Mg2+-free induced synchronized neuronal activity is initiated in CA3 of organotypic hippocampal cultures. (A) Extracellular recordings from CA1 and CA3 regions. After a short delay Mg2+-free ACSF induces a burst of firing in both regions that settles into prolonged rhythmic and synchronized events. (B) Expanded region of the trace from the region indicated by asterix. (C) Further time scale expansion (centered on asterix in B) reveals that CA3 firing precedes (vertical lines) activity in CA1 suggesting that under normal conditions synchronized events are paced from the CA3 region.