Insects, hormones action

Mammals are hardly unique in possessing hormonal signaling systems. Insects and nematode worms have highly developed systems for hormonal regulation, with fundamental mechanisms similar to those in mammals. Plants, too, use hormonal signals to coordinate the activities of their various tissues (Chapter 12). The study of hormone action is not as advanced in plants as in animals, but we do know that some mechanisms are shared. To illustrate the structural diversity and range of action of mammalian hormones, we consider representative examples of each major class listed in Table 23-1. 

As we shall see, cyclization reactions of this general type seem to be important in terpene biosynthesis. The 6,7-trans-farnesol has been shown to have hormone action in some insects. It acts to regulate the changes from caterpillar to cocoon to moth. 

Figure 2 Principal reaction paths of metabolism of ecdysone. The larger the print, the more common the reaction in known examples. Modified from R. Lafont C. Dauphin-Villemant J. T. Warren H. H. Rees, Insect Hormones. In Comprehensive Molecular Insect Science-, Elsevier Oxford, 2005 Vol. 3, pp 125-195, Fig. 15, Copyright Elsevier, 2004, in turn Modified from R. Lafont J.-L. Connat, Pathways to Ecdysone Metabolism. In Ecdysone From Chemistry to Mode of Action-, J. Koolman, Ed. Georg Thieme Stuttgart, 1989 Chapter 14, Fig. 14.1.

Enhancement of diuretic hormone actions could be fatal for those insects that have critical problems of maintaining body water, esp. soft-bodied larvae. Soft-bodied larvae rely on hemolymph volume to maintain turgor for movement, and dehydration would result in immobility followed by death, inhibition of antidiuretic hormones could have serious effects on insects that reside in extremely dry environments, e.g. stored grain. Antidiuretic hormone antagonists in combination with diuretic hormone agonists would be especially potent. 

As early as 1934 Wigglesworth showed that the evolutionary differentiation of insects is regulated by a centre in the brain. In 1956 Williams obtained from Cecropia moth a highly active extract which produced anomalies in the metamorphosis of experimental insects. Following this first description of the action of juvenile hormone, Gilbert and Schneiderman (1958), then Williams et al. (1959) described the juvenile hormone action of lipid extracts obtained from different mammalian organs. The pure active substance was isolated first by Schmialek (1961) from feces of the yellow mealworm, Tenebrio Molitor, and identified as famesol (1), known as an intermediate product in steroid biosynthesis. Six years later, Roeller and co-workers (1967) elucidated the structure of the juvenile hormone isolated from the male of Cecropia. The hormone was ascribed the structure methyl ( , )-3,11-dimethyl-10,ll-epoxy-7-ethyl-2,6-tridecadienoate (2), and was called C,g-JH. Independent of these workers, Meyer et al. (1968) isolated also from Cecropia another juvenile hormone, ( , )-10,11-epoxy farnesenate (3), called C,g-JH. 

Molting hormones secreted by endocrine glands (corpora allata) of insects for the purpose of triggering larval molting for details on the specific action, see under insect hormones. JH I [C]g-JH, methyl (2 ,6 , 10 , 11S)-10,11 -epoxy-7-ethyl-3,11 -dime-thyl-2,6-tridecatrienoate, CigHjoOa, Mr 294.43, [a] +14.9° (CHCI3)] was isolated from the silkworm the J. 

Physiological and Molecular Basis of Insect Molting Hormone Action... 

The biological evidence for a hormonal factor produced by the brain was furnished by Kopec. It was the first publication on hormonal action in the insect kingdom. Further observations followed, but nearly all investigations failed to prove the activity of extracts. Kobayashi and Kirimura were the first to succeed. Starting with 8,400 brains, they obtained 4 mg of an extract which had the activity of the brain hormone. The active substance is soluble in ether and petroleum-ether. Recently, Kobayashi has claimed that the active principle is identical with cholesterol. It is more likely that the activity is due to some contaminant of the cholesterol isolated. 

Eedysone was the first insect hormone to be obtained in pure crystalline form. It has the action of the molting and metamorphosis hormone and has also been called, by Scharrer, the growth and differentiation hormone . 

Fig. 56. Action of insect hormones. In the upper part, the hormone-producing glands are represented. Below that, the molting stages initiated by the hormones are shown.

P. Karlson, Chemistry and mechanism of action of insect hormones, 4tb Intern. Congr. Biochem. XII, 37-47 (1959). 

One of the insect neurohormones, the activation hormone, controls the secretion of the corpora allata, paired glands that synthesize the juvenile hormone (Fig. 22-4) in insect larvae. While the structure of the juvenile hormone varies somewhat with species, it is usually a polyprenyl ester. A specific binding protein provides the hormone with protection from degrada-tive enzymes. However, in the tobacco homworm an esterase, able to hydrolyze the protein-bound juvenile hormone, is produced at the start of pupal differentiation.354 The exact mechanism of action of juvenile hormones has been difficult to determine. However, it affects polyamine synthesis.355 356... 

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