Fluralaner is an isoxazoline ectoparasiticide that is used to treat flea and tick infestations in dogs and cats. We have reported that fluralaner inhibits agonist responses in both GABA-gated chloride channels (GABARs) and glutamate-gated chloride channels, with the former being more sensitive to fluralaner than the latter. Amino acid residues in the first and third transmembrane α-helices of these homopentameric channels are critical determinants of the inhibitory potency of fluralaner, suggesting that the transmembrane subunit interface is its site of action. To elucidate the mode of action of fluralaner, we examined the inhibitory effects of fluralaner on GABA-induced currents in oocytes expressing housefly GABARs using the two-electrode voltage clamp technique and various application protocols. The inhibition gradually progressed by repeating 3-s applications of GABA during the perfusion of fluralaner (i.e., in an activated state of channels). Once inhibited, the current was not restored by repeated 3-s applications with GABA alone. The inhibition by a single fluralaner application was not significant when coapplied for 3 s or 60 s during a 60-s GABA application. GABA-induced currents after the perfusion of fluralaner alone followed by a 60-s washout with buffer (i.e., in a resting state of channels) remained unchanged compared with the control currents before the perfusion. However, the GABA response was inhibited, albeit poorly, when GABA was coapplied for 3 s at the end of fluralaner perfusion. These findings suggest that fluralaner intrudes stepwise into the binding site in an activated state of the channel but not in a resting state.

Mode of action of the ectoparasiticide fluralaner: State-dependent inhibition of GABA receptor channels

ACS Fall 2021

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