Twenty-four-hour profiles of serum leptin in siberian and golden hamsters: photoperiodic and diurnal variations

Short communication: Photoperiod impacts ovarian extracellular matrix and metabolic gene expression in Siberian hamsters

Ovarian cyclicity is variable in adult Siberian hamsters (Phodopus sungorus), who respond to long breeding season photoperiods with follicle development and ovulation, while short photoperiods typical of the non-breeding season induce gonadal atrophy. Recent RNAseq results identified ovarian matrix components and regulators of metabolism as differentially regulated by photoperiod; however, the impact of photoperiod across a full cycle of ovarian regression and recrudescence had not been explored for additional regulators of ovarian metabolism and extracellular matrix components. We hypothesized that matrix and metabolism-related genes would be expressed differentially across photoperiods that mimic breeding and non-breeding season daylengths. Hamsters were housed in one of four photoperiod groups: long day (16 h of light per day: 8 h of dark; LD, controls), short day regressed (8 L:16D; SD, regressed), and females exposed to SD then transferred to LD to stimulate return of ovarian function for 2 (early recrudescence), or 8 (late recrudescence) weeks. Plasma leptin concentrations along with expression of ovarian versican and liver-receptor homolog-1/Nr582 mRNA decreased in SD compared to LD and late recrudescence, while vimentin mRNA expression peaked in early and late recrudescence. Ovarian expression of fibronectin and extracellular matrix protein-1 was low in LD ovaries and increased in regressed and recrudescing groups. Expression of hyaluronidase-2, nectin-2, liver-X receptors-α and-β, and adiponectin mRNA peaked in late recrudescence, with no changes noted for adiponectin receptor-1 and -2. The results offer a first look at the parallels between expression of these genes and the dynamic remodeling that occurs during ovarian regression and recrudescence.

Pleiotropic effects of proopiomelanocortin and VGF nerve growth factor inducible neuropeptides for the long-term regulation of energy balance

Seasonal rhythms in energy balance are well documented across temperate and equatorial zones animals. The long-term regulated changes in seasonal physiology consists of a rheostatic system that is essential to successful time annual cycles in reproduction, hibernation, torpor, and migration. Most animals use the annual change in photoperiod as a reliable and robust environmental cue to entrain endogenous (i.e. circannual) rhythms. Research over the past few decades has predominantly examined the role of first order neuroendocrine peptides for the rheostatic changes in energy balance. These anorexigenic and orexigenic neuropeptides in the arcuate nucleus include neuropeptide y (Npy), agouti-related peptide (Agrp), cocaine and amphetamine related transcript (Cart) and pro-opiomelanocortin (Pomc). Recent studies also indicate that VGF nerve growth factor inducible (Vgf) in the arcuate nucleus is involved in the seasonal regulation of energy balance. In situ hybridization, qPCR and RNA-sequencing studies have identified that Pomc expression across fish, avian and mammalian species, is a neuroendocrine marker that reflects seasonal energetic states. Here we highlight that long-term changes in arcuate Pomc and Vgf expression is conserved across species and may provide rheostatic regulation of seasonal energy balance.

Photoperiodic and Diurnal Regulation of WNT Signaling in the Arcuate Nucleus of the Female Djungarian Hamster, Phodopus sungorus

The WNT pathway was shown to play an important role in the adult central nervous system. We previously identified the WNT pathway as a novel integration site of the adipokine leptin in mediating its neuroendocrine control of metabolism in obese mice. Here we investigated the implication of WNT signaling in seasonal body weight regulation exhibited by the Djungarian hamster (Phodopus sungorus), a seasonal mammal that exhibits profound annual changes in leptin sensitivity. We furthermore investigated whether crucial components of the WNT pathway are regulated in a diurnal manner. Gene expression of key components of the WNT pathway in the hypothalamus of hamsters acclimated to either long day (LD) or short day (SD) photoperiod was analyzed by in situ hybridization. We detected elevated expression of the genes WNT-4, Axin-2, Cyclin-D1, and SFRP-2, in the hypothalamic arcuate nucleus, a key energy balance integration site, during LD compared with SD as well as a diurnal regulation of Axin-2, Cyclin-D1, and DKK-3. Investigating the effect of photoperiod as well as leptin on the activation (phosphorylation) of the WNT coreceptor LRP-6-(Ser1490) by immunohistochemistry, we found elevated activity in the arcuate nucleus during LD relative to SD as well as after leptin treatment (2 mg/kg body weight). These findings indicate that differential WNT signaling may be associated with seasonal body weight regulation and is partially regulated in a diurnal manner in the adult brain. Furthermore, they suggest that this pathway plays a key role in the neuroendocrine regulation of body weight and integration of the leptin signal.

