The Journal of Neurophysiology Vol. 83 No. 5 May 2000, pp. 2905-2915
Copyright ©2000 by the American Physiological Society

Ischemia But Not Anoxia Evokes Vesicular and Ca²⁺-Independent Glutamate Release In the Dorsal Vagal Complex In Vitro

II. Physiologisches Institut, Universität Göttingen, D-37073 Göttingen, Germany

Kulik, Anna, Stefan Trapp, and Klaus Ballanyi. Ischemia But Not Anoxia Evokes Vesicular and Ca²⁺-Independent Glutamate Release In the Dorsal Vagal Complex In Vitro. J. Neurophysiol. 83: 2905-2915, 2000. 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 [Ca²⁺]_i rise (40 nM). These responses were neither mimicked by [1S,3R]-1-aminocyclo-pentane-1,3-dicarboxylic acid nor affected by Ca²⁺-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 [Ca²⁺]_i increase that turned after several minutes into a prominent Ca²⁺ transient (0.9 µM) 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 [Ca²⁺]_i rise to values similar to those during anoxia. Each of the latter drugs delayed onset of both ischemic current and prominent [Ca²⁺]_i rise by several minutes and attenuated their magnitudes by up to 40%. Ca²⁺-free solution induced a twofold delay of the ischemic inward current and suppressed the prominent Ca²⁺ increase but not the initial moderate [Ca²⁺]_i rise. Cyclopiazonic acid or arachidonic acid in Ca²⁺-free saline delayed further the ischemic current, whereas neither inhibitors of glutamate uptake (dihydrokainate, D,L-threo--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 Ca²⁺ entry via N-methyl-D-aspartate receptors and voltage-activated Ca²⁺ channels. An early Ca²⁺-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.