Assessing the effects of the three herbicides acetochlor, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid on the compound action potential of the sciatic nerve of the frog (Rana ridibunda)

To assess the relative toxicity of the herbicides acetochlor and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) on the nervous system, the sciatic nerve of the frog (Rana ridibunda) nerve was incubated in saline inside a specially designed recording chamber. This chamber permits monitoring of the evoked compound action potential (CAP) of the nerve, a parameter that could be used to quantify the vitality of the nerve in normal conditions as well as when the nerve was exposed to the compounds under investigation. Thus, when the nerve was exposed to acetochlor, the EC50 was estimated to be 0.22 mM, while for 2,4,5-T the EC50 was 0.90 mM. Using the identical nerve preparation, the EC50 of 2,4-D was estimated to be 3.80 mM [Kouri, G., Theophilidis, G., 2002. The action of the herbicide 2,4-dichlorophenoxyacetic acid on the isolated sciatic nerve of the frog (Rana ridibunda). Neurotoxicol. Res. 4, 25–32]. The ratio of the relative toxicity for acetochlor, 2,4,5-T and 2,4-D was found to be 1:4:17.2. However, because it is well-known that the action of 2,4-D is dependent on the pH, the relative toxicity of the three compounds was tested at pH 3.3, since it has been found that the sciatic nerve of the frog is tolerant of such a low pH. Under these conditions, the EC50 was 0.77 mM (from 0.22 mM at pH 7.2) for acetochlor, 0.20 mM (from 0.90 mM) for 2,4,5-T and 0.24 mM (from 3.80 mM at pH 7.2) for 2,4-D. Thus, the relative toxicity of the three compounds changed drastically to 1:0.25:0.31. This change in the relative toxicity is due not only to the increase in the toxicity of 2,4,5-T and 2,4-D at low pH levels, but also to the decrease in the toxicity of acetochlor at pH 3.3.