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NEUROPHARMACOLOGY

Sulfone Metabolite of Fipronil Blocks -Aminobutyric Acid- and Glutamate-Activated Chloride Channels in Mammalian and Insect Neurons

Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois

Received September 14, 2004; accepted February 7, 2005.

	Abstract

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Fipronil sulfone, a major metabolite of fipronil in both insects and mammals, binds strongly to GABA receptors and is thought to play a significant role in poisoning by fipronil. To better understand the mechanism of selective insecticidal action of fipronil, we examined the effects of its sulfone metabolite on GABA- and glutamate-activated chloride channels (GluCls) in cockroach thoracic ganglion neurons and on GABA_A receptors in rat dorsal root ganglion neurons using the whole-cell patch-clamp technique. Fipronil sulfone blocked both desensitizing and nondesensitizing GluCls in the cockroach. Activation was required for block and unblock of desensitizing GluCls. In contrast, activation was not prerequisite for block and unblock of nondesensitizing channels. After repetitive activation of the receptors, the IC₅₀ of fipronil sulfone to block the desensitizing GluCls was reduced from 350 to 25 nM and that for blocking nondesensitizing GluCls was reduced from 31.2 to 8.8 nM. This use-dependent block may be explained by its slow unbinding rate. In cockroach and rat neurons, fipronil sulfone blocked GABA receptors in both activated and resting states, with IC₅₀ values ranging from 20 to 70 nM. In conclusion, although fipronil sulfone is a potent inhibitor of cockroach GABA receptors, desensitizing and nondesensitizing GluCls, and rat GABA_A receptors, its selective toxicity in insects over mammals appears to be associated with its potent blocking action on both desensitizing and nondesensitizing GluCls, which are lacking in mammals.

Under normal use conditions, fipronil can be degraded mainly to fipronil sulfone via biotic/abiotic oxidation and to a desulfinyl photoproduct via photolysis (Environmental Protection Agency, 1996; Bobe et al., 1998; Ngim et al., 2000; Fenet et al., 2001; Tingle et al., 2003). Fipronil sulfone is rapidly formed through cytochrome P450 NADPH-dependent oxidation in human recombinant CYP3A4 system and in human liver microsomes in vitro. The metabolite was also found in the brain and liver of mice treated i.p. with fipronil (Caboni et al., 2003). In European corn borer (Ostrinia nubilalis Hubner), fipronil sulfone was formed through the microsomal cytochrome P450 oxidation system in the midgut in vivo and in vitro (Durham et al., 2002). The biological conversion of fipronil to fipronil sulfone was completely blocked with piperonyl butoxide, a cytochrome P450 monooxygenase inhibitor, in vivo and in vitro (Hainzl et al., 1998; Caboni et al., 2003). This inhibitor has been used in vivo to test whether or not the metabolites of fipronil are active as insecticides.

It is still unclear whether the conversion of fipronil to fipronil sulfone represents detoxication in mammals and insects. When pretreated with piperonyl butoxide, the toxicity of fipronil increased in mice (Hainzl et al., 1998; Caboni et al., 2003) and houseflies (Cole et al., 1993; Hainzl and Casida, 1996), a result indicating that conversion to fipronil sulfone is a detoxifying process. However, in some other studies, when pretreated with piperonyl butoxide, the toxicity of fipronil mildly decreased in German cockroaches (Valles et al., 1997) and Western corn rootworms (Scharf and Siegfried, 1999), indicating that conversion to fipronil sulfone is not a detoxifying process. Consistent with this notion are the observations that the toxicity of fipronil sulfone to freshwater invertebrates, freshwater fish, and birds was indeed higher than that of the parent compound (Environmental Protection Agency, 1996; Schlenk et al., 2001; Tingle et al., 2003). Other studies showed that the toxicological and neurological effects of fipronil sulfone were comparable with those of fipronil in mammals and insects (Cole et al., 1993; Scharf and Siegfried, 1999). One way to resolve these conflicting results is to directly examine the effects of fipronil sulfone on the target sites of action.

The mechanism of selective toxicity of fipronil sulfone in insects and mammals is not well understood. Sulfone metabolite showed a similar potency to its parent compound in inhibiting the 4'-ethynyl-4-n-[2,3-³H₂]propylbicycloorthobenzoate ([³H]EBOB) binding to housefly and fruit fly GABA receptors but had a higher potency than fipronil in mammalian GABA_A receptors (Hainzl et al., 1998). Thus, based on their binding specificity for GABA receptors, fipronil sulfone is more toxic than fipronil in the vertebrate. Besides GABA-activated chloride channels, GluCls have recently been found to be very sensitive to fipronil blocking action (Raymond et al., 2000; Horoszok et al., 2001; Ikeda et al., 2003; Zhao et al., 2004b). However, no functional studies have been performed for the actions of fipronil sulfone on insect GABA receptors and GluCls. To further explore the mechanism of action of fipronil sulfone and its selective toxicity, the present study was undertaken to compare the effects of fipronil sulfone on chloride channels activated by cockroach GABA and glutamate receptors and by mammalian GABA_A receptors using the whole-cell patch-clamp technique. Our results indicate that fipronil sulfone is a potent inhibitor of cockroach GABA receptors, GluCls, and rat GABA_A receptors and that its selective toxicity in insects over mammals is associated with its potent action on multiple insect target sites.

