Photoperiod and body weight in female Syrian hamsters: skeleton photoperiods, response magnitude, and development of photorefractoriness

Two experiments examined the effects of various photoperiods on body weight and reproductive function in Syrian hamsters (Mesocricetus auratus). In Experiment 1 female hamsters were exposed to symmetrical skeleton photoperiods in which dawn and dusk were mimicked by 1-hr light pulses. A skeleton long (LD 16:8) photoperiod had no effect on body weight or estrous cyclicity (compared with animals in a complete LD 16:8 photoperiod), but exposure to a skeleton short (LD 10:14) photoperiod increased body weight and interrupted estrous cycles. Thus, body weight appears to respond to the relative timing of the two light pulses (a circadian mechanism) rather than to the absolute amount of light or darkness (an hourglass mechanism), just as does reproduction. In Experiment 2 female hamsters were exposed to a long photoperiod (LD 16:8) or to one of two short photoperiods (LD 10:14 and LD 8:16). Both short photoperiods increased body weight and interrupted estrous cycles, but the LD 8:16 photoperiod was substantially more effective at increasing body weight than was the LD 10:14. Thus, hamster body weight appears to be capable of a graded response to photoperiod, with shorter days producing a greater obesity. With prolonged exposure to the two short photoperiods (greater than 30 weeks), body weights spontaneously returned to the same level as the long-day controls, and estrous cycles resumed. When these hamsters were treated with the pineal gland hormone, melatonin, only those housed in long days exhibited the characteristic body weight gains, growth of brown and white adipose tissues, and decreases in uterine weight. Therefore, with prolonged exposure to short days, energy balance develops a photorefractoriness and an insensitivity to melatonin treatment, just as with reproductive function.     

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.     

Diet-induced obesity in the short-day-lean Brandt's vole

To test the hypothesis that mammals that show decrease in body mass under short-day condition should be resistant to high-fat induced obesity, we traced the changes of energy balance in a wild rodent, Brandt's voles (Lasiopodomys brandtii), which were acclimated to either long day (16 L: 8D, LD) or short day (8 L: 16D, SD) and fed either low-fat diet (LFD) or high-fat diet (HFD) in each photoperiodic manipulation. We found that Brandt's vole was not resistant to high-fat diet-induced obesity and SD, not HFD, induced the elevation in basal metabolic rate, the maximal rate of oxygen consumption after norepinephrine injection, and uncoupling protein 1 content in brown adipose tissue. HFD caused the increase in apparent digestibility and body fat mass, and the decrease in energy intake in both LD and SD voles. The enhancement of energy absorption associated with small intestine tissue recruitment can compensate the lower energy intake, which may contribute to the high-fat diet-induced body fat deposition. Thus, a decrease in body-weight gain but has no resistance to high-fat induced obesity implies an evolutionary and adaptive mechanism which is a benefit for their winter survival.     

Scheduled daily exercise or feeding alters the phase of photic entrainment in Syrian hamsters

Single daily bouts of appropriately timed activity can phase-shift or entrain circadian rhythms in rodents maintained in constant dark (DD). Whether this apparent feedback of behavioral activity to the circadian pacemaker has any adaptive significance is unclear; circadian rhythms are normally entrained by light-dark (LD) cycles, and this may override any effects of activity. To address this issue, the phase of entrainment to LD cycles was examined in hamsters exposed to a daily exercise schedule (3 h of induced wheel running). Hamsters exercised late in the dark showed a significant delay of entrained phase in LD (i.e., they became relative “night owls”) and lengthening of free-running periodicity in DD, compared to controls and hamsters exercised in midlight. Hamsters fed in midlight (arousal without wheel running) showed a significant advance of LD entrained phase (i.e., they became “early birds”). These observations provide the necessary rationale for further examination of the functional significance of behavioral feedback for the normal entrainment process. In addition, they raise the possibility that the entrained phase of human circadian rhythms can be similarly manipulated by behavioral procedures such as timed exercise.     

