WRIGHT, M.L.: Circadian aspects of hormonal regulation of metamorphosis in Rana catesbeiana.
Most hormones exhibit a circadian, or 24-hr, rhythm synchronized by the light/dark (LD) cycle. Hormones such as melatonin (ML), adrenal cortical steroids, and prolactin may synergize with, or antagonize, the thyroid hormones (TH) which induce metamorphosis. Consequently, the relationship of their diurnal profiles in the plasma with those of the TH might be important in controlling the rate of metamorphosis. ML, which may distribute a �night� signal and regulate hormone rhythms in vertebrates, declines precipitously in the plasma at climax coincident with the rise in TH. The fall in plasma ML, which is induced prematurely by exogenous thyroxine (T4), occurs in contrast to the rise of adrenal corticoids and prolactin at climax. Unlike T4 rhythms, which are stable throughout larval life, plasma corticoid and ML rhythms change from prometamorphosis to climax. Thus, the relationship of ML and adrenal steroid rhythms with the T4 rhythm changes during development. The acrophase of the T4 rhythms, and the diurnal profiles and climactic changes of the corticoids and ML, are specific to each LD cycle. These, and other, results of ongoing work suggest that the influence of modulators on TH promotion of metamorphosis may have chronobiological aspects, which allow fine-tuning of the response to environmental conditions, and which might explain the variations in the rate of metamorphosis that occur on different LD cycles, or when larvae are given exogenous hormones at different times of the day. (Supported by NSF IBN9723858.)
