No abstract

Recent studies on mice with mutations in the Clock gene have shown that this mutation disrupts oestrus cyclicity and interferes with successful pregnancy. In order to determine whether two other molecular components of the main clock, namely the period genes, Per1 and Per2, have an effect on the length of the oestrous cycle and the reproductive success, we used Per1-and Per2-deficient females. We show that although fecundity of young adult Per mutant females does not differ from that of wild-type females, middle-aged Per mutant mice are characterised by lower reproductive success than the control group. This may be a consequence of irregularity and acyclicity of the oestrous cycle of the middle-aged mutants. Besides, we demonstrate that Per mutant females have significantly more embryonal implantations in the uterus than successfully delivered offspring. The reproductive deficits of the middle-aged Per mutant females are comparable with those seen in aged wild-type mice. This suggests that Per1 and Per2 mutations cause an advanced ageing.Reproduction (2008) 135 559-568

In Phodopus sungorus spontaneous shallow daily torpor occurred only during winter. Frequency of torpor was not affected by low ambient temperature but the seasonal cueing seems primarily dependent on photoperiodic control. Maximum torpor frequency was found in January with 30% of all hamsters living inside or outside being torpid at a time. It is calculated that torpor will reduce long term energy requirements of Phodopus by only 5%. Therefore it is concluded that torpor is not primarily aimed to reduce energy requirements but to guarantee survival of a fraction of a population during short periods of extreme cold load or inaccessability of food.

The Per1 and Per2 genes are components of the mammalian circadian clock. Mutations in these genes alter phase resetting in response to a nocturnal light pulse, and Per2 mutant mice are known to become arrhythmic in constant darkness. We show that under constant light conditions, Per2 mutant mice exhibit robust activity rhythms as well as body temperature rhythms with a period length that is less than 24 h. In Per1 mutants, the period length of both activity and body temperature rhythms is longer than 24 h in constant light. Per1 mutants prolong their period length (tao) when illuminance is increased, whereas Per2 mutants shorten their endogenous period. Additionally, the authors show that the circadian pattern of Per1 and Per2 gene expression in mice is modified under different photoperiods and that there is a mutual influence of these genes on their timing of expression. We propose that, in mice, the phase relationship between Per1 and Per2 gene expression might be critical for transducing day length information to the organism. Per1 could be part of a morning oscillator tracking dawn, and Per2 could be part of an evening oscillator tracking dusk.

Evidence has recently begun to accumulate that photoperiodic responses of mam mals and birds may affect the control of energy balance and thermoregulation. Exposure to short photoperiod can lower the set point for body temperature regulation in birds and mam mals, as well as the voluntarily selected body temperature in ectothermic lizards. This de crease is accompanied by a reorganization of circadian or ultradian rhythms of body temper ature, particularly an increase in periods spent at rest with minimum body temperatures. Short photoperiod is also used as an environmental cue for induction of seasonal torpor or facilita tion of hibernation. During winter, cold tolerance of small mammals is improved by an in crease of nonshivering thermogenesis in brown fat. Thermogenic capacity of brown fat (res piratory enzymes, mitochondria, uncoupling protein) is enhanced in response to short pho toperiod. This response is mediated via an increase in the activity of sympathetic innervation in brown fat. Moreover, an exposure to short photoperiod prior to low temperatures may act in preparing brown fat for facilitated thermogenesis during acclimation to cold. This shows that photoperiodic control not only affects energy balance indirectly via the control of repro duction or body mass, but may directly interact with central control of thermoregulation and may influence the process of acclimatization.