Orexin receptor 2 expression in the posterior hypothalamus rescues sleepiness in narcoleptic mice

Abstract

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...

Nonpeptide orexin type-2 receptor agonist ameliorates narcolepsy-cataplexy symptoms in mouse models

“…1 F and G). These results show that YNT-185 mimics the effects of orexin on histaminergic neurons in the TMN (16).…”

“…OX2R-null mice and OXKO mice are similarly affected with behaviorally abnormal attacks of NREM sleep ("sleep attacks") and show similar degrees of fragmentation (18). Focal restoration of OX2R in neurons of the posterior hypothalamus, including the TMN, rescued the fragmentation of wakefulness in OX2R-null mice in the presence of OX1R (16), further suggesting that simultaneous activation of both OX1R and OX2R might be important for restoring the fragmentation of wakefulness.…”

“…Histaminergic neurons in the tuberomammillary nucleus (TMN) abundantly express OX2R, which plays an important role in the maintenance of wakefulness (16). Electrophysiological properties of histaminergic neurons in the TMN in mice have been well characterized, and orexin-A was shown to increase firing rates of these neurons by depolarizing them through OX2R (16). We examined whether YNT-185 excites histaminergic neurons by using wholecell patch-clamp recordings in mouse brain slices.…”

“…Although these results suggest that orexin replacement could be an effective mechanistic therapy, orexins do not efficiently cross the blood-brain barrier (15) and are therefore difficult to use as a clinical drug. Because deficiency of OX2R signaling is primarily responsible for sleepiness and cataplexy-like episodes (6,(16)(17)(18), nonpeptidic OX2R agonists are promising candidates for a therapeutic agent to treat narcolepsy. We recently discovered a potent nonpeptidic OX2R agonist, YNT-185 (compound 30 in ref.…”

Orexin system and pathogenesis of sleep-wake cycle disturbances in narcolepsy

“…При воспроизведении генетической формы нарколепсии в ýксперименте (животные, нокаут-ные по гену препроорексина) наблюдали анорек-сию, снижение уровня локомоторной активности, частоты сердечных сокращений, артериального давления и (так же, как у людей) регистрирова-ли невосприимчивость к аддиктивным расстрой-ствам и стрессорным воздействиям [21,22]. У жи-вотных, нокаутных по гену рецептора к орексину OxR2, были отмечены сходные симптомы [23].…”