A new study published in the journal Proceedings of the National Academy of Sciences reveals the link between breeding season and the hormone melatonin, made in the pineal gland in the brain during long winter nights.
Castle-Miller et al have discovered how animals link the change in seasons to their fertility.
“Changes during the year in sex hormones made in the pituitary gland control when mammals start reproducing, and other changes like growing new coats or developing antlers,” said senior co-author Prof. David Bates, from the University of Nottingham, UK.
“The length of the day is recognized in most vertebrate animals by the pineal gland in the brain, which produces melatonin.”
“However, until now, it has not been known how melatonin, which is produced at night, signals to the area of the pituitary gland that controls sex hormones.”
Using the sheep as the experimental model, Prof. Bates and his colleagues found that melatonin controls production of two different types of a protein called vascular endothelial growth factor (VEGF).
This happens within a specific region of the pituitary gland, away from the area where sex hormones are made.
In winter, sheep make forms of VEGF that stop blood vessels growing.
In summer, they make a different VEGF that makes vessels grow between the two areas of the pituitary.
This allows VEGF to act as a messenger to control sex hormone-producing cells and fertility — the first time that the molecule linking long winter nights to sex hormones has been found.
“We found that this melatonin-dependent production of different forms of VEGF has two complementary effects,” said senior co-author Dr. Domingo Tortonese, from the University of Bristol, UK.
“Firstly, it causes remodeling of the blood vessels that connect the brain with the pituitary gland, and secondly, it is used as a messenger signal to act on the cells of the pituitary that produce hormones that control seasonal fertility, which are located in a different part of the gland.”
The search for the missing link between the different parts of the pituitary has been going on for nearly three decades, because knowing how this works could, for instance, help control when sheep start lambing, enabling farmers to respond to climate change.
The team’s findings also have wider implications for how animals, and potentially humans, use this system in health and disease.
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Jennifer Castle-Miller et al. Mechanisms regulating angiogenesis underlie seasonal control of pituitary function. PNAS, published online March 7, 2017; doi: 10.1073/pnas.1618917114
