Some effects of artificial illumination on reproduction in the white-footed mouse, Peromyscus leucopus noveboracensis

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.

Photoresponsiveness affects life history traits but not oxidative status in a seasonal rodent

Background

Shortening photoperiod triggers seasonal adjustments like cessation of reproduction, molting and heterothermy. However there is a considerable among-individual variation in photoresponsiveness within one population. Although seasonal adjustments are considered beneficial to winter survival, and natural selection should favor the individuals responding to changes in photoperiod (responders), the phenotype non-responding to changes in day length is maintained in population. Assuming the same resource availability for both phenotypes which differ in strategy of winter survival, we hypothesized that they should differ in life history traits. To test this we compared reproductive traits of two extreme phenotypes of Siberian hamster Phodopus sungorus - responding and non-responding to seasonal changes in photoperiod. We bred individuals of the same phenotype and measured time to first parturition, time interval between litters, offspring body mass 3, 10 and 18 days after birth and their growth rate. We also analyzed nest-building behavior. Additionally, we estimated the correlation between reproduction, and basal metabolic rate (BMR) and oxidative status in both phenotypes to infer about the effect of reproductive output on future investments in somatic maintenance.

Results

Prior to reproduction responding individuals were smaller than non-responding ones, but this difference disappeared after reproduction. Responding pairs commenced breeding later than non-responding ones but there was no difference in time interval between consecutive litters. Responders delivered smaller offspring than non-responders and more out of responding individuals built the nest during winter than non-responding ones. Reproduction did not affect future investments in somatic maintenance. Phenotypes did not differ in BMR and oxidative status after reproduction. However, concentration of reactive oxygen metabolites (ROM) was highest in responding males, and biological antioxidant potential (BAP) was higher in males of both phenotypes than in females.

Conclusions

Delayed breeding in responding Siberian hamsters and high ROM concentration in male responders support our hypothesis that differences in adjustment to winter result in different life history characteristics which may explain coexistence of both phenotypes in a population. We propose that polymorphism in photoresponsiveness may be beneficial in stochastic environment, where environmental conditions differ between winters. We suggest that non-responding phenotype may be particularly beneficial during mild winter, whereas responders would be favored under harsh conditions. Therefore, none of the phenotypes is impaired when compared to the other.

XXV.—The Effect of Increased Daily Illumination and of Reversed Day and Night on the Œstrus Cycle of the Mouse (Mus musculus)

In the course of a recent investigation on the fertilised egg of the mouse (Gresson, 1940 a, b) it was necessary to keep a record of the time at which pairing took place. This revealed that copulation, especially in the winter, was very infrequent during the hours of daylight, and usually took place at about 12 midnight or between midnight and early morning. Similar observations were made by Lewis and Wright (1935), who found that in the autumn and winter mice seldom copulated during the day, and that while the majority of pairings took place between 10 p.m. and 1 a.m., several of their mice paired between 1 a.m. and 7 a.m., whereas in the spring pairing occurred more readily during all hours of the day.

Enduring effects of photoperiod on affective behaviors in Siberian hamsters (Phodopus sungorus)

The effects of perinatal and postweaning photoperiods on subsequent affective behaviors were examined in adult Siberian hamsters (Phodopus sungorus). Hamsters exposed perinatally to short days (8 hr light/day) exhibited mixed results for adult anxiety-like behaviors and increased some depressive-like behaviors compared with hamsters exposed to long days (16 hr light/day). Postweaning exposure to short days increased depressive- and anxiety-like behaviors compared with long days. Sex differences in affective behaviors were observed. These results suggest that anxiety-like behaviors are organized early in life and endure throughout adulthood, and anxiety- and depressive-like behaviors are modified by postweaning photoperiod. The persistence of photoperiod-induced affective behaviors in rodents supports the hypothesis that symptoms of human affective disorders may reflect ancestral adaptations to seasonal environments.

