Evidence for independence of circadian characters and extent of photoresponsiveness in the Djungarian hamster, Phodopus sungorus

Warm spells in winter affect the equilibrium between winter phenotypes

Each phenotype is a product of the interaction of the genes and the environment. Although winter phenotype in seasonal mammals is heritable, its development may be modified by external conditions. In today's world, global climate change and increasing frequency of unpredictable weather events may affect the dynamic equilibrium between phenotypes. We tested the effect of changes in ambient temperature during acclimation to short photoperiod on the development of winter phenotypes in three generations of Siberian hamsters (Phodopus sungorus). Based on seasonal changes in fur colour, body mass, and expression of daily torpor we distinguished three different winter phenotypes: responding, non-responding, and partially-responding to short photoperiod. We found that warm spells in winter can increase the proportion of non-responding individuals in the population, while stable winter conditions can increase photoresponsiveness among the offspring of non-responders. We conclude that the polymorphism of winter phenotype is an inherent characteristic of the Siberian hamster population but the development of winter phenotype is not fixed but rather a plastic response to the environmental conditions.

Polymorphism of winter phenotype in Siberian hamster: consecutive litters do not differ in photoresponsiveness but prolonged acclimation to long photoperiod inhibits winter molt

BACKGROUND

The theory of delayed life history effects assumes that phenotype of adult individual results from environmental conditions experienced at birth and as juvenile. In seasonal environments, being born late in the reproductive season affects timing of puberty, body condition, longevity, and fitness. We hypothesized that late-born individuals are more prone to respond to short photoperiod (SP) than early born ones. We used Siberian hamsters Phodopus sungorus, a model species characterized by high polymorphism of winter phenotype. We experimentally distinguished the effect of litter order (first or third) from the effect of exposure to long photoperiod (LP) before winter (3 months or 5 months) by manipulating the duration of LP acclimation in both litters. We predicted that, irrespective of the litter order, individuals exposed to long photoperiod for a short time have less time to gather energy resources and consequently are more prone to developing energy-conserving phenotypes. To assess effect of litter order, duration of acclimation to long days, and phenotype on basal cost of living we measured basal metabolic rate (BMR) of hamsters.

RESULTS

Individuals born in third litters had faster growth rates and were bigger than individuals from first litters, but these differences vanished before transfer to SP. Litter order or duration of LP acclimation had no effects on torpor use or seasonal body mass changes, but prolonged acclimation to LP inhibited winter molting both in first and third litters. Moreover, individuals that did not molt had significantly higher BMR in SP than those which molted to white fur. Although one phenotype usually predominated within a litter, littermates were often heterogeneous. We also found that over 10% of individuals presented late response to short photoperiod.

CONCLUSIONS

Our data indicate that duration of postnatal exposure to LP may define propensity to photoresponsiveness, regardless of the litter in which animal was born. Existence of littermates presenting different phenotypes suggests a prudent reproductive strategy of investing into offspring of varied phenotypes, that might be favored depending on environmental conditions. This strategy could have evolved in response to living in stochastic environment.

Predictive and reactive changes in antioxidant defence system in a heterothermic rodent

Living in a seasonal environment requires periodic changes in animal physiology, morphology and behaviour. Winter phenotype of small mammals living in Temperate and Boreal Zones may differ considerably from summer one in multiple traits that enhance energy conservation or diminish energy loss. However, there is a considerable variation in the development of winter phenotype among individuals in a population and some, representing the non-responding phenotype (non-responders), are insensitive to shortening days and maintain summer phenotype throughout a year. Differences in energy management associated with the development of different winter phenotypes should be accompanied by changes in antioxidant defence capacity, leading to effective protection against oxidative stress resulting from increased heat production in winter. To test it, we analysed correlation of winter phenotypes of Siberian hamsters (Phodopus sungorus) with facultative non-shivering thermogenesis capacity (NST) and oxidative status. We found that in both phenotypes acclimation to winter-like conditions increased NST capacity and improved antioxidant defence resulting in lower oxidative stress (OS) than in summer, and females had always lower OS than males. Although NST capacity did not correlate with the intensity of OS, shortly after NST induction responders had lower OS than non-responders suggesting more effective mechanisms protecting from detrimental effects of reactive oxygen metabolites generated during rewarming from torpor. We suggest that seasonal increase in antioxidant defence is programmed endogenously to predictively prevent oxidative stress in winter. At the same time reactive upregulation of antioxidant defence protects against reactive oxygen species generated during NST itself. It suggests that evolution of winter phenotype with potentially harmful characteristics was counterbalanced by the development of protective mechanisms allowing for the maintenance of phenotypic adjustments to seasonally changing environment.

