Independence of circadian entrainment state and responses to melatonin in male Siberian hamsters

Background

Seasonal fluctuations in physiology and behavior depend on the duration of nocturnal melatonin secretion programmed by the circadian system. A melatonin signal of a given duration, however, can elicit different responses depending on whether an animal was previously exposed to longer or shorter photoperiod signals (i.e., its photoperiodic history). This report examined in male Siberian hamsters which of two aspects of photoperiod history – prior melatonin exposure or entrainment state of the circadian system – is critical for generating contingent responses to a common photoperiodic signal.

Results

In Experiment #1, daily melatonin infusions of 5 or 10 h duration stimulated or inhibited gonadal growth, respectively, but had no effect on entrainment of the locomotor activity rhythm to long or short daylengths, thereby demonstrating that melatonin history and entrainment status could be experimentally dissociated. These manipulations were repeated in Experiment #2, and animals were subsequently exposed to a 12 week regimen of naturalistic melatonin signals shown in previous experiments to reveal photoperiodic history effects. Gonadal responses differed as a function of prior melatonin exposure but were unaffected by the circadian entrainment state. Experiment #3 demonstrated that a new photoperiodic history could be imparted during four weeks of exposure to long photoperiods. This effect, moreover, was blocked in animals treated concurrently with constant release melatonin capsules that obscured the endogenous melatonin signal: Following removal of the implants, the gonadal response depended not on the immediately antecedent circadian entrainment state, but on the more remote photoperiodic conditions prior to the melatonin implant.

Conclusions

The interpretation of photoperiodic signals as a function of prior conditions depends specifically on the history of melatonin exposure. The photoperiodic regulation of circadian entrainment state contributes minimally to the interpretation of melatonin signals.

     

Maternal entrainment of tau mutant hamsters.

Maternal entrainment of the circadian wheel-running activity rhythm was examined in Syrian hamsters heterozygous for a single gene mutation (tau) that affects the free-running period of circadian rhythms. Heterozygous tau pups were born to and raised by wild-type mothers under constant dim light. The pups' wheel-running activity was recorded after weaning on postnatal day 18 or 24. Pups weaned on day 18 had an average free-running period of 21.70 hr, demonstrating that the tau phenotype was fully expressed at this age. Using the activity onset of the postnatal free-running rhythms as a phase reference, we estimated the phase relationships between the pups and their mothers on days 18 and 24. In contrast to results with wild-type pups, the activity rhythms of tau pups were not in phase with the rhythms of their wild-type mothers; that is, activity onsets of mothers and pups did not coincide. The pups did, however, show synchrony among themselves, indicating that they had been exposed to a synchronizing signal sometime during development. It is likely that this synchronizing signal was provided by the mothers, since pups from different litters showed phase relationships similar to those of their mothers. Thus the mothers provided a signal that was sufficient to cause entrainment, despite the 2-hr difference in free-running period between the mothers and pups. Although the pups' activity rhythms appeared to have been entrained by the mothers, they were clearly free-running by postnatal day 18. The mechanism for entrainment is lost during the course of development, despite continued interaction between the mothers and pups.     

Social dominance determines estrous entrainment among female hamsters

In order to study the phenomenon of estrous entrainment and its relation to social behavior two studies were conducted with female hamsters. In the first experiment single females were placed in close proximity, but not in direct contact with three other females whose estrous cycles were already synchronous. The single female was either in phase (control) or out of phase (experimental) with the synchronous females. In the second experiment two females at the same (control) or different (experimental) points in their estrous cycles lived together until entrainment occurred. Their social interactions during the initial 30 min in this shared habitat were observed and a dominance hierarchy established. Estrous stage was determined daily for each animal by vaginal lavage. The major findings of these studies are (1) estrous entrainment occurred in all female hamsters whether living in close proximity (Expt I), or living together (Expt II); (2) regardless of the stage of estrous at the start of cohabitation, hamsters' cycles may be either accelerated or decelerated depending on whether the partner's stage is ahead or behind their own; (3) social interaction appears to accelerate the entrainment process, and (4) when examined in light of social dominance, the behaviorally submissive animals entrained to the behaviorally dominant animals.     