Photoperiod-induced differential expression of angiogenesis genes in testes of adult Peromyscus leucopus

Non-pathological angiogenesis in adults is rare and is largely thought to be restricted to wound healing and female reproductive cycles. Adult male rodents, however, display seasonal angiogenesis to support seasonal changes in reproductive tissue morphology. Non-tropical rodents use photoperiod (day length) to determine the time of year. During short days, the reproductive system undergoes involution and mating behaviours stop, adaptations which presumably allow energy resources to be shifted to processes necessary for winter survival. We compared the patterns of gene expression involved in angiogenesis in testes of white-footed mice (Peromyscus leucopus) following 7, 14, 21 or 34 weeks of long or short day lengths. Short days decreased body mass, reproductive tract mass and seminiferous tubule diameter. Potential genes involved in seasonal angiogenesis were screened by hybridizing testicular RNA from each group to angiogenesis-specific microarrays. Genes that were ≥6-fold different between long- and short-day testes (i.e. hypoxia-inducible factor 1α(Hif1α), plasminogen activator inhibitor 1 (Serpine1), transforming growth factor β receptor 3 (Tgfβr3) and tumour necrosis factor (Tnf)) were sequenced and expression differences were compared throughout gonadal regression and recrudescence using quantitative RT-PCR. Our results suggest that short days trigger expression ofHif1α,Serpine1, andTgfβr3to inhibit angiogenesis or promote apoptosis during testicular regression, and also trigger expression ofTnfto promote angiogenesis during testicular recrudescence.

Social environment modulates photoperiodic immune and reproductive responses in adult male white-footed mice (Peromyscus leucopus)

Social cues may interact with photoperiod to regulate seasonal adaptations in photoperiod-responsive rodents. Specifically, photoperiod-induced adjustments (e.g., reproduction and immune function) may differ among individuals in heterosexual pairs, same-sex pairs, or isolation. Heterosexual cues may be more influential, based on their potential fitness value, than same-sex cues or no social cues. The present study examined the effects of pair (with a male or female) or individual housing on reproductive and immune responses in male white-footed mice (Peromyscus leucopus) maintained in long or short photoperiods. Female pairing did not affect reproductive responses in short-day males. In long days, however, the presence of a female increased both testosterone concentrations and testes mass compared with individually housed and male-paired mice, respectively. Short-day, individually housed males enhanced delayed-type hypersensitivity (DTH) responses compared with single-housed mice in long days, but all paired groups decreased DTH responses regardless of photoperiod. The lack of enhanced DTH response in male mice paired with females coincided with reduced circulating corticosterone concentrations in both photoperiod treatments. Together, these results suggest that social environment may have important modulatory effects on photoperiod-regulated immune responses in male white-footed mice.

Photoperiod time measurement for activity, torpor, molt and reproduction in mice

Resonance light:dark cycles (LD 6:18, 6:30, 6:42, or 6:54) were used to establish that a circadian rhythm of light sensitivity is involved in the thermoregulatory and reproductive responses to a short day photoperiod in the mouse, Peromyscus leucopus. A fifth group was maintained on the long day photoperiod of LD 16:8. After 19 weeks animals presented with LD 6:18 or 6:42 exhibited short day photoperiod responses: gonadal regression, incidence of spontaneous daily torpor and molt to the winter pelage. In contrast animals responded to LD 6:30 and 6:54 as long day photoperiods: maintenance of gonadal system, no incidence of spontaneous daily torpor, and summer pelage. In a second study a T-experiment was conducted to determine that more than one circadian system may regulate these multiple photoperiodic effects. Mice were exposed to 1 of 8 LD cycles for 15 weeks (1:22.00, 1:22.25, 1:22.50, 1:23.00, 1:23.50, 1:23.75, 9:15, or 16:8), Entrained wheel-running activity occurred under all LD regimes. Mice on LD 1:22.50, 1:23.00, and 1:23.50, however, exhibited activity patterns similar to mice on LD 9:15, and they exhibited gonadal regression. Mice on LD 1:22.00, 1:22.25, and 1:23.75 exhibited activity patterns similar to LD 16:8 animals, and most of these animals remained reproductively competent. There was also a close association between occurrence of reproductive regression and daily torpor. In contrast, molt to the winter pelt occurred under all non-24-hr LD cycles. This dysynchrony in response suggests that at least 2 circadian systems are involved in photoperiodic time measurement in P. leucopus.

