endogenous ligand

An endogenous ligand is the body's own molecule that a receptor was built to recognize — the original key cut by nature for that particular lock. 'Endogenous' simply means 'produced inside the body', as opposed to a drug that comes from outside. Every receptor with a known job has, or is presumed to have, such a natural partner.

These native messengers include neurotransmitters such as acetylcholine and dopamine, hormones such as adrenaline and insulin, and local signals such as histamine and prostaglandins. They are the standard against which drugs are measured: a drug that mimics the endogenous ligand is an agonist, while one that blocks the receptor and keeps the natural ligand out is an antagonist.

Understanding a receptor's endogenous ligand guides both drug design and how we interpret drug effects, because a drug competes with, or substitutes for, this natural signal. The honest caveat is that the picture is rarely one-to-one: a single receptor may have several endogenous ligands of differing strength, and a single ligand often acts at several different receptors throughout the body.