Manganese inhalation by rhesus monkeys is associated with brain regional changes in biomarkers of neurotoxicity

The purpose of this study was to evaluate biochemical markers of neurotoxicity following subchronic manganese sulfate (MnSO4) inhalation. Juvenile rhesus monkeys were exposed to MnSO4 at 0, 0.06, 0.3, or 1.5 mg Mn/m³ for 65 days. Glutamine synthetase (GS), glutamate transporters (glutamate transporter-1 [GLT-1] and glutamate/aspartate transporter [GLAST]) and tyrosine hydroxylase (TH) protein levels, metallothionein (MT), GLT-1, GLAST, TH and GS mRNA levels, and total glutathione (GSH) levels were assessed in known targets (caudate, globus pallidus, putamen) as well as the cerebellum, frontal cortex, and olfactory cortex. All MnSO₄-exposed monkeys had decreased pallidal GS protein, decreased caudate GLT-1 mRNA, decreased pallidal GLAST protein, and increased olfactory cortical TH mRNA levels. Monkeys exposed to MnSO₄ at 0.06 or 0.3 mg Mn/m³ had significantly increased pallidal mRNA levels for GLT-1, GLAST, and TH. Monkeys exposed to MnSO₄ at ≥ 0.3 mg Mn/m³ had several alterations including decreased frontal cortical MT mRNA, decreased caudate, globus pallidus, olfactory cortex, and cerebellum GLT-1 protein, decreased olfactory cortex and cerebellum GLAST protein, increased cerebellar GLAST mRNA, and decreased pallidal TH protein levels. Lastly, GSH levels were significantly increased in the frontal cortex and decreased in the caudate of monkeys exposed to the 1.5-mg Mn/m³ compared to the controls. Overall, as in our previous studies, we observed that increased Mn concentrations due to airborne Mn exposure differentially affects biomarkers in each brain region (e.g., GSH was increased in the frontal cortex and decreased in the caudate despite two- to threefold increases in Mn concentrations in these regions).