Pineal melatonin is a circadian time-giver for leptin rhythm in Syrian hamsters

Nocturnal secretion of melatonin from the pineal gland may affect central and peripheral timing, in addition to its well-known involvement in the control of seasonal physiology. The Syrian hamster is a photoperiodic species, which displays gonadal atrophy and increased adiposity when adapted to short (winter-like) photoperiods. Here we investigated whether pineal melatonin secreted at night can impact daily rhythmicity of metabolic hormones and glucose in that seasonal species. For that purpose, daily variations of plasma leptin, cortisol, insulin and glucose were analyzed in pinealectomized hamsters, as compared to sham-operated controls kept under very long (16 h light/08 h dark) or short photoperiods (08 h light/16 h dark). Daily rhythms of leptin under both long and short photoperiods were blunted by pinealectomy. Furthermore, the phase of cortisol rhythm under a short photoperiod was advanced by 5.6 h after pinealectomy. Neither plasma insulin, nor blood glucose displays robust daily rhythmicity, even in sham-operated hamsters. Pinealectomy, however, totally reversed the decreased levels of insulin under short days and the photoperiodic variations in mean levels of blood glucose (i.e., reduction and increase in long and short days, respectively). Together, these findings in Syrian hamsters show that circulating melatonin at night drives the daily rhythmicity of plasma leptin, participates in the phase control of cortisol rhythm and modulates glucose homeostasis according to photoperiod-dependent metabolic state.

Food-anticipatory activity in Syrian hamsters: behavioral and molecular responses in the hypothalamus according to photoperiodic conditions

When food availability is restricted, animals adjust their behavior according to the timing of food access. Most rodents, such as rats and mice, and a wide number of other animals express before timed food access a bout of activity, defined as food-anticipatory activity (FAA). One notable exception amongst rodents is the Syrian hamster, a photoperiodic species that is not prone to express FAA. The present study was designed to understand the reasons for the low FAA in that species. First, we used both wheel-running activity and general cage activity to assess locomotor behavior. Second, the possible effects of photoperiod was tested by challenging hamsters with restricted feeding under long (LP) or short (SP) photoperiods. Third, because daytime light may inhibit voluntary activity, hamsters were also exposed to successive steps of full and skeleton photoperiods (two 1-h light pulses simulating dawn and dusk). When hamsters were exposed to skeleton photoperiods, not full photoperiod, they expressed FAA in the wheel independently of daylength, indicating that FAA in the wheel is masked by daytime light under full photoperiods. During FAA under skeleton photoperiods, c-Fos expression was increased in the arcuate nuclei independently of the photoperiod, but differentially increased in the ventromedial and dorsomedial hypothalamic nuclei according to the photoperiod. FAA in general activity was hardly modulated by daytime light, but was reduced under SP. Together, these findings show that food-restricted Syrian hamsters are not prone to display FAA under common laboratory conditions, because of the presence of light during daytime that suppresses FAA expression in the wheel.

Body mass affects seasonal variation in sickness intensity in a seasonally breeding rodent

Species that display seasonal variation in sickness intensity show the most intense response in the season during which they have the highest body mass, suggesting that sickness intensity may be limited by an animal's energy stores. Siberian hamsters (Phodopus sungorus) display lower body masses and less intense sickness when housed in short, winter-like days as opposed to long, summer-like days. To determine whether reduced sickness intensity displayed by short-day hamsters is a product of seasonal changes in body mass, we food restricted long-day hamsters so that they exhibited body mass loss that mimicked the natural photoperiod-induced loss of body mass in short-day hamsters. We then experimentally induced sickness with lipopolysaccharide (LPS) and compared sickness responses among long-day food-restricted and long- and short-day ad libitum fed groups, predicting that long-day food-restricted hamsters would show sickness responses comparable to those of short-day ad libitum fed hamsters and attenuated in comparison to long-day ad libitum fed hamsters. We found that long-day food-restricted hamsters showed attenuated LPS-induced anorexia, loss of body mass and hypothermia compared with long-day ad libitum fed animals; however, anorexia remained elevated in long-day food-restricted animals compared with short-day ad libitum fed animals. Additionally, LPS-induced anhedonia and decreases in nest building were not influenced by body mass. Results of hormone assays suggest that cortisol levels could play a role in the attenuation of sickness in long-day food-restricted hamsters, indicating that future research should target the roles of glucocorticoids and natural variation in energy stores in seasonal sickness variation.

