Sensory evoked neuronal activity in the hippocampus before and after lesions of the medial septal nuclei

The evoked activity of 142 single neurons in the hippocampus of 44 rats was recorded following presentation of one of several modalities of peripheral stimulation. In the intact rat, 41% of the neurons encountered showed some period of inhibition of neuronal firing following peripheral stimulation. In subjects that had received lesions of the medial septal nuclei, only 21% of the cells encountered showed such inhibition. Medial septal lesions were made in 23 animals while recording the activity of cells which exhibited inhibition following stimulation. Of these 23 cells, only 5 continued to exhibit response inhibition after the lesion, while 13 displayed only response excitation, and 5 showed a complete loss of responsiveness. These findings suggest that the integrity of the medial septal nucleus is necessary for the inhibition of neuronal firing observed in many hippocampal neurons following peripheral stimulation.