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

Abstract

<jats:p> 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. </jats:p>

Biomarkers of the cholinergic and dopaminergic signaling pathways in Arctic beluga whales (Delphinapterus leucas): relationship to methylmercury and selenium

“…Previous in vivo studies have found similar relationships between MeHg exposure and MAO activity, although wildlife studies are uncommon in the literature. In wild river otters, a negative correlation between MAO activity and THg or MeHg concentration was observed in the cerebral cortex but not the cerebellum (Basu et al 2007a), and in wild polar bears, MAO activity in the occipital lobe showed a negative correlative relationship with total Hg concentration (Krey et al 2014). In laboratory studies, decreased MAO activity was observed in the brain stem of rat pups exposed to MeHg chloride in utero (Beyrouty et al 2006), and in the cortex and cerebellum of rats exposed to MeHg (Chakrabarti et al 1998).…”

“…Cholinergic signaling pathways have been linked to essential physiological processes including learning, memory, stress response, and modulation of sensory information; the disturbance of dopaminergic neurotransmission is related to psychiatric symptoms in humans (Reis et al 2009). The muscarinic acetylcholine receptor (mAChR) from the cholinergic signaling pathway and monoamine oxidase B (MAO-B) from the dopaminergic signaling pathway are known to be sensitive to MeHg exposure (Chakrabarti et al 1998;Coccini et al 2000;Basu et al 2005aBasu et al , 2006aBasu et al , 2006bBasu et al , 2007aBeyrouty et al 2006;Stamler et al 2006;Coccini et al 2007). The disruption of either of these signaling pathways due to increased THg accumulation in the Arctic could adversely affect beluga whales in the wild.…”

Assessing the neurotoxicological risk of methylmercury exposure for beluga whales (delphinapterus leucas) harvested in the Mackenzie Delta Estuary.

“…A negative correlation between H gi and the dopamine receptor was also observed in wild river otters and wild mink (Basu et al, 2005a). A negative relationship between GABAa receptor binding levels and Hg was observed in captive mink , while monoamine oxidase and acetylcholinesterase activities were negatively correlated to brain Hg in wild river otters (Basu et al, 2007b). (Hamilton et al, 2011;Scheuhammer et al, 2008) Herring gulls (Larus argentatus) -ns -ns (Rutkiewicz et al, 2010) Little brown bats (Myotis lucifugus)…”

“…Neurochemical changes may represent early and reversible indicators o f neurological harm because they occur prior to the onset o f overt functional or structural damage Manzo et al, 2001). Neurochemical biomarkers from diverse signaling pathways have been used to assess potential neurotoxicological risk o f MeHg exposure in terrestrial mammals, avian species and marine mammals; results have suggested that environmental exposure to MeHg was associated with neurochemical variation in diverse species (Basu et al, 2005a;Basu et al, 2006a;Basu et al, 2005c;Basu et al, 2007b;Basu et al, 2007c;Basu et al, 2009;Hamilton et al, 2011;Rutkiewicz et al, 2010;Scheuhammer et al, 2008). Therefore, a neurochemical biomarker approach may provide valuable information about potential neurotoxicity in wildlife exposed to MeHg and other neurotoxins.…”

“…In beluga whales harvested in 2008 and 2010, variation o f molecular and/or biochemical components o f the GABAergic and glutamatergic signaling pathways was associated with MeHg exposure in different brain regions (Ostertag et al, in review). Components o f the cholinergic and dopaminergic signaling pathways are known to be sensitive to MeHg exposure (Basu et al, 2005a;Basu et al, 2006a;Basu et al, 2007b;Basu et al, 2006b;Beyrouty et al, 2006;Chakrabarti et al, 1998;Coccini et al, 2000;Coccini et al, 2007;Stamler et al, 2006); therefore, the effect o f the Hg and Hg species on these signaling pathways needs further study in beluga whales.…”