Two daily melatonin injections differentially induce nonshivering thermogenesis and gonadal regression in the mouse (Peromyscus leucopus)

Female white-footed mice (Peromyscus leucopus) were injected twice daily with 5, 10, 50, 100 micrograms melatonin (MEL) or saline. Injections were given for 7 weeks at 2 and 12 hours after lights-on under a long day (LD 16:8) photoperiod. Afternoon administration of MEL induced gonadal regression, although a dose of 50 micrograms or more was necessary to obtain a maximal response. A 5 micrograms MEL injection in the afternoon resulted in intermediate reproductive tract weights. In white-footed mice a morning MEL injection did not abolish the reproductive regression induced by an afternoon injection. Mice receiving 10, 50 or 100 micrograms MEL daily exhibited increased nonshivering thermogenesis (NST), irrespective of the timing of the injection. Daily injections of 5 micrograms MEL had little effect on NST. These observations suggest that "up and down regulation" of MEL receptors may not be important in P. leucopus. Further, the mechanism by which MEL controls reproduction is different from that for NST.

Effects of timed melatonin injections on reproduction in pinealectomized Peromyscus leucopus

Fifty reproductively mature female pinealectomized or sham-operated white-footed mice (Peromyscus leucopus) were injected daily for 7 weeks with 50 micrograms melatonin at 12 hr after lights on. The injections started either on the day following surgery or 2 weeks after surgery. All mice were maintained at room temperature under long-day photoperiod (LD 16:8). Pinealectomized and sham-operated mice injected immediately following surgery and sham-operated mice injected beginning 2 weeks after surgery had reduced reproductive tract weights (30 +/- 6, 33 +/- 6, and 33 +/- 3 mg, respectively) compared to the pinealectomized mice injected beginning 2 weeks after surgery (78 +/- 8 mg). In a second study 74 pinealectomized or sham-operated mice were injected at either 2, 7.5, 12, or 15 hr after lights on with 50 micrograms melatonin. Irrespective of surgical treatment, mice injected at 15 hr had a mean (+/- SE) reproductive tract weight (33 +/- 10 mg) which was substantially less than the 2-hr group (78 +/- 17 mg) while the 7.5- and 12-hr groups were intermediate (P less than 0.001). Pinealectomized and sham-operated animals responded similarly to melatonin injections at these four time points (P less than 0.35). The clearance rate of plasma melatonin after a single 50-micrograms melatonin injection or after 2 weeks of daily injections at either 2 or 12 hr after lights on was determined. Melatonin concentrations were determined using high-performance liquid chromatography and radioimmunoassay.(ABSTRACT TRUNCATED AT 250 WORDS)

Seasonal changes in glycogen level and size of pinealocytes of the white-footed mouse, Peromyscus leucopus: a semiquantitative histochemical study

Glycogen level in and size of pinealocytes of the feral, white-footed mouse Peromyscus leucopus, were studied by a semiquantitative histochemical method to determine whether seasonal changes exist in them under natural conditions, what temporal pattern they exhibit, and whether 24-hour changes in these parameters exist in different seasons, as shown in the laboratory dd-mice. Marked seasonal changes were seen in both glycogen levels and nuclear densities (ANOVA p less than 0.005). The size of pinealocytes at 09:00 to 10:00 showed one peak (and nadir) seasonal change, with the smallest size in winter (December and February) and a larger size in warmer seasons, with the maximum value in July. Glycogen level in pinealocytes at 09:00 to 10:00 showed bimodal seasonal changes, with lower levels in fall and spring and higher levels in winter and summer. In fall, a circadian trend in glycogen level in pinealocytes was seen, with a higher level at the end of the light period. In winter, the glycogen levels were very high at 09:00, 13:00, 17:00, and 21:00 examined and showed dampening of time-of-day differences. On the other hand, the size of pinealocytes followed a time-of-day change (P less than 0.005), being largest at 13:00 and smallest at 21:00. Thus, marked changes in quantitative structure and chemical activities, suggesting changes in functional activity, in pinealocytes were noted especially in severe, cold winter.

