Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro-dendritic gap junctions between neigh boring cells mediated both weak Lucifer yellow dye coupling and strong neur obiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 mu M) or cyclic adenosine monophosphate (cA MP) analogs and was significantly increased by the D-1 antagonist SCH 23390 (1D mu M). These results provide the first evidence in the mammalian retin a that the gap junctions between horizontal cells are endogenously regulate d by dopamine, which acts through D-1 receptors to increase the intracellul ar cAMP. It has been proposed that the gap junctional coupling between hori zontal cells is mediated by connexin 32 (Cx32), but the pattern and dopamin ergic regulation of horizontal cell coupling were unaffected in Cx32-knocko ut mice, ruling out the possible involvement of Cx32. Every tracer-coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providin g strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian ret inas, the axon-bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compen sate for the absence of axonless horizontal cells. (C) 2000 Wiley-Liss, Inc .