Frequency-related inhibitory mechanisms controlling rhythmical activity in the septal area.

1. The response patterns of identified neurones in the medical and lateral septal regions to varying rates of repetitive stimulation of the fimbria were investigated in rats anaesthetized with urethane. 2. Neurones in the lateral septum which characteristically respond to single pulse stimulation of the fimbria with an activation-inhibition sequence, exhibited a reduction or complete elimination of the inhibitory component both during and following tetanic volleys delivered at 7-12 HZ. Stimulation at lower frequencies did not alter the response. 3. Concurrently with this effect on the inhibitory component of the response exhibited by lateral septal cells, repetitive volleys eliminate the small amplitude burst discharges which are correlated with the onset of the inhibitory period and are considered to indicate the firing of inhibitory interneurones. 4. Tetanic stimulation of the fimbria at rates which eliminate this interneuronal response in the lateral septum, produce an irregular pattern of firing in medial septal neurones which previously exhibited a synchronized bursting discharge to single pulses. 5. Ipsilateral section of the fimbrial input to the septum resulted in the elimination of the burst discharge pattern exhibited by medial septal neurones. 6. The results suggest that a frequency gating mechanism in the lateral septum, activation of which is dependent upon the level of hippocampal output, is responsible for controlling the firing pattern of medical septal neurones.