Masking of circadian activity rhythms in hamsters by darkness

Summary Wheel-running activity was recorded in male golden hamsters (Mesocricetus auratus) kept in constant dim illumination. For periods of several weeks the lights in the cabinet were turned off daily at the same time of day, either for 1 h or 2 h. Despite these periodically recurring dark pulses, the circadian activity rhythms continued to free-run, and consequently crossed through the pulses at a more or less regular speed. During a dark pulse, the activity was usually enhanced. The amount of these masking effects varied with the phase of the circadian cycle at which the pulse occurred. The responses were maximal a few hours after the onset of spontaneous activity, and minimal during the rest-time of the animal.     

Twilights widen the range of photic entrainment in hamsters

The range of entrainment of the circadian rhythm of locomotor activity was compared in four groups of Syrian hamsters (eight animals per group) initially exposed to daily light-dark (LD) cycles with either abrupt transitions between light and darkness (LD-rectangular) or simulated twilights (LD-twilight). Lighting was provided by arrays of white light-emitting diodes; daytime illuminance (10 lux) and the total amount of light emitted per day were the same in the two conditions. The period (T) of the LD cycles was then gradually increased to 26.5 h or gradually decreased to 21.5 h, at the rate of 5 min/day. Under LD-rectangular, the upper and lower limits of entrainment were 25.0 to 25.5 h and 22.0 to 22.5 h, respectively, whereas under LD-twilight, 50% of the animals exposed to the lengthening cycles were still entrained at T = 26.5 h and 50% of those exposed to the shortening cycles were still entrained at T = 21.5 h. In a second experiment, two groups of hamsters were exposed to fixed T = 25 h LD-rectangular (n = 15) or LD-twilight cycles (n = 7). Only 33% of the animals entrained in LD-rectangular, whereas 86% of the animals entrained in LD-twilight. Free-running periods in constant darkness were longer following successful entrainment to T = 25 h but did not differ between the animals that entrained to LD-rectangular and those that entrained to LD-twilight. The widening of the range of entrainment observed in LD-twilight indicates that twilight transitions increase the strength of the LD zeitgeber. In LD-twilight, successful entrainment to T = 26.5 h was accompanied by an expansion of activity time to 16.52+/-1.22 h, with activity onsets preceding mid-dusk by 12.56+/-2.15 h. Together with earlier data showing similar phase response curves for hour-long dawn, dusk, and rectangular light pulses, these results suggest that the effect of twilights on the range of entrainment may involve parametric rather than nonparametric mechanisms.     

Siberian hamsters that fail to reentrain to the photocycle have suppressed melatonin levels

Siberian hamsters readily reentrain to a 3-h phase delay of the photocycle (16 h light/day) but fail to reentrain to a 5-h phase delay. This study tested whether melatonin production was suppressed in animals that failed to reentrain. Melatonin was measured on the day before, day of, or several days after each phase shift. Melatonin levels measured 4 h after dark onset were approximately 83 microg/ml on the day before each phase delay and undetectable (<6 microg/ml) during the light phase on the day of the phase shift. Activity onsets regained their prior phase relationship to the photocycle 4 (3 h) or 5 (5 h) days after the phase shift; on that day, melatonin levels were measured 4 h after dark onset. Melatonin levels were unaffected by the 3-h phase delay (>57.6 microg/ml) but were undetectable after a 5-h phase delay (<8 microg/ml). Thus melatonin remained suppressed only after the phase delay to which hamsters also fail to reentrain. This relationship suggests that the propensity for reentrainment may be influenced by changes in melatonin production following a phase shift of the photocycle.     

