title: Carrier-mediated GABA release activates GABA receptors on hippocampal neurons.
abstract: gamma-Aminobutyric acid (GABA) transporters are electrogenic and sodium-dependent and can operate in reverse when cells are depolarized or when there is reversal of the inward sodium gradient. However, the functional relevance of this phenomenon is unclear. We have examined whether depolarization induced by a physiologically relevant increase in extracellular [K+] leads to sufficient amounts of carrier-mediated GABA release to activate GABAA receptors on neurons. Patch-clamp recordings were made from rat hippocampal neurons in culture with solutions designed to isolate chloride currents in the recorded neuron. Pressure microejection was used to increase extracellular [K+] from 3 to 12 mM. After blockade of vesicular GABA release by removal of extracellular calcium, this stimulus induced a large conductance increase in hippocampal neurons [18.9 +/- 6.8 (SD) nS; n = 16]. This was blocked by the GABAA receptor antagonists picrotoxin and bicuculline and had a reversal potential that followed the Nernst potential for chloride, indicating that it was mediated by GABAA receptor activation. Similar responses occurred after block of vesicular neurotransmitter release by tetanus toxin. GABAA receptors also were activated when an increase in extracellular [K+] (from 3 to 13 mM) was combined with a reduction in extracellular [Na+] or when cells were exposed to a decrease in extracellular [Na+] alone. These results indicate that depolarization and/or reversal of the Na+ gradient activated GABA receptors via release of GABA from neighboring cells. We found that the GABA transporter antagonists 1-(4, 4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF89976A; 20-100 microM) and 1-(2-([(diphenylmethylene)amino]oxy)ethyl) -1, 2, 5, 6 - tetrahydro - 3 - pyridine - carboxylic acid hydrochloride (NO-711; 10 microM) both decreased the responses, indicating that the release of GABA resulted from reversal of the GABA transporter. We propose that carrier-mediated GABA release occurs in vivo during high-frequency neuronal firing and seizures, and dynamically modulates inhibitory tone.
authors.affiliation: Department of Neurology, Veteran's Affairs Medical Center, West Haven, Connecticut 06510, USA.
authors.forename: H L
authors.initials: HL
authors.lastname: Gaspary
authors.forename: W
authors.initials: W
authors.lastname: Wang
authors.forename: G B
authors.initials: GB
authors.lastname: Richerson
mesh_terms: D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002478:Cells, Cultured; D005333:Fetus; D018682:GABA Agents / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D058787:GABA-A Receptor Antagonists; D006624:Hippocampus / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009557:Nipecotic Acids / Q000494:pharmacology; D027361:Organic Anion Transporters; D010091:Oximes / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010852:Picrotoxin / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D001640:Bicuculline; D002352:Carrier Proteins; D018682:GABA Agents; D018756:GABA Antagonists; D050485:GABA Plasma Membrane Transport Proteins; D058787:GABA-A Receptor Antagonists; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009557:Nipecotic Acids; D027361:Organic Anion Transporters; D010091:Oximes; D010852:Picrotoxin; D011188:Potassium; D011963:Receptors, GABA-A; D012964:Sodium; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; C078793:NNC 711; C045598:N-(4,4-diphenyl-3-butenyl)nipecotic acid