The spatial distribution of excitatory and inhibitory inputs to ganglion cell dendrites in the tiger salamander retina

In response to focal stimuli, ganglion cell dendrites receive excitation over a relatively narrow extent of the inner plexiform layer (IPL). This excitation is embedded in 2 wider lateral inhibitory regions. Here we estimate the lateral dimensions of the inhibitory regions. Ganglion cells were whole-cell patch-clamped and dendrites were identified and located in retinal slices using Lucifer yellow in the pipettes. The spatial distribution of ganglion cell dendritic sensitivity was measured with puffs of transmitter substances applied at different distances along the dendrites. All ganglion cell dendrites were sensitive to glutamate, GABA, and glycine across their full extent. The responses to puffs decreased with lateral distance from the soma and were well fit by Gaussians. The responses to puffs of potassium showed a similar decrement with distance. Since potassium channels are probably uniformly distributed along the dendrites, the similarity in profiles suggests that receptor density is also uniform along the dendrites. The spatial distribution of responses of ganglion cells to excitatory and inhibitory synaptic inputs was measured by depolarizing local populations of bipolar terminals (and subsequently local populations of amacrine cells) with transretinal current (TRC). TRC-stimulating electrodes were displaced laterally, with respect to the ganglion cell soma, to generate response profiles. We estimated the dimensions of the inhibitory and excitatory signals received by the ganglion cells by removing the contributions of their dendrites, the stimulus, and other interneurons from the response profiles. The excitatory signal extended less than 100 μm, the approximate dimensions of the ganglion cell dendrites, and corresponds roughly to the width of the bipolar inputs. The GABAergic signal extended, on average, 253 μm and glycinergic signal extended, on average, 315 μm. These inhibitory signal dimensions correspond to the width of classes of amacrine cell processes measured in other studies.