Although ganglia in the heart are well known to be cholinergic, many other neurotransmitters and neuropeptides also influence (and are produced in) cardiac neurons, including adrenergic and purinergic compounds. Recently, histamine was suggested as a possible neurotransmitter in cardiac tissue. Although histamine does elicit many effects in the heart, does it stand up to rigorous scrutiny and fulfill certain criteria that are used to define neurotransmitters?