[35S]t-butylbicyclophosphorothionate binding sites in susceptible and cyclodiene-resistant houseflies.

4-aminobutyric acid (GABA)-gated chloride ion channels are important molecular targets for a number of polychlorocycloalkane compounds including cyclodiene insecticides. Previous radioligand binding studies have indicated that cyclodiene insecticides are potent inhibitors of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to housefly thorax and abdomen membranes. In the present study, a laboratory-reared, cyclodiene-resistant (CYW) housefly strain (Musca domestica) showed resistance to a number of cyclodiene insecticides. Specific, saturable [35S]TBPS binding was detected in thorax and abdomen membranes prepared from housefly strains susceptible (CSMA) and resistant (CYW) to cyclodienes. Scatchard analysis of [35S]TBPS binding data from CSMA and CYW membranes revealed no significant differences between the two strains in either the affinity (Kd) or the density (Bmax) of specific, saturable binding sites. There were no differences in the comparative effectiveness of a range of polychlorocycloalkanes, including cyclodiene insecticides, as inhibitors of specific [35S]TBPS binding to CSMA and CYW thorax and abdomen membranes. Therefore, if an alteration in target site is a mechanism for resistance to cyclodienes in the CYW strain, it is not readily measurable using [35S]TBPS.