title: Neocortical GABA release at high intracellular sodium and low extracellular calcium: an anti-seizure mechanism.
abstract: In epilepsy, the GABA and glutamate balance may be disrupted and a transient decrease in extracellular calcium occurs before and during a seizure. Flow Cytometry based fluorescence activated particle sorting experiments quantified synaptosomes from human neocortical tissue, from both epileptic and non-epileptic patients (27.7% vs. 36.9% GABAergic synaptosomes, respectively). Transporter-mediated release of GABA in human and rat neocortical synaptosomes was measured using the superfusion technique for the measurement of endogenous GABA. GABA release was evoked by either a sodium channel activator or a sodium/potassium-ATPase inhibitor when exocytosis was possible or prevented, and when the sodium/calcium exchanger was active or inhibited. The transporter-mediated release of GABA is because of elevated intracellular sodium. A reduction in the extracellular calcium increased this release (in both non-epileptic and epileptic, except Rasmussen encephalitis, synaptosomes). The inverse was seen during calcium doubling. In humans, GABA release was not affected by exocytosis inhibition, that is, it was solely transporter-mediated. However, in rat synaptosomes, an increase in GABA release at zero calcium was only exhibited when the exocytosis was prevented. The absence of calcium amplified the sodium/calcium exchanger activity, leading to elevated intracellular sodium, which, together with the stimulation-evoked intracellular sodium increment, enhanced GABA transporter reversal. Sodium/calcium exchange inhibitors diminished GABA release. Thus, an important seizure-induced extracellular calcium reduction might trigger a transporter- and sodium/calcium exchanger-related anti-seizure mechanism by augmenting transporter-mediated GABA release, a mechanism absent in rats. Uniquely, the additional increase in GABA release because of calcium-withdrawal dwindled during the course of illness in Rasmussen encephalitis. Seizures cause high Na(+) influx through action potentials. A transient decrease in [Ca(2+)]e (seizure condition) increases GABA transporter (GAT)-mediated GABA release because of elevated [Na(+)]i. This amplifies the Sodium-Calcium-Exchanger (NCX) activity, further increasing [Na(+)]i and GABA release. The reduction in [Ca(2+)]e triggers a GAT-NCX related anti-seizure mechanism by augmenting GAT-mediated GABA release. This mechanism, obvious in humans, is absent in rats.
authors.affiliation: Section of Clinical Neuropharmacology of the Department of Neurosurgery, University of Freiburg, Freiburg, Germany.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Rassner
authors.affiliation: Neurochemical Research Group, Department of Neurology, University of Lübeck, Lübeck, Germany.|Freiburg Institute for Advanced Studies (FRIAS), University of Freiburg, Freiburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Moser
authors.affiliation: Department of Medicine I, Medical Center - University of Freiburg, Freiburg, Germany.
authors.forename: Marie
authors.initials: M
authors.lastname: Follo
authors.affiliation: Section of Clinical Neuropharmacology of the Department of Neurosurgery, University of Freiburg, Freiburg, Germany.|Section of Neuroelectronic Systems of the Department of Neurosurgery, University of Freiburg, Freiburg, Germany.
authors.forename: Kevin
authors.initials: K
authors.lastname: Joseph
authors.affiliation: Department of Neurosurgery, ZU Brussels, Belgium.
authors.forename: Vera
authors.initials: V
authors.lastname: van Velthoven-Wurster
authors.affiliation: Section of Clinical Neuropharmacology of the Department of Neurosurgery, University of Freiburg, Freiburg, Germany.|Freiburg Institute for Advanced Studies (FRIAS), University of Freiburg, Freiburg, Germany.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Feuerstein
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000814:Aniline Compounds / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002648:Child; D002675:Child, Preschool; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D050485:GABA Plasma Membrane Transport Proteins / Q000378:metabolism; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D019579:Neocortex / Q000378:metabolism* / Q000473:pathology*; D009498:Neurotoxins / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D010647:Phenyl Ethers / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012640:Seizures / Q000473:pathology*; D012964:Sodium / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013744:Tetanus Toxin / Q000494:pharmacology; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology; D014316:Tritium / Q000378:metabolism; D014701:Veratridine / Q000494:pharmacology; D055815:Young Adult
chemical_list: C101670:2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate; D000814:Aniline Compounds; D004791:Enzyme Inhibitors; D050485:GABA Plasma Membrane Transport Proteins; D009498:Neurotoxins; D010647:Phenyl Ethers; C430918:SEA 0400; D013744:Tetanus Toxin; D014316:Tritium; D010042:Ouabain; D014701:Veratridine; D012964:Sodium; D013890:Thiourea; D002118:Calcium
keywords: GABA transporter; Na+/Ca2+ exchanger; Superfusion; epilepsy; fluorescence-activated sorting; human neocortical synaptosomes