Objective and quantitative evaluation of motor function in a monkey model of Parkinson's disease
Introduction
In rodents, non-human primates, and humans, systemic administration of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) causes the selective loss of dopamine (DA) neurons in the substantia nigra (SN), as seen in patients with Parkinson's disease (PD). The treatment of monkeys with MPTP has become the most successful primate model of human neurodegenerative disease. This treatment evokes a persistent syndrome in the monkey and produces virtually all of the cardinal behavioral, biochemical, and histological changes that occur within the DA system in PD (Andersson et al., 2006, Burns et al., 1983). Furthermore, most of the current anti-parkinsonism therapies were tested for efficacy and approved based on this model. Thus, MPTP-treated animals have been contributing to physiological and pharmacological investigations as PD models (Jenner, 2003).
A number of qualitative rating scales have been employed to evaluate the neurological functions of MPTP-treated monkeys. These scales, however, are functions of multiple variables, and the impact on each variable differs among the scales (Imbert et al., 2000). Therefore, more objective evaluation methods are needed for an accurate comparison of the effects of anti-parkinsonism therapies. Previous studies have used a video-based analysis system as a qualitative assessment of behavioral evaluations of rodents and non-human primates (Chassain et al., 2001, Liu et al., 2009, Togasaki et al., 2005). This type of system provides qualitative interpretations of movement, or the measurement of specific actions, as well as a quantitative assessment. Thus, the first aim of this study was to validate a video-based movement analysis system by comparing its results with behavioral evaluation using a qualitative rating scale.
An objective evaluation for indicating the function of the DA system can be also obtained using neuroimaging techniques. Specifically, positron emission tomography (PET) using 6-[18F]-fluoro-3,4-dihydroxy-l-phenylalanine ([18F]-F-DOPA), [11C]-2β-carbomethoxy-3β-(4-fluorophenyl)-tropane ([11C]-CFT), and [11C]-raclopride has been clinically used to evaluate DA synthesis capacity, DA transporter (DAT), and DA receptors, respectively. Several PET studies in humans (Bruck et al., 2009, Nurmi et al., 2003, Rinne et al., 2001) and monkeys (Oiwa et al., 2003, Wullner et al., 1994) have demonstrated a behavior-related decrease in DA synthesis or DAT density in the striatum. However, the behavioral evaluations that were used were not fully established as objective, and the anatomical focus of these studies was the striatum, where DA and DAT are most abundant. Thus, the second aim of this study was to investigate a correlation between the whole brain PET analysis and the behavior evaluations. In particular, we focused on DAT as an indicator of DA function, because it has been reported that CFT-PET is more sensitive than F-DOPA-PET in detecting DA hypofunction (Forsback et al., 2004). The DAT is a protein localized on the presynaptic membrane of DA neuron terminals; it clears DA from the interstitial space back into presynaptic elements by a process of facilitated diffusion (Kuhar et al., 1990). In addition, several studies have shown that a substantial amount of DAT also localizes in the somatodendritic membrane of DA neurons (Cheramy et al., 1981, Cobb and Abercrombie, 2002, Timmerman and Abercrombie, 1996) where it modulates local DA transmission within the SN (Vandecasteele et al., 2008) and thus may have a distinct role from that of nigro-striatal DA (Cheramy et al., 1981).
To accomplish these aims, we evaluated the neurological functions of MPTP-treated monkeys using a qualitative rating scale and video-based analysis system. We then performed an unbiased voxel-based analysis of the whole brain using [11C]-CFT-PET.
Section snippets
Animals
Adult (4 y.o.) cynomolgus monkeys (Macaca fascicularis), weighing 3.3–4.0 kg, were provided by Shin Nippon Biomedical Laboratories, Ltd., Kagoshima, Japan for this study. Animals in the MPTP-treated group (n = 13) were given intravenous injections of MPTP HCl (0.4 mg/kg as free base, Sigma–Aldrich) twice a week until persistent parkinsonian behavioral disturbances, such as tremor, bradykinesia, and impaired balance, became evident. The animals received an average of 15.6 MPTP administrations, and
Behavioral evaluation
The video-based analysis demonstrated that the amount of spontaneous movement was significantly less (an 88% reduction) in the MPTP-treated monkeys compared to normal monkeys [Fig. 1A; t(20) = 5.2, p < 0.01]. The parkinsonian monkeys were then divided into three groups by qualitative rating scores; mildly affected: less than 5 points (n = 4); moderately affected: 5–9 points (n = 4); severely affected: more than 10 points (n = 5). When analyzed by ANOVA, we found that the extent of spontaneous movement
Discussion
We showed that a video-based analysis system can detect a difference in spontaneous movements between normal and MPTP-treated monkeys. Moreover, we clearly showed a significant correlation between the quantity of spontaneous movements and the severity of qualitative rating scores. These results are consistent with previous studies which found a significant correlation of rating scale and the residual dopaminergic neurons in post-mortem evaluation (Elsworth et al., 2000). Other studies in
Acknowledgments
We thank T. Yamamoto and H. Magotani for technical assistance with the monkeys, and H. Watabe, T. Ose and H. Koshino for their technical assistance with PET scans. This study was supported by a Grant-in-Aid for Scientific Research from the JSPS, and a Kobe Cluster Project from the MEXT of Japan. The authors declare no conflicts of interest.
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These authors contributed equally.