title: Positive feedback regulation of prothoracicotropic hormone secretion by ecdysteroid--a mechanism that determines the timing of metamorphosis.
abstract: When insect larvae have fully grown, prothoracicotropic hormone (PTTH) is released from the brain, triggering the initiation of metamorphic development through stimulation of ecdysteroid secretion by the prothoracic glands. The present study analyzes the mechanism that regulates the occurrence of this PTTH surge. In the silkworm Bombyx mori, the PTTH surge occurs on day 6 of the fifth instar and is preceded by a small rise in hemolymph ecdysteroid titer, which occurs late on day 5. We therefore hypothesized that this rise of ecdysteroid titer is involved in the induction of the PTTH surge. To test this hypothesis, two experiments were conducted. First, a small amount of 20-hydroxyecdysone was injected on day 4, two days before the expected day of the PTTH surge, to simulate the small rise in hemolymph ecdysteroid titer on day 5. This injection led to a precocious surge of PTTH the next day. Next, the hemolymph ecdysteroid titer on day 5 was artificially lowered by injecting ecdysteroid-22-oxidase, which inactivates 20-hydroxyecdysone. After this treatment, the PTTH surge did not occur on day 6 in 80% of the animals. These results indicate that a small rise of the hemolymph ecdysteroid titer plays a critical role in the induction of the PTTH surge. Since basal ecdysteroidogenic activity of the prothoracic glands increases with larval growth, a circulating level of ecdysteroids may convey information about larval maturity to the brain, to coordinate larval growth and metamorphosis. This is the first report in invertebrates to demonstrate positive feedback regulation of the surge of a tropic hormone by a downstream steroid hormone.
authors.affiliation: Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan. Electronic address: mizoguch@bio.nagoya-u.ac.jp.
authors.forename: Akira
authors.initials: A
authors.lastname: Mizoguchi
authors.affiliation: National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8634, Japan.
authors.forename: Manabu
authors.initials: M
authors.lastname: Kamimura
authors.affiliation: National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8634, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Kiuchi
authors.affiliation: Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8562, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kataoka
mesh_terms: D000818:Animals; D012831:Bombyx / Q000254:growth & development* / Q000378:metabolism*; D026461:Ecdysteroids; D004441:Ecdysterone / Q000037:antagonists & inhibitors / Q000378:metabolism; D006458:Hemolymph / Q000737:chemistry; D007301:Insect Hormones / Q000378:metabolism*; D007814:Larva / Q000254:growth & development / Q000378:metabolism; D008675:Metamorphosis, Biological
chemical_list: D026461:Ecdysteroids; D007301:Insect Hormones; D004441:Ecdysterone; C009089:prothoracicotropic hormone
keywords: Bombyx mori; Ecdysteroid; Metamorphosis; Positive feedback; Prothoracicotropic hormone