Development of a stable, early stage unilateral model of Parkinson's disease in middle-aged rhesus monkeys

Abstract

An important issue raised in testing new neuroprotective/restorative treatments for Parkinson's disease (PD) is the optimal stage in the disease process to initiate therapy. Current palliative treatments are effective in the early disease stages raising ethical concerns about substituting an experimental treatment for a proven therapy. Thus, we have endeavored to create a stable 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) nonhuman primate model of early PD. The new model was created by controlling for dose and route administration of MPTP (unilateral intracarotid infusion), and age of the animals (middleaged, 16-19 years old) in 27 female rhesus monkeys. All animals showed stable parkinsonian features lasting for up to 12-month as per behavioral evaluation. Compared with late-stage PD animals, postmortem analysis demonstrated that more dopaminergic neurons remained in the substantia nigra pars compacta, and more fibers were found in the striatum. In addition, tissue levels of striatal dopamine and its metabolites were also higher. Our results support that a milder but stable PD model can be produced in middle-aged rhesus monkeys.

Figure 1

A. Parkinsonian features at 3, 6 and 12 months following MPTP administration in 27 early-stage parkinsonian monkeys. No significant differences were observed from 3 through 12 months post MPTP treatment, indicating the stability of the model. A significant difference in parkinsonism severity was seen among the early-stage (5.3 ± 0.15 points) and late-stage model animals (8.3± 0.45 points) on the rating scale. +P<0.001, late- vs. early-stage. In addition, L-dopa improved overall PD rating scale in the early-stage model animals. B. Levodopa significantly improved parkinsonian features including bradykinesia, rigidity of the upper and lower limbs, balance, and postural instability. *P<0.05, L-dopa vs. other time points.

Figure 2

Changes in locomotor function 12 months following MPTP administration in 27 early stage parkinsonian monkeys. Both the total distance traveled (A) and movement speed (B) were significantly decreased versus baseline (F=20.4, r²=0.4, P<0.001). The last bar of each panel shows that L-dopa significantly improved each measurement. **P<0.01 MPTP vs. baseline.

Figure 3

TH+ cell numbers in the substantia nigra on the uninjected and injected hemispheres of the early stage and late stage parkinsonian monkey models. Compared to the uninjected side, a loss in TH+ neurons of approximately 68% was seen in the substantia nigra in the early stage parkinsonian animals versus an 87% reduction in the late stage parkinsonian animals. ** P<0.01 vs. uninjected side.

Figure 4

Both panels show representative examples of remaining TH+ neurons in the substantia nigra in an early stage (A) versus late stage (B) monkey model. In both stages of MPTP-induced parkinsonism, the VTA on the lesioned side was less affected. In contrast, the lateral region of the substantia nigra was most affected by the MPTP lesions. Tissue loss (asterisk) during processing for immunocytochemistry produced the cavitation in b). Scale bar = 1mm. R= right side; SNc= substantia nigra compacta; SNr= substantia nigra reticulata; VTA= ventral tegmental area;

Figure 5

TH+ staining in the striatum. A. TH+ staining in a section through the striatum from an early stage parkinsonian monkey. Compared to the uninjected side, MPTP administration produced an uneven loss of TH+ fibers in the putamen. There appeared to be a greater loss in the lateral putamen and dorsal caudate nucleus. B. TH staining in a late stage parkinsonian monkey. Compared to the uninjected side, very few TH+ fibers could be observed. Scale bar =1mm. Cd= caudate nucleus; IC= internal capsule; LV=lateral ventricle; Put= putamen; R= right side.

Figure 6

TH+ fiber densities in the caudate nucleus and putamen of parkinsonian animals. No significant difference in TH+ fiber density was found between early stage and late stage parkinsonian animals in both the caudate nucleus and the putamen on the contralateral side of MPTP administration (Open bars in A &B). However, higher doses of MPTP almost completely destroyed all TH+ fibers in these structures (about 1% remained, far right close bars in A&B) while in milder parkinsonian animals (the left close bars) approximately 30% and 25% of TH+ fibers remained in the caudate nucleus and the putamen, respectively. **P<0.01

Figure 7

Whole tissue levels of DA, DOPAC and HVA in the caudate nucleus of early-stage and late-stage PD models. A) In early-stage PD animals, tissue levels of DA, DOPAC and HVA were greatly reduced in the caudate nucleus and putamen in the injected hemisphere of the animals as compared to the un-injected side of the parkinsonian animals. No significant differences in DA and DOPAC between the caudate nucleus and putamen were observed. However, a significant difference was found in HVA between the caudate nucleus and putamen. The level of HVA was about 40% higher in the putamen. B) No significant differences in DA, DOPAC, and HVA were found in late-stage animals, but they were greatly reduced as compared to the un-injected hemisphere. *P=0.009.