General discussion—Session X

General Discussion-Session X MODE OF ACTION AND COMPARATIVE TOXICITY OF INSECT HORMONES SCHOOF: More than 15 compounds from 5 commercial sources have been evaluated in the laboratory against larvae of Culex, Anopheles, and Aedes, the effective- ness being measured at the 95 % level of insect immo- bilization or incapacitation. Some of these com- pounds exerted only a lethal action against larvae, with others there was fairly normal larval develop- ment but the pre-pupae succumbed, and the prin- cipal effect of others was exerted on the pupal stage. The concentrations required to produce these effects ranged from 0.05 ppm to greater than 2.5 ppm, levels that are well above those for effective mosquito larvicides. As with insecticides, different mosquito species show differences in response to hormones. In field studies with hormones, consi- deration must be given to the organic content of the water, the behavioural characteristics of the species, the potential effect of the hormone on nontarget organisms, and formulations that will be most effective against anophelines. BARNES: The action of these hormones appears to be fairly specific and perhaps limited to certain cells in the insect. Conventional studies on mammals would have to be used to determine whether or not these compounds have any effects, such as effects on the maturation of rats. If it were to be found that large doses produced no effect in mammals, it would be most satisfactory if this could be explained in terms of, for example, the rapid metabolism and excretion of these compounds in some inactive form. DONNINGER: As I understand it, Dr Gilbert's work on the mode of action of ecdysone in the insect closely follows that on androgens and oestro- gens in mamnmalian systems. It may be premature at this stage to assume that, because ecdysone is an insect hormone, it will be devoid of effects in mammals. If I recall correctly, a report published in Natuiire some time ago suggested that ecdysone could increase protein synthesis in mammalian liver. GILBERT: This was later refuted. In fact, plants con- tain high concentrations of " phytoecdysones ", which apparently do not harm persons who eat the plants. STAAL: Mammalian toxicology studies of ecdysone were reported by J. Siddall at the Santa Barbara conference on plant-hormone interactions in 1968. These studies revealed only slight corticosteroid- like effects on the mineral balance. Although the development of resistance can be studied only in the field, preliminary investigations show a cross-resistance between carbamates and methyl- enedioxyphenyl growth hormone analogues in propoxur-resistant housefly pupae, judged from the growth hormone effect of the analogues. The same strain did not show reduced susceptibility to other growth hormone analogues not containing this synergistically active moiety. HOLAN: With respect to the development of resis- tance to hormones, the Australian plant species Podocarpus elatus contains gram amounts of moulting hormone but it is still attacked by insects. GILBERT: Is it necessary to continue synthesizing juvenile hormone analogues when more basic studies might lead to the development of inhibitors more useful as insecticides? BOWERS: The rationale behind the development of juvenile hormone analogues is three-fold. (1) Early workers suggested the use of the insect juvenile hormone itself as an insecticide. When the chemistry of the hormone became known and it became available for study, it was too unstable and its activity was too low to permit its wide- spread use. It was necessary to prepare more active and stable analogues in order to explore the possible uses of hormonal chemicals. Some of the analogues have enough of these desirable characteristics to warrant further study. (2) By the preparation of analogues, we hoped to obtain compounds so similar to the natural juvenile hormones that they would act as compe- titive or noncompetitive inhibitors at a juvenile hormone receptor site or would cause negative feedback that might interfere with biosynthesis of the natural hormones. Despite the lack of success with this approach, it was and remains valid in the absence of more positive approaches. - 399-2655 27 GENERAL DISCUSSION-SESSION X (3) The synthesis of analogues has resulted in the discovery of a number of compounds with juvenile hormone activity that are chemically un- related to the natural hormones, such as the syn- ergists. This makes it possible to undertake studies with the object of elucidating the mode of action of juvenile hormones, especially since a large number of data have been accumulated on the mode of action of synergists. Perhaps the mode of action of juvenile hormones is similar to that of some of the synergists. DONNINGER: Do you see any changes in the neuro- secretory cells following treatment with ecdysone or juvenile hormone? GILBERT: I have not studied this subject. BOWERS: How would you go about investigating the action of juvenile hormone ? GILBERT: If I knew what experiment to conduct to determine the precise mode of action of juvenile hormone, I would already have conducted it. How- ever, we are investigating the action of juvenile hormone at several levels, from the morphological to the molecular, and are attempting to answer the following specific questions. (1) How does the juve- nile hormone enter the target cell-that is, since we have demonstrated that it is transported as a haemolymph lipoprotein, does the protein moiety enter the cell with the hormone? (2) How does the juvenile hormone antagonize the action of moulting hormone in such systems as chromosome puffing, adenyl cyclase, and RNA polymerase? (3) Is sti- mulation of the prothoracic glands and corpora allata by the juvenile hormone a specific effect or does the hormone have a general metabolic effect on other tissues, such as ovaries? (4) What are the morphological and biochemical effects of the hor- mone on organ cultures and pure insect cell lines? Answers to these questions will enable us to decide whether the juvenile hormone can act by itself as a growth hormone or whether it is only a modulator of moulting hormone action. 400