Dopamine modulates S-potential amplitude and dye-coupling between external horizontal cells in carp retina

Horizontal cells in the fish retina are electrically coupled and possess gap junctions so that intracellularly injected dye normally diffuses freely to neighbouring cells. Applied dopamine (DA) alters the spatial properties of the horizontal cell responses to light, increasing the amplitude of photopic L-type S-potentials but decreasing their lateral spread. These effects have been attributed to the action of DA on horizontal cell membrane resistance, particularly at the gap junctions, and our present study on the carp retina agrees with this in showing that DA also restricts intracellular Lucifer yellow (LY) to single injected horizontal cells, an effect, like those of DA on the S-potentials, which is antagonized by the dopamine blocker haloperidol. In addition, we present evidence that dopaminergic interplexiform cells in fish normally function to regulate the spatial properties of responses in horizontal cells, possibly acting on their junctional resistance via a DA-receptor-mediated mechanism. Previous destruction of the interplexiform cells with 6-hydroxydopamine (6-OHDA) resulted in much reduced L-type S-potentials to centred lights but wider lateral spread of these responses, while the dye injected spread extensively to neighbouring cells. After 6-OHDA treatment, however, applied DA retained its normal activity, restoring large-amplitude, narrow receptive-field S-potentials and restricting LY to the injected cells, effects which were both closely mimicked by dibutyryl cyclic AMP.