LOCALIZATION OF DOPAMINE D-1-RECEPTORS IN VERTEBRATE RETINAE

We used the fluorescent labelled dopamine D-1-receptor antagonist Bodipy-SCH 23390 for the cellular localization of D-1-ligand binding sites in the retinae of different vertebrates (teleosts, Xenopus, turtle, rat and rabbit). Competition experiments with unfixed cryosections of fish retina were performed to characterize the binding conditions of Bodipy-labelled SCH 23390. Tissue bound [H-3]SCH 23390 was displaceable with increased amounts of bodipy-SCH 23390. The pharmacological specificity of the D-1 fluorescent antagonist was determined by competition experiments with an excess of unlabelled SCH 23390. This treatment significantly reduced the level of fluorescence of the retina confirming the specificity of the binding. We observed a homogeneously distributed fluorescence signal in both plexiform layers in unfixed cryosections of fish, frog, turtle, rat and rabbit. Similar staining intensities of both plexiform layers were found in frog, turtle, rat and rabbit retina. In teleosts, the label of the outer plexiform layer was markedly more intense. Non-specific label was associated with photoreceptor outer and inner segments. The specific labelling of both plexiform layers indicates a mismatch of dopamine releasing and D-1-binding sites, and suggests a possible extrasynaptic localization of the D-1-receptor. The physiological significance of the observed distribution of D-1-ligand binding sites is discussed with respect to the role of dopamine in controlling adaptational processes in the retina.