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For small non-hibernating mammals, a high thermogenic capacity is important to increase activity levels in the cold. It has been previously reported that lactating females decrease their thermogenic activity of brown adipose tissue (BAT), whereas their capacity to cope with extreme cold remains uncertain. In this study we examined food intake, body temperature and locomotor behavior, resting metabolic rate, non-shivering thermogenesis, and cytochrome c oxidase activity, and the rate of state 4 respiration of liver, skeletal muscle, and BAT in striped hamsters (Cricetulus barabensis) at peak lactation and non- breeding hamsters (controls). The lactating hamsters and non- breeding controls were acutely exposed to -15°C, and several markers indicative of thermogenic capacity were examined. In comparison to non-breeding females, lactating hamsters significantly increased food intake and body temperature, but decreased locomotor behavior, and the BAT mass, indicative of decreased BAT thermogenesis at peak lactation. Unexpectedly, lactating hamsters showed similar body temperature, resting metabolic rate, non-shivering thermogenesis with non-breeding females after acute exposure to -15°C. Furthermore, cytochrome c oxidase activity of liver, skeletal muscle and BAT, and serum thyroid hormone concentration, and BAT uncoupling protein 1 expression, in lactating hamsters were similar with that in non-breeding hamsters after acute extreme cold exposure. This suggests that lactating females have the same thermogenic capacity to survive cold temperatures compared to non-breeding animals. This is particularly important for females in the field to cope with cold environments during the period of reproduction. Our findings indicate that the females during lactation, one of the highest energy requirement periods, do not impair their thermogenic capacity in response to acute cold exposure., Competing Interests: Declaration of Competing Interest The authors declare that they have no competing interests., (Copyright © 2024 Elsevier GmbH. All rights reserved.)

Background: Hibernation is a physiological and behavioural adaptation that permits survival during periods of reduced food availability and extreme environmental temperatures. This is achieved through cycles of metabolic depression and reduced body temperature (torpor) and rewarming (arousal). Rewarming from torpor is achieved through the activation of brown adipose tissue (BAT) associated with a rapid increase in ventilation frequency. Here, we studied the rate of rewarming in the European hamster (Cricetus cricetus) by measuring both BAT temperature, core body temperature and ventilation frequency., Results: Temperature was monitored in parallel in the BAT (IPTT tags) and peritoneal cavity (iButtons) during hibernation torpor-arousal cycling. We found that increases in brown fat temperature preceded core body temperature rises by approximately 48 min, with a maximum re-warming rate of 20.9℃*h -1 . Re-warming was accompanied by a significant increase in ventilation frequency. The rate of rewarming was slowed by the presence of a spontaneous thoracic mass in one of our animals. Core body temperature re-warming was reduced by 6.2℃*h -1 and BAT rewarming by 12℃*h -1 . Ventilation frequency was increased by 77% during re-warming in the affected animal compared to a healthy animal. Inspection of the position and size of the mass indicated it was obstructing the lungs and heart., Conclusions: We have used a minimally invasive method to monitor BAT temperature during arousal from hibernation illustrating BAT re-warming significantly precedes core body temperature re-warming, informing future study design on arousal from hibernation. We also showed compromised re-warming from hibernation in an animal with a mass obstructing the lungs and heart, likely leading to inefficient ventilation and circulation.

Photoperiod is an important factor of mammalian seasonal rhythm. Here, we studied morphological differences in the Harderian gland (HG), a vital photosensitive organ, in male striped dwarf hamsters (Cricetulus barabensis) under different photoperiods (short photoperiod, SP; moderate photoperiod, MP; long photoperiod, LP), and investigated the underlying molecular mechanisms related to these morphological differences. Results showed that carcass weight and HG weight were lower under SP and LP conditions. There was an inverse correlation between blood melatonin levels and photoperiod in the order SP > MP > LP. Protein expression of hydroxyindole-O-methyltransferase (HIOMT), a MT synthesis-related enzyme, was highest in the SP group. Protein expression of bax/bcl2 showed no significant differences, indicating that the level of apoptosis remained stable. Protein expression of LC3II/LC3I was higher in the SP group than that in the MP group. Furthermore, comparison of changes in the HG ultrastructure demonstrated autolysosome formation in the LP, suggesting the lowest autophagy level in under MP. Furthermore, the protein expression levels of ATP synthase and mitochondrial fission factor were highest in the MP group, whereas citrate synthase, dynamin-related protein1, and fission1 remained unchanged in the three groups. The change trends of ATP synthase and citrate synthase activity were similar to that of protein expression among the three groups. In summary, the up-regulation of autophagy under SP and LP may be a primary factor leading to loss of HG weight and reduced mitochondrial energy supply capacity., Competing Interests: The authors have declared that no competing interests exist.

Predicted increases in global average temperature are physiologically trivial for most endotherms. However, heat waves will also increase in both frequency and severity, and these will be physiologically more important. Lactating small mammals are hypothesized to be limited by heat dissipation capacity, suggesting high temperatures may adversely impact lactation performance. We measured reproductive performance of mice and striped hamsters ( Cricetulus barabensis ), including milk energy output (MEO), at temperatures between 21 and 36 °C. In both species, there was a decline in MEO between 21 and 33 °C. In mice, milk production at 33 °C was only 18% of that at 21 °C. This led to reductions in pup growth by 20% but limited pup mortality (0.8%), because of a threefold increase in growth efficiency. In contrast, in hamsters, MEO at 33 °C was reduced to 78.1% of that at 21 °C, yet this led to significant pup mortality (possibly infanticide) and reduced pup growth by 12.7%. Hamster females were more able to sustain milk production as ambient temperature increased, but they and their pups were less capable of adjusting to the lower supply. In both species, exposure to 36 °C resulted in rapid catastrophic lactation failure and maternal mortality. Upper lethal temperature was lowered by 3 to 6 °C in late lactation, making it a critically sensitive window to high ambient temperatures. Our data suggest future heat wave events will impact breeding success of small rodents, but this is based on animals with a long history in captivity. More work should be performed on wild rodents to confirm these impacts., Competing Interests: The authors declare no competing interest.