Leptin mediates seasonal variation in some but not all symptoms of sickness in Siberian hamsters

Many seasonally breeding species, including Siberian hamsters (Phodopus sungorus), exhibit seasonal variation in sickness responses. One hypothesis regarding the mechanism of this variation is that sickness intensity tracks an animal's energetic state, such that sickness is attenuated in the season that an animal has the lowest fat stores. Energetic state may be signaled via leptin, an adipose hormone that provides a signal of fat stores. Siberian hamsters respond to extended housing in short, winter-like days by reducing fat stores and leptin levels, relative to those housed in long, summer-like days. Sickness responses are also attenuated in short-day hamsters as compared to long-day hamsters. We hypothesized that leptin provides a physiological signal by which seasonally breeding animals modulate sickness responses, such that animals with higher leptin levels show increased sickness intensity. To test this, we provided short-day hamsters with a long-day-like leptin signal and assessed their responses to lipopolysaccharide (LPS), a sickness-inducing antigen. We compared these responses to short-day vehicle-, long-day vehicle-, and long-day leptin-treated hamsters. Unexpectedly, LPS induced a hypothermic response (rather than fever) in all groups. Short-day vehicle-treated hamsters exhibited the greatest LPS-induced hypothermia, and leptin treatment attenuated this response, making hypothermia more long-day-like. Contrary to our hypothesis, short-day leptin-treated hamsters showed the least pronounced LPS-induced anorexia among all groups. These results suggest that leptin may mediate some but not all aspects of seasonal sickness variation in this species. Future studies should be targeted at determining roles of other energetic hormones in regulating seasonal sickness response variation.

Hormonal changes and energy substrate availability during the hibernation cycle of Syrian hamsters

Animals have to adapt to seasonal variations in food resources and temperature. Hibernation is one of the most efficient means used by animals to cope with harsh winter conditions, wherein survival is achieved through a significant decrease in energy expenditure. The hibernation period is constituted by a succession of torpor bouts (hypometabolism and decrease in body temperature) and periodic arousals (eumetabolism and euthermia). Some species feed during these periodic arousals, and thus show different metabolic adaptations to fat-storing species that fast throughout the hibernation period. Our study aims to define these metabolic adaptations, including hormone (insulin, glucagon, leptin, adiponectin, GLP-1, GiP) and metabolite (glucose, free fatty acids, triglycerides, urea) profiles together with body composition adjustments. Syrian hamsters were exposed to varied photoperiod and temperature conditions mimicking different phases of the hibernation cycle: a long photoperiod at 20 °C (LP20 group), a short photoperiod at 20 °C (SP20 group), and a short photoperiod at 8 °C (SP8). SP8 animals were sampled either at the beginning of a torpor bout (Torpor group) or at the beginning of a periodic arousal (Arousal group). We show that fat store mobilization in hamsters during torpor bouts is associated with decreased circulating levels of glucagon, insulin, leptin, and an increase in adiponectin. Refeeding during periodic arousals results in a decreased free fatty acid plasma concentration and an increase in glycemia and plasma incretin concentrations. Reduced incretin and increased adiponectin levels are therefore in accordance with the changes in nutrient availability and feeding behavior observed during the hibernation cycle of Syrian hamsters.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Effects of photoperiod on daily locomotor activity, energy expenditure, and feeding behavior in a seasonal mammal

The objective of this study was to determine whether the previously observed effects of photoperiod on body weight in Siberian hamsters were due to changes in the daily patterns of locomotor activity, energy expenditure, and/or feeding behavior. Adult males were monitored through a seasonal cycle using an automated comprehensive laboratory animal monitoring system (CLAMS). Exposure to a short-day photoperiod (SD; 8:16-h light-dark cycle) induced a significant decline in body weight, and oxygen consumption (Vo(2)), carbon dioxide production (Vco(2)), and heat production all decreased reaching a nadir by 16 wk of SD. Clear daily rhythms in locomotor activity, Vo(2), and Vco(2) were observed at the start of the study, but these all progressively diminished after prolonged exposure to SD. Rhythms in feeding behavior were also detected initially, reflecting an increase in meal frequency but not duration during the dark phase. This rhythm was lost by 8 wk of SD exposure such that food intake was relatively constant across dark and light phases. After 18 wk in SD, hamsters were transferred to a long-day photoperiod (LD; 16:8-h light-dark cycle), which induced significant weight gain. This was associated with an increase in energy intake within 2 wk, while Vo(2), Vco(2), and heat production all increased back to basal levels. Rhythmicity was reestablished within 4 wk of reexposure to long days. These results demonstrate that photoperiod impacts on body weight via complex changes in locomotor activity, energy expenditure, and feeding behavior, with a striking loss of daily rhythms during SD exposure.

