The dopaminergic amacrine cell

The detailed morphology of the dopaminergic amacrine cell type has been characterized in the macaque monkey retina by intracellular injection of horseradish peroxidase (HRP). This cell type was recognized by its large soma in an in vitro, wholemount preparation of the retina stained with the fluorescent dye, acridine orange. HRP‐fills revealed a large, sparsely branching, spiny dendritic tree and a number of extremely thin, axon‐like processes that arose from the soma and proximal dendrites. The axon‐like processes were studded with distinct varicosities and were traced for up to 3 mm beyond the dendritic tree. The true lengths of the axon‐like processes were greater than 3 mm, however, because the HRP reaction product consistently diminished before an endpoint was reached. Both the dendrites and the axon‐like processes were narrowly stratified close to the outer border of the inner plexiform layer, although in a few cases single axon‐like processes projected into the outer nuclear and outer plexiform layers.

The HRP‐filled amacrines appeared equivalent to a subpopulation of neurons that are intensely immunoreactive for tyrosine hydroxylase (TH). TH‐immunoreactive cells showed a nearly identical soma size and dendritic field size range, the same pattern of dendritic branching and spiny morphology, and also gave rise to distinct axon‐like processes from both the soma and proximal dendrites.

To test this correspondence more directly, the large acridine stained cells were injected with Lucifer Yellow and the retina was subsequently processed for TH immunoreactivity using diaminobenzidine as the chromagen. In all cases Lucifer Yellow injected cells also showed intense TH immunoreactivity.

Spatial densities of the TH amacrine cells were therefore used to calculate coverage factors for the dendritic trees and for the axon‐like components of the HRP‐filled cells. The axon‐like processes showed a coverage factor of at least 300, about 100 times that of the dendritic fields. This great overlap could be directly observed in TH‐immunoreacted retinal wholemounts as a dense plexus of fine, varicose processes. The density of the TH plexus is greater than the density predicted from the lengths (1–3 mm) of the HRP‐filled axon‐like processes however, and suggests that the axon‐like processes have an actual length of about 4–5 mm.

The dual morphology of the dopaminergic amacrine, coupled with the previous studies of identified dopaminergic synapses, suggests the hypothesis that the varicosities on the axon‐like processes are the major source of synaptic output, and that the spiny dendrites are the major recipients of synaptic input from cone bipolar cells and other amacrine cells.

The rare extension of the axon‐like processes into the outer plexiform layer, and current understanding of the morphology of dopaminergic interplexiform cells in teleost fish, supports the hypothesis that the dopaminergic amacrine and interplexiform cells represent a single cell type in which there is quantitative variation across species in the projection pattern of the axon‐like processes.