Action of Phenylpyrazole Insecticides at the GABA-Gated Chloride Channel

Abstract

The GABA-gated chloride channel is the proposed target for phenylpyrazole insecticides based on studies comparing four potent insecticidal compounds with two herbicidal but noninsecticidal analogs. The four insecticides used are 1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-1H-pyrazoles with the following heterocyclic substituents: 3-cyano-4-[(trifluoromethyl)sulfinyl]-5-amino (fipronil, 1), 4-[(trifluoromethyl)thiol (2), 4-[(trifluoromethyl)sulfinyl] (3), 4-[(trifluoromethyl)sulfonyl] (4). Pyrazole substituents of the two herbicidal analogs examined are 5-amino-4-nitro (5), 5-(2-chloropropanamide)-4-nitro (6). Piperonyl butoxide strongly antagonizes the toxicity of thioether 2 to houseflies, does not alter the potency of sulfoxide 3 and sulfone 4, and strongly synergizes sulfoxide 1, indicating the importance of oxidative activation and detoxification to insecticidal activity. Insecticides 1-4, but not herbicides 5 and 6, yield poisoning signs in house flies and mice similar or identical to those produced by trioxabicyclooctanes such as 1-[(4-ethynyl)phenyl]-4-n-propy-2,6,7-trioxabicyclo[2.2.2]octane (EBOB), which is known to block the γ-aminobutyric acid (GABA)-gated chloride channel. A dieldrin-resistant house fly strain with a low-affinity EBOB binding site is also tolerant to 1 and 2. The first four phenylpyrazoles are more potent than the latter two as inhibitors of [3H]EBOB binding with 50% inhibition levels of 2-20 nM in house fly head membranes and 1700-10,100 nM in mouse brain membranes. Compounds 1 and 2 are much more potent than 5 and 6 in inhibiting ivermectin-induced neurotransmitter release from cricket central nervous system synaptosomes. Compound 1 is also more effective than 2 and 5 at 20 μM in blocking GABA-dependent chloride uptake in mouse brain microvesicles; in this assay 6 is as effective as 1 although they act by different mechanisms in vivo in mice based on the symptomology. Neither 1 nor 2 inhibits [3H]avermectin binding in house fly head membranes. Phenylpyrazoles 1 and 2 block [3H]EBOB binding in house fly head membranes by reducing the Bmax but not the Kd, indicating that they are irreversible or poorly reversible inhibitors or act at a distinct but coupled binding site. The insecticidal phenylpyrazoles appear to block the GABA-gated chloride channel with higher potency for a site in house fly brain than in mouse brain, thereby providing useful selective toxicity. These findings are consistent with an earlier pharmacological investigation on the action of a structurally related phenyltriazole as a potent antagonist of GABA-activated chloride currents in insects. © 1993 Academic Press. All rights reserved.