Suprachiasmatic Nuclei May Regulate the Rhythm of Leptin Hormone Release in Syrian Hamsters (Mesocricetus Auratus)

The suprachiasmatic nuclei (SCN) generate the circadian rhythm of many hormones. The hormone leptin is a metabolic signal that informs the brain about fat and energy stores of the body. We investigated whether the rhythm of leptin hormone release in Syrian hamsters is directly controlled by the SCN. Three experiments were performed: in the first, hamsters were SCN-lesioned; in the second, hamsters were exposed to different feeding regimes; and in the third, hamsters were adrenalectomized and implanted with cortisol capsules to maintain constant glucocorticoid release. Blood samples were collected before and after the experiments at different clock times and examined for leptin levels by enzyme-linked immunosorbant assay (ELISA). Different feeding regimes and constant glucocorticoid release did not alter the rhythm of leptin release; whereas, SCN lesions abolished the rhythm. The results of the present study suggest the rhythm in leptin release in Syrian hamsters may be controlled by the SCN.

Body mass loss during adaptation to short winter-like days increases food foraging, but not food hoarding

Siberian hamsters markedly reduce their body/lipid mass ( approximately 20-45%) in short 'winter-like' days (SD). Decreases in body/lipid mass associated with food deprivation or lipectomy result in increases in foraging and food hoarding. When at their SD-induced body/lipid mass nadir, food hoarding is not increased despite their decreases in body/lipid mass, but hoarding was not tested during the dynamic period of body/lipid mass loss (first 5-6 weeks of SDs). Therefore, we tested for changes in foraging/hoarding during this initial period in Siberian hamsters housed in a simulated burrow with a wheel running-based foraging system and exposed to either long 'summer-like' days (LD) or SDs. Two foraging effort conditions were used: 10 Revolutions/Pellet (pellet delivered after running 10 revolutions) and a Free Wheel/Free Food condition (wheel available, food pellets non-contingently available). Regardless of the foraging condition, body mass was significantly reduced across 8 weeks of SDs ( approximately 15%). Foraging increased after 7 weeks in SDs, but food hoarding did not increase compared to LDs. Instead food hoarding significantly decreased in SDs at Weeks 2-5 compared with Week 0 values, with the 10 Revolutions/Pellet foraging group returning to LD levels thereafter and the Free Wheel/Free Food group remaining reduced from Weeks 2-7. Collectively, we found that SDs decreased body mass, increased foraging after 7 weeks, and increased food hoarding, but only after an initial decrease and not above that seen in LDs. These data suggest that SD-induced body/lipid mass losses do not engender similar behavioral responses as seen with food deprivation or lipectomy.

The regulation of seasonal changes in food intake and body weight

Seasonal rhythms of body weight, reflecting altered food intake, energy storage and expenditure, are a common feature of mammals inhabiting temperate and arctic latitudes. They have evolved so that predictable annual changes in the external environment can be anticipated and animals can adjust their physiology and behaviour in preparation for the changing demands of the seasons. These long-term changes in energy balance are not simply effected by the brain centres and peptidergic pathways known to underlie short-term homeostatic regulation. Screens of altered gene expression in Siberian hamsters comparing the anabolic summer state in long photoperiods and the catabolic 'winter' state in short photoperiods have identified differential gene expression in the hypothalamus. The majority of gene expression changes are confined to two restricted areas: the dorsomedial posterior arcuate nucleus, and the ventral ependymal layer of the third ventricle. Functions encoded by these 'seasonal' genes include thyroid hormone metabolism, retinoic acid and histaminergic signalling, and VGF and secretogranin production. The changes in thyroid hormone availability that are brought about by differential activity of deiodinase enzymes are of particular importance because experimental manipulation of central thyroid levels can prevent seasonal cyclicity. Given the importance of thyroid hormone in the initial development of the brain, we hypothesise that thyroid hormone-dependent plasticity of hypothalamic connections and neurogenesis underlie seasonal cycles of food intake and body weight.

