Background: Our 24-hour circadian rhythms are important for physiological health, but less is known about shorter cycles (milliseconds to hours) within a 24h day, known as ultradian rhythms. They are postulated to be endogenously generated, self-sustained oscillatory processes, allowing partitioning of metabolism within the circadian cycle, and may potentially contribute to brain maturation. Impaired ultradian rhythmicity is associated with adverse outcomes but little is known about fetal ultradian rhythms.
Aim: To evaluate the presence and maturation of ultradian rhythms in the fetal electroencephalogram (EEG).
Methods: Pregnant ewes were acclimatised to the laboratory, housed in individual metabolic crates, with ad libitum access to food and water and entrained to a 12h:12h light:dark cycle (lights off 1800h). In chronically instrumented fetuses EEG recordings (digitised at 1024Hz) were started 5d post-surgery at 104d gestation and continued for 21d. A subset of fetuses received 25min of umbilical cord occlusion (UCO) at 104d. Wavelet transformation was performed on the EEG.
Results: At 104-110d the EEG was discontinuous, with ultradian rhythms with periods of ~0.5h to ~1.5h. Between 121-125d there was development of ultradian rhythms related to sleep state cycling (cycling of rapid-eye movement (REM) episodes (~20-30min) with non-REM episodes (~10-15min)), with superimposed shorter rhythms, mostly in REM. Fetuses subjected to UCO showed altered ultradian rhythmicity.
Conclusions: These preliminary data demonstrate the progressive maturation of EEG ultradian rhythms over the last third of pregnancy in the fetal sheep. The changes in the pattern of these rhythms over time reflect key phases of neural network development, notably the well-known transition from discontinuous to continuous sleep-state activity. The pattern of maturation was markedly altered by exposure to acute hypoxia-ischemia. Further studies are required to characterise these rhythms, and to assess whether neuroprotective treatments can restore normal ultradian rhythms.