Improved husbandry and better knowledge of exotic pets have led to a gradual increase in the life span of pets, such as rats, mice, hamsters, and gerbils. Much of the information on these senior patients is derived from the laboratory animal studies and anecdotal practitioner information. Although the small size of some of the patients makes blood collection problematic for hematology and organ function testing, the advent of polymerase chain reaction testing and other molecular diagnostics is allowing practitioners to test for specific etiologies with the small biologic samples available. Radiology and ultrasonography also are valuable diagnostic modalities., Competing Interests: Disclosure The authors have nothing to disclose., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Social stress can dramatically influence the health of animals via communication between gut microbiota and the HPA system. However, this effect has been rarely investigated among different social ranked animals after chronic repeated social encounters. In this study, we evaluated changes and differences in microbiota among control, dominant, and subordinate male greater long-tailed hamsters (Tscherskia triton) over 28 successive days of repeated social encounter. Our results indicated that as compared with the control group, short-term repeated social encounters significantly altered fecal microbiota of subordinate hamsters, while chronic repeated social encounters altered colonic mucosa-associated microbiota of both dominant and subordinate hamsters. Fecal microbiota showed a transition in composition and diversity on day 2 for the subordinate group but on day 4 for the control and dominant groups under repeated encounters. Compared with their baseline, genus Lactobacillus increased in both dominant and subordinate groups, while genus Bifidobacterium increased in the subordinate group and genus Adlercreutzia increased in the dominant group. Our results suggest that chronic repeated social encounter can alter diversity and composition of gut microbiota of hamsters in both feces and colonic mucosa, but the latter performed better in reflecting the effects of chronic stress on microbiota in this species. Future studies should focus on elucidating how these microbiota alterations may affect animal behavior and fitness.

Traditionally, urbanization has been seen as a negative phenomenon for biota. However, changes in the environmental parameters induced by urbanization might be favorable for some species. Over the past half-century, the common hamster has actively populated cities, establishing populations in some European, Russian and Kazakhstan cities. Based on integrative methods, we investigated free-range common hamsters inhabiting Simferopol from 2015 to 2018 to reveal possible adaptations to the urbanized environment across several parameters, including lifespan, hibernation period, reproductive activity and body mass. Results show that in urban areas, the common hamster demonstrates an extremely short hibernation period compared to other localities, possibly due to enhanced food resources from urban forestry (walnuts, locus and hazelnut), allowing the species to start breeding very early (February) and finish as late as October. We present the first evidence of polyandry for this species: mating of receptive females with several males and subsequent confirmation of multiple paternity. Despite high reproductive potential, the lifespan of the common hamster in urban conditions is generally very short (less than 1 year). We speculate that in the process of synurbization, the common hamster's innate plasticity across many life history traits permits it to successfully colonize throughout a wide range of habitats, with the ability to form novel adaptations to urban environments., (© 2019 International Society of Zoological Sciences, Institute of Zoology/Chinese Academy of Sciences and John Wiley & Sons Australia, Ltd.)

A Quick guide to singing mice., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Understanding whether captive-reared animals destined to reintroduction are still able to discriminate predators has important implications for conservation biology. The endangered European hamster benefits from conservation programs throughout Europe, in which several thousand individuals are released into the wild every year. Despite this, the anti-predator strategy of hamsters and their ability to maintain predator discrimination in captivity remain to be investigated. Here, we explore the predator discrimination behaviour of captive-reared European hamsters and their response to different predation cues. When first exposed to the urine of cats and goats in a Y-maze test, hamsters spent more time close to the cat scent rather than to the goat scent. In a second experiment, during which hamsters were exposed to a non-mobile European ferret (inside a cage), hamsters significantly increased the time spent close to the ferret's cage and displayed aggressive behaviour towards the ferret. Furthermore, they did not take refuge inside an anti-predation tube (APT), a device designed to upgrade wildlife underpasses and reconnect wild hamster populations. Finally, when exposed to a mobile ferret (but without physical contact), hamsters displayed mobbing and aggressive behaviours towards the ferret, before taking refuge inside the APT. Taken together, our results show that captive-reared hamsters are still able to detect and react to predation cues, but that they initially adopt an offensive strategy (grunting, spitting, mobbing) during the risk-assessment phase. After risk assessment, however, hamsters used the APT as a refuge. Our study provides important insights into the anti-predator behaviour of hamsters. Testing the efficacy of the APT, a device that will allow upgrading wildlife underpasses for the hamster and other rodents, is also of great importance and is instrumental in conservation efforts for these species., Competing Interests: The authors have declared that no competing interests exist.

Agriculture intensification, marked by the generalization of crop monoculture, by the increase in plot size and by the reduction of plant diversity, has led to huge decline in wildlife in European farmlands. In such habitat, research has long been biased towards birds and invertebrates, while very few studies have investigated the effect on small mammals. Considering the European hamster, Cricetus cricetus, we therefore review the different techniques that can be used to investigate the impact of environmental changes and conservation measures in small and endangered wild mammals. We suggest that only a multidisciplinary approach will allow exploration of these effects, combining experimental laboratory work on captive-bred animals with the monitoring of wild individuals. In particular, individual energy balance has to be investigated and measured as accurately as possible, through either biochemical or bio-logging techniques. It is, indeed, the most affected physiological trait in a changing environment, as it determines both the reproductive output and the survival of the individual. We also discuss the inconvenience of capture-release approaches for such an endangered species and emphasize the disturbance that experimental protocols could impose on the hamster., (© 2018 International Society of Zoological Sciences, Institute of Zoology/Chinese Academy of Sciences and John Wiley & Sons Australia, Ltd.)

