Synaptic organization of the dopaminergic neurons in the retina of the cynomolgus monkey.

Dopaminergic neurons have previously been demonstrated in the retina of cynomolgus monkeys (Macaca fascicularis) by fluorescence microscopy. These neurons take up 5,6-dihydroxytryptamine, which alters their ultrastructure, and this technique has been used in the present study to identify the dopaminergic retinal neurons in the electron microscope. There are no indoleamine-accumulating neurons in the cynomolgus monkey retina to interfere with the analysis of the dopaminergic cells. The dopaminergic neurons have their cell bodies among those of the amacrine neurons in the inner nuclear layer. Their processes ramify mainly in the outermost sublayer of the inner plexiform layer. However, dopaminergic processes can be found occasionally to extend into the middle part of the inner nuclear layer and rarely into the innermost parts of the inner plexiform layer. All their output synapses are of the conventional kind. They appear to form synaptic connections only with other amacrine neurons, indicating that the dopaminergic amacrine cells in the retina of the cynomolgus monkey are interamacrine neurons.