Takaki Keiko, Sakurai Sho
Division of Life Sciences, Graduate School of Science and Technology, Kanazawa University, Kakumamachi, Kanazawa 920-1192, Japan.
Insect Biochem Mol Biol. 2003 Dec;33(12):1189-99. doi: 10.1016/j.ibmb.2003.06.006.
The prothoracic glands of early last (fifth) instar larvae of the silkworm are inactive with regard to ecdysteroidogenesis and unresponsive to prothoracicotropic hormone (PTTH) [J. Insect Physiol. 31 (1985) 455]. In an attempt to elucidate the hormonal mechanisms that cause the inactivity, we compared the effects of PTTH, dibutyryl cyclic AMP (dbcAMP), a cAMP phosphodiesterase inhibitor (IBMX), juvenile hormone analogue (JHA) and 20-hydroxyecdysone (20E) on secretory activity of the third, fourth and fifth instar glands. Among the factors examined, feedback inhibition by 20E was indicated to be the most likely factor. Inhibition was moderate in the third and early fourth instars while 20E strongly inhibited the glands of middle fourth instar larvae. The inhibitory effect of 20E was reduced by removal of the brain and corpora allata. Once the glands were suppressed by 20E to the degree of exhibiting neither secretory activity nor responsiveness to PTTH, dbcAMP or IBMX did not elicit ecdysone secretion at all. Thus the feedback inhibition may shut down ecdysteroidogenesis although it is obscure whether it affects the intracellular transductory cascade from the PTTH receptor through cAMP. Taken together, this evidence suggests that inactivity of the gland in the early fifth instar is brought about by feedback inhibition of the glands by 20E occurring in the late fourth instar, and that this inactivity is maintained by the juvenile hormone found in the early fifth instar.
家蚕末龄（第五龄）早期幼虫的前胸腺对蜕皮甾体生成无活性，且对促前胸腺激素（PTTH）无反应[《昆虫生理学杂志》31（1985）455]。为了阐明导致这种无活性的激素机制，我们比较了PTTH、二丁酰环磷酸腺苷（dbcAMP）、一种环磷酸腺苷磷酸二酯酶抑制剂（异丁基甲基黄嘌呤，IBMX）、保幼激素类似物（JHA）和20-羟基蜕皮酮（20E）对第三、第四和第五龄前胸腺分泌活性的影响。在所研究的因素中，20E的反馈抑制被认为是最可能的因素。在第三龄和第四龄早期，抑制作用适中，而20E强烈抑制第四龄中期幼虫的前胸腺。去除脑和咽侧体可降低20E的抑制作用。一旦前胸腺被20E抑制到既不表现出分泌活性也对PTTH、dbcAMP或IBMX无反应的程度，dbcAMP或IBMX根本不会引发蜕皮激素分泌。因此，尽管尚不清楚它是否影响从PTTH受体通过环磷酸腺苷的细胞内转导级联反应，但反馈抑制可能会关闭蜕皮甾体生成。综上所述，这些证据表明，第五龄早期前胸腺的无活性是由第四龄后期出现的20E对前胸腺的反馈抑制引起的，并且这种无活性由第五龄早期发现的保幼激素维持。
