Orexin Receptor Antagonism: A New Principle in Neuroscience

Orexins are hypothalamic neuropeptides interacting with G-protein coupled receptors in the brain. They play a role in the regulation of sleep–wake cycles in mammals, as suggested by the deficits in orexinergic function that are associated with rodent, canine and human narcolepsy. Selective or dual orexin1-receptor and/or orexin2-receptor antagonists or agonists that cross the blood-brain-barrier (BBB) may be of therapeutic interest for disorders of disturbed arousal and alertness. This article summarizes recent research to identify and characterize orexin receptor antagonists and their therapeutic potential for normalizing sleep in insomnia patients

Dual Hypocretin Receptor Antagonism Is More Effective for Sleep Promotion than Antagonism of Either Receptor Alone

The hypocretin (orexin) system is involved in sleep/wake regulation, and antagonists of both hypocretin receptor type 1 (HCRTR1) and/or HCRTR2 are considered to be potential hypnotic medications. It is currently unclear whether blockade of either or both receptors is more effective for promoting sleep with minimal side effects. Accordingly, we compared the properties of selective HCRTR1 (SB-408124 and SB-334867) and HCRTR2 (EMPA) antagonists with that of the dual HCRTR1/R2 antagonist almorexant in the rat. All 4 antagonists bound to their respective receptors with high affinity and selectivity in vitro. Since in vivo pharmacokinetic experiments revealed poor brain penetration for SB-408124, SB-334867 was selected for subsequent in vivo studies. When injected in the mid-active phase, SB-334867 produced small increases in rapid-eye-movement (REM) and non-REM (NR) sleep. EMPA produced a significant increase in NR only at the highest dose studied. In contrast, almorexant decreased NR latency and increased both NR and REM proportionally throughout the subsequent 6 h without rebound wakefulness. The increased NR was due to a greater number of NR bouts; NR bout duration was unchanged. At the highest dose tested (100 mg/kg), almorexant fragmented sleep architecture by increasing the number of waking and REM bouts. No evidence of cataplexy was observed. HCRTR1 occupancy by almorexant declined 4–6 h post-administration while HCRTR2 occupancy was still elevated after 12 h, revealing a complex relationship between occupancy of HCRT receptors and sleep promotion. We conclude that dual HCRTR1/R2 blockade is more effective in promoting sleep than blockade of either HCRTR alone. In contrast to GABA receptor agonists which induce sleep by generalized inhibition, HCRTR antagonists seem to facilitate sleep by reducing waking “drive”

Overview of orexin/hypocretin system

A series of recent studies has established the orexin/hypocretin system as a critical regulator of sleep/wake states. Its deficiency results in the sleep disorder narcolepsy in humans, dogs, and rodents. These findings have brought about the possibility of novel therapies for sleep disorders including narcolepsy and insomnia. Moreover, accumulating evidence indicates that the orexin/hypocretin system regulates sleep and wakefulness through interactions with neuronal systems that regulate emotion, reward, and energy homeostasis. Here, we briefly summarize the progress of orexin/hypocretin studies and future perspectives. Key word