Are Humans Seasonally Photoperiodic?

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here—namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.

Tau Differences between Short-Day Responsive and Short-Day Nonresponsive White-Footed Mice (Peromyscus leucopus) Do Not Affect Reproductive Photoresponsiveness

In laboratory-bred rodent populations, intraspecific variation in circadian system organization is a known cause of individual variation in reproductive photoresponsiveness. The authors sought to determine whether circadian system variation accounted for individual variation in reproductive photoresponsiveness in a single, highly genetically variable population of Peromyscus leucopusrecently derived from the wild. Running-wheel activity patterns of male and female mice, aged 70 to 90 days, from artificially selected lines of reproductively photoresponsive (R) and nonresponsive (NR) lines were monitored under short-day photoperiod (8 h light, 16 h dark), long-day photoperiod (16 h light, 8 h dark), and constant darkness (DD). NR mice displayed a significantly longer mean free-running period (24.08 h) in DD compared with R mice (23.75 h), due in large part to a difference between NR and R females (24.25 h vs. 23.74 h, respectively). All other entrainment characteristics (alpha, phase angle of activity) under short days, long days, and DD were similar between R and NR mice. Variation in free- running period and entrainment characteristics has been shown to affect photoresponsiveness in other rodent species by altering the manner in which the circadian system interprets short days. To determine whether variation in photoresponsiveness in P. leucopus is due to differences in free-running period instead of variation downstream from the central circadian clock in the pathway controlling photoresponsiveness, the authors exposed young R and NR mice to DD and measured the effect on reproductive organ development. If variation in free-running period affected how the circadian system of mice interpreted short days, then both R and NR mice exposed to DD should have exhibited a delay in gonadal development. Only R mice exhibited pubertal delay in DD. NR mice exhibited large paired testes, paired seminal vesicles, paired ovaries, and uterine weight typical of mice nonresponsive to short days, whereas R mice exhibited reproductive organ weight typical of mice responsive to short days. These data suggest that despite significant differences in free-running period between R and NR mice, individual variation in photoresponsiveness is not due to differences in how the circadian systems of R and NR mice interpret the LD cycle.

Individual differences in circadian waveform of Siberian hamsters under multiple lighting conditions

Because the circadian clock in the mammalian brain derives from a network of interacting cellular oscillators, characterizing the nature and bases of circadian coupling is fundamental to understanding how the pacemaker operates. Various phenomena involving plasticity in circadian waveform have been theorized to reflect changes in oscillator coupling; however, it remains unclear whether these different behavioral paradigms reference a unitary underlying process. To test whether disparate coupling assays index a common mechanism, we examined whether there is covariation among behavioral responses to various lighting conditions that produce changes in circadian waveform. Siberian hamsters, Phodopus sungorus, were transferred from long to short photoperiods to distinguish short photoperiod responders (SP-R) from nonresponders (SP-NR). Short photoperiod chronotyped hamsters were subsequently transferred, along with unselected controls, to 24-h light:dark:light: dark cycles (LDLD) with dim nighttime illumination, a procedure that induces bifurcated entrainment. Under LDLD, SP-R hamsters were more likely to bifurcate their rhythms than were SP-NR hamsters or unselected controls. After transfer from LDLD to constant dim light, SP-R hamsters were also more likely to become arrhythmic compared to SP-NR hamsters and unselected controls. In contrast, short photoperiod chronotype did not influence more transient changes in circadian waveform. The present data reveal a clear relationship in the plasticity of circadian waveform across 3 distinct lighting conditions, suggesting a common mechanism wherein individual differences reflect variation in circadian coupling.