	Materials and Methods

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Rat Dorsal Root Ganglion Neurons. The dorsal root ganglia (DRG) were dissected from the lumbosacral region of newborn Sprague-Dawley rats (2-5 days old) under halothane anesthesia and were immediately placed into ice-cold, Ca²⁺/Mg²⁺-free phosphate-buffered saline solution supplemented with 6 g/liter glucose. The ganglia were then digested in Ca²⁺/Mg²⁺-free phosphate-buffered saline solution containing 2.5 mg/ml trypsin (type XI; Sigma-Aldrich, St. Louis, MO) for 20 min at 37°C. Digestion was terminated by washing with Dulbecco's modified Eagle's medium (DMEM) containing 0.1 mg/ml fetal bovine serum and 0.08 mg/ml gentamycin. The ganglia were then dissociated by repeated trituration using a fire-polished Pasteur pipette in 2 ml of DMEM. The dissociated cells were placed onto coverslips coated with poly-L-lysine hydrobromide (mol. wt. > 30,000, 0.1 mg/ml; Sigma-Aldrich). Neurons were maintained in DMEM containing fetal bovine serum and gentamycin in a 90% air and 10% CO₂ atmosphere controlled at 37°C. The neurons were ready for experiments 3 to 4 h later and used in 1 day.

Cockroach Neurons. Adult cockroaches (Periplaneta americana) were purchased from Carolina Biological Supply Co. (Burlington, NC) and were maintained at room temperature (22-24°C) with free access to water and food. Isolation of neurons from thoracic ganglia was performed at room temperature using enzymatic digestion and mechanical dissociation as described previously (Zhao et al., 2003). Briefly, a cockroach was immobilized with pins dorsal side up on a dissection dish coated with wax. The dorsal cuticle, gut, and some muscles were removed to gain access to the ventral nerve cord. Three thoracic ganglia were carefully dissected and placed in cockroach saline solution containing 200 mM NaCl, 3.1 mM KCl, 4 mM MgCl₂, 20 mM D-glucose, and 10 mM HEPES acid, with pH adjusted to 7.3 with 1 mM NaOH. After removing the remaining nerve fibers and nerve sheets, the ganglia were incubated for 40 min at room temperature in the saline solution containing collagenase (type A, 0.5 mg/ml; Roche Diagnostics, Mannheim, Germany) and hyaluronidase (type I-S, 1 mg/ml; Sigma-Aldrich). The ganglia were then rinsed twice in saline solution supplemented with 5 mM CaCl₂ and 5% (v/v) fetal calf serum and were mechanically dissociated by gentle trituration through a fire-polished Pasteur pipette. The dissociated neurons, suspended in the supplemented saline solution, were allowed to settle on glass coverslips coated with poly-L-lysine. The neurons were ready for experiments 3 to 4 h later and used in 1 day.

Recording Solutions. The internal solution for rat DRG neurons contained 140 mM CsCl, 5 mM EGTA, 1 mM MgCl₂, and 10 mM HEPES. The external solution contained 136 mM choline chloride, 2 mM CaCl₂, 1 mM MgCl₂, and 10 mM HEPES. The pH of internal and external solutions was adjusted to 7.3 with Tris base, and the osmolarity of internal and external solutions was adjusted to 290 to 300 mOsm with sucrose.

The internal solution for measurements of GABA- and glutamate-activated chloride channel currents from cockroach neurons contained 15 mM NaCl, 170 mM KCl, 0.5 mM CaCl₂, 1 mM MgCl₂, 10 mM EGTA, 10 mM phosphocreatine diTris, 20 mM HEPES, and 3 mM ATP/Mg²⁺. The pH was adjusted to 7.4 with KOH, and the osmolarity was 420 mOsm. The external solution contained 167 mM NaCl, 3.1 mM KCl, 33 mM D-gluconic acid, 5 mM CaCl₂, 4 mM MgCl₂, and 10 mM HEPES acid. The pH was adjusted to 7.4 with NaOH, and the osmolarity was 420 mOsm.

Current Recordings. Membrane currents in response to agonists were recorded from rat and cockroach neurons using the whole-cell patch-clamp technique at room temperature. The agonists, GABA or glutamate, were applied at 100 µM, which was near or slightly above their EC₅₀ values to activate the respective receptors. Neurons were continuously perfused with the respective external solution. Pipette electrodes were made from 1.5-mm (o.d.) borosilicate glass capillary tubes and had a resistance of 1 to 2 M when filled with the internal solution. The membrane potential was clamped at -60 mV unless otherwise stated. The recording of whole-cell currents began at least 10 min following membrane rupture so that the intracellular milieu was adequately equilibrated with the pipette solution. Currents were recorded with an Axopatch-200A amplifier (Axon Instruments Inc., Union City, CA) interfaced to a personal computer and controlled by the ClampEX 6.0.4 program. The currents were digitalized at 5 kHz and filtered at 2 kHz, stored by a personal computer-based data acquisition system, and analyzed off-line using the Clampfit program.

Drug Application. Two drug application methods, bath perfusion and U-tube application, were used separately or in combination in the present study. A U-tube was used for rapid application of external solution containing agonist (GABA or glutamate) alone or with test compounds. External solution containing agonist alone or with test compounds was fed through the tube by gravity from a container located above the bath. Closure of a computer-operated solenoid valve in the outlet side of the tube allowed the U-tube solution to flow out of the hole located near the cell. Another valve controlling a suction tube with an opening on the other side of the cell was also opened, allowing the test solution to be sucked away quickly. The opening and closing of two solenoid valves were operated by ClampEX 6.0.4. With this method, the external solution surrounding the cell could be completely exchanged with a test solution in 30 ms.

