Vol. 296, Issue 3, 914-921, March 2001

Fipronil Modulation of -Aminobutyric Acid_A Receptors in Rat Dorsal Root Ganglion Neurons

Tomoko Ikeda , Xilong Zhao, Keiichi Nagata¹ , Yoshiaki Kono, Toshio Shono, Jay Z. Yeh and Toshio Narahashi

Institute of Agriculture and Forestry, University of Tsukuba, Tsukuba, Japan (T.I., K.N., Y.K., T.S.); and Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois (T.I., X.Z., K.N., J.Z.Y., T.N.)

The -aminobutyric acid (GABA) receptor is an important site of action of a variety of chemicals, including barbiturates, benzodiazepines, picrotoxin, bicuculline, general anesthetics, alcohols, and certain insecticides. Fipronil is the first phenylpyrazole insecticide introduced for pest control. It is effective against some insects that have become resistant to the existing insecticides. To elucidate the mechanism of fipronil interaction with the mammalian GABA system, whole-cell patch-clamp experiments were performed using rat dorsal root ganglion neurons in primary culture. Fipronil suppressed the GABA-induced whole-cell currents reversibly in both closed and activated states. The IC₅₀ values and Hill coefficients for fipronil block of the GABA_A receptor were estimated to be 1.66 ± 0.18 µM and 1.23 ± 0.14 for the closed receptor, respectively, and 1.61 ± 0.14 µM and 0.96 ± 0.06 for the activated receptor, respectively. The association rate and dissociation rate constants of fipronil effect were estimated to be 673 ± 220 M¹ s¹ and 0.018 ± 0.0035 s¹ for the closed GABA_A receptor, respectively, and 6600 ± 380 M¹ s¹ and 0.11 ± 0.0054 s¹ for the activated GABA_A receptor, respectively. Thus, both the association and dissociation rate constants of fipronil for the activated GABA_A receptor are approximately 10 times as large as those for the closed receptor. Experiments with coapplication of fipronil and picrotoxinin indicated that they did not compete for the same binding site to block the receptor. It is concluded that although fipronil binds to the GABA_A receptor without activation, channel opening facilitates fipronil binding to and unbinding from the receptor.