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Electrophysiological Properties of Mouse Horizontal Cell GABA_A Receptors

Institute of Biology, University of Oldenburg, D-26111 Oldenburg, Germany

Submitted 22 March 2004; accepted in final form 28 June 2004

GABA-induced currents have been characterized in isolated horizontal cells from lower vertebrates but not in mammalian horizontal cells. Therefore horizontal cells were isolated after enzymatical and mechanical dissociation of the adult mouse retina and visually identified. We recorded from horizontal cell bodies using the whole cell and outside-out configuration of the patch-clamp technique. Extracellular application of GABA induced inward currents carried by chloride ions. GABA-evoked currents were completely and reversibly blocked by the competitive GABA_A receptor antagonist bicuculline (IC₅₀ = 1.7 µM), indicating expression of GABA_A but not GABA_C receptors. Their affinity for GABA was moderate (EC₅₀ = 30 µM), and the Hill coefficient was 1.3, corresponding to two GABA binding sites. GABA responses were partially reduced by picrotoxin with differential effects on peak and steady-state current values. Zinc blocked the GABA response with an IC₅₀ value of 7.3 µM in a noncompetitive manner. Furthermore, GABA receptors of horizontal cells were modulated by extracellular application of diazepam, zolpidem, methyl 6,7-dimethoxy-4-ethyl--carboxylate, pentobarbital, and alphaxalone, thus showing typical pharmacological properties of CNS GABA_A receptors. GABA-evoked single-channel currents were characterized by a main conductance state of 29.8 pS and two subconductance states (20.2 and 10.8 pS, respectively). Kinetic analysis of single-channel events within bursts revealed similar mean open and closed times for the main conductance and the 20.2-pS subconductance state, resulting in open probabilities of 44.6 and 42.7%, respectively. The ratio of open to closed times, however, was significantly different for the 10.8-pS subconductance state with an open probability of 57.2%.

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