OX1 Orexin Receptors Couple to Adenylyl Cyclase Regulation via Multiple Mechanisms

In this study, the mechanism of OX1 orexin receptors to regulate adenylyl cyclase activity when recombinantly expressed in Chinese hamster ovary cells was investigated. In intact cells, stimulation with orexin-A led to two responses, a weak (21%), high potency (EC50 ≈ 1nm) inhibition and a strong (4-fold), low potency (EC50 = ≈300 nm) stimulation. The inhibition was reversed by pertussis toxin, suggesting the involvement of Gi/o proteins. Orexin-B was, surprisingly, almost equally as potent as orexin-A in elevating cAMP (pEC50 = ≈500 nm). cAMP elevation was not caused by Ca2+ elevation or by Gβγ. In contrast, it relied in part on a novel protein kinase C (PKC) isoform, PKCδ, as determined using pharmacological inhibitors. Yet, PKC stimulation alone only very weakly stimulated cAMP production (1.1-fold). In the presence of Gs activity, orexins still elevated cAMP; however, the potencies were greatly increased (EC50 of orexin-A = ≈10 nm and EC50 of orexin-B = ≈100 nm), and the response was fully dependent on PKCδ. In permeabilized cells, only a PKC-independent low potency component was seen. This component was sensitive to anti-Gαs antibodies. We conclude that OX1 receptors stimulate adenylyl cyclase via a low potency Gs coupling and a high potency phospholipase C → PKC coupling. The former or some exogenous Gs activation is essentially required for the PKC to significantly activate adenylyl cyclase. The results also suggest that orexin-B-activated OX1 receptors couple to Gs almost as efficiently as the orexin-A-activated receptors, in contrast to Ca2+ elevation and phospholipase C activation, for which orexin-A is 10-fold more potent.