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Multiple Effects of Melatonin on Rhythmic Clock Gene Expression in the Mammalian Pars Tuberalis

School of Biological Sciences (J.D.J, D.G.H.), University of Aberdeen, Zoology Building, Aberdeen AB24 2TZ, Scotland, United Kingdom; Neurobiologie des Rhythmes laboratory, Unité Mixte de Recherche 7518 Centre National de la Recherche Scientifique/Université Louis Pasteur (B.B.T., M.M.-P.),67084 Strasbourg, France; and Medical Research Council Human Reproductive Sciences Unit (H.A., G.A.L.), Centre for Reproductive Biology, The Queen’s Medical Research Institute, Edinburgh EH16 4TJ, Scotland, United Kingdom

Address all correspondence and requests for reprints to: David Hazlerigg, School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ, Scotland, United Kingdom. E-mail: d.hazlerigg{at}abdn.ac.uk.

In mammals, changing day length modulates endocrine rhythms via nocturnal melatonin secretion. Studies of the pituitary pars tuberalis (PT) suggest that melatonin-regulated clock gene expression is critical to this process. Here, we considered whether clock gene rhythms continue in the PT in the absence of melatonin and whether the effects of melatonin on the expression of these genes are temporally gated. Soay sheep acclimated to long photoperiod (LP) were transferred to constant light for 24 h, suppressing endogenous melatonin secretion. Animals were infused with melatonin at 4-h intervals across the final 24 h, and killed 3 h after infusion. The expression of five clock genes (Per1, Per2, Cry1, Rev-erb, and Bmal1) was measured by in situ hybridization. In sham-treated animals, PT expression of Per1, Per2, and Rev-erb showed pronounced temporal variation despite the absence of melatonin, with peak times occurring earlier than predicted under LP. The time of peak Bmal1 expression remained LP-like, whereas Cry1 expression was continually low. Melatonin infusion induced Cry1 expression at all times and suppressed other genes, but only when they showed high expression in sham-treated animals. Hence, 3 h after melatonin treatment, clock gene profiles were driven to a similar state, irrespective of infusion time. In contrast to the PT, melatonin infusions had no clear effect on clock gene expression in the suprachiasmatic nuclei. Our results provide the first example of acute sensitivity of multiple clock genes to one endocrine stimulus and suggest that rising melatonin levels may reset circadian rhythms in the PT, independently of previous phase.

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