Dopamine D-1 and adenosine A(1) receptors form functionally interacting heteromeric complexes

The possible molecular basis for the previously described antagonistic inte ractions between adenosine A(1) receptors (A(1)R) and dopamine D-1 receptor s (D1R) in the brain have been studied in mouse fibroblast Ltk(-) cells cot ransfected with human A(1)R and D1R cDNAs or with human A(1)R and dopamine D-2 receptor (long-form) (D2R) cDNAs and in cortical neurons in culture. A( 1)R and D1R, but not A(1)R and D2R, were found to coimmunoprecipitate in co transfected fibroblasts. This selective A(1)R/D1R heteromerization disappea red after pretreatment with the D1R agonist, but not after combined pretrea tment with D1R and A(1)R agonists. A high degree of A(1)R and D1R colocaliz ation, demonstrated in double immunofluorescence experiments with confocal laser microscopy, was found in both cotransfected fibroblast cells and cort ical neurons in culture. On the other hand, a low degree of A(1)R and D2R c olocalization was observed in cotransfected fibroblasts. Pretreatment with the A(1)R agonist caused coclustering (coaggregation) of A(1)R and D1R. whi ch was blocked by combined pretreatment with the D1R and A(1)R agonists in both fibroblast cells and in cortical neurons in culture. Combined pretreat ment with D1R and A(1)R agonists, but not with either one alone, substantia lly reduced the D1R agonist-induced accumulation of cAMP. The A(1)R/D1R het eromerization may be one molecular basis for the demonstrated antagonistic modulation of A(1)R of D1R receptor signaling in the brain. The persistence of A(1)R/D1R heteromerization seems to be essential for the blockade of A( 1)R agonist-induced A(1)R/D1R coclustering and for the desensitization of t he D1R agonist-induced cAMP accumulation seen on combined pretreatment with D1R and A(1)R agonists, which indicates a potential role of A(1)R/D1R hete romers also in desensitization mechanisms and receptor trafficking.