Introduction

Adrenergic receptors mediate the central and peripheral actions of the neurohormones norepinephrine and epinephrine. Both of these catecholamine messengers play important roles in the regulation of diverse physiological systems, and thus adrenergic receptors are widely distributed throughout the body. Stimulation of adrenergic receptors by catecholamines released from the sympathetic autonomic nervous system results in a variety of effects, such as increased heart rate, regulation of vascular tone, and bronchodilatation. In the central nervous system, adrenergic receptors are involved in many functions, including memory, learning, alertness, and the response to stress.

Although adrenergic receptors were originally divided into two major types, α and β, the current classification scheme is based on three major types, α1, α2 and β. Each of these three receptor types is further divided into three subtypes: α1A, α1B, α1D; α2A, α2B, α2C; β1, β2, β3. All of these adrenergic receptors are seven transmembrane receptors, each consisting of a single polypeptide chain with seven hydrophobic regions that are thought to form α helical structures and span or transverse the membrane.

Because the mechanism of action of adrenergic receptors includes the activation of guanine nucleotide regulatory binding proteins (G proteins), these receptors are also called G protein-coupled receptors. The binding of norepinephrine (or another agonist) to an adrenergic receptor induces (or stabilizes) a conformational change that allows the receptor to interact with and activate a G protein.