The gamma-aminobutyric acid (GABA) receptor bears sites of action for insecticides. To discover GABA receptor-directed insecticides in natural products, fungal culture extracts were screened for their ability to inhibit specific binding of the radiolabeled noncompetitive antagonist [3H]1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane to housefly head membranes. The screening efforts led to the isolation of two alkaloids from Aspergillus terreus: PF1198A (alantrypinone) and PF1198B (serantrypinone), which had IC50 values of 0.34 and 2.1 microM, respectively, in this assay. These compounds were ca. 47-61-fold selective for housefly vs rat GABA receptors. Both compounds showed insecticidal activity against Myzus persicae in the range of 100-500 ppm. Binding assay-guided screening should provide significant opportunities for the identification of novel and selective insecticides.

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The receptor for the inhibitory neurotransmitter GABA is known as a target for such insecticides and acaricides as fipronil and avermectins.1) These insecticides selectively act on the insect GABA receptor, and thus are toxic to insects but not mammals. We previously reported that naturally occurring terpenoids such as picrodendrins and anisatin exhibit insecticidal activity by binding to the antagonist site of the GABA receptor. [2][3][4][5] Several terpene lactones such as picrodendrin O exhibited selective affinity for the insect versus mammalian GABA receptors. We have also isolated insecticidal alkaloid GABA antagonists from the culture of Aspergillus terreus by binding assay-guided screening.6) This earlier screening convinced us that our fungal cultures are a rich source of GABAergic or anti-GABAergic compounds.We report here on the subsequent isolation and structural elucidation of a novel ligand for the insect GABA receptor. MATERIALS AND METHODS InstrumentsThe NMR spectra were recorded on a JEOL GSX400 instrument. Mass spectra were obtained on a JEOL JMS-700 spectrometer. The UV and IR spectra were measured on Shimadzu UV-260 and FTIR-8100 spectrophotometers, respectively. Optical rotations were obtained on a JASCO DIP-370 polarimeter. Observations by electron microscopy were performed with a JEOL JSM-6300F electron microscope. Taxonomic StudiesTaxonomic studies of strain PF1223 were performed according to the method of Horie et al.7) The fungal strain PF1223 was deposited at the National Institute of Bioscience and Human-Technology Agency of Industrial Science and Technology, Japan, under the accession number FERM P-17658. FermentationStrain PF1223 was inoculated from an agar slant into a 100-ml Erlenmeyer flask containing 20 ml of a seed medium made up of 1.0% starch, 1.0% glucose, 0.6% wheat germ, 0.2% soybean meal, 0.3% yeast extract, 0.5% polypeptone, 0.2% CaCO 3 , and tap water (pH 7.0 before sterilization). The inoculated flask was shaken on a rotary shaker (200 rpm) at 25°C Matsue, Shimane 690-8504, Japan † Pharmaceutical Research Center, Meiji Seika Kaisha, Ltd., Yokohama, Kanagawa 222-8567, Japan (Received April 27, 2004; Accepted June 2, 2004) For the purpose of discovering GABA receptor-directed insecticides in natural products, fungal culture extracts were screened for their ability to inhibit the specific binding of the noncompetitive antagonist [ 3 H]EBOB to housefly head membranes. The screening efforts led to the isolation of a derivative of dihydroisocoumarin (PF1223) from the culture of Neosartorya quadricincta. This compound at 2.2 mM inhibited [3 H]EBOB binding by 65%. This ligand might prove to be a lead compound for the identification of novel insecticides acting at the insect GABA receptor.