'Cat P300' disappears after septal lesions.

Endogenous responses were recorded from 9 awake cats with loud and soft clicks randomly presented as rare (P = 0.15) or frequent (P = 0.80) stimuli; a reinforced tone CS (P = 0.05) resulted in a conditioned eye blink response and focused the cat's attention on the auditory stimuli. Subsequent to 12 preoperative recording sessions the medial septal area was lesioned in 7 cats and similar but more rostral lesions were placed in 2 cats. Thereafter, 12 postoperative recording sessions were carried out, the animals were terminated, and the brains processed for AChE histochemistry and histology of the lesioned areas. Destruction of the medial septum and vertical limb of the diagonal band of Broca resulted in a transient postoperative 'cat P300' followed by reduction and disappearance of the response. The hippocampus of these animals was characterized by marked AChE depletion. In contrast, the animals with lesions rostral to the medial septal area showed no postoperative change in the P300 response and no depletion of hippocampal AChE. These data indicate an important role for the medial septal area as a modulator of 'cat P300' generation possibly through the cholinergic component of the septohippocampal projection system.