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

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 rec eptor-binding properties, and to their ability to activate G proteins and t o inhibit adenylyl cyclase in both cellular and animal models. Both tetrape ptides activated G proteins and inhibited adenylyl cyclase activity in memb rane preparations from cells stably expressing the mu opioid receptor, an e ffect reversed by the mu receptor antagonist CTAP (d-Phe-Cys-Tyr-d-Trp-Arg- Thr-Pen-Thr-NH2), but they had no influence on cells stably expressing the delta opioid receptor. To further establish the selectivity of these peptid es for the mu opioid receptor, brain preparations of mice lacking the mu op ioid receptor gene were used to study their binding and signalling properti es. Endomorphin 2, tritiated by a dehalotritiation method resulting in a sp ecific radioactivity of 1.98 TBq/mmol (53.4 Ci/mmol), labelled the brain me mbranes 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 mu receptor gene, no bind ing was observed, and both endomorphins failed to stimulate [S-35]guanosine -5'-O-(3-thio)triphosphate ([S-35]GTP gamma S) binding and to inhibit adeny lyl cyclase. These data show that endomorphins are capable of activating G proteins and inhibiting adenylyl cyclase activity, and all these effects ar e mediated by the mu opioid receptors.