Preweaning Mn exposure leads to prolonged astrocyte activation and lasting effects on the dopaminergic system in adult male rats

Gene deletion of nos2 protects against manganese-induced neurological dysfunction in juvenile mice

The mechanisms underlying cognitive and neurobehavioral abnormalities associated with childhood exposure to manganese (Mn) are not well understood but may be influenced by neuroinflammatory activation of microglia and astrocytes that results in nitrosative stress due to expression of inducible nitric oxide synthase (iNOS/NOS2). We therefore postulated that gene deletion of NOS2 would protect against the neurotoxic effects of Mn in vivo and in vitro. Juvenile NOS2 knockout (NOS2(-/-)) mice were orally exposed to 50 mg/kg of MnCl₂ by intragastric gavage from days 21 to 34 postnatal. Results indicate that NOS2(-/-) mice exposed to Mn were protected against neurobehavioral alterations, despite histopathological activation of astrocytes and microglia in Mn-treated mice in both genotypes. NOS2(-/-) mice had decreased Mn-induced formation of 3-nitrotyrosine protein adducts within neurons in the basal ganglia that correlated with protection against Mn-induced neurobehavioral defects. Primary striatal astrocytes from wildtype mice caused apoptosis in cocultured striatal neurons following treatment with MnCl₂ and tumor necrosis factor-α, whereas NOS2(-/-) astrocytes failed to cause any increase in markers of apoptosis in striatal neurons. Additionally, scavenging nitric oxide (NO) with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) prevented the ability of Mn- and cytokine-treated wildtype astrocytes to cause apoptosis in cocultured striatal neurons. These data demonstrate that NO plays a crucial role in Mn-induced neurological dysfunction in juvenile mice and that NOS2 expression in activated glia is an important mediator of neuroinflammatory injury during Mn exposure.

Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Altered striatal dopamine release following a sub-acute exposure to manganese

Certain metals that are necessary for regulating biological function at trace levels hold the potential to become neurotoxic when in excess. Specifically, chronic exposure to high levels of manganese leads to manganism, a neurological disorder that exhibits both motor and learning deficits similar to Parkinson's disease. Since Parkinson's disease symptomatology is primarily attributed to dopamine neurodegeneration in the striatum, dopamine system dysfunction has been implicated in the onset of manganism. In this study, dopamine system function in the dorsal striatum was evaluated in C57Bl/6 mice, 1, 7, and 21 days following repeated injections of manganese(II) chloride (50 mg/kg, subcutaneous) intermittently for 7 days. Tissue content analysis confirmed the presence of persistent accumulation of manganese in the striatum up to 21 days after cessation of treatment. In vitro fast scan cyclic voltammetry examined the effect of sub-acute manganese on electrically stimulated dopamine release and uptake in the striatum. While no difference was observed in uptake rates following manganese treatment, dopamine release was attenuated on days 7 and 21, compared to control levels. Basal levels of extracellular dopamine determined by the zero net flux microdialysis method were significantly lower in manganese-treated mice at 7 days post-treatment. On the other hand, potassium stimulated increases in extracellular dopamine were attenuated at all three time points. Together, these findings indicate that repeated manganese exposure has long-term effects on the regulation of exocytotic dopamine release in the striatum, which may be involved in the mechanism underlying manganese toxicity.