Differential blocking actions of 4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) and gamma-hexachlorocyclohexane (gamma-HCH) on gamma-aminobutyric acid- and glutamate-induced responses of American cockroach neurons.

4'-Ethynyl-4-n-propylbicycloorthobenzoate (EBOB) has been employed extensively as a radioligand in binding assays to evaluate the pharmacology of gamma-aminobutyric acid (GABA)-gated Cl- channels (GABARs) of insects and mammals, and gamma-hexachlorocyclohexane (gamma-HCH) was used as an insecticide targeting insect GABARs. Since recent studies have shown that not only GABARs but also glutamate-gated chloride channels (GluCls) are blocked by picrotoxinin, dieldrin and fipronil, the actions of EBOB and gamma-HCH on native GABARs and GluCls of terminal abdominal ganglion neurons in American cockroach (Periplaneta americana) were tested using patch-clamp electrophysiology. A marked run-down of the GABA- and glutamate-induced responses of the cockroach neurons occurred, when a standard pipette solution was employed, but addition of pyruvate to the solution permitted stable recordings of these responses. With this solution, EBOB and gamma-HCH were found to block not only the GABA- but also glutamate-gated responses, with the actions augmented by repeated co-application with the agonists. It was also found that prolonged pre-application of EBOB and gamma-HCH prior to co-application with GABA and glutamate resulted in enhanced blocking actions, indicating resting-state actions of the blockers. The blocking actions of EBOB and gamma-HCH on the GABA- and glutamate-induced responses were compared by determining IC50 values under steady state condition. The IC50 values for the actions of EBOB on GABAR and GluCls differed less than those of gamma-HCH.