Photoperiod, melatonin secretion, and sexual maturation in a tropical rodent

Descendants of a sample of cane mice (Zygodontomys brevicauda) trapped at 8 degrees latitude in Venezuela were tested for reproductive photoresponsiveness. This species breeds continuously, year around, despite living in a seasonally harsh habitat. At 50 days of age there were no differences in the weights of the testes or seminal vesicles or in sperm counts of males born and reared on 16L:8D, 13L:11D, 11L:13D, or 8L:16D photoperiods, although there were small differences in body weight. Females born and reared on 16L:8D vs. 8L:16D cycles became pregnant at the same rates and ages when paired with males at 21 or 31 days of age. The daily duration of melatonin secretion depended on the length of the dark phase of the cycle in both sexes. Circulating levels of melatonin were elevated for 8 h on a 16L:8D cycle and for between 9 and 16 h on an 8L:16D cycle. In this tropical species, the neuroendocrine pathway that links photoperiod to reproduction apparently is disconnected somewhere between melatonin and gonadotropin secretion, causing cane mice to be reproductively unresponsive to variation in photoperiod.     

Photoperiod affects reproductive responsiveness to 6-methoxy-2-benzoxazolinone in house mice

House mice (Mus musculus) have traditionally been characterized as nonphotoperiodic because reproductive function is unaffected by day length in the laboratory. In the present study, the reproductive responsiveness of CF1 mice to a naturally occurring plant metabolite was assessed in animals that were maintained in either long (16L:8D) or short (8L:16D) photoperiods from birth until the end of the study. Males and females were implanted i.p. with either an empty Silastic capsule or one containing 6-methoxy-2-benzoxazolinone (6-MBOA) at either 21 or 60 days of age for 3 days. Other 31-day-old mice were implanted with capsules for 8 wk. Three-day exposure to 6-MBOA stimulated uterine growth in short-day, adult females, but did not affect adult females housed in long photoperiods. Short-term treatment with 6-MBOA did not significantly affect reproductive parameters in either long- or short-day peripubertal house mice, or in adult males regardless of photoperiod, nor did exposure to 6-MBOA for 8 wk influence reproduction in males in either photoperiodic condition. However, short-day female mice had significantly reduced ovarian and uterine masses after 8 wk chronic 6-MBOA treatment as compared to long-day animals or mice unexposed to 6-MBOA. Short-day females exposed to 6-MBOA for 8 wk developed a denser pelage compared to long-day mice treated with this compound. Photoperiod-mediated differential responsiveness to 6-MBOA indicates that female house mice can discriminate long from short days, and these results suggest that the physiological mechanisms for photoperiodic responsiveness remain extant in this species previously characterized as nonphotperiodic.     

Does leptin mediate the effect of photoperiod on immune function in mice?

Seasonal fluctuations in immune status have been documented for avian and mammalian populations. During the late summer and early fall, immune function is bolstered to help animals cope with the more physiologically demanding winter. The environmental cue for these seasonal changes is apparently decreasing photoperiod. In the present study, we determined the potential role of leptin in mediating the effect of photoperiod on cell-mediated immune responses in male mice. Leptin-deficient (ob/ob) and littermate control mice were housed for 10 wk in either a short (8L:16D) or a long (16L:8D) photoperiod beginning at 6 wk of age. After the mice were killed, immune and reproductive organs were weighed and splenocytes isolated. The proliferative and cytokine responses (interleukin [IL]-2 and IL-4) of splenocytes to the T-cell mitogen, concanavalin A (Con A; 0-40 microg/ml), were determined. Body weights were elevated and both testes and seminal vesicle weights subnormal in ob/ob mice (by ANOVA, main effect of leptin deficiency), but thymuses and spleens were of normal size. Serum leptin levels were at minimum detection limits in ob/ob mice, but leptin levels in control mice housed at 8L:16D were higher than in control mice housed at 16L:8D. The proliferative response of splenocytes from ob/ob mice to Con A was subnormal (by ANOVA, main effect of leptin deficiency), but photoperiod had no effect on this response. Production of IL-2 in splenocytes of ob/ob mice was subnormal (by ANOVA, main effect of leptin deficiency) irrespective of photoperiod, but cells from mice housed at 8L:16D (by ANOVA, main effect of photoperiod) produced more IL-2 than cells from animals housed at 16L:8D. In contrast, a leptin deficiency did not alter IL-4 production, but cells from animals (ob/ob and controls) housed at 16L:8D produced less IL-4 than cells from animals housed at 8L:16D (by ANOVA, main effect of photoperiod). The present study suggests that both photoperiod and leptin have mutually independent effects on the proliferation of lymphocytes and cytokine production profiles. The data do not provide definitive support for the hypothesis that photoperiod-induced changes in leptin secretion mediate the effects of season on immune status.     

