Distribution and reciprocal interactions of 3H-melatonin and 125I-thyroxine in peripheral, neural, and endocrine tissues of bullfrog tadpoles.

Tissue distribution of 125I-thyroxine (T4) and 3H-melatonin and the effect of each hormone on the tissue content of the other were studied because previous work indicated that melatonin antagonized metamorphosis through peripheral, as well as thyroidal effects. Late pre- to prometamorphic Rana catesbeiana tadpoles on an 18 light:6 dark cycle were used for injection of hormones in vivo or to supply tissues for in vitro hormone administration. Labeled melatonin uptake was highest in intestine, ventral skin and pituitary; lowest in thyroid and brain and intermediate in hindlimb, tail and gills. The tissue content of labeled T4 was distributed in nearly the same way, except that the thyroid level was relatively higher, and pituitary lower, than that of labeled melatonin. The pineal, studied only in the tracer T4 experiments, had the highest content of labeled T4 of all tissues. Simultaneous injection of either 0.007 or 0.2 microgram T4 increased 3H-melatonin uptake into peripheral tissues that undergo major metamorphic changes but not into neural or endocrine organs. In contrast, 0.033, 3.75 or 15 micrograms melatonin had no significant influence on the content of 125I-T4 in any tissue studied in vivo. Results of in vitro labeling of selected tissues were generally in agreement with the in vivo work except that the 125I-T4 content of intestinal segments from late prometamorphic larvae was lower in melatonin-treated than in control groups. The results suggest that peripheral tissues are a major site for T4-melatonin interactions and that T4 may modulate its own action through influencing melatonin levels in target tissues and perhaps in the thyroid. Because melatonin had no effect on tissue T4 content in young tadpoles, retardation of metamorphic events by melatonin does not seem to involve modulation of T4 availability to the tissues.

Effect of melatonin on the anuran thyroid gland: follicle cell proliferation, morphometry, and subsequent thyroid hormone secretion in vitro after melatonin treatment in vivo.

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.