A major advantage of studying dopaminergic systems in the retina is that you can use an intact eye cup preparation instead of a brain slice preparation where lost connectivity can affect results. The retina is a laminar structure. At its back, stuck in the pigment epithelial layer reside photoreceptors. In the fast straight-through pathway the photoreceptors signal bipolar cells which in turn signal ganglion cells. All photoreceptors and bipolar cells release glutamate, like most excitatory transmission in the brain.
Ganglion cells transmit information through the optic nerve back to the brain.
Horizontal cells get input from a large number of photoreceptors and readout the average light intensity over a large area. They also feed back onto photoreceptors and modulate photoreceptor transmission to bipolar cells. Amacrine cells pick up input from many bipolar cells and can modulate transmission from bipolar to ganglion cells. Neurons in the retina do not spike except for ganglion cells. This probably affects the kind of ion channels that are expressed in retinal cells, for example.