A system of indoleamine-accumulating neurons in the rabbit retina

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Abstract
The indoleamine-accumulating neurons of the rabbit retina were labeled by intraocular injection of 5,7-dihydroxytryptamine (5,7-DHT). The retinas were fixed with 2.5% paraformaldehyde and 0.2% glutaraldehyde and inspected by fluorescence microscopy. Five kinds of cell accumulated the indoleamine. They were labeled to essentially the same brightness and remained so despite variations in the concentration at which 5,7-DHT had been applied or the duration of its application. Experiments in which 5,7-DHT was applied to retinas incubated in vitro gave identical results. To see the whole shape of the cells, we visually guided micropipettes to the fluorescent cell bodies and injected the cells with Lucifer yellow CH. To study the cells as a population, we used a new method in which the fluorescence of 5,7-DHT is photochemically converted to an insoluble diaminobenzidine product. The dendrites of all of the indoleamine-accumulating cells were then simultaneously visible. Used together, these techniques revealed an interrelated system of indoleamine-accumulating neurons. All of the cells contribute processes to a dendritic plexus that lies at the inner margin of the inner plexiform layer. The plexus is roughly 4 micron thick. It is pierced by the stalks of the Muller cells and is occasionally interrupted by ganglion cell bodies, where they extend above the average margin of the ganglion cell layer. Otherwise it fills much of the space at the junction of the plexiform and ganglion cell layers. The type 1 and type 2 cells are amacrine cells with cell bodies at the inner margin of the inner nuclear layer. They have 5–8 radially branching primary dendrites which extend horizontally across the inner plexiform layer before descending to join the dendritic plexus. They differ from each other in cell body shape, dendritic morphology, and the course of their dendrites within the inner plexiform layer. Each has a “displaced” counterpart, with a morphology similar to the type 1 or type 2 cell but with a cell body located in the ganglion cell layer. The displaced cells are separate functional elements because, in contrast to the type 1 and type 2 cells, they have no dendrites (and hence can have no synaptic connections) in the outer part of the inner plexiform layer. The fifth kind of cell (type 3) appears not to have been described before. Its cell body is located at the outer margin of the inner nuclear layer.(ABSTRACT TRUNCATED AT 400 WORDS)