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The decrease in plasma melatonin at metamorphic climax in Rana catesbeiana (bullfrog) tadpoles is induced by

M L Wright1, C D Alves

  • 1Biology Department, College of Our Lady of the Elms, Chicopee, MA 01013, USA. wrightm@elms.edu

Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology
|June 26, 2001
PubMed
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Thyroid hormone (T4) levels rise during bullfrog tadpole metamorphosis, causing a decrease in melatonin. This study shows T4 induces the melatonin decline, not the other way around.

Area of Science:

  • Endocrinology
  • Developmental Biology
  • Comparative Physiology

Background:

  • Melatonin levels decrease in bullfrog tadpoles during metamorphic climax as thyroxine (T4) levels peak.
  • Melatonin is known to inhibit thyroid function in vitro, suggesting a potential role in regulating metamorphosis.

Purpose of the Study:

  • To investigate the reciprocal relationship between melatonin and thyroxine (T4) during tadpole metamorphosis.
  • To determine if declining melatonin permits increased thyroid hormone secretion or if rising T4 causes melatonin decrease.

Main Methods:

  • Bullfrog tadpoles were treated with exogenous T4 or melatonin prior to metamorphic climax.
  • Treatments included daily injections or immersion in solutions of T4 or melatonin.
  • Plasma T4 and melatonin levels, along with metamorphic progress, were monitored.

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Main Results:

  • T4 administration (injection or immersion) accelerated metamorphosis, increased plasma T4, and induced a premature decrease in plasma melatonin.
  • Melatonin immersion accelerated metamorphosis and decreased plasma melatonin but did not affect plasma T4.
  • Daily melatonin injections had minimal effects on metamorphosis or hormone levels, except for a transient melatonin rise.

Conclusions:

  • Rising thyroxine (T4) levels are the primary driver for the decrease in plasma melatonin observed during metamorphic climax.
  • Exogenous melatonin's effects on metamorphosis depend on the administration method, explaining conflicting previous findings.
  • The study clarifies the hormonal regulation of amphibian metamorphosis, highlighting the role of T4 in modulating melatonin.