Action Potentials Facilitate Lateral Gabaergic Inhibition of Ganglion Cells in the Salamander Retina

Abstract

Abstract: : Purpose: Steady illumination of a ganglion cell receptive field evokes GABAergic inhibition from amacrine cells. Since amacrine cells fire action potentials it is likely that the lateral propagation of the GABAergic signal would require action potentials in amacrine cell processes. We wanted to determine whether amacrine cells were immuno-positive for voltage-gated Na+ channels and whether blocking action potentials would also block lateral GABAergic inhibition. Methods: We used standard immunocytochemical techniques to stain frozen, fixed sections (20 µm thick) of larval tiger salamander retina with primary antibodies for Pan-specific, typeI or type II voltage-gated Na+ CCC Channels and conjugated a fluorescent secondary antibody for visualization using confocal microscopy. We also recorded whole-cell currents from ganglion cells in retinal slices from larval tiger salamanders. Synaptic currents were evoked by focal electrical stimulation of the outer retina from electrodes positioned directly above the ganglion cell and lateral to the recorded cell. GABAergic currents were isolated by holding the potential at 0 mV and perfusing with Strychnine (10µM) in all experiments. Results: Both Pan-specific and type II antibodies labeled numerous somas in the inner nuclear layer and nearly all cells in the ganglion cell layer while type I staining was weak or absent. Local and lateral stimulation (1-14µA, 2 msec duration) evoked large outward currents that were blocked by bicuculline (150µM). In contrast the laterally-evoked currents were reduced by TTX (.5µM) while the centrally-evoked currents remained unchanged. Conclusions: Amacrine cells, which stain for Pan-specific and type II voltage-gated sodium channels, generate action potentials that are essential for the propagation of GABAergic inhibition laterally across the inner plexiform layer.

Keywords: ganglion cells • amacrine cells • receptive fields