Seasonal and fasting induced changes in iron metabolism in Djungarian hamsters.

Djungarian hamsters are small rodents that show pronounced physiological acclimations in response to changes in photoperiod, and unfavorable environmental conditions such as reduced food availability and low external temperature. These include substantial adjustments, such as severe body weight loss and the use of daily torpor. Torpor is a state of decreased physiological activity in eutherms, usually marked by low metabolic rate and a reduced body temperature. In this study, we investigated the effects of photoperiodic acclimation and food deprivation on systemic iron metabolism in Djungarian hamsters. Our study illustrates the association between liver iron levels and the incidence of torpor expression during the course of the experiment. Moreover, we show that both, acclimation to short photoperiods and long-term food restriction, associated with iron sequestration in the liver. This effect was accompanied with hypoferremia and mild reduction in the expression of principal iron-hormone, hepcidin. In addition to iron, the levels of manganese, selenium, and zinc were increased in the liver of hamsters under food restriction. These findings may be important factors for regulating physiological processes in hamsters, since iron and other trace elements are essential for many metabolic and physiological processes.

Sex and seasonal differences in neural steroid sensitivity predict territorial aggression in Siberian hamsters.

Many animals display marked changes in physiology and behavior on a seasonal timescale, including non-reproductive social behaviors (e.g., aggression). Previous studies from our lab suggest that the pineal hormone melatonin acts via steroid hormones to regulate seasonal aggression in Siberian hamsters (Phodopus sungorus), a species in which both males and females display increased non-breeding aggression. The neural actions of melatonin on steroids and aggressive behavior, however, are relatively unexplored. Here, we housed male and female hamsters in long-day photoperiods (LDs, characteristic of breeding season) or short-day photoperiods (SDs, characteristic of non-breeding season) and administered timed melatonin (M) or control injections. Following 10 weeks of treatment, we quantified aggressive behavior and neural steroid sensitivity by measuring the relative mRNA expression of two steroidogenic enzymes (aromatase and 5α-reductase 3) and estrogen receptor 1 in brain regions associated with aggression or reproduction [medial preoptic area (MPOA), anterior hypothalamus (AH), arcuate nucleus (ARC), and periaqueductal gray (PAG)] via quantitative PCR. Although LD-M and SD males and females displayed increased aggression and similar changes in gene expression in the ARC, there were sex-specific effects of treatment with melatonin and SDs on gene expression in the MPOA, AH, and PAG. Furthermore, males and females exhibited different relationships between neural gene expression and aggression in response to melatonin and SDs. Collectively, these findings support a role for melatonin in regulating seasonal variation in neural steroid sensitivity and aggression and reveal how distinct neuroendocrine responses may modulate a similar behavioral phenotype in male and female hamsters.

Photoperiod alters testicular methyltransferase complex mRNA expression in Siberian hamsters.

Exposure to long photoperiods stimulates, whereas exposure to short photoperiods transiently inhibit testicular function in Siberian hamsters via well-described neuroendocrine mechanisms. However, less is known about the intra-testicular regulation of these photoperiod-mediated changes. N6-methyladenosine (m6A) is one of the most common mRNA modifications in eukaryotes, with alterations in m6A mRNA methylation affecting testis function and fertility. We hypothesized that genes controlling m6A methylation such as methyltransferase-like-3 (Mettl3) and -14 (Mettl14) and Wilms' tumor-1 associated protein (Wtap), part of an mRNA methylating methyl-transferase complex, or the fat-mass-and-obesity-associated (Fto) and the α-ketoglutarate-dependent dioxygenase alkB homolog-5 (Alkbh5) genes responsible for m6A demethylation, may be differentially regulated by photoperiod in the testis. Male hamsters were exposed to long (LD, control) photoperiod for 14-weeks, short (SD) photoperiod for 2, 5, 8, 11 and 14-weeks to induce regression, or SD for 14-weeks followed by transfer to LD for 1, 2, 4 or 8-weeks to induce recrudescence (post-transfer, PT). SD exposure significantly reduced body, testis, and epididymal masses compared to all other groups. Spermatogenic index, seminiferous tubule diameters and testosterone concentrations significantly decreased in SD as compared to LD, returning to levels no different than LD in post-transfer groups. SD exposure significantly decreased Wtap, Fto, Alkbh5, but increased Mettl14 mRNA expression as compared to LD, with values in PT groups restored to LD levels. Mettl3 mRNA expression did not change. These results suggest that testicular recovery induced by stimulatory photoperiod is relatively rapid, and that the methyltransferase complex may play a role during photostimulated testicular recrudescence.

