Metabotropic glutamate receptor 5 antagonist protects dopaminergic and noradrenergic neurons from degeneration in MPTP-treated monkeys

Abstract

Degeneration of the dopaminergic nigrostriatal system and of noradrenergic neurons in the locus coeruleus are important pathological features of Parkinson's disease. There is an urgent need to develop therapies that slow down the progression of neurodegeneration in Parkinson's disease. In the present study, we tested whether the highly specific metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, reduces dopaminergic and noradrenergic neuronal loss in monkeys rendered parkinsonian by chronic treatment with low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Weekly intramuscular 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections (0.2-0.5 mg/kg body weight), in combination with daily administration of 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine or vehicle, were performed until the development of parkinsonian motor symptoms in either of the two experimental groups (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine versus 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle). After 21 weeks of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment, all 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals displayed parkinsonian symptoms, whereas none of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys were significantly affected. These behavioural observations were consistent with in vivo positron emission tomography dopamine transporter imaging data, and with post-mortem stereological counts of midbrain dopaminergic neurons, as well as striatal intensity measurements of dopamine transporter and tyrosine hydroxylase immunoreactivity, which were all significantly higher in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated animals than in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated monkeys. The 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine treatment also had a significant effect on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced loss of norepinephrine neurons in the locus coeruleus and adjoining A5 and A7 noradrenaline cell groups. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals, almost 40% loss of tyrosine hydroxylase-positive norepinephrine neurons was found in locus coeruleus/A5/A7 noradrenaline cell groups, whereas the extent of neuronal loss was lower than 15% of control values in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys. Our data demonstrate that chronic treatment with the metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, significantly reduces 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity towards dopaminergic and noradrenergic cell groups in non-human primates. This suggests that the use of metabotropic glutamate receptor 5 antagonists may be a useful strategy to reduce degeneration of catecholaminergic neurons in Parkinson's disease.

Figure 1

Study design.

Figure 2

Histograms illustrating the progressive increase of (A) Parkinsonism rating scores and (B) food picking ‘entry’ time (i.e. the time between entering the apparatus and retrieval of a food object) in MPTP-treated monkeys (mean ± SD). Four weeks of vehicle (V) or MTEP (10 mg/kg) treatment (4 W-V/MTEP) did not alter behaviour. Significant motor impairments were observed in MPTP/vehicle-treated monkeys after 21 weekly MPTP injections, in the vehicle-treated group (‘21 MPTP’) and were maintained throughout the rest of the study period. *P < 0.001 for differences between the vehicle and MTEP-treated animals. No significant difference was found between control and MPTP/MTEP- treated monkeys. BL = baseline.

Figure 3

Representative post-commissural coronal in vivo ¹⁸F-FECNT PET images. (A) Baseline, (B) MPTP/vehicle, (C) MPTP/MTEP-treated monkeys. (D, E) Bar graphs showing the percentages of striatocerebellar ¹⁸F-FECNT uptake ratios (means ± SD) for the motor putamen (D) and the limbic putamen (E), obtained from the five different PET scans collected throughout the study. **P < 0.001; *P < 0.05 for differences between control, MPTP/vehicle- and MPTP/MTEP-treated animals. ^#P < 0.05 for differences between vehicle- and MTEP-treated animals. CA = associative caudate nucleus; PM = motor putamen; PL = limbic putamen. Scale bar = 10 mm (A–C).

Figure 4

Representative pre-commissural coronal planes of in-vivo ¹⁸F-FECNT PET images. (A) Baseline, (B) MPTP/vehicle, (C) MPTP/MTEP-treated monkeys. (D–F) Bar graphs showing the percentages of striatocerebellar ¹⁸F-FECNT uptake ratios (means ± SD) for the pre-commissural putamen (D), caudate nucleus (E) and the nucleus accumbens (F) obtained from the five PET scans throughout the study. **P < 0.001; *P < 0.05 for differences from control, MPTP/vehicle-, MPTP/MTEP-treated animals. ^#P < 0.05 for differences between the vehicle and MTEP-treated animals. No significant difference was found in the nucleus accumbens between control and MPTP/MTEP-treated monkeys. AL = limbic nucleus accumbens; CA = associative caudate nucleus; PA = associative putamen. Scale bar = 10 mm (A–C).

Figure 5

Representative midbrain coronal in-vivo ¹⁸F-FECNT PET images. (A) Baseline, (B) MPTP/vehicle, (C) MPTP/MTEP-treated monkeys. (D) Bar graph showing the percentage of nigrocerebellar ¹⁸F-FECNT uptake ratios (means ± SD) for the substantia nigra pars compacta obtained from the five PET scans throughout the study. *P < 0.05 for differences from control, MPTP/vehicle, MPTP/MTEP-treated animals. ^#P < 0.05 for differences between the vehicle and MTEP-treated animals. CA = associative caudate nucleus; PM = motor putamen; SNC = substantia nigra pars compacta. Scale bar = 10 mm (A–C).

Figure 6

Representative coronal sections of pre-commissural (A–C) and post-commissural (D–F) striatum showing dopamine transporter (DAT) immunoreactivity in Controls, MPTP/vehicle and MPTP/MTEP-treated monkeys. (G–H) Densitometry analysis of control, MPTP/vehicle and MPTP/MTEP-treated monkeys. *P < 0.001 and **P < 0.05 for differences between control and MPTP/vehicle-treated monkeys. ^#P < 0.05 for differences between the vehicle and MTEP-treated animals. No significant difference was found between control and MPTP/MTEP-treated monkeys. AL = limbic nucleus accumbens; CA = associative caudate nucleus; GPe = external pallidal segment; GPi = internal pallidal segment; PA = associative putamen; PL = limbic putamen; PM = motor putamen; ROD = relative optical density; ROI = region of interest; TA = thalamus; TH-ir = thyrosine hydroxylase immunoreactivity. Scale bar = 5 mm (A–F).

Figure 7

Photomicrographs showing tyrosine hydroxylase labelling in the mesencephalon. (A) Control, (B) MPTP/vehicle, (C) MPTP/MTEP-treated monkeys. (D) Stereological estimate of the total number of tyrosine hydroxylase-positive (TH+) neurons (means ± SD) in the ventral substantia nigra pars compacta, dorsal substantia nigra pars compacta and ventral tegmental area regions of control, MPTP/vehicle and MPTP/MTEP-treated monkeys. *P < 0.001; **P < 0.05 for differences between control, MPTP/vehicle and MPTP/MTEP-treated animals by using one-way ANOVA with Tukey’s post hoc test. There was no significant difference found in the dorsal substantia nigra pars compacta and ventral tegmental area between control and MPTP/MTEP-treated monkeys. SNCd = dorsal substantia nigra compacta; SNCv = ventral substantia nigra compacta; SNR = substantia nigra pars reticulata; VTA = ventral tegmental area. Scale bar = 5 mm (A–C).

Figure 8

Tyrosine hydroxylase labelling in noradrenergic cells groups. (A) Control, (B) MPTP/vehicle, (C) MPTP/MTEP-treated monkeys. (D) Stereological estimate of the total number of tyrosine hydroxylase-positive (TH+) neurons (means ± SD) in locus coeruleus, A5 and A7 regions of control, MPTP/vehicle and MPTP/MTEP-treated monkeys, representing noradrenergic neurons. *P < 0.05 for differences between control, MPTP/vehicle- and MPTP/MTEP-treated animals. ^#P < 0.05 for differences between the vehicle and MTEP-treated animals. LC = locus coeruleus; A5 and A7 = catecholaminergic areas A5 and A7; ROI = region of interest. Scale bar = 5 mm (A–C).