The effects of short periods of immobilization on the hamster circadian clock

Seasonality in Biological Rhythms in Scandinavian brown Bears

Biological rhythms, such as rhythms in activity and body temperature, are usually highly synchronized and entrained by environmental conditions, such as photoperiod. However, how the expression of these rhythms changes during hibernation, when the perception of environmental cues is limited, has not yet been fully understood for all hibernators, especially in the wild. The brown bear (Ursus arctos) in Scandinavia lives in a highly seasonal environment and adapts to harsh winter conditions by exhibiting hibernation, characterized by reduced metabolism and activity. In this study, we aimed to explore the expression of biological rhythms in activity, body temperature and heart rate of free-ranging brown bears over the annual cycle, including active, hibernation and the transition states around den entry and exit. We found that rhythms in physiology and activity are mostly synchronized and entrained by the light-dark cycle during the bears’ active state with predominantly diel and ultradian rhythms for body temperature, activity and heart rate. However, during hibernation, rhythms in body temperature and heart rate were considerably slowed down to infradian rhythms, influenced by the amount of snow in the denning area, whereas rhythms in activity remained diel. Rhythms in the transition states when bears prepared for entering or coming out of hibernation state displayed a combination of infradian and diel rhythms, indicating the preparation of the body for the change in environmental conditions. These results reveal that brown bears adjust their biological rhythms to the seasonal environment they inhabit. Rhythms in physiology and activity show simultaneity during the active state but are partly disconnected from each other during hibernation, when bears are most sheltered from the environment.

A High Cattle-Grazing Density Alters Circadian Rhythmicity of Temperature, Heart Rate, and Activity as Measured by Implantable Bio-Loggers

Six cows managed under extensive grazing conditions were used to study the effect of moving the animals to a higher grazing density on the circadian rhythms of temperature (T), heart rate (HR), and activity (ACT), which were recorded by implantable bio-loggers. Cows were maintained at a density of 1.5 livestock units per hectare (LSUs/ha; low density, LD) until they were moved to a grazing area at 128 LSUs/ha (high density, HD). Animals were implanted subcutaneously with a T, HR, and ACT bio-logger, which was programmed to record data at 5-min intervals. For each animal, cosinor rhythmometry (the study of circadian rhythms by fitting a sine wave to a time series) was applied to the data recorded over 5 days in LD and HD. Mean Midline Estimating Statistic of Rhythm (MESOR; the average value around which the variable oscillates), amplitude (difference between the peak and the mean value of a wave), and acrophase (timing of peak activity) were calculated and evaluated statistically. Differences between mean day and nighttime values, and mean LD and HD values were calculated. Cows presented cosinor curves that fit a 24-h rhythm (p < 0.001) in T, HR, and ACT at both densities. MESOR (T: 37.98 vs. 38.02°C; HR: 69.12 vs. 65.91 bpm; ACT: 49.39 vs. 40.41 mg, for LD and HD, respectively) and amplitude (T: 0.28 vs. 0.28°C; HR: 4.12 vs. 3.14 bpm; ACT: 18.14 vs. 11.28 mg, respectively) did not differ significantly between the two densities; however, significant (p < 0.05) differences between densities occurred in the acrophase of the three variables; specifically, the T acrophase was 2 h later at HD (22:45 h) than LD (20:45 h), and HR (LD: 19:51; HD: 16:49 h) and ACT acrophases 3 and 2 h earlier at HD than LD (LD: 14:47; HD: 12:49 h), respectively. T and ACT differed significantly (p < 0.01) between daytime (mean ± SE; 37.92 ± 0.19°C, 40.39 ± 4.74 mg) and nighttime (38.14 ± 0.17°C, 29.93 ± 5.66 mg). In conclusion, our study suggests that a high animal grazing density might exacerbate the social competence for valuable resources for animals, resulting in shifting the circadian rhythmicity of temperature, heart rate, and activity of the cows, advancing or delaying their acrophases.

Chronic social defeat stress suppresses locomotor activity but does not affect the free-running circadian period of the activity rhythm in mice

In mammals, daily rhythms in behavior and physiology are under control of an endogenous clock or pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN assures an optimal temporal organization of internal physiological process and also synchronizes rhythms in physiology and behavior to the cyclic environment. The SCN receives direct light input from the retina, which is capable of resetting the master clock and thereby synchronizes internally driven rhythms to the external light-dark cycle. In keeping with its function as a clock and pacemaker, the SCN appears to be well buffered against influences by other stimuli and conditions that contain no relevant timing information, such as acute stressors. On the other hand, it has been suggested that chronic forms of stress may have gradually accumulating effects that can disturb normal clock function and thereby contribute to stress-related disorders. Therefore, in the present study we investigated whether chronic intermittent social stress affects the endogenous period and phase of the free-running activity rhythm in mice. Adult male mice were maintained in constant dim red light conditions and exposed to a daily 20 min social defeat stress session for 10 consecutive days, either during the first half of their activity phase or the first half of their resting phase. The overall amount of running wheel activity was strongly suppressed during the 10 days of social defeat, to about 50% of the activity in non-defeated control mice. Activity levels gradually normalized during post-defeat recovery days. Despite the strong suppression of activity in defeated animals, the endogenous free-running circadian period of the activity rhythm and the phase of activity onset were not affected. These findings are thus in agreement with earlier studies suggesting that the circadian pacemaker in the SCN that is driving the rhythmicity in activity is well-protected against stress. Even severe social defeat stress for 10 consecutive days, which has a major effect on the levels of activity, does not affect the pace of the endogenous clock.

•

Chronic intermittent social defeat stress strongly reduces locomotor activity in mice.

•

Chronic intermittent social defeat stress does not affect the circadian period and phase of the free-running activity rhythm under constant conditions.

•

Social defeat stress does not appear to affect the endogenous clock in the SCN driving the rhythmicity in activity.

