In vivo photocontrol of orexin receptors with a nanomolar light-regulated analogue of orexin-B

Orexinergic neurons are critically involved in regulating arousal, wakefulness, and appetite. Their dysfunction has been associated with sleeping disorders, and non-peptide drugs are currently being developed to treat insomnia and narcolepsy. Yet, no light-regulated agents are available to reversibly control their activity. To meet this need, a photoswitchable peptide analogue of the endogenous neuroexcitatory peptide orexin-B was designed, synthesized, and tested in vitro and in vivo. This compound (photorexin) is the first photo-reversible ligand reported for orexin receptors. It allows dynamic control of activity in vitro (including full agonism, nanomolar activity, and subtype selectivity to human OX2 receptors) and in vivo in zebrafish larvae by direct application in water. Photorexin induces dose- and light-dependent changes in locomotion and a reduction in the successive induction reflex that is associated with sleep behavior. Molecular dynamics calculations indicate that trans and cis photorexins adopt similar bent conformations and that the only discriminant between their structures and activities is the positioning of the N-terminus at the extracellular region of the orexin receptor. Thus, our approach could be extended to a broad and important family of neuropeptides that share a “message-address” mechanism when binding to their cognate receptors.