Characterization of the spontaneous synaptic activity of amacrine cells inthe 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 recept ors, GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs). To charac terize the functional contribution of these different receptors, spontaneou s postsynaptic currents (sPSCs) were recorded with the whole cell configura tion of the patch-clamp technique in acutely isolated slices of the adult m ouse retina. All amacrine cells investigated (n=47) showed spontaneous syna ptic activity. In six amacrine cells, spontaneous excitatory postsynaptic c urrents could be identified by their sensitivity to kynurenic acid. They we re 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 poten tial of sPSCs characterized by slow decay kinetics (amplitude-weighted time constant, tau (w), >4 ms) was dependent on the intracellular Cl- concentra tion (n=7), indicating that they were spontaneous inhibitory postsynaptic c urrents (sIPSCs). In 14 of 34 amacrine cells sIPSCs were blocked by bicucul line (10 muM), indicating that they were mediated by GABA(A)Rs. Only four a macrine cells showed glycinergic sIPSCs that were inhibited by strychnine ( 1 muM). In one amacrine cell, sIPSCs mediated by GABAARs and GlyRs were fou nd simultaneously. GABAergic sIPSCs could be subdivided into one group best fit by a monoexponential decay function and another biexponentially decayi ng 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 kinet ics of monoexponentially decaying GABA(A)Rs (mean tau (w): 20.3 +/-0.50), b iexponentially decaying GABA(A)Rs (mean tau (w): 30.7 +/-0.95), and GlyRs ( mean tau (w) = 25.3 +/-1.94) were significantly different. These difference s in the activation and decay kinetics of sIPSCs indicate that amacrine cel ls of the mouse retina express at least three types of functionally differe nt inhibitory receptors: GlyRs and possibly two subtypes of GABA(A)Rs.