Repeated Psychosocial Stress at Night Affects the Circadian Activity Rhythm of Male Mice

We have recently shown that molecular rhythms in the murine suprachiasmatic nucleus (SCN) are affected by repeated social defeat (SD) during the dark/active phase (social defeat dark [SDD]), while repeated SD during the light/inactive phase (social defeat light [SDL]) had no influence on PERIOD2::LUCIFERASE explant rhythms in the SCN. Here we assessed the effects of the same stress paradigm by in vivo biotelemetry on 2 output rhythms of the circadian clock (i.e., activity and core body temperature) in wild-type (WT) and clock-deficient Period (Per)1/2 double -mutant mice during and following repeated SDL and SDD. In general, stress had more pronounced effects on activity compared to body temperature rhythms. Throughout the SD procedure, activity and body temperature were markedly increased during the 2 h of stressor exposure at zeitgeber time (ZT) 1 to ZT3 (SDL mice) and ZT13 to ZT15 (SDD mice), which was compensated by decreased activity during the remaining dark phase (SDL and SDD mice) and light phase (SDL mice) in both genotypes. Considerable differences in the activity between SDL and SDD mice were seen in the poststress period. SDD mice exhibited a reduced first activity bout at ZT13, delayed activity onset, and, consequently, a more narrow activity bandwidth compared with single-housed control (SHC) and SDL mice. Given that this effect was absent in Per1/2 mutant SDD mice and persisted under constant darkness conditions in SDD WT mice, it suggests an involvement of the endogenous clock. Taken together, the present findings demonstrate that SDD has long-lasting consequences for the functional output of the biological clock that, at least in part, appear to depend on the clock genes Per1 and Per2.

Activation of growth hormone secretagogue receptor induces time-dependent clock phase delay in mice

Early studies have reported a phase-shifting effect of growth hormone secretagogues (GHSs). This study aimed to determine the mechanism of action of GHSs. We examined the response of the hypothalamic suprachiasmatic nuclei (SCN) to growth hormone releasing peptide-6 (GHRP-6) by assessing effects on the phase of locomotor activity rhythms, SCN neuronal discharges, and the potential signaling pathways involved in the drug action on circadian rhythms. The results showed that bolus administration of GHRP-6 (100 μg/kg ip) at the beginning of subjective night (CT12) induced a phase delay of the free-running rhythms in male C57BL/6J mice under constant darkness, but did not elicit phase shift at other checked circadian time (CT) points. The phase-delay effect of GHRP-6 was abolished by d-(+)-Lys-GHRP-6 (GHS receptor antagonist), KN-93 [calcium/calmodulin-dependent protein kinase II (CaMK) II inhibitor], or anti-phosphorylated (p)-cAMP response element-binding protein (CREB) antibody. Further analyses demonstrated that GHRP-6 at CT12 induced higher calcium mobilization and neuronal discharge in the SCN compared with that at CT6, decreased the levels of glutamate and γ-aminobutyric acid, increased the levels of p-CaMKII, p-CREB, and period 1, and delayed the circadian expressions of circadian locomotor output cycles kaput, Bmal1, and prokineticin 2 in the SCN; these signaling changes resulted in behavioral phase delay. Collectively, GHRP-6 induces a CT-dependent phase delay via activating GHS receptor and the downstream signaling, which is partially similar to the signaling cascade of light-induced phase delay at early night. These novel observations may help to better understand the role of GHSs in circadian physiology.

Alterations in the circadian system in advanced age

In addition to light, a variety of non-photic stimuli can induce phase shifts in the circadian clock of rodents. We have examined the effects of advanced age on the response of the circadian clock to both photic and non-photic stimuli in old hamsters (i.e., over 16 months of age). Among the age-related changes in the circadian rhythm of locomotor activity are: (1) alterations in the phase angle of entrainment to the light-dark cycle; (2) an altered response to the phase-shifting effects of light pulses; (3) changes in the time it takes to re-entrain to a new light-dark cycle; and (4) a loss of responsiveness to the phase-shifting or entraining effects of stimuli which induce an acute increase of activity. Many of the effects of ageing on the circadian clock system can be simulated in young animals by depleting brain monoamine levels, suggesting that ageing alters monoaminergic inputs to the clock. Some of the age-related changes in the response of the clock to an activity-inducing stimulus can be reversed by implanting old animals with fetal suprachiasmatic nuclear tissue. Determining the physiological basis of age-related changes in the responsiveness of the clock to both internal and external stimuli, and the mechanisms by which normal circadian functioning can be restored, should lead to new insight into the functioning of the circadian clock and may suggest new approaches to the normalization of disturbed circadian rhythms.

Reciprocal relationships between general (Propofol) anesthesia and circadian time in rats

Several common postdischarge symptoms, such as sleep disorders, headache, drowsiness or general malaise, evoke disturbances of circadian rhythms due to jet lag (ie crossing time zones) or shift work rotation. Considering that general anesthesia is associated with numerous effects on the central nervous system, we hypothesized that it may also act on the circadian timing system. We first determined the effects of the circadian timing on general anesthesia. We observed that identical doses of propofol showed marked circadian fluctuations in duration of effects, with a peak at the middle of the resting period (ie 7 h after lights on). Then, we examined the effects of general anesthesia on circadian timing, by analysing stable free-running circadian rhythms (ie in constant environmental conditions), an experimental approach used widely in circadian biology. Free-running rats were housed in constant darkness and temperature to assess possible phase-shifting effects of propofol anesthesia according to the time of the day. When administered around (+/-2 h) the daily rest/activity transition point, a 30-min propofol anesthesia induced a 1-h phase advance in the free-running rest-activity rhythm, while anesthesia had no significant resetting effect at other times of the day. Anesthesia-induced hypothermia was not correlated with the phase-shifting effects of propofol anesthesia. From our results, anesthesia itself can reset circadian timing, and acts as a synchronizing cue for the circadian clock.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

