Dopamine plays an important role in modulating various aspects of retinal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. In an attempt to better understand dopaminergic mechanisms at the cellular level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit, cow, chick, turtle, frog, and two fish species with immunofluorescence techniques. Antisera were raised in rabbits to two oligopeptides predicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed towards the extracellular amino terminal region shared by both short and long isoforms. Preadsorption with the synthetic peptide resulted in a significant reduction of label, indicating the presence of specific binding in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner and outer segments, and in the outer and inner plexiform layers of all species. In mammals and chick, strongly fluorescent perikarya were observed in the ganglion cell layer and at the proximal margin of the inner nuclear layer. Label may be present in the pigment epithelium but could not be established beyond doubt. This pattern of labelling is in accordance with previous observations on D2 receptor localization by means of radioactive ligand binding and in situ hybridization techniques. It suggests that retinal dopamine acts as a neuromodulator as well as a transmitter. In the distal retina, it may reach its targets via diffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synaptic transmission seems to coexist with paracrine addressing of more distant targets, and D2 receptors are expressed by both amacrine and ganglion cells.
We re-investigated the occurrence of substance P-like immunoreactivity in the retina of the goldfish Carassius auratus using antisera to substance P and other tachykinins. Most antisera labelled a previously described single class of mono-stratified amacrine cells arborizing in layer 3 of the inner plexiform layer. Preabsorption experiments showed that these amacrine cells contained at least one tachykinin-like peptide. One antiserum (INC 353) to substance P labelled not only these amacrine cells but also fibres in layer 1 of the inner plexiform layer and fibres in the optic nerve. These fibres were identified as retinopetal projections of the nervus terminalis, in part because of colocalized labelling with antisera against gonadotropin-releasing hormone and FMRFamide. Preabsorption experiments showed that the substance P-immunoreactive material in the nervus terminalis was not substance P or any other typical tachykinin. Labelling of the nervus terminalis with INC 353 was blocked by preabsorption with two bovine FMRF-amide-like peptides, F8Famide and A18Famide, which contain a substance P(4-7)-like region. Antisera to F8Famide and A18Famide strongly labelled ganglia of the nervus terminalis and retinopetal fibres. We suggest that labelling of the nervus terminalis by antisera to substance P and FMRFamide occurs because of homologies between these antigens and a non-tachykinin, endogenous peptide that is similar to F8Famide and A18Famide.
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