Behavioral and histopathological consequences of paraquat intoxication in mice: effects of alpha-synuclein over-expression

Abstract

Genetic variability in the alpha-synuclein gene and long-term exposure to the pesticide paraquat constitute possible risk factors for sporadic Parkinson's disease. The goal of the present study was to further characterize the effects of paraquat in mice as a model of Parkinson's disease and to determine whether it acted synergistically with alpha-synuclein over-expression to cause nigrostriatal cell death or dysfunction. Paraquat (10 mg/kg i.p.) was administered once a week for 3 weeks to mice over-expressing human alpha-synuclein under the Thy1 promoter and their wild-type littermates. The effect of paraquat on catecholaminergic neurons was reminiscent of that of Parkinson's disease, with preferential loss of dopaminergic neurons in the ventral tier of the substantia nigra pars compacta and loss of tyrosine hydroxylase staining in the locus coeruleus. alpha-Synuclein over-expression did not increase paraquat-induced cell loss, and paraquat did not worsen the behavioral deficits observed in the transgenic mice. However, paraquat markedly increased proteinase-K-resistant alpha-synuclein aggregates in substantia nigra of the transgenic mice. The data further validate the use of paraquat to model Parkinson's disease in mice and show that although paraquat and alpha-synuclein over-expression act synergistically to increase protein aggregation in vivo, this interaction does not result in short-term neuroprotection or increased vulnerability of nigrostriatal neurons.

Fig. 1

Behavioral assessment before and after administration of saline (grey bars) or paraquat (black bars) in WT and α-synuclein over-expressing mice (Thy1-aSYN). A: Errors per step on the challenging Beam. B, C: Time to orient downward (T-Turn, B) and descend (T-Total, C) on the pole test. #, P < 0.01 compared with WT mice using two-way ANOVA with repeated measures followed by Fisher’s LSD test, n = 6–7 mice per group.

Fig. 2

Stereological counting of TH-IR neurons in the substantia nigra after administration of saline (grey bars) or paraquat (black bars) in WT and α-synuclein over-expressing mice (Thy1-aSYN). A: Illustration of subregions (white dashed lines) defined within the SNc (bold line). B: Stereological estimate in the whole SNc. C: Stereological estimate in the lateral part of the SNc. D: stereological estimate in the medial part of the SNc. E: Stereological estimate in the ventral part of the SNc. *, P < 0.05, #, P < 0.01 compared with corresponding saline group, using two-way ANOVA followed by Fisher’s LSD test, n = 6–7 mice per group. Scale bar = 200 µm.

Fig. 3

Stereological estimate of the number of TH-IR neurons (A) and Nissl stained neurons (B) in the locus coeruleus after administration of saline (grey bars) or paraquat (black bars) in WT and α-synuclein over-expressing mice (Thy1-aSYN). #, P < 0.01 compared with corresponding saline group, using two-way ANOVA followed by Fisher’s LSD test, n = 6–7 mice per group. Representative histological images of the locus coreuleus in saline-treated wild-type (C) or Thy1-aSYN mice (D) and paraquat-treated wild-type (E) and Thy1-aSYN mice (F). Scale bar = 200 µm.

Fig. 4

Proteinase-K-resistant α-synuclein immunoreactive inclusions in the substantia nigra of a saline-treated (A) and paraquat-treated (B) Thy1-aSYN mouse. C: Increased the number of inclusions after administration of paraquat (black bar) compared with saline (grey bar). *, P < 0.001 compared with saline using student t-test, n = 6–7 mice per group. Scale bar = 20 µm.