The enigma of behavioral inputs to the circadian clock: A test of function using restraint

Wheel running stimulated during the daily rest period can acutely shift circadian rhythms in Syrian hamsters. Spontaneous running, defining the active phase of the circadian rest-activity cycle, can shorten the circadian periodicity in constant light or dark in several nocturnal rodent species. The adaptive significance of these behavioral effects on pacemaker phase and period is unclear. Here we consider a hypothesis that behavioral inputs to the circadian pacemaker serve primarily to enhance the precision of light-dark entrainment and maintain daily activity onset close to lights-off (i.e., dusk) by stabilizing entrainment on a steeper portion of the delay zone of the phase-response curve to light. This hypothesis rests on the evidence that spontaneous activity early in the active period feeds back on the pacemaker to advance its motion. If so, then preventing activity at this time should induce a phase delay shift. Such delay shifts have been reported in Syrian hamsters physically restrained early in the active period. We show here that restraint can induce phase delays but that, using the Aschoff Type 2 procedure for measuring shifts, these delays are very small, are inversely related to behavioral sleep during restraint, and are positively correlated with 'rebound' increases in running following restraint, at a circadian time when stimulated running is known to induce phase delay shifts. Repeated bouts of restraint, to promote habituation, were associated with strong attenuation of 'rebound' running and no significant delay shifts. These results suggest that, in Syrian hamsters, spontaneous activity early at night has little effect on pacemaker motion, and argue against the stated hypothesis.

Restraint stress delays reentrainment in male and female diurnal and nocturnal rodents

A temporary loss of normal circadian entrainment, such as that associated with shift work and transmeridian travel, can result in an array of detrimental symptoms, making rapid reentrainment of rhythmicity essential. While there is a wealth of literature examining the effects of stress on the entrained circadian system, less is known about the influence of stress on circadian function following a phase shift of the light: dark (LD) cycle. The authors find that recovery of locomotor activity synchronization is altered by restraint stress in the diurnal rodent Octodon degus (degu) and the nocturnal rat. In the first experiment, degus were subjected to a 6-h phase advance of the LD cycle. Sixty minutes after the new lights-on, animals underwent 60 min of restraint stress. The number of days it took each animal to reentrain its activity rhythms to the new LD cycle was recorded and compared to the number of days it took the animal to reentrain under control conditions. When subjected to restraint stress, degus took 30% longer to reentrain their activity rhythms (p < 0.01). In a second experiment, rats underwent a similar experimental paradigm. As with the degus, stress significantly delayed the reentrainment of rats' activity rhythms (p < 0.01). There was no interaction between sex and stress on the rate of reentrainment for either rats or degus. Furthermore, there was no effect of stress on the free-running activity rhythm of degus, suggesting that the effect of stress on reentrainment rate is not secondary to alterations of period length. Together, these data point to a detrimental effect of stress on recovery of entrainment of circadian rhythms, which is independent of activity niche and sex.

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as 'zeitgebers' (time-cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory-specific nonphotic inputs to the clock. The circadian phase-dependence of clock resetting to social stimuli or arousal (the 'nonphotic' phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light-dark cycles, humans may ignore social cues and free-run independently, although cases of mutual synchrony among two or more group-housed individuals have been reported. Social cues may affect circadian timing by controlling sleep-wake states, but the phase of entrainment observed to fixed sleep-wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light-dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near-24 h sleep-wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.

Reward and Aversive Stimuli Produce Similar Nonphotic Phase Shifts

Circadian rhythms in rodents respond to arousing, nonphotic stimuli that contribute to daily patterns of entrainment. To examine whether the motivational significance of a stimulus is important for eliciting nonphotic circadian phase shirts in Syrian hamsters (Mesocricetus auratus), the authors compared responses to a highly rewarding stimulus (lateral hypothalamic brain stimulation reward [BSR]) and a highly aversive stimulus (footshock). Animals were housed on a 14:10-hr light-dark cycle until test day, when they were given a 1-hr BSR session (trained animals) or a 1-mA electric footshock at 1 of 8 circadian times, and were maintained in constant dark thereafter. Both BSR pulses and footshock produced nonphotic phase response curves. These results support the hypothesis that arousal resulting from the motivational significance of a stimulus is a major factor in nonphotic phase shifts.

Circadian clock resetting by arousal in Syrian hamsters: the role of stress and activity

Circadian rhythms in the Syrian hamster can be markedly phase shifted by 3 h of wheel running or arousal stimulation during their usual daily rest period ("subjective day"). Continuous wheel running is predictive but not necessary for phase shifts of this "nonphotic" type; hamsters aroused by gentle handling without running can also show maximal shifts. By contrast, physical restraint, a standard stress procedure and thus presumably arousing, is ineffective. To resolve this apparent paradox, phase-shifting effects of 3-h sessions of restraint or other stress procedures were assessed. In a preliminary study, phase shifts to arousal by gentle handling were significantly potentiated by the cortisol synthesis inhibitor metyrapone, suggesting that stress-related cortisol release may inhibit phase shifts to arousal. Next, it was confirmed that restraint in the subjective day does not induce phase shifts, but behavioral observations revealed that it also does not sustain arousal. Restraint combined with noxious compressed air blasts did sustain arousal and induced a significant cortisol response compared with arousal by gentle handling but did not induce shifts. Restraint combined with continuous horizontal rotation was also ineffective, as was EEG-validated arousal via confinement to a pedestal over water. However, 3 h of resident-intruder interactions (an intense psychosocial stress) or exposure to an open field (a mild stress) did induce large shifts that were positively correlated with indexes of forward locomotion. The results indicate that large phase shifts associated with arousal in the usual sleep period are neither induced nor prevented by stress per se, but are dependent on the expression of at least low levels of locomotor activity. Sustained arousal alone is not sufficient.