The Djungarian hamsters of our breeding colony show unstable daily activity patterns when kept under standard laboratory conditions. Moreover, part of them develops a delayed activity onset (DAO) or an arrhythmic phenotype. In former studies, we have shown that the system of photic entrainment works at its limits. If the period length (tau) increases, which is the case in DAO hamsters, the light-induced phase advances are too small to compensate the daily delay of the activity rhythm caused by tau being longer than 24 h. Accordingly, under natural conditions, there must be further (environmental) factors to enable a stable entrainment. One of these may be the higher level of motor activity. Animals must cover long distances to search for food, sexual partners and others. In the laboratory, hamsters are kept singly in small cages. This does restrict animals' options for motor activity. Also, there is less need for moving around as the hamsters are fed ad libitum. In the present study, a series of experiments was performed to investigate the putative effect of the activity level. To begin with, wild type (WT) and DAO animals were given access to running wheels. 50% of DAO hamsters developed a WT activity pattern. As the main reason for the DAO phenomenon is their long tau together with a too weak photic phase response, the effect of wheel running on these parameters was investigated in further experiments. With higher activity level, tau decreased in WT hamsters but increased in DAO animals even though the increase for the activity onset was only close to significance. Moreover, the photic phase responses were weaker though significant only for the activity offset of DAO hamsters. Based on the assumptions that running wheel activity will affect the phase response and/or the free running period, the results of the present paper do not provide an explanation for why part of DAO hamsters developed a WT phenotype when they had access to running wheels. Obviously, mechanisms downstream from the suprachiasmatic nuclei must be taken into account when investigating the stabilizing, improving circadian entrainment effect of motor activity.

Breeding of golden hamsters is classically performed at thermal conditions ranging from 20 to 24 °C. However, growing evidence suggests that lactating females suffer from heat stress. We hypothesised that shaving females dorsally to maximise heat dissipation may reduce stress during reproduction. We thus compared faecal cortisol metabolites (FCM) from shaved golden hamster mothers with those from unshaved controls. We observed significantly lower FCM levels in the shaved mothers (F 1,22  = 8.69, p = 0.0075) pointing to lower stress due to ameliorated heat dissipation over the body surface. In addition, we observed 0.4 °C lower mean subcutaneous body temperatures in the shaved females, although this effect did not reach significance (F 1,22  = 1.86, p = 0.18). Our results suggest that golden hamsters having body masses being more than four times that of laboratory mice provide a very interesting model to study aspects of lactation and heat production at the same time.

Background/aims: The mouse is widely used as an animal model for studying human embryo implantation. However, the mouse is unique in that both ovarian progesterone and estrogen are critical to implantation, whereas in the majority of species (e.g. human and hamster) implantation can occur in the presence of progesterone alone., Methods: In this study, we analyzed embryo-induced transcriptomic changes in the hamster uterus during embryo implantation by using RNA-seq. Differentially expressed genes were characterized by bioinformatic analysis., Results: We identified a total of 781 differentially expressed genes, of which 367 genes were up-regulated and 414 genes were down-regulated at the implantation site compared to the inter-implantation site. Functional clustering and gene network analysis highlighted the cell cycle process in uterus upon embryo implantation. By examining of the promoter regions of differentially expressed genes, we identified 7 causal transcription factors. Additionally, through connectivity map (CMap) analysis, multiple compounds were identified to have potential anti-implantation effects due to their ability to reverse embryo-induced transcriptomic changes., Conclusion: Our study provides a valuable resource for in-depth understanding of the mechanism underlying embryo implantation., (© 2018 The Author(s). Published by S. Karger AG, Basel.)

From 1735 to 1940, maize-based diets led to the death of hundreds of thousands of people from pellagra, a complex disease caused by tryptophan and vitamin B3 deficiencies. The current cereal monoculture trend restricts farmland animals to similarly monotonous diets. However, few studies have distinguished the effects of crop nutritional properties on the reproduction of these species from those of other detrimental factors such as pesticide toxicity or agricultural ploughing. This study shows that maize-based diets cause high rates of maternal infanticides in the European hamster, a farmland species on the verge of extinction in Western Europe. Vitamin B3 supplementation is shown to effectively restore reproductive success in maize-fed females. This study pinpoints how nutritional deficiencies caused by maize monoculture could affect farmland animal reproduction and hence their fitness., (© 2017 The Author(s).)

It has been well known that metabolic thermogenesis plays an important role in the thermoregulation of small mammals under different temperatures, while its role in fat accumulation is far from clear. In the present study, several physiological, hormonal, and biochemical measures indicative of metabolic thermogenesis were measured in the weaning striped hamsters after acclimated to a warm condition (30°C) for 1, 3 and 4months. The warm-acclimated groups significantly decreased energy intake, and simultaneously decreased nonshivering thermogenesis compared to those housed at 21°C. Body fat content increased by 29.9%, 22.1% and 19.6% in the hamsters acclimated to 1, 3 or 4months, respectively relative to their counterparts maintain at 21°C (P<0.05). The cytochrome c oxidase (COX) activity of brain, liver, heart and skeletal muscle, and the ratio of serum tri-iodothyronine to thyroxine significantly decreased in warm-acclimated groups compared with 21°C group. COX activity and uncoupling protein 1 (UCP1) mRNA expression of brown adipose tissue (BAT) were significantly down-regulated under the warm conditions. COX activity of BAT, liver, heart and muscle were significantly negatively correlated with body fat content, and the correlation between UCP1 expression and body fat content tended to be negative. These findings suggest that the decrease in the energy spent on metabolic thermogenesis plays an important role in the fat accumulation. The attenuation of COX and UCP1-based BAT activity may be involved in body fat accumulation in animals under warm conditions., (Copyright © 2016 Elsevier Inc. All rights reserved.)

In this study, we investigated the timing and duration of hibernation as well as body temperature patterns in free-ranging common hamsters (Cricetus cricetus) with regard to sex and age differences. Body temperature was recorded using subcutaneously implanted data loggers. The results demonstrate that although immergence and vernal emergence sequences of sex and age groups resembled those of most hibernators, particularly adult females delayed hibernation onset until up to early January. Thus, in contrast to other hibernators, female common hamsters hibernated for shorter periods than males and correspondingly spent less time in torpor. These sex differences were absent in juvenile hamsters. The period between the termination of hibernation and vernal emergence varied among individuals but did not differ between the sex and age groups. This period of preemergence euthermy was related to emergence body mass: individuals that terminated hibernation earlier in spring and had longer euthermic phases prior to emergence started the active season in a better condition. In addition, males with longer periods of preemergence euthermy had larger testes at emergence. In conclusion, females have to rely on sufficient food stores but may adjust the use of torpor in relation to the available external energy reserves, whereas males show a more pronounced energy-saving strategy by hibernating for longer periods. Nonetheless, food caches seem to be important for both males and females as indicated by the euthermic preemergence phase and the fact that some individuals, mainly yearlings, emerged with a higher body mass than shortly before immergence in autumn.