Estradiol and estrogen receptor α in the mPOA and MeA in dwarf hamster (Phodopus campbelli) fathers.

E2 and its alpha receptor (ERα) have an essential role in the regulation of maternal behavior. In dwarf hamster (Phodopus campbelli), E2 facilitates the display of paternal care, and it is possible that ERα is part of the neuroendocrine mechanisms that regulate this behavior. The aim of this study was to analyze the influence of copulation, cohabitation with the pregnant mate and the presence of the pups on paternal behavior, circulating E2 levels and the presence of ERα in the medial preoptic area (mPOA) and medial amygdala (MeA) in dwarf hamsters. Eight males were mated with intact females (IFs), 8 with tubally ligated females (TLFs) and 8 with ovariectomized females (OFs). In males mated with IFs, paternal behavior tests were performed after copulation, halfway through pregnancy and 24 h after the birth of their pups. Males mated with TLFs were subjected to paternal behavior tests at equivalent periods as the males mated with IFs. In males mated with OFs, paternal behavior tests were performed on days 1, 5 and 10 of cohabitation. After the last paternal behavior tests, blood samples were taken for quantification of E2 by radioimmunoassay (RIA), and the brains were dissected to determine ERα immunoreactivity (ir) in the mPOA and MeA. Fathers mated with IFs had higher serum E2 concentrations and more ERα-ir cells in the mPOA than those of males mated with TLFs and OFs. These results suggest that E2 and its ERα may be associated with paternity in the dwarf hamster.

Prepubertal ovarian inhibition of Light/Dark Box exploration and novel object investigation in juvenile Siberian hamsters.

The overwhelming majority of research on the role of gonadal hormones in behavioral development has focused on perinatal, pubertal, or adult life stages. The juvenile period has been overlooked because it is thought to be a time of gonadal quiescence. In the present study, we tested whether prepubertal gonadectomy impacts the behavior of male and female juvenile hamsters on the Light/Dark Box, Novel Object, and Social Approach tests (Experiment 1) and compared these findings to those obtained after adult gonadectomy (Experiment 2). Prepubertal ovariectomy increased exploration (i.e. time spent in the light zone of the Light/Dark Box) and novel object investigation of juveniles indicating an inhibitory role for the juvenile ovary; social approach was unaffected. In contrast, adult ovariectomy and castration (both prepubertal and adult) had no effect on any behavioral measure. Experiment 3 tested whether rearing hamsters in a short day length (SD), which delays puberty in this species, extends the interval of juvenile ovarian inhibition on exploration and novelty seeking. We also tested whether provision of estradiol reverses the effects of prepubertal ovariectomy. Hormonal manipulations and behavioral tests of Experiment 3 were conducted at ages when long day-reared hamsters are adult (as in Experiment 2), but SD-reared hamsters remain reproductively immature. Ovariectomy again increased exploration in the SD-reared juveniles despite the older age of surgery and testing. Estradiol treatment had no effect. These findings reveal a novel role for the juvenile ovary in exploration and novelty seeking that is unlikely to be mediated exclusively by estradiol.

Kisspeptin and RFRP3 modulate body mass in Phodopus sungorus via two different neuroendocrine pathways.