Circadian clock functioning is linked to acute stress reactivity in rats

At least two major physiological systems are involved in the adaptation of the organism to environmental challenges: the circadian system and the stress reaction. This study addressed the possibility that interindividual differences in stress sensitivity and in the functioning of the circadian system are related. At 2 months of age, corticosterone secretion in response to a 20-min restraint stress was assessed in 9 Sprague-Dawley rats for which running wheel activity was recorded as a rhythmic behavioral marker of the circadian clock. Two weeks later, the adaptive response of the circadian system to an abrupt shift in the light:dark (LD) cycle was assessed in those rats using a jet-lag paradigm. Finally, after resynchronization to the new LD cycle, rats were transferred to constant darkness to assess the free-running period of their circadian rhythm of running-wheel activity. Results indicate that stress-induced corticosterone secretion was (1) positively correlated with the number of days to resynchronize the circadian activity rhythm to the new LD cycle, and with the value of its free-running period, and (2) negatively correlated with the intensity of daily locomotor activity. Those data, emphasizing the interactions between the stress response of an organism and the functioning of its circadian system, could explain interindividual differences in humans' susceptibility to shift work or other circadian-related disorders.

Phase shifting the hamster circadian clock by 15-minute dark pulses

The mammalian circadian pacemaker can be phase shifted by exposure to a period of darkness interrupting otherwise continuous light. Circadian phase shifting by dark pulses was interpreted originally as reflecting a photic mirror-image mechanism, but more recent observations suggest that dark pulse-induced phase shifting may be mediated by a nonphotic, behavioral state-dependent mechanism. The authors recently presented evidence indicating that the dark-pulse phase response curve (PRC) is in fact a complex function, reflecting both photic mirror image and nonphotic mechanisms at different phases of the circadian cycle. Previous studies of dark pulse-induced phase shifting have universally employed relatively long (2 to 6 h) pulses, which complicates PRC analysis due to the extended segment of the underlying PRC spanned by such a long pulse. The present study was therefore designed to examine the phase-shifting effects of brief 15-min dark pulses presented at both mid-subjective day and subjective dusk, and to explore the possible activity dependence of these effects by using physical restraint to prevent evoked locomotor activity. The results indicate that 15-min dark pulses are effective phase-shifting stimuli at both midday and dusk. Furthermore, as with longer dark pulses, phase shifting by 15-min dark pulses is completely blocked by physical restraint during subjective day but combines in a simple additive manner with the independent phase-shifting effect of restraint at subjective dusk.

Adenosine and caffeine modulate circadian rhythms in the Syrian hamster

Extracellular adenosine accumulates in some brain areas during sleep deprivation. In Syrian hamsters, both sleep deprivation and adenosine A1 agonists can inhibit phase shifts of circadian rhythms to light at night. Sleep deprivation in the day (sleep period) can shift circadian phase. We examined whether the A1 agonist N-CHA mimics this effect. N-CHA (i.p. or i.c.) in the mid-sleep period induced dose-dependent shifts similar to those induced by 3 h sleep deprivation. The adenosine antagonist caffeine administered systemically at the mid-sleep period induced arousal without shifts, and dose-dependently attenuated shifts to a 3 h sleep deprivation procedure (running in a novel wheel). Adenosine may participate in resetting of the circadian clock by manipulations of behavioral state.

Sleep deprivation decreases phase-shift responses of circadian rhythms to light in the mouse: role of serotonergic and metabolic signals

The circadian pacemaker in the suprachiasmatic nuclei is primarily synchronized to the daily light-dark cycle. The phase-shifting and synchronizing effects of light can be modulated by non-photic factors, such as behavioral, metabolic or serotonergic cues. The present experiments examine the effects of sleep deprivation on the response of the circadian pacemaker to light and test the possible involvement of serotonergic and/or metabolic cues in mediating the effects of sleep deprivation. Photic phase-shifting of the locomotor activity rhythm was analyzed in mice transferred from a light-dark cycle to constant darkness, and sleep-deprived for 8 h from Zeitgeber Time 6 to Zeitgeber Time 14. Phase-delays in response to a 10-min light pulse at Zeitgeber Time 14 were reduced by 30% in sleep-deprived mice compared to control mice, while sleep deprivation without light exposure induced no significant phase-shifts. Stimulation of serotonin neurotransmission by fluoxetine (10 mg/kg), a serotonin reuptake inhibitor that decreases light-induced phase-delays in non-deprived mice, did not further reduce light-induced phase-delays in sleep-deprived mice. Impairment of serotonin neurotransmission with p-chloroamphetamine (three injections of 10 mg/kg), which did not increase light-induced phase-delays in non-deprived mice significantly, partially normalized light-induced phase-delays in sleep-deprived mice. Injections of glucose increased light-induced phase-delays in control and sleep-deprived mice. Chemical damage of the ventromedial hypothalamus by gold-thioglucose (600 mg/kg) prevented the reduction of light-induced phase-delays in sleep-deprived mice, without altering phase-delays in control mice. Taken together, the present results indicate that sleep deprivation can reduce the light-induced phase-shifts of the mouse suprachiasmatic pacemaker, due to serotonergic and metabolic changes associated with the loss of sleep.

Circadian clock resetting by sleep deprivation without exercise in the Syrian hamster

Circadian rhythms in several species can be phase-shifted by procedures that stimulate locomotor activity ("exercise") during the usual sleep period. The role of arousal or sleep loss, independent of activity, in this effect has not been adequately resolved. We show here, using the sleep deprivation procedure of gentle handling, that comparably large phase shifts (up to 240 min advances) of the rest-activity cycle can be induced in Syrian hamsters by 3 hr of behavioral arousal, with minimal locomotion, beginning 6 hr before the usual active period. Horizontal distance traveled during the deprivation procedure averaged approximately 0.08 km, compared to 2. 5 km typical in exercise studies. Hamsters requiring fewer interventions exhibited larger shifts, suggesting that the level or continuity of spontaneous arousal determines shift size. The circadian rhythm of light-induced c-fos expression in the suprachiasmatic nucleus (SCN) was used as a phase marker to further demonstrate that the clock is reset within 1 hr after a 3 hr deprivation. Sleep deprivation mimicked the effects of exercise on basal c-fos expression in two components of the circadian system, suppressing basal Fos immunoreactivity in the SCN, and increasing Fos in the intergeniculate leaflet. Sleep deprivation without exercise in hamsters can rapidly reset the circadian clock and alter gene expression within the circadian system.