Chemicals. Agonist (GABA or glutamate) was first dissolved in deionized water as stock solution and then diluted with the external solution immediately before experiment. Fipronil sulfone was prepared according to the method of Hainzl et al. (1998) and had a purity of >99%. Fipronil sulfone was first dissolved in dimethylsulfoxide to make stock solutions and stored in freezer in glass vials. The fipronil sulfone stock solution was diluted to various concentrations with the external solution immediately before each experiment. The concentration of dimethylsulfoxide in test solutions was 0.1% (v/v) and did not cause any noticeable effects on GABA- or glutamate-activated chloride channel currents in DRG and cockroach neurons.

Data Analysis. Whole-cell currents were initially analyzed with Clampfit 6.0.4 program to measure the current amplitude and decay kinetics. The statistical analysis and the nonlinear regression analysis were carried out using Sigmaplot software (version 8.0; SPSS Inc., Chicago, IL). The dose-response relationship for test chemicals to modulate the GABA or glutamate response was evaluated by fitting the data to the Hill equation: Y = Y_maxIC₅₀^h/(C^h + IC₅₀^h), where Y is the response in the percentage of control, Y_max the maximum response, C the test compound concentration, IC₅₀ the concentration of test compound to inhibit the currents 50%, and h the Hill coefficient. Data, where appropriate, were expressed as the mean ± S.E.M.

	Results

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Differential Responses of Desensitizing and Nondesensitizing Components of GluCl Currents to Fipronil Sulfone. The majority of GluCl currents evoked by glutamate in cockroach neurons comprised variable fractions of desensitizing and nondesensitizing currents (Zhao et al., 2004a). Figure 1A depicts these mixed type currents evoked by a 5-s application of 100 µM glutamate every 60 s at a holding potential of -60 mV. The actions of fipronil sulfone on nondesensitizing currents were examined mainly on the mixed type GluCl currents because it was difficult to find neurons that generated nondesensitizing currents only. At least 10 min following rupture of the membrane and after several stable control recordings were obtained, fipronil sulfone at 100 nM was applied via bath and U-tube perfusion. Both desensitizing component (peak current) and nondesensitizing components (measured at a 5-s time point during the glutamate pulse) were decreased gradually during application of glutamate and fipronil sulfone. However, it took a longer time for the desensitizing component to reach a steady-state block than the nondesensitizing component. Taking advantage of different decaying kinetics, the peak amplitude of the desensitizing current could be parsed from the total current by subtracting out the nondesensitizing component. The degrees of steady-state inhibition of currents by 10 and 100 nM fipronil sulfone were 62.0 ± 1.5% (n = 6) and 92.2 ± 4.5% (n = 7), respectively, for the nondesensitizing component, and 44.0 ± 5.2% (n = 6) and 79.8 ± 6.4% (n = 7), respectively, for the desensitizing component (Fig. 1, B and C). From these results, the IC₅₀ for fipronil sulfone against the desensitizing current was calculated to be 25 ± 2 nM (n = 4), and the IC₅₀ against the nondesensitizing current was calculated to be 8.5 ± 0.4 nM (n = 4).

View larger version (21K): [in this window] [in a new window]   Fig. 1. Differential blocking actions of fipronil sulfone on the desensitizing and nondesensitizing GluCls in cockroach neurons. A, sample traces of chloride currents with a desensitizing and a nondesensitizing component were evoked by 5-s applications of 100 µM glutamate at an interval of 60 s at -60 mV holding potential. Fipronil sulfone at 100 nM was applied via bath perfusion and U-tube. B and C, time courses of blocking action of 10 and 100 nM fipronil sulfone on the desensitizing (filled circles) and nondesensitizing (open circles) components (n = 6 for B and n = 7 for C).

Block of Desensitizing GluCl Current by Fipronil Sulfone. To study fipronil sulfone block of the desensitizing GluCl current, fipronil sulfone at a concentration of 1 µM was coapplied for 30 s with 100 µM glutamate via a U-tube at an interval of 3 min. The peak amplitude of current was gradually decreased in a use-dependent manner to 97.7 ± 3.5%, 65.2 ± 2.0%, 35.0 ± 2.2%, and 25.9 ± 4.4% (n = 4) of the control following four consecutive coapplications of fipronil sulfone (Fig. 2, A and B). Meanwhile, the decay phase of current was gradually accelerated from the control time constant of 575 ± 28 (n = 4) to 205 ± 23 (n = 4) ms at the fourth coapplication of fipronil sulfone. After washing with insecticide-free external solutions for 10 min in the absence of repetitive glutamate stimulations, the decay time constant increased slightly, but no recovery of current amplitude was seen (Fig. 2, A and B). However, recovery of the current amplitude and decay time constant was accelerated by activation of the receptors by glutamate. The results suggested that block and unblock of the desensitizing currents by fipronil sulfone required the activation or opening of GluCls.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Block and unblock of desensitizing GluCls by 1000 nM fipronil sulfone in cockroach neurons. Desensitizing currents were evoked by 100 µM glutamate applied for 30 s at an interval of 3 min at -60 mV holding potential. A, peak current was gradually suppressed during repetitive coapplications of fipronil sulfone with glutamate (left panel). The neuron was then washed with drug-free external solutions for 10 min, during which period no glutamate was applied. The recovery of the GluCls occurred during repetitive applications of glutamate in continuing washout (right panel). B, time course of block and unblock of desensitizing GluCls by fipronil sulfone in four cells. The peak currents were suppressed to 29.0 ± 4.3% of the control after four consecutive coapplications of fipronil sulfone. The GluCls did not recover after 10-min washout without activation of the receptors by glutamate but recovered after repetitive activations of GluCls by glutamate.