Early photoperiod history and short-day responsiveness in Siberian hamsters

Siberian hamsters exhibit seasonal, photoperiod influenced cycles of reproductive activity, body size, pelage characteristics, and thermoregulatory behavior. Laboratory populations generally exhibit inter-individual variability in expression of photoperiod responsiveness, with a subset of individuals that fail to show the species typical responses to short photoperiod. This variability is partly explained by a genetic component, as it has been possible to increase the number of short-day nonresponders by artificial selection. Responsiveness to short photoperiod is also substantially influenced by photoperiod history in this species; hamsters that have been raised under long (16L) or very long (18L) day lengths are less likely to exhibit winter-type responses to short days as compared to hamsters raised under an intermediate (14L) day length. In the present experiment, we examined effects of age and early photoperiod history in a strain of Siberian hamsters that had been selected for short-day nonresponsiveness. Hamsters transferred into short photoperiod on the day of birth were uniform in exhibiting winter-type responses. However, hamsters raised until 25 days of age in either continuous illumination or in 16L exhibited variation in responsiveness when subsequently moved into short photoperiod. We conclude that virtually all hamsters of the short-day nonresponsive strain are born responsive to short days. Subsequent development of resistance to potential short day effects is dependent on age and/or photoperiod history.

Genetic and environmental influences on short-day responsiveness in Siberian hamsters (Phodopus sungorus)

Siberian hamsters are photoperiodic rodents that typically exhibit several physiological changes when exposed to a short-day photoperiod. However, development of the winter phenotype in short days is largely conditional on prior photoperiod history: Hamsters that have been reared in an exceptionally long day length (18 L) do not usually exhibit the winter phenotype after transfer to short days, whereas animals reared under "moderately" long days (16 L) are more variable in responsiveness to subsequent short-day exposure, with 20% to 30% generally failing to exhibit winter-type responses. Hamsters reared exclusively in an "intermediate" day length (14 L) are almost uniformly responsive to short photoperiod. In the present study, the authors examine the influence of photoperiod history on short-day responsiveness in a breeding line of hamsters that has been subjected to artificial selection for resistance to the effects of short days. The results demonstrate that photoperiod history is an important determinant of short-day responsiveness in both random-bred (UNS) hamsters and animals artificially selected and bred for nonresponsiveness to short photoperiod (PNR). The PNR hamsters have a reduced requirement for long-day exposure to evoke a state of unresponsiveness to short days. The results are discussed in relation to possible significance for the origin of population and species differences in photoperiod responsiveness.

Urinary 6-sulfatoxymelatonin profiles in male Djungarian hamsters (Phodopus sungorus) responding and not responding to short-day photoperiods: possible role of elevated daytime levels

The lack of endocrine and physiological responses of some Djungarian hamsters (Phodopus sungorus) to the transition from long to short photoperiods (L:D 16:8 --> L:D 8:16) has been known for a long time but is not yet understood. We investigated the role of melatonin synthesis in this context because melatonin, as part of the circadian system, may play a role in non-responsiveness. In ten responding and ten non-responding male hamsters, the urinary 24 hr 6-sulfatoxymelatonin (aMT6s) profiles under L:D 8:16 and L:D 16:8 were measured. Both short day responding and non-responding hamsters showed diurnal aMT6s excretion rhythms. Whereas responders reacted to the transition L:D 16:8 --> L:D 8:16 with a marked elevation of aMT6s excretion, in non-responders no adjustment of the melatonin rhythm to the change of the photoperiod was seen. Furthermore, under L:D 16:8 the daytime levels of aMT6s were significantly (P<0.001) lower in responders compared to non-responders whereas under L:D 8:16 these levels were higher (P<0.01). It is speculated that high daytime levels of aMT6s under long-day photoperiods in non-responders result in down-regulation of melatonin receptors of the nucleus suprachiasmaticus, the pacemaker for the pineal gland, leading to a lack of response to the transition to short-day photoperiods.

