The two known families of dopamine receptors, the Dj-like and the D2-like (26,43-45,85,86), are structurally similar to all the G protein-coupled receptors in that they have seven-transmembrane domains with cytoplasmic carboxy terminal domains and a glycosylated extracellular amino terminal domain. The Dj-like receptor family consists of the Dj and D5 subtypes. Because the central Dj and D5 receptors have the same primary structure as the peripheral dopamine receptors (Dja and Dm, respectively in rodents) (87), distinguishing "central" from "peripheral" dopamine receptors is no longer necessary. Despite similar affinities for dopamine, the distinct actions of the Dj and D5 receptors are a result of differential coupling to various intracellular G proteins associated with increases in intracellular enzymatic activity. For example, both the Dj and D5 couple to intracellular adenylyl cyclase to increase the conversion of ATP to cyclic adenosine monophosphate (cAMP) which in turn stimulates numerous intracellular events (87). However, the Dj receptor (in the presence of calcyon) but not the D5 receptor activates phospholipase C leading to the generation of inositol phosphates and diacylglycerol.
The D2-like dopamine receptor family consists of the D2, D3, and D4 receptors, all which couple to the inhibitory G proteins Gai and Go. Unlike the Dj receptors, the D2-like receptors inhibit adenylyl cyclase and calcium channel activities, and modulate potassium channel activity (45,88,89).
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