Block of Nondesensitizing GluCl Current by Fipronil Sulfone. To study fipronil sulfone block of the nondesensitizing GluCl current, the same protocols as those for the desensitizing GluCl current were used to generate the mixed desensitizing and nondesensitizing currents (Fig. 3). Since the nondesensitizing currents were more sensitive to fipronil sulfone than the desensitizing currents (Fig. 1, B and C), a low concentration of fipronil sulfone was used. Fipronil sulfone at 10 nM was coapplied for 30 s with 100 µM glutamate to the neuron at an interval of 3 min following stable control recordings (Fig. 3A). Fipronil sulfone suppressed the nondesensitizing current in a use-dependent manner. The steady-state current amplitude measured at a 30-s time point during the glutamate pulse gradually decreased to 31.9 ± 7.4% (n = 4) of the control after five coapplications of fipronil sulfone and glutamate, whereas the desensitizing component remained almost unchanged (Fig. 3, A and B). After washing the neuron with insecticide-free external solutions for 10 min without glutamate application, the nondesensitizing currents recovered to 69.1 ± 3.2% (n = 4) of the control. The current further recovered with prolonged washout. The results suggested that a large proportion of the nondesensitizing receptors were able to recover from fipronil sulfone block without receptor activation.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Block and unblock of nondesensitizing GluCls by fipronil sulfone. The current with desensitizing and nondesensitizing components was evoked by 100 µM glutamate applied for 30 s at an interval of 3 min in cockroach neurons. A, fipronil sulfone at a concentration of 10 nM was coapplied with glutamate. The amplitude of nondesensitizing component of current decreased gradually and reached a steady state after four coapplications of fipronil sulfone, whereas the desensitizing component remained almost constant. The neuron was then washed with drug-free external solutions for 10 min, during which period no glutamate was applied to activate the receptors. The recovery of the nondesensitizing currents occurred by repetitive applications of glutamate during continuing washout. The nondesensitizing component partially recovered after 10-min washout without glutamate activation and recovered almost completely after an additional three applications of glutamate. B, time course of fipronil sulfone block and unblock of nondesensitizing GluCls in four cells. The steady-state current amplitudes measured at the 30-s time point during glutamate pulse decreased to 31.9 ± 7.4% of the control after five consecutive coapplications of fipronil sulfone and glutamate. The GluCls recovered to 69.1 ± 3.2% of the control after 10-min washout without activation of the receptors by glutamate. Further recovery was obtained with repetitive activations of GluCls by glutamate.

View larger version (11K): [in this window] [in a new window]   Fig. 4. Use-dependent block of GluCls by fipronil sulfone in cockroach neurons. Two protocols were used. A, GluCl currents with a desensitizing and a nondesensitizing component were induced by 5-s applications of 100 µM glutamate at an interval of 60 s. Fipronil sulfone at 100 nM was bath-perfused and repetitively coapplied with glutamate for 10 min. B, similar protocol to A, but no glutamate was applied during 10-min bath perfusion of 100 nM fipronil sulfone. A test pulse of glutamate was applied to determine the degree of block. After 10-min bath perfusion of 100 nM fipronil sulfone, the nondesensitizing current amplitude was almost completely suppressed in protocol A, whereas it was suppressed only partially in protocol B. Thus, activation of the receptors significantly increases the blocking action of fipronil on the nondesensitizing component.

The result of experiments using the second protocol is shown in Fig. 4B. Without receptor activation during 10-min bath perfusion of 100 nM fipronil sulfone, the amplitudes of the peak current and the current measured at 3 s during glutamate pulse were decreased to 96.9 ± 6.9% (n = 7) and 75.4 ± 7.0% (n = 7) of the control, respectively. The reduction in peak current was not significant (p > 0.05), whereas the reduction in current measured at 3 s was significant (p < 0.05). The latter reduction is much smaller than the reduction observed in the activated receptor as subsequent repetitive U-tube coapplications of glutamate and fipronil sulfone reduced the desensitizing currents to 5.2 ± 1.3% of the control (n = 6, data not shown). These results suggested that the activation of receptors is prerequisite for the block of the desensitizing current, whereas it significantly enhanced the blocking action of fipronil sulfone on nondesensitizing GluCl currents.

Kinetic and Steady-State Analysis of Fipronil Sulfone Suppression of Activated Nondesensitizing GluCls. To examine the fipronil sulfone block of activated nondesensitizing GluCls in more detail, fipronil sulfone at various concentrations was coapplied with 100 µM glutamate for 30 s at an interval of 3 min. Coapplications of fipronil sulfone caused use- and dose-dependent decreases in current amplitude (Fig. 5A). At the end of the first, second, and third coapplications, fipronil sulfone suppressed the nondesensitizing currents with IC₅₀s of 31.2 ± 4.1, 12.3 ± 1.4, and 8.8 ± 1.1 (n = 3-5) nM, respectively (Fig. 5B). The IC₅₀ of 8.8 nM probably represents the affinity of fipronil sulfone for the activated receptors.

