The human body is made up of many different types of tissues and many different kinds of cells. To co-ordinate body functions, cells signal to other cells in the same organ and in different organs by releasing chemical messengers that travel through the bloodstream. The chemical messengers control all of the vital body processes. For example, they determine the force of a heartbeat and the number of beats per minute, the height of the blood pressure, and the propulsive energy of the intestine. In the brain these chemicals profoundly influence our moods and our behavior, including our drives for food and sex. When Lefkowitz began his work in the late 1960’s, scientists had already identified several chemical messengers but they did not know how these chemicals affected the target cells so as to alter their behavior. Over the subsequent 35 years Lefkowitz and his students painstakingly elucidated a family of molecules on the surface of target cells that receive the chemical messages. These receiving molecules are known as G protein-coupled receptors (GPCRs).
GPCRs are proteins embedded in the surface membrane of target cells with their receiving ends facing the outside fluid. Each cell produces many different GPCRs, each tuned to respond to different chemical messengers. For example, certain GPCRs called beta-adrenergic receptors located on heart muscle cells recognize adrenalin secreted by the adrenal gland and thereby control the heartbeat. When a human is physically threatened, the adrenal gland releases adrenalin which travels through the bloodstream and attaches to beta-adrenergic receptors on heart muscle. Once stimulated by the adrenalin, the receptors initiate a cascade of events that causes the heart to beat stronger and faster. This prepares the threatened person for “fight or flight”.