Allatostatins: Diversity in structure and function of an insect neuropeptide family

The juvenile hormones (JHs) are a unique group of sesquiterpenoids, identified definitively only in insects, that are responsible for the maintenance of juvenile characteristics. A reduction in JH titer is generally believed to be required for metamorphosis to the adult form. Reproductive functions in most adult female insect species are also regulated by JH, and oocyte growth and maturation, including vitellogenesis, show an absolute dependency on JH-in the absence of the hormone, oocyte growth is arrested.(1) JH titer is regulated in part by the rate of biosynthesis within the endocrine glands known as the corpora allata (CA), which can be considered analogous to the adenohypophysis of vertebrates. Based on the now classic experiments involving severance of nerve tracts originating in the brain and innervating the CA, Scharrer demonstrated that CA function was under close-range neutral control. Scharrer suggested that signals in the hemolymph might also act to regulate the CA.(2) Consistent with Scharrer's original observations, the regulation of JH biosynthesis by CA is now known to be stimulated or inhibited by two groups of peptides, allatotropins and allatostatins (ASTs), respectively. This review will examine the ASTs which occur in multiple forms, are pleiotropic in function, and are widely distributed in both neural and nonneural tissue. The structural and functional features of the ASTs appear to parallel the vertebrate somatostatins and may provide one of the best examples of the parallel evolution of peptides.(2)