The role of GABA and glutamate receptors in susceptibility and resistance to chloride channel blocker insecticides

Despite a point mutation in the pore-forming segment of the Rdl GABA receptor subunit that is widespread and persistent in insect populations and confers high levels of resistance to dieldrin and other polychlorocycloalkane (PCCA) insecticides, the phenylpyrazole insecticide fipronil, which binds at same site, has proven to be effective in controlling many insects, including dieldrin-resistant populations. Fipronil and its major sulfone metabolite are unique among chloride channel blocking insecticides in that they also potently block GluCls. We present here a patch clamp study of the action of fipronil sulfone on native GABA receptors and GluCl receptors from susceptible and dieldrin-resistant German cockroaches, to provide a better understanding of the effect of the Rdl mutation on the function and insecticide sensitivity of these two targets, and its role in resistance. Dieldrin blocked GABA currents with an IC50 of 3 nM in wild-type cockroaches, and 383 nM in resistant insects, yielding a resistance ratio of 128. Fipronil sulfone blocked GABA currents with an IC50 of 0.8 nM in susceptible insects and 12.1 nM, or 15-fold higher, in resistant insects. While both GluClD (desensitizing) and GluClN (non-desensitizing) receptors were found in German cockroach neurons, GluClN receptors were rare and could not be included in this study. GluClD receptors from resistant insects had reduced sensitivity to glutamate and a lower rate of desensitization than those from susceptible insects, but their sensitivity to block by fipronil sulfone was not significantly changed, with an IC50 of 38.5 ± 2.4 nM (n = 8) in the susceptible strain and 40.3 ± 1.0 nM (n = 7) in the resistant strain. Fipronil sulfone also slowed the decay time course of GluClD currents. These results suggest that GluClD receptors contain the Rdl subunit, but their sensitivity to fipronil sulfone is not altered in resistant insects.