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Visual information is processed in the retina to a remarkable degree before it is transmitted to higher visual centres. Several types of retinal ganglion cells (the output neurons of the retina) respond preferentially to image motion in a particular direction, and each type of direction-selective ganglion cell (DSGC) is comprised of multiple subtypes with different preferred directions. The direction selectivity of the cells is generated by diverse mechanisms operating within microcircuits that rely on independent neuronal processing in individual dendrites of both the DSGCs and the presynaptic neurons that innervate them.

In mammalian retina, the rod bipolar cells synapse on the AII amacrine cells, which are therefore the third-order neurons in the rod-signal pathway. The AII amacrine cells are connected by gap junctions, both to each other and to fourth-order, On-center cone bipolar cells. They also receive synaptic input from the dopaminergic amacrine cells, and in this study, we investigated whether dopamine modulates the permeability of the gap junctions between AII amacrine cells in the isolated rabbit retina. The small biotinylated tracer Neurobiotin was injected into nuclear yellow-labeled AII cells under direct microscopic control. The extent of tracer coupling to neighboring AII cells, 40-60 min after Neurobiotin injection (0.5 nA for 60 sec), provided a standard measure of the permeability of the homologous gap junctions. Under control conditions, individual AII amacrine cells were coupled to 73 +/- 15 neighboring cells, and this was unaffected by changes in pH from 6.6 to 7.8. Exogenous dopamine significantly reduced the tracer coupling at concentrations as low as 10 nM (26 +/- 16 cells), with the effect increasing with dopamine concentration up to 10 microM (6 +/- 4 cells). The uncoupling effect of dopamine was both blocked by the selective D1 antagonist SCH-23390 (10 microM) and mimicked by the specific D1 agonist SKF-38393 (500 microM). Moreover, the AII amacrine cells were also uncoupled when the retina was incubated in forskolin (60 microM) and isobutylmethylxanthine (200 microM). Taken together, these results indicated that the uncoupling was mediated by a D1-like receptor that stimulates cAMP production. Although the selective D1 antagonist on its own did not increase tracer coupling, suggesting that there was little release of endogenous dopamine in the superfused photo-bleached retina, veratridine-evoked release of endogenous transmitters did uncouple the AII amacrine cells, and this effect was blocked by the specific D1 antagonist.