GABA-ergic and glycinergic pathways in the inner plexiform layer of the goldfish retina

GABA-ergic and glycinergic circuitry in the inner plexiform layer of the goldfish retina was evaluated by electron microscopic autoradiography of 3H-GABA and 3H-glycine uptake, combined with retrograde horseradish peroxidase (HRP) labeling of ganglion cells. GABA-ergic and glycinergic synapses were found on labeled ganglion cells throughout the inner plexiform layer. This reinforces the idea that physiological evidence of GABA-ergic and glycinergic influence on a variety of ganglion cells in goldfish and carp often reflects direct inputs. Double-labeled synapses are presented as evidence of direct type Ab amacrine cell input to on-center ganglion cells. At least one population of type Aa sustained-off GABA-ergic amacrine cell is proposed, on the basis of profuse GABA-ergic inputs onto bipolar cells in sublamina a. Similar GABA-labeled profiles are shown to synapse onto HRP-labeled probable off-center ganglion cells. Thus GABA-ergic amacrine cells not only provide the predominant feedback to depolarizing (on-center) and hyperpolarizing (off-center) bipolar cells but also provide feed-forward inputs to on- and off-center ganglion cells. Large-caliber GABA-ergic dendrites present in both sublaminae a and b resemble those expected of a previously described bistratified, transient amacrine cell. These processes synapse onto HRP-labeled ganglion cell profiles in both sublaminae. Two morphologies of glycinergic amacrine cell are proposed on the basis of light microscopic autoradiography, 1) the previously described small pyriform cell and 2) a multipolar cell. The differential connectivity of the glycinergic neurons described, however, remains indistinguishable. Whereas abundant glycinergic inputs to ganglion cells occur throughout the inner plexiform layer, contacts between glycinergic profiles and bipolar cells are extremely rare. Therefore, interpreting the meaning of glycinergic input to ganglion cells will require further study of amacrine cell circuitry.