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This version published online on May 26, 2005
Endocrinology, doi:10.1210/en.2005-0132
A more recent version of this article appeared on September 1, 2005
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Submitted on February 2, 2005
Accepted on May 18, 2005

Photorefractoriness in mammals: dissociating a seasonal timer from the circadian-based photoperiod response

Gerald A Lincoln*, Jonathan D. Johnston, Hakan Andersson, Gabriela Wagner, and David G Hazlerigg

MRC Human Reproduction Sciences Unit, The Chancellor's Building, 49 Little France Crescent, Edinburgh EH16 4SB, Scotland; School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ, Scotland

* To whom correspondence should be addressed. E-mail: g.lincoln{at}hrsu.mrc.ac.uk.

In seasonal animals, prolonged exposure to constant photoperiod induces photorefractoriness, causing spontaneous reversion in physiology to that of the previous photoperiodic state. This study tested the hypothesis that the onset of photorefractoriness is correlated with a change in circadian expression of clock genes in the suprachiasmatic nucleus (SCN, circadian pacemaker) and the pars tuberalis (PT, a melatonin target tissue). Soay sheep were exposed to summer photoperiod (16-h light) for either 6 or 30 weeks to produce a photostimulated and photorefractory physiology, and seasonal changes were tracked by measuring the long-term prolactin cycles. Animals were killed at 4-h intervals throughout 24-h. Contrary to the hypothesis the 24-h rhythmic expression of clock genes (Rev-erb{alpha}, Per1, Per2, Bmal1, Cry1) in the SCN and PT reflected the ambient photoperiod/melatonin signal, and not the changing physiology. Contrastingly, the PT expression of {alpha}-glycoprotein hormone subunit ({alpha}-GSU) and {beta}-TSH declined in photorefractory animals toward a short-day like endocrinology. We conclude that the generation of long-term endocrine cycles depends on the interaction between a circadian-based, melatonin-dependent timer that drives the initial photoperiodic response, and a non-circadian-based timer that drives circannual rhythmicity in long-lived species. Under constant photoperiod the two timers can dissociate leading to the apparent refractory state.
Key words: clock genes • hypothalamus • melatonin • pars tuberalis • photoperiodism • pineal gland • suprachiasmatic nucleus




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