Some small mammals limit energy expenditure during winter conditions through torpor bouts, which are characterized by a decrease in body temperature and metabolic rate. Individuals arise periodically from torpor to restore critical functions requiring euthermia. Although most of the species involved do not feed during hibernation and rely on body reserves to fulfil energy requirements (fat-storing species), others hoard food in a burrow (food-storing species) and can feed during interbout euthermy. Whereas fat-storing species undergo a marked atrophy of the digestive tract, food-storing species have to maintain a functional digestive system during hibernation. Our study aimed to evaluate the absorption capacities of a food-storing species, the European hamster, throughout the annual cycle. In vivo intestinal perfusions were conducted in different groups of hamsters (n=5) during the different life periods, namely before hibernation, in torpor, during interbout euthermy, and during summer rest. The triglyceride, non-esterified free fatty acid, starch, glucose and protein composition of the perfusate was evaluated before and after the 1h perfusion of a closed intestinal loop. Triglyceride, starch and protein hydrolysis rates were similar in hibernating (torpid and euthermic) and non-hibernating hamsters. Intestinal absorption of free fatty acid was also similar in all groups. However, glucose uptake rate was higher during hibernation than during the summer. In contrast with fat-storing species, the intestinal absorption capacities of food-storing species are fully maintained during hibernation to optimize nutrient assimilation during short interbout euthermy. In particular, glucose uptake rate is increased during hibernation to restore glycaemia and ensure glucose-dependent pathways., (Copyright © 2016 Elsevier Inc. All rights reserved.)

The capacity of small mammals to sustain periods of food shortage largely depends on the adaptive regulation of energy budget in response to the decrease in food supply. In addition to food availability, ambient temperature (Ta) is an important factor affecting the rates of both energy intake and expenditure. To examine the effect of Ta on energy strategy and the capacity to sustain food shortage, striped hamsters were exposed to a warm condition (30°C) and were then restricted to 70% of ad libitum food intake. Body mass, energy intake and expenditure and physiological markers indicative of thermogenesis were measured. Warm exposure had no effect on body mass and digestibility, but decreased energy intake, basal metabolic rate and maximum nonshivering thermogenesis. The mitochondria protein content, cytochrome c oxidase activity and uncoupling protein 1 level of brown adipose tissue were significantly lower in hamsters at 30°C than at 21°C. Food restriction induced a significant decrease in body mass, but the decreased body mass was attenuated at 30°C relative to 21°C. This suggests that striped hamsters could not compensate for the limited food supply by decreasing daily energy expenditure at 21°C, whereas they could at 30°C. The significant reductions in the rates of metabolism and thermogenesis in warm-acclimated hamsters increase the capacity to cope with food shortage. Although, it remains uncertain whether this response represents some generalized evolutionary adaptation, the Ta-dependent adjustment in the capacity to survive food restriction may reflect that warm acclimation plays an important role in adaptive regulation of both physiology and behavior in response to the variations of food availability., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Seasonal reproduction is a common adaptive strategy among mammals that allows for breeding to occur at times of the year when it is most advantageous for the subsequent survival and growth of offspring. A major mechanism responsible for seasonal reproduction is a striking increase in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the negative feedback effects of estradiol. The neural and neuroendocrine circuitry responsible for mammalian seasonal reproduction has been primarily studied in three animal models: the sheep, and two species of hamsters. In this review, we first describe the afferent signals, neural circuitry and transmitters/peptides responsible for seasonal reproductive transitions in sheep, and then compare these mechanisms with those derived from studies in hamsters. The results suggest common principles as well as differences in the role of specific brain nuclei and neuropeptides, including that of kisspeptin cells of the hypothalamic arcuate nucleus, in regulating seasonal reproduction among mammals., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Why some females choose to mate with a 'preferred' male, whereas others choose to mate with an 'inferior' male is not always clear. Generally, the choosiness of females is thought to decline with advanced age, but relatively few studies have investigated this concept, and reports of this phenomenon in mammals are lacking. To address this deficiency, young and old female golden hamsters were evaluated for their preference for dominant vs. subordinate males. Females observed male dyads as a dominance relationship was established. Dominant and subordinate males were then placed within enclosures at the opposite ends of a Y-maze, and the first approach, scent marking, and time spent near each male were evaluated in young and old females during pro-oestrus-a time when females solicit visits by prospective mates by leaving vaginal and flank scent marks. Whereas the proportion of time spent near the dominant male was significantly greater than random for both young and old females, the proportions of vaginal and flank scent marks left for the dominant male were significantly greater than random for young females, but not for old females. Overall, these results are consistent with a decline in the preference for dominant males by old female hamsters., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Animals living in temperate zones anticipate seasonal environmental changes to adapt their biological functions, especially reproduction and metabolism. Two main physiological mechanisms have evolved for this adaptation: intrinsic long-term timing mechanisms with an oscillating period of approximately 1 year, driven by a circannual clock [1], and synchronization of biological rhythms to the sidereal year using day length (photoperiod) [2]. In mammals, the pineal hormone melatonin relays photoperiodic information to the hypothalamus to control seasonal physiology through well-defined mechanisms [3-6]. In contrast, little is known about how the circannual clock drives endogenous changes in seasonal functions. The aim of this study was to determine whether genes involved in photoperiodic time measurement (TSHβ and Dio2) and central control of reproduction (Rfrp and Kiss1) display circannual rhythms in expression under constant conditions. Male European hamsters, deprived of seasonal time cues by pinealectomy and maintenance in constant photoperiod, were selected when expressing a subjective summer or subjective winter state in their circannual cycle of body weight, temperature, and testicular size. TSHβ expression in the pars tuberalis (PT) displayed a robust circannual variation with highest level in the subjective summer state, which was positively correlated with hypothalamic Dio2 and Rfrp expression. The negative sex steroid feedback was found to act specifically on arcuate Kiss1 expression. Our findings reveal TSH as a circannual output of the PT, which in turn regulates hypothalamic neurons controlling reproductive activity. Therefore, both the circannual and the melatonin signals converge on PT TSHβ expression to synchronize seasonal biological activity., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

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., (© 2013.)

