Connexions between retinal neurons with identified neurotransmitters.

Dopaminergic and indoleamine accumulating neurons can be demonstrated both in the light and the electron microscopes. Considerable differences have been found between different animal species. There are two types of dopaminergic neurons, the interamacrine cells and the interplexiform cells. The interamacrine cells contact only other amacrine cells. They receive synapses from other amacrine cells which are likely to operate with, e.g. GABA or glycine as neurotransmitter. The dopamine turnover in the dopaminergic interamacrine cells is very rapidly activated by light. Dopaminergic interplexiform neurons are known only in teleost fish and New World monkeys. They have approximately the same contacts in the inner plexiform layer as the interamacrine cells, but, in addition, send processes to the outer plexiform layer and there contact both horizontal cells and bipolar cells. The function of the dopaminergic neurons has not been determined. The indoleamine accumulating amacrine neurons are in Cebus monkeys, cats and rabbits contacted by bipolar cells in dyads and form reciprocal synapses with them. They are also contacted by amacrine cells and make synapses on other with them. They are also contacted by amacrine cells and make synapses on other amacrine cells and on ganglion cells. The contacts are different in teleost fish, where the indoleamine accumulating cells mainly contact other amacrine cells only. The transmitter of the indoleamine accumulating neurons is debated in mammals but is most likely 5-hydroxytryptamine in other vertebrates.