DIFFERENT CONTRIBUTIONS OF GABAA AND GABAC RECEPTORS TO ROD AND CONE BIPOLAR CELLS IN A RAT RETINAL SLICE PREPARATION. Thomas Euler and Heinz W„ssle. Max-Planck-Institut fr Hirnforschung, Deutschordenstr. 46, D-60528 Frankfurt, Germany.
APStracts 4:360N, 1997.
ABSTRACT
Whole cell currents were recorded from rod and cone bipolar cells in a slice preparation of the rat retina. Use of the Gramicidin D perforated-patch technique prevented loss of intracellular compounds. The recorded cells were identified morphologically by injection with Lucifer Yellow. During the recordings the cells were synaptically isolated by extracellular cobalt. To distinguish the GABA receptors pharmacologically, the GABA_A_ receptor antagonist, Bicuculline, and the GABA_C receptor antagonist, 3-APMPA (3- Aminopropyl(methyl)phosphinic acid), were used. In all bipolar cells tested, application of GABA-induced postsynaptic chloride currents which hyperpolarized the cells from their resting potential of about -40 mV. GABA was applied at different concentrations to allow for the different affinity of GABA at GABA_A_ and GABA_C_ receptors. At a GABA concentration of 25?_ć_M, in the case of rod bipolar cells, about 70% of the current was found to be mediated by GABA_C_ receptors. In the case of cone bipolar cells, only about 20% of the current was mediated by GABA_C receptors. Furthermore, this GABA_C_-mediated fraction varied among the different morphological types of cone bipolar cells, supporting the hypothesis of distinct functional roles for the different types of cone bipolar cells. There is evidence that the efficacy of GABA_C_ receptors is modulated by glutamate through metabotropic glutamate receptors (Feigenspan and Bormann, 1994b). We tested this hypothesis by applying agonists of mGluR1/5 receptors to rod bipolar cells. The specific agonist (ń)-trans-azetidine-2,4-dicarboxylic acid (tADA) and the potent mGluR agonist quisqualic acid (QA) reduced the amplitude of the GABA_C_ responses by 10 to 30 percent. This suggests a functional role for the modulation of GABA_C receptors by the metabotropic glutamate receptors mGluR1/5. 

Received 20 June 1997; accepted in final form 4 December 1997.
APS Manuscript Number J511-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997