In contrast to photoperiodic rodent species, adult circannual European hamsters (Cricetus cricetus) do not rely on melatonin as transducer of the photoperiodic message. Instead, seasonal entrainment involves a special circadian organisation which characterizes a photoperiod-sensitive phase. When days shorten a precise activity pattern ("summer pattern") switches to a weak or arrhythmic "winter pattern". At the very same day gonadal regression is initiated and the circannual clock is reset. In contrast to this difference in photoperiodic time measurement, the broad time span in which offspring are born and the birth-season dependent timing of puberty is similar to photoperiodic rodents. We investigated how juvenile European hamsters measure photoperiod to situate themselves at the proper position in the annual cycle. Activity and 6-sulphatoxymelatonin (aMT6s) excretion were recorded in pups of five litters born at different seasons. Pups of all litters showed an activity pattern identical with the adults' summer pattern until postnatal day 78, suggesting that the pathway known to reset the circannual clock in adults is functional. The synchronous start of reproduction in yearlings supports this. However, since puberty and gonadal regression occurred before the switch in the activity pattern, the timing of reproduction in the birth year must be controlled by other means. As in photoperiodic species melatonin might be involved, since the aMT6s excretion showed daily and seasonal rhythms from early life on., (Copyright © 2014 Elsevier Inc. All rights reserved.)

A number of field and laboratory studies have shown that the social environment influences daily rhythms in numerous species. However, underlying mechanisms, including the circadian system's role, are not known. Obstacles to this research have been the inability to track and objectively analyse rhythms of individual animals housed together. Here, we employed temperature dataloggers to track individual body temperature rhythms of pairs of cohabiting male Syrian hamsters (Mesocricetus auratus) in constant darkness and applied a continuous wavelet transform to determine the phase of rhythm onset before, during, and after cohabitation. Cohabitation altered the predicted trajectory of rhythm onsets in 34% of individuals, representing 58% of pairs, compared to 12% of hamsters single-housed as 'virtual pair' controls. Deviation from the predicted trajectory was by a change in circadian period (τ), which tended to be asymmetric-affecting one individual of the pair in nine of 11 affected pairs-with hints that dominance might play a role. These data implicate a change in the speed of the circadian clock as one mechanism whereby social factors can alter daily rhythms. Miniature dataloggers coupled with wavelet analyses should provide powerful tools for future studies investigating the principles and mechanisms mediating social influences on daily timing.

The crucial step for the fatal neurodegenerative prion diseases involves the conversion of a normal cellular protein, PrP(C), into a fibrous pathogenic form, PrP(Sc), which has an unusual stability against heat and resistance against proteinase K digestion. A successful challenge to reverse the reaction from PrP(Sc) into PrP(C) is considered valuable, as it would give a key to dissolving the complex molecular events into thermodynamic and kinetic analyses and may also provide a means to prevent the formation of PrP(Sc) from PrP(C) eventually in vivo. Here we show that, by applying pressures at kbar range, the "proteinase K-resistant" fibrils (rHaPrP(res)) prepared from hamster prion protein (rHaPrP [23-231]) by seeding with brain homogenate of scrapie-infected hamster, becomes easily digestible. The result is consistent with the notion that rHaPrP(res) fibrils are dissociated into rHaPrP monomers under pressure and that the formation of PrP(Sc) from PrP(C) is thermodynamically controlled. Moreover, the efficient degradation of prion fibrils under pressure provides a novel means of eliminating infectious PrP(Sc) from various systems of pathogenic concern.

Turkish hamsters (Mesocricetus brandti) are a model organism for studies of hibernation, yet a detailed account of their torpor characteristics has not been undertaken. This study employed continuous telemetric monitoring of body temperature (T(b)) in hibernating male and female Turkish hamsters at ambient temperatures (T(a)s) of 5 and 13 °C to precisely characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (T(b) > 20 °C), followed by deep torpor bouts lasting 4-6 days at T(a) = 5 °C and 2-3 days at T(a) = 13 °C. Females at T(a) = 5 °C had longer bouts than males, but maintained higher torpor T(b); there were no sex differences at T(a) = 13 °C. Neither body mass loss nor food intake differed between the two T(a)s. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both T (a)s generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.

Two experiments were conducted to assess the effect of lever height on lever pressing that was not explicitly reinforced - i.e., operant-level responding. Two rodent species were used as subjects, rats (Experiment 1) and hamsters (Experiment 2), aiming to compare the behavioral support offered by one lever at various heights relative to the subjects' body size. Results showed that lever height had a substantial effect on response rate. The rate of lever pressing varied similarly for rats and hamsters as a function of lever height, when lever height was re-scaled relative to body size. The distribution of inter-response times showed that lever pressing was organized in bouts separated by pauses. This pattern of responding was accurately described in both experiments by a mixture of two exponential distributions. These findings support an analysis of affordances in non-human species., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Seasonal adaptation is widespread among mammals of temperate and polar latitudes. The changes in physiology, morphology and behaviour are controlled by the photoneuroendocrine system that, as a first step, translates day lengths into a hormonal signal (melatonin). Decoding of the humoral melatonin signal, i.e. responses on the cellular level to slight alterations in signal duration, represents the prerequisite for appropriate timing of winter acclimatization in photoperiodic animals. Corresponding to the diversity of affected traits, several hormone systems are involved in the regulation downstream of the neural integration of photoperiodic time measurement. Results from recent studies provide new insights into seasonal control of reproduction and energy balance. Most intriguingly, the availability of thyroid hormone within hypothalamic key regions, which is a crucial determinant of seasonal transitions, appears to be regulated by hormone secretion from the pars tuberalis of the pituitary gland. This proposed neuroendocrine pathway contradicts the common view of the pituitary as a gland that acts downstream of the hypothalamus. In the present overview of (neuro)endocrine mechanisms underlying seasonal acclimatization, we are focusing on the dwarf hamster Phodopus sungorus (long-day breeder) that is known for large amplitudes in seasonal changes. However, important findings in other mammalian species such as Syrian hamsters and sheep (short-day breeder) are considered as well.

