General toxicant exposure and cholinesterase activity as a biomarker for neurotoxicity in tissues of four Gadidae species in the Oslofjord

Neurotoxic chemicals have the potential to cause devastating effects on marine organisms and ecosystems. The continued synthesis of new chemicals for commercial use highlights the need for improved efforts in monitoring and risk assessment regarding ecotoxicity. The use of biomarkers, such as cholinesterase activity, is commonly used to investigate potential exposure to neurotoxic agents. However, the use of these biomarkers is often limited to a single species and tissue. Therefore, the aims of this study were to investigate potential differences in acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activity between four tissues (liver, brain, heart and muscle) of four closely related fish species in the Gadidae family (Atlantic cod, whiting, haddock and Norway pout) in the Oslofjord. General exposure to toxicants was also measured using hepatic mercury concentrations and biliary PAH metabolite concentrations (four-ringed and five-ringed species). Fish samples were collected at two locations in the Oslofjord, separated by a 20-meter-deep sill outside Drøbak: Holmestrandsfjorden in the outer part of the fjord, and Midtmeie in the inner part. Samples were collected in spring and in autumn to account for potential seasonal variations. Hepatic mercury concentrations were highest in Atlantic cod, with little variation between locations and seasons. Four-ringed PAH metabolite concentrations were higher in the inner fjord in both Atlantic cod and whiting, while five-ringed metabolite concentrations were higher in the inner fjord for whiting. The tissue distribution of AChE was similar between the species, where activity was highest in brain and lowest in muscle. Significant seasonal differences were seen in all tissues with higher activity in spring than in autumn in the liver and muscle, while the opposite was observed for hearts and brains, suggesting variations in the seasonal cycle of the enzyme activity between tissues. There were no significant differences in the activity of AChE or BChE between the two locations, but differences between species were observed for all tissues. BChE activity was low overall, and non-detectable in several samples. The use of BChE activity as a biomarker in these species is therefore debatable, but analysing for enzymatic activities allows for a more accurate determination of the specific AChE activity in the tissues. A significant negative correlation between hepatic mercury concentration and hepatic AChE activity was seen in Norway pout, but apart from this there were no signs of cholinesterase inhibition from exposure to contaminants, using mercury and PAHs as proxies, in the Gadidae species in the Oslofjord.