Geographic differences for delay of sexual maturation in Peromyscus leucopus: effects of photoperiod, pinealectomy, and melatonin

Effects of short-day photoperiod, pinealectomy, and melatonin on sexual maturation were tested in Peromyscus leucopus from either Connecticut (CT) or Georgia (GA). Laboratory reared-stocks from CT and GA were exposed to short daylength (photoperiod) from birth or 25 days of age. At 12 wk of age, delay in sexual maturation was indicated in most CT mice by decreased testis length, combined testes weight, and seminal vesicle weight. Conversely, GA animals did not delay sexual maturation when exposed to short-day photoperiod from either birth or 25 days of age. These results indicate that responses to short daylengths differ for juvenile CT and GA populations. In a second experiment, pinealectomized or sham-operated CT males were exposed to short-day (9L:15D) or long-day (16L:8D) photoperiod from birth. Pinealectomy blocked the effect of short daylength on reproduction. Therefore, the pineal must be involved in the delay of sexual maturation observed for short-day CT mice. The effects of melatonin, a pineal gland hormone, were tested with chronic s.c. implants or daily injections. In CT mice given either melatonin implants or afternoon injections, sexual maturation was delayed. GA mice were insensitive to all melatonin treatments. Further, no differences in circadian organization (phase angle, duration of activity, period under constant dark) between GA and CT animals were apparent. Collectively, these studies indicate that melatonin is involved in the mechanism responsible for delay of sexual maturation in CT mice. Short-day insensitivity of GA Peromyscus leucopus probably results from a deficiency in the melatonin effector pathway and is not due to a disruption of circadian organization.     

Photoperiod influences the critical caloric intake necessary to maintain reproduction among male deer mice (Peromyscus maniculatus).

The concept of critical day length is well established among rodents; reproductive function is maintained when day lengths are greater than some specific threshold. In addition to day length cues, seasonal breeding in deer mice can also be regulated by food availability. The caloric threshold necessary to support reproduction remains unspecified for seasonally breeding rodents. The present study examined the interaction between photoperiod and food availability on reproductive function in adult male deer mice (Peromyscus maniculatus). A critical caloric intake profile was constructed in long (16L:8D) and short (8L:16D) photoperiods; groups of deer mice in both photoperiods either received food ad libitum or 90, 80, or 70% of their individual ad libitum food intake for 10 wk. At autopsy, paired testes, epididymides, and seminal vesicles were removed and weighed. Body mass, total body fat, and total body water contents were also obtained. Short, as compared to long, day lengths inhibited the reproductive systems of male deer mice. However, food consumption interacted with photoperiod to affect reproductive function. Significant reductions in reproductive organ size as well as spermatogenic activity were observed among short-day mice after a 10% reduction in ad libitum food intake. Long-day animals required a 20% reduction in caloric intake to depress reproductive function. Body mass and total body water content were generally unaffected by either photoperiod or food consumption. Total body fat content was reduced in short- as compared to long-day mice. Individual reproductive responsiveness to short days increased as food availability decreased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoperiodic stimulation of pubertal development in male deer mice: involvement of the circadian system

