The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

The role of G protein-coupled receptor kinases (GRKs) in the regulation of dopamine D1A receptor responsiveness is poorly understood. To explore the potential role played by the GRKs in the regulation of the rat dopamine D1A receptor, we performed whole cell phosphorylation experiments and cAMP assays in 293 cells cotransfected with the receptor alone or with various GRKs (GRK2, GRK3, and GRK5). The agonist-dependent phosphorylation of the rat D1A receptor was substantially increased in cells overexpressing GRK2, GRK3, or GRK5. Moreover, we report that cAMP formation upon receptor activation was differentially regulated in cells overexpressing either GRK2, GRK3, and GRK5 under conditions that elicited similar levels of GRK-mediated receptor phosphorylation. Cells expressing the rat D1A receptor with GRK2 and GRK3 displayed a rightward shift of the dopamine dose-response curve with little effect on the maximal activation when compared with cells expressing the receptor alone. In contrast, cells expressing GRK5 displayed a rightward shift in the EC 50 value with an additional 40% reduction in the maximal activation when compared with cells expressing the receptor alone. Thus, we show that the dopamine D1A receptor can serve as a substrate for various GRKs and that GRK-phosphorylated D1A receptors display a differential reduction of functional coupling to adenylyl cyclase. These results suggest that the cellular complement of G protein-coupled receptor kinases may determine the properties and extent of agonist-mediated responsiveness and desensitization.Phosphorylation is an important mechanism involved in the regulation of numerous cellular responses, notably the responsiveness of G protein-coupled receptors (1). This phosphorylation process is believed to be the triggering mechanism that leads to receptor desensitization. The cellular responses elicited upon activation of G protein-coupled receptors are regulated in a dynamic fashion by the action of two classes of serine/threonine kinases. The first class consists of the second messenger-dependent kinases such as protein kinase A and protein kinase C (1). The second class consists of receptorspecific kinases that phosphorylate the agonist-occupied or activated form of the G protein-coupled receptors (1-3). These receptor kinases were originally described for rhodopsin (rhodopsin kinase) and the ␤ 2 -adrenergic receptor (␤-adrenergic receptor kinase) and are referred to as the G protein-coupled receptor kinases or GRKs 1 (1-3). This large family of kinases includes six members (GRK1 to GRK6) whose activities are regulated by phospholipids, posttranslational modifications, or G protein ␤␥ subunits (2-6). The GRKs are widely distributed in brain and periphery, suggesting an important role in the regulation of responsiveness of various G protein-coupled receptors (2, 7). Moreover, Arriza et al. (7) have shown that ␤-adrenergic receptor kinase 1 (GRK2) and ␤-adrenergic receptor kinase 2 (GRK3) are found in presynaptic and postsynaptic localizations in various brain r...