Historically, dopamine (DA) receptors have been divided into two major subtypes referred to as D₁ and D₂, which differ in (1) their coupling to G-proteins, (2) their distribution within the central nervous system (CNS), and (3) their pharmacology. D₁ receptors activate the enzyme adenyl cyclase, and increase intracellular levels of cAMP, whereas D₂ receptors exert an inhibitory influence on this enzyme; D₂ receptors also may be linked to additional second-messenger systems, including activation of K⁺ channels, inhibition of Ca²⁺ channels, and phosphatidylinositol turnover. The cloning of these receptors and their genes in the last decade and the subsequent identification of multiple DA receptors referred to as D₁, D₂, D₃, D₄, and D₅ have profoundly changed our understanding of the anatomy and pharmacology of DA receptors. Two of these cloned receptors exhibit the functional and pharmacologic properties of classic D₁ receptors, whereas the other three present the pharmacologic characteristics of D₂ receptors. Thus, it is now recognized that subfamilies of D₁ and D₂ receptors exist rather than single receptor subtypes. Indeed, as can be seen in Table 1, DA elicits both similar and distinct responses compared with D₁-like (D₁ and D₅) and D₂-like (D₂, D₃, and D₄) receptors in fibroblast cells, expressing the recombinant receptors.