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Hypocretin (Hcrtr, HCRTR) / orexin (OX) receptors modulate a range of neurobiological functions and are drug targets for several disorders. Mapping the distribution of receptors in the brain can inform their function and guide targeting of specific disorders. Although studied in rodents, orexin receptor distribution has remained relatively unexplored in humans, and thus there is also a paucity of comparative anatomy. The aim of this study was therefore to map the distribution of hypocretin/orexin receptor mRNA in selected regions of the mouse and human brain by non-radioactive in situ hybridization (ISH) using digoxigenin (DIG)-labelled cRNA anti-sense probes. Data revealed both distinct and overlapping patterns of distributions of Hcrtr1/HCRTR1 and Hcrtr2/HCRTR2 mRNA suggesting that the functions of the orexin system are mediated differently by each receptor. In the mouse brain, the highest expression of Hcrtr1 mRNA was in the locus coeruleus (LC) whereas Hcrtr2 mRNA was most abundant in the lateral hypothalamus (LH). The human caudate nuclei showed significant expression of both HCRTR1 and HCRTR2 mRNA, whereas the mouse predominantly expressed Hcrtr2 mRNA. The noradrenergic neurons of the human LC showed high signals for both HCRTR1 (71.7%) and HCRTR2 (81.5%) mRNA. Expression of HCRTR2 mRNA in non-noradrenergic human LC cells was also notable. The distribution pattern in mouse and human brains is consistent with the involvement of the orexin system in arousal and the sleep/wake cycle in both species, however, variations in receptor subtype expression profiles suggests that species differences in responses to orexin receptor ligands may be expected.

Orexin receptor antagonists are on the market or under development for the treatment of insomnia and a number of other neuropsychiatric disorders. Currently, suvorexant, lemborexant and daridorexant, three dual orexin receptor antagonists (DORAs) have received market approval by regulatory authorities in the USA, Australia, Europe and/or Japan for the treatment of insomnia. More DORAs and Selective Orexin Receptor Antagonists (SORAs) in addition to orexin receptor agonists are in various stages of preclinical and clinical development: for instance, 1SORAs (selective orexin 1 receptor antagonists) are being developed for the treatment of anxiety, panic, eating disorders, whereas 2SORAs (selective orexin 2 receptor antagonists) are in late clinical stage for the treatment of insomnia and insomnia-related depression. On the other hand, selective orexin 2 receptor agonists are in clinical trials for the treatment of narcolepsy with (NT1) or without cataplexy (NT2) and other aspects of extreme day time sleepiness.Traditionally, the medium to high throughput screening procedures used to screen / characterize orexin receptor antagonists or agonists (and for that matter ligands acting on a variety of potential drug targets), frequently ignore two aspects of new drugs candidates: possible functional selectivity (biased agonism or antagonism) and intrinsic receptor-ligand kinetic properties (i.e. association and dissociation features at the target level), since most screening protocols are conducted under short incubation time conditions with usually a single functional readout. Here, we report on strategies to characterise orexin receptor ligands (agonists or antagonists) in radioligand binding and calcium mobilization assays (e.g. using the FLIPR ® /Fluorescent Imaging Plate Reader assay) and for a few select DORAs, on ERK activation. We studied clinically effective and/or tool orexin receptor antagonists (almorexant, suvorexant, filorexant, SB-649868, MK1064…), which have been or are being evaluated in clinical trials or are on the market, in these signalling pathways with an emphasis on kinetics. Thus, we investigated calcium mobilization and pERK elevation triggered by orexin A (OXA) in HEK293 cells stably transfected with human OX1R or OX2R. We confirmed that the ligands behave as antagonists in either assay. Most ligands do not show significant functional selectivity between the two pathways, except MK-1064, which inhibits calcium mobilization about 35 times more potently than ERK phosphorylation. We also estimated the kinetic properties of the antagonists in radioligand binding, calcium mobilization and pERK assays. The results of radioligand binding and calcium mobilization assays indicate consistently that several of the tested antagonists bind to/dissociate from either or the two orexin receptors very slowly, with equilibrium reached only after several hours. Thus, SB-649868 is a very slow binder at the OX1R, whereas almorexant is a very slow binder at the OX2R. By contrast, all tested antagonists (except almorexant at OX2R), seem to equilibrate with both orexin receptors relatively rapidly in the pERK assay. The different results drawn from the various test systems may relate to experimental design, with different temperature and buffer conditions: some assays were performed with live cells at 37 °C, while others were carried out at room temperature with either intact cells or cell membranes. Our data suggest that orexin receptor ligands may have entirely different effects in different brain cells / nuclei, if biased signalling was to occur. Independent of biased signalling, the kinetic properties of orexin receptor ligands will influence the actual receptor selectivity and duration of action of the compound, as is strongly suggested for clinically relevant antagonists such as almorexant, SB-649868, filorexant or suvorexant. Thus, this paper stresses the importance of studying orexin receptor ‘antagonists’ in different functional assays and to determine their kinetic properties under conditions that are as physiological as feasible.