Messenger biomolecules or ‘keys’ (e.g., hormones, neuropeptides, neurotransmiitters) that bind specifically and reversibly to receptors or ‘locks’ (e.g., proteins) to form a larger complex, and thereby induce a signal in a cell (thus sometimes referred to as a ‘signal triggering molecule’). In a sense, they ‘float’ around in the bloodstream and in the extracellular fluid ‘looking for’ receptor sites to ‘lock’ on to. As the key turns inside the lock, the information encoded in the messenger is transmitted to the receptor site. In turn, the cell ‘door’ of the receptor opens so that the information is transmitted from the outside to the inside of the cell. Informationally, messengers function in one of two different ways, either as agonists or antagonists. An agonist ligand triggers the physiological function of another molecule (e.g., acetycholine is an agonist for a cholinergic receptor). An antagonist is a drug that binds to cell receptors and thereby prevents the agonist from eliciting a biological response. Continuing the metaphor, opening the door triggers the specific response for which the receptor site is responsible (e.g., pain relief). When a lligand has a antagonistic function, the messenger fits into the the lock, but is incapable of opening it. Consequently, the action of the receptor site is blocked and no message is transmitted. The term ‘ligands’ was first used by two German inorganic chemists, Alfred Stock and Carl Somiesky, in an article published in 1919 on the chemical properties of silicon. How it got from there to its use in molecular biology is unknown.
See Acetycholine (AcH), Boss (or bride of sevenless), Brain (neuro-) imaging, Cell, Cell adhesion molecules (CAMs), Cholinergic neurotransmitter system, Extracellular matrix, Hormones, Information, Molecular biology, Neurotransmitters, Peptides, Proteins