ADENOSINE AND ANOXIA REDUCE N-METHYL-D-ASPARTATE RECEPTOR OPEN PROBABILITY IN TURTLE CEREBROCORTEX

During normoxia, glutamate and the glutamate family of ion channels pl ay a key role in mediating rapid excitatory synaptic transmission in t he central nervous system, However, during hypoxia, intracellular [Ca2 +] increases to neurotoxic levels, mediated largely by the N-methyl-D- aspartate (NMDA) subfamily of glutamate receptors, Adenosine has been shown to decrease the magnitude of the hypoxia-induced increase in [Ca 2+](i) in mammalian brain slices, delaying tissue injury, Turtle brain is remarkably tolerant of anoxia, maintaining a pre-anoxic [Ca2+](i) while cerebral adenosine levels increase 12-fold, Employing cell-attac hed single-channel patch-clamp techniques, we studied the effect of ad enosine (200 mu mol l(-1)) and anoxia on NMDA receptor open probabilit y (P-open) and current amplitude, After 60 min of anoxic perfusion, ch annel P-open decreased by 65 % (from 6.8+/-1.6 to 2.4+/-0.8 %) an effe ct that could also be achieved with a normoxic perfusion of 200 mu mol l(-1) adenosine (P-open decreased from 5.8+/-1.1 to 2.3+/-1.2 %). The inclusion of 10 mu mol l(-1) 8-phenyltheophylline, an Al receptor blo cker, prevented the adenosine-and anoxia-induced decrease in P-open. M ean single-channel current amplitude remained at approximately 2.7+/-0 .23 pA conditions, To determine membrane potential could be part of th e mechanism by which P-open decreases, membrane and threshold potentia l were measured following each experiment, Membrane potential did not change significantly under any condition, ranging from -76.8 to -80.6 mV, Therefore, during anoxia, NMDA receptors cannot be regulated by Mg 2+ ina manner dependent on membrane potential, Threshold potentials di d decrease significantly following 60 min of anoxic or adenosine perfu sion (control -33.3+/-1.9 mV, anoxia -28.4+/-1.5 mV, adenosine -23.4+/ -2.8 mV), We conclude that anoxia modulates NMDA receptor activity and that adenosine plays a key role in mediating this change, This is the first direct measurement of ion channel activity in anoxic turtle bra in and demonstrates that ion channel regulation is part of the natural ly evolved anoxic defence mechanism of this species.