Pubertal development in prairie deer mice (Peromyscus maniculatus bairdii) is accelerated by exposure of juveniles to a long-day photoperiod, and, conversely, retarded by exposure to short days. The purpose of the present study was to evaluate the possible involvement of the circadian system in the photoperiodic regulation of puberty. Weanling males, previously housed on a short-day light cycle of 6L:18D, were subjected to a "resonance" protocol in which they received one of the following light cycles: 6L:18D, 6L:30D, 6L:42D, 6L:54D, or 16L:8D. Post-weaning exposure to cycles of 16L:8D, 6L:30D, and 6L:54D stimulated reproductive organ growth as measured at 6 weeks of age. Exposure to cycles of 6L:18D and 6L:42D failed to stimulate reproductive development. These data support the hypothesis that young male deer mice use a circadian rhythm of responsiveness to light to measure photoperiodic time and, consequently, regulate pubertal development.     

Characteristics of a genetic polymorphism for reproductive photoresponsiveness in the white-footed mouse (Peromyscus leucopus)

Wild populations of Peromyscus are often composed of individuals that vary greatly in their reproductive response to photoperiod. A population of white-footed mice (P. leucopus) from Michigan (43 degrees N) was subjected to mass selection in the laboratory both for and against reproductive photoresponsiveness for four generations. The first generation of selection yielded one line of mice in which about 80% of the individuals were classified as reproductively photoresponsive (i.e., with undeveloped reproductive tracts when reared in short days, 8L: 16D) and another in which only about 20% were reproductively photoresponsive. Some and perhaps most of this difference was accounted for by changes in degree of responsiveness to photoperiod rather than by alterations in the proportion of discrete responsive vs. unresponsive phenotypes. Alteration of critical day length was not a factor. Three more generations of selection failed to change the proportions noted above significantly. Although the genetic control of reproductive photoresponsiveness is undoubtedly complex, a single variable locus may be responsible for much of the heritable variation present in this population. These results also suggest that natural populations contain genetically determined phenotypes that are intermediate between absolutely photoresponsive and absolutely unresponsive. The factors that might promote maintenance of heterogeneity of reproductive photoresponsiveness in a wild population of rodents are considered.     

European hamsters (Cricetus cricetus) show a transient phase of insensitivity to long photoperiods after gonadal regression

Annual rhythms of body weight and reproduction in the European hamster (Cricetus cricetus) are the result of an interaction between seasonal changes in day length (photoperiod) and seasonal changes in the responsiveness of animals to these photoperiods. The present study demonstrates that under natural conditions European hamsters are not able to perceive long photoperiods (i.e., a 16L:8D cycle) before mid-November. This is an important difference to other hamster species, in which regrowth of the gonads can be stimulated by exposure to long photoperiods at any stage of gonadal regression. The experiments also demonstrate the existence of an annual phase of sensitivity to long photoperiods that starts around mid-November and extends until March/April. During this phase of sensitivity, exposure to a long photoperiod (16L:8D) induced gonadal regrowth within 3 wk. Additional experiments with an accelerated photoperiodic lighting regimen indicated that a photoperiod of approximately 13 h is necessary to stimulate gonadal regrowth. Under natural light conditions in Stuttgart (48.46 degrees N), a photoperiod of 13 h is reached by the beginning of April, which fits well with the finding that the majority of animals kept under a natural light:dark cycle had well-developed gonads by the end of April. Nevertheless, these animals showed a rather variable timing of gonadal regrowth, ranging from early January to late April. This is most likely the result of two processes: first, an endogenous mechanism (photorefractoriness) that induces gonadal recrudescence without any photoperiodic information while the animals are still in their hibernation burrows, and second, a direct stimulatory effect of long photoperiods.     

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.     

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 leucopus recently 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.     

