Abstract
Objectives: Under constant sleep pressure, caused by repeated 2h sleep deprivation in constant darkness (DD) in the rat, slow wave activity (SWA; 1-4 Hz) in the NREM sleep electroencephalogram (EEG) reduces its daily amplitude, whereas the higher frequencies preserve their circadian modulations (Yasenkov&Deboer, SLEEP, 2010, in press). To investigate the level of interrelations between different EEG frequencies under these conditions a correlation analysis was applied. Methods: EEG and electromyogram recordings were performed in freely moving rats (n=8) adapted to DD. After a 24 h baseline day (BL) recording, the repeated short sleep deprivation (SD) protocol was applied, where 2h of SD is alternated with 2h of rest (2h/2h) for 48 hours. Vigilance states were determined and spectral analysis of EEG frequencies between 0.1-25 Hz and a correlation analysis (Pearson’s correlation coefficient, r) between 1 Hz frequency bins over 4h intervals were performed. Results: In BL during NREM sleep, the correlation matrix revealed significant positive relationships between 1 Hz frequency bins within three frequency ranges – SWA, 9-14 Hz and 15-25 Hz (p<0.01). In addition, a tendency for a negative correlation between SWA and higher frequencies (8-25 Hz) was found. Under the 2h/2h protocol the r values decreased within SWA, and increased in the higher frequency ranges where significant positive correlations within a broader frequency range (8-25 Hz; p<0.01) were observed. Both for REM sleep and waking during BL, the correlation analysis showed significant positive relationships within 12-15 Hz and 19-22 Hz. This disappeared during the rest phases of 2h/2h for both waking and REM sleep and was slightly reduced for waking in the deprivation phases of the 2h/2h protocol. Conclusions: In NREM sleep during BL three correlation clusters were found, which may represent slow-waves, spindles, and high frequencies. These were modified under constant sleep pressure: the reduction of correlation within SWA confirms its homeostatic component involved in sleep regulation, while a pronounced increase of correlation level within the higher frequencies (8-25 Hz) depicts an endogenous circadian influence. Both REM sleep and waking showed in BL two correlation clusters within the high frequency range (12-22 Hz) which dissociated under the 2h/2h protocol, therefore demonstrating loss of their interdependency under constant sleep pressure.