Specific activation of the μ opioid receptor (MOR) by endomorphin 1 and endomorphin 2

Abstract

The recently discovered endomorphin 1 (Tyr-Pro-Trp-Phe-NH₂) and endomorphin 2 (Tyr-Pro-Phe-Phe-NH₂) were investigated with respect to their direct receptor-binding properties, and to their ability to activate G proteins and to inhibit adenylyl cyclase in both cellular and animal models. Both tetrapeptides activated G proteins and inhibited adenylyl cyclase activity in membrane preparations from cells stably expressing the μ opioid receptor, an effect reversed by the μ receptor antagonist CTAP (d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH₂), but they had no influence on cells stably expressing the δ opioid receptor. To further establish the selectivity of these peptides for the μ opioid receptor, brain preparations of mice lacking the μ opioid receptor gene were used to study their binding and signalling properties. Endomorphin 2, tritiated by a dehalotritiation method resulting in a specific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), labelled the brain membranes of wild-type mice with a K_d value of 1.77 nm and a B_max of 63.33 fmol/mg protein. In membranes of mice lacking the μreceptor gene, no binding was observed, and both endomorphins failed to stimulate [³⁵S]guanosine-5′-O-(3-thio)triphosphate ([³⁵S]GTPγS) binding and to inhibit adenylyl cyclase. These data show that endomorphins are capable of activating G proteins and inhibiting adenylyl cyclase activity, and all these effects are mediated by the μ opioid receptors.