Responses In Siberian Hamsters Research Articles

Artificial light at night alters delayed-type hypersensitivity reaction in response to acute stress in Siberian hamsters

Several physiological and behavioral processes rely on precisely timed light information derived from the natural solar cycle. Using this information, traits have adapted to allow individuals within specific niches to optimize survival and reproduction, but urbanization by humans has significantly altered natural habitats. Nighttime light exposure alters immune function in several species, which could lead to decreased fitness or survival, particularly in the face of an environmental challenge. We exposed male Siberian hamsters (Phodopus sungorus) to five lux of light at night for four weeks, and then administered six hours of acute restraint stress. Delayed-type hypersensitivity (DTH) response was assessed immediately following stress. Acute restraint increased the DTH reaction in dark nights, but exposure to nighttime light prevented this response. Exposure to light at night prolonged the DTH response in non-stressed control hamsters. These results suggest that light pollution may significantly alter physiological responses in Siberian hamsters, particularly in response to a salient environmental challenge such as stress.

Chronic exposure to dim light at night suppresses immune responses in Siberian hamsters.

Species have been adapted to specific niches optimizing survival and reproduction; however, urbanization by humans has dramatically altered natural habitats. Artificial light at night (LAN), termed 'light pollution', is an often overlooked, yet increasing disruptor of habitats, which perturbs physiological processes that rely on precise light information. For example, LAN alters the timing of reproduction and activity in some species, which decreases the odds of successful breeding and increases the threat of predation for these individuals, leading to reduced fitness. LAN also suppresses immune function, an important proxy for survival. To investigate the impact of LAN in a species naive to light pollution in its native habitat, immune function was examined in Siberian hamsters derived from wild-caught stock. After four weeks exposure to dim LAN, immune responses to three different challenges were assessed: (i) delayed-type hypersensitivity (DTH), (ii) lipopolysaccharide-induced fever, and (iii) bactericide activity of blood. LAN suppressed DTH response and reduced bactericide activity of blood after lipopolysaccharide treatment, in addition to altering daily patterns of locomotor activity, suggesting that human encroachment on habitats via night-time lighting may inadvertently compromise immune function and ultimately fitness.

An Intact Dorsomedial Hypothalamic Nucleus, but Not the Subzona Incerta or Reuniens Nucleus, Is Necessary for Short-Day Melatonin Signal-Induced Responses in Siberian Hamsters

Siberian hamsters provide a useful model to define mechanisms underlying obesity reversal as they naturally transition from their extreme seasonal obesity in long ‘summer-like’ days (LDs) to a leaner state in short ‘winter-like’ days (SDs). These day length changes are coded into durational melatonin (MEL) signals by the pineal gland resulting in stimulation of MEL receptors (MEL_1a-Rs). MEL_1a-R mRNA is colocalized centrally in sympathetic nervous system (SNS) outflow neurons comprising a chain of neurons that ultimately innervates white adipose tissue (WAT). Neural components in this circuit include the subzona incerta (subZI), dorsomedial hypothalamic nucleus (DMH) and thalamic reuniens nucleus (ReN). SD, long-duration MEL signals induce gonadal regression and increase WAT SNS drive triggering lipolysis and thereby reversing LD obesity. We attempted to block the reversal of SD MEL signal-induced obesity by making electrolytic or sham lesions of the subZI, ReN or DMH in LD-housed hamsters. To create SD-like, long-duration MEL signals, we injected MEL 3 h before lights out, thereby lengthening the naturally occurring nocturnal duration of circulating MEL. ReN and subZI lesions did not block SD-like MEL signal-induced decreases in body, WAT, testicular masses or food intake; by contrast, DMH lesions blocked decreases in WAT and testicular mass. This nonresponsiveness was not due to lesion-induced inappropriate nocturnal LD MEL secretion that would have altered our creation of SD-like signals. Therefore, the DMH appears to participate in the control of both SD energy and reproductive responses, and joins the suprachiasmatic nucleus as sites necessary for SD responses in this species.

An intact dorsomedial hypothalamic nucleus is necessary for short day melatonin signal-induced responses in Siberian hamsters

An intact dorsomedial hypothalamic nucleus is necessary for short day melatonin signal-induced responses in Siberian hamsters

Distributed Forebrain Sites Mediate Melatonin-Induced Short-Day Responses in Siberian Hamsters

