Temporal dynamics of the transition period between nonrapid eye movement and rapid eye movement sleep in the rat

Study Objectives: We studied the sequence of events that occur in electroencephalogram (EEG), electromyogram (EMG), and ocular activities along the transition period between nonrapid eye movement and rapid eye movement (REM) sleep and their temporal relationships. Methods: Twenty adult Wistar rats underwent polygraphic recordings of EEG, EMG, and eye movements during the transition periods. The studied EEG variables included the time of occurrence of the last sleep spindle and the evolution of the amplitudes of delta (1-5 Hz), theta (6-9 Hz), and high-frequency (110-160 Hz) oscillations (HFO). Motor variables included the development of atonia in the acromiotrapezius muscle and the occurrence of the first REM. The phase-amplitude coupling between theta and HFO was also analyzed. Results: The transition periods started with the onset of., followed by HFO onset and the decline of delta activities. From onset, HFO amplitude was coupled to the phase of theta. The first REM and the last sleep spindle coincided with the complete extinction of delta. At that point, the muscle tonus started to decrease, reaching its atonia when HFO achieved maximum amplitude. Conclusions: As the development of HFO delimited the duration of all transitional changes with precision, it can be considered a good delimiter for the transition period. Furthermore, as transitional changes occurred in a precise sequence, alterations in their temporal dynamics could be of interest in the study of sleep disorders and pathologies.