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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

AMPA receptors undergo channel arrest in the anoxic turtle cortex

¹Department of Cellular and Systems Biology, University of Toronto, Toronto, Ontario, Canada; ²Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada; and ³Division of Fundamental Neurobiology, Toronto Western Research Institute, Toronto, Ontario, Canada

Submitted 20 June 2007 ; accepted in final form 21 November 2007

Without oxygen, all mammals suffer neuronal injury and excitotoxic cell death mediated by overactivation of the glutamatergic N-methyl-D-aspartate receptor (NMDAR). The western painted turtle can survive anoxia for months, and downregulation of NMDAR activity is thought to be neuroprotective during anoxia. NMDAR activity is related to the activity of another glutamate receptor, the -amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR). AMPAR blockade is neuroprotective against anoxic insult in mammals, but the role of AMPARs in the turtle's anoxia tolerance has not been investigated. To determine whether AMPAR activity changes during hypoxia or anoxia in the turtle cortex, whole cell AMPAR currents, AMPAR-mediated excitatory postsynaptic potentials (EPSPs), and excitatory postsynaptic currents (EPSCs) were measured. The effect of AMPAR blockade on normoxic and anoxic NMDAR currents was also examined. During 60 min of normoxia, evoked peak AMPAR currents and the frequencies and amplitudes of EPSPs and EPSCs did not change. During anoxic perfusion, evoked AMPAR peak currents decreased 59.2 ± 5.5 and 60.2 ± 3.5% at 20 and 40 min, respectively. EPSP frequency (EPSP) and amplitude decreased 28.7 ± 6.4% and 13.2 ± 1.7%, respectively, and EPSC and amplitude decreased 50.7 ± 5.1% and 51.3 ± 4.7%, respectively. In contrast, hypoxic (PO₂ = 5%) AMPAR peak currents were potentiated 56.6 ± 20.5 and 54.6 ± 15.8% at 20 and 40 min, respectively. All changes were reversed by reoxygenation. AMPAR currents and EPSPs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In neurons pretreated with CNQX, anoxic NMDAR currents were reversibly depressed by 49.8 ± 7.9%. These data suggest that AMPARs may undergo channel arrest in the anoxic turtle cortex.

N-methyl-D-aspartate receptor; excitotoxic cell death; spike arrest

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