Diurnal profiles of hypothalamic energy balance gene expression with photoperiod manipulation in the Siberian hamster, Phodopus sungorus

Hypothalamic energy balance genes have been examined in the context of seasonal body weight regulation in the Siberian hamster. Most of these long photoperiod (LD)/short photoperiod (SD) comparisons have been of tissues collected at a single point in the light-dark cycle. We examined the diurnal expression profile of hypothalamic genes in hamsters killed at 3-h intervals throughout the light-dark cycle after housing in LD or SD for 12 wk. Gene expression of neuropeptide Y, agouti-related peptide, proopiomelanocortin, cocaine- and amphetamine-regulated transcript, long-form leptin receptor, suppressor of cytokine signaling-3, melanocortin-3 receptor, melanocortin-4 receptor, and the clock gene Per1 as control were measured by in situ hybridization in hypothalamic nuclei. Effects of photoperiod on gene expression and leptin levels were generally consistent with previous reports. A clear diurnal variation was observed for Per1 in the suprachiasmatic nucleus in both photoperiods. Temporal effects on expression of energy balance genes were restricted to long-form leptin receptor in the arcuate nucleus and ventromedial nucleus, where similar diurnal expression profiles were observed, and melanocortin-4 receptor in the paraventricular nucleus; these effects were only observed in LD hamsters. There was no variation in serum leptin concentration. The 24-h profiles of hypothalamic energy balance gene expression broadly confirm photoperiodic differences that were observed previously, based on single time point comparisons, support the growing consensus that these genes have a limited role in seasonal body weight regulation, and further suggest limited involvement in daily rhythms of food intake.

Phodopus campbelli detect reduced photoperiod during development but, unlike Phodopus sungorus, retain functional reproductive physiology

Golden (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamsters are widely used as animal models for seasonal reproduction; butM. auratusshows no developmental delay in short days until after sexual maturity, whereasP. sungorusjuveniles delay development in short days. As the photoperiodic response ofPhodopus campbelliis not well established, litters of the twoPhodopusspecies were gestated and reared under long days (14 h light:10 h darkness) or short days (10 h light:14 h darkness) until 70 days of age. As expected, under short photoperiodP. sungorusshowed reduced body, testes, epididymides, uterus, and ovary weight; antral follicles and corpora lutea were absent and vaginae remained closed. Animals moulted to winter pelage, and low concentrations of each of leptin, testosterone, and prolactin were present in male serum.Phodopus campbellijuveniles also responded to the short photoperiod as measured by reduced body, testes, epididymides, and ovary weight. The summer pelage persisted. However, both sexes ofP. campbellideveloped functional reproduction under 10 h light:14 h darkness. All females had a patent vagina by 10 weeks; ovaries contained antral follicles and corpora lutea, and uteri were not reduced in weight. In males, the concentrations of testosterone, leptin, and prolactin were not reduced by short photoperiod. Developmental patterns in the three species of hamster, therefore, differ and are not predicted by relatedness or latitude of origin. Other ecological traits, such as predictability of summer rainfall, ambient temperature, and differential responses to social cues might be important.

Short photoperiod influences energy intake and serum leptin level in Brandt's voles (Microtus brandtii)

Photoperiod cues play important roles in the regulation of seasonal variations in body mass (BM) and energy balance for many small mammals. The present study was designed to examine the effects of photoperiod acclimation on BM, energy intake, and serum leptin levels in Brandt's voles (Microtus (Lasiopodomys) brandtii). After 4 weeks of acclimation to either long (LD; light:dark, 16:8) or short (SD; 8:16) photoperiod, SD voles had lower BM, body fat mass, and dry mass of liver and kidneys, but higher digestible energy intake in comparison to LD voles. SD voles also showed a lower level of serum leptin than did LD voles. Furthermore, the level of serum leptin was correlated positively with body fat mass and negatively with gross energy intake. Together, these data suggest that Brandt's voles employ a strategy of minimizing body growth, increasing energy intake, and mobilizing fat deposition in response to cues associated with short photoperiod. Furthermore, leptin seems to be involved in the regulation of BM and energy balance mediated by photoperiod.

Effects of Photoperiod and Food Availability on Growth, Leptin, Sexual Maturation and Maintenance in the Mongolian Gerbils (Meriones unguiculatus)

