Abstract
It has been demonstrated in the rodent hippocampus that rhythmic slow activity (theta) predominantly occurs during rapid eye movement (REM) sleep, while sharp waves and associated ripples occur mainly during non-REM sleep. However, evidence is lacking for correlates of sleep stages with electroencephalogram (EEG) in the hippocampus of monkeys. In the present study, we recorded hippocampal EEG from the dentate gyrus in monkeys overnight under conditions of polysomnographical monitoring. As result, the hippocampal EEG changed in a manner similar to that of the surface EEG: during wakefulness, the hippocampal EEG showed fast, desynchronized waves, which were partly replaced with slower waves of intermediate amplitudes during the shallow stages of non-REM sleep. During the deep stages of non-REM sleep, continuous, slower oscillations (0.5–8 Hz) with high amplitudes were predominant. During REM sleep, the hippocampal EEG again showed fast, desynchronized waves similar to those found during wakefulness. These results indicate that in the monkey, hippocampal rhythmic slow activity rarely occurs during REM sleep, which is in clear contrast to that of rodents. In addition, the increase in the slower oscillations of hippocampal EEG during non-REM sleep, which resembled that of the surface EEG, may at least partly reflect cortical inputs to the dentate gyrus during this behavioral state.
Highlights
The hippocampus is important for certain types of learning and memory [1,2,3,4,5,6]
Rhythmic slow activity in the hippocampus has different behavioral correlates in different animal species in an awake condition, rapid eye movement (REM) sleep is the behavioral state under which rhythmic slow activity is invariably observed across many nonprimate animals [13], [14]
In REM sleep (REM), frequent deflections appeared again in the EOG, reflecting rapid eye movements, while EMG activity remained at its lowest level
Summary
The hippocampus is important for certain types of learning and memory [1,2,3,4,5,6]. There is a growing body of evidence for the contribution of sleep to memory function (for review, see [7,8,9,10,11]). Several studies have dealt with the sleep-stage correlates of hippocampal EEG recorded in patients suffering from temporal lobe epilepsy (e.g., [20,21,22,23,24,25]). The findings from these studies are relatively inconsistent. We think that it is of value to investigate sleep-stage correlates of hippocampal EEG under controlled conditions using monkeys as experimental animals. We implanted recording electrodes in the dentate gyrus of monkeys using this technique, and recorded hippocampal EEG while the monkeys were asleep
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