Effects of light intensity and restraint on dark-pulse-induced circadian phase shifting during subjective night in Syrian hamsters

Dark pulses presented on a background of constant light (LL) result in phase advances during midsubjective day and early subjective night, and phase delays during late subjective night, as shown in the dark-pulse phase response curve. In hamsters, the phase-shifting effects of dark pulses are thought to be mediated by increased activity, as previous studies have shown that restraining animals during dark pulses blocks the phase shifts observed in midsubjective day and late subjective night. This study focuses on dark-pulse-induced phase shifting during early subjective night, examining the influence of both LL intensity and restraint on the magnitude of these phase shifts. Syrian hamsters were maintained in LL of four different illumination levels (1, 10, 100, or 600 lux) and periodically presented with 6-h pulses (dark pulse alone, restraint alone, or dark pulse plus restraint) beginning at circadian time 11. Phase advances were observed in response to dark pulses alone, and the magnitude of these shifts was dependent on background illumination, with significantly larger advances seen under higher intensities. No relationship was found between the amount of activity displayed during dark pulses and phase shift magnitude. Six-hour periods of restraint resulted in phase delays, the magnitude of which was also dependent on background illumination. Restraining hamsters during dark pulses reduced the magnitude of phase advances, but the extent of this reduction could be predicted from the additive effects of the dark-pulse-alone and restraint-alone conditions. These results indicate that the phase-shifting effects of dark pulses during early subjective night are not mediated by behavioral activation and may instead reflect a mirror image of the phase-delaying effects of light pulses at this phase.

Effects of hypoxia and hypercapnia on circadian rhythms in the golden hamster (Mesocricetus auratus)

This study was designed to determine whether respiratory stimuli can influence the mammalian circadian timing system. Three-hour pulses of hypoxia (inspired O(2) concentration = 8%) or hypercapnia (inspired CO(2) concentration = 11%) were presented for 7 days at mid-subjective day (circadian time 6-9) under constant darkness. Hypoxic and hypercapnic pulses caused cumulative phase delays of 46. 4 +/- 6.9 and 25.9 +/- 12.3 min, respectively. Distance run per day was significantly reduced on hypoxic and hypercapnic pulse days, compared with nonpulsed days. Phase shifts were correlated with the reduction in daily running activity (multiple r(2) = 0.521, P = 0.036), metabolic depression (multiple r(2) = 0.772, P < 0.001), and reduction in body temperature (multiple r(2) = 0.539, P = 0.027), but not lung ventilation (multiple r(2) = 0.306, P = 0.414) during pulses. We conclude that hypoxia and hypercapnia can influence the phase and quantity of activity in free-running hamsters.

Daytime naps in darkness phase shift the human circadian rhythms of melatonin and thyrotropin secretion

To systematically determine the effects of daytime exposure to sleep in darkness on human circadian phase, four groups of subjects participated in 4-day studies involving either no nap (control), a morning nap (0900-1500), an afternoon nap (1400-2000), or an evening nap (1900-0100) in darkness. Except during the scheduled sleep/dark periods, subjects remained awake under constant conditions, i.e., constant dim light exposure (36 lx), recumbence, and caloric intake. Blood samples were collected at 20-min intervals for 64 h to determine the onsets of nocturnal melatonin and thyrotropin secretion as markers of circadian phase before and after stimulus exposure. Sleep was polygraphically recorded. Exposure to sleep and darkness in the morning resulted in phase delays, whereas exposure in the evening resulted in phase advances relative to controls. Afternoon naps did not change circadian phase. These findings indicate that human circadian phase is dependent on the timing of darkness and/or sleep exposure and that strategies to treat circadian misalignment should consider not only the timing and intensity of light, but also the timing of darkness and/or sleep.

Neuroactive steroids alter the circadian system of the Syrian hamster in a phase-dependent manner

Several steroid compounds affect neuronal function, primarily by modulating the GABAA receptor complex. A circadian variation in the brain concentration of neurosteroids has been reported in rats and humans. We have previously reported that natural occurring or synthetic neuroactive steroids such as androsterone and alphaxalone also have a rhythmic effect on behavior (anesthetic and anticonvulsant activity) and GABAergic activity. In the present work, we have tested the ability of neuroactive steroids to phase shift circadian rhythms in hamsters. The GABA(A) negative modulator dehydroepiandrosterone sulphate (DHEAS) elicited phase advances when administered at CT 6, while the positive modulator androsterone lacked any effect at this time. A complete phase response curve for DHEAS revealed a nonphotic-like effect. DHEAS also blocked the circadian effects of light, while androsterone induced photic-like responses. There is also evidence that neurosteroids may be present and even synthesized in the SCN. Collectively, the results so far indicate that some neuroactive steroids might modulate the activity of the circadian clock.