Homeostatic regulation of sleep in arrhythmic Siberian hamsters

Sleep is regulated by independent yet interacting circadian and homeostatic processes. The present study used a novel approach to study sleep homeostasis in the absence of circadian influences by exposing Siberian hamsters to a simple phase delay of the photocycle to make them arrhythmic. Because these hamsters lacked any circadian organization, their sleep homeostasis could be studied in the absence of circadian interactions. Control animals retained circadian rhythmicity after the phase shift and re-entrained to the phase-shifted photocycle. These animals displayed robust daily sleep-wake rhythms with consolidated sleep during the light phase beginning about 1 h after light onset. This marked sleep-wake pattern was circadian in that it persisted in constant darkness. The distribution of sleep in the arrhythmic hamsters over 24 h was similar to that in the light phase of rhythmic animals. Therefore, daily sleep amounts were higher in arrhythmic animals compared with rhythmic ones. During 2- and 6-h sleep deprivations (SD), it was more difficult to keep arrhythmic hamsters awake than it was for rhythmic hamsters. Because the arrhythmic animals obtained more non-rapid eye movement sleep (NREMS) during the SD, they showed a diminished compensatory response in NREMS EEG slow-wave activity during recovery sleep. When amounts of sleep during the SD were taken into account, there were no differences in sleep homeostasis between experimental and control hamsters. Thus loss of circadian control did not alter the homeostatic response to SD. This supports the view that circadian and homeostatic influences on sleep regulation are independent processes.     

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

This article is part of a Special Issue “Energy Balance”.     

Short photoperiod exposure increases adipocyte sensitivity to noradrenergic stimulation in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit a naturally occurring, reversible seasonal obesity with body fat peaking in long "summerlike" days (LDs) and reaching a nadir in short "winterlike" days (SDs). These SD-induced decreases in adiposity are mediated largely via sympathetic nervous system (SNS) innervation of white adipose tissue (WAT), as indicated by increased WAT norepinephrine (NE) turnover. We examined whether SDs also increase sensitivity to NE-stimulated lipolysis. This was accomplished by measuring NE- and beta3-adrenoceptor (beta3-AR) agonist (BRL-37344)-induced lipolysis (glycerol release) as well as NE-induced cAMP accumulation by inguinal, epididymal, and retroperitoneal WAT (IWAT, EWAT, and RWAT) in isolated adipocytes of LD- and SD-housed hamsters. SDs increased potency/efficacy of NE-triggered lipolysis in a temporally and fat pad-specific manner. Thus when WAT pad mass decreased most rapidly (5 wk of SDs), potency (sensitivity/EC50) and efficacy (maximal response asymptote) of NE-stimulated lipolysis were increased for all WAT pads and also at 10 wk for IWAT compared with their LD counterparts. SD enhancement of lipolysis was similar for NE and BRL-37344 in IWAT adipocytes. These results, coupled with our previous demonstration that SDs upregulate WAT beta3-AR mRNA expression, suggest that increased beta3-ARs mediated the SD-induced increased NE sensitivity. NE-stimulated adipocyte accumulation of cAMP was greater after 5 wk of SDs for IWAT and EWAT and after 10 wk of SDs for IWAT compared with LDs, with no photoperiod effect for RWAT. Therefore, the SD-induced increase in SNS drive to WAT and increased sensitivity to this drive may work together to increase lipolysis in SDs.     

Non-parametric photic entrainment of Djungarian hamsters with different rhythmic phenotypes

