Phase Angle Of Entrainment Research Articles

Intrinsic Activity Rhythms in Macaca mulatta: Their Entrainment to Light and Melatonin

Mounting evidence that circadian abnormalities are a risk factor for cancer and for cardiovascular, psychiatric, and other disorders calls for in-depth investigation of intrinsic clock-dependent processes in diurnal animal models phylogenetically close to humans. Rhesus monkey (Macaca mulatta) is the most extensively studied diurnal nonhuman primate. Similar to humans, it features consolidated nighttime sleep and advanced cardiovascular, neuroendocrine, and cognitive responses. However, the intrinsic circadian rhythmicity in this species remains to be fully characterized. Here it is demonstrated that under constant dim light (~10 lx) conditions, young adult rhesus monkeys maintain robust intrinsic circadian rhythms of activity, with periods ranging from 23.4 to 25.1 h. Constant environmental light of moderate intensity (~100 lx) slows down the circadian clock in rhesus monkeys. The exposure to light or melatonin shifts the phase of intrinsic circadian rhythms, with the direction and magnitude of the shift dependent on the circadian phase at which a stimulus was administered. The length of the intrinsic period largely defines an individual's chronotype (morningness or eveningness) and affects the stability of intrinsic rhythms and the phase angle of entrainment to melatonin and light. This first detailed characterization of intrinsic circadian rhythms of activity and their responses to light and melatonin in rhesus monkeys shows principal similarities to those in humans. These findings should provide new opportunities for translational research on the effects of diverse agents, environmental conditions, aging, and disease on the circadian clock and its outputs.

DISSOCIATION OF THE CIRCADIAN SYSTEM OF OCTODON DEGUS BY T28 AND T21 LIGHT-DARK CYCLES

Octodon degus is a primarily diurnal rodent that presents great variation in its circadian chronotypes due to the interaction between two phase angles of entrainment, diurnal and nocturnal, and the graded masking effects of environmental light and temperature. The aim of this study was to test whether the circadian system of this diurnal rodent can be internally dissociated by imposing cycles shorter and longer than 24 h, and to determine the influence of degus chronotypes and wheel-running availability on such dissociation. To this end, wheel-running activity and body temperature rhythms were studied in degus subjected to symmetrical light-dark (LD) cycles of T28h and T21h. The results show that both T-cycles dissociate the degus circadian system in two different components: one light-dependent component (LDC) that is influenced by the presence of light, and a second non–light-dependent component (NLDC) that free-runs with a period different from the external lighting cycle. The LDC was more evident in the nocturnal than diurnal chronotype, and also when wheel running was available. Our results show that, in addition to rats and mice, degus must be added to the list of species that show an internal dissociation in their circadian rhythms when exposed to forced desynchronization protocols. The existence of a multioscillatory circadian system having two groups of oscillators with low coupling strength may explain the flexibility of degus chronotypes. (Author correspondence: angerol@um.es)

Sex Differences in Phase Angle of Entrainment and Melatonin Amplitude in Humans

Studies of sex differences in the timing of human circadian rhythms have reported conflicting results. This may be because the studies conducted to date have not controlled for the masking effects of the rest activity cycle on the circadian rhythms being assessed. In the present analysis of data collected under controlled conditions, we examined sex differences in the timing of circadian rhythms while minimizing masking from behavioral and environmental factors using a constant routine (CR) protocol. All participants (28 women and 28 men paired by habitual wake time; age range, 18 30 years) maintained a regular self selected sleep wake schedule at home prior to the study. After 3 baseline days in the laboratory, participants began a CR. Women were found to have a significantly higher melatonin amplitude and lower temperature amplitude than men. While sleep timing was the same between the 2 groups, the timing of the circadian rhythms of core body temperature and pineal melatonin secretion was earlier relative to sleep time in women as compared to men. Sleep therefore occurred at a later biological time for women than men, despite being at the same clock time. Given that sleep propensity and structure vary with circadian phase and are impacted by circulating melatonin, these findings may have important implications for understanding sex differences in sleep timing and duration, diurnal preference, and the prevalence of sleep disorders such as insomnia.

