Receptor.

A protein that binds a signalling molecule and triggers a cellular response.

A receptor is a protein, often sitting in the cell surface membrane, that binds a specific ligand such as a peptide hormone and converts that binding event into an intracellular signal. Most peptide hormones act on G-protein-coupled receptors, a large family that translates binding on the outside of the cell into second-messenger cascades on the inside. Because peptides are generally too large and too water-soluble to cross cell membranes freely, receptors are how they act at all, which is why almost every mechanistic claim about a peptide should come with a named receptor. The GLP-1 receptor, the GIP receptor, and the growth-hormone-secretagogue receptor that ghrelin and GHRPs use are the ones that appear most often in this field. Two features are worth understanding. Receptors are distributed unevenly across tissues, so the same molecule can produce effects in the pancreas, stomach, and brain simultaneously depending on where its receptor is expressed. And receptors adapt, becoming less responsive under sustained stimulation, which is the basis of tolerance. The common misunderstanding is that a compound binding a receptor in a laboratory dish implies a meaningful effect in a living body. Binding is necessary but nowhere near sufficient.