Avermectin B-1a (AVM B-1a), a widely used insecticide and acaricide, is reported to both activate and inhibit gamma-aminobutyric acid, (GABA(A)) receptor function in mammalian brain. This study attempts to resolve these seemingly contradictory results by examining the binding properties of AVM B-1a and its effects on the GABA-gated chloride channel with primary cultures of rat cerebellar granule neurons as a model system. Specific binding of [H-3]AVM B-1a in intact neuron cultures is time- and concentration-dependent and is displaceable by AVM analogs. Scatchard analysis of [H-3]AVM B-1a binding reveals high- and low-affinity sites with K-D values of 5 and 815 nM, respectively. AVM B-1a alters the binding of [H-3]ethynylbicycloorthobenzoate at the noncompetitive blocker site in a biphasic manner; activation is evident with 10 to 300 nM AVM B-1a after 5 to 10 min incubation and inhibition with an IC50 of 866 nM after 60 min incubation. Consistent with this observation, Cl-36(-) influx is stimulated by AVM B-1a at 3 to 100 nM and inhibited at 1 to 3 mu M. GABA-stimulated Cl-36(-) influx is completely blocked by both [H-3]ethynylbicycloorthobenzoate and 12-ketoendrin (two GABA-gated chloride channel blockers) and AVM B-1a at 1 to 1.5 mu M. Also, Cl-36(-) influx induced by AVM B-1a at 10 nM is suppressed by the two channel blockers. Thus, AVM B-1a binds to two different sites in the GABA-gated chloride channel with dual effects, i.e., activating the channel on binding to the high-affinity site and blocking it on further binding to the low-affinity site.
