Mechanism of action of the insecticides, lindane and fipronil, on glycine receptor chloride channels

BACKGROUND AND PURPOSE Docking studies predict that the insecticides, lindane and fipronil, block GABAA receptors by binding to 6' pore-lining residues. However, this has never been tested at any Cys-loop receptor. The neurotoxic effects of these insecticides are also thought to be mediated by GABA(A) receptors, although a recent morphological study suggested glycine receptors mediated fipronil toxicity in zebrafish. Here we investigated whether human alpha 1, alpha 1 beta, alpha 2 and alpha 3 glycine receptors were sufficiently sensitive to block by either compound as to represent possible neurotoxicity targets. We also investigated the mechanisms by which lindane and fipronil inhibit alpha 1 glycine receptors. EXPERIMENTAL APPROACH Glycine receptors were recombinantly expressed in HEK293 cells and insecticide effects were studied using patch-clamp electrophysiology. KEY RESULTS Both compounds completely inhibited all tested glycine receptor subtypes with IC50 values ranging from 0.2-2 mu M, similar to their potencies at vertebrate GABA(A) receptors. Consistent with molecular docking predictions, both lindane and fipronil interacted with 6' threonine residues via hydrophobic interactions and hydrogen bonds. In contrast with predictions, we found no evidence for lindane interacting at the 2' level. We present evidence for fipronil binding in a non- blocking mode in the anaesthetic binding pocket, and for lindane as an excellent pharmacological tool for identifying the presence of beta subunits in ab heteromeric glycine receptors. CONCLUSIONS AND IMPLICATIONS This study implicates glycine receptors as novel vertebrate toxicity targets for fipronil and lindane. Furthermore, lindane interacted with pore-lining 6' threonine residues, whereas fipronil may have both pore and non-pore binding sites.