PET imaging a MPTP-induced mouse model of Parkinson's disease using the fluoropropyl-dihydrotetrabenazine analog [18F]-DTBZ (AV-133)

Abstract

Parkinson's disease (PD) is characterized by the loss of dopamine-producing neurons in the nigrostriatal system. Numerous researchers in the past have attempted to track the progression of dopaminergic depletion in PD. We applied a quantitative non-invasive PET imaging technique to follow this degeneration process in an MPTP-induced mouse model of PD. The VMAT2 ligand (18)F-DTBZ (AV-133) was used as a radioactive tracer in our imaging experiments to monitor the changes of the dopaminergic system. Intraperitoneal administrations of MPTP (a neurotoxin) were delivered to mice at regular intervals to induce lesions consistent with PD. Our results indicate a significant decline in the levels of striatal dopamine and its metabolites (DOPAC and HVA) following MPTP treatment as determined by HPLC method. Images obtained by positron emission tomography revealed uptake of (18)F-DTBZ analog in the mouse striatum. However, reduction in radioligand binding was evident in the striatum of MPTP lesioned animals as compared with the control group. Immunohistochemical analysis further confirmed PET imaging results and indicated the progressive loss of dopaminergic neurons in treated animals compared with the control counterparts. In conclusion, our findings suggest that MPTP induced PD in mouse model is appropriate to follow the degeneration of dopaminergic system and that (18)F-DTBZ analog is a potentially sensitive radiotracer that can used to diagnose changes associated with PD by PET imaging modality.

Figure 1. Changes in levels of dopamine and its main metabolites in samples of brain striatum from MPTP treated mice.

The degree of dopaminergic neurodegeneration caused by MPTP administration was evaluated by examining the levels of dopamine (A), DOPAC (B), and HVA (C). As a control, the effect of MPTP administration on noradrenaline levels (D) was also evaluated in the same samples. Results of striatum values from control and MPTP treated mice were analyzed by ANOVA followed by Tukey’s post-hoc test. Lowercase letters above the bars indicate the results of that analysis; values that share any letter in common are not significantly different (P ≥ 0.05), and values that do not share any letter in common are significantly different (P < 0.05).

Figure 2. Immunohistochemical visualization of mouse brain striatum samples from control and MPTP treated mice.

Tyrosine hydroxylase expression was evaluated by immunofluorescence staining of a brain section from a control untreated mouse (A) or a corresponding brain section from a mouse treated with 17 mg/kg MPTP (B).

Figure 3. Non-invasive PET imaging of control and MPTP treated mice.

A control male C57BL/6NCrl mouse and mice treated with 15 mg/kg MPTP or 17 mg/kg MPTP were imaged by PET using [¹⁸F]-DTBZ. (A) Shown are coronal brain slices depicting the uptake of [¹⁸F]-DTBZ corresponding to the anatomical localization of VMAT sites in the striatum. (B) Shown are coronal brain slices from the same animals depicting immunohistochemical visualization of tyrosine hydroxylase activity in dopaminergic neurons.

Figure 4. Results from quantification of PET imaging.

Control male C57BL/6NCrl mice (m1 and m2) and mice treated with 15 mg/kg MPTP (m3 and m4) or 17 mg/kg MPTP (m5 and m6) were imaged by PET using [18F]-DTBZ. Normalized mean voxel values from regions of interest corresponding to each slice of striatum and cerebellum were obtained, and values shown are means±SD for all image slices from each animal. Results of striatum values from control and MPTP treated mice were analyzed by ANOVA followed by Tukey’s post-hoc test. Lowercase letters above the symbols indicate the results of that analysis; values that share any letter in common are not significantly different (P ≥ 0.05), and values that do not share any letter in common are significantly different (P < 0.05). In a similar fashion, results of cerebellum values from control and MPTP treated mice were analyzed by ANOVA followed by Tukey’s post-hoc test.