Orexin B, rat, mouse is an endogenous agonist at Orexin receptor with Kis of 420 and 36 nM for OX1 and OX2, respectively.
Orexin B is derived by proteolytic amino acid precursor, prepro-orexin, which is encoded by a gene localized to chromosome 17q21 in humans. In radioligand binding studies, Orexin B has a higher affinity for the OX2 receptor[1]. Human Orexin B has two amino acid substitutions when compared with the rodent sequence. OX2 receptor is indeed a high-affinity receptor for human orexin B, with an IC50 of 36 nM in the binding assay and an EC50 of 60 nM in the [Ca2+]i transient assay. Human Orexin B has significantly lower affinity for the human OX1: the calculated IC50 in the competitive binding assay and the EC50 in the [Ca2+]i transient assay are 420 nM and 2500 nM for human orexin-B, respectively[2].
Human Orexin B significantly augments food intake; at the 2 hr time point, 5- and 12-fold stimulation of food consumption is observed by 3 nM and 30 nM orexin-B, respectively[2].
References:
[1]. Smart D, et al. Orexins: a new family of neuropeptides. Br J Anaesth. 1999 Nov;83(5):695-7.
[2]. Sakurai T, et al. Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior. Cell. 1998 Feb 20;92(4):573-85.