Characterization of the spontaneous synaptic activity of amacrine cells in the mouse retina

Amacrine cells are a heterogeneous class of interneurons that modulate the transfer of the light signals through the retina. In addition to ionotropic glutamate receptors, amacrine cells express two types of inhibitory receptors, GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs). To characterize the functional contribution of these different receptors, spontaneous postsynaptic currents (sPSCs) were recorded with the whole cell configuration of the patch-clamp technique in acutely isolated slices of the adult mouse retina. All amacrine cells investigated (n=47) showed spontaneous synaptic activity. In six amacrine cells, spontaneous excitatory postsynaptic currents could be identified by their sensitivity to kynurenic acid. They were characterized by small amplitudes [mean -13.7 +/-1.5 (SE) pA] and rapid decay kinetics (mean tau. 1.35 +/-0.16 ms). In contrast, the reversal potential of sPSCs characterized by slow decay kinetics (amplitude-weighted time constant, tau (w), >4 ms) was dependent on the intracellular Cl- concentration (n=7), indicating that they were spontaneous inhibitory postsynaptic currents (sIPSCs). In 14 of 34 amacrine cells sIPSCs were blocked by bicuculline (10 muM), indicating that they were mediated by GABA(A)Rs. Only four amacrine cells showed glycinergic sIPSCs that were inhibited by strychnine (1 muM). In one amacrine cell, sIPSCs mediated by GABAARs and GlyRs were found simultaneously. GABAergic sIPSCs could be subdivided into one group best fit by a monoexponential decay function and another biexponentially decaying group. The mean amplitude of GABAergic sIPSCs (-42.1 +/-5.8 pA) was not significantly different from that of glycinergic sIPSCs (-28.0 +/-8.5 pA). However, GlyRs (mean T10/90: 2.4 +/-0.08 ms) activated significantly slower than GABA(A)Rs (mean T10/90: 1.2 +/-0.03 ms). In addition, the decay kinetics of monoexponentially decaying GABA(A)Rs (mean tau (w): 20.3 +/-0.50), biexponentially decaying GABA(A)Rs (mean tau (w): 30.7 +/-0.95), and GlyRs (mean tau (w) = 25.3 +/-1.94) were significantly different. These differences in the activation and decay kinetics of sIPSCs indicate that amacrine cells of the mouse retina express at least three types of functionally different inhibitory receptors: GlyRs and possibly two subtypes of GABA(A)Rs.