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 - (α1A -, α1B -, and α1D -ARs) couple with Gq 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 α2A -, α2B -, and α2C -AR. On binding with agonists, α2 -AR inhibit adenylyl cyclase and calcium channels, but activate potassium channels through coupling to the Gi family of G-proteins (Gi1 , Gi2 , Gi3 , and Go ). Finally, the three groups of β-AR are designated β1 -, β2 -, and β3 -AR: these increase the intracellular cAMP content by activating Gs , which is coupled to the enzyme, adenylyl cyclase (1 ).