Background: Testosterone (T) controls male Syrian hamster sexual behavior, however, neither of T's primary metabolites, dihydrotestosterone (DHT) and estradiol (E(2)), even in highly supraphysiological doses, fully restores sexual behavior in castrated hamsters. DHT and T apparently interact with androgen receptors differentially to control male sexual behavior (MSB), but whether these two hormones act synergistically or antagonistically to control MSB has received scant experimental attention and is addressed in the present study., Methodology/principal Findings: Sexually experienced male Syrian hamsters were gonadectomized and monitored 5 weeks later to confirm elimination of the ejaculatory reflex (week 0), at which time they received subcutaneous DHT-filled or empty capsules that remained in situ for the duration of the experiment. Daily injections of a physiological dose of 25 µg T or vehicle commenced two weeks after capsule implantation. MSB was tested 2, 4 and 5 weeks after T treatment began. DHT capsules were no more effective than control treatment for long-term restoration of ejaculation. Combined DHT + T treatment, however, restored the ejaculatory reflex more effectively than T alone, as evidenced by more rapid recovery of ejaculatory behavior, shorter ejaculation latencies, and a greater number of ejaculations in 30 minute tests., Conclusions/significance: DHT and T administered together restored sexual behavior to pre-castration levels more rapidly than did T alone, whereas DHT and vehicle were largely ineffective. The additive actions of DHT and T on MSB are discussed in relation to different effects of these androgens on androgen receptors in the male hamster brain mating circuit.

Recent work has shown that microinjections of the cholinergic agonist carbachol into the preoptic area (MPOA) increase 20-30 kHz ultrasonic vocalization in rats. This response could be interpreted as a quite specific and direct effect on a central mechanism for vocalization or ultrasound production. Alternatively, it could reflect a more general drug-induced increase in defensiveness or defensive aggression. Two studies were conducted to distinguish these possibilities. In both, MPOA carbachol treatments very similar to those in previous work (unilateral injections into the MPOA/AH of 1 microg of carbachol in 0.2 microl of saline) were administered to female hamsters. The first study focused on the rate of ultrasonic courtship vocalization, finding it to decrease after treatment. This result is inconsistent with the view that cholinergic agonists directly stimulate vocalization. In contrast, although it does not completely exclude an effect on a mechanism for vocalization, it provides at least suggestive support for an interpretation in terms of defensiveness: Because hamster ultrasounds convey sexual readiness rather than alarm or aggressiveness, they would be expected to decrease in the event of a drug-induced increase in an incompatible motivational or emotional state. The second study exploited the aggressiveness of hamsters to even more directly test carbachol's ability to increase defensiveness or defensive aggression. Its results showed that minimally-estrous female subjects bit their male pursuers more quickly if recently treated with carbachol rather than placebo. Taken together, both results support the existence of a cholinergic system that extends through the MPOA and regulates defensiveness or a related motivational or emotional state.

Recently, we demonstrated that pyruvate dehydrogenase A2 (PDHA2) is tyrosine phosphorylated in capacitated hamster spermatozoa. In this report, using bromopyruvate (BP), an inhibitor of PDHA, we demonstrated that hamster sperm hyperactivation was blocked regardless of whether PDHA was inhibited prior to or after the onset of hyperactivation, but the acrosome reaction was blocked only if PDHA was inhibited prior to the onset of the acrosome reaction. Further, inhibition of PDHA activity did not inhibit capacitation-associated protein tyrosine phosphorylation observed in hamster spermatozoa. It is demonstrated that the essentiality of PDHA for sperm capacitation is probably dependent on its ability to generate effectors of capacitation such as reactive oxygen species (ROS) and cAMP, which are significantly decreased in the presence of BP. MICA (5-methoxyindole-2-carboxylic acid, a specific inhibitor of dihydrolipoamide dehydrogenase [DLD]), another component of the pyruvate dehydrogenase complex (PDHc), also significantly inhibited ROS generation and cAMP levels thus implying that these enzymes of the PDHc are required for ROS and cAMP generation. Furthermore, dibutryl cyclic adenosine monophosphate could significantly reverse the inhibition of hyperactivation observed in the presence of BP and inhibition of acrosome reaction observed in the presence of BP or MICA. The calcium ionophore, A23187, could also significantly reverse the inhibitory effect of BP and MICA on sperm acrosome reaction. These results establish that PDHA is required for hamster sperm hyperactivation and acrosome reaction, and DLD is required for hamster acrosome reaction. This study also provides evidence that ROS, cAMP, and calcium are involved downstream to PDHA.

Circadian activity rhythms in hamsters are entrained to the daily light:dark cycle by photic information arriving from the retina to the suprachiasmatic nucleus, the site of the master circadian pacemaker in mammals. The effects of light on adjusting the timing of the circadian pacemaker is modified, both positively and negatively, by a variety of transmitter systems, but the effects of endocannabinoids have not been reported. Therefore, in this study we evaluated cannabinoids specific for the cannabinoid type 1 receptor (CB(1)) for their ability to modulate light-induced phase advances in hamster circadian activity rhythms. All compounds were administered intraperitoneally. The CB(1) agonist CP55940 potently inhibited light-induced phase shifts with near 90% inhibition achieved with a dose of 0.125 mg/kg. The inhibitory effect of CP55940 was partially reversed by the CB(1) antagonist LY320135 and completely reversed with 1 mg/kg of the CB(1) antagonist AM 251. Neither LY320135 nor AM 251 had any effect on light-induced phase shifts when administered alone. Further evidence for CB(1) involvement in hamster circadian rhythms was provided by immunohistochemical detection of CB(1) receptors in four separate nuclei comprising the principal components of the hamster circadian system: the suprachiasmatic nucleus, intergeniculate leaflet of the thalamus, and dorsal and median raphe nuclei. Altogether these data indicate that the endocannabinoid system has the capability to modulate circadian rhythms in the hamster and cannabis use should be evaluated for adverse effects on circadian rhythms in humans.

Defects in preimplantation embryonic development, uterine receptivity, and implantation are the leading cause of infertility, pregnancy problems and birth defects. Significant progress has been made in our basic understanding of these processes using the mouse model, where implantation is ovarian estrogen-dependent in the presence of progesterone. However, an animal model where implantation is progesterone-dependent must also be studied to gain a full understanding of the embryo and uterine events that are required for implantation. In this regard, the hamster is a useful model and this review summarizes the information currently available regarding mechanisms involved in synchronous preimplantation embryo and uterine development for implantation in this species.