Effect of various photoperiods on testicular weight, weekly sperm output and plasma levels of LH and testosterone over the reproductive season in male turkeys

The effects of duration and variation in photoperiod on testis weight, testicular sperm production, semen output, and hormone status over the reproductive season in male turkeys were investigated. In Experiment 1, four groups of males raised from 17 to 23 wk of age under a constant short photoperiod were subjected to a constant short (Group 1: 7L:17D; Group 2: 10.5L:13.5D), constant long (Group 3: 14L:10D) or progressively increasing photoperiod (Group 4: 7L:17D to 14L:10D) up to 60 wk of age. In Experiment 2, four groups of males first raised as in Experiment 1 up to 23 wk of age were placed under a constant short (Group 5: 10.5L:13.5D), constant long (Group 6: 14L:10D), or night-interrupted photoperiod (Group 7: 6L:2.5D:1L:14.5D, referred to as subjective 9.5L:14.5D; Group 8: 6L:3.5D:1L:13.5D), referred to as subjective 10.5L:13.5D) up to 60 wk of age. Males in Groups 2-4 had similar reproductive characteristics, whereas sexual maturity was delayed from 29 to 49 wk in males from Group 1. In Experiment 2, males in Groups 5 and 8 had similar reproductive characteristics, whereas sexual maturity was delayed in males in Group 7 in a manner similar to that observed in Group 1. In both experiments, plasma LH and testosterone concentrations were poor indicators of testis development and semen production, irrespective of age and photoperiod. We conclude that a moderately short photoperiod such as 10.5L:13.5D or subjective 10.5L:13.5D may stimulate reproductive characteristics of male turkeys in a manner comparable to constant long or increasing photoperiods. We inferred the existence of a threshold of photosensitivity in male turkeys for photoperiods longer than 9.5L:14.5D, but shorter than or equal to 10.5L:13.5D.     

Sustained reproductive responses in Djungarian hamsters (Phodopus sungorus) exposed to a single long day

Male and female Djungarian hamsters maintained from birth in a short photoperiod (8 h light per day; 8L:16D) showed substantial testicular and uterine growth in response to a single long photoperiod or a 15-min light pulse that interrupted the 16-h dark period at 18 days of age. These light regimens resulted in heavier testes and uteri at 30 and 35 days of age when compared with those of control animals. Similar results were obtained in hamsters maintained from birth to Day 18 in a long photoperiod (16L:8D), given a single longer day (20L:4D) or constant light on Day 18 and then transferred to a short photoperiod (8L:16D) on Day 19. At 35 days of age animals that received extended light treatment on Day 18 had significantly more developed reproductive structures than did control hamsters. The marked effects of brief light treatment in producing long-term changes in the reproductive axis provide a convenient mammalian model system in which to study neuroendocrine events that underlie photoperiodism.     

Termination of gonadal refractoriness in Turkish hamsters, Mesocricetus brandti

The Turkish hamster (Mesocricetus brandti) is a photoperiodic species. In this investigation, we characterized the photoperiodic requirements for termination of gonadal refractoriness, defined as the inability of the animal to respond to short-day treatment with gonadal regression. Paired testes weights were reduced to less than 20% of their original weight by 10 wk of 12L:12D treatment. This was followed by spontaneous testicular recrudescence (completed by Week 25 of 12L:12D treatment), the overt indication of refractoriness to short photoperiods. Next, the period of long-day exposure sufficient for termination of refractoriness was determined. Refractory males were exposed to 16L:8D for 5 to 20 wk. Ten weeks of 16L:8D treatment was enough for the animals to regain the sensitivity to a second challenge of 12L:12D treatment. Fifteen weeks of 20L:4D or 16L:8D terminated refractoriness in female Turkish hamsters; 20L:4D therefore was not interpreted as a short day by refractory hamsters. This was unexpected because in photosensitive animals this photoperiod acts like a short day, causing gonadal regression. These results suggest that Turkish hamsters are similar to Syrian hamsters in that both species require two or more months of long days in summer to recover sensitivity to the short days of the following fall.     

