Pharmacological analysis of the activation and receptor properties of the tonic GABA cr current in retinal bipolar cell terminals

Abstract

GABAergic inhibition in the central nervous system (CNS) can occur via rapid, transient postsynaptic currents and via a tonic increase in membrane conductance, mediated by synaptic and extrasynaptic GABA A receptors (GABA ARs) respectively. Retinal bipolar cells (BCs) exhibit a tonic current mediated by GABA CRs in their axon terminal, in addition to synaptic GABA AR and GABA CR currents, which strongly regulate BC output. The tonic GABA CR current in BC terminals (BCTs) is not dependent on vesicular GABA release, but properties such as the alternative source of GABA and the identity of the GABA CRs remain unknown. Following a recent report that tonic GABA release from cerebellar glial cells is mediated by Bestrophin 1 anion channels, we have investigated their role in non-vesicular GABA release in the retina. Using patch-clamp recordings from BCTs in goldfish retinal slices, we find that the tonic GABA CR current is not reduced by the anion channel inhibitors NPPB or flufenamic acid but is reduced by DIDS, which decreases the tonic current without directly affecting GABA CRs. All three drugs also exhibit non-specific effects including inhibition of GABA transporters. GABA CR ρ subunits can form homomeric and heteromeric receptors that differ in their properties, but BC GABA CRs are thought to be ρ1-ρ2 heteromers. To investigate whether GABA CRs mediating tonic and synaptic currents may differ in their subunit composition, as is the case for GABA ARs, we have examined the effects of two antagonists that show partial ρ subunit selectivity: picrotoxin and cyclothiazide. Tonic and synaptic GABA CR currents were differentially affected by both drugs, suggesting that a population of homomeric ρ1 receptors contributes to the tonic current. These results extend our understanding of the multiple forms of GABAergic inhibition that exist in the CNS and contribute to visual signal processing in the retina. © 2011 Jones, Palmer.