Isoform-specific uncoupling of the D2 dopamine receptors subtypes.

Dopaminergic transmission is fundamental to many neural pathways of clinical interest. We have analyzed the alternatively-spliced isoforms of the D(2) dopamine receptor, D(2) long (D(2l)) and D(2) short (D(2s)), which differ only by a 29-amino acid insertion in the third cytoplasmic loop. Well-known determinants for GPCR signal transduction--the third intracellular loop regions--were co-expressed with the wild-type receptors to test for their ability to antagonize parent receptor function. We found that the D(2l)-mediated inhibition of forskolin-stimulated adenylyl cyclase was blocked by the co-expression of the third cytoplasmic loop of D(2l). However, expression of the third cytoplasmic loop of D(2s) did not inhibit D(2l)-mediated signal transduction. Conversely, expression of the D(2s) third cytoplasmic loop antagonized the D(2s) receptor's function and the D(2l) third cytoplasmic loop did not. In contrast, expression of the alternatively-spliced insert region had no effect when co-expressed with either wild-type receptor isoform. These results suggest that the third cytoplasmic loops of each receptor adopt unique conformations and that the primary sequence of the insert region is not the basis for differences in signaling between D(2s) and D(2l). These findings further support previous studies suggesting that the D2 receptor isoforms use distinct signal transduction mechanisms.