Double-pulse experiments with nonphotic and photic phase-shifting stimuli

Reflections on Several Landmark Advances in Circadian Biology

Circadian Biology intersects with diverse scientific domains, intricately woven into the fabric of organismal physiology and behavior. The rhythmic orchestration of life by the circadian clock serves as a focal point for researchers across disciplines. This retrospective examination delves into several of the scientific milestones that have fundamentally shaped our contemporary understanding of circadian rhythms. From deciphering the complexities of clock genes at a cellular level to exploring the nuances of coupled oscillators in whole organism responses to stimuli. The field has undergone significant evolution lately guided by genetics approaches. Our exploration here considers key moments in the circadian-research landscape, elucidating the trajectory of this discipline with a keen eye on scientific advancements and paradigm shifts.

Exposure to Short Wavelength-Enriched White Light and Exercise Improves Alertness and Performance in Operational NASA Flight Controllers Working Overnight Shifts

OBJECTIVE

We evaluated the efficacy of a combined short-wavelength-enriched white light and exercise fatigue countermeasure during breaks for flight controllers working overnight shifts.

METHODS

Twenty NASA flight controllers were studied for two blocks of nightshifts in ISS mission control, randomized to either the control or countermeasure condition. The countermeasure constituted passive exposure to blue-enriched polychromatic lighting for three 20-minute intervals, which included 10 minutes of exercise and occurred before and twice during their shifts. Alertness, performance, and mood were evaluated.

RESULTS

Flight controllers reported being significantly more alert (P < 0.0001) and happy (P = 0.003) and had faster reaction times (10% slowest responses; P < 0.05) during the countermeasure condition compared to control.

CONCLUSIONS

The combined light and exercise countermeasure improved alertness, performance, and mood in shift workers overnight. Further research is necessary to determine their relative contribution.

Examination of Zinc in the Circadian System

Zinc is a trace element that is essential for a large number of biological and biochemical processes in the body. In the nervous system zinc is packaged into synaptic vesicles by the ZnT3 transporter, and synaptic release of zinc can influence the activity of postsynaptic cells, either directly through its own cognate receptors, or indirectly by modulating activation of receptors for other neurotransmitters. Here, we explore the anatomical and functional aspects of zinc in the circadian system. Melanopsin-containing retinal ganglion cells in the mouse retina were found to colocalize ZnT3, indicating that they can release zinc at their synaptic targets. While the master circadian clock in the hamster suprachiasmatic nucleus (SCN) was found to contain, at best, sparse zincergic input, the intergeniculate leaflet (IGL) of hamsters and mice were found to have prominent zincergic input. Levels of zinc in these areas were not affected by time of day. Additionally, IGL zinc staining persisted following enucleation, indicating other prominent sources of zinc instead of, or in addition to, the retina. Neither enhancement nor chelation of free zinc at either the SCN or IGL altered circadian responses to phase-shifting light in hamsters. Finally, entrainment, free-running, and circadian responses to light were explored in mice lacking the ZnT3 gene. In every aspect explored, the ZnT3 knockout mice were not significantly different from their wildtype counterparts. These findings highlight the presence of zinc in areas critical for circadian functioning but have yet to identify a role for zinc in these areas.

Exercise Distributed across Day and Night Does Not Alter Circadian Period in Humans

In rodents, increased activity due to running-wheel access is associated with a change in observed circadian period. In humans, exposure to exercise has failed to demonstrate similar effects on period. Methodological issues with prior studies such as light exposure during exercise, length of study, and method of measuring period confounded those evaluations of the effect of exercise on period in humans. In the present experiment, the authors examined the effect of exercise on period in 8 subjects using a 44-day within-subjects inpatient study. They used a 20-h forced desynchrony protocol, in which subjects were exposed to exercise across circadian phases under dim light conditions. Exercise consisted of three 45-min sessions per wake period on an ergometer. Target exercise intensity was ~65% of maximal heart rate. Intrinsic circadian period was measured using both core body temperature and hourly plasma melatonin samples. Consistent with previous reports, the authors find no effect of exercise on endogenous circadian period as measured by either core body temperature or melatonin. Exercise distributed across biological day and night does not appear to affect circadian period.

Non-Photic Modulation of Phase Shifts to Long Light Pulses

Circadian rhythms can be reset by both photic and non-photic stimuli. Recent studies have used long light exposure to produce photic phase shifts or to enhance non-photic phase shifts. The presence or absence of light can also influence the expression of locomotor rhythms through masking; light during the night attenuates locomotor activity, while darkness during the day induces locomotor activity in nocturnal animals. Given this dual role of light, the current study was designed to examine the relative contributions of photic and non-photic components present in a long light pulse paradigm. Mice entrained to a light/dark cycle were exposed to light pulses of various durations (0, 3, 6, 9, or 12 h) starting at the time of lights-off. After the light exposure, animals were placed in DD and were either left undisturbed in their home cages or had their wheels locked for the remainder of the subjective night and subsequent subjective day. Light treatments of 6, 9, and 12 h produced large phase delays. These treatments were associated with decreased activity during the nocturnal light and increased activity during the initial hours of darkness following light exposure. When the wheels were locked to prevent high-amplitude activity, the resulting phase delays to the light were significantly attenuated, suggesting that the activity following the light exposure may have contributed to the overall phase shift. In a second experiment, telemetry probes were used to assess what effect permanently locking the wheels had on the phase shift to the long light pulses. These animals had phase shifts fully as large as animals without any form of wheel lock, suggesting that while non-photic events can modulate photic phase shifts, they do not play a role in the full phase-shift response observed in animals exposed to long light pulses. This paradigm will facilitate investigations into non-photic responses of the mouse circadian system.