Laser Doppler flowmetry studies showed that blood flow in the microcirculation bed of paired buccal pouches in hamsters decreased under conditions of reduced systemic blood pressure and this decrease is more pronounced in the left pouch. The transition of the circulatory bed to the new state is step-wise in the left pouch and continuous in the right one.

We monitored individual reproductive timing and output in a common hamster population in Vienna over a 3-year period. Animals were live-trapped, weighed, individually marked, and reproductive status was determined at capture. Costs of reproduction were investigated by measuring body condition shortly before hibernation and overwinter survival. Our results indicated that early emerging females had more litters and weaned more offspring per season. Body mass throughout the active season did not differ significantly between females with high and low reproductive output. High reproductive output seemed to affect the duration of the active season. Successful females had a longer postreproduction period before hibernation than less successful ones, probably serving to balance the costs of reproductive effort by extended preparation for hibernation. Also, females that had emerged early in spring and had high reproductive success were more likely to survive the subsequent winter. Hence, we found female common hamsters to vary strongly in maternal investment capacity and to tailor reproductive strategies accordingly.

Siberian hamsters (Phodopus sungorus) exhibit seasonal cycles of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short-duration Mel signals stimulate reproduction and long-duration signals inhibit reproduction. The mechanism by which Mel signals are decoded at the level of neural target tissues remains uncharacterized. In Siberian hamsters, exposure to short day lengths or injections of Mel in long days results in a decrease in hypothalamic expression of type 2 iodothyronine deiodinase (Dio2) mRNA. Dio2 catalyzes the conversion of the thyroid hormone thyroxine to triiodothyronine (T3). Thus exposure to short and long day lengths should decrease and increase hypothalamic T3 concentrations, respectively. We tested the hypothesis that exogenous T3 administered to short-day hamsters would mimic exposure to long day lengths with respect to gonadal stimulation. Hamsters gestated and raised in short day lengths that exhibited photoinhibition of the testes were given daily subutaneous injections of T3 or saline vehicle for 4 wk beginning at week 12 of life. The results indicate that exogenous T3 induced gonadal growth in short-day hamsters and delayed spontaneous gonadal development by an interval equal to the number of weeks during which T3 was administered. T3 injections delayed gonadal regression if given coincident with the transfer of hamsters from long to short day lengths. These results suggest that T3 mimics long day exposure in Siberian hamsters and may serve as an intermediate step between the Mel rhythm and the reproductive response.

Sexual dimorphism in lacrimal gland (LG) gene expression is believed to be due to direct inductive effects of androgens mediated by androgen receptors (AR) but hypophysectomy dramatically curtails these inductive effects. Since, functional estrogen receptors (ER) could not be detected in LG, estrogen effects on LG are believed to be indirectly mediated by changes in levels of pituitary hormones. We found that two lipocalins expressed in female hamster LG display an unusual and marked repression by both androgens and estrogens, which could be detected both at the level of transcripts and proteins. Here, we investigate whether these repressions, (i) require presence of pituitary and (ii) are mediated by androgen and estrogen receptors. Pituitary-ablation but not gonadectomy reduced LG weights in hamster. However, both pituitary-ablation and gonadectomy induced abundant expression of the LG lipocalins, which were markedly repressed by androgen or estrogen treatment. AR- and ER-antagonists prevented these repressions and only ER-alpha- but not ER-beta-specific agonist could mimic the estrogenic repression. AR transcript and protein and ER-alpha transcript were also detected in hamster LG. Thus, pituitary factors are neither essential for the expression of these LG lipocalins nor for their estrogenic or androgenic repressions and these repressions are very likely mediated by functional ER and AR present in LG.

We have investigated the signalling properties of the chemokine receptor, CCR5, using several assays for agonism: stimulation of changes in intracellular Ca(2+) or CCR5 internalisation in CHO cells expressing CCR5 or stimulation of [(35)S]GTPgammaS binding in membranes of CHO cells expressing CCR5. Four isoforms of the chemokine CCL3 with different amino termini (CCL3, CCL3(2-70), CCL3(5-70), CCL3L1) were tested in these assays in order to probe structure/activity relationships. Each isoform exhibited agonism. The pattern of agonism (potency, maximal effect) was different in the three assays, although the rank order was the same with CCL3L1 being the most potent and efficacious. The data show that the amino terminus of the chemokine is important for signalling. A proline at position 2 (CCL3L1) provides for high potency and efficacy but the isoform with a serine at position 2 (CCL3(2-70)) is as efficacious in some assays showing that the proline is not the only determinant of high efficacy. We also increased the sensitivity of CCR5 signalling by treating cells with sodium butyrate, thus increasing the receptor/G protein ratio. This allowed the detection of a change in intracellular Ca(2+) after treatment with CCL7 and Met-RANTES showing that these ligands possess measurable but low efficacy. This study therefore shows that sodium butyrate treatment increases the sensitivity of signalling assays and enables the detection of efficacy in ligands previously considered as antagonists. The use of different assay systems, therefore, provides different estimates of efficacy for some ligands at this receptor.

Gonadotropin-releasing hormone (GnRH) regulates secretion of both of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone. Thus, it is a key hormone for vertebrate reproduction. GnRH was considered to be unusual among hypothalamic neuropeptides in that it appeared to have no direct antagonist, although some neurochemicals and peripheral hormones (opiates, GABA, gonadal steroids, inhibin) can modulate gonadotropin release to a degree. Five years ago, a vertebrate hypothalamic neuropeptide that inhibited pituitary gonadotropin release in a dose-dependent manner was discovered in quail by Tsutsui et al. (2000. Biochem Biophys Res Commun 275:661-667). We now know that this inhibitory peptide, named gonadotropin-inhibitory hormone, or GnIH, is a regulator of gonadotropin release in vitro and in vivo. Its discovery has opened the door to an entirely new line of research within the realm of reproductive biology. In our collaborative studies, we have begun to elucidate the manner in which GnIH interacts with GnRH to time release of gonadotropins and thus time reproductive activity in birds and mammals. This paper reviews the distribution of GnIH in songbirds relative to GnRHs, and our findings on its modes of action in vitro and in vivo, based on laboratory and field studies. These data are simultaneously compared with our findings in mammals, highlighting how the use of different model species within different vertebrate classes can be a useful approach to identify the conserved actions of this novel neuropeptide, along with its potential importance to vertebrate reproduction., ((c) 2006 Wiley-Liss, Inc.)

