Hypocretin/Orexin Receptor Pharmacology and Sleep Phases

Abstract

The hypocretins/orexins are two excitatory neuropeptides, alternately called HCRT1 or orexin-A and HCRT2 or orexin-B, that are the endogenous ligands for two G-protein-coupled receptors, HCRTR1/OX1R and HCRTR2/OX2R. Shortly after the discovery of this system, degeneration of hypocretin/orexin-producing neurons was implicated in the etiology of the sleep disorder narcolepsy. The involvement of this system in a disorder characterized by the loss of control over arousal state boundaries also suggested its role as a critical component of endogenous sleep-wake regulatory circuitry. The broad projections of the hypocretin/orexin-producing neurons, along with differential expression of the two receptors in the projection fields of these neurons, suggest distinct roles for these receptors. While HCRTR1/OX1R is associated with regulation of motivation, reward, and autonomic functions, HCRTR2/OX2R is strongly linked to sleep-wake control. The association of hypocretin/orexin with these physiological processes has led to intense interest in the therapeutic potential of compounds targeting these receptors. Agonists and antagonists for the hypocretin/orexin receptors have shown potential for the treatment of disorders of excessive daytime somnolence and nocturnal hyperarousal, respectively, with the first antagonists approved by the US Food and Drug Administration (FDA) in 2014 and 2019 for the treatment of insomnia. These and related compounds have also been useful tools to advance hypocretin/orexin neurobiology.

Fig. 1.. Hypocretin/orexin-containing neurons and their projections, receptor distributions, and downstream behavioral outcomes.

Hypocretin/orexin-producing neurons are located in the hypothalamus (Hy) and project to diverse brain regions differentially expressing hypocretin/orexin receptors 1 and 2 (OX₁R and OX₂R; receptor expression is indicated by the background color of each brain region), including the cerebral cortex, cingulate cortex, bed nucleus of the stria terminalus (BNST), nucleus accumbens (NAc), tuberomammillary nucleus (TMN), dorsal raphé (DR), locus coeruleus (LC), paraventricular thalamus (PVT), ventrolateral periaqueductal grey (vlPAG), the dorsal deep mesencephalic nucleus (dDPMe), pedunculopontine and laterodorsal tegmental nuclei (PPT and LDT), substantia nigra (SN), ventral tegmental area (VTA), central area of the amygdala (CeA), and the paraventricular nucleus (PVN). Through these projections, hypocretin/orexin-containing neurons are able to elicit diverse behavioral outputs.