Horizontal and amacrine cells in the isolated carp retina were impaled with micropipette electrode, identified by their characteristic light responses, and injected iontophoretically with markers for morphological study. Both Lucifer Yellow CH and biocytin were injected simultaneously. Lucifer Yellow was seen by its own fluorescence while biocytin was visualized by binding with Texas Red-linked or horseradish peroxidase-conjugated avidin. For cone-connected horizontal cells, biocytin-coupled cells were found to be approximately five-times more numerous than Lucifer Yellow-coupled cells. Coupling for both tracers was consistently hampered by intravitreally applied dopamine. In untreated retinas, the injected Lucifer Yellow was restricted within one rod-connected horizontal cell, while biocytin revealed several coupled neighbors. Amacrine cells, labeled by the tracers, were morphologically grouped into eight types, based on our earlier classification.(29) Among them, amacrine cells, belonging to three types (Fnd, Pmb or Pma), were confirmed to be Lucifer Yellow-coupled, and the number of biocytin-coupled cells was more numreous (about 2.5 times) than that of Lucifer Yellow-coupled cells. Most amacrine cells (i.e. Pwd, Fnb and Fna) showed biocytin-coupling with no Lucifer Yellow-coupling. A few classified (i.e. Pwb and Fwa) and unclassified cells did not show any coupling. Since the tracer coupling takes place via gap junctions, the majority of amacrine cells, belonging to certain homologous types, appear to be functionally coupled with each other in the inner plexiform layer. However, dopamine did not influence the range of tracer coupling between amacrine cells in the carp retina under the present experimental conditions.
