Orexin Maintains Ventilation During a Critical Window of Postnatal Life

Abstract

Orexinergic neurons in the lateral and perifornical hypothalamus project widely to respiratory regions of the brainstem. Also, orexin neurons are active in wakefulness and silent in sleep. In adult animals, chronic orexin deficiency reduces the respiratory response to CO2, but has no apparent effect on resting ventilation. The acute effects of orexin on breathing have not been described in adult or infant animals. This topic is important because narcoleptic patients have a higher incidence of sleep apnea, and there is pathological evidence of orexinergic dysfunction in Sudden Infant Death Syndrome (SIDS). SIDS is a leading cause of infant death that occurs in sleep, or in transitions between wakefulness and sleep. Most cases occur between 2–6 months of age (i.e. a “critical window” of development). There is evidence of respiratory defects associated with SIDS, including severe hypoventilation prior to death. We hypothesized that orexin receptor blockade would lead to acute hypoventilation in a vigilance state and age‐dependent manner. To test this hypothesis, we used whole‐body plethysmography to monitor the breathing of infant (postnatal days 7–8, 12–14, 17–19) and adult rats (~2 months old) treated with suvorexant, a selective orexin 1 and 2 receptor antagonist, via an intra‐abdominal catheter. In pups, we determined vigilance state using standard behavioral criteria, confirmed by video. In adults, vigilance state was determined with EMG and EEG, and by applying Fast Fourier Transform to the EEG and EMG signals. Pups were studied during the light period, and adults were tested during the transition from the dark to light period, when orexin levels peak. Rats cycled through quiet wakefulness (QW), quiet sleep (QS, or NREM in adults) and active sleep (AS, or REM in adults) for at least 1 hour, at which point suvorexant (5 mg/kg in 50% DMSO, for pups; 20 mg/kg in 100% DMSO, for adults) or vehicle alone was administered. Pups and adults were monitored for another 1 hr or 2 hr, respectively. We analyzed the effects of suvorexant on respiratory frequency (f), tidal volume (VT), and the ventilatory equivalent (VE/VO2) in QW, QS/NREM, and AS/REM. Separate groups of 2‐week‐old pups were used to test the effects of suvorexant on resting blood gases, and the ventilatory chemoreflex responses to 5% CO2 or hyperoxia (100% O2). In P12‐14 pups, suvorexant significantly reduced f in all states, VT only in QS, and VE/VO2 in QW and QS. These effects were absent at all other ages. Hypoventilation in P12‐14 pups was confirmed by blood gas analysis. Suvorexant appears to have no effect on ventilatory responses to CO2 or hyperoxia. These results suggest that orexin preserves eupneic breathing in QW and QS only during a critical window of neonatal life (P12‐14). Orexin stimulates breathing independently of the drive provided by the central or peripheral chemoreceptors. Defects in the orexin system may predispose an infant to SIDS during a critical window of development by increasing the risk of hypoventilation during periods of sleep, or transitions between sleep and wakefulness.Support or Funding InformationFunding: National Institute of Health (5R01HL136710‐02) to Kevin James Cummings