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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science, Ltd
    Journal of neuroendocrinology 15 (2003), S. 0 
    ISSN: 1365-2826
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Seasonal mammals commonly exhibit robust annual cycles of adiposity, food intake and energy metabolism. These cycles are driven by changes in the external daylength signal, which generates a diurnal melatonin profile and acts on neuroendocrine pathways. The white adipose tissue hormone leptin reflects overall adiposity in seasonal mammals, and consequently undergoes significant seasonal fluctuations in secretion. The seasonally breeding Siberian (Djungarian) hamster is a convenient laboratory model to study the effect of a seasonal time-keeping clock on energy metabolism, appetite regulation and the control of adiposity. We have shown that administration of exogenous leptin at physiological doses induces significant loss of adipose tissue for short-day housed winter-like hamsters in which endogenous adipose tissue and leptin concentrations are already low. By contrast, long-day housed hamsters with high adipose tissue reserves are refractory to the effects of leptin. This phenomenon of seasonal leptin resistance appears to be a general feature of other seasonally breeding mammals, and may reflect the operation of an annual timer controlling leptin uptake and/or action on central nervous system signal transduction pathways. The mobilization of fat by leptin in short-day housed hamsters is not associated with changes in expression in either anorexic or anabolic peptides expressed in leptin-receptor rich structures in the arcuate region of the hypothalamus, and suggests that leptin may target other structures. These data contrast with studies, which show that homeostatic mechanisms in response to feed-restriction induce changes in hypothalamic peptides in a similar manner to nonphotoperiodic species. Thus, the long-term seasonal regulation of body weight set point and leptin feedback may operate through separate pathways to those responsible for acute responses to food restriction.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science, Ltd
    Journal of neuroendocrinology 14 (2002), S. 0 
    ISSN: 1365-2826
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Cocaine and amphetamine-regulated transcript (CART) mRNA and immunoreactivity are expressed abundantly in the hypothalamus. Central administration of various fragments of this neuropeptide decreases food intake in rodents. To find out whether CART might play a role in the physiological regulation of energy balance, we used in situ hybridization to investigate whether CART mRNA abundance changed in two chronic obese/fat versus lean states and after acute dietary restriction. In the first study, mice were treated with goldthioglucose to destroy glucose-responsive neurones in the ventromedial hypothalamus. This produced hyperphagia and obesity: 7 weeks after treatment, those receiving goldthioglucose weighed 70% more than the controls. CART mRNA abundance in the arcuate nucleus of goldthioglucose-treated mice was decreased by 71% compared to levels in the control mice, but CART expression was unaffected in the dorsolateral hypothalamus. In the second study, male Siberian hamsters were exposed to short days to induce a physiological winter response in which body weight decreases as fat reserves are catabolized, and food intake correspondingly declines. After 8 weeks in short days, body weight had declined by 18% relative to controls maintained in long days in a summer fat state. CART mRNA levels did not differ significantly between the two groups in any hypothalamic areas. In the third study, male Siberian hamsters, either in long days or after 12 weeks exposure to short days to induce weight loss, were subject to a 48-h period of fasting. Although photoperiod per se did not affect CART expression, fasting produced a significant decrease in CART mRNA in the arcuate nucleus of hamsters in both the long- and short-day state. We conclude that CART-producing cells are involved in energy homeostasis: the marked decrease in CART expression in the arcuate nucleus in goldthioglucose-lesioned mice may contribute to the development of obesity, and the decrease following acute dietary restriction in hamsters may reflect a compensatory mechanism to reduce caloric expenditure, but our results do not indicate that CART is involved in long-term seasonal regulation of body weight.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-2826
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Syrian hamsters exhibit a marked seasonal variation in prolactin secretion. The aim of this study was to analyse the nature of the photoperiodic regulation of prolactin gene expression, and to define the role of melatonin and the pars tuberalis of the anterior pituitary in this process. Pituitary prolactin gene expression, restricted to the pars distalis, was increased in hamsters maintained in long daylengths (16 h : 8 h, light : dark) compared to hamsters exposed to short daylengths (8 h : 16 h, light : dark) for 8–12 weeks. Analysis of single cells by in situ hybridization showed that photoperiod had no effect on the percentage of pars distalis cells expressing prolactin mRNA, but shifted the frequency distribution of prolactin mRNA expression per cell, such that in long photoperiods a greater proportion of cells were recruited to a higher expressing population. In vitro coculture of hamster pars tuberalis fragments increased prolactin promoter-driven luciferase activity in stably transfected GH3 cells in a dose- and duration-dependent manner. Conditioned medium from hamster and ovine pars tuberalis also activated the prolactin promoter. Furthermore, basal and forskolin-stimulated conditioned medium from hamster pars tuberalis increased prolactin mRNA expression in primary cultures of pars distalis cells. Melatonin attenuated the activity of pars tuberalis-conditioned medium but had no direct effect on either prolactin mRNA expression or secretion in pars distalis cell cultures. Finally, pars tuberalis fragments from long photoperiod hamsters stimulated prolactin gene promoter activity to a greater extent than those from short photoperiod hamsters. In conclusion, this study provides the first evidence in a seasonal mammal that the synthesis of prolactin depends on photoperiodic modulation of a pars tuberalis-derived factor. Our data support further the hypothesis that seasonal modulation of prolactin gene expression depends upon a melatonin-dependent paracrine action of the pars tuberalis on pars distalis lactotrophic cells.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 307 (1984), S. 296-296 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] LOUDON ET AL. REPLY-In our paper we drew attention to the influence of pasture type on the milk yield, suckling patterns and fertility of red deer hinds. We were alerted to the physiological implications of the differences in suckling pattern when we considered our data on prolactin. These data, in ...
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of pineal research 5 (1988), S. 0 
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The Bennett's wallaby is a seasonal breeder in which photoperiod is an important proximate factor involved in regulating the timing of the breeding season. The present study was undertaken in order to describe the melatonin profiles of female wallabies under natural photoperiod and to examine the effects of abrupt changes in day length on the melatonin profile. Under artificial and natural photoperiods, plasma melatonin concentrations were low or nondetectable during the day but increased after the onset of darkness to values of up to 50 pg/ml. Under natural photoperiod, melatonin profiles were obtained from animals on December 17, April 3, June 26, and October 7. The duration of the nocturnal melatonin rise was positively correlated with the duration of the night, and there was no significant effect of time of year on the amplitude of the nocturnal melatonin peak. In a further study, 12 animals were placed in 9L: 15D and after 7 days, one-half were transferred to 18.5L: 5.5D for 6 weeks. Before and after each change of photoperiod, melatonin profiles were obtained from both groups. As demonstrated for animals on natural photoperiod, duration but not amplitude of the nocturnal melatonin rise was affected by day length. It is noteworthy that two of the 17 animals used in this study did not show evidence of a nocturnal rise in melatonin levels.
    Type of Medium: Electronic Resource
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