The European hamster (Cricetus cricetus) is a circannual species in which the synchronization of the circannual cycle to the natural year occurs during 2 annual phases of sensitivity. Around the summer solstice, the animals are sensitive to a shortening of photoperiod. During this sensitive phase, pronounced changes in circadian output parameters are observed, indicating a different functional state of the circadian system. This special state is assumed to be necessary to develop the extreme sensitivity to short day length in European hamsters during this phase. In natural conditions, the animals are able to recognize the shortening of photoperiod already in mid-July, when the photoperiod is reduced only by 30 min. To investigate the short-day response in sensitive European hamsters on the basis of the 2-coupled oscillator model of Pittendrigh and Daan (1976), daily activity and the reproductive state of European hamsters were recorded after an asymmetrical reduction of photoperiod from long (LD 16:08) to short (LD 08:16) photoperiods. The activity pattern of the animals showed an immediate response to the short photoperiod at the day of transfer when the night was extended only into the evening, but there was a significant delay in the response time when the night was extended into the morning. Thus, the evening oscillator E is more important in inducing the photoperiodic response than the morning oscillator M. Moreover, the broad intragroup variation in the latter conditions strongly suggests that the changes in the activity pattern were endogenously induced and that the animals were not able to recognize a lengthening of the night into the morning. Gonadal regression started in both groups 3 weeks after the change in the activity pattern, indicating that this process is initiated when the circadian system has received the short-day signal either through changes in photoperiod or through the circannual clock.

The suprachiasmatic nuclei (SCNs) control circadian rhythms of numerous behavioral and physiological responses. In hamsters, constant light causes "splitting" of circadian rhythms, such that a single daily bout of activity separates into two components, 12 h apart, with antiphase circadian oscillations in the left and right SCN. Given the phenotypic and functional heterogeneity of the SCN, in which ventrolateral but not dorsomedial neurons are retinorecipient, we asked how these two compartments respond to the constant lighting conditions that produce splitting, using three different phase markers of neuronal activity: PER1 (Period 1), c-FOS, and pERK (phosphorylated extracellular signal-regulated kinase). We report the emergence of a coherent novel network in which each side of the SCN exhibits two antiphase oscillating subregions, here termed "core-like" and "shell-like," in addition to the known antiphase oscillation between the right and left SCN. The novel SCN response entails a coherent rhythm in a core-like region of the SCN, which otherwise is not cycling. A mathematical model is presented, and this model interprets the observed changes in the proportion of in-phase and antiphase populations of SCN oscillators and suggests novel testable hypotheses. Finally, the functional significance of this network was explored by investigating the adjacent hypothalamus. Activation of the paraventricular nucleus is in-phase with the ipsilateral core-like SCN, whereas activation of the lateral subparaventricular zone is in-phase with the ipsilateral shell-like SCN, pointing to a multiplicity of SCN output signals. These results suggest a neural basis for internal coincidence of SCN oscillators, and a novel mechanism of plasticity in SCN neural networks and outputs.

European hamsters (Cricetus cricetus) show pronounced seasonal changes in their physiology and behavior. The present study provides a detailed analysis of the temporal relationship between seasonal cycles of reproduction and body mass and seasonal changes of two circadian parameters, i.e., locomotor activity and 6-sulphatoxymelatonin (aMT6s) excretion, in individual animals kept under natural light conditions. Our results demonstrate a characteristic pattern of locomotor activity and aMT6s excretion observed around the summer solstice, i.e., from mid-May to mid-July. During this time, locomotor activity was characterized by a high level of activity and an early activity onset, while the nightly elevation of melatonin was reduced to baseline levels. These seasonal changes in aMT6s excretion and locomotor activity were only loosely related to changes in the reproductive status of the animals, but correlated well with a period of the annual cycle during which the animals were sensitive to short days. They may therefore reflect a specific state of the circadian pacemaker system within the SCN and can thus be a valuable tool to further characterize molecular and physiological mechanisms of photoperiodic time measurements in European hamsters.

When Syrian hamsters (Mesocricetus auratus) are bred in a cold and short-day environment, most animals go into hibernation after a certain period of time. However, to date it has not been possible to predict which hamster will enter hibernation. In this study, we subcutaneously implanted thermo-loggers in hamsters bred in the cold environment, and recorded the subcutaneous temperature at short intervals until they went into hibernation. A time series analysis of temperature disclosed that a fall of 0.4 to 0.8 degrees C in subcutaneous temperature was seen 5 to 16 days before entering hibernation, and this phenomenon continued for three days or more. No hamster went into the hibernation without displaying this signal. Although the mechanism by which this phenomenon takes place is not clear, it is a sign from the body, which is useful for indicating if a hamster will enter hibernation shortly.

To clarify whether the common alpha-subunit of glycoprotein hormones is involved in photic signal transduction, alpha-subunit mRNA levels in the pars tuberalis (PT) of both hamsters and chickens were estimated at different time points of the day/night cycle by laser capture microdissection (LCM) and real-time quantitative polymerase chain reaction (PCR). Distinct diurnal rhythms were found for alpha-subunit mRNA expression in both species. In the hamster PT, alpha-subunit mRNA levels gradually increased during the dark phase; the diurnal peak was found at time (ZT) 21. The lowest value was obtained at ZT 5 during the day. In the chicken PT, alpha-subunit mRNA levels were maintained at a low constant level at night between ZT 13 and 21. Thus, alpha-subunit mRNA expression in the PT depends on the light-dark cycle and may be controlled by the pineal hormone melatonin. The effect of various photoperiods on the hamster PT was examined by real-time PCR, immunohistochemistry, and electron microscopy. In hamsters kept under short photoperiod (L/D=8 h:16 h) or complete darkness, a dramatic decrease of alpha-subunit mRNA level was induced, and the PT-specific cells accumulated glycogen-like particles and enlarged secretory granules. Under long photoperiods (L/D=16 h:8 h), however, the alpha-subunit mRNA level was elevated and the PT-specific cells exhibited highly active features, i.e., piles of lamellar cisternae of rough endoplasmic reticulum and well-developed Golgi complexes. The alpha-subunit synthesized by the PT-specific cells may therefore participate in the circadian and seasonal regulation of endocrine activities.