A second messenger is a small intracellular molecule generated in response to receptor activation whose function is to relay and amplify the primary (extracellular) signal to downstream effector proteins. The classical second messengers include cyclic AMP (cAMP), cyclic GMP (cGMP), inositol trisphosphate (IP₃), diacylglycerol (DAG), calcium ions (Ca²⁺) and nitric oxide.
Because second messengers can diffuse rapidly within the cell and each activated receptor can generate many messenger molecules, this layer of signalling provides substantial signal amplification. Compartmentalisation — mediated by scaffolding proteins, phosphodiesterases and localised production — restricts the messenger's action to specific subcellular regions.
Second-messenger systems (particularly cAMP) are the primary readouts in most functional GPCR assays used to characterise research peptides.