Effects of histamine on hippocampal pyramidal cells of the rat in vitro

The actions of bath applied histamine on CA1 pyramidal cells were investigated in hippocampal slices of the rat. Histamine caused a) a slight depolarization but no significant change in resting membrane conductance; b) an abbreviation of long afterhyperpolarizations after single action potentials, bursts of action potentials or TTX resistant spikes; c) a loss of accommodation of firing. In the presence of TEA or barium, histamine prolonged and increased the size and number of the slow TTX resistant spikes. A depolarizing plateau which follows such spikes was also increased by histamine. Evoked synaptic potentials were unaffected by histamine, but the population spike was increased. The frequency of spontaneous chloride dependent potentials, which reflect interneurone firing, was also increased. These effects considerably outlasted histamine application and were mimicked by the H₂-agonist impromidine but not the H₁-agonist thiazolethylamine, and blocked by the H₂-antagonists cimetidine and metiamide but not the H₁-antagonists mepyramine or the beta-antagonist propranolol. It is concluded that histamine, by activating H₂-receptors, antagonizes a calcium mediated potassium conductance in hippocampal pyramidal cells without affecting calcium current. By this mechanism histaminergic afferent fibres could effectively regulate cortical responsiveness by selectively potentiating large excitatory inputs of target neurones.