Elevated manganese (Mn) exposure is associated with attentional deficits in children, and is an environmental risk factor for attention deficit hyperactivity disorder (ADHD). We have shown that developmental Mn exposure causes lasting attention and sensorimotor deficits in a rat model of early childhood Mn exposure, and that these deficits are associated with a hypofunctioning catecholaminergic system in the prefrontal cortex (PFC), though the mechanistic basis for these deficits is not well understood. To address this, male Long-Evans rats were exposed orally to Mn (50 mg/kg/d) over PND 1-21 and attentional function was assessed in adulthood using the 5-Choice Serial Reaction Time Task. Targeted catecholaminergic system and epigenetic gene expression, followed by unbiased differential DNA methylation and gene regulation expression transcriptomics in the PFC, were performed in young adult littermates. Results show that developmental Mn exposure causes lasting focused attention deficits that are associated with reduced gene expression of tyrosine hydroxylase, dopamine transporter, and DNA methyltransferase 3a. Further, developmental Mn exposure causes broader lasting methylation and gene expression dysregulation associated with epigenetic regulation, inflammation, cell development, and hypofunctioning catecholaminergic neuronal systems. Pathway enrichment analyses uncovered mTOR and Wnt signaling pathway genes as significant transcriptomic regulators of the Mn altered transcriptome, and Western blot of total, C1 and C2 phospho-mTOR confirmed mTOR pathway dysregulation. Our findings deepen our understanding of the mechanistic basis of how developmental Mn exposure leads to lasting catecholaminergic dysfunction and attention deficits, which may aid future therapeutic interventions of environmental exposure associated disorders.

BACKGROUND: Environmental manganese (Mn) exposure is associated with impair-ments in attention and psychomotor function as well as impulsivity/hyperactivity in children and adolescents. We have shown previously that developmental Mn exposure can cause these same areas of dysfunction in a rat model. Although treatment with methylphenidate (MPH) is well documented to lessen impairments in attention, im-pulse control, and sensorimotor function in children, it is unknown whether this treat-ment is effective in improving these areas of dysfunction induced by developmental Mn exposure. OBJECTIVES: Determine if oral MPH therapy ameliorates the lasting attention and sensorimotor impairments caused by developmental Mn exposure and elucidate the underlying mechanisms of Mn neurotoxicity and MPH effectiveness, if observed. Rats were given 50 mg Mn/kg/d orally over PND 1 - 21 and assessed as adults in a series of attention, impulse control and sensorimotor tasks during chronic oral MPH treatment (0, 0.5, 1.5, or 3.0 mg/kg/d), followed by continued behavioral testing in the presence of selective catecholaminergic receptor antagonists (SCH 23390, raclopride, and BRL 44408 s.c.) and MPH treatment, using a quasi-Latin-Square design. RESULTS: Developmental Mn exposure caused persistent attention and sensorimotor impairments in adult rats. MPH therapy at 0.5 mg/kg/d completely ameliorated the Mn-induced attentional dysfunction; notably this benefit was not seen following acute ad-ministration, but was apparent after prolonged (i.e., >8 days) treatment. The Mn-induced sensorimotor dysfunction was also ameliorated by MPH treatment but only at the 3 mg/kg/d MPH dose. Furthermore, selectively antagonizing D1, D2, or α2A recep-tors had no effect on the Mn-induced attentional deficits, or MPH efficacy to ameliorate those deficits. However, antagonism of D2R attenuated the sensorimotor deficits pro-duced by Mn, whereas the efficacy of MPH to ameliorate those sensorimotor deficits was diminished by D1R antagonism. CONCLUSIONS: These findings demonstrate that MPH treatment is effective in treat-ing the lasting attention and sensorimotor deficits caused by developmental Mn expo-sure, and they clarify the mechanisms underlying developmental Mn neurotoxicity and MPH efficacy. Given that the cause of attention and co-morbid psychomotor deficits in children is often unknown, these findings may have implications for the treatment of environmentally-induced attentional and psychomotor dysfunction in children more broadly.