Circadian Phase-Shifting Effects of Bright Light, Exercise, and Bright Light + Exercise

Limited research has compared the circadian phase-shifting effects of bright light and exercise and additive effects of these stimuli. The aim of this study was to compare the phase-delaying effects of late night bright light, late night exercise, and late evening bright light followed by early morning exercise. In a within-subjects, counterbalanced design, 6 young adults completed each of three 2.5-day protocols. Participants followed a 3-h ultra-short sleep-wake cycle, involving wakefulness in dim light for 2h, followed by attempted sleep in darkness for 1 h, repeated throughout each protocol. On night 2 of each protocol, participants received either (1) bright light alone (5,000 lux) from 2210-2340 h, (2) treadmill exercise alone from 2210-2340 h, or (3) bright light (2210-2340 h) followed by exercise from 0410-0540 h. Urine was collected every 90 min. Shifts in the 6-sulphatoxymelatonin (aMT6s) cosine acrophase from baseline to post-treatment were compared between treatments. Analyses revealed a significant additive phase-delaying effect of bright light + exercise (80.8 ± 11.6 [SD] min) compared with exercise alone (47.3 ± 21.6 min), and a similar phase delay following bright light alone (56.6 ± 15.2 min) and exercise alone administered for the same duration and at the same time of night. Thus, the data suggest that late night bright light followed by early morning exercise can have an additive circadian phase-shifting effect.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Interspecific contact affects phase response and activity in Desert hamsters

Circadian rhythms enhance survival and reproductive fitness of animals by promoting optimal timing of behavior and physiology with reference to geophysical changes in environment. Although light is considered the dominant stimulus for entraining circadian rhythms, social stimuli can also act as zeitgebers. The aim of this study was to analyze how Desert hamsters (Phodopus roborovskii) coordinate their behavior in time with that of animals of another competing species (Mongolian gerbils, Meriones unguiculatus). First, the behavior of hamsters was analyzed during a step-wise avoidance test. Two effects were observed: a) spatial separation if it was possible or b) shortening of the activity period due to contact without chance for avoidance. The latter finding was now further analyzed using a phase response curve (PRC). Here, phase shifts of Desert hamsters caused by single social interactions with Mongolian gerbils were quantified. Phase advances during the rest period were found at CT3 and CT9, a similar tendency was observed at CT6. A second phase advance was determined at CT18, coinciding with the end of the activity period. Then, it was tested whether additional activity during the stimulus was a trigger for the phase response. Although an increase in activity occurred especially when stimuli were applied during the rest period, there was no general relation between additional activity measured and the phase response shown. Overall, relevance of interspecific contact as nonphotic zeitgeber was indicated by phase shifts in a phase response curve. The shape of it can be explained by two behavioral adaptations; stress and contact avoidance.

Serotonergic potentiation of dark pulse-induced phase-shifting effects at midday in hamsters

In mammals, resetting of the suprachiasmatic clock (SCN) by behavioral activation or serotonin (5-HT) agonists is mimicked by dark pulses, presented during subjective day in constant light (LL). Because behavioral resetting may be mediated in part by 5-HT inputs to the SCN, here we determined whether 5-HT system can modulate dark-induced phase-shifts in Syrian hamsters housed in LL. Two hours of darkness at mid-subjective day (circadian time 6; CT-6) resulted in increased concentrations of 5-HT in the SCN tissue and induction of c-FOS expression in the raphe nuclei. Injections of the 5-HT(1A/7) agonist +8-OH-DPAT or dark pulses at CT-6 induced phase-advances of the wheel-running activity rhythm and down-regulated the expression of the clock genes Per1-2 and c-FOS in the SCN in a similar way. The combination of both treatments [+8-OH-DPAT + dark pulses], however, resulted in larger phase-advances, while associated molecular changes were not significantly modified, except for the gene Dbp, in comparison to +8-OH-DPAT or dark pulses alone. Dark resetting was blocked by pre-treatment with a 5-HT(7) antagonist, but not with a 5-HT(1A) antagonist. The additive phase-shifts of two different cues to reset the SCN clock open wide the gateway for non-photic shifting, leading to new strategies in chronotherapy.

Acute ethanol modulates glutamatergic and serotonergic phase shifts of the mouse circadian clock in vitro

Alcohol abuse is associated with sleep problems, which are often linked to circadian rhythm disturbances. However, there is no information on the direct effects of ethanol on the mammalian circadian clock. Acute ethanol inhibits glutamate signaling, which is the primary mechanism through which light resets the mammalian clock in the suprachiasmatic nucleus (SCN). Glutamate and light also inhibit circadian clock resetting induced by nonphotic signals, including 5-HT. Thus, we investigated the effects of acute ethanol on both glutamatergic and serotoninergic resetting of the mouse SCN clock in vitro. We show that ethanol dose-dependently inhibits glutamate-induced phase shifts and enhances serotonergic phase shifts. The inhibition of glutamate-induced phase shifts is not affected by excess glutamate, glycine or d-serine, but is prevented by excess brain-derived neurotrophic factor (BDNF). BDNF is known to augment glutamate signaling in the SCN and to be necessary for glutamate/light-induced phase shifts. Thus, ethanol may inhibit glutamate-induced clock resetting at least in part by blocking BDNF enhancement of glutamate signaling. Ethanol enhancement of serotonergic phase shifts is mimicked by treatments that suppress glutamate signaling in the SCN, including antagonists of glutamate receptors, BDNF signaling and nitric oxide synthase. The combined effect of ethanol with these treatments is not additive, suggesting they act through a common pathway. Our data indicate further that the interaction between 5-HT and glutamate in the SCN may occur downstream from nitric oxide synthase activation. Thus, acute ethanol disrupts normal circadian clock phase regulation, which could contribute to the physiological and psychological problems associated with alcohol abuse.

A non-photic gateway to the circadian clock of hamsters

This paper considers the neural mechanisms underlying a particular kind of non-photic phase shifting, that produced by novelty-induced wheel running in the hamster. The projection from the intergeniculate leaflet (IGL) to the suprachiasmatic nucleus (SCN) appears to be an important part of the mechanism mediating such phase shifts. A number of experiments support this view. First, expression of immediate-early genes in the IGL is induced by non-photic phase-shifting stimuli. Second, Fos-like immunoreactivity in the IGL co-localizes with neuropeptide Y (NPY) immunoreactivity. Third, direct application of NPY to the SCN produces phase shifts which do not depend on the hamsters becoming active following the injections. Fourth, blocking the normal actions of NPY at the SCN blocks or greatly attenuates the phase shifting that is normally produced by novelty-induced wheel running. Progress on the physiological basis of phase shifts associated with activity, or a correlate, depends on understanding the behavioural aspects of this phenomenon. The activity-shift response curve is especially useful.

Melatonin marks circadian phase position and resets the endogenous circadian pacemaker in humans

Measuring the dim light melatonin onset (DLMO) is a useful and practical way to assess circadian phase position in humans. As a marker for the phase and period of the endogenous circadian pacemaker, the DLMO has been shown to advance with exposure to bright light in the morning and to delay with exposure to bright light in the evening. This 'phase response curve' (PRC) to light has been applied in the treatment of winter depression, jet lag and shift work, as well as circadian phase sleep disorders. Exogenous melatonin has phase-shifting effects described by a PRC that is about 12 h out of phase with the PRC to light. That is, melatonin administration in the morning causes phase delays and in the afternoon causes phase advances. All of the circadian phase disorders that have been successfully treated with appropriately timed exposure to bright light can be treated with appropriately scheduled melatonin administration. Melatonin administration is more convenient and therefore may be the preferred treatment.

