Developmental divergence of sleep-wake patterns in orexin knockout and wild-type mice

Narcolepsy, a disorder characterized by fragmented bouts of sleep and wakefulness during the day and night as well as cataplexy, has been linked in humans and nonhuman animals to the functional integrity of the orexinergic system. Adult orexin knockout mice and dogs with a mutation of the orexin receptor exhibit symptoms that mirror those seen in narcoleptic humans. As with narcolepsy, infant sleep-wake cycles in humans and rats are highly fragmented, with consolidated bouts of sleep and wakefulness developing gradually. Based on these common features of narcoleptics and infants, we hypothesized that the development of sleep-wake fragmentation in orexin knockout mice would be expressed as a developmental divergence between knockouts and wild-types, with the knockouts lagging behind the wild-types. We tested this hypothesis by recording the sleep-wake patterns of infant orexin knockout and wild-type mice across the first three postnatal weeks. Both knockouts and wild-types exhibited age-dependent, and therefore orexin-independent, quantitative and qualitative changes in sleep-wake patterning. At 3 weeks of age, however, by which time the sleep and wake bouts of the wild-types had consolidated further, the knockouts lagged behind the wild-types and exhibited significantly more bout fragmentation. These findings suggest the possibility that the fragmentation of behavioural states that characterizes narcolepsy in adults reflects reversion back toward the more fragmented sleep-wake patterns that characterize infancy.