GABAergic synapses made by a retinal dopaminergic neuron.

In the retina, dopaminergic amacrine (interplexiform) cells establish multiple synapses on the perikarya 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 receptors contained within the postsynaptic active zone of these synapses in both mouse and rat retinas. We found that at the interface between the dendrites of the dopaminergic neurons and the AII amacrine cell perikarya clusters of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors are situated in register with aggregates of presynaptic organelles immunoreactive for GABA, the GABA vesicular transporter, and the vesicular monoamine transporter-2. D1 and D23 dopamine receptors, on the other hand, do not form clusters on the surface of the perikarya of AII amacrine cells. We suggest that the synapses between retinal dopaminergic neurons and AII amacrine cells are GABAergic and that both GABA and dopamine are released by the presynaptic endings. GABA acts on the ionotropic receptors clustered at the postsynaptic active zone, whereas dopamine diffuses to more distant, slower-acting metabotropic receptors.