Circadian gene expression is suppressed during sevoflurane anesthesia and the suppression persists after awakening

General anesthesia is routinely used as a surgical procedure and its safety has been endorsed by clinical outcomes; however, its effects at the molecular level have not been elucidated. We previously showed that inhalation anesthesia affects the expression of 1.5% of 10,000 genes, which included so-called circadian genes. In the current study, we confirmed that inhalation of sevoflurane alters circadian gene expression, and investigated whether this alteration persists after awakening from anesthesia. Rats were anesthetized with 4.0% sevoflurane for 0 h, 2 h and 6 h (n=9 each group), before being sacrificed. Rats were also anesthetized for 6 h and allowed to recover after anesthesia, then sacrificed 2 h, 6 h and 24 h after awakening (n=9 each group). Anesthesia was started for each group so that all rats would be sacrificed at 13:00, and gene expression in the whole brain was examined using real-time RT-PCR. Expression of the genes encoding Per2, Dbp, Arc, Egr1, Krox20 and NGFI-B was suppressed during inhalation of sevoflurane for 2 h and 6 h. Although the suppression tended to be alleviated after awakening from anesthesia, the expression levels after a recovery period of 24 h remained significantly lower than the control levels of these genes, except for Krox20. We demonstrated that circadian gene expression is suppressed in whole brain during sevoflurane anesthesia, and the suppression continues for at least 24 h after termination of sevoflurane treatment. This suggests that sevoflurane anesthesia may have effects at the molecular level and that these effects are long lasting.

A molecular explanation of interactions between photic and non-photic circadian clock-resetting stimuli

Non-photic clock-resetting events (arousal and locomotor activity) in the subjective day reduced expression of Period genes in the suprachiasmatic nucleus of hamsters. This decrease was attenuated by a 30-min light pulse occurring during the last 0.5 h of 3.5 h of confinement to a novel running wheel. This provides an example at the molecular level of an interaction between different modalities of synchronizing agents.

Gastrin releasing peptide and neuropeptide Y exert opposing actions on circadian phase

Microinjection of gastrin releasing peptide (GRP) into the third ventricle or the suprachiasmatic nucleus (SCN) induces circadian phase shifts similar to those produced by light. Administration of GRP during the day does not alter circadian phase. In contrast, neuropeptide Y (NPY) induces phase shifts of circadian rhythms during the day but has little effect when administered at night, similar to the effects of most non-photic stimuli. NPY inhibits the phase shifting effects of light, and GRP is thought to be part of the photic signaling system within the SCN. This experiment was designed to test whether GRP and NPY inhibit each other's effects on circadian phase. Adult male Syrian hamsters equipped with guide cannulas aimed at the SCN were housed in constant darkness until stable free-running rhythms of wheel running activity were apparent. Microinjection of GRP during the early subjective night induced phase delays that were blocked by simultaneous administration of NPY. During the middle of the subjective day, microinjection of NPY caused phase advances that were blocked by simultaneous administration of GRP. These data suggest that GRP and NPY oppose each other's effects on the circadian clock, and that the actions of NPY on the photic phase shifting mechanism in the SCN occur at least in part downstream from retinorecipient cells.

Effects of exercise on sleep

Historically, perhaps no daytime behavior has been more closely associated with better sleep than exercise. The assumption that exercise promotes sleep has also been central to various hypotheses about the functions of sleep. Hypotheses that sleep serves an energy conservation function, a body tissue restitution function, or a temperature down-regulation function all have predicted a uniquely potent effect of exercise on sleep because no other stimulus elicits greater depletion of energy stores, tissue breakdown, or elevation of body temperature, respectively. Exercise offers a potentially attractive alternative or adjuvant treatment for insomnia. Sleeping pills have a number of adverse side effects and are not recommended for long-term use, partly on the basis of a significant epidemiologic association of chronic hypnotic use with mortality. Other behavioral/cognitive treatments are more effective for chronic insomnia treatment, but difficult and costly to deliver. By contrast, exercise could be a healthy, safe, inexpensive, and simple means of improving sleep.

Intergeniculate leaflet: contributions to photic and non-photic responsiveness of the hamster circadian system

The circadian visual system is able to integrate light energy over time, enabling phase response and Fos induction in the suprachiasmatic nucleus to increase in proportion to the total energy of the photic stimulus. In the present studies, the contribution of the intergeniculate leaflet to light energy integration by the hamster circadian rhythm system was evaluated. Fos protein is induced in intergeniculate leaflet neurons at much lower irradiance levels than seen in suprachiasmatic nucleus neurons. Bilateral N-methyl-d-aspartate lesions of the intergeniculate leaflet decreased phase response of the circadian locomotor rhythm to high irradiance and, in animals exposed to long duration light stimuli, reduced Fos induction in the suprachiasmatic nucleus. Normal photon integration, as indicated by attenuated rhythm phase shifts and Fos induction in suprachiasmatic nucleus cells in response to the energy in light stimuli, does not occur in the absence of the intergeniculate leaflet and is likely to be a property of the circadian rhythm system, rather than solely of the suprachiasmatic nucleus. Anatomical analysis showed that virtually no intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus contain Fos induced by either light or locomotion in a novel wheel. However, cells projecting to the pretectum were found to contain novel-wheel induced Fos. The intergeniculate leaflet is implicated in the normal assessment of light by the circadian rhythm system, but the circuitry by which either photic or non-photic information gains access to the suprachiasmatic nucleus may be more complex than previously thought.

Potentiation of the resetting effects of light on circadian rhythms of hamsters using serotonin and neuropeptide Y receptor antagonists

Circadian rhythms are entrained by light/dark cycles. In hamsters, the effects of light on circadian rhythms can be modulated by serotonergic input to the suprachiasmatic nucleus from the raphe nuclei and by neuropeptide Y containing afferents to the suprachiasmatic nucleus from the intergeniculate leaflet in the thalamus. In this study we measured effects of compounds acting on serotonergic 1A and neuropeptide Y Y5 receptors to determine if combined serotonergic-neuropeptide Y inhibition could synergistically potentiate effects of light on rhythms. We used mixed serotonergic agonist/antagonists BMY 7378 or NAN-190 as well as a neuropeptide Y Y5 antagonist CP-760,542. Both BMY 7378 and NAN-190 are thought to block serotonin release via acting as agonists at the 5-hydroxytryptamine 1A (5-HT1A) autoreceptors on cells in the raphe, and also block response of target cells by acting as antagonists at post-synaptic 5-HT1A receptors, for example, in the suprachiasmatic nuclei or the intergeniculate leaflet. Replicating prior work, we found that pretreatment with either drug alone increased the phase shift to light at circadian time 19. The combined effect of BMY 7378 and CP-760,542 given prior to light at circadian time 19 was to further potentiate the subsequent phase shift in wheel-running rhythms (the phase shift was 317% of controls; light alone: 1.35 h phase shift vs. BMY 7378, CP-760,542, and light: 4.27 h phase shift). Combined treatment with NAN-190 and CP-760,542 produced a light-induced phase shift 576% of controls (phase shift to light alone: 1.23 h vs. NAN-190, CP-760,542, and light: 7.1 h phase shift). These results suggest that the resetting effects of light on circadian rhythms can be greatly potentiated in hamsters by using pharmacological treatments that block both serotonergic and neuropeptide Y afferents to the suprachiasmatic nuclei.

