Carbachol induces fast oscillations in the medial but not in the lateral entorhinal cortex of the isolated guinea pig brain

Carbachol induces fast oscillations in the medial but not in the lateral en torhinal cortex of the isolated guinea pig brain. J. Neurophysiol. 82: 2441 -2450, 1999. Fast oscillations at 25-80 Hz (gamma activity) have been propo sed to play a role in attention-related mechanisms and synaptic plasticity in cortical structures. Recently, it has been demonstrated that the preserv ation of the entorhinal cortex is necessary to maintain gamma oscillations in the hippocampus. Because gamma activity can be reproduced in vitro by ch olinergic activation, this study examined the characteristics of gamma osci llations induced by arterial perfusion or local intracortical injections of carbachol in the entorhinal cortex of the in vitro isolated guinea pig bra in preparation. Shortly after carbachol administration, fast oscillatory ac tivity at 25.2-28.2 Hz was observed in the medial but not in the lateral en torhinal cortex. Such activity was transiently associated with oscillations in the theta range that showed a variable pattern of distribution in the e ntorhinal cortex. No oscillatory activity was observed when carbachol was i njected in the lateral entorhinal cortex. Gamma activity in the medial ento rhinal cortex showed a phase reversal at 200-400 mu m, had maximal amplitud e at 400-500 mu m depth, and was abolished by arterial perfusion of atropin e (5 mu M). Local carbachol application in the medial entorhinal cortex ind uced gamma oscillations in the hippocampus, whereas no oscillations were ob served in the amygdala and in the piriform, periamygdaloid, and perirhinal cortices ipsilateral and contralateral to the carbachol injection. Hippocam pal oscillations had higher frequency than the gamma activity recorded in t he entorhinal cortex, suggesting the presence of independent generators in the two structures. The selective ability of the medial but not the lateral entorhinal cortex to generate gamma activity in response to cholinergic ac tivation suggests a differential mode of signal processing in entorhinal co rtex subregions.