To investigate the role of non-parametric light effects in entrainment, Djungarian hamsters of two different circadian phenotypes were exposed to skeleton photoperiods, or to light pulses at different circadian times, to compile phase response curves (PRCs). Wild-type (WT) hamsters show daily rhythms of locomotor activity in accord with the ambient light/dark conditions, with activity onset and offset strongly coupled to light-off and light-on, respectively. Hamsters of the delayed activity onset (DAO) phenotype, in contrast, progressively delay their activity onset, whereas activity offset remains coupled to light-on. The present study was performed to better understand the underlying mechanisms of this phenomenon. Hamsters of DAO and WT phenotypes were kept first under standard housing conditions with a 14:10 h light–dark cycle, and then exposed to skeleton photoperiods (one or two 15-min light pulses of 100 lx at the times of the former light–dark and/or dark–light transitions). In a second experiment, hamsters of both phenotypes were transferred to constant darkness and allowed to free-run until the lengths of the active (α) and resting (ρ) periods were equal (α:ρ = 1). At this point, animals were then exposed to light pulses (100 lx, 15 min) at different circadian times (CTs). Phase and period changes were estimated separately for activity onset and offset. When exposed to skeleton-photoperiods with one or two light pulses, the daily activity patterns of DAO and WT hamsters were similar to those obtained under conditions of a complete 14:10 h light–dark cycle. However, in the case of giving only one light pulse at the time of the former light–dark transition, animals temporarily free-ran until activity offset coincided with the light pulse. These results show that photic entrainment of the circadian activity rhythm is attained primarily via non-parametric mechanisms, with the “morning” light pulse being the essential cue. In the second experiment, typical photic PRCs were obtained with phase delays in the first half of the subjective night, phase advances in the second half, and a dead zone during the subjective day. ANOVA indicated no significant differences between WT and DAO animals despite a significantly longer free-running period (tau) in DAO hamsters. Considering the phase shifts induced around CT0 and the different period lengths, it was possible to model the entrainment patterns of both phenotypes. It was shown that light-induced phase shifts of activity offset were sufficient to compensate for the long tau in WT and DAO hamsters, thus enabling a stable entrainment of their activity offsets to be achieved. With respect to activity onsets, phase shifts were sufficient only in WT animals; in DAO hamsters, activity onset showed increasing delays. The results of the present paper clearly demonstrate that, under laboratory conditions, the non-parametric component of light and dark leads to circadian entrainment in Djungarian hamsters. However, a stable entrainment of activity onset can be achieved only if the free-running period does not exceed a certain value. With longer tau values, hamsters reveal a DAO phenotype. Under field conditions, therefore, non-photic cues/zeitgebers must obviously be involved to enable a proper circadian entrainment.     

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

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

Early photoperiod history and short-day responsiveness in Siberian hamsters

Siberian hamsters exhibit seasonal, photoperiod influenced cycles of reproductive activity, body size, pelage characteristics, and thermoregulatory behavior. Laboratory populations generally exhibit inter-individual variability in expression of photoperiod responsiveness, with a subset of individuals that fail to show the species typical responses to short photoperiod. This variability is partly explained by a genetic component, as it has been possible to increase the number of short-day nonresponders by artificial selection. Responsiveness to short photoperiod is also substantially influenced by photoperiod history in this species; hamsters that have been raised under long (16L) or very long (18L) day lengths are less likely to exhibit winter-type responses to short days as compared to hamsters raised under an intermediate (14L) day length. In the present experiment, we examined effects of age and early photoperiod history in a strain of Siberian hamsters that had been selected for short-day nonresponsiveness. Hamsters transferred into short photoperiod on the day of birth were uniform in exhibiting winter-type responses. However, hamsters raised until 25 days of age in either continuous illumination or in 16L exhibited variation in responsiveness when subsequently moved into short photoperiod. We conclude that virtually all hamsters of the short-day nonresponsive strain are born responsive to short days. Subsequent development of resistance to potential short day effects is dependent on age and/or photoperiod history.     

