Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABA(A) receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogeneous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signalling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA(A) receptors of A17 amacrines using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10%-90% rise time similar to 740 mu s) and double-exponential decays (tau similar to 4.5 ms [43% of amplitude]; tau(slow) similar to 22 ms). Ultra-fast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (tau(1) similar to 5.3 ms [55% of amplitude]; tau(2) similar to 48 ms [32% of amplitude]; tau(3) similar to 187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of similar to 21 and similar to 25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an alpha 1 beta gamma 2 subunit composition and the somatic receptors have an alpha 2 beta gamma 2 and/or alpha 3 beta gamma 2 composition. These results demonstrate the presence of synaptic GABA(A) receptors on A17s, which may play an important role in signal integration in these cells.