The ghrelin receptor [growth hormone secretagogue receptor (GHSR)‐1a] represents a promising pharmacologic target for the treatment of metabolic disorders, including obesity and cachexia, via central appetite modulation. The GHSR‐1a has a complex pharmacology, highlighted by G‐protein–dependent and—independent downstream signaling pathways and high basal constitutive activity. The functional selectivity and signaling bias of many GHSR‐1a–specific ligands has not been fully characterized. In this study, we investigated the pharmacologic properties of ghrelin, MK‐0677, L692,585, and [d‐Lys3]‐growth hormone–releasing peptide‐6 (Dlys), JMV2959, and [d‐Arg(1),d‐Phe(5),d‐Trp(7, 9), Leu(11)]‐substance P (SP‐analog). We investigated their effect on basal GHSR‐1a constitutive signaling, ligand‐directed downstream GHSR‐1a signaling, functional selectivity, and signaling bias. Dlys behaved as a partial antagonist with a strong bias toward GHSR‐1a–β‐arrestin signaling, whereas JMV2959 acted as a full unbiased GHSR‐1a antagonist. Moreover, the SP‐analog behaved as an inverse agonist increasing G‐protein–dependent signaling, but only at high concentrations, whereas, at low concentrations, the SP‐analog attenuated β‐arrestin–dependent signaling. Considering the limited success in the clinical development of GHSR‐1a–targeted drugs so far, these findings provide a novel insight into the pharmacologic characteristics of GHSR‐1a ligands and their signaling bias, which has important implications in the design of novel, more selective GHSR‐1a ligands with predictable functional outcome and selectivity for preclinical and clinical drug development.—Ramirez, V. T., van Oeffelen, W. E. P. A., Torres‐Fuentes, C., Chruścicka, B., Druelle, C., Golubeva, A. V., van de Wouw, M., Dinan, T. G., Cryan, J. F., Schellekens, H. Differential functional selectivity and downstream signaling bias of ghrelin receptor antagonists and inverse agonists. FASEB J. 33, 518–531 (2019). http://www.fasebj.org
