Sleep, wake, and signaling: Functional profiling of orexin agonists and antagonists using newly developed orexin β-arrestin 2 and miniGαq recruitment assays.

The excitatory neuropeptides orexin-A and -B interact with their target G protein-coupled receptors (GPCRs), the orexin 1 and orexin 2 (OX1 and OX2) receptors, which are widely expressed throughout the central nervous system. The orexin system plays a critical role in regulating several physiological processes such as sleep-wake cycles, feeding behaviour, and arousal, and is implicated in a variety of (neurological) disorders. In particular dysregulation of the orexin system is linked to sleep disorders such as narcolepsy (often associated with orexin deficiency) and insomnia (characterized by an overactivity of sleep-wake regulation). This has prompted a growing interest in orexin-targeting therapeutics. This study is the first to report the development of four OX1 and OX2 receptor luminescence bioassays based on functional complementation of a split-nanoluciferase enzyme, capable of monitoring β-arrestin 2 (βarr2) and Gαq recruitment to activated OX1 and OX2 receptors. These assays were successfully applied to evaluate the pharmacological profiles of both agonists and antagonists, including the endogenous ligands orexin-A and -B, the clinically approved small molecule antagonists suvorexant and daridorexant, as well as EMPA (N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulphonyl)-amino]-N-pyridin-3-ylmethyl-acetamide) and four other compounds described in literature to act at orexin receptors. The obtained receptor activation patterns and selectivity profiles were consistent with literature data, indicating the reliability and robustness of the assay systems. Overall, the newly developed assays expand the toolkit for orexin receptor research by allowing the characterization of both agonists and antagonists, thereby contributing to the functional characterization of potential new drug candidates for various pathological conditions.