After the recent chronotherapy threads and other discussion about circadian rhythms, etc., this paper appears to be of some interest.
I can't pretend to have read it yet!
J Endocrinol. 2014 Jun 2. pii: JOE-14-0141. [Epub ahead of print]
Clocks for all seasons: unwinding the roles of circadian and interval timers in the pituitary.
Wood SH1, Loudon A2.
1S Wood, Biological sciences, University of manchester, manchester, United Kingdom.
2A Loudon, Biological Sciences, University of manchester, manchester, m13 9pt, United Kingdom
Adaption to the environment is essential for survival, in all wild animal species seasonal variation in temperature and food availability needs to be anticipated. This has led to the evolution of deep-rooted physiological cycles, driven by internal clocks, which can track seasonal time with remarkable precision. Evidence has now accumulated that a seasonal change in thyroid hormone (TH) availability within the brain is a crucial element. This is mediated by local control of TH metabolizing enzymes within specialized ependymal cells lining the 3rd ventricle of the hypothalamus. Within these cells, de-iodinase type-2 enzyme is activated on summer day-lengths, converting metabolically inactive thyroxine (T4) to tri-iodothyronine (T3), and driving seasonal reproductive responses. Remarkably, in both birds and mammals, the pars tuberalis (PT) of the pituitary gland plays an essential role. A specialized endocrine thyrotroph cell (TSH expressing) is regulated by the changing day length signal, leading to activation of TSH by long-days. This acts on adjacent TSH-receptors expressed in the hypothalamic ependymal cells, causing local regulation of de-iodinase enzymes and conversion of TH to the metabolically active T3. In mammals, the PT is regulated by the nocturnal melatonin signal. Summer-like melatonin signals activate a PT-expressed clock-regulated transcription regulator (Eya3), which in turn drives expression of the TSH-beta sub-unit, leading to a sustained increase in TSH expression. In this manner, a local pituitary timer, driven by melatonin, initiates a cascade of molecular events, led by Eya3 and which translates to seasonal changes of neuroendocrine activity in the hypothalamus.
Full paper currently available here:
Image: By Michael Gwyther-Jones from UK (Dandelion) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons