Hippocampal electrical activity following local tetanization. I. Afterdischarges

Following a short (1-10 s) train of repetitive stimulation delivered to the hippocampal CA1 region, the following sequelae of afterdischarges (ADs) was seen: (1) a silent period of 2-4 s, (2) a large primary (1 degree) AD usually alvear-surface negative and deep positive, (3) a period of suppressed hippocampal EEG, (4) a secondary (2 degrees) hippocampal AD, and after 3-6 min, (5) 15-25 min of enhanced (up to 10 times normal) fast (30-70 Hz) waves. The 2 degrees hippocampal AD was preceded by or simultaneous with large AD at the amygdaloid electrodes. Electrolytic lesions (n = 7) or large heat lesions of the amygdala (n = 5) or electrolytic lesions of the medial septum (n = 10) were not successful in suppressing the 2 degrees hippocampal AD. However, 4 rats with radiofrequency lesion and 3 rats with bilateral aspiration lesion of the entorhinal cortex had diminished or no 2 degrees hippocampal AD. The fast waves after tetanization were reversed 180 degrees across surface and deep CA1 electrodes. The fast wave increase was blocked by atropine sulfate (25-50 mg/kg i.p.), scopolamine hydrochloride (5 mg/kg i.p.) and medial septal lesions. It was concluded that the 2 degrees hippocampal AD may depend on a reverberation of neural circuitry involving the entorhinal cortex. The 2 degrees AD recorded from amygdala electrodes may partly reflect spreading of activities from the entorhinal cortex. On the other hand, the increase in fast waves after tetanization requires an intact septohippocampal, muscarinic cholinergic input, and may depend on an enhanced cholinergic input or an increased response.