Many animals exhibit remarkable metabolic and reproductive adaptations to seasonal changes in their environment. When day length shortens, Djungarian hamsters (Phodopus sungorus) reduce their body weight and inhibit their reproductive activity, whereas the opposite occurs in springtime. These physiological adaptations are considered to depend on photoperiodic changes in hypothalamic genes encoding the peptides kisspeptin (Kp) and RFamide-related peptide 3 (RFRP3) for the control of reproduction, as well as pro-opiomelanocortin and somatostatin for metabolic regulation. The present study investigates the effect of Kp and RFRP3 on long-term body weight regulation, aiming to establish whether metabolic and reproductive hypothalamic networks may interact during adaptation to seasonal physiology. We found that chronic central administration of both Kp and RFRP3 in short photoperiod-adapted male Djungarian hamsters increased body weight, although via different pathways. The effect of Kp was dependent on testicular activity because castration prevented the body weight increase and was associated with an increase in pro-opiomelanocortin and neuropeptide Y expression. On the other hand, the orexigenic effect of RFRP3 was associated with an increase in circulating insulin and leptin levels, although it had no effect on any of the hypothalamic metabolic genes investigated, and did not change circulating levels of sex steroids. Notably, neither Kp, nor RFRP3 altered female hamster metabolic parameters. Thus, using a rodent model exhibiting seasonal changes in reproduction and metabolism, the present study demonstrates that, in addition to its role in the central control of reproduction, Kp also participates in body weight control in a sex-dependent manner via an anabolic action of testosterone. Conversely, RFRP3 affects body weight control in males mostly by acting on adiposity, with no overt effect on the reproductive system in both sexes.

Endotoxin rapidly desensitizes the gonads to kisspeptin-induced luteinizing hormone release in male Siberian hamsters (Phodopus sungorus).

Activation of the immune system induces rapid reductions in hypothalamic-pituitary-gonadal (HPG) axis activity, which in turn decreases secretion of sex steroids. This response is likely adaptive for survival by temporarily inhibiting reproduction to conserve energy; however, the physiological mechanisms controlling this response remain unclear. The neuropeptide kisspeptin is a candidate to mediate the decrease in sex hormones seen during sickness through its key regulation of the HPG axis. In this study, the effects of acute immune activation on the response to kisspeptin were assessed in male Siberian hamsters (Phodopus sungorus). Specifically, an immune response was induced in animals by a single treatment of lipopolysaccharide (LPS), and reproductive hormone concentrations were determined in response to subsequent injections of exogenous kisspeptin. Saline-treated controls showed a robust increase in circulating testosterone in response to kisspeptin; however, this response was blocked in LPS-treated animals. Circulating luteinizing hormone (LH) levels were elevated in response to kisspeptin in both LPS- and saline-treated groups and, thus, were unaffected by LPS treatment, suggesting gonad-level inhibition of testosterone release despite central HPG activation. In addition, blockade of glucocorticoid receptors by mifepristone did not attenuate the LPS-induced inhibition of testosterone release, suggesting that circulating glucocorticoids do not mediate this phenomenon. Collectively, these findings reveal that acute endotoxin exposure rapidly renders the gonads less sensitive to HPG stimulation, thus effectively inhibiting sex hormone release. More broadly, these results shed light on the effects of immune activation on the HPG axis and help elucidate the mechanisms controlling energy allocation and reproduction.

An increase in estradiol facilitates the onset of paternal behavior in the dwarf hamster (Phodopus campbelli).

In the dwarf hamster (Phodopus campbelli), activational effects of testosterone (T) and estradiol (E2) in the regulation of paternal behavior have been repeatedly rejected because peripheral concentrations of E2 do not change across the reproductive cycle of males. Further, castration no affected paternal behavior despite that both T and E2 concentrations decreased significantly. However, the role of these hormones has not been evaluated in models of castration and hormonal replacement in virgin males. Here, we analysed the effects of E2 and T in paternal behavior in virgin male dwarf hamster (Phodopus campbelli). Thirty paternal (PAT) males were bilaterally castrated; of them, 10 were implanted with T, 10 with E2 and 10 males received no treatment. Other 10 PAT males underwent sham-castration. Seventeen aggressive (AGG) males were also bilaterally castrated; of these, 10 AGG received E2 replacement, 7 were not treated. Other 7 AGG males were submitted to sham-castration. Following treatments, paternal behavior tests were conducted again. T and E2 levels in plasma were quantified by radioimmunoassay (RIA). The results showed that the treatments did not affect the paternal behavior of males that were initially paternal. Neither castration nor sham-castration surgery affected the behavior of AGG males. However, when these males were treated with E2 and the concentrations of this hormone increase significantly they became paternal. Our data suggest that an increase in E2 levels shifted infanticidal behavior to paternal behavior in dwarf hamster.

