Orexin neurons are exclusively localized in the lateral hypothalamic area and project their fibers to the entire central nervous system, including the histaminergic tuberomammillary nucleus (TMN). Dysfunction of the orexin system results in the sleep disorder narcolepsy, but the role of orexin in physiological sleep–wake regulation and the mechanisms involved remain to be elucidated. Here we provide several lines of evidence that orexin A induces wakefulness by means of the TMN and histamine H ₁ receptor (H1R). Perfusion of orexin A (5 and 25 pmol/min) for 1 hr into the TMN of rats through a microdialysis probe promptly increased wakefulness for 2 hr after starting the perfusion by 2.5- and 4-fold, respectively, concomitant with a reduction in rapid eye movement (REM) and non-REM sleep. Microdialysis studies showed that application of orexin A to the TMN increased histamine release from both the medial preoptic area and the frontal cortex by ≈2-fold over the baseline for 80 to 160 min in a dose-dependent manner. Furthermore, infusion of orexin A (1.5 pmol/min) for 6 hr into the lateral ventricle of mice produced a significant increase in wakefulness during the 8 hr after starting infusion to the same level as the wakefulness observed during the active period in wild-type mice, but not at all in H1R gene knockout mice. These findings strongly indicate that the arousal effect of orexin A depends on the activation of histaminergic neurotransmission mediated by H1R.

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 ∼45 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.

Sleep architecture is often disturbed after a stressful event; nevertheless, little is known about the brain circuitry responsible for the sleep perturbations induced by stress. We exposed rats to a psychological stressor (cage exchange) that initially causes an acute stress response, but several hours later generates a pattern of sleep disturbances similar to that observed in stress-induced insomnia in humans: increased sleep latency, decreased non-REM (nREM) and REM sleep, increased fragmentation, and high-frequency EEG activity during nREM sleep. We examined the pattern of Fos expression to identify the brain circuitry activated, and found increased Fos in the cerebral cortex, limbic system, and parts of the arousal and autonomic systems. Surprisingly, there was simultaneous activation of the sleep-promoting areas, most likely driven by ongoing circadian and homeostatic pressure. The activity in the cerebral cortex and arousal system while sleeping generates a novel intermediate state characterized by EEG high-frequency activity, distinctive of waking, during nREM sleep. Inactivation of discrete limbic and arousal regions allowed the recovery of specific sleep components and altered the Fos pattern, suggesting a hierarchical organization of limbic areas that in turn activate the arousal system and subsequently the cerebral cortex, generating the high-frequency activity. This high-frequency activity during nREM was eliminated in the stressed rats after inactivating parts of the arousal system. These results suggest that shutting down the residual activity of the limbic-arousal system might be a better approach to treat stress-induced insomnia, rather than potentiation of the sleep system, which remains fully active.

It has long been suspected that sensory signal transmission is inhibited in the mammalian brain during sleep. We hypothesized that Ca _v 3.1 T-type Ca ²⁺ channel currents inhibit thalamic sensory transmission to promote sleep. We found that T-type Ca ²⁺ channel activation caused prolonged inhibition (>9 s) of action-potential firing in thalamic projection neurons of WT but not Ca _v 3.1 knockout mice. Inhibition occurred with synaptic transmission blocked and required an increase of intracellular Ca ²⁺ . Furthermore, focal deletion of the gene encoding Ca _v 3.1 from the rostral–midline thalamus by using Cre/ loxP recombination led to frequent and prolonged arousal, which fragmented and reduced sleep. Interestingly, sleep was not disturbed when Ca _v 3.1 was deleted from cortical pyramidal neurons. These findings support the hypothesis that thalamic T-type Ca ²⁺ channels are required to block transmission of arousal signals through the thalamus and to stabilize sleep.