Roles of suprachiasmatic nuclei and intergeniculate leaflets in mediating the phase-shifting effects of a serotonergic agonist and their photic modulation during subjective day

Serotonin (5-HT) has been implicated in the phase adjustment of the circadian system during the subjective day in response to nonphotic stimuli. Two components of the circadian system, the suprachiasmatic nucleus (SCN) (site of the circadian clock) and the intergeniculate leaflet (IGL), receive serotonergic projections from the median raphe nucleus and the dorsal raphe nucleus, respectively. Experiment 1, performed in golden hamsters housed in constant darkness, compared the effects of bilateral microinjections of the 5-HT1A/7 receptor agonist, 8-hydroxydipropylaminotetralin (8-OH-DPAT; 0.5 microgram in 0.2 microliter saline per side), into the IGL or the SCN during the mid-subjective day. Bilateral 8-OH-DPAT injections into either the SCN or the IGL led to significant phase advances of the circadian rhythm of wheel-running activity (p < .001). The phase advances following 8-OH-DPAT injections in the IGL were dose department (p < .001). Because a light pulse administered during the middle of the subjective day can attenuate the phase-resetting effect of a systemic injection of 8-OH-DPAT, Experiment 2 was designed to determine whether light could modulate 5-HT agonist activity at the level of the SCN and/or the IGL. Serotonergic receptor activation within the SCN, followed by a pulse of light (300 lux of white light lasting 30 min), still induced phase advances. In contrast, the effect of serotonergic stimulation within the IGL was blocked by a light pulse. These results indicate that the respective 5-HT projections to the SCN and IGL subserve different functions in the circadian responses to photic and nonphotic stimuli.

Neuropeptide Y blocks light-induced phase advances but not delays of the circadian activity rhythm in hamsters

In mammals, the suprachiasmatic nuclei (SCN) are the anatomical site of localization of the light-entrainable circadian clock responsible for the generation of daily rhythms in physiology and behavior. In addition to direct retinohypothalamic innervation, the SCN receive a prominent projection of fibers from the intergeniculate leaflet (IGL) of the thalamus, the geniculohypothalamic tract (GHT), some of which contain the neurotransmitter, neuropeptide Y (NPY). Since the GHT has been suggested to play a role in the modulation of photic entrainment of the SCN circadian clock in rodents, we investigated the effects of local administration of NPY into the region of the SCN on light-induced phase shifts of the free-running activity rhythm in hamsters. Injection of 60 nmol of NPY into the SCN region 10 min prior to light exposure at circadian time 19 completely blocked light-induced phase advances. Similar treatment at circadian time 14 had no significant effect on the magnitude of light-induced phase delays. Injection of NPY at either time point without light exposure did not alter circadian phase. The findings support a modulatory role for NPY in the photic entrainment of the SCN circadian clock.

Comparative effects of a melatonin agonist on the circadian system in mice and Syrian hamsters

S-20098 has potent and specific agonist properties on melatonin receptors both in vitro and in vivo. Behavioral studies on rodents already showed that repeated intraperitoneal administration of S-20098 could dose-dependently alter the functioning of the circadian clock. To determine whether single administration of S-20098 could alter the circadian rhythms of rodents, we first used the phase-response curve (PRC) approach in two different species: Syrian hamsters and mice (C3H/HeJ). Our results show that the shape, circadian times and extent of the PRC to S-20098 look very similar in mice and hamsters. In both species, the phase advance portion of the PRC to S-20098 is limited to a 3 h window preceding the onset of locomotor activity, but the magnitude of phase shifts is larger in mice. We also tested the phase shifting effects of increasing doses of S-20098 during the interval of maximal sensitivity to this compound. Treatment with S-20098 induces dose-dependent phase shifts, with maximal shifts observed after injections of 20 and 25 mg/kg S-20098 i.p., respectively, in mice and hamsters. Those results are in agreement with the limited distribution of melatonin-binding sites within the circadian clock of adult Syrian hamsters, as compared to other rodents.

Multiunit activity recordings in the suprachiasmatic nuclei: in vivo versus in vitro models

The suprachiasmatic nuclei (SCN) of the hypothalamus continue to oscillate when they are isolated in a brain slice preparation. We recorded multiunit activity in the SCN of the rat both in vivo and in vitro to determine the circadian discharge pattern. The variability of the discharge pattern is larger and the amplitude of the rhythm is smaller in vivo than in vitro. Moreover we found evidence for a direct effect of the animal's behavioural activity on electrical activity of the SCN in vivo. These findings may provide an electrophysiological basis for the known effects of behavioural stimuli on the circadian pacemaker. This study underscores the importance of recordings in intact preparations in addition to in vitro work when generalisations to physiological conditions are to be made.

Stress-induced changes in circadian rhythms of body temperature and activity in rats are not caused by pacemaker changes

Previous work has shown that social stress in rats (i.e., defeat by an aggressive male conspecific) causes a variety of behavioral and physiological changes including alterations in the daily rhythms of body temperature and activity. To study the role of the circadian pacemaker in these stress-induced changes, three experiments were performed, successively addressing pacemaker period, phase, and sensitivity to light. In all experiments, rats were subjected to social stress by placing them in the home cage of a dominant conspecific for 1 h. This was done on 2 consecutive days, between the second and fifth hours of the activity phase. Experimental animals were attacked by the resident and lost the fight as indicated by submissive behavior. Control animals were placed in an unfamiliar but clean and empty cage for 1 h. In Experiment 1, the effects of social stress on the period of the free-running activity rhythm were studied. Rats were individually housed under constant dim red light. Activity was measured with infrared detectors. Social defeat caused a reduction of activity for a number of days, but the period of the free-running rhythm was not affected. In Experiment 2, the authors studied whether social defeat induced acute phase shifts. Body temperature and activity were measured by means of radiotelemetry with intraperitoneally implanted transmitters. After the social interactions, experimental animals were kept under constant dim red light. Social stress caused a profound reduction in the amplitude of the body temperature and activity rhythm, but no significant phase shifts occurred. In Experiment 3, the authors studied whether social defeat affected the circadian pacemaker's sensitivity to light given that the size of light-induced phase shifts is thought to reflect pacemaker amplitude. Again, body temperature and activity were measured by means of telemetry. After double social defeat, animals were kept under continuous dim red light. One day after the second conflict, animals were subjected to a single 1-h light pulse (300 lux) at circadian time 14. The light pulse induced a phase delay of the body temperature rhythm, but there were no significant differences between the stress and control groups. The data indicate that stress-induced changes in activity and temperature rhythm, as well as behavioral and physiological changes found in earlier experiments, are not caused by changes in the circadian pacemaker. More generally, the data support the notion that overt rhythms are not always a reliable indication of pacemaker function.