Octodon degus: a diurnal, social, and long-lived rodent

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.

Circadian Phase Alteration by GABA and Light Differs in Diurnal and Nocturnal Rodents During the Day

These studies investigated the circadian effects of light and gamma aminobutyric acid-A (GABAA) receptor activation in the suprachiasmatic nucleus (SCN) of the diurnal unstriped Nile grass rat (Arvicanthis niloticus). Microinjection of the GABAA agonist muscimol into the SCN during the day produced phase shifts that were opposite in direction to those previously reported in nocturnal rodents. In addition, light had no significant effect on the magnitude of muscimol-induced phase delays during the daytime. Injection of muscimol during the night, however, significantly inhibited light-induced phase delays and advances in a manner similar to that previously reported in nocturnal rodents. Therefore, the circadian effects of GABAA receptor activation are similar in diurnal and nocturnal species during the night but differ significantly during the day.

Phase Shifts of the Circadian Locomotor Rhythm Induced by Pigment-Dispersing Factor in the Cricket Gryllus bimaculatus

Pigment-dispersing factors (PDFs) are octadeca-peptides widely distributed in insect optic lobes and brain. In this study, we have purified PDF and determined its amino acid sequence in the cricket Gryllus bimaculatus. Its primary structure was NSEIINSLLGLPKVLNDA-NH(2), homologous to other PDH family members so far reported. When injected into the optic lobe of experimentally blinded adult male crickets, Gryllus-PDF induced phase shifts in their activity rhythms in a phase dependent and dose dependent manner. The resulted phase response curve (PRC) showed delays during the late subjective night to early subjective day and advances during the mid subjective day to mid subjective night. The PRC was different in shape from those for light, serotonin and temperature. These results suggest that PDF plays a role in phase regulation of the circadian clock through a separate pathway from those of other known phase regulating agents.

Behavioral arousal blocks light-induced phase advances in locomotor rhythmicity but not light-induced Per1 and Fos expression in the hamster suprachiasmatic nucleus

Both photic and nonphotic stimuli entrain circadian rhythms. Although the adaptive significance of nonphotic clock resetting is unknown, one possibility is that nonphotic cues modulate circadian responses to light. Results of studies on the interaction between photic and nonphotic stimuli support this idea. During the day, light blocks the effects of nonphotic stimuli on the phase of locomotor rhythms and on expression of clock genes in suprachiasmatic nucleus (SCN) neurons. At night, novelty-induced activity prior to and during exposure to light attenuates the phase-shifting response to that light, but the effects of this manipulation on clock gene expression are unknown. The present experiments explore the interaction between behavioral state and response to light at the molecular level. We show that confining hamsters to novel wheels immediately after a light pulse during the late subjective night attenuates light-induced phase advances of wheel-running rhythms and the transient effects on circadian period. In contrast to the striking effect of novelty-induced activity on behavioral responses to light, Fos protein and Per1 mRNA were robustly expressed in the SCN of all light-pulsed animals, regardless of behavioral treatment. Our results are inconsistent with the idea that light and nonphotic stimuli block each other's effects on phase shifts by inducing or attenuating transcription of Per1. Photic regulation of clock genes and spontaneous rhythmic expression of clock genes are probably mediated by different mechanisms.

Nonphotic phase shifting in female Syrian hamsters: interactions with the estrous cycle

Nonphotic phase shifting of circadian rhythms was examined in female Syrian hamsters. Animals were stimulated at zeitgeber time 4.5 by either placing them in a novel running wheel or by transferring them to a clean home cage. Placement in a clean home cage was more effective than novel wheel treatment in stimulating large (> 1.5 h) phase shifts. Peak phase shifts (ca. 3.5 h) and the percentage of females showing large phase shifts were comparable to those found in male hamsters stimulated with novel wheels. The amount of activity induced by nonphotic stimulation and the amount of phase shifting varied slightly with respect to the 4-day estrous cycle. Animals tended to run less and shift less on the day of estrus. Nonphotic stimulation on proestrus often resulted in a 1-day delay of the estrous cycle reflected in animals' postovulatory vaginal discharge and the expression of sexual receptivity (lordosis). This delay of the estrous cycle was associated with large phase advances and high activity. These results extend the generality of nonphotic phase shifting to females for the first time and raise the possibility that resetting of circadian rhythms can induce changes in the estrous cycle.

The art of entrainment

The circadian system actively synchronizes the temporal sequence of biological functions with the environment. The oscillatory behavior of the system ensures that entrainment is not passive or driven and therefore allows for great plasticity and adaptive potential. With the tools at hand, we now can concentrate on the most important circadian question: How is the complex task of entrainment achieved by anatomical, cellular, and molecular components? Understanding entrainment is equal to understanding the circadian system. The results of this basic research will help us to understand temporal ecology and will allow us to improve conditions for humans in industrialized societies.

Serotonin phase-shifts the mouse suprachiasmatic circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) receives multiple afferent signals that could potentially modulate its phase. One input, the serotonin (5-HT) projection from the raphe nuclei, has been extensively investigated in rats and hamsters, yet its role(s) in modulating circadian clock phase remains controversial. To expand our investigation of 5-HT modulation of the SCN clock, we investigated the phase-shifting effects of 5-HT and its agonist, (+)8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), when applied to mouse SCN brain slices. 5-HT induced 2-3 h phase advances when applied during subjective day, while non-significant phase shifts were seen after 5-HT application at other times. These phase shifts were completely blocked by the 5-HT antagonist, metergoline. DPAT also induced phase shifts when applied during mid-subjective day, and this effect appeared dose-dependent. Together, these results demonstrate that the mouse SCN, like that of the rat, is directly sensitive to in vitro phase-resetting by 5-HT.

