Shapes and distributions of the catecholamine-accumulating neurons in the rabbit retina.

Rabbit retinas were fixed with mixed aldehydes and examined for the fluorescence of catecholamines. Labeled cell bodies were present in the layer of the amacrine cells. A band of fluorescent processes was present in layer 1 of the inner plexiform layer. Weaker labeling was present in two deeper strata, one near the middle of the inner plexiform layer (presumably layer 3) and one at the junction of layers 4 and 5. Immunohistochemistry showed tyrosine hydroxylase (TH) to be present in the same cells and the same strata of the inner plexiform layer as the endogenous catecholamines. Exposing the retina to exogenous dopamine or norepinephrine resulted in stronger labeling in the middle and deep levels of the inner plexiform layer. At the same time a second population of amacrine cell bodies became visible. Catecholamine fluorescence contained in the amacrine cell bodies was used as a guide to their injection with Lucifer yellow CH. The filled dendritic arbors revealed two main types of cells. The type 1 cells are monostratified at the most distal level of the inner plexiform layer. They have relatively uncomplicated, radially branching dendritic trees. They are the cells densely stained by immunohistochemistry with antibodies against TH. The type 2 cells are tristratified, with minor branching in layer 1 of the inner plexiform layer and major branching in the two deeper sublayers. The descending dendrites follow a complicated course, and it is not uncommon for intermediate dendrites to cross between strata more than once. The relationship of the cells to their dendritic plexuses was further studied in retinas in which the aldehyde-induced fluorescence of catecholamines was photoconverted to a diaminobenzidine product. The type 1 cells were found to dominate the plexus of dendrites in layer 1 of the inner plexiform layer. The catecholaminergic plexuses in the middle and deep levels of the inner plexiform layer are formed by dendrites of the type 2 cells. The position of every type 1 cell was mapped in retinas stained with antibodies directed against TH. In one retina we counted 5,613 type 1 cells, distributed evenly across the retina. In another retina, all of the catecholamine-accumulating cells were counted. There were 9,058 with a distribution that peaks in the visual streak. The type 1 cells appear to be the dopaminergic cells previously studied by others and thought to regulate the flow of information from rod bipolar cells to ganglion cells. The low density and wide spread of type 2 cells suggests that they, too, perform a generalized control function, presumably a novel one that dictates their intricate, tristratified shape.