Adrenergic receptors (ARs) belong to a superfamily of the G-protein-coupled receptors and are categorized by their binding to endogenously occurring catecholamines, i.e., norepinephrine and epinephrine. Adrenergic receptors are classified into three groups (α₁-, α₂-, and β-ARs), each of which is further divided into three subtypes. The α₁- (α_1A-, α_1B-, and α_1D-ARs) couple with G_q family of G-proteins (G₁₁, G₁₄, G₁₅, and G₁₆) and result in activation of phospholipase C-βs that liberate two second messengers, diacylglycerol and inositol-1,4,5-trisphosphate. The three subtypes of α₂-ARs are designated α_2A-, α_2B-, and α_2C-AR. On binding with agonists, α₂-AR inhibit adenylyl cyclase and calcium channels, but activate potassium channels through coupling to the G_i family of G-proteins (G_i1, G_i2, G_i3, and G_o). Finally, the three groups of β-AR are designated β₁-, β₂-, and β₃-AR: these increase the intracellular cAMP content by activating G_s, which is coupled to the enzyme, adenylyl cyclase (1).