INOSITOL 1,4,5-TRIPHOSPHATE RECEPTORS AND NAD(P)H MEDIATE Ca2+ SIGNALING REQUIRED FOR HYPOXIC PRECONDITIONING OF HIPPOCAMPAL NEURONS

Exposure of neurons to a non-lethal hypoxic stress greatly reduces cell death during subsequent severe ischemia (hypoxic preconditioning, HPC). In organotypic cultures of rat hippocampus, we demonstrate that HPC requires inositol triphosphate (IP3) receptor-dependent Ca2+ release from the endoplasmic reticulum (ER) triggered by increased cytosolic NAD(P)H. Ca2+ chelation with intracellular BAPTA, ER Ca2+ store depletion with thapsigargin, IP3 receptor block with xestospongin, and RNA interference against subtype I of the IP3 receptor all blunted the moderate increases in [Ca2+](i) (50-100 nM) required for tolerance induction. Increases in [Ca2+](i) during HPC and neuroprotection following HIPC were not prevented with NMDA receptor block or by removing Ca2+ from the bathing medium. Increased NAD(P)H fluoresce ice in CA1 neurons during hypoxia and demonstration that NADH manipulation increases [Ca2+](i) in an Ip(3)R-dependent manner revealed a primary role of cellular redox state in liberation of Ca2+ from the ER. Blockade of IP(3)Rs and intracellular Ca2+ chelation prevented phosphorylation of known HPC signaling targets, including MAPK p42/44 (ERK), protein kinase B (Akt) and CREB. We conclude that the endoplasmic. reticulum, acting via redox/NADH-dependent intracellular Ca2+ store release, is an important mediator of the neuroprotective, response to hypoxic stress. (C) 2009 IBRO. Published by Elsevier Ltd. All rights reserved.