Retinal indoleamine accumulating neurons

A previously unknown set of neurons, characterized by their ability to accumulate indoleamines, has been identified in the retina of Cebus monkeys, rabbits, cats, pigeons, chicken, goldfish and lampreys. They are not demonstrable with presently available techniques in humans, Cynomolgus monkeys, cows, pigs and rats. The neurons are called indoleamine accumulating neuron and form a subset of amacrine cells, distinguishable from all other subsets of amacrines with known transmitter. By electron microscopy they have been shown to be contacted by bipolar cells in the dyad arrangement and to form reciprocal contacts on the bipolar cells. A procedure is available for destroying selectively the processes of the indoleamine accumulating neurons. The indoleamine accumulating neurons do not show any formaldehyde induced fluorescence in the normal animals or in animals in which the 5-hydroxytryptamine concentration in the brain has been elevated pharmacologically, and the 5-hydroxytryptamine concentration in the normal retina is too low to make it a likely neurotransmitter. What little is present is presumably in blood platelets in most cases (chicken may be an exception). The rate limiting enzyme in the 5-hydroxytryptamine synthesis, tryptophan hydroxylase, is not detectable in the retina. 5-hydroxytryptamine does not elicit any increase in retinal cyclic AMP. There is an energy dependent, high affinity uptake system for indoleamines but the effect of various inhibitors is different from that on the uptake into brain tissue. Several lines of evidence thus disfavour 5-hydroxytryptamine as a retinal neurotransmitter. Nevertheless, the active uptake of indoleamines suggests that the transmitter of the indoleamine accumulating neurons is an indole which, however, at present remains unidentified.