Cholinesterase and monoamine oxidase activity in relation to mercury levels in the cerebral cortex of wild river otters

Mercury (Hg) is a global pollutant that is neurotoxic to many mammalian species. The present study was conducted to determine if the bioaccumulation of Hg by wild river otters (Lontra canadensis) could be related to variations in the activities of key neurochemical enzymes. River otters were collected from Ontario and Nova Scotia (Canada) during the trapping seasons, spanning 2002-2004, and their brains were dissected into the cerebral cortex and cerebellum. The activities of cholinesterase (ChE) and monoamine oxidase (MAO) were measured from each sample and correlated with concentrations of brain Hg from the same animal. Significant negative correlations were found between concentrations of brain Hg and ChE (total Hg: r = -0.42; MeHg: r = -0.33) and MAO (total Hg: r = -0.31; MeHg: r = -0.42) activity in the cerebral cortex. The scatterplots relating concentrations of brain Hg and enzyme activity in the cerebral cortex were wedge-shaped, and could be fitted with quantile regression modeling, suggesting that Hg may act as a limiting factor for ChE and MAO activity. No relationships were found in the cerebellum. These data suggest that environmentally relevant concentrations of Hg may influence the activities of ChE and MAO in the cerebral cortex of river otters, and by extension, other fish-eating mammals.