APA Dictionary of Psychology

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.