Plasticity of hamster circadian entrainment patterns depends on light intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (approximately 350 lux) were alternated with either dim (< 0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (approximately 45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.

Attenuation of phase shifts to light by activity or neuropeptide Y: a time course study

Circadian rhythms in mammals can be synchronised to photic and non-photic stimuli. Interactions between photic and behavioural stimuli were investigated during the late subjective night, 6 h after activity onset in Syrian hamsters (CT18). Light pulses of 130 lx for 15 min at this time resulted in phase advance shifts. Novel wheel exposure, for a period of 3 h, following photic stimulation was able to attenuate the phase advancing effects of light. A time delay of up to 60 min between photic and behavioural stimuli also resulted in significant attenuation of light-induced phase shifts (P<0.05). A 90-min interval between stimuli resulted in no significant attenuation. Novel wheel exposure mediates its effects via the intergeniculate leaflet, which conveys information to the SCN and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Phase shifts to light pulses given at CT18 were attenuated by NPY administration. Neuropeptide Y injections up to 60 min post-light exposure significantly attenuated phase shifts by 50% on average. However a 90-min interval between light and NPY microinjection did not significantly affect light-induced phase shifts. These results confirm previous work indicating that novel wheel exposure and NPY administration can modulate light-induced phase shifts during the late night. Further, they show for the first time that the time course for this interaction is similar between wheel running and NPY. Most significantly, our work indicates that the time course in vivo in the late night is similar to that shown previously in vitro during the early night.

Removal of polysialic acid from the SCN potentiates nonphotic circadian phase resetting

The adult suprachiasmatic nucleus (SCN) expresses a polysialylated form of neural cell adhesion molecule (PSA-NCAM) that modulates cell interactions. Previous studies have shown that PSA is important for photic entrainment of the SCN circadian clock, suggesting that changes in cell-cell interactions may contribute to the phase-resetting capacity of this system. A possible role for PSA in nonphotic circadian phase resetting was evaluated using the enzyme endoneuraminidase (endo N) to selectively remove PSA from the SCN. Pretreatment of rat brain slices containing the SCN with endo N enhanced the daytime phase-advancing effects of the serotonin agonist, 8-hydroxy-2-dipropylaminotetralin [8-OH-DPAT] (41% greater than inactivated enzyme controls; P<.05). Similarly, removal of PSA from the Syrian hamster SCN in vivo potentiated the daytime phase-advancing effects of behavioral arousal (sleep deprivation) and of systemic application of 8-OH-DPAT (61% and 220% greater, respectively, than inactivated enzyme controls; both P<.05). While endo N perturbation of both photic and nonphotic phase resetting suggests that PSA plays a central role in clock regulation, it is striking that the effects on the two inputs are opposite in direction. This difference could reflect the antagonistic relationship between photic and nonphotic signaling pathways. It could also be explained by a permissive role of PSA common to both inputs, which in and of itself would not specify direction of response. Such a bidirectional control mechanism, based on PSA's attenuation of cell-cell interactions, is well documented in the developing nervous system, and may be retained in plastic regions of the adult brain.

Is novel wheel inhibition of per1 and per2 expression linked to phase shift occurrence?

We studied whether access to a novel running wheel in vivo could reset the suprachiasmatic nuclei (SCN) in vitro. Golden hamsters were transferred to dim red light at Zeitgeber time (ZT) 4, given their first exposure to a running wheel for 3 h, and killed at either ZT7 or ZT9. Using a brain slice preparation, the SCN firing rate rhythm in vitro was advanced relative to controls only in the slices prepared at ZT9 (phase shift: 2.36+/-0.06 h, n=4) but not ZT7 (-0.26+/-0.16 h, n=4). Transitions to dim red light or brain slice preparation at ZT7 or ZT9 alone do not shift the rhythm. Hamsters with wheels had significantly lower levels of SCN per1 mRNA compared with controls at ZT7, and lower per2 mRNA when examined at ZT9. We conclude that 3 h of novel wheel access appears to require some extended time in vivo in order for the SCN to be reset, even beyond the time when per1 mRNA levels have been altered.

Three days of novel wheel access diminishes light-induced phase delays in vivo with no effect on per1 induction by light

The mammalian circadian clock, located in the hypothalamic suprachiasmatic nuclei, synchronizes endogenous behavioral and physiological rhythms to a 24 h period through responses to two types of stimuli: photic (light) and nonphotic (behaviorally induced arousal and/or increases in activity). Photic stimuli can block nonphotic effects and vice versa, although the mechanisms and levels of interactions between these two stimuli types are unknown. Here, we investigated whether 3 d of access to a novel running wheel alters the phase shift to light in vivo, and whether this effect could be seen on induction by light of the circadian gene per1. Through measurement of running wheel activity of golden hamsters, access to a new wheel for 3 d was shown to diminish photic phase delays with no effect on phase advances. As seen using in situ hybridization, however, there was no effect on levels of light-induced per1 mRNA. This study indicates a possible role for this paradigm as a model of interactions between photic and nonphotic stimuli.

Circadian clock resetting by sleep deprivation without exercise in Syrian hamsters: dark pulses revisited

Circadian rhythms in Syrian hamsters can be phase shifted by procedures that stimulate wheel running ("exercise") in the mid-subjective day (the hamster's usual sleep period). The authors recently demonstrated that keeping hamsters awake by gentle handling, without continuous running, is sufficient to mimic this effect. Here, the authors assessed whether wakefulness, independent of wheel running, also mediates phase shifts to dark pulses during the midsubjective day in hamsters free-running in constant light (LL). With running wheels locked during a 3 h dark pulse on day 3 of LL, hamsters (N = 16) averaged only 43+/-15 min of spontaneous wake time and phase shifted only 24+/-43 min. When wheels were open during a dark pulse, two hamsters remained awake, ran continuously, and showed phase advance shifts of 7.3 h and 8.7 h, respectively, whereas the other hamsters were awake <60 min and shifted only 45+/-38 min. No animals stayed awake for 3 h without running. Additional time in LL (10 and 20 days) did not potentiate the waking or phase shift response to dark pulses. When all hamsters were sleep deprived with wheels locked during a dark pulse, phase advance shifts averaged 261+/-110 min and ranged up to 7.3 h. These shifts are large compared to those previously observed in response to the 3 h sleep deprivation procedure. Additional tests revealed that this potentiated shift response is dependent on LL prior to sleep deprivation but not LL after sleep deprivation. A final sleep deprivation test showed that a small part of the potentiation may be due to suppression of spontaneous wheel running by LL. These results indicate that some correlate of waking, other than continuous running, mediates the phase-shifting effect of dark pulses in the mid-subjective day. The mechanism by which LL potentiates shifting remains to be determined. The lack of effect of subsequent LL on the magnitude of shifts to sleep deprivation in the dark suggests that LL reduces responsivity to light by processes that take >3 h of dark to reverse.

