The endocrine mechanisms that determine the timing of initiation of larval-pupal transformation

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When final instar larvae of insects have fully grown, their brain secretes the prothoracicotropic hormone (PTTH) to stimulate the prothoracic glands, which in turn secrete ecdysteroid leading to initiation of larval-pupal transformation. In the present study, the mechanism that leads to this surge of PTTH at the mid fifth instar of the silkmoth, Bombyx mori was analyzed. Since small peaks of hemolymph titers of PTTH and ecdysteroids were detected at the same time on the day before the occurrence of PTTH surge, we hypothesized that this rise in ecdysteroid titer affects the brain enhancing its PTTH secretory activity. When a small dose of ecdysteroid was injected to larvae two days before the occurrence of PTTH surge, indicating that ecdysteroid is responsible for the increase in PTTH secretory activity of the brain. This is the first to demonstrate the positive feedback regulation of the brain by the prothoracic glands. We also studied the mechanisms by which the prothoracic gland activity is regulated during early fifth instar using a newly developed long-term in vitro culture system of the gland. This study revealed that the prothoracic gland of fourth instar larvae is inactivated by ecdysteroids released by the gland at mid fourth instar through negative feedback mechanism and that, after ecdysis to teh fifth instar, the gland is spontaneously reactivated to some extent without stimulation by PTTH. We speculate that the prothoracic glands of fifth instar larvae gradually increase their ecdysone secretory activity and PTTH sensitivity in accord with larval growth so that the glands of fully grown larvae can secrete enough amounts of ecdysone, in response to a small secretion of PTTH, to stimulate the brain.