Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons.

Although large increases in neuronal intracellular calcium concentrations (Ca(2+)) are lethal, moderate increases in Ca(2+) 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+), 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+) was increased in CA1 neurons by 40-200 nM with 1 nM-1 microM 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+) was similar (approximately 2 microM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD Ca(2+) was lowered to 25 nM (10 nM ionophore in Ca(2+)-free medium) or increased to 8 microM (10 microM 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 microM increase in Ca(2+) elicited by 1 microM ionomycin. Thus, a zone of moderately increased Ca(2+) before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in Ca(2+) from initiating cell death processes.