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• W2085742008 abstract "A recent study has revealed new insight into how the annual clock may drive seasonal hormone rhythms in mammals; the data suggest that melatonin-receptor-containing cells in the pituitary gland may operate as key calendar cells, transmitting seasonal temporal information to the endocrine system. A recent study has revealed new insight into how the annual clock may drive seasonal hormone rhythms in mammals; the data suggest that melatonin-receptor-containing cells in the pituitary gland may operate as key calendar cells, transmitting seasonal temporal information to the endocrine system. In the past decade key features of the core molecular oscillators involved in the regulation of circadian (daily) clocks have been defined in mammals and flies, and elegant studies conducted to examine their role in neural and behavioural pathways, as well as peripheral tissue physiology. In contrast, we still have a very poor understanding of how animals tell time on an annual time-scale. That circannual clocks exist is not in doubt, but little is known about them, partly because of the extraordinary time scale required for their study. Research by Gwinner and colleagues (reviewed in [1Gwinner E. Circannual rhythms in birds.Curr. Opin. Neurobiol. 2003; 13: 770-778Crossref PubMed Scopus (142) Google Scholar]) on migratory birds has shown that long-term reproductive and migratory behavioural annual rhythms persist in individual birds kept for up to 12 years in artificial constant environments, while other researchers studying mammals have reported long-term cycles of metabolism, hibernation behaviour and reproduction following many years of exposure to constant conditions [2Davis D.E. Hibernation and circannual rhythms of food consumption in marmots and ground squirrels.Q. Rev. Biol. 1976; 51: 477-514Crossref PubMed Scopus (119) Google Scholar]. Such circannual cycles tend to run with slightly shorter periods than 12 months. A recent paper by Lincoln et al. [3Lincoln G.A. Clarke I.J. Hut R.A. Hazlerigg D.G. Characterizing a mammalian circannual pacemaker.Science. 2006; 314: 1941-1944Crossref PubMed Scopus (172) Google Scholar] on sheep now offers new insight into this circannual timer, and suggests an anatomical substrate based in the pituitary gland (Figure 1). Sheep are seasonally breeding mammals, and as such use seasonal changes in day-length to cue reproductive and hormonal rhythms. We have known for more than 20 years that photoperiod regulates the nocturnally secreted pineal melatonin signal, generating long-duration signals in winter and short-duration signals in summer, and this provides the brain with an accurate internal representation of external photoperiod, which drives seasonal physiology. Curiously, the brain is not rich in structures expressing melatonin receptors, and in seasonal mammals, a major site of receptor expression is in the proximal pituitary gland region, at the point where it joins the hypothalamus, the pars tuberalis [4Morgan P.J. Barrett P. Howell H.E. Helliwell R. Melatonin receptors: localization, molecular pharmacology and physiological significance.Neurochem. Int. 1994; 24: 101-146Crossref PubMed Scopus (582) Google Scholar] (Figure 1). An elegant surgical approach pioneered by Lincoln and Clarke [5Lincoln G.A. Clarke I.J. Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland.J. Neuroendocrinol. 1994; 6: 251-260Crossref PubMed Scopus (246) Google Scholar] showed that in ‘hypothalamo-disconnected’ animals, in which the hypothalamus was surgically disconnected from the pituitary with the part tuberalis attached, the photoperiod-controlled melatonin signal could still regulate seasonal rhythms of the hormone prolactin. This demonstrated that blood borne melatonin signals must directly regulate this axis. In the new study [3Lincoln G.A. Clarke I.J. Hut R.A. Hazlerigg D.G. Characterizing a mammalian circannual pacemaker.Science. 2006; 314: 1941-1944Crossref PubMed Scopus (172) Google Scholar], the same group has now extended these earlier observations by studying a cohort of hypothalamo-disconnected sheep kept on constant long photoperiods for just under three years. Despite an invariant long-day melatonin signal, most of the animals underwent robust circannual rhythms of prolactin secretion with an average period for each cycle of 9 to 10 months. When exposed to short photoperiods, prolactin levels fell and the cycle was re-set. Together, these studies suggest that the circannual machinery driving prolactin rhythms can operate in the absence of direct neural links from the hypothalamus, that it is likely to be pituitary specific, and that it remains sensitive to changes in the melatonin signal. In a further group of hypothalamo-disconnected animals, rhythmic production of the melatonin signal was blocked by removal of the superior cervical ganglia, thus inhibiting sympathetic drive to the pineal gland (blue arrow in Figure 1). Deprived of the melatonin signal, levels of prolactin fell and remained basal for just under three years, and were no longer sensitive to photoperiod change [3Lincoln G.A. Clarke I.J. Hut R.A. Hazlerigg D.G. Characterizing a mammalian circannual pacemaker.Science. 2006; 314: 1941-1944Crossref PubMed Scopus (172) Google Scholar]. This suggests that, although the daily melatonin signal does not need to change in duration to drive the circannual oscillator, it nonetheless serves an important permissive function within the pituitary gland maintaining the circannual oscillation. Other studies have investigated whether the local melatonin-regulated clock gene rhythm within the pituitary exhibits changes in phasing over the circannual cycle [6Lincoln G.A. Johnston J.D. Andersson H. Wagner G. Hazlerigg D.G. Photorefractoriness in mammals: dissociating a seasonal timer from the circadian-based photoperiod response.Endocrinology. 2005; 146: 3782-3790Crossref PubMed Scopus (89) Google Scholar, 7Johnston J.D. Cagampang F.R. Stirland J.A. Carr A.J. White M.R. Davis J.R. Loudon A.S. Evidence for an endogenous per1- and ICER-independent seasonal timer in the hamster pituitary gland.FASEB J. 2003; 17: 810-815Crossref PubMed Scopus (46) Google Scholar]. It appears that in both rodents and sheep, the clock gene rhythm faithfully reflects the ambient melatonin signal, and thus the circannual component probably lies downstream. The search is now on for molecular pathways involved in such seasonal timing and for this, the pars tuberalis is the ideal starting point." @default.
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• W2085742008 title "Circannual Clocks: Annual Timers Unraveled in Sheep" @default.
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