GTP regulation of (-)-stepholidine binding to R-H of D-1 dopamine receptors in calf striatum

(-)-Stepholidine (SPD) exhibits antagonist effects on normosensitive dopamine (DA) receptors, but it has an agonist action on rotation in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. The present work endeavors to further elucidate the mechanism of its agonist action on D-1 receptors. [H-3]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1-H-3-benzazepine ([H-3]SCH-23390) and [H-3]spiperone were used, respectively, as radioligands in D-1 and D-2 DA receptor binding assays in calf striatal membranes. Experimental data were analyzed by a non linear regression computer program, GraphPAD InPlot 3.15. The competition curves were fitted first by a single-site equation and then by a two-site equation. The results showed that both apomorphine (APO) and SPD competitively inhibited [H-3]SCH-23390 binding. Their competition curves fitted best to the two-site equation (P < 0.05) with a high-affinity site (R-H) and a low-affinity site (R-L) to DA receptors. The K-H and K-L values (nM) were 2.7 +/- 0.45 and 378 +/- 62 for APO, and 3.9 +/- 2.2 and 126 +/- 25 for SPD, respectively. In contrast, the competition curve of SCH-23390, a selective D-1 DA receptor antagonist, fitted best to a single-site model with a K-i value of 1.7 +/- 0.5 nM. The R-H Of APO or SPD could be decreased by the addition of 450 mu M GTP. In the [H-3]spiperone binding test, the APO curve was modeled best by the two-site equation, while the SPD curve fitted best to a single site model. In the rotational behavior test, APO induced 441 +/- 20 turns/30 min in the 6-OHDA-lesioned rats, and SPD induced 310 +/- 42 turns/30 min, while SCH-23390 antagonized the SPD induced rotation but did not induce rotational behavior. These results suggest that SPD possesses agonist actions on D-1 but antagonist effects on D-2 DA receptors. (C) 1997 Elsevier Science Inc.