Effects of monoterpenoid insecticides on [3H]-TBOB binding in house fly GABA receptor and 36Cl− uptake in American cockroach ventral nerve cord

Monoterpenoids and their derivatives from plant essential oils showed good insecticidal activities in previous studies, but the mechanisms of their action as natural insecticides are not known yet. In the present work, we evaluated the pharmacological action of five monoterpenoids (α-terpineol, carvacrol, linalool, pulegone, and thymol) on native insect GABA receptors from house flies and American cockroaches using radiotracer methods. In the [3H]-TBOB binding assay, carvacrol, pulegone, and thymol all enhanced the [3H]-TBOB binding to membrane preparation of house fly heads with EC50 values of 48 μM, 432 μM, and 6 mM, respectively. Moreover, these three monoterpenoids at concentrations of 500 μM and 1 mM also significantly increased the 36Cl− uptake induced by GABA in membrane microsacs prepared from American cockroach ventral nerve cords. These results revealed that carvacrol, pulegone, and thymol are all positive allosteric modulators at insect GABA receptors. The other two monoterpenoids that were tested, α-terpineol and linalool, showed little or no effect in both the [3H]-TBOB binding and 36Cl− uptake assays.

Research highlights
► Insect GABA receptors are targets of three monoterpenoid insecticides.
► Carvacrol, thymol, pulegone are positive allosteric modulators at GABA receptors.
► Three monoterpenoids enhanced [3H]-TBOB binding in house fly head membrane prep.
► Three monoterpenoids enhanced 36Cl− uptake induced by GABA in cockroach nerve cord.
► α-Terpineol and linalool show only minor effects on the insect GABA receptors.