Abstract
Competition by endogenous dopamine with the binding of D~2~ dopamine receptor ligands may be important in the interpretation of positron emission tomography (PET) neuroreceptor studies. PET studies with Nβmethylspiperone (NMSP) have revealed increased D~2~ dopamine receptors in schizophrenia, whereas studies with raclopride (RAC) have not detected such differences. This may be due, at least in part, to differences in competition with endogenous dopamine for ligand binding. To determine effects of endogenous dopamine on in vivo receptor binding, adult male rats were preinjected with amphetamine and reserpine prior to [^3^H]NMSP or [^3^H]RAC. Striatal to cerebellar ratios of ligand binding were determined. To approximate the conditions of a PET study, a kinetic model was employed to examine effects of pharmacologically increasing brain dopamine levels (amphetamine pretreatment) on PET ligand binding. In these experiments, tail veins and arteries were cannulated and kinetic parameters determined from normalized integral plots in rats treated with amphetamine prior to radioligand injection. Both [^3^H]NMSP (43.5%) and [^3^H]RAC (41.5%) binding were significantly decreased after amphetamine pretreatment, whereas after reserpine pretreatment [^3^H]RAC binding was increased (52.7%). Kinetic studies revealed a marked resistance of [^3^]NMSP to competition with endogenous dopamine released by amphetamine. In contrast, kinetic parameters of [^3^H]RAC were markedly reduced at all time intervals. This suggests significant differences in competition with endogenous dopamine by [^3^H]NMSP and [^3^H]RAC, determined kinetically. These findings may have important implications for the interpretation of PET neuroreceptor studies.