D2 dopamine receptor internalization prolongs the decrease of radioligand binding after amphetamine: a PET study in a receptor internalization-deficient mouse model

Abstract

Dopamine released by amphetamine decreases the in vivo binding of PET radioligands to the dopamine D(2) receptor. Although concentrations of extracellular dopamine largely return to baseline within 1 to 2 h after amphetamine treatment, radioligand binding remains decreased for several hours. The purpose of this study was to determine whether the prolonged decrease of radioligand binding after amphetamine administration is caused by receptor internalization. To distinguish dopamine displacement from receptor internalization, we used wild-type and arrestin3 (arr3) knockout mice, which are incapable of internalizing D(2) receptors. In addition, we used both the D(2) selective agonist [(11)C]MNPA (which is thought to bind to the high affinity state of the receptor) and the D(2) selective antagonist [(18)F]fallypride (which does not differentiate between high and low affinity state). After an initial baseline scan, animals were divided in three groups for a second scan: either 30 min or 4 h after amphetamine administration (3 mg/kg, i.p.) or as retest. At 30 min, [(11)C]MNPA showed greater displacement than [(18)F]fallypride, but each radioligand gave similar displacement in knockout and wild-type mice. At 4 h, the binding of both radioligands returned to baseline in arr3 knockout mice, but remained decreased in wild-type mice. Radioligand binding was unaltered on retest scanning. Our results suggest that the prolonged decrease of radioligand binding after amphetamine is mainly due to internalization of the D(2) receptor rather than dopamine displacement. In addition, this study demonstrates the utility of small animal PET to study receptor trafficking in vivo in genetically modified mice.

Fig. 1

Representative parametric images of binding potential (BP_ND) from wild-type mice for [¹⁸F]fallypride (A) and for [¹¹C]MNPA (B). The corresponding coronal MRI image (C) is fused with the image of BP_ND for [¹¹C]MNPA (D) and shows high uptake in striatum.

Fig. 2

Baseline BP_ND values and time course of radioactivity in striatum and cerebellum of arr3 wild-type (n = 15) and knockout mice (n = 15). Panels A & B are for [¹¹C]MNPA. Panels C & D are for [¹⁸F]fallypride. The average BP_ND values (mean ± SD) for [¹¹C]MNPA were 0.91 ± 0.18 in wild-type and 0.92 ± 0.14 in knockout mice (A). The average BP_ND values for [¹⁸F]fallypride were 3.08 ± 0.77 in wild-type and 3.31 ± 0.99 in knockout mice. As expected from its lower affinity, [¹¹C]MNPA (B) washed out of striatum more quickly than did [¹⁸F]fallypride (D). Time activity curves show mean ± SD. Symbols: wild-type striatum (△), wild-type cerebellum (○), knockout striatum (■), knockout cerebellum (▼;).

Fig. 3

Striatal binding potential (BP_ND) of [¹¹C]MNPA in arr3 wild-type (open bars) and knockout (solid bars) mice on retest or at 30 min and 4 hours after amphetamine. BP_ND of knockout mice returned to baseline at 4 hours, whereas that of wild-type mice was still decreased. The dashed line represents the average BP_ND of test scan expressed as 100%; the bars show the percentage of this baseline for retest, amphetamine 30 min, and amphetamine 4 hours. Bars represent mean ± SD, n = 5 of each genotype in each group.

Fig. 4

Striatal binding potential (BP_ND) of [¹⁸F]fallypride in arr3 wild-type (open bars) and knockout (solid bars) mice on retest or at 30 min and 4 hours after amphetamine. BP_ND of knockout mice was slightly increased at 4 hours, whereas that of wild-type mice was slightly decreased. The dashed line represents the average BP_ND of test scan expressed as 100%; the bars show the percentage of this baseline for retest, amphetamine 30 min, and amphetamine 4 hours. Bars represent mean ± SD, n = 5 of each genotype in each group.