Denver R J
Department of Integrative Biology, University of California at Berkeley 94720.
Gen Comp Endocrinol. 1993 Jul;91(1):38-51. doi: 10.1006/gcen.1993.1102.
Despite substantial information on the role of the pituitary-thyroid and pituitary-interrenal axes in controlling amphibian metamorphosis, the hypothalamic hormones responsible for controlling the activity of these axes have not been identified. The mammalian thyrotropin-releasing hormone (TRH) does not regulate the thyroid axis of tadpoles; however, corticotropin releasing hormone (CRH) stimulates the release of thyrotropin from bullfrog tadpole pituitary glands in vitro and may thus function as a central regulator of the thyroid axis during metamorphosis. I tested the possibility that a CRH-like peptide is involved in controlling amphibian development by treating tadpoles of two anuran species, the western spadefoot toad Scaphiopus hammondii, and the North American bullfrog, Rana catesbeiana, with neuropeptides and monitoring their effects on metamorphosis. Injection of spadefoot toad tadpoles with ovine (o) CRH (2 micrograms/animal every other day for 3 weeks) or the amphibian CRH-like peptide sauvagine (SV) significantly decreased their time from hatching to metamorphic climax (Gosner stage 42; frontlimb emergence) and their body weight and body length at climax compared with vehicle-injected controls; whereas, TRH had no effect and arginine vasotocin produced a small but significant lengthening of the larval period but did not alter body size at climax. In an acute response experiment, S. hammondii tadpoles (in Gosner stages 36-38--late prometamorphosis) treated with oCRH or SV (2 micrograms/animal) exhibited significantly elevated whole-body thyroxine (T4) content at 2 and 6 hr after injection; whereas, treatment with TRH (2 micrograms/animal) did not significantly alter whole-body T4. R. catesbeiana tadpoles treated with oCRH or SV (surgical implantation of ELVAX pellets impregnated with 100 micrograms peptide and injections of peptides at 5 micrograms/animal once every 3 days) exhibited accelerated spontaneous and triiodothyronine (T3)-induced metamorphosis as assessed by changes in tail height, hind limb development, and body weight; TRH had no effect. Injections of a pool of antisera generated against CRH-like peptides (rat/human CRH, oCRH, SV) slowed T3-induced metamorphosis when compared with normal serum-injected controls. These results support the hypothesis that a CRH-like peptide(s) is involved in the central control of metamorphosis of anuran amphibians, and may act, at least in part, through stimulation of the thyroid axis.
尽管关于垂体 - 甲状腺轴和垂体 - 肾上腺轴在控制两栖动物变态发育中的作用已有大量信息，但负责控制这些轴活动的下丘脑激素尚未被确定。哺乳动物的促甲状腺激素释放激素（TRH）并不调节蝌蚪的甲状腺轴；然而，促肾上腺皮质激素释放激素（CRH）在体外可刺激牛蛙蝌蚪垂体释放促甲状腺激素，因此可能在变态发育过程中作为甲状腺轴的中枢调节因子发挥作用。我通过用神经肽处理两种无尾目物种的蝌蚪，即西部锄足蟾（Scaphiopus hammondii）和北美牛蛙（Rana catesbeiana），并监测它们对变态发育的影响，来测试一种CRH样肽参与控制两栖动物发育的可能性。给锄足蟾蝌蚪注射羊（o）CRH（每只动物每隔一天注射2微克，共3周）或两栖类CRH样肽蛙皮素（SV），与注射溶剂的对照组相比，显著缩短了从孵化到变态高潮（戈斯纳42期；前肢出现）的时间，以及高潮期的体重和体长；而TRH没有效果，精氨酸血管催产素使幼虫期有小幅但显著的延长，但未改变高潮期的体型。在急性反应实验中，用oCRH或SV（每只动物2微克）处理的西部锄足蟾蝌蚪（戈斯纳36 - 38期——前变态晚期）在注射后2小时和6小时全身甲状腺素（T4）含量显著升高；而用TRH（每只动物2微克）处理并未显著改变全身T4含量。用oCRH或SV处理的北美牛蛙蝌蚪（通过植入含100微克肽的ELVAX小丸以及每隔3天每只动物注射5微克肽进行手术植入），通过尾高、后肢发育和体重变化评估，显示出自发性和三碘甲状腺原氨酸（T3）诱导的变态发育加速；TRH没有效果。与注射正常血清的对照组相比，注射针对CRH样肽（大鼠/人CRH、oCRH、SV）产生的一组抗血清减缓了T3诱导的变态发育。这些结果支持了这样一种假说，即一种CRH样肽参与无尾目两栖动物变态发育的中枢控制，并且可能至少部分地通过刺激甲状腺轴发挥作用。
