Activation of a D2 receptor increases electrical coupling between retinal horizontal cells by inhibiting dopamine release.

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Abstract
In the fish retina, interplexiform cells release dopamine onto cone-driven horizontal cells. Dopamine decreases the electrical coupling between horizontal cells by activating adenylate cyclase through dopamine D1 receptors. Using intracellular recording, we have studied the effect of dopamine D2 receptor activation on horizontal cell electrical coupling in the intact goldfish retina. Superfusion of the D2 agonist LY171555 (quinpirole; 0.2-10 muM) increased horizontal cell coupling, as indicated by a decrease in responses to centered spots or slits of light. The length constant of the horizontal cell network increased an average of 31 %. Although dopamine (0.5-20 muM) uncoupled horizontal cells, lower concentrations (e.g., 0.2 muM) initially uncoupled and then subsequently increased coupling beyond initial control levels. The coupling effect of LY171555 (10 muM) was blocked completely by prior application of the D1 agonist SKF 38393 at saturating (20 muM) or nonsaturating (2.5-5.0 muM) doses. Prior treatment of the retinas with 6-hydroxydopamine, which destroyed dopaminergic neurons, eliminated the coupling effect of LY171555 but not the uncoupling effect of SKF 38393. These results suggest that goldfish horizontal cells contain D1, but not D2, receptors and that dopamine activation of D2 autoreceptors on interplexiform cells inhibits dopamine release onto horizontal cells so that the electrical coupling between horizontal cells increases.