On the influence of prenatal hypoxia on formation of the orexinergic system and sleep–wake cycle in early ontogenesis of rats

The role of orexin in the organization of the sleep–wake cycle (SWC) is well known. The aim of this study was to examine the timing of the orexinergic system formation in rat postnatal ontogenesis and to assess the role of orexin A in the SWC organization under normal conditions and after prenatal hypoxia undergone on days 14 and 19 of embryogenesis. The SWC was investigated in 30-day-old rats with electrodes implanted into the somatosensory and occipital cortex. Immunoreactivity within the orexigenic structures of the lateral hypothalamus was analyzed. It was shown that in control 14-day-old animals the orexinergic structures were in their formative stage, whereas in 30-day-old rats they were already as formed as in adults. In 14-day-old rats, prenatal hypoxia evoked retarded formation of the orexinergic system. In 30-day-old animals, hypoxia undergone in the prenatal period increased the activity of the orexinergic system, which was higher in animals exposed to hypoxia on day 19 than on day 14 of gestation. In 30-day-old rats, these changes were reflected in the SWC formation in the form of shorter slow-wave sleep, more fitful sleep and increased number of transitions from slow- to fast-wave sleep. The results obtained are discussed in the light of the adaptive-compensatory role of the orexigenic system in postnatal ontogenesis after prenatal damage to the central nervous system.