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Tiffany A. Mathews


Manganese (Mn), an abundant metal found in soil, air, and many foods is essential for normal body development and functioning. However, excess exposure to Mn results in a neurological disorder known as manganism, which is associated with marked motor impairments that are similar to Parkinson's disease. The similarities between manganism and Parkinson's disease are most likely due to dopaminergic alterations in the basal ganglia. Understanding the neurobiological consequences of overexposure to Mn is critical to develop better treatment strategies. The principle objective of this research is to study the impact of Mn exposure on the striatal dopamine. The first objective was to develop a sensitive method to prepare tissue samples for parallel measurements of metals (i.e. Mn) and neurotransmitters including monoamines (specifically dopamine) and amino acids (GABA and glutamate). The second objective was to investigate the effect of 50 mg/kg of MnCl2 delivered in an intermittent regimen on striatal dopamine dynamics. A subcutaneous injection of MnCl2 (0.5 or 50 mg/kg) or saline (0.9% NaCl) was administered to male C57/Bl6 mice every 2 days for a period of 7 days. Analysis of dopamine and Mn was determined 24 h from the last Mn injection. When the higher dose of Mn was administered (50 mg/kg), an additional two time points were evaluated: 7 and 21 days from last Mn treatment. To assess the effect of excess Mn exposure on the dopamine system in the dorsal striatum, the following methods were used: ex vivo tissue content, in vitro fast scan cyclic voltammetry, in vivo microdialysis, and graphite furnace atomic absorption spectroscopy. The results demonstrate a bi-phasic striatal DA response depending on the dose of Mn administered.

More specifically, the low dose of Mn (II) demonstrated a significant increase in Mn levels in the kidney, liver, and brain 1 day post-treatment. However, only an increase in intracellular dopamine and GABA levels was observed in the cortex and midbrain, respectively. On the other hand, when a high dose of Mn (II) was administered, Mn levels were elevated as a function of time. One day post-treatment, Mn levels were elevated in the kidney, liver, and specific brain regions including the midbrain and cortex. Notably, only the cortex and the striatum showed elevated Mn levels beyond 1 day post-treatment. In the cortex a significant reduction in intracellular dopamine and DOPAC levels were observed at all three time points. Interestingly, no matter the dose of MnCl2 administered intracellular cortical dopamine levels were significantly altered. These results suggest that a target brain region of excess Mn exposure is cortical dopamine. These cortical alterations appear to agree with human clinical research which suggests that the earliest symptoms of manganism are psychological disorders associated with the cortex. Although ex vivo tissue content studies highlight dopaminergic alterations in the cortical region, previous studies have demonstrated that overexposure to Mn has a greater impact on the striatal dopamine. Therefore, in this study dopamine dynamics in the striatum were investigated to better understand the extracellular DA in C57BL/6 mice. Although, after Mn exposure there was a persistent intracellular accumulation of Mn for up to 21 days post-treatment, no differences were observed in intracellular dopamine levels. Extracellular Mn levels measured using in vivo microdialysis were reduced indicating enhanced transport of Mn from the extracellular to the intracellular space. Concomitantly, extracellular dopamine levels in the dorsal striatum were decreased in Mn-treated animals. Additionally, in vivo microdialysis showed that dopamine release by high K+ stimulation was blunted in Mn-treated mice at all three time points examined. However, electrically-evoked dopamine release was only attenuated 7 days post Mn-treatment. Even though significant alterations were observed with extracellular dopamine levels by in vivo microdialysis, this did not correlate to alterations in dopamine uptake or functionality of dopamine D2 autoreceptors. Since Mn-treatment significantly influenced stimulated dopamine release and extracellular dopamine levels without interfering with dopamine metabolism or uptake, the next study evaluated dopamine biosynthesis. L-DOPA accumulation showed no difference between the Mn- and saline-treated mice indicating that excess Mn exposure does not influence dopamine synthesis. In conclusion, these findings highlight that an intermittent sub-acute exposure to Mn (II) impacts stimulated dopamine release and extracellular dopamine levels. However, these dopamine alterations in the dorsal striatum do not appear to be a result of Mn influencing dopamine synthesis, uptake, or metabolism. These results reveal extracellular striatal DA is sensitive to Mn, but that tissue content masks these DA alterations.

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