Reproductive activity of Mongolian gerbils is regulated by photoperiod nevertheless body weight regulation is controlled without ambient photoperiod. Food intake is a major factor affecting rodent reproductive efficiency. Leptin is a hormone secreted by adipose tissue and modulates food intake, energy expenditure and body fat stores. In this study we studied the interaction of photoperiod and food availability on growth, sexual maturation and leptin concentration in the male and female gerbils. Gerbils were gestated and reared in either 14L:10D or 2L:22D. At weaning, gerbils were housed individually and divided into three groups: fed ad libitum, fed 80% of ad libitum or fed 60% of ad libitum. Body weights were recorded at weaning and every week thereafter. After twelve weeks of treatment, animals were sacrificed and testes and uterine weights were determined and blood was collected for leptin measurement. Food restriction reduced body weight and inhibited reproductive development. Absolute paired testis weights were similar in ad lib and 80% of ad lib groups but significantly different compared with the 60% of ad lib group in both photoperiods. Body weights were also directly dependent upon the level of food restriction. Uterine mass was only affected in the 60% of ad lib group in 14L but both food restriction levels significantly affected the uterine weights in 2L. Significant variations were found in leptin profiles. Leptin concentration was highest in ad lib and 80% of ad lib groups and lowest in 60% of ad lib groups. These results suggest that the reproductive activity of Mongolian gerbils is sensitive to food intake and multiple potential environmental cues (e.g., food availability, temperature) can be utilized.

Circulating ghrelin levels and central ghrelin receptor expression are elevated in response to food deprivation in a seasonal mammal (Phodopus sungorus)

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor (GHSR). However, the functional interaction of ligand and receptor is not very well understood. We demonstrate that GHSR mRNA is up-regulated after food deprivation (48 h) in the hypothalamic arcuate nucleus and ventromedial nucleus of the seasonal Siberian hamster, Phodopus sungorus. This increase is accompanied by a two-fold elevation of circulating ghrelin concentration. Chronic changes in feeding state imposed by food restriction over a period of 12 weeks during long day-length induced increased GHSR gene expression, whereas food restriction for 6 weeks had no effect. Phodopus sungorus reveals remarkable seasonal changes in body weight, fat mass and circulating leptin levels. Ghrelin is generally regarded as having opposing effects on appetite and body weight with respect to those exhibited by leptin. However, our study revealed that seasonal adaptations were not accompanied by changes in either GHSR gene expression or circulating ghrelin concentration. Therefore, we suggest that ghrelin only plays a minor role in modulating long-term seasonal body weight cycles. Our findings imply that ghrelin predominantly acts as a short-term regulator of feeding.

Daily and annual variations of free fatty acid, glycerol and leptin plasma concentrations in goats (Capra hircus) under different photoperiods

The aim of the study was to differentiate the impact of lighting conditions and feeding times on the regulation of lipid metabolism of goats under different photoperiods throughout the year. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (the longest light period 18 h, the shortest 6 h). Ambient temperature and feeding regime were kept constant. Blood samples were collected six times a year at 2-h intervals for 2 days, first in light/dark (LD) conditions and then after 3 days in constant darkness (DD). Significant daily variations were detected in the concentrations of plasma free fatty acids (FFA) and glycerol throughout the year. The nocturnal decrease and morning rise of FFA levels were related to the photoperiod, while the trough levels of glycerol were associated with the concentrate meal times. In DD conditions, FFA and glycerol rhythms were unstable. A significant seasonal variation was detected in the overall FFA and glycerol levels suggesting decreased lipogenesis in winter, increased lipolysis in spring and high lipogenesis in summer and fall. There was no significant daily rhythm in serum leptin levels, nor did the profiles in LD and DD conditions differ. The leptin level was slightly lower in early fall than in the other seasons, paralleling a small decrease of body mass in the goats after the grazing season. The daily or annual variations of FFA and glycerol levels were not clearly related to leptin concentrations. The results suggest that lipid metabolism of goats is regulated by light even in constant temperature and feeding conditions; however, no significant contribution of leptin levels could be shown.

Pheromone-induced anorexia in male Syrian hamsters

Transition from long days (LDs) to short days (SDs) triggers seasonal obesity in Syrian hamsters. We report here that SD-exposed males housed near females exhibit obesity resistance, episodic weight loss, and reduced adiposity. Negative energy balance is achieved by reduced eating, elevated motor activity, and increased caloric efficiency without metabolic compensation. Circulating leptin, insulin, testosterone, corticosterone, and cortisol are normal or reduced in obesity-resistant hamsters. When males are housed in chambers that block physical, visual, and auditory, but not pheromonal, signals from females, resistance to seasonal obesity persists. Moreover, inhalation of extracts from pheromone-releasing flank glands of females suppresses eating and weight gain in SD-exposed males. This novel phenomenon, pheromone-induced anorexia, shows that female pheromones play a critical role in the seasonal energy balance of male hamsters. These findings provide a model to study neural and endocrine mechanisms that underlie eating disorders.