The pineal hormone melatonin (MEL) is the key initiator in regulating seasonal photoperiodic responses; however, the central sites that mediate short day (SD) winter-like responses, such as testicular regression and decreases in white adipose tissue (WAT) mass, by Siberian hamsters are not precisely known. WAT is innervated by the sympathetic nervous system, and several forebrain sites that are part of the sympathetic nervous system outflow to WAT coexpress MEL(1a) receptor mRNA [e.g. suprachiasmatic nucleus, subzona incerta (SubZi), dorsomedial nucleus of the hypothalamus, nucleus reunions and paraventricular nuclei of the thalamus]. We tested the involvement of these sites in MEL-triggered SD responses. A long duration, SD-like MEL signal was applied site specifically for 5 wk, with sc and third ventricle MEL application serving as positive controls. Whereas SD MEL signals delivered to each of these sites were able to induce testicular regression, all but the paraventricular nuclei of the thalamus also trigger SD-induced decreases in body mass. Third ventricle, sc, suprachiasmatic nucleus, or SubZi MEL application also decreased WAT mass, and only sc and SubZi MEL application decreased food intake. Collectively these data suggest a distributed system of MEL-sensitive brain sites sufficient to mediate these SD responses, the redundancy of which suggests its importance for appropriate seasonal responses critical for overwintering.

Social defeat differentially affects immune responses in Siberian hamsters ( Phodopus sungorus)

Social defeat is a complex, multi-faceted behavioral interaction capable of eliciting a wide range of physiological and behavioral responses. The behavioral components responsible for eliciting these changes, however, remain unspecified. The goal of the present study was to examine the effects of chronic social defeat on serum cortisol concentration as well as innate and acquired immune responses in adult male Siberian hamsters ( Phodopus sungorus). Specifically, we experimentally manipulated the nature of the social interaction among conspecific animals (i.e., no social interaction, exposure to the sight or smell of a conspecific, or full social defeat) in order to determine the important components contributing to potential stress-induced changes in immunity. We found that immunoglobulin G (IgG) levels were decreased in defeated animals relative to both control animals and those exposed only to the olfactory cues of conspecifics. In contrast, serum bactericidal activity was increased in the defeated animals relative to controls. Prolonged social defeat did not elevate serum cortisol levels as compared with control animals. The results of this study suggest that social defeat alters immune responses and that specific behavioral components (i.e., defeat) contribute to this response. Importantly, these findings also demonstrate that social defeat exerts opposite effects on innate and acquired measures of immunity. Collectively, these results contribute to our understanding of complex social behaviors and their differential effects on endocrine and immune responses in vertebrates.

Photoperiod and food restriction differentially affect reproductive and immune responses in Siberian hamsters Phodopus sungorus

Summary 1. Organisms must contend with seasonal fluctuations in energy availability. To maintain a positive energy balance year‐round, a number of adaptations have evolved including seasonal changes in reproduction, energetics and immunity. Photoperiod is the primary environmental signal most animals use to predict seasonal events. Despite the established link between energetics and immune function, little is known regarding how changes in energy availability affect immunity. 2. The goal of the present study was to determine the effects of food restriction on photoperiodic changes in reproduction and immune function in the Siberian hamster (Phodopus sungorus). Adult hamsters were housed in long or short days and were food restricted or fed ad libitum. Immune responses were quantified by measuring specific antibody production and bacterial killing capacity. 3. Food restriction decreased body and relative reproductive masses in long‐day animals. Antibody responses, but not bacterial killing ability, were enhanced in food restricted short‐day animals as compared with ad libitum fed controls. We also found differential effects of body fat on immune responses depending on the immune measure. 4. The effects of food restriction on immune function appear to vary based on the restriction regimen, the response measured, and the physiological state of the organism including energy balance, metabolic rate and reproductive status. 5. In conclusion, these results suggest that a wide range of factors can differentially affect immune function. In addition, these effects may vary based on the specific response examined. Future studies should include a variety of measurements to provide a more integrative and accurate picture of reproductive, energetic, and photoperiodic effects on immune function.

Social interactions differentially affect reproductive and immune responses of Siberian hamsters

Social interactions can have pronounced effects on reproductive physiology and behavior in a wide range of species. Much less is known about the effects of social interactions on immunity. The goal of the present study was to test the effects of social interactions on both reproductive and immune responses in Siberian hamsters ( Phodopus sungorus). Male and female hamsters were housed alone, in same-sex pairs or in mixed-sex pairs for 4 weeks. Animals were then immunized with the antigen keyhole limpet hemocyanin (KLH) and blood samples were drawn 5 days postinoculation. Reproductive tissue masses, testosterone, 17β-estradiol and cortisol concentrations were measured and immunity was assessed by measuring serum anti-KLH IgM, and mitogen-stimulated splenocyte proliferation. Male hamsters housed with a female had increased testosterone and anti-KLH IgM and elevated splenocyte proliferation compared with males housed alone. Female hamsters housed in same-sex pairs had increased serum IgM compared with females housed with males. Cortisol was elevated in both sexes housed with male conspecifics compared with the other experimental groups. Serum estradiol concentrations did not differ among females in any group. Collectively, the results of the present study suggest that social interactions can alter reproductive responses, but that these changes appear unrelated to changes in immunity. In contrast, the presence of a male conspecific elicits social-stress-induced elevations in serum cortsiol in both males and females, which is generally immunosuppressive. These data support the notion of social-stress-induced suppression of immune humoral and cell-mediated immune responses.

Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin.

Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuroendocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secretion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 degrees C or 22 degrees C. Locomotor activity was recorded continuously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (alpha), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 degrees C than 22 degrees C. Cold exposure increased the proportion of hamsters that were photoresponsive: gonadal regression in short days increased from 44% at 22 degrees C to 81% at 5 degrees C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 degrees C temporarily had longer alphas during the first 4 weeks in short days and significant decreases in body mass and testicular size that were reversed during the ensuing weeks when alpha decreased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes masses when maintained at 5 degrees C but not 22 degrees C. Low-ambient temperature appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser extent by augmenting the rate at which the duration of nocturnal melatonin secretion increases in short day lengths.

Parameters of the circadian rhythm of pineal melatonin secretion affecting reproductive responses in Siberian hamsters

The major function of the mammalian pineal gland appears to be its central role in photoperiodism. The pineal hormone, melatonin, is synthesized and secreted primarily at night, under the control of a circadian oscillator that is entrained to the light-dark cycle. Both the circadian phase and the duration of the nocturnal peak of melatonin secretion are established primarily by interactions between the endogenous circadian oscillator and the daily photic cycle. The duration of the melatonin peak varies inversely with day length, and this relationship between day length and the duration of each circadian melatonin peak appears to be an integral part of the photoperiodic mechanism. When pinealectomized animals are given daily melatonin infusions of long duration, they exhibit physiologic responses that normally are observed during exposure to short day photoperiods; when administered short-duration melatonin infusions, the animals display long photoperiod-type responses. In addition to the importance of the duration of each melatonin peak, certain other parameters appear to be significant. If a long-duration infusion of melatonin is interrupted by a period of 2 hours or more without melatonin (i.e., to produce two short duration infusions), the responses are those typical for long day-exposed animals. Thus, to elicit short day-type responses, each long-duration melatonin peak must he relatively continuous; responses are not determined simply by the total time of exposure to melatonin in each circadian cycle. Also, long-duration melatonin peaks may not he effective to elicit photoperiod-type responses unless they are present at frequencies of nearly once every 24 hours or more.

Effects of diabetes and insulin on photoperiodic responses in Siberian hamsters

Short day pineal melatonin secretion patterns cause gonadal regression in Siberian hamsters, an effect due to alterations in gonadotropin secretion that ultimately affects the gonads. An analogous endocrine response sequence may underlie short day (melatoni)-induced decrease in body weight (fat) in this species. A potential mediating hormone for this effect of melatonin may be insulin. The purpose of the present experiments was to determine whether alterations in insulin status might affect the photoperiodic control of body weight (fat) and reproductive condition. Insulin was controlled by inducing experimental diabetes mellitus via injections of streptozotocin across several days, followed by treatment with one of two doses of long-lasting insulin. Both insulin doses normalized urinary glucose, but not body weight in long days. Seven weeks of short day exposure caused gonadal regression in all groups regardless of insulin status. The magnitude of the short day-induced decrease in body weight and carcass lipid was directly related to long day baseline body weights; however, the body weight nadir reached was similar among the groups. A ‘regulation’ of seasonally appropriate body weight (fat) is implied by these findings.

Suprachiasmatic and paraventricular control of photoperiodism in Siberian hamsters

The effects of lesions of the suprachiasmatic (SCN) and paraventricular nuclei (PVN) of the hypothalamus on photoperiodic responses were examined in adult Siberian hamsters. SCN lesions reduced nocturnal water intake in long days, whereas PVN lesions increased body weight and food intake in both short and long days. SCN or PVN lesions blocked short-day-induced decreases in body, fat pad, and testes weights and in food intake. Serum prolactin (PRL), but not follicle-stimulating hormone, levels were increased. The distribution of immunostained neurons and fibers for gonadotropin-releasing hormone (GnRH), beta-endorphin, arginine vasopressin (AVP), and vasoactive intestinal polypeptide (VIP) resembled that of other rodent species. Short-day exposure reduced AVP staining in lateral septum, medial amygdala, and bed nucleus of the stria terminalis but not in the PVN of the thalamus or the SCN. Short-day-exposed hamsters had fewer beta-endorphin-positive arcuate nucleus cells and tended to have fewer GnRH-positive preoptic cells than long-day controls. VIP staining was unaffected by photoperiod. Most day length effects on immunostaining were eliminated by either lesion. These results establish the importance of the SCN and PVN in the photoperiodic control of several seasonal responses in Siberian hamsters.