Acetylcholinesterase activity in Gammarus fossarum (Crustacea Amphipoda): Linking AChE inhibition and behavioural alteration

Relations between whole-body acetylcholinesterase (AChE) inhibition and changes in feeding and locomotor behaviours were investigated in adult male Gammarus fossarum during short-term exposure (96 h) to the organophosphorous pesticide chlorpyrifos (CPE) and the carbamate pesticide methomyl (MT). AChE activity was measured after 24, 48 and 96 h of exposure. The feeding rate was assessed after 48 and 96 h of exposure and locomotor activity was measured at the end of the experiment. A concentration-dependent decrease of AChE activity and behavioural parameters was observed for both CPE and MT. However, these two compounds presented dissimilar modes of action since MT-induced effects appeared rapidly during the first 48 h of the experiment and remained constant until the end of experiment, contrary to CPE-induced effects, which occurred gradually during the last 48 h. For CPE, significant mortality was observed from 50% AChE inhibition, contrary to MT for which no mortality was observed despite 66% inhibition. These results suggest that, for CPE, the observed mortality was not directly related to AChE inhibition but that an additional toxic mode of action occurred. On the contrary, the feeding rate and locomotion impairment were directly correlated to levels of AChE inhibition for both chemicals, except for the lowest concentrations of MT for which an induction of the behavioural parameters was observed. Although CPE and MT have different modes of action (acting as indirect and direct inhibitors), the relations between AChE activity and inhibition of behavioural parameters were relatively close for these two compounds. This study provides a basis to interpret the biomarker AChE at the upper biological organisation level, on both the feeding rate and locomotor behaviour, which are known to be relevant ecological responses.