Sequential release of GABA by exocytosis and reversed uptake leads to neuronal swelling in simulated ischemia of hippocampal slices.
GABA release during cerebral energy deprivation (produced by anoxia or ischemia) has been suggested either to be neuroprotective, because GABA will hyperpolarize neurons and reduce release of excitotoxic glutamate, or to be neurotoxic, because activation of GABA(A) receptors facilitates Cl- entry into neurons and consequent cell swelling. We have used the GABA(A) receptors of hippocampal area CA1 pyramidal cells to sense the rise of [GABA](o) occurring in simulated ischemia. Ischemia evoked, after several minutes, a large depolarization to approximately -20 mV. Before this "anoxic depolarization," there was an increase in GABA release by exocytosis (spontaneous IPSCs). After the anoxic depolarization, there was a much larger, sustained release of GABA that was not affected by blocking action potentials, vesicular release, or the glial GABA transporter GAT-3 but was inhibited by blocking the neuronal GABA transporter GAT-1. Blocking GABA(A) receptors resulted in a more positive anoxic depolarization but decreased cell swelling at the time of the anoxic depolarization. The influence of GABA(A) receptors diminished in prolonged ischemia because glutamate release evoked by the anoxic depolarization inhibited GABA(A) receptor function by causing calcium entry through NMDA receptors. These data show that ischemia releases GABA initially by exocytosis and then by reversal of GAT-1 transporters and that the resulting Cl- influx through GABA(A) receptor channels causes potentially neurotoxic cell swelling.
Allen
Nicola J
NJ
Department of Physiology, University College London, London WC1E 6BT, United Kingdom.
Rossi
David J
DJ
Attwell
David
D
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002545:Brain Ischemia / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D048429:Cell Size / Q000502:physiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005089:Exocytosis / Q000502:physiology*; D018682:GABA Agents / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D058787:GABA-A Receptor Antagonists; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism*; D066298:In Vitro Techniques; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D049990:Membrane Transport Modulators; D026901:Membrane Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors / Q000378:metabolism; D015091:beta-Alanine / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000493:pharmacokinetics
D002352:Carrier Proteins; D018691:Excitatory Amino Acid Antagonists; D018682:GABA Agents; D018756:GABA Antagonists; D050485:GABA Plasma Membrane Transport Proteins; D058787:GABA-A Receptor Antagonists; D008565:Membrane Proteins; D049990:Membrane Transport Modulators; D026901:Membrane Transport Proteins; D011963:Receptors, GABA-A; D016194:Receptors, N-Methyl-D-Aspartate; C493415:Slc6a1 protein, rat; D015091:beta-Alanine; D005680:gamma-Aminobutyric Acid; D002118:Calcium