Dynamic Changes in Striatal mGluR1 But Not mGluR5 during Pathological Progression of Parkinson's Disease in Human Alpha-Synuclein A53T Transgenic Rats: A Multi-PET Imaging Study

Abstract

Parkinson's disease (PD) is a prevalent degenerative disorder affecting the CNS that is primarily characterized by resting tremor and movement deficits. Group I metabotropic glutamate receptor subtypes 1 and 5 (mGluR1 and mGluR5, respectively) are important targets for investigation in several CNS disorders. In the present study, we investigated the in vivo roles of mGluR1 and mGluR5 in chronic PD pathology by performing longitudinal positron emission tomography (PET) imaging in A53T transgenic (A53T-Tg) rats expressing an abnormal human α-synuclein (ASN) gene. A53T-Tg rats showed a dramatic decline in general motor activities with age, along with abnormal ASN aggregation and striatal neuron degeneration. In longitudinal PET imaging, striatal nondisplaceable binding potential (BPND) values for [(11)C]ITDM (N-[4-[6-(isopropylamino) pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]methylbenzamide), a selective PET ligand for mGluR1, temporarily increased before PD symptom onset and dramatically decreased afterward with age. However, striatal BPND values for (E)-[(11)C]ABP688 [3-(6-methylpyridin-2-ylethynyl)-cyclohex-2-enone-(E)-O-[(11)C]methyloxime], a specific PET ligand for mGluR5, remained constant during experimental terms. The dynamic changes in striatal mGluR1 BPND values also showed a high correlation in pathological decreases in general motor activities. Furthermore, declines in mGluR1 BPND values were correlated with decreases in BPND values for [(18)F]FE-PE2I [(E)-N-(3-iodoprop-2E-enyl)-2β-carbo-[(18)F]fluoroethoxy-3β-(4-methylphenyl) nortropane], a specific PET ligand for the dopamine transporter, a biomarker for dopaminergic neurons. In conclusion, our results have demonstrated for the first time that dynamic changes occur in mGluR1, but not mGluR5, that accompany pathological progression in a PD animal model.

Significance statement: Synaptic signaling by glutamate, the principal excitatory neurotransmitter in the brain, is modulated by group I metabotropic glutamate receptors, including the mGluR1 and mGluR5 subtypes. In the brain, mGluR1 and mGluR5 have distinct functional roles and regional distributions. Their roles in brain pathology, however, are not well characterized. Using longitudinal PET imaging in a chronic rat model of PD, we demonstrated that expression of mGluR1, but not mGluR5, dynamically changed in the striatum accompanying pathological PD progression. These findings imply that monitoring mGluR1 in vivo may provide beneficial information to further understand central nervous system disorders.

Keywords: PET; Parkinson's disease; dopamine transporter; mGluR1; mGluR5; nondisplaceable binding potential.

Figure 1.

Chemical structures of PET ligands used in this study.

Figure 2.

Experimental schedule for open-field testing and PET scans. A, Experimental time points in noncarrier and A53T-Tg rats. B, PET scan schedule for a given rat over the course of a week.

Figure 3.

General motor activities in noncarrier and A53T-Tg rats. A, B, Locomotion (number of square crossings per min; A) and rearing (number of times per min; B) scores in 4- to 16-month-old noncarrier (n = 6; open circles) and A53T-Tg (n = 12; filled circles) rats. C, Correlation between weight gain and locomotion in animals for PET experiments. *p < 0.05 (two-way ANOVA). Correlation coefficients (r), p values, and R² values are shown adjacent to the scatter plot.

Figure 4.

A–D, Representative immunohistochemical images for human ASN and TH on brain sections of noncarrier (16-month-old; A, C) and A53T-Tg (16-month-old; B, D) rats. Brain sections containing the caudate/putamen (A, B) and substantia nigra (C, D) were prepared. Red arrows indicate notable human ASN aggregation. Dotted red lines show the area of the substantia nigra.

Figure 5.

