Ischemia but not anoxia evokes vesicular and Ca(2+)-independent glutamate release in the dorsal vagal complex in vitro.
Whole cell recordings of fura-2 dialyzed vagal neurons of brain stem slices were used to monitor interstitial glutamate accumulation within the dorsal vagal complex. Anoxia produced a sustained outward current (60 pA) and a moderate [Ca(2+)](i) rise (40 nM). These responses were neither mimicked by [1S,3R]-1-aminocyclo-pentane-1, 3-dicarboxylic acid nor affected by Ca(2+)-free solution, 6-cyano-7-nitroquino-xaline-2,3-dione (CNQX), 2-amino-5-phosphonovalerate (APV), or tetrodotoxin. Anoxia or cyanide in glucose-free saline (in vitro ischemia) as well as ouabain or iodoacetate elicited an initial anoxia-like [Ca(2+)](i) increase that turned after several minutes into a prominent Ca(2+) transient (0.9 microM) and inward current (-1.8 nA). APV plus CNQX (plus methoxyverapamil) inhibited this inward current as well as accompanying spontaneous synaptic activity, and reduced the secondary [Ca(2+)](i) rise to values similar to those during anoxia. Each of the latter drugs delayed onset of both ischemic current and prominent [Ca(2+)](i) rise by several minutes and attenuated their magnitudes by up to 40%. Ca(2+)-free solution induced a twofold delay of the ischemic inward current and suppressed the prominent Ca(2+) increase but not the initial moderate [Ca(2+)](i) rise. Cyclopiazonic acid or arachidonic acid in Ca(2+)-free saline delayed further the ischemic current, whereas neither inhibitors of glutamate uptake (dihydrokainate, D,L-threo-beta-hydroxyaspartate, L-transpyrrolidone-2,4-dicarboxylate) nor the Cl(-) channel blocker 5-nitro-2-(3-phenylpropyl-amino) benzoic acid had any effect. In summary, the response to metabolic arrest is due to activation of ionotropic glutamate receptors causing Ca(2+) entry via N-methyl-D-aspartate receptors and voltage-activated Ca(2+) channels. An early Ca(2+)-dependent exocytotic phase of ischemic glutamate release is followed by nonvesicular release, not mediated by reversed glutamate uptake or Cl(-) channels. The results also show that glycolysis prevents glutamate release during anoxia.
Kulik
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A
II. Physiologisches Institut, Universität Göttingen, Germany.
Trapp
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S
Ballanyi
K
K
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000200:Action Potentials; D000818:Animals; D002545:Brain Ischemia / Q000378:metabolism*; D001933:Brain Stem / Q000166:cytology / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D018118:Chloride Channels / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism* / Q000494:pharmacology; D007004:Hypoglycemic Agents / Q000494:pharmacology; D002534:Hypoxia, Brain / Q000378:metabolism*; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D013453:Sulfonylurea Compounds / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects; D014044:Tolbutamide / Q000494:pharmacology; D014630:Vagus Nerve / Q000166:cytology / Q000378:metabolism*
D018118:Chloride Channels; D004791:Enzyme Inhibitors; D018691:Excitatory Amino Acid Antagonists; D007004:Hypoglycemic Agents; D007211:Indoles; D013453:Sulfonylurea Compounds; D018698:Glutamic Acid; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate; D014044:Tolbutamide; C010969:gliquidone; D002118:Calcium; C000543:cyclopiazonic acid