Freshwater microcrustacean Daphnia magna Straus as an early screen model to compare toxicity of acetylcholinesterase inhibitors

Abstract

Daphnia magna is a freshwater microcrustacean which is often used for acute and chronic toxicity testing in aquatic ecotoxicology. Recently, tests with daphnids have been used to prescreen the toxicity of newly synthesized acetylcholinesterase reactivators (oximes), which appear as weak inhibitors of acetylcholinesterase (AChE). In our study we investigated and compared the toxicity of five reversible acetylcholinesterase inhibitors, two of them non ionic (tacrine and 7-MEOTA) and three ionic (berberine chloride, 1-methyltacridium iodide and ethidium bromide). Tests were carried out according to the methodology described in the standard EN ISO 6341. EC50 values as well as dose-response curves were calculated for all tested inhibitors. In general, non ionic substances were found to be less toxic than ionic substances. Estimated EC50 values in tests taking respectively 24 h and 48 h were as follows: tacrine 3000 resp. 1515 μg.l-1, 7-MEOTA 3606 resp. 707.2 μg.l-1, berberine chloride 903.6 resp. 822.4 μg.l-1, 1-methylacridinium iodide 714.3 resp. 233.5 μg.l-1 and ethidium bromide 644.3 resp. 291.1 μg.l-1. The higher toxicity of 7-MEOTA in time compared to tacrine and the similar toxicity of berberine chloride in both tests are explained as a consequence of their metabolism in the daphnid's body.