Two types of tyrosine hydroxylase-immunoreactive amacrine cell in the rhesus monkey retina

Two types of amacrine cell immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine (CA)-synthetic pathway, are described in the rhesus monkey retina with the indirect-immunofluorescent method. These 2 types of neuron differ in soma size, plane of arborization in the inner plexiform layer, levels of the enzyme TH as quantified by microspectrofluorometry, and population density. Type 1 CA cells have comparatively large cell bodies almost exclusively in the innermost row of the inner nuclear layer; their processes arborize in the outermost stratum of the inner plexiform layer; they give rise to fine predominantly radially oriented fibers in the inner nuclear layer; and there are about 26 type 1 CA cells/mm2. Type 2 CA amacrine cells have relatively small cell bodies located in the inner nuclear layer (44.4%), the inner plexiform layer (35.6%) and the ganglion cell layer (20%), and their processes arborize in the center of the inner plexiform layer. Although type 2 CA amacrine cells are more numerous (35 cells/mm2) than type 1 CA cells, type 1 CA amacrine cells are 3.5 x brighter than type 2 CA cells and therefore likely to contain 3.5 X more TH. Thus the primate retina contains 2 distinct catecholaminergic neuronal pathways that could have different functional roles in vision.