Adenosine and anoxia reduce N-methyl-D-aspartate receptor open probability in turtle cerebrocortex.

Abstract

During normoxia, glutamate and the glutamate family of ion channels play a key role in mediating rapid excitatory synaptic transmission in the 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 [Ca2+]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-attached single-channel patch-clamp techniques, we studied the effect of adenosine (200 micromol l-1) and anoxia on NMDA receptor open probability (Popen) and current amplitude. After 60 min of anoxic perfusion, channel Popen decreased by 65 % (from 6.8+/-1.6 to 2.4+/-0.8 %) an effect that could also be achieved with a normoxic perfusion of 200 micromol l-1 adenosine (Popen decreased from 5.8+/-1.1 to 2.3+/-1.2 %). The inclusion of 10 micromol l-1 8-phenyltheophylline, an A1 receptor blocker, prevented the adenosine- and anoxia-induced decrease in Popen. Mean single-channel current amplitude remained at approximately 2.7+/-0.23 pA under all experimental conditions. To determine whether a change in the membrane potential could be part of the mechanism by which Popen decreases, membrane and threshold potential 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 Mg2+ in a manner dependent on membrane potential. Threshold potentials did decrease significantly following 60 min of anoxic or adenosine perfusion (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 brain and demonstrates that ion channel regulation is part of the naturally evolved anoxic defence mechanism of this species.