Abstract
Avermectin B1a (AVM B1a), a widely used insecticide and acaricide, is reported to both activate and inhibit γ-aminobutyric acidA (GABAA) receptor function in mammalian brain. This study attempts to resolve these seemingly contradictory results by examining the binding properties of AVM B1a and its effects on the GABA-gated chloride channel with primary cultures of rat cerebellar granule neurons as a model system. Specific binding of [3H]AVM B1a in intact neuron cultures is time- and concentration-dependent and is displaceable by AVM analogs. Scatchard analysis of [3H]AVM B1a binding reveals high- and low-affinity sites with K D values of 5 and 815 nM, respectively. AVM B1a alters the binding of [3H]ethynylbicycloorthobenzoate at the noncompetitive blocker site in a biphasic manner; activation is evident with 10 to 300 nM AVM B1a after 5 to 10 min incubation and inhibition with an IC50 of 866 nM after 60 min incubation. Consistent with this observation, 36Cl− influx is stimulated by AVM B1a at 3 to100 nM and inhibited at 1 to 3 μM. GABA-stimulated 36Cl− influx is completely blocked by both [3H]ethynylbicycloorthobenzoate and 12-ketoendrin (two GABA-gated chloride channel blockers) and AVM B1a at 1 to 1.5 μM. Also, 36Cl−influx induced by AVM B1a at 10 nM is suppressed by the two channel blockers. Thus, AVM B1a 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.
Footnotes
-
Send reprint requests to: Dr. John E. Casida, Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management, 114 Wellman Hall, University of California, Berkeley, CA 94720-3112.
-
↵1 The project described was supported by grant PO1 ES 00049 from the National Institute of Environmental Health Sciences, National Institutes of Health.
- Abbreviations:
- AVM B1a
- avermectin B1a
- [3H]AVM B1a
- AVM B1a labeled with tritium in the 5-position
- AVMs
- avermectins
- EBOB
- ethynylbicycloorthobenzoate or, more specifically, 4′-ethynyl-4-n-propylbicycloorthobenzoate
- [3H]EBOB
- EBOB labeled with tritium in the propyl substituent
- GABA
- γ-aminobutyric acid
- GABAA receptor
- type A GABA receptor
- [3H]TBOB
- [3H]t-butylbicycloorthobenzoate
- [35S]TBPS
- [35S]t-butylbicyclophosphorothionate
- HEPES
- N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid
- IC50
- molar concentration for 50% inhibition
- Received August 20, 1996.
- Accepted December 2, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|