The dopamine D2 receptor subfamily in rat retina: ultrastructural immunogold and in situ hybridization studies.

Dopamine, a major neurotransmitter in the vertebrate retina, is released from interplexiform cells and a restricted subset of amacrine cells. Dopamine effects vary between different retinal cell types, most likely due to differences in cell-specific receptor subtype expression. Identification of cells expressing receptors of the D2-subfamily (D2R, D3R, D4R) on a light microscopical level has rendered equivocal results, and no information is as yet available concerning the subcellular distribution of receptor protein. In the present study, D2R and D2/3R subtype-specific antisera, and D2R-, D3R- and D4R-specific oligonucleotide probes were used for ultrastructural and in situ hybridization analyses of the receptor subtype distribution in the rat retina. Light and electron microscopy showed that in addition to the known localization of intense D2R-immunoreactivity in all dopaminergic cells immunoreactive for tyrosine hydroxylase (TH), homogeneous, less intense D2R-immunoreactivity was also seen throughout the inner plexiform layer (IPL). Ultrastructurally, many additional amacrine cell processes devoid of TH-immunoreactivity at all levels of the inner plexiform layer were immunoreactive. D2R-immunoreactivity was found mainly on intracellular vesicles, and immunoreactivity associated with the plasma membrane was always extrasynaptic. No D2R-immunoreactivity was found in amacrine cell somata postsynaptic to the so-called dopaminergic 'ring endings'. Many D2R-mRNA reactive cells were observed throughout the inner nuclear layer. Morphologically, labelled cells resemble amacrines and bipolars but not horizontal cells. Reactivity with splice variant-specific oligonucleotide probes suggested that the D2LR variant is the predominant if not the only D2R isoform in the rat retina. D2R-mRNA reactivity was not observed in other retinal layers, in particular not in photoreceptor inner segments, which displayed D4R-mRNA reactivity. D3R-mRNA reactivity was not detected. The results indicate that D2-like responses are mediated through the D2R subtype, by an autoreceptor mechanism in dopaminergic cells, and by volume transmission in non-dopaminergic cells of the inner retina. D2-like responses in photoreceptors probably represent D4R activation.