title: The K+-Cl cotransporter KCC2 promotes GABAergic excitation in the mature rat hippocampus.
abstract: GABAergic excitatory [K(+)](o) transients can be readily evoked in the mature rat hippocampus by intense activation of GABA(A) receptors (GABA(A)Rs). Here we show that these [K(+)](o) responses induced by high-frequency stimulation or GABA(A) agonist application are generated by the neuronal K(+)-Cl() cotransporter KCC2 and that the transporter-mediated KCl extrusion is critically dependent on the bicarbonate-driven accumulation of Cl() in pyramidal neurons. The mechanism underlying GABAergic [K(+)](o) transients was studied in CA1 stratum pyramidale using intracellular sharp microelectrodes and extracellular ion-sensitive microelectrodes. The evoked [K(+)](o) transients, as well as the associated afterdischarges, were strongly suppressed by 0.5-1 mm furosemide, a KCl cotransport inhibitor. Importantly, the GABA(A)R-mediated intrapyramidal accumulation of Cl(), as measured by monitoring the reversal potential of fused IPSPs, was unaffected by the drug. It was further confirmed that the reduction in the [K(+)](o) transients was not due to effects of furosemide on the Na(+)-dependent K(+)-Cl() cotransporter NKCC1 or on intraneuronal carbonic anhydrase activity. Blocking potassium channels by Ba(2+) enhanced [K(+)](o) transients whereas pyramidal cell depolarizations were attenuated in further agreement with a lack of contribution by channel-mediated K(+) efflux. The key role of the GABA(A)R channel-mediated anion fluxes in the generation of the [K(+)](o) transients was examined in experiments where bicarbonate was replaced with formate. This anion substitution had no significant effect on the rate of Cl() accumulation, [K(+)](o) response or afterdischarges. Our findings reveal a novel excitatory mode of action of KCC2 that can have substantial implications for the role of GABAergic transmission during ictal epileptiform activity.
authors.affiliation: Department of Biosciences, PO Box 65, FI-00014 University of Helsinki, Helsinki, Finland.
authors.forename: Tero
authors.initials: T
authors.lastname: Viitanen
authors.forename: Eva
authors.initials: E
authors.lastname: Ruusuvuori
authors.forename: Kai
authors.initials: K
authors.lastname: Kaila
authors.forename: Juha
authors.initials: J
authors.lastname: Voipio
mesh_terms: D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology; D001464:Barium / Q000494:pharmacology; D001639:Bicarbonates / Q000378:metabolism; D056547:CA1 Region, Hippocampal / Q000378:metabolism; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D002712:Chlorides / Q000378:metabolism; D004232:Diuretics / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D004827:Epilepsy / Q000503:physiopathology; D005665:Furosemide / Q000494:pharmacology; D018755:GABA Agonists / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007539:Isonicotinic Acids / Q000494:pharmacology; D008839:Microelectrodes; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology; D017966:Pyramidal Cells / Q000378:metabolism; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects / Q000502:physiology; D013050:Spectrometry, Fluorescence; D027981:Symporters / Q000187:drug effects / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000502:physiology*; D000096922:K Cl- Cotransporters
chemical_list: D000927:Anticonvulsants; D001464:Barium; D001639:Bicarbonates; D002257:Carbonic Anhydrase Inhibitors; D002712:Chlorides; D004232:Diuretics; D005665:Furosemide; D018755:GABA Agonists; D007539:Isonicotinic Acids; D011188:Potassium; D011963:Receptors, GABA-A; D027981:Symporters; D005680:gamma-Aminobutyric Acid; D000096922:K Cl- Cotransporters; C015300:isoguvacine