GTP regulation of (-)-stepholidine binding to R(H) of D1 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 D1 receptors. [3H]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine ([3H]SCH-23390) and [3H]spiperone were used, respectively, as radioligands in D1 and D2 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 [3H]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 D1 DA receptor antagonist, fitted best to a single-site model with a Ki value of 1.7 +/- 0.5 nM. The R(H) of APO or SPD could be decreased by the addition of 450 microM GTP. In the [3H]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 D1 but antagonist effects on D2 DA receptors.
Dong
Z J
ZJ
Shanghai Institute of Materia Medica, Chinese Academy of Sciences.
Chen
L J
LJ
Jin
G Z
GZ
Creese
I
I
D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D001058:Apomorphine / Q000378:metabolism; D001552:Benzazepines / Q000378:metabolism; D001599:Berberine / Q000031:analogs & derivatives* / Q000378:metabolism; D001667:Binding, Competitive; D002417:Cattle; D003342:Corpus Striatum / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000494:pharmacology*; D008297:Male; D016627:Oxidopamine / Q000008:administration & dosage; D051381:Rats; D017207:Rats, Sprague-Dawley; D017447:Receptors, Dopamine D1 / Q000378:metabolism*; D012399:Rotation; D013134:Spiperone / Q000378:metabolism
D001552:Benzazepines; D017447:Receptors, Dopamine D1; D001599:Berberine; C010513:stepholidine; D013134:Spiperone; D006160:Guanosine Triphosphate; D016627:Oxidopamine; D000262:Adenylyl Cyclases; D001058:Apomorphine