Occupancy of dopamine D_2/3 receptors in rat brain by endogenous dopamine measured with the agonist positron emission tomography radioligand [¹¹C]MNPA

Abstract

Estimates of dopamine D_2/3 receptor occupancy by endogenous dopamine using positron emission tomography (PET) in animals have varied almost threefold. This variability may have been caused by incomplete depletion of dopamine or by the use of antagonist radioligands, which appear less sensitive than agonist radioligands to changes in endogenous dopamine. PET scans were performed in rats with the agonist PET radioligand [¹¹C]MNPA ([O-methyl-¹¹C]2-methoxy-N-propylnorapomorphine). [¹¹C]MNPA was injected as a bolus plus constant infusion to achieve steady-state concentration in the body and equilibrium receptor binding in the brain. Radioligand binding was compared at baseline and after treatment with reserpine plus α-methyl-para-tyrosine, which cause ∼95% depletion of endogenous dopamine. Depletion of dopamine increased radioligand binding in striatum but had little effect in cerebellum. Striatal [¹¹C]MNPA binding potential was 0.93 ± 0.12 at baseline and increased to 1.99 ± 0.25 after dopamine depletion. Occupancy of D_2/3 receptors by endogenous dopamine at baseline was calculated to be ∼53%. Striatal binding was displaceable with raclopride, but not with BP 897 (a selective D₃ compound), thus confirming the D₂ receptor specificity of [¹¹C]MNPA binding. Radioactivity extracted from rat brain contained only 8–10% radiometabolites and was insignificantly altered by administration of reserpine plus α-methyl-para-tyrosine. Hence, dopamine depletion did not increase the PET measurements via an effect on radiotracer metabolism. Our in vivo estimate of dopamine's occupancy of D_2/3 receptors at baseline is higher than that previously reported using antagonist radioligands and PET, but is similar to that reported using agonist radioligands and ex vivo measurements. Synapse 62:756–763, 2008. Published 2008 Wiley-Liss, Inc.