DIFFERENTIAL-EFFECTS OF NONSTEROIDAL ECDYSTEROID AGONISTS IN COLEOPTERA AND LEPIDOPTERA - ANALYSIS OF EVAGINATION AND RECEPTOR-BINDING IN IMAGINAL DISCS

The effects of 20-hydroxyecdysone (20E) and the first nonsteroidal ecd ysteroid agonists, RH-5849 and tebufenozide (=RH-5992), were tested on in vitro cultured mesothoracical imaginal wing discs of last-instar l arvae of the Colorado potato beetle, Leptinotarsa decemlineata Say, an d the greater mar moth, Galleria mellonella L. In general, the respons e of the nonsteroidal agonists was qualitatively similar to that induc ed by 20E, although the concentrations required to induce evagination varied widely. The EC(50) of 20E, RH-5849 and tebufenozide, that indic ates the compound concentration which elicits evagination in 50% of th e isolated imaginal discs, was 60.7, 461 and 757 nM, respectively, in L. decemlineata. In vying discs of G. mellonella the order of biologic al activity (expressed as EC(50)) resulted in tebufenozide (8.9 nM) > 20E (321 nM) > RH-5849 (865 nM). In a second series of experiments, a competition receptor binding assay was developed with whole imaginal d iscs, and showed that the I-50 value of 20E, RH-5849 and tebufenozide to compete 50% of the specifically bound amount of H-3-ponasterone A i n imaginal discs of L. decemlineata was 425, 740 and 1316 nM, respecti vely, In wing discs of G. mellonella that respective value was 106, 91 1 and 22 nM. The current results showed that the binding affinity (I-5 0) of 20E reached a similar level as the concentration to elicit a bio logical response (EC(50)) in imaginal wing discs of the two insect spe cies; a similar conclusion could be drawn for the two nonsteroidal ecd ysteroid agonists. In this regard, it is suggested that the imaginal d isc assay allows us to measure the potency of an ecdysteroid active co mpound on the one hand and its affinity to bind on the ecdysteroid rec eptors (EcRs) in competition with H-3-ponasterone A on the other. In a ddition, the current data suggest that the specific activity and insec t selectivity of the nonsteroidal agonists, especially the high toxici ty of tebufenozide for Lepidoptera and the nonsusceptibility of Coleop tera, is related to selective binding on the EcRs. Copyright (C) 1996 Elsevier Science Ltd.