Behavioral State Instability in Orexin Knock-Out Mice

Abstract

Narcolepsy is caused by a lack of orexin (hypocretin), but the physiologic process that underlies the sleepiness of narcolepsy is unknown. Using orexin knock-out (KO) mice as a model of narcolepsy, we critically tested the three leading hypotheses: poor circadian control of sleep and wakefulness, inadequate activation of arousal regions, or abnormal sleep homeostasis. Compared with wild-type (WT) littermates, orexin KO mice had essentially normal amounts of sleep and wake, but wake and non-rapid eye movement (NREM) bouts were very brief, with many more transitions between all behavioral states. In constant darkness, orexin KO mice had normal amplitude and timing of sleep-wake rhythms, providing no evidence for disordered circadian control. When placed in a new, clean cage, both groups of mice remained awake for ϳ45 min, demonstrating that, even in the absence of orexin, fundamental arousal regions can be engaged to produce sustained wakefulness. After depriving mice of sleep for 2-8 hr, orexin KO mice recovered their NREM and rapid eye movement sleep deficits at comparable rates and to the same extent as WT mice, with similar increases in EEG delta power, suggesting that their homeostatic control of sleep is normal. These experiments demonstrate that the fragmented wakefulness of orexin deficiency is not a consequence of abnormal sleep homeostasis, poor circadian control, or defective fundamental arousal systems. Instead, the fragmented behavior of orexin KO mice may be best described as behavioral state instability, with apparently low thresholds to transition between states.

Abnormal Sleep/Wake Dynamics in Orexin Knockout Mice

“…and implanted with electroencephalogram (EEG) and electromyogram (eMg) electrodes as described previously. 10 eeg signals were recorded using 2 ipsilateral stainless steel screws (1.5 mm to the right of the sagittal suture; 1 mm anterior to bregma and 1 mm anterior to lambda). eMg signals were acquired by a pair of multistranded stainless steel wires inserted into the neck extensor muscles.…”

“…We scored epochs as Cataplexy using the recently published consensus definition: wake preceding cataplexy onset had to last ≥ 40 sec 17,18 ; cataplexy onset was marked by an abrupt transition from wakefulness to periods of high eeg theta activity (4-9 hz) and atonia in the nuchal muscles. 10 simultaneous video recordings showed that during cataplexy, the mouse was often prone or lying on its side in a posture atypical of sleep, and that the cataplexy often occurred outside of the usual nest. these episodes were always followed by a direct transition back to wakefulness.…”

“…with previously described behavioral state instability in these animals. 10,11 analysis of intra-state transitions provided useful insights into the cohesion of behavioral states. cluster cores were in the middle of the larger state clusters, and intra-state transition trajectories originated and terminated all around the core boundaries (data not shown), demonstrating cohesion of states even outside the core region.…”

“…9 in addition, orexins probably stabilize wake and sleep; narcoleptic people, dogs, and mice lacking orexins have great difficulty remaining awake for long periods and also experience fragmented sleep. [10][11][12][13] in earlier work, we found that the fragmented wakefulness of orexin deficiency is not a consequence of abnormal sleep homeostasis, poor circadian control, or defective fundamental arousal systems. 10 however, conventional sleep scoring in 10-to 30-second epochs reveals little about the process of transitioning between states as cortical activity and behavior can change quite rapidly.…”

“…[10][11][12][13] in earlier work, we found that the fragmented wakefulness of orexin deficiency is not a consequence of abnormal sleep homeostasis, poor circadian control, or defective fundamental arousal systems. 10 however, conventional sleep scoring in 10-to 30-second epochs reveals little about the process of transitioning between states as cortical activity and behavior can change quite rapidly. Furthermore, conventional scoring simply identifies discrete states, so it can overlook important variations within states, such as the distinctions between light and deep nreM sleep or between drowsy wake and high levels of arousal.…”