Gap junctions in the inner plexiform layer of the goldfish retina.

Oblique thin sections of the goldfish inner plexiform layer (IPL) were examined to characterize the processes engaging in gap junctions. Omission of en bloc uranyl acetate staining enhanced the relative visibility of gap junctions, thus facilitating surveys. Three general categories were identified: (1) extensive gap junctions between large caliber amacrine cell dendrites presynaptic to ganglion cells and other amacrine cells in distal and proximal IPL; (2) moderate to small gap junctions between amacrine cell synaptic terminals or small amacrine cell-like processes of similar cytoplasmic appearance; and (3) gap junctions between the telodendria/terminals of bipolar cells. Most instances of gap junctions in the goldfish IPL seem to represent cases of homologous coupling. We have not found unambiguous cases of heterologous coupling as in the mammalian retina, nor have we yet found mixed synapses. Taken together with previous physiological and anatomical findings, homologous coupling occurs between pairs of rods, cones, horizontal cells, bipolar cells and amacrine cells in the goldfish retina, and this in turn implies that direct intercellular communication among like cell types is a common theme in retinal circuitry.