Two physiologically distinct, HRP-marked turtle retinal ganglion cells were examined for their morphology, GABAergic, glycinergic, and bipolar cell synaptic inputs, using electron-microscopic autoradiography and postembedding immunocytochemistry. One cell was a color-opponent, transient ON/OFF ganglion cell. Its center response to red was a sustained hyperpolarization, and its center response to green was a depolarization with increased spiking at onset. The HRP-injected cell most resembled G6, from previous Golgi-impregnation studies (Kolb, 1982; Kolb et al., 1988). It was a narrow-field bistratified cell, whose two broad dendritic strata peaked at approximately levels L20–25 (sublamina a) and L60 (sublamina b) of the inner plexiform layer. Bipolar cell synapses onto G6 were found evenly distributed between its distal and proximal dendritic strata, spanning L20–75. These inputs probably originated from several different bipolar cells, reflecting the complexity of the center response. GABAergic inputs were found onto both the distal and proximal strata, from near L20–L85. Only a few glycinergic inputs, confined to dendrites at L50–70, were observed.
A second ganglion cell type that we physiologically characterized and HRP-injected had sustained ON-center, sustained OFF-surround responses. Two examples were studied; both were bistratified in sublamina b, near L60–70 and L85–100, with branches up to near L40. They resembled G10, from previous Golgi-impregnation studies (Kolb, 1982; Kolb et al., 1988). One cell was partially reconstructed to look at the distributions of GABAergic and glycinergic amacrine cell, and bipolar cell inputs. Although synapses from bipolar cells were equally divided between the two major dendritic strata of G10, the inputs to the distal stratum were close to the soma, and the inputs to the more proximal stratum were on the peripheral dendrites. This arrangement may reflect input from two distinct types of ON-bipolar cell. GABAergic and glycinergic inputs to G10 costratified to both strata and to the distal branches; but where glycinergic inputs were found distributed throughout the arbor, GABAergic inputs appeared to be confined to peripheral dendrites. We hypothesize on the neural elements involved and the circuitry that may underlie the physiologically recorded receptive fields of these two very different ganglion cell types in the turtle retina.