Circadian phase-delaying effects of bright light alone and combined with exercise in humans

In a within-subjects (n = 18), counterbalanced design, the circadian phase-shifting effects of 3 h of 1) bright light (3,000 lx) alone 2) and bright light combined with vigorous exercise were compared. For each treatment, volunteers spent 3 nights and 2 days in the laboratory, typically receiving the treatment from approximately 2300 to 0200 on night 2. Bedtimes and waketimes were fixed to the volunteers' habits. Illumination was 50 lx during other wake hours and 0 lx during sleep. Bright Light Alone elicited a significant phase delay in rectal temperature minimum (70 min), but not in urinary 6-sulphatoxymelatonin (6-SMT) acrophase (20 min). Bright Light + Exercise elicited a significant phase delay in 6-SMT (68 min), but did not result in a significant difference in shift compared with Bright Light Alone. The study had adequate statistical power (80%) to detect phase-shift differences between treatments of approximately 2-2.5 h. Thus any antagonism of light shifts with exercise could not have been revealed. Within the limited exercise and light parameters of this study, the results suggest that exercise does not reliably modulate phase-shifting effects of late night bright light in humans.

Hypoxia affects expression of circadian genes PER1 and CLOCK in mouse brain

The key elements of circadian clockwork and oxygen homeostasis are the PAS protein family members PER and CLOCK and hypoxia-inducible factor 1alpha (HIF-1alpha). The PAS domain serves as an interface for protein-protein interactions. We asked whether a cross-talk exists between the PAS components of hypoxic and circadian pathways. We found several isoforms of PER1 protein that exhibit tissue-specific size differences. In the mouse brain, a predominantly nuclear 48 kDa isoform that followed a daily rhythm was observed. The 48 kDa form was found in the nuclear fractions derived from mouse liver, Swiss3T3 fibroblasts, and N2A neuroblastoma cells. In mouse kidney and human 293 kidney cells, a 55 kDa PER1 form was detected. CLOCK was observed as a predicted 100 kDa protein in rat-1 cells and in all analyzed mouse tissues including brain, liver, kidney, and spleen. In contrast to PER1, CLOCK protein expression was not rhythmic. Exposure to hypoxia led to increased PER1 and CLOCK protein levels in mice. Based on coimmunoprecipitation experiments that showed protein-protein interaction between PER1 and the alpha subunit of HIF-1, we suggest that these hypoxic effects may be modulated by HIF-1alpha.-Chilov, D., Hofer, T., Bauer, C., Wenger, R. H., Gassmann, M. Hypoxia affects expression of circadian genes PER1 and CLOCK in mouse brain.

Daily novel wheel running reorganizes and splits hamster circadian activity rhythms

The phenomenon of splitting of locomotor activity rhythms in constant light has implied that the mammalian circadian pacemaker is composed of multiple interacting circadian oscillators. Exposure of male Syrian hamsters to novel running wheels also induces splitting in some reports, although novel wheel running (NWR) is better known for its effects on altering circadian phase and the length of the free-running period. In three experiments, the authors confirm and extend earlier reports of split rhythms induced by NWR. Male Syrian hamsters, entrained to LD 14:10, were transferred for 6 to 11 consecutive days to darkened novel Wahmann wheels at ZT 4 and were returned to their home cages at ZT 9. All hamsters ran robustly in the novel wheels. NWR caused a marked reorganization of home cage wheel-running behavior: Activity onsets delayed progressively with each additional day of NWR. After 11 days, activity onset in the nighttime scotophase was delayed by 7 h and disappeared completely in 2 hamsters (Experiment 1). After 6 to 7 days of NWR (Experiment 2), activity onset delayed by 5 h. Transfer of hamsters to constant darkness (DD) after 7 days of NWR revealed clearly split activity rhythms: The delayed nighttime activity bout was clearly identifiable and characterized by a short duration. A second bout associated with the former time of NWR was equally distinct and exhibited a similarly short duration. These components rejoined after 3 to 5 days in DD accomplished via delays and advances of the nighttime and afternoon components, respectively. The final experiment established that rejoining of activity components could be prevented by perpetuating the light-dark:light-dark cycle used to induce split rhythms. The data suggest that NWR causes selective phase shifting of some circadian oscillators and that component oscillators interact strongly in constant darkness.

Glutamate blocks serotonergic phase advances of the mammalian circadian pacemaker through AMPA and NMDA receptors

The phase of the mammalian circadian pacemaker, located in the suprachiasmatic nucleus (SCN), is modulated by a variety of stimuli, most notably the environmental light cycle. Light information is perceived by the circadian pacemaker through glutamate that is released from retinal ganglion cell terminals in the SCN. Other prominent modulatory inputs to the SCN include a serotonergic projection from the raphe nuclei and a neuropeptide Y (NPY) input from the intergeniculate leaflet. Light and glutamate phase-shift the SCN pacemaker at night, whereas serotonin (5-HT) and NPY primarily phase-shift the pacemaker during the day. In addition to directly phase-shifting the circadian pacemaker, SCN inputs have been shown to modulate the actions of one another. For example, 5-HT can inhibit the phase-shifting effects of light or glutamate applied to the SCN at night, and NPY and glutamate inhibit phase shifts of one another. In this study, we explored the possibility that glutamate can modulate serotonergic phase shifts during the day. For these experiments, we applied various combinations of 5-HT agonists, glutamate agonists, and electrical stimulation of the optic chiasm to SCN brain slices to determine the effect of these treatments on the rhythm of spontaneous neuronal activity generated by the SCN circadian pacemaker. We found that glutamate agonists and optic chiasm stimulation inhibit serotonergic phase advances and that this inhibition involves both AMPA and NMDA receptors. This inhibition by glutamate may be indirect, because it is blocked by both tetrodotoxin and the GABA(A) antagonist, bicuculline.

