Carbachol-Induced Rhythms in the Hippocampal Slice: Slow (.5-2HZ), Theta (4-10HZ) and Gamma (80-100HZ) Oscillations

Abstract

The hippocampus is the locus of various in vivo brain rhythms. Early work in the freely moving rat showed that three types of hippocampal oscillatory activity could be detected, depending on the behavior of the animal¹. Low frequency (.5–2 Hz) irregular oscillations predominate during slow wave sleep, and are completely absent during walking. A medium frequency (5–10 Hz) rhythmical component predominates during walking behavior or REM sleep, and is absent during slow wave sleep. Finally, a fast oscillatory component (40–100 Hz) can be observed during REM sleep or walking. The neuronal circuits involved in each of these oscillations are still largely unknown, and are likely to involve the complex interplay between intrinsic cellular and synaptic hippocampal properties, and external rhythmic inputs from subcortical areas. In particular, the septal cholinergic projection to the hippocampus has been shown to significantly contribute to some of these rhythms. Here we focus on the effects of carbachol (CCH, a cholinergic agonist) on the intrinsic hippocampal circuitry. Using field recordings, we show that rhythms in these three frequency ranges may be observed in vitro, and are therefore likely to be the result of synchronized population activity. Further work in this system will allow for a detailed exploration of the neural circuitry involved and its computational role in learning and memory.