Pharmacological characterization of dopamine-stimulated [35S]-guanosine 5'(gamma-thiotriphosphate) ([35S]GTPgammaS) binding in rat striatal membranes.

Functional activation of dopamine receptors in the crude membranes from rat striatum was studied by a [35S]-guanosine 5'-O-(gamma-thiotriphosphate) ([35S]GTPgammaS) binding assay. Binding of [35S]GTPgammaS could be characterized with a dissociation constant (Kd) = 14.6+/-0.8 nM and this did not depend on the presence of dopamine. The displacement of [35S]GTPgammaS binding by GDP could be characterized with an inhibition constant (K(i)) = 78+/-15 microM in the presence of 10 microM of butaclamol, while the presence of 100 microM of dopamine decreased it to a K(i) = 0.13+/-0.02 mM. Dopamine increased the association rate of [35S]GTPgammaS binding in the presence of GDP in a dose-dependent manner with an EC50 = 1.45+/-0.48 microM. Other dopamine receptor agonists studied displayed a potency to stimulate the [35S]GTPgammaS binding in the order R(-)-10,11dihydroxy-N-n-propylnorapomorphine (NPA) > pergolide > or = apomorphine > dopamine approximately quinpirole > R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) > S(+)(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]be nzopyrano[4,3-b]-1,4-oxazin-9-ol hydrocholoride (PD 128,907). The dopamine-induced stimulation of [35S]GTPgammaS binding was inhibited by different dopamine receptor antagonists in the potency order: (+)butaclamol > haloperidol approximately clorpromazine > or = raclopride > (-)-sulpride > remoxipride > 5,6-dimethoxy-2-(dipropylamine)indan (U 991944A) > R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine (SCH-23390). Comparison of the obtained data with the dissociation constants of these ligands to different subtypes of dopamine receptors gave a good correlation only with constants for the D2 subtype, supporting the idea that this subtype is most likely responsible for the dopaminergic activation of [35S]GTPgammaS binding in rat striatal membranes.