Paired stimulation between CA3 and CA1 alters excitability of CA3 in the rat hippocampus

► To measure excitability of CA3 extracellularly, optogenetically evoked firing rate was used. ► The excitability of CA3 was up-regulated by synchronous CA3 and CA1 pairing stimulation. ► The excitability of CA3 was down-regulated by asynchronous CA3 and CA1 pairing stimulation.