Hormonal responses to fighting in hamsters: separation of physical and psychological causes.

Male Syrian hamsters were paired and allowed to interact with a conspecific for 15 min a day for 4 days. On the fifth day, the animals were again paired, but they were kept physically separated by a mesh partition that allowed visual, olfactory, and auditory contact between the animals. Controls were placed with conspecifics on each of the 5 testing days, but the partition between them was never removed. Hamsters that were submissive on days 1-4 exhibited elevated plasma adrenocorticotropin-like immunoreactivity (ACTH-LI), beta-endorphin-like immunoreactivity (B-EP-LI), and cortisol on day 5 even though no fighting occurred on that day. Dominant hamsters did not differ from controls. These data support the hypothesis that there is an important psychological component to the pituitary-adrenocortical response in defeated hamsters.     

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.     

Three daily melatonin infusions alter gonadal development but not GnRH neuron number in the medial preoptic area or diagonal band of Broca in Siberian hamsters

Among juvenile Siberian hamsters reproductive development is associated with an increased number of unipolar GnRH-immunoreactive neurons in the diagonal band of Broca and medial preoptic area. In the present study, GnRH neuron morphology was assessed in male juvenile hamsters which were treated with three daily melatonin (MEL) infusions to initiate or delay gonadal development. Hamsters gestated in short days were transferred to constant light (LL) at day 14 of age and infused on days 18–20 with MEL for 6 h/day. This treatment stimulated testis development compared to that in saline-infused controls. By contrast, testis growth was suppressed by three 12 h MEL infusions in long-day gestated hamsters in LL from day 14. The number of unipolar and bipolar GnRH neuron subtypes was the same irrespective of MEL infusion or preweaning photoperiod. Thus, gonadal response to three MEL infusions is independent of changes in GnRH neuron number.     

Environmental enrichment enhances delayed-type hypersensitivity in both short- and long-day Siberian hamsters

Darwinian fitness reflects trade-offs between reproduction and survival. Mechanisms have evolved in small nontropical mammals and birds to maximize reproductive output during the summer when thermoregulatory demands are relatively low and food is abundant and to shunt energy to processes that presumably increase the odds of survival during the winter when thermoregulatory demands are high and food is scarce. In order to predict the onset of winter, many seasonally-breeding mammals use day length (photoperiod) information. Seasonal adjustments of immune responses may be one mechanism to enhance survival; short days enhance cell-mediated immune function in seasonally-breeding rodents. The goal of the present study was to determine whether delayed-type hypersensitivity in hamsters is constrained or if photoperiod merely establishes a baseline level of immune response that can then be fined tuned by other environmental conditions. To test this, we used environmental enrichment, a manipulation that enhances many aspects of immune function. Hamsters were assigned to either long or short photoperiods and further assigned into either singly-housed or environmentally-enriched cages. After 10 weeks of concurrent photoperiod and housing treatment, delayed-type hypersensitivity (DTH) was induced. Although short days enhanced DTH responses compared with long days, environmental enrichment enhanced swelling responses in both short days and long days, suggesting that even during potential energetic bottlenecks or during maximal reproductive investment, hamsters can modulate their investment in immune function.     

