Synaptic inputs to identified color-coded amacrine and ganglion cells in the turtle retina.

Previous studies have proposed models of the specific synaptic circuitry responsible for color processing in the turtle retina. To determine the accuracy of these models of the circuits underlying color opponency in the inner retina of the turtle (Pseudemys scripta), we have studied the physiology, morphology, and synaptic connectivity of identified amacrine and ganglion cells. These cells were first characterized electrophysiologically and were then stained with horseradish peroxidase. Postembedding electron immunocytochemistry for gamma-aminobutyric acid (GABA) and glycine was used to reveal the neurochemical identity of their synaptic inputs. The red-ON/green, blue-OFF small-field ganglion cell, classified as G24, branched primarily in strata S1, S4, and S5 of the inner plexiform layer (IPL). Ganglion cell G24 showed a complex receptive field organized into a red-ON center surrounded by an inhibitory region, which, in turn, was surrounded by a second excitatory region. Only the center responses were color opponent. The red-OFF/green, blue-ON large-field, stellate amacrine cell, classified as A23b, stratified exclusively in stratum S2, near the S2/S3 border. The color-coded center was surrounded by a luminosity, red-sensitive surround. Synaptic input to G24 and A23b was dominated by amacrine cells (89% and 87%, respectively). G24 received significant input from amacrine cell profiles with GABA (13% of total) as well as glycine (11% of total) immunoreactivity, mostly in the proximal stratum S5 of the IPL (64% and 67% of the total GABA- and glycine-immunoreactive input, respectively). Bipolar cell synaptic input was also found predominantly in S4 and S5 (89%). In contrast, we found no glycine-immunoreactive input to A23b, and the density of the GABA-immunoreactive amacrine cell synaptic input revealed a central (15%) to peripheral (3%) gradient within the dendritic tree. The results of the present study support the previous models of the synaptic circuitry responsible for color-opponent signal processing in the inner retina of the turtle.