Dopaminergic neurotoxicity following pulmonary exposure to manganese-containing welding fumes

The potential for development of Parkinson's disease (PD)-like neurological dysfunction following occupational exposure to aerosolized welding fumes (WF) is an area of emerging concern. Welding consumables contain a complex mixture of metals, including iron (Fe) and manganese (Mn), which are known to be neurotoxic. To determine whether WF exposure poses a neurological risk particularly to the dopaminergic system, we treated Sprague-Dawley rats with WF particulates generated from two different welding processes, gas metal arc-mild steel (GMA-MS; low Mn, less water-soluble) and manual metal arc-hard surfacing (MMA-HS; high Mn, more water-soluble) welding. Following repeated intratracheal instillations (0.5 mg/rat, 1/week x 7 weeks) of GMA-MS or MMA-HS, elemental analysis and various molecular indices of neurotoxicity were measured at 1, 4, 35 or 105 days after last exposure. MMA-HS exposure, in particular, led to increased deposition of Mn in striatum and midbrain. Both fumes also caused loss of tyrosine hydroxylase (TH) protein in the striatum (similar to 20%) and midbrain (similar to 30%) by 1 day post-exposure. While the loss of TH following GMA-MS was transient, a sustained loss (34%) was observed in the midbrain 105 days after cessation of MMA-HS exposure. In addition, both fumes caused persistent down-regulation of dopamine D2 receptor (Drd2; 30-40%) and vesicular monoamine transporter 2 (Vmat2; 30-55%) mRNAs in the midbrain. WF exposure also modulated factors associated with synaptic transmission, oxidative stress, neuroinflammation and gliosis. Collectively, our findings demonstrate that repeated exposure to Mn-containing WF can cause persistent molecular alterations in dopaminergic targets. Whether such perturbations will lead to PD-like neuropathological manifestations remains to be elucidated.