Developmental exposure to the Parkinson's disease-associated organochlorine pesticide dieldrin alters dopamine neurotransmission in α-synuclein pre-formed fibril (PFF)-injected mice

Abstract

Parkinson's disease (PD) is the fastest-growing neurological disease worldwide, with increases outpacing aging and occurring most rapidly in recently industrialized areas, suggesting a role of environmental factors. Epidemiological, post-mortem, and mechanistic studies suggest that persistent organic pollutants, including the organochlorine pesticide dieldrin, increase PD risk. In mice, developmental dieldrin exposure causes male-specific exacerbation of neuronal susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and synucleinopathy. Specifically, in the α-synuclein (α-syn) pre-formed fibril (PFF) model, exposure leads to increased deficits in striatal dopamine (DA) turnover and motor deficits on the challenging beam. Here, we hypothesized that alterations in DA handling contribute to the observed changes and assessed vesicular monoamine transporter 2 (VMAT2) function and DA release in this dieldrin/PFF 2-hit model. Female C57BL/6 mice were exposed to 0.3 mg/kg dieldrin or vehicle every 3 days by feeding, starting at 8 weeks of age and continuing throughout breeding, gestation, and lactation. Male offspring from independent litters underwent unilateral, intrastriatal injections of α-syn PFFs at 12 weeks of age, and vesicular 3H-DA uptake assays and fast-scan cyclic voltammetry were performed 4 months post-PFF injection. Dieldrin-induced an increase in DA release in striatal slices in PFF-injected animals, but no change in VMAT2 activity. These results suggest that developmental dieldrin exposure increases a compensatory response to synucleinopathy-triggered striatal DA loss. These findings are consistent with silent neurotoxicity, where developmental exposure to dieldrin primes the nigrostriatal striatal system to have an exacerbated response to synucleinopathy in the absence of observable changes in typical markers of nigrostriatal dysfunction and degeneration.

Keywords: Parkinson disease; alpha-synuclein; developmental neurotoxicity; dieldrin; dopamine; pesticides.

Figure 1.

Experimental design including dosing schedule, weaning strategy, cage, and group assignments. A, Timeline of dieldrin-PFF 2-hit model: At 8 weeks of age, female C57BL/6 mice dieldrin exposure began via oral administration of 0.3 mg/kg dissolved in corn oil and injected into peanut butter pellets. At 12 weeks of age, mating began, and exposure continued through weaning of pups. F1 pups were weaned 3 weeks after birth and separated by litter and sex (2–4 animals per cage). At 3 months of age, male pups underwent intrastriatal injections of PFFs and were individually housed after surgery. B, Cage, group assignments, and group numbers: Male F1 offspring (F1) that underwent intrastriatal PFF-injections were assigned to endpoints such that every animal for each endpoint came from an independent litter. The Fourth F1 litter is an example of a litter excluded from endpoint assignments due to individual housing.

Figure 2.

Verification of α-syn PFF size. A, PFF length distribution determined via TEM. Each point represents a measured fibril length, the error bars denote standard deviation. B, Representative TEM image of sonicated fibrils. C, Frequency distribution of PFF lengths post-sonication.

Figure 3.

Confirmation of PFF-induced seeding in FSCV animals. A, Representative images from nigral tissue sections stained with TH (top panels) and pSyn (middle panels) from a vehicle/PFF (A) and dieldrin/PFF (B) animals 4 months post-PFF injection. C, pSyn counts in the SNpc show no effect of dieldrin on pSyn-positive objects in the SNpc ipsilateral to the PFF injection (p = .2441). D, As expected, there were no pSyn-positive objects contralateral to the injection in either group of animals. All data are shown as mean ± SD.

Figure 4.

Dieldrin/PFFs increase peak dopamine and upward velocity in striatal tissues measured using FSCV. Four months post-PFF injection, animals were killed and FSCV was performed in dorsal striatum. A, Example dopamine versus time graph showing each quantified metric. B, Representative dopamine versus time graph for the groups vehicle/PFF (black) and dieldrin/PFF (red). C, D, Representative dopamine concentration vs time plot for (C) vehicle/PFF and (D) dieldrin/PFF following stimulation at t = 5 s. E–H, FSCV metrics represented as ipsilateral values normalized to contralateral values. E, Quantification of peak dopamine showed a significant dieldrin-related increase (p = .0394). F, Quantification of upward velocity showed a significant dieldrin-related increase (p = .0434). G, Quantification of downward velocity showed no significant effect of dieldrin (p = .5303). H, Quantification of tau showed no significant effect of dieldrin (p = .6435). Each individual data point represents a sum of 20 recordings per animal. All data shown as mean ± SD. A color version of this figure appears in the online version of this article.

Figure 5.

Dieldrin does not affect VMAT2 uptake velocity in PFF-injected male F1 offspring. There was no difference in uptake velocity ipsilateral to injection site 4 months post-PFF injection (p = .4524). All data shown as mean ± SD.

Figure 6.

Overview of developmental dieldrin/PFF 2-hit model.

Figure 7.

Summary of observed changes in the dieldrin PFF 2-hit model. Timelines show representative changes synuclein pathology, microglial activation, striatal loss, and nigral degeneration in the PFF model based on published literature, shown as the percent change in these markers compared to a saline/monomer injected mouse. Grey boxes indicate previous results from our lab in the dieldrin PFF 2-hit model. White boxes indicate FSCV and uptake results reported here at 4 months post-PFF injection. Light grey and dark grey squares represent results from vehicle: PFF and dieldrin: PFF animals, respectively.