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The gamma-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 IC(50) values and Hill coefficients for fipronil block of the GABA(A) receptor were estimated to be 1.66 +/- 0.18 microM and 1.23 +/- 0.14 for the closed receptor, respectively, and 1.61 +/- 0.14 microM 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 M(-1) s(-1) and 0.018 +/- 0.0035 s(-1) for the closed GABA(A) receptor, respectively, and 6600 +/- 380 M(-1) s(-1) and 0.11 +/- 0.0054 s(-1) 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.

Nitromethylene heterocycle insecticides are known to act on the nicotinic acetylcholine receptor-channel. The effects of the nitromethylene heterocycle, imidacloprid, on the nicotinic acetylcholine receptor-channel of clonal rat phaeochromocytoma (PC12) cells were studied using whole-cell and single-channel patch clamp methods. Imidacloprid suppressed carbachol-induced whole-cell currents in a dose-dependent manner, and this compound itself generated small currents. Multiple conductance states of single-channel currents were also evoked by imidacloprid at the nicotinic acetylcholine receptor-channels. The most frequently generated single-channel currents showed two conductance states, 25.4 and 9.8 pS, which were identical to the conductance states of acetylcholine-generated currents. The mean open time and burst duration of the main conductance currents induced by imidacloprid were shorter than those induced by acetylcholine. Co-application of imidacloprid and acetylcholine caused some interactions at the two conductance states. Mean open time and mean burst duration of the main conductance state currents evoked by acetylcholine were decreased by the co-application of imidacloprid as compared with those induced by acetylcholine alone. In conclusion, imidacloprid has both multiple agonist and antagonist effects on the neuronal nicotinic acetylcholine receptor-channels.