Light Exposure Patterns in Healthy Older and Young Adults

Aging is associated with an earlier timing of circadian rhythms and a shorter phase angle between wake time and the timing of melatonin secretion or the core body temperature nadir. Light has a phase-dependent effect on the circadian pacemaker, and modifications of habitual light exposure in older people could contribute to a change in the timing of circadian rhythms or in the phase angle of entrainment. In this study, we compare natural light exposure of community-dwelling older and young subjects studied at the same time of year, focusing on the pattern of light exposure across the waking day. We recorded light exposure data for 3 to 8 days from 22 older (aged 66.01 +/- 5.83) and 22 young subjects (aged 23.41 +/- 4.57), living at home on self-selected sleepwake schedules, and matched for time of year. All subjects were from New England (latitude 42.3 degrees N to 43 degrees N). We compared the percentage of the waking day spent by older and young subjects at 4 different light levels (from very dim to very bright). We compared hourly averaged light exposure data in each group according to clock time and with respect to each subject's daily sleepwake times. Although both age groups spent more than half of their waking hours in dim or moderate room light intensity (<100 lux), we found that the older subjects spent a significantly greater percentage of their waking day in the brighter light levels (> or =1000 lux); their hourly averaged light exposure levels were also significantly greater whether we examined the data with respect to absolute clock time, to wake time, or to bed time, and this was true across all seasons. We found that healthy older people were exposed to significantly higher levels of light throughout their waking day than young people. Differences in natural light exposure may contribute to the age-related phase advance of the circadian pacemaker and its later timing relative to the sleepwake cycle. This hypothesis should be explored further in carefully designed prospective studies.

Timing of sleep and its relationship with the endogenous melatonin rhythm.

While much research has investigated the effects of exogenous melatonin on sleep, less is known about the relationship between the timing of the endogenous melatonin rhythm and the sleep–wake cycle. Significant inter-individual variability in the phase relationship between sleep and melatonin rhythms has been reported although the extent to which the variability reflects intrinsic and/or environmental differences is unknown. We examined the effects of different sleeping schedules on the time of dim light melatonin onset (DLMO) in 28 young, healthy adults. Participants chose to maintain either an early (22:30–06:30 h) or a late (00:30–08:30 h) sleep schedule for at least 3 weeks prior to an overnight laboratory visit. Saliva samples were collected under dim light (<2 lux) and controlled posture conditions to determine salivary DLMO. The 2-h difference between groups in the enforced sleep–wake schedule was associated with a concomitant 1.75-h delay in DLMO. The mean phase relationship between sleep onset and DLMO remained constant (~2 h). The variance in DLMO time, however, was greater in the late group (range 4.5 h) compared to the early group (range 2.4 h) perhaps due to greater effect of environmental influences in delayed sleep types or greater intrinsic instability in their circadian system. The findings contribute to our understanding of individual differences in the human circadian clock and have important implications for the diagnosis and treatment of circadian rhythm sleep disorders, in particular if a greater normative range for phase angle of entrainment occurs in individuals with later sleep–wake schedules.

Twilight and Photoperiod Affect Behavioral Entrainment in the House Mouse (Mus musculus)

The effect of twilight transitions on entrainment of C57BL/6JOlaHsd mice (Mus musculus) was studied using light-dark cycles of different photoperiods (6, 12, and 18 h) and twilight transitions of different durations (0, 1, and 2 h). Phase angle differences of the onset, center of gravity, and offset of activity, activity duration (alpha), as well as free-running period (tau) in continuous darkness were analyzed. The main finding was that for all conditions the onset of activity was close to dusk or lights-off except for the short photoperiod with 2 h of twilight where activity onset was on average 5.3 (SEM 1.07) h after lights-off. This finding contrasts with the results of Boulos and Macchi for Syrian hamsters where a 5.9-h earlier activity onset was observed when similar photoperiod and twilight conditions are compared with a rectangular LD cycle. The authors suggest the opposite effects of 2 h of twilight in the 2 species may be related to their different free-running periods under DD conditions following entrainment to short photoperiod with 2-h twilight conditions. Since the authors observed larger variation in phase angle of entrainment in longer twilight conditions, twilight does not necessarily form a stronger zeitgeber.

