Amacrine cells of the retina are central nervous system interneurons. Knowledge of their morphology, connections and neurotransmitters could aid in the understanding of the local-circuit neurons in the brain. Amacrine cells of the rhesus monkey, Macaca mulatta, were studied in 38 retinas Golgi-impregnated as whole, flat preparations. By using criteria of dendritic morphology, span of arborization, and level of arborization in the inner plexiform layer, 26 types of amacrine cell ranging in size of dendritic span from 30 mm to 2 mm were identified. This study provides evidence for an unprecedented number of amacrine cell types in the primate retina. The similar morphologies of different types of amacrine cell types within a group suggests other common features within these groups such as neurotransmitter phenotype. Two types of amacrine cell (type 1CA and type 2CA) immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine (CA) synthetic pathway, are present in the retina of the rhesus monkey, Macaca mulatta. Although type 2CA cells are more numerous than type 1CA amacrines, type 2 cells contain 3.5 times less TH than the type 1 cells. Electron microscopy of retinal tissue immunoreactive for TH by the peroxidase-antiperoxidase (PAP) method revealed synaptic input from amacrine cells at conventional synapses, and bipolar cells at ribbon synapses onto the type 2CA amacrine cells. Curiously, although the synaptic input is comparatively easily found, the output synapses, or synapses of the type 2CA amacrine cells onto other neuronal elements, are rarely found. Some synapses of the type 2CA cells onto non-immunoreactive amacrine cells have been identified however. This unusual pattern of synaptic organization, with many identifiable input synapses but few morphologically characterizable output synapses, suggests a paracrine role for nonsynaptic release of dopamine by the type 2CA amacrine cells in the primate retina.