In vivo resetting of the hamster circadian clock by 5-HT7 receptors in the suprachiasmatic nucleus

Serotonin (5-HT) has been strongly implicated in the regulation of the mammalian circadian clock located in the suprachiasmatic nuclei (SCN); however, its role in behavioral (nonphotic) circadian phase resetting remains elusive. Central to this issue are divergent lines of evidence that the SCN may, or may not, be a target for the phase-resetting effects of 5-HT. We have addressed this question using a novel reverse-microdialysis approach for timed perfusions of serotonergic and other agents to the Syrian hamster SCN with durations equivalent to the increases in in vivo 5-HT release during phase-resetting behavioral manipulations. We found that 3 hr perfusions of the SCN with either 5-HT or the 5-HT(1A,7) receptor agonist 2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydro-naphthalene (8-OH-DPAT) at midday advanced the phase of the free-running circadian rhythm of wheel-running assessed using an Aschoff type II procedure. Phase shifts induced by 8-OH-DPAT were enhanced more than threefold by pretreatment with the 5-HT synthesis inhibitor para-chlorophenylalanine. Phase advances induced by SCN 8-OH-DPAT perfusion were significantly inhibited by the 5-HT(2,7) receptor antagonist ritanserin and by the more selective 5-HT(7) receptor antagonist DR4004, implicating the 5-HT(7) receptor in mediating this phase resetting. Concurrent exposure to light during the 8-OH-DPAT perfusion abolished the phase advances. Furthermore, coperfusion of the SCN with TTX, which blocked in vivo 5-HT release, did not suppress intra-SCN 8-OH-DPAT-induced phase advances. These results indicate that 5-HT(7) receptor-mediated phase resetting in the SCN is markedly influenced by the degree of postsynaptic responsiveness to 5-HT and by photic stimulation. Finally, 5-HT may act directly on SCN clock cells to induce in vivo nonphotic phase resetting.

Opposing effects of behavioural activity and light on neurons of the suprachiasmatic nucleus

The mammalian circadian pacemaker is located in the suprachiasmatic nuclei. It can be shifted in phase by photic cues and by the behavioural activity of the animal. When presented together, light and behavioural activity attenuate each others' phase-shifting effect. Still unclear is how behavioural activity affects the suprachiasmatic nuclei and how it interacts with photic information. Previously, we reported the occurrence of behaviourally induced suppressions of neuronal activity. The present study investigates the characteristics of these suppressions as a function of circadian time and, additionally, in the presence of photic cues. We performed long-term multiunit activity recordings of neurons in freely moving rats and found that these suppressions of neuronal firing in the suprachiasmatic nucleus occurred at every phase of the circadian cycle. The magnitude of the suppressions showed a circadian variation, with larger suppressions during subjective day. When a light pulse was applied during a suppression, light and activity appeared to oppose each others' effects within the recorded population of neurons. The resulting discharge level appeared to be the sum of both responses. The opposing effects of light and activity were also found in single unit recordings, indicating that photic and behavioural stimuli interact at the level of a single neuron.

Rapid down-regulation of mammalian period genes during behavioral resetting of the circadian clock

The pervasive role of circadian clocks in regulating physiology and behavior is widely recognized. Their adaptive value is their ability to be entrained by environmental cues such that the internal circadian phase is a reliable predictor of solar time. In mammals, both light and nonphotic behavioral cues can entrain the principal oscillator of the hypothalamic suprachiasmatic nuclei (SCN). However, although light can advance or delay the clock during circadian night, behavioral events trigger phase advances during the subjective day, when the clock is insensitive to light. The recent identification of Period (Per) genes in mammals, homologues of dperiod, which encodes a core element of the circadian clockwork in Drosophila, now provides the opportunity to explain circadian timing and entrainment at a molecular level. In mice, expression of mPer1 and mPer2 in the SCN is rhythmic and acutely up-regulated by light. Moreover, the temporal relations between mRNA and protein cycles are consistent with a clock based on a transcriptional/translational feedback loop. Here we describe circadian oscillations of Per1 and Per2 in the SCN of the Syrian hamster, showing that PER1 protein and mRNA cycles again behave in a manner consistent with a negative-feedback oscillator. Furthermore, we demonstrate that nonphotic resetting has the opposite effect to light: acutely down-regulating these genes. Their sensitivity to nonphotic resetting cues supports their proposed role as core elements of the circadian oscillator. Moreover, this study provides an explanation at the molecular level for the contrasting but convergent effects of photic and nonphotic cues on the clock.

Intermittent bright light and exercise to entrain human circadian rhythms to night work

Bright light can phase shift human circadian rhythms, and recent studies have suggested that exercise can also produce phase shifts in humans. However, few studies have examined the phase-shifting effects of intermittent bright light, exercise, or the combination. This simulated night work field study included eight consecutive night shifts followed by daytime sleep/dark periods (delayed 9 h from baseline). There were 33 subjects in a 2 x 2 design that compared 1) intermittent bright light (6 pulses, 40-min long each, at 5,000 lx) versus dim light and 2) intermittent exercise (6 bouts, 15-min long each, at 50-60% of maximum heart rate) versus no exercise. Bright light and exercise occurred during the first 6 h of the first three night shifts. The circadian phase marker was the demasked rectal temperature minimum. Intermittent bright-light groups had significantly larger phase delays than dim-light groups, and 94% of subjects who received bright light had phase shifts large enough for the temperature minimum to reach daytime sleep. Exercise did not affect phase shifts; neither facilitating nor inhibiting phase shifts produced by bright light.

Morphine-induced activity attenuates phase shifts to light in C57BL/6J mice

Circadian rhythms can be phase shifted and entrained by daily schedules of light and by non-photic stimuli such as locomotor activity. Relatively little is known of how photic and non-photic stimuli interact to regulate circadian phase. Morphine injections were used to examine the effects of locomotor activity on phase shifts to light pulses in mice free-running in constant dark. Morphine injections scheduled early or late in the active period (subjective night) induced hyperactivity, but did not induce phase shifts. Light pulses late in the subjective night induced phase advance shifts that were significantly attenuated (63% smaller, p<0. 01) by pretreatment with morphine. This inhibitory effect of morphine on light-induced phase advances was blocked by preventing mice from running for 6 h after the injections. Light pulses early in the subjective night induced phase delay shifts that were only weakly attenuated (15% smaller, p=0.06) by morphine. These results demonstrate behavioral inhibition of light-induced phase resetting of circadian rhythms in mice, and suggest that the strength of this effect may be phase dependent, although other interpretations are possible.

Reduced glucose availability attenuates circadian responses to light in mice

To test whether circadian responses to light are modulated by decreased glucose availability, we analyzed photic phase resetting of the circadian rhythm of locomotor activity in mice exposed to four metabolic challenges: 1) blockade of glucose utilization induced by 2-deoxy-D-glucose (2-DG), 2) fasting (food was removed for 30 h), 3) insulin administration, and 4) insulin treatment after fasting. In mice housed in constant darkness, light pulses applied during early subjective night induced phase delays of the rhythm of locomotor activity, whereas light pulses applied during late subjective night caused phase advances. There was an overall reduction of light-induced phase shifts, with a more pronounced effect for delays, in mice pretreated with 500 mg/kg ip 2-DG compared with mice injected with saline. Administration of glucose with 2-DG prevented the reduction of light-induced phase delays. Furthermore, phase delays were reduced in fed mice pretreated with 5 IU/kg sc insulin and in fasted mice injected with saline or insulin compared with control fed mice. These results show that circadian responses to light are reduced when brain glucose availability is decreased, suggesting a metabolic modulation of light-induced phase shifts.