View larger version (22K): [in this window] [in a new window]   Fig. 5. Kinetic analysis of fipronil sulfone block of the activated nondesensitizing GluCls in cockroach neurons. A, glutamate (100 µM) was applied for 30 s to induce currents (control), with coapplication of various concentrations of fipronil sulfone at an interval of 3 min. B, dose-response relationships for fipronil sulfone block of the activated nondesensitizing GluCls. The current amplitude was measured at the end of 30-s coapplication during the first, second, and third pulses of fipronil sulfone and glutamate. The fit to the dose-response data gave IC50 values/Hill coefficients of 31.2 ± 4.1 nM/0.9 ± 0.1 after the first (circles), 12.3 ± 1.4 nM/1.7 ± 0.4 after the second (triangles), and 8.8 ± 1.1 nM/2.2 ± 0.9 after the third (squares) applications (n = 4-5). C, slow decay phase of the currents was normalized to the control and was fitted with a single exponential function to obtain the time constant of fipronil sulfone block. The reciprocal of the time constant (1/) is plotted as a function of fipronil concentration to calculate the blocking and unblocking rate constants for fipronil sulfone interaction with the activated receptor. Data points are best fitted to the solid line according to the equation: 1/ = k+1[fipronil sulfone] + k-1, where k+1 and k-1 are the blocking and unblocking rate constants, respectively, and [fipronil sulfone] is the fipronil sulfone concentration. The correlation coefficient of 0.995 gives a significance level of p < 0.05. k+1 = 3.1 x 105 M-1 s-1; k-1 = 1.9 x 10-2 s-1; the calculated Kd = 61.7 nM (n = 4-8).

Actions of Fipronil Sulfone on Insect GABA Receptors. GABA receptors of insects and mammals were also blocked by fipronil sulfone as indicated by binding studies (Hainzl et al., 1998). To further investigate the actions of fipronil sulfone on the function of insect GABA receptors, a whole-cell patch-clamp study was conducted using cockroach neurons. Currents were evoked by 1-s applications of 100 µM GABA at an interval of 1 min (Fig. 6, A and B). After several stable control recordings were obtained, fipronil sulfone at a concentration of 10 or 100 nM was applied through bath perfusion and U-tube. The current gradually decreased during a 10-min fipronil sulfone treatment in a use- and concentration-dependent manner (Fig. 6C). The GABA-induced currents were suppressed by 23.4 ± 4.6% (n = 5) and 95.3 ± 0.8% (n = 5) by fipronil sulfone at 10 and 100 nM, respectively. In another protocol in which GABA stimulations were not applied during a 10-min bath perfusion of 100 nM fipronil sulfone, the GABA currents were suppressed by 83.3 ± 3.8% (n = 6, data not shown). Thus, like fipronil, fipronil sulfone is capable of inhibiting both the resting and activated GABA receptors.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Fipronil sulfone block of cockroach GABA receptors. Chloride currents were evoked by 1-s applications of 100 µM GABA at a holding potential of -60 mV. Fipronil sulfone at concentrations of 10 and 100 nM was applied via bath perfusion and U-tube at an interval of 60 s. A, control current sample and current recorded at the end of 10-min bath perfusion of 10 nM fipronil sulfone. B, similar to A but with 100 nM fipronil sulfone. C, time course of the blocking action of fipronil sulfone. The currents were normalized to the respective control to calculate the blocking percentages. Each data point was the average value obtained from five cells. The peak current was decreased to 76.6 ± 4.6% (open circles, n = 5) of the control by 10 nM and to 4.7 ± 0.8% (filled circles, n = 5) by 100 nM fipronil sulfone after 10-min bath perfusion.

The blocking action of fipronil sulfone on the activated state of GABA receptors was also determined with a 30-s coapplication of GABA and test drug via U-tube. Following having recorded several stable control currents, fipronil sulfone at various concentrations was coapplied with 100 µM GABA (Fig. 7A). The steady-state current amplitude measured at 30-s time point of GABA application was suppressed by 6.4 ± 1.8% (n = 4), 22.4 ± 5.6% (n = 4), 64.9 ± 5.6% (n = 7), 75.7 ± 2.7% (n = 4), and 87.7 ± 4.2% (n = 5) after the first coapplication of fipronil sulfone at 10, 30, 100, 300, and 1000 nM, respectively. The concentration-response relationships for fipronil sulfone block established for the first, second, and third coapplications of fipronil sulfone were fitted by a sigmoid curve with IC₅₀ values of 53.7 ± 8.0, 26.6 ± 4.9, and 15.4 ± 2.1 nM and Hill coefficients of 1.6, 1.4, and 1.3, respectively (Fig. 7B). The results indicated that fipronil sulfone was a potent blocker of the cockroach GABA receptor in the activated state and that repetitive activation of the receptors enhanced the block.

View larger version (21K): [in this window] [in a new window]   Fig. 7. The blocking action of fipronil sulfone on the activated cockroach GABA receptors. A, currents evoked by 100 µM GABA applied for 30 s every 3 min at a holding potential of -60 mV without and with coapplication of various concentrations of fipronil sulfone. B, dose-response relationships of fipronil sulfone block of the activated GABA receptors. The blocking percentages were calculated from the steady-state current amplitude measured at the 30-s time point of GABA pulses. The dose-response curves for the first, second, and third coapplications of fipronil sulfone were fitted by the sigmoid curves with IC50 values of 53.7 ± 8.0, 26.6 ± 4.9, and 15.4 ± 2.1 nM and Hill coefficients of 1.6, 1.4, and 1.3, respectively (n = 4-7). C, slow decay phase of the GABA currents was normalized to the control and was fitted with a single exponential function to obtain the time constant of fipronil sulfone block. The reciprocal of the time constant (1/) is plotted as a function of fipronil concentration to calculate the blocking and unblocking rate constants for fipronil sulfone interaction with the activated receptor. Data points are best fitted to the solid line with the blocking and unblocking rate constants of 7.3 x 104 M-1 s-1 and 3.2 x 10-2 s-1, respectively. The calculated dissociate constant (Kd) was 440 nM (n = 4-7).

