Effect of reserpine-induced depletion of synaptic dopamine on [11C]Raclopride binding to D2-dopamine receptors in the monkey brain

Positron emission tomography was used to examine the in vivo binding of [11C]raclopride to D2-dopamine (DA) receptors in the striatum of two Cynomolgus monkeys after a single dose of reserpine (1 mg/kg, i.v.). A Scatchard procedure was repeated five times to follow D2 receptor density and apparent affinity for 7 weeks after reserpine. Reserpine-induced depletion of DA lead to a marked increase in [11C]raclopride binding, which was still detectable 20 days after treatment. Scatchard analyses indicated that the measured increase in [11C]raclopride binding reflected an increase in receptor affinity but no evident change in receptor density (Bmax). Thus, the increase in [11C]raclopride binding after reserpine should correspond to a reduced competition with endogenous DA for binding to D2 receptors. The results were used to estimate the DA-induced D2 occupancy to be about 40% at physiological conditions.