Neuropeptide Y blocks serotonergic phase shifts of the suprachiasmatic circadian clock in vitro

The mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) can be reset in vitro by various neurochemical stimuli. This study investigated the phase-shifting properties of neuropeptide Y (NPY) and serotonin (5-HT) agonists when applied alone, as well as their combined effects on clock resetting. These neurotransmitters have both been shown to advance the SCN clock in vitro when applied during the daytime. By monitoring the SCN neuronal activity rhythm in vitro, I first confirm that the 5HT1A/5HT7 agonist (+)DPAT maximally advances the SCN clock when applied at zeitgeber time 6 (ZT6). Conversely, NPY only phase advances the neuronal activity rhythm when applied at ZT 10. This effect occurs through stimulation of Y2 receptors. NPY, again acting through Y2 receptors, blocks (+)DPAT-induced phase shifts at ZT 6, while neither (+)DPAT nor 5-HT affect NPY-induced phase shifts at ZT 10. NPY appears to block (+)DPAT-induced phase shifts by preventing increases in cyclic AMP. These data are the first to demonstrate in vitro interactions between daytime resetting stimuli in the rat, and provide critical insights into mechanisms controlling circadian clock phase.

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.

Behavioral inhibition of light-induced circadian phase resetting is phase and serotonin dependent

Circadian rhythms in Syrian hamsters can be phase shifted by light exposure during the subjective night and by a bout of wheel running induced during the subjective day. Interactions between photic and behavioral stimuli were examined by comparing phase shifts to 15 min, 50 lux light pulses with and without a bout of running induced by confinement to a novel wheel 30 min prior to and extending through light exposure. Light pulses 6 h after dark onset on the first night of constant dark induced phase advance shifts averaging 80 min. Wheel running attenuated these shifts by 45% on average (p<0.01). Light pulses 1 h or 2.25 h after dark onset induced phase delay shifts averaging 50 min and 20 min, respectively, that were not affected by stimulated running. A significant running response to the novel wheel was evident at all 3 time points, but was greater to wheel confinement at both times early in the night. Stimulated running alone early or late in the night did not produce significant phase shifts. Behavioral attenuation of phase advances to light late in the night was prevented by pretreatment with the general 5HT1 antagonist metergoline (2 mg/kg i.p.). Metergoline did not significantly attenuate running in novel wheels. These results indicate that modulation of light-induced phase shifts by behavior is phase dependent and may involve direct or indirect actions of serotonin within the circadian system.

Bright light and LEET effects on circadian rhythms, sleep and cognitive performance

Shift work and jet lag can disrupt cicadian rhythms, with detrimetnal effects on alertness, performance and sleep. This study examined the effects of two interventions to adapt circadian rhythms, sleep and performance to a 10-h phase delay of the work-rest cycle. Bright light was administered from 2200 to 0200 each night to promote phase delay of circadian rhythms. Low energy emission therapy (LEET) was administered for 20 min prior to daytime sleep periods to promote sleep. Twelve subjects received bright light, 12 subjects received LEET, 11 received both interventions and 10 control subjects received only placebo treatments. Bright light accelerated phase delay of the circadian melatonin rhythms after the work-rest schedule shift. Further, subjects who received bright light had greater total sleep time (TST) and improved sleep continuity. LEET treatment produced a trend (p = 0.16) for increased TST, but LEET did not affect the melatonin circadian rhythm. After the schedule shift, cognitive performance measures showed few significant differences. Some minor improvements in cognitive performance were producced by light treatments but not by LEET.

Does exercise truly enhance sleep?

Two recent studies support the commonsense view that exercise improves sleep in individuals with insomnia, but questions still abound. Do patients have to be fit to reap the benefits of exercise? Does exposure to bright light during exercise enhance sleep? Is shorter, more intense exercise more helpful than longer, more moderate activities? Does exercising too close to bedtime inhibit sleep? In considering recent research on sleep and exercise, this article addresses such questions and also looks at the relationship of exercise and sleep to anxiety, depression, circadian rhythms, and age-associated problems, such as sleep apnea.

Lengthening of circadian period in hamsters by novelty-induced wheel running

Phase shifts resulting from nonphotic events can be accompanied by sizable changes in the free-running period. This study examined the relationship between tau changes and phase shifts produced by confining Syrian hamsters to a novel running wheel in the mid-subjective day. Both phase shifting and tau lengthening were higher in animals that made a high number of wheel turns in the 3 h in the novel wheel. Hamsters that ran little during the activity pulse, and did not subsequently exhibit either phase shifts or tau lengthening, had low baseline activity and long taus before the pulse. However, long taus did not preclude hamsters from running in a novel wheel and subsequently phase shifting. This was demonstrated by finding the phase shifts after activity pulses in animals whose tau had already been lengthened by previous activity pulses in novel wheels. The possibility is discussed that feedback from locomotor activity influences the period of the clock in hamsters, but it is concluded that, in addition, there must be other mechanisms accounting for the relationships between activity and tau.

Scheduled activity reorganizes circadian phase of Syrian hamsters under full and skeleton photoperiods

Circadian rhythms can be shifted or entrained by light and by arousing nonphotic stimuli. Interactions between photic and nonphotic stimuli were examined by subjecting hamsters to a daily 3 h bout of induced running under full (FPP) or skeleton (two daily light pulses; SPP) photoperiods. Activity scheduled in mid-day of a FPP induced large phase delays (260 +/- 63 min) in hamsters that ran more than 4000 rev/3 h. Split rhythms were not evident in constant dark (DD) tests. Activity scheduled in mid-subjective day of a SPP induced 180 degrees inversions of circadian phase, apparently achieved by oscillator splitting in some cases. Activity scheduled late-day and early-night induced a mix of phase delays, advances and no responses. Activity scheduled at two phase ranges late in the night had no effect, but scheduled 1 h later (beginning the last hour of darkness) induced large phase delays (238 +/- 30 min). There was no evidence of oscillator splitting during DD tests, but free-running period was significantly longer in groups that showed large phase delays. Induced running schedules have powerful effects on the phase of photic entrainment and can alter intrinsic pacemaker properties, including internal oscillator coupling and period.