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K Rousseau, Z Atcha, J Denton, F R A Cagampang, A R Ennos, A J Freemont and A S I Loudon

involving the production of a daily pineal melatonin signal. This, in turn, acts on pathways that regulate the set-point for energy expenditure and food intake. As a result, seasonal mammals such as Siberian hamsters exhibit profound photoperiod

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Perry Barrett, Elena Ivanova, E Scott Graham, Alexander W Ross, Dana Wilson, Helene Plé, Julian G Mercer, Francis J Ebling, Sandrine Schuhler, Sandrine M Dupré, Andrew Loudon and Peter J Morgan

important source of neural stem cells which give rise to differentiated neuronal cells that migrate to appropriate locations in the hypothalamus ( Xu et al. 2005 ). The Siberian hamster undergoes major physiological and behavioural changes to

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HA Kenny, DJ Bernard, TH Horton and TK Woodruff

Follicle-stimulating hormone (FSH) stimulates ovarian follicle development and the production of protein hormones including inhibin A and inhibin B. The inhibins are dimeric proteins (alpha-beta(A) or alpha-beta(B)) secreted by growing follicles that suppress FSH in a classical endocrine negative feedback loop. Siberian hamsters, Phodopus sungorus, exhibit seasonal variation in FSH levels. Given the role of inhibin in FSH regulation, we hypothesized that ovarian inhibin expression differs between animals reared in long (16 h light:8 h darkness) and short (6 h light:18 h darkness) photoperiods. To examine inhibin expression in animals housed under long or short photoperiods, hamster inhibin alpha-, beta(A)-, and beta(B)-subunits were cloned and used to detect and localize inhibin subunit mRNA in developing follicles. Ovarian inhibin alpha-subunit mRNA levels were significantly higher in long day-exposed (LD) than in short day-exposed (SD) hamsters. In addition, dimeric inhibin, as well as inhibin alpha-, beta(A)-, and beta(B)-subunit protein levels were higher in the LD than in the SD hamster ovaries.

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HA Kenny, DJ Bernard, TH Horton and TK Woodruff

Inhibin production differs in ovaries of Siberian hamsters (Phodopus sungorus) exposed to long days (LD) or short days (SD). We believe that seasonal differences in serum follicle-stimulating hormone contribute to this difference. However, given the profound photoperiodic differences in follicle maturation, serum gonadotropins alone may not account for all of the observed differences in inhibin processing. To test this hypothesis, we challenged LD and SD female hamsters with exogenous gonadotropins. While both groups responded with increased inhibin expression, the effects were muted in ovaries of SD females and there was no evidence of ovulation in these animals. These data indicate that the ovaries of SD females are not immediately equipped to respond to gonadotropin stimulation. More generally, these data suggest that photoperiodic history affects ovarian inhibin production and secretion in response to gonadotropins.

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Ricardo J Samms, Michelle Murphy, Maxine J Fowler, Scott Cooper, Paul Emmerson, Tamer Coskun, Andrew C Adams, Alexei Kharitonenkov, Francis J P Ebling and Kostas Tsintzas

, FGF21 is now being evaluated as a potential treatment for both obesity and diabetes and their associated comorbidities ( Gaich et al . 2013 , Gimeno & Moller 2014 ). The Siberian hamster ( Phodopus sungorus ) is a natural animal model of adaptive

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Perry Barrett, Elena Ivanova, E Scott Graham, Alexander W Ross, Dana Wilson, Helene Plé, Julian G Mercer, Francis J Ebling, Sandrine Schuhler, Sandrine M Dupré, Andrew Loudon and Peter J Morgan

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PJ Morgan, AW Ross, JG Mercer and P Barrett

The photoperiodic mammal undergoes quite remarkable changes in physiology as part of its natural adaptations to seasonal fluctuations in the environment. Changes in energy balance and body weight are among these adaptations. In some seasonal mammals, such as the Siberian hamster (Phodopus sungorus), these changes in body weight have been explored in detail, and there is evidence for tightly controlled systems of energy balance that are coordinated by photoperiod acting via the temporal pattern of melatonin secretion from the pineal gland. The pathways and systems involved appear to be quite distinct from the hypothalamic pathways identified to regulate energy balance in studies of both mice and rats thus far. Instead it appears that in the Siberian hamster a tightly regulated system under the control of photoperiod is able to reset the tone of the systems involved in energy balance regulation. Understanding how photoperiod and melatonin act within the hypothalamus to regulate energy balance offers potentially fundamental and important new insights into the control of energy balance. This review describes the current state of our knowledge.

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ANN LOGAN and BRIAN WEATHERHEAD

α-Melanocyte-stimulating hormone (α-MSH) has been shown to act directly on the mammalian melanocyte in short-term cultures of hair follicles obtained from the Siberian hamster. Melanogenesis was stimulated through an increase in tyrosinase activity which resulted in an increase in melanin production. The response of hair follicle melanocytes to α-MSH occurred only in follicles taken from moulting animals, implying that they show a discontinuous expression of MSH receptors during the hair follicle growth cycle. Synthetic 1–24 ACTH had no effect on melanogenesis regardless of whether the follicles came from moulting or non-moulting animals. The pineal peptide, [8-arginine]-vasotocin (AVT), inhibited melanin production without a concomitant decrease in tyrosinase activity. In this respect AVT resembled melatonin, although AVT showed a potency ratio of less than half on a molar basis. The action of AVT, like that of melatonin, must ultimately be on some post-tyrosinase step in melanin biosynthesis. In these hair follicle melanocytes AVT seems to bind to specific receptors since neither of the closely related peptides, oxytocin and [8-arginine]-vasopressin, displayed any activity in our culture system.

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GA Lincoln, H Andersson and A Loudon

Melatonin-based photoperiod time-measurement and circannual rhythm generation are long-term time-keeping systems used to regulate seasonal cycles in physiology and behaviour in a wide range of mammals including man. We summarise recent evidence that temporal, melatonin-controlled expression of clock genes in specific calendar cells may provide a molecular mechanism for long-term timing. The agranular secretory cells of the pars tuberalis (PT) of the pituitary gland provide a model cell-type because they express a high density of melatonin (mt1) receptors and are implicated in photoperiod/circannual regulation of prolactin secretion and the associated seasonal biological responses. Studies of seasonal breeding hamsters and sheep indicate that circadian clock gene expression in the PT is modulated by photoperiod via the melatonin signal. In the Syrian and Siberian hamster PT, the high amplitude Per1 rhythm associated with dawn is suppressed under short photoperiods, an effect that is mimicked by melatonin treatment. More extensive studies in sheep show that many clock genes (e.g. Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the PT, and their expression oscillates through the 24-h light/darkness cycle in a temporal sequence distinct from that in the hypothalamic suprachiasmatic nucleus (central circadian pacemaker). Activation of Per1 occurs in the early light phase (dawn), while activation of Cry1 occurs in the dark phase (dusk), thus photoperiod-induced changes in the relative phase of Per and Cry gene expression acting through PER/CRY protein/protein interaction provide a potential mechanism for decoding the melatonin signal and generating a long-term photoperiodic response. The current challenge is to identify other calendar cells in the central nervous system regulating long-term cycles in reproduction, body weight and other seasonal characteristics and to establish whether clock genes provide a conserved molecular mechanism for long-term timekeeping.

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Elżbieta Król and John R Speakman

exposure showed no significant response, indicating a state of leptin resistance. The changes in responsiveness to leptin treatment induced by photoperiod were similar to those observed in Siberian hamsters ( Atcha et al. 2000 , Klingenspor et al. 2000