and amacrine cells that are known to have tetrodotoxin (TTX)-sensitive Na(v) channels. Pan-Na(v), Na(v)1.2, and Na(v)1.6 labeling was found in horizontal cells and processes in all three species. Weaker Na(v)1.1 labeling was observed in rodent horizontal cells, but some rabbit horizontal cells and processes were prominently labeled. Additional labeling for Na(v)1.1, Na(v)1.2, and Na(v)1.6 that was not attributable to horizontal cells was also present in the OPL. Much of this labeling was diffusely distributed. Some of the additional Na(v)1.1 labeling was associated with photoreceptor terminals. By exclusion using photoreceptor and horizontal cell markers, some of this labeling could have been associated with bipolar cell dendrites, although colocalization was not directly established due to the diffuse nature of the labeling and limits on anatomical resolution. No Na(v)1 alpha-subunit labeling was observed in bipolar cell bodies. Testing for functional Na(v) channels was performed by recording full field flash electroretinograms from dark-adapted rats before and after intravitreal injections of TTX, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or TTX+CNQX. TTX and CNQX+TTX, but not CNQX alone, greatly attenuated the dark-adapted cone-driven b-waves.
Conclusions: Horizontal cells from three different mammalian retinas showed prominent labeling for Na(v)1 alpha-subunits. Some additional diffuse Na(v)1 alpha-subunit labeling in the OPL was associated with photoreceptor terminals. Na(v)1 alpha-subunit labeling also may have been present on bipolar cell dendrites, although it was not possible to establish this localization unequivocally by immunostaining. However, cone-driven b-waves in rats were reduced in maximum amplitude by TTX in the presence of CNQX which blocks synaptic input to horizontal, amacrine, and ganglion cells. This finding is consistent with TTX effects on the b-wave being due to blockade of Na(v) channels in cone bipolar cell dendrites in the OPL. The role of Na(v) channels in horizontal cells remains to be determined.