Photoperiod-peptide interactions in the energy intake of Siberian hamsters

Siberian hamsters (Phodopus sungorous sungorous) decrease their food intake when exposed to short (“winter-like”) photoperiods. The cause of this naturally-occurring hypophagia is unknown, but it may be due to a heightened sensitivity to the factors that normally terminate food intake in long photoperiods, such as the putative satiety peptides. The purpose of the present investigation was to test whether there would be an enhanced sensitivity to the inhibitory effects of some of these peptides on food intake in short relative to long days. Ad lib-fed, adult female Siberian hamsters were housed in a long photoperiod (LD 14:10) and injected with bombesin, glucagon, cholecystokinin octapeptide (CCK-8) and calcitonin (CT). Food intake was monitored 1, 2, 4, 6, and 24 hr post-injection. Bombesin and glucagon had no effect on food intake in long day-housed hamsters. CCK-8 and CT inhibited food intake; however, CCK-8 did so without any apparent behavioral disruption, while CT produced a marked and prolonged depression of behavior. After 10 weeks of exposure to a short photoperiod (LD 8:16) the hamsters were tested again. The previously ineffective dose of bombesin greatly inhibited food intake following short photoperiod exposure. In addition, an increased inhibition of food intake by CCK-8 was also found. In contrast, glucagon did not decrease food intake and CT still produced its non-specific, behaviorally disruptive effects. To our knowledge, this is the first demonstration that the effectiveness of a putative satiety peptide can be dependent upon a change in the photoperiod. This heightened responsiveness of short photoperiod-exposed Siberian hamsters to the inhibitory effects of bombesin and cholecystokinin may account for the reduction in food intake that accompanies short day exposure in this species.     

Interval timer control of puberty in photoinhibited Siberian hamsters

Puberty, which is markedly delayed in male Siberian hamsters (Phodopus sungorus) born into short day lengths, is controlled by an interval timer regulated by the duration of nocturnal melatonin secretion. Properties of the interval timer were assessed by perturbing normal patterns of melatonin secretion in males gestated and maintained thereafter in 1 of 2 short day lengths, 10 h light/day (10 L) or 12L. Melatonin secretion of short-day hamsters was suppressed by constant light treatment or modified by daily injection of propranolol to mimic nocturnal melatonin durations typical of long-day hamsters. Constant light treatment during weeks 3 to 5 induced early incomplete gonadal growth in 12L but not 10 L hamsters but did not affect late onset of gonadal development indicative of puberty in either photoperiod. Propranolol treatment during postnatal weeks 3 to 5 induced transient growth of the testes and ultimately delayed the timing of puberty by 3 weeks. Similar treatments between weeks 5 and 7 or on alternate weeks for 24 weeks did not affect the interval timer. The first 2 weeks after weaning may constitute a critical period during which the interval timer is highly responsive to photoperiod. Alternatively, the hamsters' photoperiodic history rather than age or developmental stage may be the critical variable. The interpolation of long-day melatonin signals at the time of weaning does not appear to reset the interval timer to its zero position but may reduce timer responsiveness to long-day melatonin signals several weeks later.     

Torpor characteristics and energy requirements of furless Siberian hamsters.

After approximately 10 wk of exposure to decreasing day lengths, Siberian hamsters (Phodopus sungorus) begin to display spontaneous torpor bouts several times each week. Torpor is associated with reduced daily energy expenditure and lower food consumption and ameliorates the thermoregulatory challenges of winter. We tested the extent to which the energy savings conferred by daily torpor depend on the presence of an insulative pelage. Female hamsters were housed in a winter day length (8L:16D) at 5 degrees C; daily food intake and torpor characteristics were recorded for 5 wk in shaved (furless) or normal hamsters. Torpor-bout incidence decreased by 62% in furless hamsters, but the duration of individual bouts and the minimum body temperature attained during torpor were unaffected by loss of pelage. Body temperature declined more rapidly during entry into torpor and increased more slowly during arousal from torpor in furless than in control hamsters. Energy savings per torpor bout, assessed by the amount of food consumed on days that included a torpor bout, was substantially greater in normal than in furless hamsters (16.0% vs. 3.3%); this difference likely reflects the increased cost of thermoregulation during torpor, as well as the increased caloric expenditure incurred by furless hamsters during arousal from torpor. An insulative pelage may be a prerequisite for the energetic benefits derived from heterothermy in this species.     

Exogenous T3 mimics long day lengths in Siberian hamsters

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.     