Leptin and seasonal mammals

Seasonal mammals commonly exhibit robust annual cycles of adiposity, food intake and energy metabolism. These cycles are driven by changes in the external daylength signal, which generates a diurnal melatonin profile and acts on neuroendocrine pathways. The white adipose tissue hormone leptin reflects overall adiposity in seasonal mammals, and consequently undergoes significant seasonal fluctuations in secretion. The seasonally breeding Siberian (Djungarian) hamster is a convenient laboratory model to study the effect of a seasonal time-keeping clock on energy metabolism, appetite regulation and the control of adiposity. We have shown that administration of exogenous leptin at physiological doses induces significant loss of adipose tissue for short-day housed winter-like hamsters in which endogenous adipose tissue and leptin concentrations are already low. By contrast, long-day housed hamsters with high adipose tissue reserves are refractory to the effects of leptin. This phenomenon of seasonal leptin resistance appears to be a general feature of other seasonally breeding mammals, and may reflect the operation of an annual timer controlling leptin uptake and/or action on central nervous system signal transduction pathways. The mobilization of fat by leptin in short-day housed hamsters is not associated with changes in expression in either anorexic or anabolic peptides expressed in leptin-receptor rich structures in the arcuate region of the hypothalamus, and suggests that leptin may target other structures. These data contrast with studies, which show that homeostatic mechanisms in response to feed-restriction induce changes in hypothalamic peptides in a similar manner to nonphotoperiodic species. Thus, the long-term seasonal regulation of body weight set point and leptin feedback may operate through separate pathways to those responsible for acute responses to food restriction.

Different photoperiods affect proliferation of lymphocytes but not expression of cellular, humoral, or innate immunity in hamsters

In seasonal mammals, photoperiod change is associated with a suite of alterations in physiology. It has recently been proposed that the immune response is one of the systems regulated by changes in photoperiod, although this hypothesis has not been rigorously challenged by assays of functional immune responses. The aim of this study was to test the hypothesis that photoperiod modulates immune responsiveness in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamsters. Consistent with previously reported data, short-day-housed (SD) animals exhibited a significant increase in lymph node cell (LNC) numbers and increased cellular proliferation in response to the polyclonal mitogen concanavalin A compared to long-day-housed (LD) animals. In contrast, LNC numbers from intact or gonadectomized SD animals that had been sensitized with the antigen dinitrofluorobenzene (DNFB) exhibited a reduced ex vivo proliferative response and reduced production of interleukin-6 (IL-6) compared to LD animals. In vivo studies of the contact hypersensitivity response of animals that had previously been sensitized, and subsequently challenged, with DNFB were similar in SD and LD animals, as was the proliferative activity of LNC recovered from these animals. There were also no photoperiodic differences in the antidinitrophenyl antibody response of animals sensitized with DNFB, or the anti-sheep red blood cell (srbc) response of animals immunized with srbc. Furthermore, no differences could be detected in the activity of natural killer cells from spleens of LD and SD Siberian hamsters, or in lipopolysaccharide-induced IL-6 production by LD and SD Syrian hamsters in vivo. Thus, although photoperiod is able to influence factors regulating the gross number and non-antigen-specific proliferation of lymphocytes in seasonally breeding mammals, day length does not directly influence activation of an effective immune response. The authors conclude, therefore, that expression of the immune response is not directly modified or compromised by photoperiod in these seasonally breeding hamster species.

Cocaine and amphetamine-regulated transcript mRNA regulation in the hypothalamus in lean and obese rodents

Cocaine and amphetamine-regulated transcript (CART) mRNA and immunoreactivity are expressed abundantly in the hypothalamus. Central administration of various fragments of this neuropeptide decreases food intake in rodents. To find out whether CART might play a role in the physiological regulation of energy balance, we used in situ hybridization to investigate whether CART mRNA abundance changed in two chronic obese/fat versus lean states and after acute dietary restriction. In the first study, mice were treated with goldthioglucose to destroy glucose-responsive neurones in the ventromedial hypothalamus. This produced hyperphagia and obesity: 7 weeks after treatment, those receiving goldthioglucose weighed 70% more than the controls. CART mRNA abundance in the arcuate nucleus of goldthioglucose-treated mice was decreased by 71% compared to levels in the control mice, but CART expression was unaffected in the dorsolateral hypothalamus. In the second study, male Siberian hamsters were exposed to short days to induce a physiological winter response in which body weight decreases as fat reserves are catabolized, and food intake correspondingly declines. After 8 weeks in short days, body weight had declined by 18% relative to controls maintained in long days in a summer fat state. CART mRNA levels did not differ significantly between the two groups in any hypothalamic areas. In the third study, male Siberian hamsters, either in long days or after 12 weeks exposure to short days to induce weight loss, were subject to a 48-h period of fasting. Although photoperiod per se did not affect CART expression, fasting produced a significant decrease in CART mRNA in the arcuate nucleus of hamsters in both the long- and short-day state. We conclude that CART-producing cells are involved in energy homeostasis: the marked decrease in CART expression in the arcuate nucleus in goldthioglucose-lesioned mice may contribute to the development of obesity, and the decrease following acute dietary restriction in hamsters may reflect a compensatory mechanism to reduce caloric expenditure, but our results do not indicate that CART is involved in long-term seasonal regulation of body weight.

