Differential signalling of both wild-type and Thr(343) Arg dopamine D-2short receptor by partial agonists in a G-protein-dependent manner

G-protein activation and Ca2+ responses by the wild-type D-2short receptor and a mutation Thr(343)Argg, in the distal BBXXB motif of its third intrace llular loop, were investigated in CHO-K1 cells in terms of ligand:receptor: G-protein interactions. No evidence was obtained for constitutive, agonist- independent receptor activation, but differences in the ligand-mediated act ivation profiles of both the wild-type and mutant Thr(343) Arg D-2short rec eptor were observed. Most of the partial agonists, but not bromocriptine, d isplayed an enhanced response at the mutant D-2short receptor, suggesting t hat the mutation brings the receptor in a more active state. This enhanceme nt was apparent both at the level of G-protein activation ([S-35]GTP gammaS binding) and at the effector (Ca2+ response) and occurred with different G (alpha)-proteins. Partial agonists were also found to act differently via t he wild-type D-2short receptor depending on the involved G(alpha)-protein. Compared with higher efficacy agonists, partial agonists displayed Ca2+ res ponses with slower and dissimilar kinetic properties. Lisuride and in parti cular bromocriptine produced a more potent response in the co-presence of a G(alphao) protein instead of a chimeric G(alphaq/o)- or a promiscuous G(al pha 15)-protein. S(+)-propylnorapomorphine showed a similar partial respons e irrespective of the combined G(alpha)-protein. Bromerguride and (+)-UH 23 2 induced weak (16 to 21% versus dopamine) intrinsic activity in the co-pre sence of a G(alphaq/o)-protein in contrast to their silent properties with a G(alpha 15)- or a G(alphao)Cys(351) Ile-protein. In conclusion, the prese nt data strongly suggest that multiple activation binding sites are involve d with these ligands at the D-2short receptor, and that their activation ma y be unravelled by either the mutation or co-expressed G.-proteins being in vestigated. (C) 2001 Elsevier Science Inc. All rights reserved.