Racial Differences in the Human Endogenous Circadian Period

The length of the endogenous period of the human circadian clock (tau) is slightly greater than 24 hours. There are individual differences in tau, which influence the phase angle of entrainment to the light/dark (LD) cycle, and in doing so contribute to morningness-eveningness. We have recently reported that tau measured in subjects living on an ultradian LD cycle averaged 24.2 hours, and is similar to tau measured using different experimental methods. Here we report racial differences in tau. Subjects lived on an ultradian LD cycle (1.5 hours sleep, 2.5 hours wake) for 3 days. Circadian phase assessments were conducted before and after the ultradian days to determine the change in circadian phase, which was attributed to tau. African American subjects had a significantly shorter tau than subjects of other races. We also tested for racial differences in our previous circadian phase advancing and phase delaying studies. In the phase advancing study, subjects underwent 4 days of a gradually advancing sleep schedule combined with a bright light pulse upon awakening each morning. In the phase delaying study, subjects underwent 4 days of a gradually delaying sleep schedule combined with evening light pulses before bedtime. African American subjects had larger phase advances and smaller phase delays, relative to Caucasian subjects. The racial differences in tau and circadian phase shifting have important implications for understanding normal phase differences between individuals, for developing solutions to the problems of jet lag and shift work, and for the diagnosis and treatment of circadian rhythm based sleep disorders such as advanced and delayed sleep phase disorder.

Temporally Restricted Role of Retinal PACAP: Integration of the Phase-Advancing Light Signal to the SCN

Circadian rhythms in physiology and behavior are temporally synchronized to the day/night cycle through the action of light on the circadian clock. In mammals, transduction of the photic signal reaching the circadian oscillator in the suprachiasmatic nucleus (SCN) occurs through the release of glutamate and pituitary adenylate cyclase-activating peptide (PACAP). The authors' study aimed at clarifying the role played by PACAP in photic resetting and entrainment. They investigated the circadian response to light of PACAPnullmice lacking the 5th exon of the PACAP coding sequence. Specifically, they examined free-running rhythms, entrainment to 12-h light:12-h dark (LD)cycles, the phase-response curve (PRC) to single light pulses, entrainment to a23-h T-cycle, re-entrainment to 6-h phase shifts in LD cycles, and light-induced c-Fos expression. PACAP-null and wild-type mice show similar free-running periods and similar entrainment to 12:12 LD cycles. However, the PRC of PACAP-null mice lacks a phase-advance portion. Surprisingly, despite the absence of phase advance to single light pulses, PACAP-null mice are able to entrain to a 23-h T-cycle, but with a significantly longer phase angle of entrainment than wild types. In addition, PACAP-null mice re-entrain more slowly to a 6-h phase advance of the LD cycle. Nevertheless, induction of c-Fos by light in late night is normal. In all experiments, PACAP-null mice show specific behavioral impairments in response to phase-advancing photic stimuli. These results suggest that PACAP is required for the normal integration of the phase advancing light signal by the SCN.

Phase Angle of Entrainment in Morning‐ and Evening‐Types under Naturalistic Conditions

Differences in morningness‐eveningness among humans are commonly ascribed to circadian parameters, such as circadian period and responsivity to environmental time cues, as well as homeostatic sleep drive. Light is the primary synchronizer of the human biological clock, and if circadian differences exist between morning and evening types, they should have different phase angles of entrainment to the light/dark cycle; that is, morning and evening types should have different patterns of light exposure relative to endogenous circadian phase (ECP). When phase angle of entrainment is strictly defined as the relationship between a marker of ECP and the timing of light exposure, such differences have been demonstrated in the laboratory under controlled light/dark cycles and have recently been shown under conditions of spring and summer light exposure outside the laboratory, taking into account the variable intensity of light. Here, we report similar results from a large (n=66), diverse cohort of morning and evening types across the age span studied at all different times of the year. Differences between morning and evening types in light exposure relative to ECP, indicative of a difference in the phase angle of entrainment to the external light/dark cycle, were found. Specifically, evening types, compared to morning types, had a higher ratio of phase advancing to phase delaying by light. We interpret this as indicating a longer circadian period (τ) in evening types.

