second messenger
an ion or molecule inside a cell whose concentration increases or decreases in response to stimulation of a cell receptor by an agonist (e.g., a neurotransmitter, hormone, or drug). The second messenger acts to relay and amplify the signal from the agonist (the first messenger) by triggering a range of cellular activities. For example, receptors for the catecholamine neurotransmitters epinephrine and norepinephrine are coupled to G proteins, whose activation in postsynaptic neurons affects levels of second messengers that act to open or close certain ion channels. Second messengers include cyclic AMP, cyclic GMP, IP3 (see inositol phosphates), and calcium ions.