Distinctive and Synergistic Signaling of Human Adenosine A2a and Dopamine D2L Receptors in CHO Cells

Abstract

Adenosine A2a receptor (A2aR) colocalizes with dopamine D2 receptor (D2R) in the basal ganglia and modulates D2R-mediated dopaminergic activities. A2aR and D2R couple to stimulatory and inhibitory G proteins, respectively. Their opposing roles in regulating neuronal activities, such as locomotion and alcohol consumption, are mediated by their opposite actions on adenylate cyclase, which often serves as “co-incidence detector” of various activators. On the other hand, the neural actions of A2aR and D2R are also, at least partially, independent of each other, as indicated by studies using D2R and A2aR knock-out mice. Here we co-expressed human A2aR and human D2LR in CHO cells and examined their signaling characteristics. Human A2aR desensitized rapidly upon agonist stimulation. A2aR activity (80%) was diminished after 2 hr of pretreatment with its agonist CGS21680. In contrast, human D2LR activity was sustained even after 2 hr and 18 hr pretreatment with its agonist quinpirole. Long-term (18 hr) stimulation of human D2LR also increased basal cAMP levels in CHO cells, whereas long-term (18 hr) activation of human A2aR did not affect basal cAMP levels. Furthermore, long-term (18 hr) activation of D2LR dramatically sensitized A2aR-induced stimulation of adenylate cyclase in a pertussis toxin-sensitive way. Forskolin-induced cAMP accumulation was significantly increased after short-term (2 hr) human D2LR stimulation and further elevated after long-term (18 hr) D2LR activation. However, neither short-term (2 hr) nor long-term (18 hr) stimulation of A2aR affected the inhibitory effects of D2LR on adenylate cyclase. Co-stimulation of A2aR and D2LR could not induce desensitization or sensitization of D2LR either. In summary, signaling t hrough A2aR and D2LR is distinctive and synergistic, supporting their unique and yet integrative roles in regulating neuronal functions when both receptors are present.