δ-Opioid receptor antagonism induces NMDA receptor-dependent excitotoxicity in anoxic turtle cortex

Abstract

delta-Opioid receptor (DOR) activation is neuroprotective against short-term anoxic insults in the mammalian brain. This protection may be conferred by inhibition of N-methyl-d-aspartate receptors (NMDARs), whose over-activation during anoxia otherwise leads to a deleterious accumulation of cytosolic calcium ([Ca(2+)](c)), severe membrane potential (E(m)) depolarization and excitotoxic cell death (ECD). Conversely, NMDAR activity is decreased by approximately 50% with anoxia in the cortex of the painted turtle, and large elevations in [Ca(2+)](c), severe E(m) depolarization and ECD are avoided. DORs are expressed in high quantity throughout the turtle brain relative to the mammalian brain; however, the role of DORs in anoxic NMDAR regulation has not been investigated in turtles. We examined the effect of DOR blockade with naltrindole (1-10 micromol l(-1)) on E(m), NMDAR activity and [Ca(2+)](c) homeostasis in turtle cortical neurons during normoxia and the transition to anoxia. Naltrindole potentiated normoxic NMDAR currents by 78+/-5% and increased [Ca(2+)](c) by 13+/-4%. Anoxic neurons treated with naltrindole were strongly depolarized, NMDAR currents were potentiated by 70+/-15%, and [Ca(2+)](c) increased 5-fold compared with anoxic controls. Following naltrindole washout, E(m) remained depolarized and [Ca(2+)](c) became further elevated in all neurons. The naltrindole-mediated depolarization and increased [Ca(2+)](c) were prevented by NMDAR antagonism or by perfusion of the G(i) protein agonist mastoparan-7, which also reversed the naltrindole-mediated potentiation of NMDAR currents. Together, these data suggest that DORs mediate NMDAR activity in a G(i)-dependent manner and prevent deleterious NMDAR-mediated [Ca(2+)](c) influx during anoxic insults in the turtle cortex.