Adrenergic receptors are seven-transmembrane receptors, which consist of a single polypeptide chain with seven hydrophobic regions that form alpha-helical structures that span or transverse the membrane. These receptors mediate the central and peripheral actions of the neurotransmitter norepinephrine (noradrenaline) and the hormone epinephrine (adrenaline), which are catecholamine messengers that play important roles in the regulation of diverse physiological systems. The adrenergic receptors are divided into three major types: alpha-1, alpha-2, and beta. Each of these three receptors types is further divided into three subtypes. Each of these receptors couples to a distinct class of G proteins and they can also interact with other signaling proteins and pathways.