Estrogen modifies the circadian timing and amplitude of the luteinizing hormone surge in female hamsters exposed to short photoperiods

LH surges occur 3 h later in intact anovulatory hamsters exposed to nonstimulatory photoperiods (6L:18D) for 8 wk than the proestrous LH surges from the same hamsters housed in 6L:18D for 3 weeks. In ovariectomized hamsters housed in 6L:18D for 3 wk, the LH surge was observed at the same time of day as in intact anovulatory hamsters at 8 wk. Implanting Silastic capsules containing estradiol benzoate (EB) advanced the timing of the daily surge of LH in ovariectomized hamsters housed in 6L:18D for 8 wk. EB also affected the magnitude of the LH surge in hamsters housed in 6L:18D for 8 wk. Two days after receiving EB implants, daily LH surges in anovulatory hamsters were suppressed by 75% and in ovariectomized "regressed" hamsters by 37%. This difference between groups was probably due to ovarian progesterone in intact animals. Estrogen is not required for LH surges in anovulatory hamsters but suppresses LH release when administered exogenously. The delay in the timing of the LH surge in anovulatory hamsters may result from the decline in estrogen resulting from short photoperiod exposure.     

Prior experience with photostimulation enhances photo-induced reproductive development in female European starlings: a possible basis for the age-related increase in avian reproductive performance

Reproductive performance in female birds improves with age, and this is generally attributed to experiences obtained during breeding. In temperate-zone species, experience with photostimulation during the first breeding year may prime the hypothalamo-pituitary-gonadal axis to respond to photic cues more rapidly or robustly in subsequent years. To test this idea, we captured 32 photorefractory juvenile (hence naive to photostimulation) female European starlings (Sturnus vulgaris) and held half of them (naive group) on a photoperiod of 8L:16D for 32 wk and the other half (experienced group) on 8L:16D for 12 wk, 16L:8D for 12 wk, and then 8L:16D for 8 wk. When we subsequently transferred all birds to 16L:8D, the increase in body mass, which may presage egg laying in the wild, was more robust in experienced than in naive females. Experienced females also showed a more robust elevation in plasma concentrations of the yolk-precursor protein vitellogenin, although naive females showed an initial rapid but transient rise in vitellogenin that we attribute to their extended exposure to short-day photoperiods prior to photostimulation. Finally, the photo-induced increase in diameter of the largest ovarian follicle, in plasma concentrations of luteinizing hormone, and in the number of septo-preoptic fibers relative to the number of cell bodies immunoreactive to GnRH was greater in experienced than in naive females. Thus, prior experience with photostimulation enhances some initial phases of photo-induced reproductive development and may explain, in part, why reproductive performance improves with age in temperate-zone birds.     

Maternal transfer of photoperiodic information influences the photoperiodic response of prepubertal Djungarian hamsters (Phodopus sungorus sungorus)

Daylengths during the spring are repeated in reverse order in the autumn. For some photoperiodic species, a given photoperiod may be stimulatory for reproduction in the spring and inhibitory in the autumn. The mechanisms regulating this type of seasonal response have, until recently, remained a mystery. Horton (1984a) showed in Microtus montanus that the photoperiod experienced by the mother influences the gonadal development of her young after weaning. To determine if this phenomenon is characteristic of other photoperiodic rodents, adult Djungarian hamsters were paired on 16L:8D, 14L:10D, or 12L:12D. Young males born from these pairings were killed at 15, 28, and 34 days of age to assess gonadal development (testes weight). At 15 days testicular development was identical in all groups; by 28 days, however, males raised in 16L:8D or 14L:10D exhibited a greater degree of testicular development than those raised in 12L:12D. Next, females maintained on each of the three photoperiods throughout gestation were transferred, with their offspring, to the other two photoperiods at birth. Postnatal exposure to 14L:10D or 12L:12D inhibited testicular development in young that had been gestated on 16L:8D. Both 16L:8D and 14L:10D stimulated testicular growth in animals that had been gestated on 12L:12D or 14L:10D. Therefore, a) 16L:8D stimulates testicular growth in all animals, b) 12L:12D inhibits testicular growth in all animals, and c) the testicular response to 14L:10D depends on the photoperiod experienced by the mother during pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus.