Effect of photoperiod and pineal indoles on the reproductive system of young female white-footed mice

Exposure of prepubertal female mice to short photoperiod (LD 8:16) for 3 weeks resulted in a significant inhibition of reproductive tract growth compared with mice maintained in long photoperiod (LD 16:8). Chronic subcutaneous melatonin-beeswax implants containing 1 mg melatonin also produced an inhibition of the reproductive system of these animals. The repressive effects of melatonin appear not to be influenced by photoperiod since comparable results were obtained from groups of mice bearing implants in long and short photoperiods. In addition, the ability of beeswax implants containing various indolic compounds (melatonin, serotonin, 5-methoxytryptamine, 5-methoxyindole-3-acetic acid, and N-acetylserotonin) to inhibit the growth of the reproductive system of this species were compared. Only implants containing melatonin or serotonin caused any significant repression of uterine and ovarian weights.

Melatonin: identification of sites of antigonadal action in mouse brain

Long-term implants releasing a small quantity of melatonin (45 nanograms per day) were used to determine the brain sites of the hormone's antigonadal action in a photoperiodic species, the white-footed mouse (Peromyscus leucopus). Implants in the medial preoptic and supra- and retrochiasmatic areas elicited completed gonadal regression after 7 weeks. Implants in other brain regions had little effect on the animals' reproductive state.

Effects of 18 weeks of daily melatonin injection on reproduction and temperature regulation in the mouse, Peromyscus leucopus

Mice injected daily with 50 microgram of melatonin, 12 hr after lights on, for 18 weeks, underwent gonadal regression after 4-7 weeks and reproductive recrudescence after 15 weeks. Most treated animals molted to the winter pelt after 9-11 weeks of injections, and 2 of the 10 mice experienced bouts of daily torpor after 14 weeks.

Daylength and sexual maturation in female house mice

A series of 4 experiments investigated the relationship between day length and sexual maturation in female house mice. Mice reared in constant light matured significantly later than mice maintained under photoperiods involving 0, 6, 12, or 18 hr of light/day. Results of an experiment involving cross-classified combinations of pre- and postweaning photoperiods revealed that only the postweaning photoperiod treatment was a significant factor influencing the attainment of puberty. Studies of body growth, food consumption, and wheel-running activity in young females housed under different photoperiods indicated that mice kept in constant light grew more slowly during the first 10 days after weaning, generally consumed more food than mice at other photoperiod conditions, and failed to exhibit regular daily activity patterns like those of mice maintained under photoperiods of 0 or 12 hr of light/day. Constant illumination apparently disrupts the normal physiological and behavioral activity rhythms and delays the attainment of sexual maturity.

Seasonal changes in thermogenesis, organ weights, and body composition in the white-footed mouse,Peromyscus leucopus

1. Seasonal adjustments in wild-caughtPeromyscus leucopus include autumn increases in basal metabolic, nonshivering thermogenesis, and interscapular brown fat and decreases in weights of gonads, liver, adrenal glands, and total lipid. Body weight and nonextractable dry weight do not change. 2. Basal metabolic rate, nonshivering thermogenesis, and interscapular brown fat increase following initial cold exposure in mid-September and are maintained at similar levels through January. 3. There is a positive correlation between nonshivering thermogenesis and interscapular brown fat, and negative correlations for both nonshivering thermogenesis and interscapular brown fat with body weight. 4. These seasonal changes in wild-caughtP. leucopus are compared with laboratory studies on the effects of chronic exposure to differences in temperature and photoperiod on these characters. It is concluded that disparities between the effects of cold acclimatization and cold acclimation could result from the influence of additional environmental cues, such as photoperiod, in cold acclimatized mice.