We report evidence that mitochondrially produced superoxide (O-2(-)) is involved in signaling in hippocampal neurons by examining the relationship between strong but physiological increases in cytosolic free Ca2+, mitochondrial calcium accumulation, O-2(-) production, and CREB phosphorylation. Strong depolarization-induced Ca2+ entry through NMDA or L-type Ca2+ channels evoked large Ca2+ transients, a sustained increase in O-2(-), and a large rise in nuclear CaM and pCREB. Under these conditions, inhibition of mitochondrial Ca2+ uptake and consequent O-2(-) production suppressed Ca2+ entry-induced pCREB elevation, indicating that O-2(-) produced by mitochondria supports CREB phosphorylation. Similarly, inhibiting mitochondrial respiration blocked O-2 production and also depressed the elevation of pCREB. Blocking calcineurin reversed this depression. We conclude that strong Ca2+ entry promotes mitochondrial calcium accumulation and the subsequent enhancement of mitochondrial O-2(-) production, which in turn prolongs the lifetime of pCREB by suppressing calcineurin-dependent pCREB dephosphorylation. (C) 2003 Elsevier Inc. All rights reserved.