Endogenous dopamine (DA) competes with the binding of a radiolabeled D3 receptor partial agonist in vivo: a positron emission tomography study.

A series of microPET imaging studies were conducted in anesthetized rhesus monkeys using the dopamine D3-selective partial agonist, [¹8F]5. There was variable uptake in regions of brain known to express a high density of D3 receptors under baseline conditions. Pretreatment with lorazepam (1 mg/kg, i.v. 30 min) to reduce endogenous dopamine activity before tracer injection resulted in a dramatic increase in uptake in the caudate, putamen, and thalamus, and an increase in the binding potential (BP) values, a measure of D3 receptor binding in vivo. These data indicate that there is a high level of competition between [¹8F]5 and endogenous dopamine for D3 receptors in vivo.