D2-LIKE DOPAMINE-RECEPTORS IN AMPHIBIAN RETINA - LOCALIZATION WITH FLUORESCENT LIGANDS

Dopamine induces several light adaptive changes in amphibian retina vi a receptors with D2-like pharmacology, but the identity of the primary target cells has not been determined. Using a fluorescent probe consi sting of a selective D2 antagonist, N-(p-aminophenethyl)spiperone (NAP S), derivatized with the fluorophore Bodipy (NA-PS-Bodipy), we identif ied the distribution of dopamine binding sites in the retina of two am phibians, post-metamorphic Xenopus laevis and larval Ambystoma tigrinu m. Specific labeling was defined as staining that was displaced by D2 selective ligands (eticlopride or sulpiride), but insensitive to D1 se lective drugs (SCH 23390), adrenergic catecholamines (epinephrine or n orepinephrine), or serotoninergic analogues (ketanserin). Both rod and cone cells showed specific dopamine D2-like binding sites arranged in clustered arrays on discrete membrane domains of the inner segment. L abeling of photoreceptor outer segments was continuous and was not dis placed by competition with D2 selective ligands; this labeling was con sidered nonspecific. In addition, in both species, clustered binding o f the D2-probe was found on Muller cells and on a subset of inner reti nal cells with the morphology of amacrine/interplexiform cells. Our da ta provide direct evidence for D2 receptors on both rods and cones, an d suggest that the receptors may be clustered into patches within a di screte cellular domain, the inner segment.