Acute exposure to aroclor 1016 or 1260 differentially affects dopamine transporter and vesicular monoamine transporter 2 levels

Abstract

Polychlorinated biphenyls (PCBs) have been shown to specifically target the dopaminergic nervous system, resulting in long-term reduction of striatal dopamine (DA) levels. However, the mechanism(s) by which PCBs exert this effect is not known. Here we report that decreased striatal dopamine levels are observed 1, 7, and 14 days after acute exposure to the common PCB mixtures Aroclor 1016 or 1260. Dopamine transporter (DAT) levels were decreased at all time points in Aroclor 1016 treated animals, and on Days 1 and 7 in Aroclor 1260 treated animals. Vesicular monoamine transporter 2 (VMAT2) levels were not affected by Aroclor 1016, but were significantly decreased 14 days after exposure to Aroclor 1260. Tyrosine hydroxylase expression, a marker of dopamine neuron integrity, was not significantly affected by PCB exposure at any time. These data suggest that PCB-induced reductions in striatal dopamine may be mediated by alterations in DAT and VMAT2 expression.

Introduction

Polychlorinated biphenyls (PCBs) are a family of halogenated hydrocarbons that were used extensively as industrial lubricants, dispersants for pesticide application, and fluid insulators prior to being banned in the 1970’s (Erickson, 1986). PCBs are complex mixtures comprised of specific PCBs referred to as congeners. Because of their physiochemical properties, PCBs are ubiquitous environmental contaminants and bioaccumulate in higher trophic levels of the food chain, leading to an increased risk of human exposure (Kamrin and Ringer, 1994). In fact, several epidemiological studies have shown that PCBs are neurotoxic to humans (reviewed by Seegal, 1996).While much study has been focused on developmental toxicity from exposure to PCBs (reviewed by Faroon et al., 2001), few studies have focused on the neurotoxic consequences of PCB exposure to adults. Acute exposure of adult rats to a mixture of Aroclor 1254 and 1260 has been shown to significantly decrease striatal dopamine (DA) levels (Seegal et al., 1986). Additionally, subchronic exposure of non-human primates to lower levels of Aroclor 1016 or 1260 results in long-term decreases of striatal DA levels (Seegal et al., 1991). It has been suggested that reduction of DA levels by PCBs occurs through a common mechanism, i.e. inhibition of DA synthesis (Seegal et al., 1989, Chisti et al., 1996). However, this conclusion was made through the use of in vitro culture systems, which may not provide a true representation of what may occur in vivo and DA synthesis is only one contributing factor in the maintenance of dopamine levels.

DA neurotransmission is terminated by the reuptake of released DA into pre-synaptic neurons through the action of the dopamine transporter (DAT). Once inside the cell, recaptured DA is sequestered into synaptic vesicles by the vesicular monoamine transporter (VMAT2), where it is once again available for release (Amara and Kuhar, 1993, Miller et al., 1999b). Therefore, any interference with this tightly coupled process could be expected to have a significant impact on DA levels. Indeed, transgenic mice lacking DAT or VMAT2 exhibit significant reductions of DA levels (Giros et al., 1996, Jones et al., 1998, Fon et al., 1997, Wang et al., 1997). Similar to these genetic models, pharmacological inhibition of DAT or VMAT2 has also been shown to reduce DA levels (Sivam, 1995, Fumigalli et al., 1999), suggesting that any chemical that affects DAT or VMAT2 function or expression may significantly impact DA neuron homeostasis.

Recent in vitro studies have shown that lower chlorinated ortho-substituted PCB congeners reduce DA uptake into rat brain synaptosomes, while more highly chlorinated ortho-substituted congeners reduced DA uptake into synaptic vesicles (Mariussen and Fonnum, 2001, Mariussen et al., 1999). Since DAT and VMAT2 are the specific transporters responsible for dopamine uptake in synaptosomes and vesicles, respectively, it appears that different PCB congeners may exert differential effects on DAT and VMAT2. As stated previously, it has been hypothesized that Aroclor 1016 and 1260 decrease DA levels by a common mechanism. However, the brains of animals treated with Aroclor 1016 contain primarily more lowly chlorinated congeners (Seegal et al., 1991), which appear to target DAT (Mariussen and Fonnum, 2001). In contrast, brains of animals treated with Aroclor 1260 contain primarily more highly chlorinated congeners, which appear to target VMAT2 (Mariussen et al., 1999). Therefore, we chose an acute exposure paradigm based on previous studies by Rosin and Martin (1981) and Seegal et al. (1986) to determine whether in vivo exposure to Aroclor 1016 or 1260 results in differential effects on DAT and VMAT2.