Effect of Melatonin on the Anuran Thyroid Gland: Follicle Cell Proliferation, Morphometry, and Subsequent Thyroid Hormone Secretionin Vitroafter Melatonin Treatmentin Vivo

Abstract

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 tipsin 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 μg/day for 12 days) of premetamorphicRana pipienstadpoles 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 μg/day for 5 days) injected into prometamorphicRana catesbeianatadpoles at 1930 hr on 18L:6D significantly altered subsequentin vitrothyroid 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 T4secretion during the first 30 hr, but not during the last 24 hr of culture, suppressed 3,5,3‘-triiodothyronine (T3) secretion for 12 hr, and then raised T3output into the media above the control for the remainder of the culture period, increasing the T3:T4ratio. Injection alone increased both T3and T4secretion for the first 30 hr, but did not change the T3:T4ratio. The findings show that exogenous melatonin administeredin vivosignificantly modulated thyroid activity and morphometry directly and/or indirectly and comprise the first demonstration of an ef-fect of melatonin on the amphibian thyroid gland.