Specific binding of [11C]raclopride and N-[3H]propyl-norapomorphine to dopamine receptors in living mouse striatum:: Occupancy by endogenous dopamine and guanosine triphosphate-free G protein

According to the ternary complex model of G-protein linkage to receptors. agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radio-ligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should hake a greater effect on the binding of the D-2,D-3 partial agonist N-[H-3]propylnorapomorphine than on the binding of the D-2,D-3 antagonist [C-11]raclopride. The baseline binding potential (p(B(O))), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine. A as simultaneously measured in mouse striatum for [C-11]raclopride (p(B(O)) = 8.5) and N-[H-3]propylnorapomorphine (p'(B(O)) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice. the pB of N-[H-3]propylnorapomorphine decreased 52% whereas the p(B) of [C-11]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore. amphetamine decreased the p(B) of N-[H-3]propylnorapomorphine to a greater extent (73%) than that of [C-11]raclopride (43%) relative to the reserpine condition. For both radioligands. the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the phamacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine ()occupancy and decreased to a value 1.66 times greater for N-[H-3]propylnoramorphine than for [C-11]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist bindin-g is more sensitise than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of doparnine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[H-3]propylnorapomprphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.