The kinetic analysis of the blocking action of fipronil sulfone on the activated cockroach GABA receptors was carried out on the acceleration of current decay in the presence of various concentrations of fipronil sulfone (Fig. 7C). The fit to the data gave the blocking rate constant of 7.3 x 10⁴ M^-1 s^-1 and the unblocking rate constant of 3.2 x 10^-2 s^-1.

Actions of Fipronil Sulfone on Mammalian GABA_A Receptors. Differences in GABA receptor sensitivity to fipronil blocking action appear to be one of the major determinants in fipronil selective toxicity to insects relative to the vertebrates (Hainzl et al., 1998). In electrophysiological studies, insect GABA receptors were shown to have a higher sensitivity than mammalian GABA_A receptors to the blocking action of fipronil (Ikeda et al., 2001; Zhao et al., 2003). In a study of the displacement of [³H]EBOB binding (Hainzl et al., 1998), fipronil bound to insect GABA receptors with an affinity at least 100-fold higher than it bound to mammalian GABA receptors. Fipronil sulfone was less selective but was nevertheless 20-fold more active against insect as opposed to mammalian GABA receptors, as measured by [³H]EBOB binding. To further examine whether the sensitivity of GABA receptors to fipronil sulfone plays a role in selective toxicity in insects and mammals, the effect of fipronil sulfone on rat DRG neuron GABA_A receptors was studied. Figure 8A depicts the currents evoked by 2-s applications of 100 µM GABA at an interval of 30 s. After several stable control recordings were obtained, fipronil sulfone at 100 nM was applied via both bath perfusion and U-tube. The peak amplitudes of currents were gradually decreased to 41.9% of the control and were maintained at a steady-state level (Fig. 8, A and B). The currents gradually recovered during 10-min washout with drug-free external solutions. The GABA_A receptors were almost completely blocked by a higher concentration of fipronil sulfone at 1000 nM (Fig. 8, A and B). The dose-response relationship for fipronil sulfone block of the rat GABA_A receptors is shown in Fig. 8C, and the data were fitted with an IC₅₀ of 70.2 ± 3.9 nM (n = 4-5). To determine the relative contribution of resting state block and activated state block to the steady-state block obtained in Fig. 8, two protocols as described earlier (Fig. 4) were used. Currents were induced by 1-s applications of 100 µM GABA at an interval of 30 s. With repetitive activation of GABA_A receptors during 10-min bath perfusion of 1000 nM fipronil sulfone, GABA currents were suppressed by 95.3 ± 2.6% (n = 5). Without receptor activation by GABA, currents were suppressed by 78.6 ± 2.3% (n = 5). The difference between the two suppressions is statistically significant (p < 0.01). The results indicated that GABA_A receptors were blocked by fipronil sulfone in both resting and activated states and that the activated state of GABA_A receptors was slightly more sensitive than the resting state to the blocking action of fipronil sulfone. The resting state block, however, contributes mostly to the steady-state block observed with the protocol in Fig. 8.

View larger version (18K): [in this window] [in a new window]   Fig. 8. Fipronil sulfone block of rat DRG GABAA receptors. A, GABA currents evoked by 2-s applications of 100 µM GABA were recorded before and during bath perfusion and U-tube coapplication of fipronil sulfone at 100 and 1000 nM. B, time course of the blocking action of fipronil sulfone. The currents were normalized to the control. The peak amplitude of the currents decreased to 41.9% of the control after bath perfusion and coapplication of 100 nM fipronil sulfone and completely recovered after 10-min washout with drug-free solution. When 1000 nM fipronil sulfone was applied after washout, the GABA current decreased to 5.0% of the control. C, dose-response relationship of the blocking action of fipronil sulfone. Each data point was the average value obtained at a steady state from separate cells (n = 4-5). The dose-response relationship was fitted with an IC50 of 70.2 ± 3.9 nM and a Hill coefficient of 0.4.

To analyze the blocking action of fipronil sulfone on the activated state of rat GABA_A receptors, GABA was applied for 30 s to evoke currents at an interval of 3 min. Fipronil sulfone at various concentrations ranging from 1 to 3000 nM was coapplied with 100 µM GABA after recording several stable control currents. The decrease in the current amplitude measured at the 30-s time point from the beginning of GABA pulse application was taken as a measure of fipronil sulfone block of the activated state. The steady-state current amplitude decreased in a concentration-dependent manner with coapplications of fipronil sulfone (Fig. 9A). The concentration-response relationship was fitted by a sigmoid curve with an IC₅₀ of 46.6 nM (n = 4-7) (Fig. 9B). Following the second 30-s application of GABA and fipronil sulfone, the IC₅₀ was reduced to 20 nM. Thus, mammalian GABA_A receptors showed similar sensitivities to insect GABA receptors to the blocking action of fipronil sulfone.