Photoperiod- and Triiodothyronine-dependent Regulation of Reproductive Neuropeptides, Proinflammatory Cytokines, and Peripheral Physiology in Siberian Hamsters (Phodopus sungorus).

Seasonal trade-offs in reproduction and immunity are ubiquitous in nature. The mechanisms that govern transitions across seasonal physiological states appear to involve reciprocal switches in the local synthesis of thyroid hormone. In long-day (LD) summer-like conditions, increased hypothalamic triiodothyronine (T3) stimulates gonadal development. Alternatively, short-day (SD) winter-like conditions increase peripheral leukocytes and enhance multiple aspects of immune function. These data indicate that the localized effects of T3 in the hypothalamus and leukocytes are photoperiod dependent. We tested the hypothesis that increased peripheral T3 in SD conditions would increase aspects of reproductive physiology and inhibit immune function, whereas T3 injections in LD conditions would facilitate aspects of immune function (i.e., leukocytes). In addition, we also examined whether T3 regulates hypothalamic neuropeptide expression as well as hypothalamic and splenic proinflammatory cytokine expression. Adult male Siberian hamsters were maintained in LD (15L:9D) or transferred to SD (9L:15D) for 8 weeks. A subset of LD and SD hamsters was treated daily with 5 µg T3 for 2 weeks. LD and SD controls were injected with saline. Daily T3 administration in SD hamsters (SD+T3) resulted in a rapid and substantial decrease in peripheral leukocyte concentrations and stimulated gonadal development. T3 treatment in LD (LD+T3) had no effect on testicular volumes but significantly increased leukocyte concentrations. Molecular analyses revealed that T3 stimulated interleukin 1ß messenger RNA (mRNA) expression in the spleen and inhibited RFamide Related Peptide-3 mRNA expression in the hypothalamus. Moreover, there was a photoperiod-dependent decrease in splenic tumor necrosis factor-α mRNA expression. These findings reveal that T3 has tissue-specific and photoperiod-dependent regulation of seasonal rhythms in reproduction and immune function.

Rapid changes in ovarian mRNA induced by brief photostimulation in Siberian hamsters (Phodopus sungorus).

This study sought to characterize the rapid intraovarian mRNA response of key folliculogenic factors that may contribute to the restoration of folliculogenesis during 2-10 days of photostimulation in Siberian hamsters. Adult hamsters were exposed to short photoperiod (8L:16D) for 14 weeks (SD). A subset were then transferred to long photoperiod (16L:8D) for 2 (PT day-2), 4 (PT day-4), or 10 days (PT day-10). Quantitative real-time PCR was used to measure intraovarian mRNA expression of: gonadotropin releasing hormone (GnRH), follicle stimulating hormone ß-subunit (FSHß-subunit), luteinizing hormone ß-subunit (LHß-subunit), FSH and LH receptors, estrogen receptors α and ß (Esr1 and Esr2), matrix metalloproteinase (MMP)-2 and -9, anti-Müllerian hormone (AMH), inhibin-α subunit, fibroblast growth factor-2 (FGF-2) and proliferating cell nuclear antigen (PCNA). Compared to SD, plasma FSH concentrations increased on PT day-4 and the number of antral follicles and corpora lutea increased on PT day-10. FSHR and inhibin-α mRNA expression also increased on PT day-4, whereas LHR and proliferation marker PCNA both increased on PT day-10 as compared to SD. Esr1 mRNA increased on PT day-2 and remained significantly increased as compared to SD, whereas Esr1 mRNA increased only on PT day-2, similar to FGF-2 and MMP-2 results. No differences were observed in mRNA expression in ovarian GnRH, FSHß- and LHß-subunits, AMH, and MMP-9 mRNA with 2-10 days of photostimulation. Rapid increases in intraovarian FSHR and inhibin-α mRNA and antral follicle/corpora lutea numbers suggest that the ovary is primed to react quickly to the FSH released in response to brief periods of photostimulation.