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.     

光照周期对叙利亚地鼠生长发育和繁殖的影响

对离乳雄性叙利亚地鼠（Ｍｅｓｏｃｒｉｃｅｔｕｓａｕｒａｔｕｓ）和雌性叙利亚地鼠各３０只分别给以长光照（１６Ｌ：８Ｄ）、中光照（１２Ｌ：１２Ｄ）和短光照（８Ｌ：１６Ｄ）三种光周期处理,研究光周期对叙利亚地鼠生长发育和繁殖的影响。１３周龄时,短光照处理组雄性地鼠的体重显著（Ｐ＜０．０５）高于长光照和中光照组的体重;雌性地鼠９周龄时长光照组的体重显著（Ｐ＜０．０１）高于另外两个处理组的体重,较高的日食量可能是长光照组具有较大体重的原因之一,雄性地鼠的睾丸、副睾和肾上腺重量及雌性地鼠的卵巢、子宫和肾上腺重量表现了随着光照时间的延长而递增的趋势。长光照条件有利于雌性地鼠的正常发情和繁殖。每天１６ｈ的光照对繁殖地鼠是比较适宜的。     

Latitude of origin influences photoperiodic control of reproduction of deer mice (Peromyscus maniculatus)

Three subspecies of Peromyscus maniculatus originating from different latitudes were maintained from birth in light dark cycles that provided between 10 and 18 h of light per day. At 50 days of age, Chihuahua, Mexico mice (latitude of origin 27 degrees N) and South Dakota, U.S.A. mice (44 degrees N) kept in the 10L:14D photoperiod had reduced gonadal and seminal vesicle weights and a lower spermatogenic index than corresponding mice kept in a 14L:10D photoperiod. Some Chihuahua and South Dakota mice, apparently constituting nonphotoperiodic subpopulations, developed their gonads while kept in the short-day photoperiod. The critical day length for stimulation of sexual maturation was greater for mice from Manitoba, Canada (55 degrees N) than for mice from the lower latitudes. At 70 days of age, testes and seminal vesicle weights, and the spermatogenic index of Manitoba mice in the 14L:10D photoperiod, were lower than those of animals maintained in 16L:8D and 18L:6D photoperiods. Responsiveness to short day lengths was greater among adult South Dakota than adult Chihuahau mice and melatonin treatment significantly reduced testes weights of South Dakota but not of Chihuahua adult mice. Photoperiodic regulation of the reproductive system varies with latitude of origin. Differences in the critical day length necessary for stimulating development of functional reproductive activity and variations in the percent of photoperiodic animals within each subspecies, appear to contribute to latitudinal gradients in reproduction.     

Short-day enhancement of immune function is independent of steroid hormones in deer mice (Peromyscus maniculatus)

The effects of photoperiod and steroid hormones on immune function were assessed in male and female deer mice (Peromyscus maniculatus). In experiment 1, male deer mice were castrated, castrated and given testosterone replacement, or sham-operated. Half of each experimental group were subsequently housed in either long (LD 16:8) or short days (LD 8:16) for 10 weeks. Short-day deer mice underwent reproductive regression and displayed elevated lymphocyte proliferation in response to the T-cell mitogen concanavalin A, as compared to long-day mice. In experiment 2, female deer mice were ovariectomized, ovariectomized and given estrogen replacement, or sham-operated. Animals from each of these experimental groups were subsequently housed in either LD 16:8 or LD 8:16 for 10 weeks. Short-day deer mice underwent reproductive regression and displayed reduced serum estradiol concentrations and elevated lymphocyte proliferation in response to concanavalin A, as compared to long-day mice. Surgical manipulation had no effect on lymphocyte proliferation in either male or female deer mice. Neither photoperiod nor surgical manipulation affected serum corticosterone concentrations. These results confirm that both male and female deer mice housed in short days enhance immune function relative to long-day animals. Additionally, short-day elevation in splenocyte proliferation appears to be independent of the influence of steroid hormones in this species. Accepted: 17 April 1998