Nonphotic phase-shifting and the motivation to run: cold exposure reexamined

Circadian rhythms in rodents can be phase shifted by appropriately timed activity. This may be dependent on motivational context; running induced by a novel wheel is effective, whereas running induced by cold has been inferred to be ineffective. This issue was reexamined using a different cold exposure procedure. On the first day of constant dark, 6 h before usual dark onset, Syrian hamsters were exposed to cold (+/- 4 degrees C) in their home cages, or were confined to novel wheels for 3 h. Activity rhythms were significantly phase advanced by 92 +/- 10 min following cold exposure and 86 +/- 17 min following novel wheel running, compared to 13 +/- 18 min in a control condition. Most hamsters exhibited eating, drinking, and modest levels of wheel running (1367 +/- 292 counts/6 h) during and for 3 h after cold exposure. Phase shifts following cold were not affected by food and water deprivation but were significantly attenuated by locking the wheel for 6 h beginning at cold onset (24 +/- 12 min). These data indicate that cold-induced running, even at modest levels, is an effective nonphotic Zeitgeber and do not provide support for a hypothesis that motivational contexts determine the phase-shifting value of physical activity.

Vitamin B12 amplifies circadian phase shifts induced by a light pulse in rats

Vitamin B12 (VB12) is a putative modulator of the human circadian clock, improving entrainability to the 24 h light-dark cycle. The present study was intended to elucidate the mechanism of VB12 action in an animal model. In male rats free-running under constant dim illumination, a single light pulse of 50-1000 lux for 20 min given at circadian time (CT) 20 induced a 0.28 to 1.08 h phase advance and at CT 14 induced a 0.54 to 2.10 h phase delay. A 3 h intracerebroventricular (icv) infusion of 30 nmol VB12 starting 2 h prior to a 20 min 200 lux light pulse significantly amplified phase shifts in comparison with saline-treated or untreated controls. The mean phase advance (1.13 h) was 1.8-fold greater than that of saline-infused controls, whereas the mean phase delay (2.28 h) was 2.9-fold greater. These values were comparable to the maximal phase shifts caused by 1000 lux light pulses in untreated rats. Since the same VB12 treatment alone had failed to induce a phase shift in a previous experiment, these results indicate that VB12 strongly enhanced light pulse-induced phase shifts and thus augmented the entrainability of the circadian clock to light.

Serotonergic stimulation and nonphotic phase-shifting in hamsters

Stimuli that make hamsters active, such as dark pulses or triazolam administration, also phase shift their circadian clocks, producing phase advances during the subjective day and phase delays during the subjective night. Activity or its correlate appears to be important in producing the shifts because preventing locomotion blocks the phase shifts associated with these stimuli. The physiological basis of clock resetting induced by activity is not fully understood. The serotonergic (5-HT) projection from the raphe to the suprachiasmatic nucleus (SCN) is a possible route by which nonphotic information could reach the pacemaker. Administration of 8-HYDROXY-2-(DI-N-PROPYLAMINO) TETRALIN HYDROBROMIDE (8-OH-DPAT), a 5-HT1A and 5-HT7 receptor agonist, at circadian time (CT) 8 produces phase advances in the circadian rhythms of hamsters. Before concluding that 5-HT mediates the effect of activity on the pacemaker, it must be shown that 5-HT agonist do not produce shifts simply because they make animals more active. Therefore, we investigated the contribution of activity to 8-OH-DPAT-produced shifts. Preventing hamsters from moving around after administering 8-OH-DPAT did not abolish phase shifts. Moreover, higher doses of 8-OH-DPAT diminished activity on the day of injection but did not affect the amplitude of phase shifts. Suprisingly, quipazine (a non specific 5-HT agonist), when injected in the middle of subjective day did not phase shift the activity rhythm of hamsters, as it has been reported to do in rats.

Effect of NMDA receptor blockade on melatonin and activity rhythm responses to a light pulse in rats

The possible role of the excitatory amino acids as mediators of the acute suppression and subsequent delay by light of pineal melatonin production was studied in rats using the NMDA receptor antagonist MK-801. Saline or MK-801 in doses up to 3 mg/kg (IP), was administered 15 min before a 15-min light pulse (200 lx), 4 h after dark onset, and the excretion of 6-sulphatoxymelatonin (aMT.6S) determined. Under these conditions saline injected/light exposed animals exhibited an acute, total but transient suppression of urinary aMT.6S excretion and a delay in the onset of aMT.6S the following night of 1.5 +/- 0.2 h. MK-801 failed to block either the acute or phase delaying effect of light (onset delayed by 2.2 +/- 0.4 h). Pretreatment with MK-801 (3 mg/kg) failed to block the effects of shorter, less intense light pulses 15 min before the pulse (e.g., 1 min/2 lx; onset delayed by 2.0 +/- 0.4 h following saline, 1.5 +/- 0.1 h following MK-801) or 60 min before a short duration low intensity pulse. In other experiments MK-801 (1 and 3 mg/kg) failed to affect aMT.6S excretion when injected in the dark at the time of lights out or 4 h after dark onset. NMDA (10 and 30 mg/kg) injection at the time of lights out or 4 h after darkness did not mimic the effects of a light pulse by decreasing aMT.6S excretion or causing a delay in the onset of excretion the following night. Finally MK-801 (3 mg/kg) injected 4 h after dark failed to block the phase delaying effects of a 15 min light pulse (200 lx) on running activity in rats. These results do not support the hypothesis that excitatory amino acids in the retino-hypothalamic tract acting on the NMDA receptor subtype and terminating in the suprachiasmatic nucleus mediate the photic influences upon rat pineal melatonin and activity rhythms.

