Influence of the medial septal nucleus on the excitability of the commissural path-CA1 pyramidal cell synapse in the hippocampus of freely moving mice.

Changes in the excitability of the commissural path-CA1 pyramidal cell synapse were studied by varying the interval between the application of a conditioning pulse in the medial septal nucleus and a test pulse in the contralateral hippocampus in freely moving mice. The results showed that septal prestimulation results in marked changes in the excitability of pyramidal cells (population spikes) without any associated changes in the averaged evoked excitatory post synaptic potentials. Thus, as a result of septal stimulation, population spikes were first potentiated for interpulse intervals ranging from 10 to 30 ms; this was followed by inhibition at intervals between 40 and 60 ms and then by another phase of hyperexcitability between 100 and 160 ms. Possible mechanisms underlying this phenomenon are discussed and these successive changes in excitability are compared to the phase-locked ones which occur spontaneously during theta waves.