GABAA Receptor Subunit Composition Drives Its Sensitivity to the Insecticide Fipronil

Fipronil (FPN) is a worldwide-used neurotoxic insecticide, targeting, and blocking GABA(A) receptors (GABA(A)Rs). Beyond its efficiency on insect GABA(A)Rs, FPN causes neurotoxic effects in humans and mammals. Here, we investigated the mode of action of FPN on mammalian alpha 6-containing GABA(A)Rs to understand its inhibitory effects on GABA-induced currents, as a function of the synaptic or extrasynaptic localization of GABA(A)Rs. We characterized the effects of FPN by electrophysiology using Xenopus oocytes which were microtransplanted with cerebellum membranes or injected with alpha 6 beta 3, alpha 6 beta 3 gamma 2S (synaptic), and alpha 6 beta 3 delta (extrasynaptic) cDNAs. At micromolar concentrations, FPN dose-dependently inhibited cerebellar GABA currents. FPN acts as a non-competitive antagonist on ternary receptors. Surprisingly, the inhibition of GABA-induced currents was partial for extra-synaptic (alpha 6 beta 3 delta) and binary (alpha 6 beta 3) receptors, while synaptic alpha 6 beta 3 gamma 2S receptors were fully blocked, indicating that the complementary gamma or delta subunit participates in FPN-GABA(A)R interaction. FPN unexpectedly behaved as a positive modulator on beta 3 homopentamers. These data show that FPN action is driven by the subunit composition of GABA(A)Rs-highlighting the role of the complementary subunit-and thus their localization within a physiological synapse. We built a docking model of FPN on GABA(A)Rs, which reveals two putative binding sites. This is consistent with a double binding mode of FPN on GABA(A)Rs, possibly one being of high affinity and the other of low affinity. Physiologically, the gamma/delta subunit incorporation drives its inhibitory level and has important significance for its toxicity on the mammalian nervous system, especially in acute exposure.

Automated summary

Fipronil, a neurotoxic insecticide, inhibits GABA-induced currents on mammalian GABA(A) receptors by acting as a non-competitive antagonist. The subunit composition of the receptors plays a crucial role in determining the inhibitory effects of Fipronil, with complementary subunits affecting its interaction and localization within physiological synapses.