Rod-signal interneurons in the rabbit retina: 1. Rod bipolar cells

The cellular morphology and topographic distribution of the rod bipolar cells in the rabbit retina have been investigated by selective labelling with protein kinase C-immunohistochemistry (Negishi et al., Neurosci, Lett. 94:247-252, 1988) and by Lucifer Yellow injection of microscopically identified cells in a superfused retinal preparation. The distribution of the rod bipolar cells parallels that of their input neurons, the rod photoreceptors, in that the rod bipolars reach maximum densities of 5,000-7,000 cells/mm2 on the inferior and superior flanks of the visual streak, dropping to slightly lower densities at the peak visual streak. The centre-to-periphery density gradient of the rod bipolars is about 2.5:1, and the density ratio of rods to rod bipolars shows little variation across the retina, ranging from 43:1 in superior retina to 58:1 in inferior retina. The dendritic field area of the rod bipolar cells increases from 600 microns2 on the visual streak to 1,200 microns2 in the far-superior retina, with each point on the retina overlapped by 2.5-3.5 dendritic fields. The axonal field area of the rod bipolar cells increases from about 100 microns2 at the peak visual streak to about 250 microns2 at the retina edge, and the axonal field coverage ranges from 0.55 in the visual streak to about 0.8 in peripheral retina. Although there appear to be gaps in the local array of rod bipolar somata, these areas are covered by the axonal arbours of neighbouring rod bipolar cells.