Embryo cryopreservation and in vitro culture of preimplantation embryos in Campbell's hamster (Phodopus campbelli).

The aims of this study were to compare different protocols of Campbell's hamster (Phodopus campbelli) embryos freezing-thawing and to explore the possibilities of their in vitro culture. First, the embryos were flushed from the reproductive ducts 2 days post coitum at the two-cell stage and cultured in rat one-cell embryo culture medium (R1ECM) for 48 hours. Most (86.7%) of the two-cell embryos developed to blastocysts in R1ECM. Second, the embryos at the two- to eight-cell stages were flushed on the third day post coitum. The eight-cell embryos were frozen in 0.25 mL straws according to standard procedures of slow cooling. Ethylene glycol (EG) was used either as a single cryoprotectant or in a mixture with sucrose. The survival of frozen-thawed embryos was assessed by double staining with fluorescein diacetate and propidium iodide. The use of EG as a single cryoprotectant resulted in fewer alive embryos when compared with control (fresh embryos), but combined use of EG and sucrose improved the survival rate after thawing. Furthermore, granulocyte-macrophage colony-stimulating factor rat (2 ng/mL) improved the rate of the hamster frozen-thawed embryo development in vitro by increasing the final cell number and alleviating nuclear fragmentation. Our data show the first attempt in freezing and thawing Campbell's hamster embryos and report the possibility of successful in vitro culture for this species in R1ECM supplemented with granulocyte-macrophage colony-stimulating factor.

Adaptation to short photoperiods augments circadian food anticipatory activity in Siberian hamsters.

This article is part of a Special Issue "Energy Balance". Both the light-dark cycle and the timing of food intake can entrain circadian rhythms. Entrainment to food is mediated by a food entrainable circadian oscillator (FEO) that is formally and mechanistically separable from the hypothalamic light-entrainable oscillator. This experiment examined whether seasonal changes in day length affect the function of the FEO in male Siberian hamsters (Phodopus sungorus). Hamsters housed in long (LD; 15 h light/day) or short (SD; 9h light/day) photoperiods were subjected to a timed-feeding schedule for 10 days, during which food was available only during a 5h interval of the light phase. Running wheel activity occurring within a 3h window immediately prior to actual or anticipated food delivery was operationally-defined as food anticipatory activity (FAA). After the timed-feeding interval, hamsters were fed ad libitum, and FAA was assessed 2 and 7 days later via probe trials of total food deprivation. During timed-feeding, all hamsters exhibited increases FAA, but FAA emerged more rapidly in SD; in probe trials, FAA was greater in magnitude and persistence in SD. Gonadectomy in LD did not induce the SD-like FAA phenotype, indicating that withdrawal of gonadal hormones is not sufficient to mediate the effects of photoperiod on FAA. Entrainment of the circadian system to light markedly affects the functional output of the FEO via gonadal hormone-independent mechanisms. Rapid emergence and persistent expression of FAA in SD may reflect a seasonal adaptation that directs behavior toward sources of nutrition with high temporal precision at times of year when food is scarce.

Increased photic sensitivity for phase resetting but not melatonin suppression in Siberian hamsters under short photoperiods.

Light regulates a variety of behavioral and physiological processes, including activity rhythms and hormone secretory patterns. Seasonal changes in the proportion of light in a day (photoperiod) further modulate those functions. Recently, short (SP) versus long days (LP) were found to markedly increase light sensitivity for phase shifting in Syrian hamsters. To our knowledge, photoperiod effects on light sensitivity have not been studied in other rodents, nor is it known if they generalize to other circadian responses. We tested whether photic phase shifting and melatonin suppression vary in Siberian hamsters maintained under LP or SP. Select irradiances of light were administered, and shifts in activity were determined. Photic sensitivity for melatonin suppression was examined in a separate group of animals via pulses of light across a 4 log-unit photon density range, with post-pulse plasma melatonin levels determined via RIA. Phase shifting and melatonin suppression were greater at higher irradiances for both LP and SP. The lower irradiance condition was below threshold for phase shifts in LP but not SP. Melatonin suppression did not vary by photoperiod, and the half saturation constant for fitted sigmoid curves was similar under LP and SP. Thus, the photoperiodic modulation of light sensitivity for phase shifting is conserved across two hamster genera. The dissociation of photoperiod effects on photic phase shifting and melatonin suppression suggests that the modulation of sensitivity occurs downstream of the common retinal input pathway. Understanding the mechanistic basis for this plasticity may yield therapeutic targets for optimizing light therapy practices.

