Whole brain mapping of orexin receptor mRNA expression visualized by branched in situ hybridization chain reaction

Orexins, which are produced within neurons of the lateral hypothalamic area, play a pivotal role in the regulation of various behaviors, including sleep/wakefulness, reward behavior, and energy metabolism, via orexin receptor type 1 (OX1R) and type 2 (OX2R). Despite the advanced understanding of orexinergic regulation of behavior at the circuit level, the precise distribution of orexin receptors in the brain remains unknown. Here, we develop a new branched in situ hybridization chain reaction (bHCR) technique to visualize multiple target mRNAs in a semiquantitative manner, combined with immunohistochemistry, which provided comprehensive distribution of orexin receptor mRNA and neuron subtypes expressing orexin receptors in mouse brains. Only a limited number of cells expressing both Ox1r and Ox2r were observed in specific brain regions, such as the dorsal raphe nucleus and ventromedial hypothalamic nucleus. In many brain regions, Ox1r-expressing cells and Ox2r-expressing cells belong to different cell types, such as glutamatergic and GABAergic neurons. Moreover, our findings demonstrated considerable heterogeneity in Ox1r- or Ox2r-expressing populations of serotonergic, dopaminergic, noradrenergic, cholinergic, and histaminergic neurons. The majority of orexin neurons did not express orexin receptors. This study provides valuable insights into the mechanism underlying the physiological and behavioral regulation mediated by the orexin system, as well as the development of therapeutic agents targeting orexin receptors. Significance statementThe neuropeptide orexin regulates sleep and other behaviors through its receptors, OX1R and OX2R, which are targets for the development of therapeutic agents for sleep and related disorders. However, the cellular distribution of orexin receptors in the brain is only partially known. We applied a newly developed branched in situ hybridization chain reaction (bHCR) technique and conducted a whole-brain mapping of orexin receptor mRNA expression in the brain with neuron subtype markers. Few cells expressed both OX1R and OX2R, and OX1R and OX2R were expressed in the different neuronal subtypes in many brain regions. This study fills an important gap in understanding and modulating the orexin system.