Wright M L, Pikula A, Cykowski L J, Kuliga K
Biology Department, College of Our Lady of the Elms, Chicopee, Massachusetts 01013, USA.
Gen Comp Endocrinol. 1996 Aug;103(2):182-91. doi: 10.1006/gcen.1996.0109.
The thyroid gland controls the progress of metamorphosis, although other hormones influence metamorphic rate, including melatonin, which may coordinate metamorphosis with seasonal and light conditions. Melatonin directly antagonized the action of thyroxine (T4) in promoting regression of tadpole tail tips in vitro, and this study sought to determine if it affects the thyroid axis of tadpoles as well. In an experiment sampling at 8-hr intervals for 24 hr, after melatonin treatment (15 micrograms/day for 12 days) of premetamorphic Rana pipiens tadpoles at approximately 1100 hr on 18L:6D, thyroid follicle cell height and lumen diameter were lowered by melatonin, but follicle cell proliferation was not significantly depressed. In a second experiment conducted under the same conditions, but sampling at 3-hr intervals for 24 hr, melatonin significantly lowered follicle cell labeling index and suppressed its ultradian (7.6 hr) rhythm, while shifting the peak of follicle lumen diameter to the dark instead of the light. Thus, melatonin tended to depress the thyroid of young tadpoles and suppress or shift its rhythms. Melatonin (10 micrograms/day for 5 days) injected into prometamorphic Rana catesbeiana tadpoles at 1930 hr on 18L:6D significantly altered subsequent in vitro thyroid function as determined by radioimmunoassay of media collected at intervals for 54 hr from cultured thyroids of injected control and melatonin groups, and a noninjected control group. Melatonin decreased T4 secretion during the first 30 hr, but not during the last 24 hr of culture, suppressed 3,5,3'-triidothyronine (T3) secretion for 12 hr, and then raised T3 output into the media above the control for the remainder of the culture period, increasing the T3:T4 ratio. Injection alone increased both T3 and T4 secretion for the first 30 hr, but did not change the T3:T4 ratio. The findings show that exogenous melatonin administered in vivo significantly modulated thyroid activity and morphometry directly and/or indirectly and comprise the first demonstration of an effect of melatonin on the amphibian thyroid gland.
甲状腺控制着变态发育的进程，尽管其他激素也会影响变态发育的速率，包括褪黑素，它可能会使变态发育与季节和光照条件相协调。褪黑素在体外直接拮抗甲状腺素（T4）促进蝌蚪尾尖退化的作用，本研究旨在确定它是否也会影响蝌蚪的甲状腺轴。在一项实验中，对处于前变态期的北美豹蛙蝌蚪在18小时光照：6小时黑暗（18L:6D）条件下于大约上午11点进行褪黑素处理（15微克/天，持续12天），之后每隔8小时取样一次，共取样24小时，结果发现褪黑素使甲状腺滤泡细胞高度和管腔直径降低，但滤泡细胞增殖并未显著受抑制。在相同条件下进行的第二项实验中，每隔3小时取样一次，共取样24小时，褪黑素显著降低了滤泡细胞标记指数并抑制了其超日节律（7.6小时），同时将滤泡管腔直径的峰值从光照期转移至黑暗期。因此，褪黑素倾向于抑制幼体蝌蚪的甲状腺并抑制或改变其节律。在18L:6D条件下于晚上7点30分对处于前变态期的牛蛙蝌蚪注射褪黑素（10微克/天，持续5天），通过对注射对照组、褪黑素组以及未注射对照组的培养甲状腺每隔一定时间收集的培养基进行放射免疫分析来测定，结果显示褪黑素显著改变了后续的体外甲状腺功能。在培养的前30小时，褪黑素降低了T4分泌，但在培养的最后24小时未出现这种情况；它抑制了三碘甲状腺原氨酸（T3）分泌12小时，然后在培养期的剩余时间将T3分泌量提高到对照组之上，从而增加了T3:T4比值。单独注射在培养的前30小时增加了T3和T4分泌，但未改变T3:T4比值。这些发现表明，体内给予外源性褪黑素会直接和/或间接显著调节甲状腺活性和形态测量，这是褪黑素对两栖类甲状腺产生影响的首次证明。