Dim light at night disrupts the short-day response in Siberian hamsters.

Photoperiodic regulation of physiology, morphology, and behavior is crucial for many animals to survive seasonally variable conditions unfavorable for reproduction and survival. The photoperiodic response in mammals is mediated by nocturnal secretion of melatonin under the control of a circadian clock. However, artificial light at night caused by recent urbanization may disrupt the circadian clock, as well as the photoperiodic response by blunting melatonin secretion. Here we examined the effect of dim light at night (dLAN) (5lux of light during the dark phase) on locomotor activity rhythms and short-day regulation of reproduction, body mass, pelage properties, and immune responses of male Siberian hamsters. Short-day animals reduced gonadal and body mass, decreased spermatid nuclei and sperm numbers, molted to a whiter pelage, and increased pelage density compared to long-day animals. However, animals that experienced short days with dLAN did not show these short-day responses. Moreover, short-day specific immune responses were altered in dLAN conditions. The nocturnal activity pattern was blunted in dLAN hamsters, consistent with the observation that dLAN changed expression of the circadian clock gene, Period1. In addition, we demonstrated that expression levels of genes implicated in the photoperiodic response, Mel-1a melatonin receptor, Eyes absent 3, thyroid stimulating hormone receptor, gonadotropin-releasing hormone, and gonadotropin-inhibitory hormone, were higher in dLAN animals than those in short-day animals. These results suggest that dLAN disturbs the circadian clock function and affects the molecular mechanisms of the photoperiodic response.

Oestrus cycle of the Desert hamster (Phodopus roborovskii, Satunin, 1903).

The Desert hamster, Phodopus roborovskii, is one of the lesser-known laboratory animal models and therefore knowledge of its reproductive system and physiology is limited. This study investigated the time course of vaginal and behavioural oestrus cycles by means of vaginal smear cytology, serum luteinizing hormone (LH) levels, wheel-running activity patterns, and pairing tests. The oestrus cycle lasts between four and six days and follows a pattern rather similar to that described for other rodent species. An important finding of this study is the existence of a fifth cycle stage between pre-oestrus and oestrus, the so-called early oestrus. Early oestrus differs from pre-oestrus by a complete lack of leucocytes in the vaginal cytology and a dramatic increase of serum LH levels just before activity onset. The early oestrus stage lasts 4-6 h, but was not observed in every female. When present it affects the length of the whole oestrus cycle. With early oestrus the pre-oestrus stage lasts only 14-18 h and the total length of the oestrus cycle length is four days. Without early oestrus, the pre-oestrus is prolonged to 18-36 h and the oestrus cycle length varies between four and six days. Desert hamsters showed only subtle oestrus-correlated changes in wheel-running activity, i.e. they failed to show the characteristic scalloping of activity onset, but showed prolonged activity during early oestrus. Pairing tests revealed characteristic changes in the relative frequencies of socio-positive, neutral, aggressive and sexual behaviour during the course of the oestrus cycle, with an elevated level of sexual behaviour during oestrus.

Sleep deprivation attenuates endotoxin-induced cytokine gene expression independent of day length and circulating cortisol in male Siberian hamsters (Phodopus sungorus).

