A PET study of D1-like dopamine receptor ligand binding

Rationale: Several positron emission tomography (PET) studies have shown that radioligand binding to D₂-like dopamine receptors competes with endogenous dopamine. Objective: The purpose of this PET study was to examine the effect of amphetamine and reserpine on D₁-like dopamine receptor binding. Methods: Three Cynomolgus monkeys were examined with the radioligands ¹¹C]SCH 23390 or ¹¹C]NNC 112 at baseline condition and after pretreatment with amphetamine (2 mg/kg IV). The B/F values (binding potential) in the striatum and the neocortex were calculated at transient equilibrium. In two monkeys, the effect of the long-lasting dopamine depletion after reserpine (1 mg/kg IV) was followed by a repeated Scatchard procedure in up to 77 days after drug administration. The Scatchard analysis was based on two PET measurements with high and low specific radioactivity and allowed the calculation of D₁-like dopamine receptor density (B_max) and apparent affinity (K_D ^app). Results: The effect of amphetamine on the B/F values was between –14 and 6%. These changes can be considered as within the range of the test-retest reliability. Thus, there was no evident effect of amphetamine-induced dopamine release on D₁-like dopamine receptor binding. Five hours after reserpine administration, there was no change in B_max or K_D ^app. At 3, 23, and 28 days after reserpine administration, the Scatchard analyses indicated a 13–20% reduction in B_max without any evident change in K_D ^app in both the striatum and the neocortex. Conclusions: The lack of evident effects of amphetamine and reserpine on D₁-like dopamine receptor binding is markedly different from the 20% amphetamine-induced decrease and 50% reserpine-induced increase that has been consistently reported for D₂-like dopamine receptor binding. The data indicated that D₁-like dopamine receptor occupancy of endogenous dopamine is low at physiological condition. It is thus unlikely that D₁-like dopamine receptor radioligands can be used to measure changes in the concentration of endogenous dopamine. Received: 8 March 1999 / Final version: 12 May 1999