Amphetamine pretreatment induces a change in both D2-Receptor density and apparent affinity: a [11C]raclopride positron emission tomography study in cats

Background Measuring changes in dopamine (DA) levels in humans using radioligand-displacement studies and positron emission tomography (PET) has provided important empirical findings in disease and normal neurophysiology. These studies are based on the assumption that DA exerts a competitive inhibition on radioligand binding. To test this, we used PET and a Scatchard approach to investigate whether the decrease in [11C]raclopride binding following amphetamine results from competitive or noncompetitive interactions with DA. Methods Scatchard analyses of [11C]raclopride/PET data were used to quantify changes in apparent D2-receptor density (Bmax) and radioligand apparent affinity (K′D) at baseline and after amphetamine pretreatment (2 mg/kg; intravenous) in cats. Results Amphetamine induced a 46% decrease in [11C]raclopride binding in the striatum of five cats. Scatchard analyses revealed that this decrease in binding was due to a 28% decrease in Bmax and a concomitant 35% increase in K′D. Conclusions Competition with DA is an insufficient explanation for the decrease in [11C]raclopride binding observed after amphetamine. Noncompetitive interactions, likely representing D2-receptor internalization, also play an important role in this phenomenon. This finding may have important implications for the interpretation of amphetamine–raclopride PET studies in schizophrenia because dysregulation of the agonist-induced internalization of D2 receptors was recently suggested in this disorder.