Receptor
A protein molecule, typically embedded in a cell membrane or located inside a cell, that binds to a specific signaling molecule (ligand) and triggers a corresponding biological response.
Receptors are the molecular gatekeepers of cellular signaling — a peptide or hormone produces its effect only when it successfully binds to its target receptor with sufficient affinity. Receptor specificity determines which tissues a compound affects: GHS-R receptors are concentrated in the pituitary and hypothalamus, which is why GHRP-type peptides specifically stimulate growth hormone release rather than acting throughout the body indiscriminately.
Receptor pharmacology concepts like agonism (activating a receptor), antagonism (blocking it), and partial agonism are essential for understanding how different peptides interact with the same receptor family. Receptor density, affinity, and downstream signaling efficiency can all be modulated by factors including chronic compound exposure (desensitization or upregulation), age, and physiological state.