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

Abstract

δ-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 ([Ca2+]c), severe membrane potential (Em) depolarization and excitotoxic cell death (ECD). Conversely, NMDAR activity is decreased by ∼50% with anoxia in the cortex of the painted turtle, and large elevations in [Ca2+]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 (1-10μmol l-1) on Em, NMDAR activity and [Ca2+] c homeostasis in turtle cortical neurons during normoxia and the transition to anoxia. Naltrindole potentiated normoxic NMDAR currents by 78±5% and increased [Ca2+]c by 13±4%. Anoxic neurons treated with naltrindole were strongly depolarized, NMDAR currents were potentiated by 70±15%, and [Ca2+]c increased 5-fold compared with anoxic controls. Following naltrindole washout, E m remained depolarized and [Ca2+]c became further elevated in all neurons. The naltrindole-mediated depolarization and increased [Ca2+]c were prevented by NMDAR antagonism or by perfusion of the Gi 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 Gi-dependent manner and prevent deleterious NMDAR-mediated [Ca2+]c influx during anoxic insults in the turtle cortex.