Differential regulation of luteinizing hormone and follicle-stimulating hormone in male siberian hamsters by exposure to females and photoperiod

Siberian hamsters have decreased gonadotropin levels and testis size after short-day (SD) exposure. Upon transfer from short to long days, FSH and testis weight increase rapidly, whereas LH and T remain low for much longer. We investigated whether an additional environmental stimulus, specifically a female, could trigger an earlier release of LH and whether the response to the female was dependent on photoperiod. An increase in serum LH was induced in long day (LD), but not SD, males within minutes of female exposure. The ability of SD males to secrete LH upon female exposure was regained within 4 d of photostimulation. FSH was not secreted after female exposure, but varied with photoperiod. Thus, FSH and LH are differentially regulated by photoperiod and female exposure. In subsequent studies melatonin injections and a GnRH antagonist were used to show that photoperiod modulates the endocrine responsiveness of a male to a female via melatonin and that female-induced LH release is GnRH dependent. Collectively, these results suggest separation of gonadotropin signaling pathways by environmental stimuli and provide an excellent model to elucidate the effects of photoperiod on the processing of social and chemosensory inputs to the GnRH neurons of the hypothalamus.

Daily rhythm in serum melatonin and leptin levels in the Syrian hamster (Mesocricetus auratus)

Melatonin is produced and secreted by the pineal gland in a rhythmic manner; circulating levels are high at night and low in the day. Leptin is a hormone secreted by adipocytes as a product of the obese gene and plays an important role in regulating body energy homeostasis and reproductive function in rodents and humans. The present study was conducted to examine daily fluctuations in serum levels of melatonin and leptin in Syrian hamster. We measured serum leptin and melatonin levels by ELISA in (a) intact and pinealectomized (pinx) male hamsters kept under long daylight conditions [14 h of light (14L)]; (b) intact and pinx hamsters under short daylight (10L); and (c) intact hamsters in constant light (24L). Blood samples were obtained every 2 h throughout a 24-h period. Statistically significant circadian variations were found in both melatonin and leptin profiles. Their relationship was inverse, i.e. when melatonin was high in the serum, leptin was comparably low. These results suggest that there is a rhythm in leptin levels in the adult male Syrian hamster and this rhythm is pineal gland (melatonin) and/or photoperiod dependent.

Seasonal changes in adiposity: the roles of the photoperiod, melatonin and other hormones, and sympathetic nervous system

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity-Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Seasonally inappropriate body weight induced by food restriction: effect on hypothalamic gene expression in male Siberian hamsters

Male Siberian hamsters undergo physiological weight change in changing photoperiod. Weight loss was induced by food restriction in long days to mimic short-day weight loss, or by food restriction superimposed on short-day weight loss, to test the hypothesis that the hypothalamus differentiates between weight change induced by imposed negative energy balance (inappropriate body weight) and seasonal, appropriate, body weight change, even when these are of similar magnitude. Short-day weight loss was accompanied by reduced POMC and leptin receptor (OB-Rb) mRNA in the arcuate nucleus but elevated cocaine- and amphetamine-regulated transcript. Melanocortin 3-receptor gene expression was reduced in the arcuate nucleus but elevated in the ventromedial nucleus compared with ad libitum-fed long-day controls. Weight loss in long-day restricted animals generated a gene expression profile typical of negative energy balance with low cocaine- and amphetamine-regulated transcript mRNA and elevated OB-Rb. Melanocortin 3-receptor mRNA levels were indistinguishable in short-day and long-day food-restricted hamsters. The hypothalamic correlates of food restriction in short days included up-regulated anabolic neuropeptides and increased OB-Rb mRNA. Low plasma leptin is integrated differently in short-day and long-day restricted animals, and seasonally-inappropriate body weight in either photoperiod engages the compensatory neuropeptide systems involved in the defense of body weight.

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.