Sleep is restorative, whereas reduced sleep leads to negative health outcomes, such as increased susceptibility to disease. Sleep deprivation tends to attenuate inflammatory responses triggered by infection or exposure to endotoxin, such as bacterial lipopolysaccharide (LPS). Previous studies have demonstrated that Siberian hamsters (Phodopus sungorus), photoperiodic rodents, attenuate LPS-induced fever, sickness behavior and upstream pro-inflammatory gene expression when adapted to short day lengths. Here, we tested whether manipulation of photoperiod alters the suppressive effects of sleep deprivation upon cytokine gene expression after LPS challenge. Male Siberian hamsters were adapted to long (16 h:8 h light:dark) or short (8 h:16 h light:dark) photoperiods for >10 weeks, and were deprived of sleep for 24 h using the multiple platform method or remained in their home cage. Hamsters received an intraperitoneal injection of LPS or saline (control) 18 h after starting the protocol, and were killed 6 h later. LPS increased liver and hypothalamic interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF) gene expression compared with vehicle. Among LPS-challenged hamsters, sleep deprivation reduced IL-1 mRNA levels in liver and hypothalamus, but not TNF. IL-1 attenuation was independent of circulating baseline cortisol, which did not increase after sleep deprivation. Conversely, photoperiod altered baseline cortisol, but not pro-inflammatory gene expression in sleep-deprived hamsters. These results suggest that neither photoperiod nor glucocorticoids influence the suppressive effect of sleep deprivation upon LPS-induced inflammation.

[Seasonal peculiarities of female hormonal response of females to conspecific chemical signals of the male in two species of the genus Phodopus].

Changes in diestrous female blood plasma progesterone, estradiol, and cortisol in response to odors of urine and midventral gland secretion (MVGS) of conspecific males during different seasons have been studied in the desert hamster (Phodopus roborowskii and Djungarian hamster (Phodopus sungorus). In desert hamster females, a statistically significant (p < or = 0.05) increase in the plasma hormonal level was observed: for progesterone in response to conspecific male MGVS in all seasons except winter, and to conspecific male urine in spring and summer; for cortisol in response to male MVGS and urine in all seasons except spring; and for estradiol in response to male MGVS in autumn. Djungarian hamster females demonstrated a statistically significant increase in the plasma progesterone level in response to conspecific male MVGS and urine in winter and summer, and no response to the estradiol and cortisol level for both odors in all seasons.

Short-day response in Djungarian hamsters of different circadian phenotypes.

In Djungarian hamsters (Phodopus sungorus) bred at the authors' institute, a certain number of animals show activity patterns incompatible with proper entrainment of their endogenous circadian pacemaker to the environmental light-dark (LD) cycle. Even though the activity-offset in these animals is stably coupled to "light-on," activity-onset is increasingly delayed, leading to a compression of the activity time (α). If α falls below a critical value, the circadian rhythm in these so called delayed activity-onset (DAO) hamsters starts to free-run and finally breaks down. Animals then show an arrhythmic activity pattern (AR hamsters). Previous studies revealed the mechanisms of photic entrainment have deteriorated (DAO) or the suprachiasmatic nucleus (SCN) does not generate a rhythmic signal (AR). The aim of the present study was to investigate the consequences that these deteriorations have upon photoperiodic time measurement. Animals were bred and kept under standardized housing conditions with food and water ad libitum and a 14L/10D (long day, LD) regimen. Locomotor activity was recorded continuously using passive infrared motion detectors. Body mass, testes size, and fur coloration were measured weekly or biweekly to further quantify the photoperiodic reaction. In a first experiment, adult male wild-type (WT), DAO, and AR hamsters were transferred initially to a 16L/8D cycle. After 3-4 wks, the light period was shortened symmetrically by 8 h. After 14 wks, none of the DAO and AR hamsters, and only 1 of 8 WT hamsters showed short-day (SD) traits. Therefore, in a second experiment, hamsters were transferred to SD conditions (8L/16D cycle) for 8 wks directly from standard LD conditions. In 6 of 7 WT hamsters, activity time expanded, body mass and testes size decreased, and fur coloration changed from summer to winter pelage. In contrast, none of the DAO and AR hamsters displayed an SD response. In a third experiment, DAO and AR hamsters were kept in constant darkness (DD) for 8 and 14 wks. After 8 wks, DAO hamsters showed a similar photoperiodic reaction to WT hamsters that had been kept for 8 wks under SD conditions. However, the level of adaptation was still less compared to WT hamsters, but this difference was not apparent after 14 wks. In contrast, AR animals did not display any photoperiodic reaction, even after 14 wks in DD. Type VI phase response curves (PRCs) were constructed to better understand the mechanism behind the SD response. In WT hamsters, the photosensitive phase, where light pulses induce phase shifts, was lengthened in SD condition. In DAO hamsters, in contrast, the PRCs were similar under LD and SD conditions with a compressed photosensitive phase corresponding to α. Also, "light-on" induced only weak phase advances of activity-onset, insufficient to compensate for the long endogenous period. The results show that physiological mechanisms necessary for seasonal adaptation are working in DAO hamsters and that it is the inadequate interaction of the LD cycle with the SCN that prevents the photoperiodic reaction. AR hamsters, on the other hand, are incapable of measuring photoperiodic time due to a complete disruption of circadian rhythmicity.

