Effects of the Xenoestrogens, 4-nonylphenol, and the organophosphorus pesticide, chlorpyrifos, on acetylcholinesterase activity in the clam Tapes philippinarum

In our recent studies we observed that exposure to 4-nonylphenol (NP), a compound known for its estrogenicity to aquatic organisms, caused several adverse effects in the clam Tapes philippinarum. In particular, in exposed clams both siphons and foot were often cut off, the animals being unable to withdraw them before shell closure (Environ. Res. 2003, 91:179-185). Alterations in normal burrowing behaviour were also observed in NP-exposed clams. Indeed, marked delay in re-burrowing was found at the highest NP concentrations (Mar. Pollut. Bull. 2004, 48:563-571). These results suggested that NP may act via narcosis and/or neurotoxic mechanisms. To confirm the hypothesis that NP may exert neurotoxic effects, in the present study, specimens of T. philippinarum were exposed to sublethal NP concentrations (for 7 days to 0, 0+acetone, 0.025, 0.05, 0.1, and 0.2 mg NP/L and for 14 days to 0, 0+acetone, 0.025, 0.05, and 0.1 mg NP/L) and acetylcholinesterase (AChE) activity was evaluated in gills. Moreover, effects on AChE activity were studied after exposure of clams for 96 h to chlorpyrifos (12 and 24 µg/L), an organophosphorus (OP) pesticide known to inhibit AChE in aquatic organisms. The results obtained showed that exposure to NP for 7 days significantly increased AChE activity in 0.025, 0.1 and 0.2 mg NP/L-treated clams, whereas no significant alteration in the enzyme activity was recorded after 14 days exposure. This allowed us to exclude that NP acted via AChE inhibition. Therefore, alterations observed in the previous studies were probably due to changes in other neurotransmitters, such as serotonin and gamma-aminobutyric acid, already identified in nervous system of bivalves. Interestingly, neither chlorpyrifos affected negatively AChE activity and significant increases in enzyme activity were recorded in 12 µg chlorpyrifos/L-exposed animals with respect to controls. Similarly, increased AChE activity was observed in Mytilus edulis exposed for 96 h to 15 µg chlorfenvinphos/L, however mussels exhibited higher sensitivity than T. philippinarum to OPs, 100% mortality being recorded at 30 µg chlorfenvinphos/L (Aquat. Toxicol., 2004, 67: 45-56). On the basis of these unclear results, further research will be performed on T. philippinarum by increasing exposure levels, in terms of both time and concentration, to better define possible use of AChE activity in clams as biomarker of exposure to OPs in biomonitoring.