Localization of sodium channels in axon hillocks and initial segments of retinal ganglion cells.

Affinity-purified antibodies against the sodium channel from rat brain were employed to localize sodium channels in the retina by immunocytochemical procedures. In rat retina, intense staining was observed in the ganglion cell axon layer and light staining was detected in fibers of the inner plexiform layer. In frog retina, only the ganglion cell axon layer was stained. Examination at higher magnification revealed that axon hillocks and initial segments of ganglion cells had a high density of immunoreactive sodium channels, whereas the cell bodies were devoid of stain. The sharply defined region of high sodium channel density at the axon hillock is likely to be responsible for the low threshold for action potential initiation in this region of vertebrate central neurons.