Retinal organization: Neurotransmitters as physiological probes.

Neurons in the goldfish retina which use GABA, dopamine and glycine as neurotransmitters have been tentatively identified. Our studies show that H1 cone horizontal cells, which receive input predominantly from red-sensitive cones, fulfill most of the criteria to be identified as GABA-ergic. In contrast, all the other types of horizontal cells in this retina are not GABA-ergic. Additionally, at least some Ab pyriform amacrine cells, which are sustained and red-depolarizing with synaptic terminals contacting center-depolarizing bipolar cells in sublamina b of the inner plexiform layer, may be GABA-ergic. Although identifications of putative dopaminergic and glycinergic neurons are far less complete, our results and those of other investigators suggest that: (1) one type of interplexiform cell (I1) may be dopaminergic; (2) another type of interplexiform cell (12), which is morphologically distinct from I1, may be glycinergic; and (3) at least some sustained, red-hyperpolarizing amacrine cells with synaptic terminals in sublamina a of the inner plexiform layer may also be glycinergic. Furthermore, we show in this paper how the use of these neurotransmitters as physiological probes lead to further understanding of transmitter-specific pathways and functional organization of the retina.