The dopamine D-2 receptor subfamily in rat retina: ultrastructural immunogold and in situ hybridization studies

Dopamine, a major neurotransmitter in the vertebrate retina, is released fr om interplexiform cells and a restricted subset of amacrine cells. Dopamine effects vary between different retinal cell types, most likely due to diff erences in cell-specific receptor subtype expression. Identification of cel ls expressing receptors of the D-2-subfamily (D2R, D3R, D4R) on a light mic roscopical level has rendered equivocal results, and no information is as y et available concerning the subcellular distribution of receptor protein. I n 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 localiza tion of intense D2R-immunoreactivity in all dopaminergic cells immunoreacti ve for tyrosine hydroxylase (TH), homogeneous, less intense D2R-immunoreact ivity was also seen throughout the inner plexiform layer (IPL), Ultrastruct urally, many additional amacrine cell processes devoid of TH-immunoreactivi ty at all levels of the inner plexiform layer were immunoreactive. D2R-immu noreactivity was found mainly on intracellular vesicles, and immunoreactivi ty associated with the plasma membrane was always extrasynaptic, No D2R-imm unoreactivity was found in amacrine cell somata postsynaptic to the so-call ed dopaminergic 'ring endings'. Many D2R-mRNA reactive cells were observed throughout the inner nuclear layer. Morphologically, labelled cells resembl e amacrines and bipolars but not horizontal cells, Reactivity with splice v ariant-specific oligonucleotide probes suggested that the D2LR variant is t he predominant if not the only D2R isoform in the rat retina. D2R-mRNA reac tivity was not observed in other retinal layers, in particular not in photo receptor inner segments, which displayed D4R-mRNA reactivity. D3R-mRNA reac tivity was not detected. The results indicate that D-2-like responses are mediated through the D2R s ubtype, by an autoreceptor mechanism in dopaminergic cells, and by volume t ransmission in non-dopaminergic cells of the inner retina. D-2-like respons es in photoreceptors probably represent D4R activation.