Homotypic Gap Junction Connections Between Retinal Amacrine Cells

Abstract

Network properties of gap junction connections between retinal amacrine cells in the inner plexiform layer (IPL) were evaluated by techniques of intracellular recording, dye injection, and electron microscopy conducted on isolated retinas of cyprinid fish. First, amacrine cells were identified with their light-evoked responses to light flashes, either transient ON-OFF or sustained. Second, tracer-coupled networks of the cell populations were visualized by transfer of intracellularly injected Neurobiotin into neighbors. Contacts between the cells were then investigated with high-voltage as well as with conventional electron microscopy. Cell-type specific, homotypic connections in the populations were found. Dendrodendritic contacts were seen either in a tip- or a cross-contact manner. In some experiments, cells belonging to the same cell category (e.g., generating a similar photoresponse profile and expressing similar cell morphology) were stained with other tiny cells, in addition to cells of the same morphology. However, no direct contact between the major and tiny cells was light microscopically identified in these preparations. Electron microscopical analysis revealed gap junctions between light microscopically documented dendrites, but not in dendrites apart from interconnected sites. These results demonstrate that gap junctions between amacrine cells have a homotypical manner in cyprinid fish retinas. The homotypic lateral gap junctions between these cells may play important roles in the inhibitory synaptic behavior in cells in the IPL.