Histamine increases the bursting activity of pyramidal cells in the CA3 region of mouse hippocampus.

An excitatory action of histamine was investigated by intracellular recording in the CA3 region of hippocampal slices. Bath application of histamine or impromidine, a H2 receptor agonist, had the following effects: (1) a depolarisation in 60% and no changes in membrane potential in 40% of the CA3 pyramids; (2) single cell firing and burst activity were evoked or more than doubled when spontaneously present; (3) the bursts were prolonged and often followed by afterdischarges instead of the normal afterhyperpolarisations (AHPs); (4) synaptic stimulation evoked large bursts instead of excitatory synaptic potentials (EPSPs) and primary burst responses became prolonged. CA3 bursts may play a decisive role in memory trace formation, their facilitation and potentiation is in keeping with a positive role of the histaminergic system in attention and learning.