Effects of stress on immune function: the good,the bad,and the beautiful

Although the concept of stress has earned a bad reputation, it is important to recognize that the adaptive purpose of a physiological stress response is to promote survival during fight or flight. While long-term stress is generally harmful, short-term stress can be protective as it prepares the organism to deal with challenges. This review discusses the immune effects of biological stress responses that can be induced by psychological, physiological, or physical (including exercise) stressors. We have proposed that short-term stress is one of the nature’s fundamental but under-appreciated survival mechanisms that could be clinically harnessed to enhance immunoprotection. Short-term (i.e., lasting for minutes to hours) stress experienced during immune activation enhances innate/primary and adaptive/secondary immune responses. Mechanisms of immuno-enhancement include changes in dendritic cell, neutrophil, macrophage, and lymphocyte trafficking, maturation, and function as well as local and systemic production of cytokines. In contrast, long-term stress suppresses or dysregulates innate and adaptive immune responses by altering the Type 1–Type 2 cytokine balance, inducing low-grade chronic inflammation, and suppressing numbers, trafficking, and function of immunoprotective cells. Chronic stress may also increase susceptibility to some types of cancer by suppressing Type 1 cytokines and protective T cells and increasing regulatory/suppressor T cell function. Here, we classify immune responses as being protective, pathological, or regulatory, and discuss “good” versus “bad” effects of stress on health. Thus, short-term stress can enhance the acquisition and/or expression of immunoprotective (wound healing, vaccination, anti-infectious agent, anti-tumor) or immuno-pathological (pro-inflammatory, autoimmune) responses. In contrast, chronic stress can suppress protective immune responses and/or exacerbate pathological immune responses. Studies such as the ones discussed here could provide mechanistic targets and conceptual frameworks for pharmacological and/or biobehavioral interventions designed to enhance the effects of “good” stress, minimize the effects of “bad” stress, and maximally promote health and healing.     

Interaction of estradiol and photoperiod on activity patterns in the female hamster

The extent of the effects of estradiol on the locomotor rhythm in hamsters is unknown. This study was designed to determine the ability of estradiol alone and in combination with the photoperiod to alter activity patterns. Intact animals displayed higher levels of activity in LD 16:8 than in LD 6:18. Ovariectomy without estradiol treatment caused a sharp decline in activity in LD 16:8; estradiol treatment prevented this. In LD 6:18, activity levels were unaffected by either estradiol or ovariectomy. Estradiol altered the temporal distribution of activity only in LD 6:18, advancing peak activity towards lights-off. Estradiol decreased the variability in day to day onsets of activity in LD 6:18, but did not affect onsets in LD 16:8. The data suggest that the activity rhythm of the hamster is comprised of discrete components; the response of each component to estradiol is differentially altered by photoperiod. Furthermore, estradiol is vital for the expression of high activity levels in the LD 16:8 female.     

Small effect of brown adipose tissue and major effect of photoperiod on body weight in hamsters (Mesocricetus auratus)

It has been suggested that brown adipose tissue plays a special role in the control of body weight. This hypothesis was investigated by measuring the body weight of female hamsters over a two month period following removal of interscapular brown adipose tissue. Two groups of animals were used, one maintained on a short day (10:14 light: dark ratio) and the other on a long day (16:8 light: dark ratio) photoperiod. Under both photoperiod conditions hamsters with interscapular brown adipose tissue removed gained somewhat more weight than their corresponding sham operated controls. Photoperiod, however, had a much larger effect on body weight. Hamsters maintained on short days increased their body weight by about 60 percent during the two months of the experiment whereas the hamsters maintained on long days increased their body weight by only about 17 percent. It was concluded that brown adipose tissue plays a small non special role in the control of body weight in the female hamster, but that the light-dark ratio is quantitatively much more important under these experimental conditions.     

The use of wound healing assessment methods in psychological studies: A review and recommendations

Purpose. To provide a critical review of methods used to assess human wound healing in psychological research and related disciplines, in order to guide future research into psychological influences on wound healing. Methods. Acute wound models (skin blister, tape stripping, skin biopsy, oral palate biopsy, expanded polytetrafluoroethylene tubing), surgical wound healing assessment methods (wound drains, wound scoring), and chronic wound assessment techniques (surface area, volumetric measurements, wound composition, and assessment tools/scoring systems) are summarized, including merits, limitations, and recommendations. Results. Several dermal and mucosal tissue acute wound models have been established to assess the effects of psychological stress on the inflammatory, proliferative, and repair phases of wound healing in humans, including material‐based models developed to evaluate factors influencing post‐surgical recovery. There is a paucity of research published on psychological factors influencing chronic wound healing. There are many assessment techniques available to study the progression of chronic wound healing but many difficulties inherent to long‐term clinical studies. Conclusions. Researchers need to consider several design‐related issues when conducting studies into the effects of psychological stress on wound healing, including the study aims, type of wound, tissue type, setting, sample characteristics and accessibility, costs, timeframe, and facilities available. Researchers should consider combining multiple wound assessment methods to increase the reliability and validity of results and to further understand mechanisms that link stress and wound healing.     

