Acute toxicity of frog-skin-homologous synthetic alkaloids and frog-skin extracts: miniaturized test to evaluate toxicity with Daphnia magna

The determination of toxicity in samples from natural extracts or products of organic synthesis represents a challenge due to the low available volume of these kinds of matrices. A miniaturized immobilization acute test in Daphnia magna was implemented at microplate-scale to determine the relative toxicity in 13 synthetic alkaloids, analogous to those found in the skin of frogs, and skin-extracts from 18 individuals of the frog Oophaga vicentei. The test revealed moderate or moderate to low toxicity towards the daphnids for all tested synthetic alkaloids after a 48-h exposure. The correlation of relative toxicity and several parameters in frog samples revealed that: (i) toxicity is not associated with frog coloration (aposematism) or their geographical location; and (ii) toxicity is dependent on the profile of alkaloid composition rather than the total alkaloid content. Results from this work provide an alternative method to assay toxicity in limited-volume samples, useful to elucidate potential aposematic-related differences in dendrobatid frogs.