Photoperiod nonresponsive Siberian hamsters: effect of age on the probability of nonresponsiveness

Groups from three different breeding lines of Siberian hamsters (UNS = general colony animals, PNRa = selected for photoperiod nonresponsiveness as adults, PNRj = selected for photoperiod nonresponsiveness as juveniles) were exposed to short days at weaning and again as adults (Experiment 1) or only as adults (Experiment 2). The proportion of photoperiod nonresponsive individuals in each line was determined by measuring testis length after 6 weeks of exposure to short days (juveniles) or by paired testis weights after 12 weeks in short photoperiod (adults). Adults were blood sampled on the day of sacrifice (Experiment 1) or on Weeks 3, 4, 5, 6, 8, 10, and 12 (Experiment 2) for determination of serum prolactin (PRL) and follicle-stimulating hormone (FSH) concentrations. Nonresponsive individuals were present in all three lines of hamsters. Furthermore, all three lines of hamsters showed an increase in the proportion of nonresponders with age; some individuals are responsive to short days as juveniles, but become nonresponsive in adulthood. The two PNR lines exhibited a greater proportion of nonresponders at both ages compared to the UNS line, with the PNRj line exhibiting the greatest proportion of nonresponders at each age. During exposure to short days, nonresponders exhibited significantly higher serum PRL and FSH concentrations that did the UNS line; nonresponders also exhibited larger testis size, and fewer animals molted to winter-type pelage. The results indicate that (a) in all three lines, a significantly higher proportion of animals are nonresponsive to short photoperiod as adults than as juveniles; (b) selection for nonresponsiveness as juveniles can produce a line of hamsters that, as adults, are nearly all nonresponsive to short days; and (c) some individuals from each line are responsive to short photoperiod early in life, but become nonresponsive as adults.

Evidence that the circadian system mediates photoperiodic nonresponsiveness in Siberian hamsters: the effect of running wheel access on photoperiodic responsiveness

Juvenile male Siberian hamsters from a line of hamsters selected for nonresponsiveness to short photoperiod (PNRj) and animals from the general colony (UNS) were separated at weaning into two groups. Group 1 males were moved into short days (10 h light:14 h dark [10L:14D]) with free access to running wheels (RW). Group 2 animals were the male siblings of Group 1 hamsters; they were moved at the same time into the same room, but were housed in cages without access to RW. Group 2 hamsters only had access to RW for the final week of short-day exposure (Week 8). Animals were blood sampled at the time of sacrifice for analysis of serum prolactin (PRL) and follicle-stimulating hormone (FSH) concentrations. At sacrifice, paired testis weights were obtained and pelage color was scored. Animals from the UNS line showed the expected declines in testis weight, body weight, and serum concentrations of both PRL and FSH, regardless of the presence or absence of RW. These animals also exhibited a high proportion of individuals molting to winter-type pelage. By contrast, a marked difference was noted between siblings from the PNRj line depending on whether RW access was provided at the time of weaning. Animals with access to RW exhibited identical responses to those of the UNS responder animals, whereas PNRj animals without access to RW showed no adjustments to short days (i.e., testis regression, pelage molt, expansion of alpha). In a second experiment, PNRj and UNS males were placed in constant darkness (DD), with or without RW access. The results of this experiment indicated that PNRj animals respond to DD regardless of the presence or absence of RW. In DD, PNRj hamsters also exhibited significantly longer free-running period lengths (taus) than did UNS hamsters; all the PNRj hamsters had taus > 24 h, whereas none of the UNS hamsters had a tau > 24 h. These results indicate that PNRj hamsters retain the proper neural pathways for responding to short day lengths and establish a role for locomotor activity feedback in modulating the circadian system and, subsequently, photoperiodic responsiveness in PNRj hamsters.

Evidence for genetic variation in the occurrence of the photoresponse of the Djungarian hamster, Phodopus sungorus

The Djungarian hamster generally responds to a short-day photoperiod with a complex syndrome of physiological and behavioral changes; however, not all hamsters are photoresponsive. The phenotypic difference is, in part, genetically determined. Parent-offspring regression on a number of continuous and discontinuous measures indicated significant heritability for photoresponsiveness. Four generations of replicated bidirectional selection on a photoresponse index (PI) resulted in significant shifts in the percentage of responsive hamsters, although the average PI of responsive individuals was not significantly changed. Eight estimates of heritability ranged from 0.20 to 0.52. We hypothesize that the circadian system is responsible for the occurrence of the photoresponse, but that the extent of photoresponse is controlled by a separate functional system.