Neurotransmitter localization in the skate retina

The retina of the skate (Raja clavata, R. radiata and R. oscellata) was studied by autoradiography following intraocular injections or incubations with [3H]GABA, [3H]isoguvacine, [3H]glycine, [3H]dopamine or [3H]5-hydroxytryptamine. Fluorescence immunohistochemistry was also used to demonstrate the endogenous content, accumulation, and retention of 5-hydroxytryptamine. The [3H]GABA was taken up by glia, and [3H]isoguvacine failed to appreciably label any neurons. [3H]Glycine was accumulated by amacrine cells, possibly of two subtypes. The [3H]dopamine was taken up by a few rare cells in the inner nuclear layer, which sent processes into the inner plexiform layer. Both autoradiography and immunohistochemistry showed 5-hydroxytryptamine to be efficiently accumulated by two types of cells in the inner nuclear layer: a bipolar cell type and an amacrine cell type. The morphology of the bipolar cells suggests they are of the ON depolarizing type. Immunohistochemistry also demonstrated the retention of accumulated 5-hydroxytryptamine by these two cell types, and that the bipolar cells contained far less endogenous 5-hydroxytryptamine than the amacrine cells did. The latter cell type can be presumed to use 5-hydroxytryptamine as its neurotransmitter. The results show the distribution of presumed glycinergic, dopaminergic and indoleaminergic neurons. They also show that there are two fundamentally distinct types of indoleamine neurons, a bipolar cell type with a low and an amacrine cell type with a high content of 5-hydroxytryptamine.