Melatonin, diel rhythms, and metamorphosis in anuran amphibians

Melatonin is present in picogram quantities in the plasma of anuran tadpoles, although the origin of circulating melatonin is not known. Melatonin may have a role in metamorphosis because it is a thyroid antagonist, whose level falls at the metamorphic climax when the thyroid hormones peak. Melatonin rhythms in plasma and eyes are entrained to the light/dark (LD) cycle and affected by temperature. Consequently, melatonin could transduce environmental information to regulate endocrine periodicity and larval circadian organization and influence metamorphic rate. Contradictory results of exogenous melatonin treatment may be largely due to a regulation of the plasma melatonin level which rapidly clears high melatonin concentrations and which can even result in lower circulating melatonin than in controls. Excess exogenous melatonin concentrates in tissues and glands, although the fall in melatonin at metamorphic climax does not occur by this mechanism. There may be thyroxine (T(4))-melatonin interactions at the tissue level that affect metamorphic progress. The rhythms of T(4) and the corticosteroids are also entrained to the LD cycle, and these rhythms, and those of melatonin, change during development, in a specific way on each LD cycle. Differences in the direction and magnitude of phase shifts during development place the peaks of thyroid modulators, such as the corticosteroids and melatonin, in different relationships to the T(4) peaks, which could be an important aspect of the hormonal regulation of metamorphosis.