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

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

Melatonin production accompanies arousal from daily torpor in Siberian hamsters

Arousal from deep hibernation is accompanied by a transient rise of melatonin (Mel) in circulation; there are no comparable analyses of Mel concentrations in species that undergo much shallower, shorter duration episodes of daily torpor. Serum Mel concentrations were determined during arousal from both natural daily torpor and torpor induced by 2-deoxy-D-glucose (2-DG) treatment (2,500 mg/kg, intraperitoneal [IP]); blood samples were drawn from the retro-orbital sinus of anesthetized Siberian hamsters. For animals kept in darkness during torpor, Mel concentrations were highest during early arousal when thermogenesis is maximal, and they decreased as body temperature increased during arousal and returned to baseline once euthermia was reestablished. In hamsters kept in the light during the torpor bout, Mel concentrations were elevated above basal values during arousal, but the response was significantly blunted in comparison with values recorded in darkness. Increased Mel concentrations were detected in hamsters only during arousal from torpor (either natural or 2-DG induced) and were not simply a result of the drug treatment; hamsters that remained euthermic or manifested mild hypothermia after drug treatment maintained basal Mel concentrations. We propose that increased Mel production may reflect enhanced sympathetic activation associated with intense thermogenesis during arousal from torpor rather than an adjustment of the circadian rhythm of Mel secretion.     

Pregnancy-induced changes in ultradian rhythms persist in circadian arrhythmic Siberian hamsters

The impact of pregnancy and lactation on ultradian rhythms (URs) and circadian rhythms (CRs) of locomotor activity was assessed in circadian rhythmic and arrhythmic Siberian hamsters maintained in a long-day photoperiod (16 h light/day). Progressive decrements in CR robustness and amplitude over the course of gestation were accompanied by enhanced URs. Dark-phase UR period and amplitude increased during early gestation and complexity and robustness increased during late gestation. The persistence of pregnancy-associated enhancements of URs in circadian arrhythmic (ARR) hamsters suggests that reproductive modulation of the UR waveform is not dependent on coherent circadian organization. The increased incidence of dark-phase URs appeared more rapidly in ARR dams than entrained (ENTR) dams. Throughout gestation, the percentage of dams with dark-phase URs was significantly greater in the ARR group. Gestational increases in UR complexity and robustness emerged earlier and were greater in ARR than ENTR dams. The attenuation of CRs during lactation is correlated with increased expression of URs. Relaxation of circadian control of the dam's behavior may increase fitness by permitting more efficient interactions with circadian arrhythmic pups.     

Photoperiodic regulation of the orexigenic effects of ghrelin in Siberian hamsters

Animals living in temperate climates with predictable seasonal changes in food availability may use seasonal information to engage different metabolic strategies. Siberian hamsters decrease costs of thermoregulation during winter by reducing food intake and body mass in response to decreasing or short-day lengths (SD). These experiments examined whether SD reduction in food intake in hamsters is driven, at least in part, by altered behavioral responses to ghrelin, a gut-derived orexigenic peptide which induces food intake via NPY-dependent mechanisms. Relative to hamsters housed in long-day (LD) photoperiods, SD hamsters consumed less food in response to i.p. treatment with ghrelin across a range of doses from 0.03 to 3 mg/kg. To determine whether changes in photoperiod alter behavioral responses to ghrelin-induced activation of NPY neurons, c-Fos and NPY expression were quantified in the arcuate nucleus (ARC) via double-label fluorescent immunocytochemistry following i.p. treatment with 0.3 mg/kg ghrelin or saline. Ghrelin induced c-Fos immunoreactivity (-ir) in a greater proportion of NPY-ir neurons of LD relative to SD hamsters. In addition, following ghrelin treatment, a greater proportion of ARC c-Fos-ir neurons were identifiable as NPY-ir in LD relative to SD hamsters. Changes in day length markedly alter the behavioral response to ghrelin. The data also identify photoperiod-induced changes in the ability of ghrelin to activate ARC NPY neurons as a possible mechanism by which changes in day length alter food intake.