Does exercise affect female hamster gonadotropins by reducing estradiol negative feedback?

Exercise stimulates reproductive function in hamsters exposed to short-day photoperiod (SDP) in contrast to its inhibitory effects in women and rats. SDP inhibits hamster reproduction in part by increasing the sensitivity of the hypothalamo-pituitary-gonadal axis (HPGA) to the negative feedback of gonadal steroids. To determine whether EX facilitates reproduction in female hamsters by affecting this mechanism, we examined the influence of estradiol (E2) on basal LH and FSH concentrations in exercising and sedentary hamsters maintained on long-day photoperiod (LD 14:10, LDP) or SDP (LD 8:16). In the LDP, serum LH and FSH were unaffected or reduced by exercise in ovariectomized (OVX) nonhormone-replaced hamsters, and LH was increased after tonic E2 replacement compared to sedentary controls. In the SDP, serum LH and FSH were significantly higher in OVX exercising than in sedentary hamsters, whether the exercisers were injected with a high dose of E2 or not. Thus, the effects of exercise on basal gonadotropin secretion in female hamsters appear to depend on the level of estradiol negative feedback (ENF). When this feedback is low (LDP OVX condition), exercise is either ineffective or inhibitory. When the ENF is increased by exposure to SDP and/or by treatment with E2, exercise has a stimulatory effect on basal gonadotropin secretion. Exercise may stimulate hamster gonadotropin secretion by reducing the ENF either by lowering the sensitivity of the HPGA to steroid negative feedback or by other means.     

Comparison of pineal melatonin rhythms in young adult and old Djungarian hamsters (Phodopus sungorus) under long and short photoperiods

The pattern of the pineal melatonin rhythm was studied in 3-6- and 17-22-month-old female Djungarian hamsters under light-dark (LD) schedules, LD 16:8 and LD 8:16. Under both photoperiods, the amplitude of the melatonin rhythm was 2.5 times higher in young adult hamsters than in old ones. Two weeks after the change from LD 16:8 to LD 8:16, the period of elevated nocturnal melatonin levels lengthened by about 3 h in both age groups. It is suggested that old Djungarian hamsters, as well as young adults, might be able to recognize the length of a photoperiod and its change.     

Induction of tumor-specific cell-mediated immunity by a noninfectious type-C virus.

Hamsters immunized with either noninfectious hamster type-C virus (D9) or irradiated D9 tumor cells were tested for cell-mediated immune reactivity by the macrophage migration inhibition assay. The migration of peritoneal exudate cells from immunized hamsters was significantly inhibited by either D9 virus or D9 tumor extract, but not by extract of an unrelated CELO virus-induced tumor. The virus and tumor extracts had little or no effect on the migration of peritoneal exudate cells from normal hamsters. Noninfectious D9 virus produces a cell-mediated immune response in the hamster and shares antigenicity with D9 lymphoma, which releases the virus.     