View larger version (22K): [in this window] [in a new window]   Fig. 9. The blocking action of fipronil sulfone on the activated rat DRG GABAA receptors. A, currents evoked by 100 µM GABA applied for 30 s every 3 min at a holding potential of -60 mV without and with coapplication of various concentrations of fipronil sulfone. B, dose-response relationships of fipronil sulfone block of the activated GABAA receptors. The blocking percentages were calculated from the steady-state current amplitude measured at a 30-s time point of GABA pulses. The dose-response relationship was fitted by a sigmoid curve with an IC50 of 46.6 nM and a Hill coefficient of 0.6 (n = 4-7). C, decay phase of currents was fitted with two-exponential function to obtain fast and a slow time constants of fipronil sulfone block. The reciprocal of the slow time constant (1/) is plotted as a function of fipronil concentration to calculate the blocking and unblocking rate constants for fipronil sulfone interaction with the activated receptor. Data points are best fitted to the solid line with blocking and unblocking rate constants of 4.3 x 104 M-1 s-1 and 1.9 x 10-2 s-1, respectively. The calculated Kd = 1930 nM (n = 3-16).

The kinetic analysis of the fipronil sulfone block of the GABA_A receptor-mediated currents in DRG neurons was complex because the control current decayed with a two-exponential time course (Table 1). The control current activated by 100 µM GABA decayed with a fast time constant of 1.67 ± 0.13 s and a slow time constant of 13.8 ± 0.92 s (n = 16). The fast time constant was reduced to around 1 s by fipronil sulfone in a concentration-independent manner, whereas the slow time constant was decreased by fipronil sulfone in a concentration-dependent manner. We interpreted the acceleration of the nondesensitizing component of current decay as being due to the blocking action of fipronil sulfone on the activated receptor. As such, the reciprocal values of the slow time constants were linearly related to the concentrations of fipronil sulfone (Fig. 9C). The regression analysis of the data gave a blocking rate constant of 4.3 x 10⁴ M^-1 s^-1 and an unblocking rate constant of 8.4 x 10^-2 s^-1. Thus, the unblocking rate constant of fipronil sulfone is similar to that of fipronil, whereas the blocking rate constant of fipronil sulfone is almost 7-fold larger than that of fipronil.

	Discussion

Top Abstract Materials and Methods Results Discussion References

 
In both mammals and insects exposed to fipronil, much of the insecticide is oxidized to the sulfone (Hainzl et al., 1998) so that the toxic effects of fipronil are in fact mediated to a large extent by the sulfone. Accordingly, it is important to study the sulfone on the known targets of its parent compound. The present study has demonstrated that fipronil sulfone potently blocked the cockroach glutamate- and GABA-activated chloride channels in a dose-dependent manner. It also blocked the rat GABA-activated chloride channels at concentrations similar to those for cockroach GABA-activated chloride channels.

Cockroach GluCls. Inhibitory glutamate-activated receptor chloride channels are a member of the ligand-gated anion channel superfamily that is found in invertebrates but not in vertebrates (Cleland, 1996; Raymond and Sattelle, 2002). GluCl gene sequences have been successfully cloned in Caenorhabditis elegans (Cully et al., 1994), Drosophila melanogaster (Cully et al., 1996), and Hemonchus contortus (Forrester et al., 1999). Although the GluCls in cockroaches have not been cloned yet, the functional study using the whole-cell patch-clamp technique revealed the existence of two pharmacologically distinct GluCls in dorsal unpaired median neurons of the cockroach, with the two GluCls differing in their sensitivity to the blocking action of picrotoxin and 3,3-bis(trifluoromethyl)bicyclo[2,2,1]heptane-2,2-dicarbonitrile (Raymond et al., 2000). More recently, two GluCls were further characterized kinetically as desensitizing and nondesensitizing chloride currents in unidentified thoracic ganglion neurons of American cockroaches (Ikeda et al., 2003; Zhao et al., 2004a). Both types of GluCls were activated by glutamate and ibotenic acid, an analog of glutamate, whereas the nondesensitizing GluCls were much more sensitive than the desensitizing GluCls to the blocking action of picrotoxinin and dieldrin (Ikeda et al., 2003; Zhao et al., 2004a).

Fipronil inhibited both desensitizing and nondesensitizing GluCls in cockroach neurons but with different potencies. The steady-state inhibition of the desensitizing currents by repetitive coapplications of 1000 nM fipronil was 43% (Zhao et al., 2004b), whereas the inhibition caused by its sulfone metabolite at 1000 nM was 90% (present study). The estimated IC₅₀ value for fipronil sulfone to block the desensitizing current was 25 nM, which is 30 times less than that for fipronil. These results indicate that fipronil sulfone is much more potent than fipronil to block the desensitizing currents. The difference in the potency to block the nondesensitizing GluCls between fipronil sulfone and fipronil is rather small; their IC₅₀ values were 31.2 and 27.0 nM (Zhao et al., 2004b), respectively, for fipronil sulfone and fipronil, as measured with a single coapplication with glutamate. The potency was increased following subsequent activation of the receptors, as reflected in the decrease in the IC₅₀ value to 8.8 and 11.0 nM, respectively, for fipronil sulfone and fipronil, after the third application.

Similar to fipronil (Zhao et al., 2004b), use-dependent fipronil sulfone block of the desensitizing and nondesensitizing GluCls required receptor activation. Recovery of the desensitizing current from fipronil sulfone block required channel opening, whereas recovery of the nondesensitizing current from block was independent of channel opening. Kinetic analysis revealed that the blocking rate constant (3.1 x 10⁵ M^-1 s^-1) for fipronil sulfone interaction with the activated receptor is equal to that for fipronil (3.0 x 10⁵ M^-1 s^-1), whereas the unblocking rate constant (1.9 x 10^-2 s^-1) for fipronil sulfone is 3-fold lower than that for fipronil (5.8 x 10^-2 s^-1). Thus, the slower unbinding rate of fipronil sulfone might explain its higher potency and stronger use-dependent block of nondesensitizing GluCls compared with fipronil. The potent blocking action of fipronil sulfone on both desensitizing and nondesensitizing currents indicates that both GluCls are crucial targets of fipronil metabolite.

