Abstract

The possible molecular basis for the previously described antagonistic interactions between adenosine A 1 receptors (A 1 R) and dopamine D 1 receptors (D 1 R) in the brain have been studied in mouse fibroblast Ltk − cells cotransfected with human A 1 R and D 1 R cDNAs or with human A 1 R and dopamine D 2 receptor (long-form) (D 2 R) cDNAs and in cortical neurons in culture. A 1 R and D 1 R, but not A 1 R and D 2 R, were found to coimmunoprecipitate in cotransfected fibroblasts. This selective A 1 R/D 1 R heteromerization disappeared after pretreatment with the D 1 R agonist, but not after combined pretreatment with D 1 R and A 1 R agonists. A high degree of A 1 R and D 1 R colocalization, demonstrated in double immunofluorescence experiments with confocal laser microscopy, was found in both cotransfected fibroblast cells and cortical neurons in culture. On the other hand, a low degree of A 1 R and D 2 R colocalization was observed in cotransfected fibroblasts. Pretreatment with the A 1 R agonist caused coclustering (coaggregation) of A 1 R and D 1 R, which was blocked by combined pretreatment with the D 1 R and A 1 R agonists in both fibroblast cells and in cortical neurons in culture. Combined pretreatment with D 1 R and A 1 R agonists, but not with either one alone, substantially reduced the D 1 R agonist-induced accumulation of cAMP. The A 1 R/D 1 R heteromerization may be one molecular basis for the demonstrated antagonistic modulation of A 1 R of D 1 R receptor signaling in the brain. The persistence of A 1 R/D 1 R heteromerization seems to be essential for the blockade of A 1 R agonist-induced A 1 R/D 1 R coclustering and for the desensitization of the D 1 R agonist-induced cAMP accumulation seen on combined pretreatment with D 1 R and A 1 R agonists, which indicates a potential role of A 1 R/D 1 R heteromers also in desensitization mechanisms and receptor trafficking.