A serotonin neurotoxin attenuates the phase-shifting effects of triazolam on the circadian clock in hamsters

Several lines of evidence suggest the potential involvement of serotonergic pathways in mediating the effects of activity-inducing stimuli on the circadian clock in rodents. The aim of the present 3 experiments was to examine the effects of the serotonergic neurotoxin, p-chloroamphetamine (PCA, 10 mg/kg) on: (1) the monoamine levels of the hypothalamus, frontal cortex and hippocampus in the hamster; (2) the phase shifts in the circadian rhythm of locomotor activity of hamsters in response to treatment with the short-acting benzodiazepine, triazolam (7.5 mg/kg); and (3) the magnitude of the acute increase in locomotor activity associated with triazolam administration in this species. The administration of PCA to hamsters caused changes of specific monoaminergic systems in the hypothalamus, that were limited to a selective decrease in serotonin levels 7 days post-treatment. The phase shifts of the circadian clock in response to triazolam treatment at CT 6 were considerably attenuated following the administration of the 5-HT neurotoxin. The total amount and the profiles of triazolam-induced wheel-running and general cage activity between CT 6 and CT 12 were not significantly affected by the PCA treatment. The finding that a 5-HT neurotoxin can attenuate the phase-shifting effects of triazolam in hamsters, without interfering with its activity-inducing properties, suggests that serotonergic afferents might be involved in the mechanism for non-photic phase-shifting of the circadian system.

Circadian rhythms

Changes in behavior which occur on a daily or circadian basis represent one of the most ubiquitous strategies by which most living organisms have adapted to their environment. Underlying the daily changes in behavior are a multitude of endocrine and metabolic rhythms which provide adaptively significant temporal organization within the organism. In mammals there appears to be a central circadian clock in the SCN which is responsible for generating and coordinating the entire 24-hour temporal organization of the animal. The circadian clock regulates the timing, duration, and characteristics of sleep, and together the circadian clock and sleep interact to control the timing of endocrine secretions. While the impact of disturbed endocrine circadian rhythms for the survival of the species has received very little attention, the almost universal presence of circadian rhythmicity within the endocrine system argues in support of the hypothesis that a disruption of the normal circadian organization within the endocrine system can have serious consequences for the health and well-being of the organism. It is particularly noteworthy that in advanced age, various alterations in circadian endocrine rhythms have been observed and these alterations may impair the ability of the animal to adapt normally to the environment. Relatively speaking, the study of circadian rhythms is a new field of biology, and as a result, much remains to be discovered about the physiological mechanisms that underlie rhythmicity, as well as the functional significance of 24-hour temporal organization for the survival of the species.

Circadian phase-shifts induced by chlordiazepoxide without increased locomotor activity

Phase advances of circadian locomotor rhythms in response to the benzodiazepine triazolam (TRZ) administered at circadian time 6 appear to be mediated by locomotor activity. The possibility that increased activity also mediates shifts induced by the benzodiazepine chlordiazepoxide (CDZ) was investigated. Hamsters injected with CDZ 7 h before dark onset exhibited phase advances despite being confined to their nest boxes for 3 h after receiving the drug. These shifts did not differ significantly from advances seen in hamsters which were allowed access to the rest of the cage after injections. In other experiments, the behavioral effects of both CDZ and TRZ were examined during the 5 h after injection. There was significantly more motor activity observed after injection of TRZ than CDZ. These results suggest that phase-shifting effects of some benzodiazepines can occur without inducing activity.

Wheel running raises body temperature and changes the daily cycle in golden hamsters

The purpose of this study was to characterize the influence of wheel-running activity on temperature circadian rhythm in golden hamsters. Amplitude of body temperature rhythm was significantly higher in animals that had access to the running wheel. Qualitatively, wheel running resulted in squaring of temperature rhythm waveform, a phenomenon that was independent of ambient illumination. When the hamsters had access to an immobilized wheel, the effect on temperature was no longer observed. Fast Fourier Transformation after subtracting 24-h sine waveforms (to rule out harmonics of the principal frequency), indicated distinct secondary components of the rhythms, i.e., a 12-h component in animals that had access to the wheels and an 8-h component in animals that did not. The rise in body temperature could be dissociated from the start of the activity period, body temperature augmenting before the time of light-dark transition, while activity increased about 12 min later. On the first night after presentation of an estrous female, the temperature cycle in male hamsters that did not access to the wheels was similar to that found in animals running on the wheels; the effect was no longer observed in subsequent nights. The results contribute to the view that the spontaneous locomotor activity in hamsters has a significant effect on temperature cycle.

Aging alters the entraining effects of an activity-inducing stimulus on the circadian clock

In young hamsters, a single injection of the short-acting benzodiazepine, triazolam, can induce permanent phase shifts in the circadian clock, while repeated injections of triazolam entrain the circadian clock to the period of the injections. Triazolam appears to act on the circadian clock by inducing an acute increase in the activity of the animals, which in turn phase-shifts the circadian clock. Surprisingly, single injections of benzodiazepines do not phase-shift the activity rhythm of old hamsters, despite the fact that such treatment induces similar acute changes in the activity state of young and old animals. We compared the entraining effects of repeated injections of triazolam on the circadian clock of young and old hamsters; while six out of seven young hamsters were entrained to the triazolam injections, only one out of seven old animals was entrained by this treatment. Three of the remaining six old hamsters showed a lengthening of the activity rhythm, while no consistent effect on the period of the activity rhythm was observed in the remaining three old animals. These results indicate that the circadian system of old hamsters becomes selectively unresponsive to synchronizing signals mediated by the activity-rest state, and suggest that aging is associated with a weakened coupling between the activity-rest cycle and the circadian clock.

Behavioral inhibition of circadian responses to light

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.