Chronic, daytime sleepiness is a major, disabling symptom for many patients with traumatic brain injury (TBI), but thus far, its etiology is not well understood. Extensive loss of the hypothalamic neurons that produce the wake-promoting neuropeptide hypocretin (orexin) causes the severe sleepiness of narcolepsy, and partial loss of these cells may contribute to the sleepiness of Parkinson's disease and other disorders. We have found that the number of hypocretin neurons is significantly reduced in patients with severe TBI. This observation highlights the often overlooked hypothalamic injury in TBI and provides new insights into the causes of chronic sleepiness in patients with TBI.

Narcolepsy is caused by a loss of orexin/hypocretin signaling, resulting in chronic sleepiness, fragmented non-rapid eye movement sleep, and cataplexy. To identify the neuronal circuits underlying narcolepsy, we produced a mouse model in which a loxP-flanked gene cassette disrupts production of the orexin receptor type 2 (OX2R; also known as HCRTR2), but normal OX2R expression can be restored by Cre recombinase. Mice lacking OX2R signaling had poor maintenance of wakefulness indicative of sleepiness and fragmented sleep and lacked any electrophysiological response to orexin-A in the wake-promoting neurons of the tuberomammillary nucleus. These defects were completely recovered by crossing them with mice that express Cre in the female germline, thus globally deleting the transcription-disrupter cassette. Then, by using an adeno-associated viral vector coding for Cre recombinase, we found that focal restoration of OX2R in neurons of the tuberomammillary nucleus and adjacent parts of the posterior hypothalamus completely rescued the sleepiness of these mice, but their fragmented sleep was unimproved. These observations demonstrate that the tuberomammillary region plays an essential role in the wakepromoting effects of orexins, but orexins must stabilize sleep through other targets.arcolepsy is caused by a selective loss of the hypothalamic neurons producing the orexin (i.e., hypocretin) neuropeptides and is one of the most common causes of chronic sleepiness (1). In humans and mice, loss of orexin signaling results in unstable sleep/wake states, with poor maintenance of wakefulness, fragmented sleep, and intrusions into wakefulness of elements of rapid eye movement (REM) sleep, including brief episodes of paralysis known as cataplexy. The orexin neuropeptides strongly excite many brain regions that regulate sleep/wake behavior, yet the key pathways through which orexins stabilize wakefulness and sleep remain unknown.Orexins act through two receptors, OX1R and OX2R (also known as HCRTR1 and HCRTR2), and the OX2R seems to play a critical role in the maintenance of wakefulness. Mice constitutively lacking OX2R are unable to maintain long bouts of wakefulness and can fall asleep rapidly (2). In addition, an OX2R antagonist strongly promotes sleep, whereas an OX1R antagonist has no effect (3).Although it is clear that OX2R signaling is necessary for the normal maintenance of wakefulness, the anatomic sites through which this occurs remain unknown. OX2R is expressed in many wake-promoting brain regions, including the histaminergic neurons of the tuberomammillary nucleus (TMN), other monoaminergic regions, cholinergic systems, and forebrain regions, including the thalamus and cortex (4). Several researchers have hypothesized that the TMN is a key site because orexin-A excites the TMN neurons and infusion of orexin-A near this region promotes wakefulness (5-7). However, this perspective is controversial as optogenetic activation of the orexin neurons promotes arousal in mice lacking histamine (8), and mice lacking both OX1...

These results indicate that the orexin neurons are necessary for the circadian suppression of REM sleep. Blunting of the REM sleep rhythm in Atx mice but not in orexin KO mice suggests that other signaling molecules such as dynorphin or glutamate may act in concert with orexins to suppress REM sleep during the active period.

Mice lacking orexin/hypocretin signaling have sudden episodes of atonia and paralysis during active wakefulness. These events strongly resemble cataplexy, episodes of sudden muscle weakness triggered by strong positive emotions in people with narcolepsy, but it remains unknown whether murine cataplexy is triggered by positive emotions. To determine whether positive emotions elicit murine cataplexy, we placed orexin knockout (KO) mice on a scheduled feeding protocol with regular or highly palatable food. Baseline sleep/wake behavior was recorded with ad lib regular chow. Mice were then placed on a scheduled feeding protocol in which they received 60% of their normal amount of chow 3 hr after dark onset for the next 10 days. Wild-type and KO mice rapidly entrained to scheduled feeding with regular chow, with more wake and locomotor activity prior to the feeding time. On day 10 of scheduled feeding, orexin KO mice had slightly more cataplexy during the foodanticipation period and more cataplexy in the second half of the dark period, when they may have been foraging for residual food. To test whether more palatable food increases cataplexy, mice were then switched to scheduled feeding with an isocaloric amount of Froot Loops, a food often used as a reward in behavioral studies. With this highly palatable food, orexin KO mice had much more cataplexy during the food-anticipation period and throughout the dark period. The increase in cataplexy with scheduled feeding, especially with highly palatable food, suggests that positive emotions may trigger cataplexy in mice, just as in people with narcolepsy. Establishing this connection helps validate orexin KO mice as an excellent model of human narcolepsy and provides an opportunity to better understand the mechanisms that trigger cataplexy.

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