Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons

Abstract

Although large increases in neuronal intracellular calcium concentrations ([Ca 2+] i) are lethal, moderate increases in [Ca 2+] 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 [Ca 2+] i, cell death, and Ca 2+-dependent neuroprotective signals before and after a 45 min period of oxygen and glucose deprivation (OGD). Thirty minutes before OGD, [Ca 2+] i was increased in CA1 neurons by 40–200 nM with 1 nM–1 μM of a Ca 2+-selective ionophore (calcimycin or ionomycin-“Ca 2+ preconditioning”). Ca 2+ preconditioning greatly reduced cell death in CA1, CA3 and dentate during the following 7 days, even though [Ca 2+] i was similar (approximately 2 μM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD [Ca 2+] i was lowered to 25 nM (10 nM ionophore in Ca 2+-free medium) or increased to 8 μM (10 μM 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 Ca 2+ influx, inhibiting calmodulin, and preventing Akt or MAP kinase p42/44 upregulation prevented Ca 2+ preconditioning, supporting a specific role for Ca 2+ 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 μM increase in [Ca 2+] i elicited by 1 μM ionomycin. Thus, a zone of moderately increased [Ca 2+] i before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in [Ca 2+] i from initiating cell death processes.