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Imaging the competition between D(2/3) radioligands and endogenous dopamine is so far the only way to measure dopamine release in the living human brain. The dopamine D(2) receptor exists in a high (D(2)(high)) and a low-affinity state for dopamine. Under physiological conditions, dopamine is expected to bind to D(2)(high) only. [(11)C]-(+)-4-propyl-9-hydroxynaphthoxazine ((+)-PHNO) is the first D(2/3) agonist radioligand for positron emission tomography (PET) imaging in humans. Since [(11)C]-(+)-PHNO is expected to bind preferentially to D(2)(high), it should be particularly vulnerable to competition with endogenous dopamine. Nine healthy subjects participated in two PET scans, one after administration of d-amphetamine and one after placebo. [(11)C]-(+)-PHNO PET test re-test variability was determined in 11 healthy subjects. Binding potentials (BPs) were calculated for caudate, putamen, ventral striatum, and globus pallidus. d-Amphetamine led to a significant decrease of [(11)C]-(+)-PHNO BPs in caudate (-13.2%), putamen (-20.8%), and ventral striatum (-24.9%), but not in globus pallidus (-6.5%). d-Amphetamine-induced displacement correlated with serum d-amphetamine levels in all regions but caudate. This is the first report on competition between endogenous dopamine and a D(2/3) agonist radioligand in humans. [(11)C]-(+)-PHNO PET might be a superior measure for release of endogenous dopamine than PET employing conventional D(2/3) antagonist radioligands.

First Human Evidence of d-Amphetamine Induced Displacement of a D2/3 Agonist Radioligand: A [11C]-(+)-PHNO Positron Emission Tomography Study