Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons

Although large increases in neuronal intracellular calcium concentrations ([Ca2+](i)) are lethal, moderate increases in [Ca2+](i) of 50-200 nM may induce immediate or long-term tolerance of ischemia or other stresses. In neurons in rat hippocampal slice cultures, we determined the relationship between [Ca2+](i), cell death, and Ca2+-dependent neuroprotective signals before and after a 45 min period of oxygen and glucose deprivation (OGD). Thirty minutes before OGD, [Ca2+](i) was increased in CA1 neurons by 40-200 nM with I nM-1 muM of a Ca2+-selective ionophore (calcimycin or ionomycin-"Ca2+ preconditioning"). Ca2+ preconditioning greatly reduced cell death in CA1, CA3 and dentate during the following 7 days, even though [Ca2+]i was similar (approximately 2 muM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD [Ca2+](i) was lowered to 25 nM (10 nM ionophore in Ca2+-free medium) or increased to 8 muM (10 muM ionophore), more than 90% of neurons died. Increased levels of the anti-apoptotic protein protein kinase B (Akt) and the MAP kinase ERK (p42/44) were present in preconditioned slices after OGD. Reducing Ca2+ influx, inhibiting calmodulin, and preventing Akt or MAP kinase p42/44 upregulation prevented Ca2+ preconditioning, supporting a specific role for Ca2+ in the neuroprotective process. Further, in continuously oxygenated cultured hippocampal/cortical neurons, preconditioning for 30 min with 10 nM ionomycin reduced cell death following a 4 muM increase in [Ca2+](i) elicited by 1 muM ionomycin. Thus, a zone of moderately increased [Ca2+](i) before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in [Ca2+](i) from initiating cell death processes. (C) 2004 IBRO. Published by Elsevier Ltd. All rights reserved.