Representative parametric PET and MRI images for mGluR1, mGluR5, and DAT at 4, 8, 12, and 16 months in the striatum of the same noncarrier (top) and A53T-Tg (bottom) rats. A, Parametric images based on mGluR1 BP_ND using [¹¹C]ITDM. B, Parametric images based on mGluR5 BP_ND using (E)-[¹¹C]ABP688. C, Parametric images based on BP_ND using [¹⁸F]FE-PE2I. All slices were located −0.12 mm from bregma.

Figure 6.

Dynamic changes in striatal BP_ND values in 4- to 16-month-old noncarrier and A53T-Tg rats. A, BP_ND values represent mGluR1-specific binding to [¹¹C]ITDM (48–67 MBq, 0.2–0.9 nmol). B, BP_ND values represent mGluR5-specific binding to (E)-[¹¹C]ABP688 (40–54 MBq, 0.1–0.6 nmol). C, BP_ND values represent DAT-specific binding to [¹⁸F]FE-PE2I (11–16 MBq, 0.1–1.2 nmol). *p < 0.05, ***p < 0.001 (two-way ANOVAs); ^#p < 0.05, ^##p < 0.01 (post hoc analyses). For noncarrier rats, n = 4 per time point. For A53T-Tg rats, n = 6 at 4, 6, 8, and 10 months; n = 4 at 12, 14, and 16 months.

Figure 7.

A–D, In vitro autoradiography using noncarrier (A–C) and A53T-Tg (D–F) rat brain sections. A, D, Autoradiograms use [¹¹C]ITDM (18 MBq, 1.0 nm) for mGluR1. B, E, Autoradiograms use (E)-[¹¹C]ABP688 (6 MBq, 0.4 nm) for mGluR5. C, F, Autoradiograms use [¹⁸F]FE-PE2I (0.7 MBq, 16.5 pm) for DAT. G, Quantitative measurements (femtomoles per square millimeter) of binding for each radioligand on brain sections. ***p < 0.001 (Student's t test).

Figure 8.

Scatter plots between general motor activities and BP_ND values. A, Locomotion plotted against [¹¹C]ITDM BP_ND values specific to mGluR1. B, Locomotion plotted against (E)-[¹¹C]ABP688 BP_ND values specific to mGluR5. C, Locomotion plotted against [¹⁸F]FE-PE2I BP_ND values specific to DAT. D, Rearing plotted against [¹¹C]ITDM BP_ND values specific to mGluR1. E, Rearing plotted against (E)-[¹¹C]ABP688 BP_ND values specific to mGluR5. F, Rearing plotted against [¹⁸F]FE-PE2I BP_ND values specific to DAT. The regression lines in each graph show the 95% confidence intervals (dotted lines). Respective correlation coefficients (r), p values, and R² values are shown adjacent to each scatter plot.

Figure 9.

Scatter plot depicting BP_ND values compared between with [¹¹C]ITDM (specific to mGluR1) and [¹⁸F]FE-PE2I (specific to DAT). The regression line shows 95% confidence intervals (dotted lines). The correlation coefficient (r), p value, and R² value from our analysis are shown adjacent to the scatter plot.

Figure 10.

Percentage of regional change in BP_ND with [¹¹C]ITDM in A53T-Tg rats at 4, 8, 12, and 16 months compared with same-aged noncarrier rats. Regions of interest were selected on mGluR1-rich regions, such as the striatum, hippocampus, cingulate cortex, thalamus, and cerebellum.

Figure 11.

Changes in cerebellar [¹¹C]ITDM BP_ND. A, B, Cerebellar binding values for noncarrier and A53T-Tg rats are shown in relation to age (A) and as correlation plots against locomotion (B). The regression line shows 95% confidence intervals (dotted lines), with correlation coefficients (r), p values, and R² values from our analysis. For noncarrier rats, n = 4 in each time point. For A53T-Tg rats, n = 6 at 4, 6, 8, and 10 months; n = 4 at 12, 14, and 16 months.