Cockroach GABA Receptors. Cockroach GABA receptors are another member of the ligand-gated chloride channel superfamily and are distinctly different from mammalian GABA_A receptors in kinetics and pharmacological properties (Lees et al., 1987; Sattelle et al., 1991). Although picrotoxin blocks both GABA receptors, bicuculline, which blocks the mammalian GABA_A receptor, does not block the insect GABA receptor (Buckingham et al., 1994). Cockroach GABA receptors were also blocked by fipronil sulfone. In binding studies with insect GABA receptor, fipronil sulfone and fipronil were found to be equipotent in blocking [³H]EBOB binding with IC₅₀ values varying from 4 to 9 nM (Hainzl et al., 1998). They exerted a similar blocking action on the GABA-activated currents as well. For example, the cockroach resting GABA-gated chloride channels were suppressed by 9.7 and 88.7%, respectively, by bath perfusion of fipronil at 10 and 100 nM (Zhao et al., 2003). In the present study, 23.4 and 95.3% suppressions were observed by fipronil sulfone at concentrations of 10 and 100 nM, respectively. Thus, the blocking action of fipronil sulfone on activated GABA receptors was comparable with that of fipronil with IC₅₀ values of 53.0 and 35 nM, respectively, after a single coapplication with glutamate. These results suggest that fipronil sulfone is as effective as fipronil in blocking the insect GABA receptors.

Mammalian GABA_A Receptors. Mammalian GABA_A receptors were also reversibly blocked by fipronil sulfone in a dose-dependent manner, with an IC₅₀ of 70.2 nM for the receptors at the resting state and with an IC₅₀ of 46.6 nM for the receptors at the activated state. Our previous study showed that fipronil blocks the resting and activated GABA_A receptors with an IC₅₀ of 1600 nM (Ikeda et al., 2001). Thus, fipronil sulfone is at least 20 times more potent than fipronil to block mammalian GABA_A receptors. The different sensitivities of mammalian GABA_A receptors to fipronil and its sulfone metabolite were also supported by the [³H]EBOB binding study, which showed that fipronil sulfone was 6-fold more potent than fipronil in vertebrate brain membrane preparations with an IC₅₀ of 175 and 1103 nM, respectively (Hainzl et al., 1998). The kinetic analysis of block of the activated GABA_A receptors showed that although fipronil and fipronil sulfone did not differ in the unbinding rate constant, the binding rate constant of fipronil sulfone was almost 7-fold larger than that of fipronil. Therefore, the higher potency of fipronil sulfone compared with fipronil in blocking the GABA-activated chloride channels appears to be related to its higher binding rate constant for the GABA_A receptor.

Relations to the Selective Toxicities. The action of fipronil is made complex by the facts that the actions of two active molecules at multiple receptors need to be considered. Table 2 summarizes the fipronil sulfone potency data obtained in this article with patch clamp, with literature data for fipronil itself, and with [³H]EBOB binding data. Furthermore, the potency was measured by patch clamp with two different protocols, and both are listed here. Fipronil sulfone blocked the cockroach GABA receptor by coapplication with a first-pulse IC₅₀ of 53.7 nM, whereas fipronil blocked it with a first-pulse IC₅₀ of 35 nM. By the third pulse, the IC₅₀ of the sulfone was reduced to 15 nM, and with more pulses, it would approach an even lower steady-state value, comparable with the EBOB result. It would likewise be expected that the IC₅₀ of fipronil itself would decrease with repetitive pulses, but the data are not available. The nondesensitizing GluCl was also similarly sensitive to both the parent and the sulfone and slightly more sensitive than the GABA receptor. More importantly, fipronil sulfone is 30 times more potent than fipronil to block the desensitizing GluCl, with an IC₅₀ of 25 nM when applied in the bath and U-tube.

Fipronil sulfone was significantly more potent than fipronil itself against rat DRG GABA receptors, as it also was in the [³H]EBOB binding study of Hainzl et al. (1998). In this respect, the finding that the sulfone and parent are equitoxic to mice (Hainzl et al., 1998), even when pretreatment with piperonyl butoxide prevents metabolism of fipronil to the sulfone, remains surprising.

Taking these and our previous results together and with the assumption that these three receptors have critical roles in the insect central nervous system, we can conclude that fipronil sulfone has three highly sensitive target sites in insects, whereas fipronil itself has two. If these target sites are coded by distinct genes, it should greatly slow the development of target-based resistance.

	Acknowledgements

 
We thank Julia Irizarry for secretarial assistance.

	Footnotes

 
This study was supported by National Institutes of Health Grant NS14143.

Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.104.077891.

ABBREVIATIONS: GluCl, glutamate-activated chloride channel; [³H]EBOB, 4'-ethynyl-4-n-[2,3-³H₂]propylbicycloorthobenzoate; DRG, dorsal root ganglion; DMEM, Dulbecco's modified Eagle's medium.

Address correspondence to: Dr. Toshio Narahashi, Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, 303 East Chicago Avenue, Chicago, IL 60611. E-mail: narahashi{at}northwestern.edu

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