Gap-junctional contacts of luminosity-type horizontal cells in the carp retina: a novel pathway of signal conduction from the cell body to the axon terminal.

L-type horizontal cells (271) of the carp retina were studied with the aid of intracellular injections of Lucifer yellow and horseradish peroxidase (HRP). Coupling among these cells was revealed by passage of the dye out of the injected cell. Thick processes that crossed axons filled with Lucifer yellow also contained the stain. These thick processes appear to be the axon terminals of other horizontal cells. Serial reconstruction of an axon terminal by electron microscopy revealed two groups of elements connected to the axon terminal with putative gap junctions. One large-diameter group was identified as axon terminals of other cells. From the results of the Lucifer dye-coupling experiments, the smaller-diameter elements are thought to be the axons of other horizontal cells. Thus, the axon terminals of carp horizontal cells have putative gap-junctional contacts not only with other axon terminals but also with the axons of other horizontal cells. This newly discovered connection could serve as a pathway for signal transfer between the cell body and the axon terminal of different horizontal cells.