Reciprocal interactions between adenosine A2A and dopamine D2 receptors in Chinese hamster ovary cells co-transfected with the two receptors

Human adenosine A(2A) and ratdopamine D-2 receptors (A(2A) and D-2 receptors) were co-transfected in Chinese hamster ovary (CHO) cells to study the interactions between two receptors that are co-localized in striatopallidal gamma aminobutyric acid-(GABA)ergic neurons. Membranes from transfected cells showed a high density of D-2 (3.6 pmol per mg protein) and A(2A) receptors (0.56 pmol per mg protein). The D-2 receptors were functional: an agonist, quinpirole, could stimulate GTP gamma S binding and reduce stimulated adenylyl cyclase activity. The A(2A) receptor agonist 2-p-( 2-carboxyethyl)phenethylamino-5 '-N-ethylcarboxamidoadenosine (CGS 21580) decreased high-affinity binding of the agonist dopamine at D-2 receptors. Activation of adenosine Az, receptors shifted the dose-response curve for quinpirole on adenosine 3',5'-cyclic monophosphate (cAMP) to the right. However, CGS 21680 did not affect dopamine D-2 receptor induced GTP gamma S binding, but did cause a concentration-dependent increase in cAMP accumulation. The maximal cAMP response was decreased by the D-2 agonist quinpirole in a concentration-dependent manner, but there was no change in EC50, and no effect in cells transfected only with adenosine A(2A) receptors. A(2A) receptor activation also increased phosphorylation of cAMP response element-binding protein and expression of c-fos mRNA. These effects were also strongly counteracted by quinpirole. These results show that the antagonistic actions between adenosine A(2A) and dopamine D-2, receptors noted previously in vivo can also be observed in CHO cells where the two receptors are co-transfected. Thus, no brain cell-specific factors are required for such interactions. Furthermore, the interaction at the second messenger level and beyond may be quantitatively more important than A,, receptor-mediated inhibition of high affinity D-2, agonist binding to the receptor. (C) 1999 Elsevier Science Inc.