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Journal of Biological Rhythms
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Thyroidectomy Does Not Affect the Daily or Free-Running Rhythms of Plasma Melatonin in European Starlings

A. Dawson

NERC Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS, United Kingdom

V. King

AFRC Research Group on Photoperiodism and Reproduction, Department of Zoology, University of Bristol, Bristol BS8 1UG, United Kingdom

Thyroidectomy results in the suppression of reproductive photoperiodic responses in starlings. Could this be a consequence of an effect on perception of daylength or on circadian pacemakers? Daily changes in plasma melatonin concentrations were monitored in intact and thyroidectomized starlings held in long days (LD 16:8) and short days (LD 8:16), and in intact and thyroidectomized starlings allowed to free-run in constant darkness from long days or short days. In long days and short days, melatonin was low during the light period and high during darkness. There was no difference between intact and thyroidectomized birds. In free-running birds, the melatonin profile of the preceding long day or short day was retained during the first day of constant darkness, with peak levels occurring at the same time they did during the light-dark cycles. Again there was no difference between intact and thyroidectomized birds. These data demonstrate that either the photoreceptive and circadian mechanisms driving melatonin secretion are independent of those concerned with reproductive photoperiodic responses, or that thyroidec tomy affects reproduction "downstream" from the photoreceptive-circadian apparatus.

Key Words: starling • thyroid • melatonin • photoperiodism • circadian rhythm

Journal of Biological Rhythms, Vol. 9, No. 2, 137-144 (1994)
DOI: 10.1177/074873049400900204
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