局部应用胰岛素对小鼠创面髓过氧化物酶及丙二醛的影响

目的观察小鼠创面局部应用小剂量胰岛素对创面髓过氧化物酶、丙二醛的影响,从对炎症调控角度探索胰岛素促进创面愈合的机制。方法48只C57BL／6J小鼠背部两侧对称部位造成两个全层皮肤缺损创面。按自身对照方法,每只小鼠背部两个创面随机分为胰岛素组和对照组。伤后即刻及伤后每24h,胰岛素组创面局部应用0．03U胰岛素（20μl生理盐水配置）,对照组应用20山生理盐水。随机选取12只小鼠用药至创面愈合,观察愈合时间和速率。余36只小鼠于伤后1、2、3d分别处死12只后取创面及创周5mm组织,免疫印记化学发光法检测创面髓过氧化物酶（MPO）、生化方法检测丙二醛（MDA）含量。结果胰岛素组创面愈合时间显著短于对照组（P〈0．05）;伤后第5、7天胰岛素组的创面愈合百分率显著高于对照组（P〈0．05）。伤后第2、3天胰岛素组和对照组创面MPO含量分别为1．52±0．37、1．95±0．53和1．2±049、1．25±0．39,两组比较差异有统计学意义（P〈0．05）。胰岛素组与对照组创面MDA含量在伤后1、2、3d比较,差异均无统计学意义。结论胰岛素在增强创面杀菌功能的同时减轻细胞损伤,可能是胰岛素促进创面愈合的机制之一。     

Dietary fish oil does not protect rats exposed to restraint or sleep deprivation stress

It has been suggested that fish oil (FO) prevents weight loss caused by physiological stress such as cancer, injury, or cardiovascular disorders. Previously, we observed that a high-fat diet containing corn and coconut oil exaggerated weight loss caused by the mixed physiological and psychological stress of repeated restraint (RR). This experiment tested the effects of a high-fat diet containing FO as the predominant lipid source in rats exposed to the mixed physiological and psychological stress of either RR or sleep deprivation (SD). FO did not prevent stress-induced hypophagia or weight loss in RR or SD rats but exaggerated the negative effects of stress on body weight in SD rats by promoting loss of lean body mass. RR caused a reduction in body fat content irrespective of dietary treatment. In SD rats, both stress and FO independently reduced body fat mass. FO did not have any effect on adrenal and thymus weights during RR or SD and did not influence corticosterone levels after 1 h of RR or after 48 or 96 h of SD. In conclusion, our results suggest that high levels of dietary FO do not improve the response to stress in rats exposed to mixed stressors.     

Photoperiod, temperature and melatonin effects on thermoregulatory behavior in Djungarian hamsters

Nesting and burrowing activity were measured in hamsters acclimated to either long or short day photoperiod in thermoneutrality and at 10 degrees C. Hamsters build larger nests under short day photoperiod or at 10 degrees C as compared to long day photoperiod or thermoneutrality. Both environmental cues contributed about 50% to a total increase in nest size from 1.8 g cotton/day to 7.7 g cotton/day (long day thermoneutral versus short day at 10 degrees C). Burrowing activity was suppressed by both cold or short day exposure. Daily melatonin injections, effective in inducing physiological short day adjustment under a long day photoperiod, also increased nesting scores. Hamsters which did not respond to short day conditions or to melatonin treatment physiologically lacked behavioral adjustments as well. Collectively, these results demonstrate analogies in the environmental control of physiological thermoregulatory adjustment and nesting behavior. Burrowing activity seems to be more related to reproductive needs than to thermoregulatory requirements in this hamster.     

Predator and restraint stress during gestation facilitates pilocarpine-induced seizures in prepubertal rats

Stress during gestation can result in early and long-term developmental aberrations. This study aimed to assess the impact of prenatal restraint or predator stress on pilocarpine-induced epileptic behavior. Pregnant rats were exposed to stressors on gestational days 15, 16, and 17. Restraint stress consisted of daily restraint of the dam. During predator stress, caged rats were exposed to a cat in a cage. On postnatal day 25, male pups were injected with pilocarpine and the behavior of each rat was observed. Prenatal stress led to low birth weight and increased blood corticosterone levels. Both stressors significantly potentiated pilocarpine-induced seizures. Predator-stressed pups exhibited significantly severe tonic-clonic seizures compared with restraint-stressed animals. These data emphasize the impact of prenatal stress on fetal growth, and neural and endocrine function. The results also suggest that psychosocial stressors have a greater impact on neural and endocrine function than physical stressors do.