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

当昆虫终龄幼虫发育完全时，它们的大脑会分泌促前胸腺激素(PTTH)来刺激前胸腺，而前胸腺又会分泌蜕皮激素，从而启动幼虫-蛹的转变。本研究分析了家蚕五龄中期PTTH升高的机制。由于在PTTH峰发生的前一天同时检测到PTTH和蜕皮类固醇的血淋巴效价的小峰，我们推测这种蜕皮类固醇滴度的上升影响了大脑增强其PTTH的分泌活性。当幼虫在PTTH峰出现前两天注射小剂量蜕皮激素时，表明蜕皮激素是导致脑中PTTH分泌活性增加的原因。这是首次证明了前胸腺对大脑的正反馈调节。我们还利用新开发的前胸腺长期体外培养系统研究了前胸腺在五龄早期活动的调节机制。本研究表明，4龄幼虫的前胸腺在4龄中期通过负反馈机制释放的蜕皮类固醇失活，蜕皮至5龄后，腺体在不受PTTH刺激的情况下有一定程度的自发复活。我们推测，5龄幼虫的前胸腺随着幼虫的生长逐渐增加其蜕皮激素的分泌活性和对PTTH的敏感性，从而使成年幼虫的腺体能够分泌足够数量的蜕皮激素，作为对PTTH少量分泌的反应，以刺激大脑。

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.