Characterization of Mice with Altered Dopamine Transporter and Vesicular Monoamine Transporter 2 Levels

Dopamine is a key neurotransmitter that regulates motor coordination and dysfunction of the dopamine system gives rise to Parkinsonâ s disease. Nigrostriatal dopamine neurons are vulnerable to various genetic and environmental insults, suggesting that these cells are inherently at-risk. A cell-specific risk factor for these neurons is the neurotransmitter, dopamine itself. If intracellular dopamine is not appropriately sequestered into vesicles, it can accumulate in the cytosol. Cytosolic dopamine is highly reactive and can trigger oxidative stress, leading to cellular toxicity. Cytosolic dopamine levels are modulated by the plasma membrane dopamine transporter (DAT) that takes up dopamine from the extracellular space, and the vesicular monoamine transporter 2 (VMAT2) that stores dopamine into vesicles. In this thesis, we altered DAT and VMAT2 levels to investigate the detrimental consequences of potentially amplifying cytosolic dopamine in transgenic mice. Project 1 focused on selective over-expression of DAT in dopaminergic cells of transgenic mice (DAT-tg). DAT-tg mice displayed phenotypes of dopaminergic damage: increased dopamine-specific oxidative stress, L-DOPA-reversible fine motor deficits and enhanced sensitivity to toxicant insult, suggesting that increasing DAT-mediated dopamine uptake is detrimental for dopamine cells. As an extension of Project 1, Project 2 focused on mice that simultaneously over-express DAT and under-express VMAT2 (DAT-tg/VMAT2-kd mice). These animals were hypothesized to demonstrate exacerbated dopaminergic toxicity due to buildup of cytosolic dopamine caused by increased uptake and decreased packaging. While DAT-tg/VMAT2-kd mice displayed detrimental phenotypes (poor survival, decreased body weight, reduced dopamine tissue content and release) and compensatory changes (increased dopamine receptors and metabolism), they did not show dopamine cell loss. This is due to unexpected loss of phenotypes in DAT-tg mice from a new colony that no longer displayed dopaminergic neurodegeneration. Thus, instead of Parkinsonian behavior, DAT-tg/VMAT2-kd mice showed novel phenotypes such as hyperactivity and improved fine-motor and cognitive skills compared to other genotypes. DAT-tg/VMAT2-kd mice were also highly sensitive to amphetamine-induced locomotion. Hence, in the absence of neurodegeneration, altered DAT and VMAT2 levels produced unique behavioral changes in DAT-tg/VMAT2-kd mice, shedding light on the complex function of the dopamine system. Collectively, these studies demonstrate how perturbations in dopamine compartmentalization can impact dopamine homeostasis and behavior.

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