– Ac-RYYRIK-NH2 TFA acts as both a potent partial agonist for ORL1 when transfected in CHO cells (K d = 1.5 nM) and an endogenous ligand of ORL1. Moreover, it serves as a specific antagonist that inhibits G protein activation and competitively blocks the stimulation of [35S]-GTPγS binding to G proteins by nociceptin/orphanin FQ (noc/OFQ) in rat brain membranes and sections [1].