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M Contini, E Raviola; GABAergic Synapses Made by Retinal Dopaminergic Neurons . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3785.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Retinal dopaminergic neurons synapse on the perikaryon of AII amacrines, the neurons that distribute rod signals to on- and off-cone bipolars. We used triple label immunocytochemistry and confocal microscopy to identify the transmitter molecules released at these synapses and the nature of the postsynaptic receptors. Methods: Retinas of adult rats were fixed for 15-17s in 2% formaldehyde under microwave irradiation. After freezing, radial and horizontal sections were stained with the indirect fluorescence antibody technique. Dopaminergic (DA) amacrines were identified with antibodies to tyrosine hydroxylase, AII amacrines with antibodies to parvalbumin. At the interface between DA cell dendrites and cell bodies of AII amacrines, we studied the distribution of the following molecules: α1 and α3 subunits of the GABAA receptor, GABA, vesicular GABA transporter (VGAT), vesicular monoamine transporter 2 (VMAT2), dopamine receptor D1 and dopamine receptor D2/3. Results: At the site of apposition between the dendrites of DA cells and the vitreal surface of the cell body of AII amacrines, clusters of GABAA receptors are situated in register with aggregates of organelles immunoreactive for GABA, VGAT and VMAT2. In contrast, D1 and D2/3 dopamine receptors are uniformly distributed throughout the surface of AII amacrine cells. These findings support the notion that the DA-to-AII amacrine cell synapses are GABAergic because: (i) they are located within the rings formed by the dendrites of DA cells around the collar of AII amacrines, both very distinctive regions of the pre- and postsynaptic neurons; (ii) they represent the only synapses received by the perikarya of AII amacrines; (iii) these perikarya do not contain presynaptic active zones; (iv) the clusters of GABAA receptor subunits have the expected size for postsynaptic active zones; (v) the three presynaptic markers (GABA, VGAT and VMAT2) are exclusively expressed in DA cells. Conclusions: At their synapses, retinal dopaminergic neurons release both GABA and dopamine. GABA acts on ionotropic receptors confined to the postsynaptic membrane, whereas dopamine diffuses to more distant, slower-acting metabotropic receptors. Supported by NIH grant EY01344.
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