Hippocampus and prefrontal cortex exhibit spindle-like waves during REM sleep

Abstract

Objectives Spindles are considered as a major hallmark of non REM sleep (NREMS). These 10–16 Hz phasic oscillations are generated by thalamocortical loops. They are distributed in superficial and deep cortical structures, but also in the hippocampus, as observed in stereo-encephalographic (S-EEG) recordings in epileptic patients [1] , [2] . Our team recently found phasic oscillations in the sigma range (10–15 Hz) in the hippocampus and prefrontal cortex (PFC) during REM sleep (REMS) in rats. We aimed to probe the presence of this activity in the same areas in Human during REMS using S-EEG recordings. Methods Two-hundred and seventy-one drug-resistant epileptic patients explored in Lyon since 2007 with S-EEG recordings were retrospectively screened. Among them, 179 patients with leads in at least one hippocampus were selected. Recordings were visually analyzed to discard epileptic hippocampus. Finally, 12 patients with full night recordings were included. For each patient, 5 minutes of N2 and N3 were selected, as well as all REM sleep episodes. A Matlab script designed to detect and characterize spindle-like waves (SLW) was applied to the hippocampus, PFC and thalamus recordings. Results As expected, SLW were observed in the hippocampus, thalamus and PFC during NREMS. SLW were found in the hippocampus and PFC but not in the thalamus during REMS. SLW were less frequent in REMS than in NREMS. Their frequency was similar during both stages in the hippocampus (12.9 ± 0.5 Hz) but slightly faster during REM sleep than during NREM sleep in PFC (12.7 ± 0.5 vs 12.1 ± 0.5 Hz). REMS SLW and NREMS spindles density decreased with hippocampal spiking, suggesting that these activities are linked to physiological processes. Conclusion Our analysis revealed SLW in the hippocampus and PFC but not in the thalamus during REMS, suggesting that SLW results from different mechanisms than those involved in NREMS spindles generation.