title: Modulation of Tonic GABA Currents by Anion Channel and Connexin Hemichannel Antagonists.
abstract: Anion channels and connexin hemichannels are permeable to amino acid neurotransmitters. It is hypothesized that these conductive pathways release GABA, thereby influencing ambient GABA levels and tonic GABAergic inhibition. To investigate this, we measured the effects of anion channel/hemichannel antagonists on tonic GABA currents of rat hippocampal neurons. In contrast to predictions, blockade of anion channels and hemichannels with NPPB potentiated tonic GABA currents of neurons in culture and acute hippocampal slices. In contrast, the anion channel/hemichannel antagonist carbenoxolone (CBX) inhibited tonic currents. These findings could result from alterations of ambient GABA concentration or direct effects on GABAAreceptors. To test for effects on GABAAreceptors, we measured currents evoked by exogenous GABA. Coapplication of NPPB with GABA potentiated GABA-evoked currents. CBX dose-dependently inhibited GABA-evoked currents. These results are consistent with direct effects of NPPB and CBX on GABAAreceptors. GABA release from hippocampal cell cultures was directly measured using HPLC. Inhibition of anion channels with NPPB or CBX did not affect GABA release from cultured hippocampal neurons. NPPB reduced GABA release from pure astrocytic cultures by 21%, but the total GABA release from astrocytes was small compared to that of mixed cultures. These data indicate that drugs commonly used to antagonize anion channels and connexin hemichannels may affect tonic currents via direct effects on GABAAreceptors and have negligible effects on ambient GABA concentrations. Interpretation of experiments using NPPB or CBX should include consideration of their effects on tonic GABA currents.
authors.affiliation: Epilepsy Center of Excellence, VA Puget Sound HealthCare System, Seattle, WA, 98108, USA. cbr5@uw.edu.|Department of Neurology, University of Washington, Seattle, WA, 98105, USA. cbr5@uw.edu.|Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA. cbr5@uw.edu.
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Ransom
authors.affiliation: Department of Neurology, University of Washington, Seattle, WA, 98105, USA.
authors.forename: Zucheng
authors.initials: Z
authors.lastname: Ye
authors.affiliation: Epilepsy Center of Excellence, VA Puget Sound HealthCare System, Seattle, WA, 98108, USA.|Department of Neurology, University of Washington, Seattle, WA, 98105, USA.|Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA.
authors.forename: William J
authors.identifier: http://orcid.org/0000-0002-7528-5261
authors.initials: WJ
authors.lastname: Spain
authors.affiliation: Department of Neurology, University of Iowa, Iowa City, IA, 52242, USA.|Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, IA, USA.
authors.forename: George B
authors.initials: GB
authors.lastname: Richerson
mesh_terms: D062365:Aminobenzoates / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D002229:Carbenoxolone / Q000494:pharmacology; D002478:Cells, Cultured; D017630:Connexins / Q000037:antagonists & inhibitors* / Q000502:physiology*; D005260:Female; D058787:GABA-A Receptor Antagonists / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology; D008297:Male; D009579:Nitrobenzoates / Q000494:pharmacology; D009924:Organ Culture Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000502:physiology*; D050994:Voltage-Dependent Anion Channels / Q000037:antagonists & inhibitors* / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: C475624:5-nitro-2-(4-phenylbutylamino)benzoic acid; D062365:Aminobenzoates; D017630:Connexins; D058787:GABA-A Receptor Antagonists; D009579:Nitrobenzoates; D011963:Receptors, GABA-A; D050994:Voltage-Dependent Anion Channels; D005680:gamma-Aminobutyric Acid; D002229:Carbenoxolone
keywords: Ambient GABA; GABAA receptor; Seizure; Tonic inhibition