Electron microscopy of aminergic retinal neurons.

The synaptic connections of the dopaminergic retinal neurons of Cynomolgus monkeys and rabbits were studied in the electron microscope. In the Cynomolgus monkeys, a labelling method with 5,6-dihydroxytryptamine was used, whereas an autoradiographic technique with a monoamine oxidase inhibitor and (3H)-dopamine was used in the rabbits. The labelling method with 5,6-dihydroxytryptamine altered the normal ultrastructure of the dopaminergic neurons, but their synaptic structures were still easily recognized. The autoradiographic method left the ultrastructure of the labelled neurons unchanged, and large dense-cored vesicles could therefore be seen to be associated with the dopaminergic neurons in the rabbits. Other morphological characteristics could also be seen. The distribution of dopaminergic perikarya in the inner nuclear layer and dopaminergic processes in the inner plexiform layer was consistent with that of amacrine cells in both Cynomolgus monkeys and rabbits. Furthermore, the dopaminergic processes had output synapses of the conventional kind characteristic of amacrine cells. In both animals, the dopaminergic processes were presynaptic to amacrine cell bodies and both pre- and postsynaptic to other amacrine cell processes. The number of output synapses was larger than the number of input synapses. Serial and convergent synaptic arrangements were common. The synaptic organization of the indoleamine-accumulating neurons in the retinae of rabbits, cats, goldfish and carp were also investigated in the electron microscope. The neurons were labelled with 5,6-dihydroxytryptamine after the otherwise interfering dopaminergic processes had been destroyed. The indoleamine-accumulating neurons of the three species had a distribution of perikarya and processes like that of amacrine cells. They also had synapses of the conventional kind. In the rabbit and the cat, the indoleamine-accumulating processes had mainly reciprocal synapses with rod bipolar terminals in the innermost part of the inner plexiform layer. They were, however, also also pre- and postsynaptic to other amacrine cells, and in the cat, they had synaptic contacts with cone bipolar cells and the dendrites of ganglion cells. The indoleamine-accumulating neurons of the goldfish and carp were mainly pre- and postsynaptic to other amacrine cells, even if contacts with bipolar terminals were also seen. In the retinae of goldfish and carp, a subset of indoleamine-accumulating processes were discovered in the middle part of the inner plexiform layer that had a previously not recognized type of conventional output synapse with two postsynaptic elements. This synapse was called a branched conventional synapse. It was found and investigated in the retinae of normal goldfish and carp. Its presynaptic structures were found to be similar to those seen in certain conventional synapses with presynaptic "globular clusters" and only one postsynaptic element.