Identification, expression, and physiological functions of Siberian hamster gonadotropin-inhibitory hormone.

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that inhibits gonadotropin secretion in birds and mammals. To further understand its physiological roles in mammalian reproduction, we identified its precursor cDNA and endogenous mature peptides in the Siberian hamster brain. The Siberian hamster GnIH precursor cDNA encoded two RFamide-related peptide (RFRP) sequences. SPAPANKVPHSAANLPLRF-NH(2) (Siberian hamster RFRP-1) and TLSRVPSLPQRF-NH(2) (Siberian hamster RFRP-3) were confirmed as mature endogenous peptides by mass spectrometry from brain samples purified by immunoaffinity chromatography. GnIH mRNA expression was higher in long days (LD) compared with short days (SD). GnIH mRNA was also highly expressed in SD plus pinealectomized animals, whereas expression was suppressed by melatonin, a nocturnal pineal hormone, administration. GnIH-immunoreactive (-ir) neurons were localized to the dorsomedial region of the hypothalamus, and GnIH-ir fibers projected to hypothalamic and limbic structures. The density of GnIH-ir perikarya and fibers were higher in LD and SD plus pinealectomized hamsters than in LD plus melatonin or SD animals. The percentage of GnRH neurons receiving close appositions from GnIH-ir fiber terminals was also higher in LD than SD, and GnIH receptor was expressed in GnRH-ir neurons. Finally, central administration of hamster RFRP-1 or RFRP-3 inhibited LH release 5 and 30 min after administration in LD. In sharp contrast, both peptides stimulated LH release 30 min after administration in SD. These results suggest that GnIH peptides fine tune LH levels via its receptor expressed in GnRH-ir neurons in an opposing fashion across the seasons in Siberian hamsters.

Thermal physiology and energetics in male desert hamsters (Phodopus roborovskii) during cold acclimation.

The adjustments in thermal physiology and energetics were investigated in male desert hamsters (Phodopus roborovskii) which were acclimated to 5°C for 4 weeks. Mean core body temperature in cold acclimated animals decreased by 0.21°C compared with controls. Further analysis revealed that the decrease mainly occurred in the scotophase, while in the photophase core body temperature remained constant during the whole cold acclimation. Thermogenic capacity, represented by resting metabolic rate and nonshivering thermogenesis increased in cold acclimated hamsters from initial values of 1.38 ± 0.05 and 5.32 ± 0.30 to 1.77 ± 0.08 and 8.79 ± 0.31 mlO(2) g(-1) h(-1), respectively. After cold acclimation, desert hamsters maintained a relative stable body mass of 21.7 ± 0.1 g very similar to the controls kept at 23°C (21.8 ± 0.1 g). The mean values of food intake and digestible energy (metabolisable energy) in cold acclimated hamsters were 5.3 ± 0.1 g day(-1) and 76.3 ± 0.9 kJ day(-1) (74.8 ± 0.9), respectively, which were significantly elevated by 76.7 and 80.4% compared to that in control group. The apparent digestibility was 81.0 ± 0.3% in cold acclimated animals which was also higher than the 79.7 ± 0.2% observed in controls. This increase corresponded with adaptive adjustments in morphology of digestive tracts with 20.2 and 36.8% increases in total length and wet mass, respectively. Body fat mass and serum leptin levels in cold acclimated hamsters decreased by 40.7 and 67.1%, respectively. The wheel running turns and the onset of wheel running remained unchanged. Our study indicated that desert hamsters remained very active during cold acclimation and displayed adaptive changes in thermal physiology and energy metabolism, such as enhanced thermogenic and energy processing capacities.