Effects of chronic expression of the HIV-induced protein, transactivator of transcription, on circadian activity rhythms in mice, with or without morphine

Patients with human immunodeficiency virus (HIV) infection exhibit changes in sleep patterns, motor disorders, and cognitive dysfunction; these symptoms may be secondary to circadian rhythm abnormalities. Studies in mice have shown that intracerebral injection of an HIV protein, transactivator of transcription (Tat), alters the timing of circadian rhythms in a manner similar to light. Therefore, we tested the hypothesis that chronic Tat expression alters circadian rhythms, especially their entrainment to a light-dark (LD) cycle, by using transgenic mice in which Tat expression in the brain was induced via a doxycycline (DOX)-sensitive, glial fibrillary-associated, protein-restricted promoter. Because opiate substance abuse, which shares comorbidity with HIV infection, also disrupts sleep, a final experiment assessed the effects of morphine exposure on circadian rhythms in wild-type and Tat transgenic mice. Mice housed in cages equipped with running wheels were fed chow with or without DOX. Experiment 1 revealed a small but significant (P < 0.05) difference between groups in the phase angle of entrainment and a 15% decrease in the wheel running in the DOX group (P < 0.005). During exposure to constant darkness, DOX did not alter the endogenous period length of the circadian rhythm. Experiment 2 investigated the effect of DOX on circadian rhythms in wild-type and Tat(+) mice during exposure to a normal or phase-shifted LD cycle, or morphine treatment without any change in the LD cycle. Tat induction significantly decreased wheel running but did not affect entrainment to the normal or shifted LD cycle. Morphine decreased wheel running without altering the phase angle of entrainment, and the drug's effects were independent of Tat induction. In conclusion, these findings suggest that chronic brain expression of Tat decreases locomotor activity and the amplitude of circadian rhythms, but does not affect photic entrainment or reentrainment of the murine circadian pacemaker.

Variation in nocturnality and circadian activity rhythms between photoresponsive F344 and nonphotoresponsive Sprague Dawley rats

BackgroundVariation in circadian rhythms and nocturnality may, hypothetically, be related to or independent of genetic variation in photoperiodic mediation of seasonal changes in physiology and behavior. We hypothesized that strain variation in photoperiodism between photoperiodic F344 rats and nonphotoperiodic Harlan Sprague Dawley (HSD) rats might be caused by underlying variation in clock function. We predicted that HSD rats would have more activity during the day or subjective day, longer free-running rhythms, poor entrainment to short day length, and shorter duration of activity, traits that have been associated with nonphotoperiodism in other laboratory rodent species, relative to F344 rats. An alternative hypothesis, that differences are due to variation in melatonin secretion or responses to melatonin, predicts either no such differences or inconsistent combinations of differences.MethodsWe tested these predictions by examining activity rhythms of young male F344 and HSD rats given access to running wheels in constant dark (DD), short day length (L8:D16; SD), and long day length (L16:D8; LD). We compared nocturnality (the proportion of activity during night or subjective night), duration of activity (alpha), activity onset and offset, phase angle of entrainment, and free running rhythms (tau) of F344 and HSD rats.ResultsHSD rats had significantly greater activity during the day, were sometimes arrhythmic in DD, and had significantly longer tau than F344 rats, consistent with predictions. However, HSD rats had significantly longer alpha than F344 rats and both strains entrained to SD, inconsistent with predictions.ConclusionThe ability of HSD rats to entrain to SD, combined with longer alpha than F344 rats, suggests that the circadian system of HSD rats responds correctly to SD. These data offer best support for the alternative hypothesis, that differences in photoresponsiveness between F344 and HSD rats are caused by non-circadian differences in melatonin secretion or the response to melatonin.

Human Tau in an Ultradian Light-Dark Cycle

Multiple experimental approaches have been used to assess the free-running period, or “tau,” of humans. One approach has been to study circadian rhythms in free-running blind humans. For example, Sack et al. (1992) measured the plasma melatonin rhythm in 11 free-running blind people and reported an average tau of 24.55 h.

Blue light phase advances entrained rhythms of brain temperature and sleep in rhesus monkeys

Blue light sensitive retinal ganglion cells and their neural projections appear to be the main pathway for the effects of light on circadian entrainment and alerting. The effects of blue light on rhythms and sleep in a 24 hour light‐dark cycle were studied in a rhesus monkey model. Six individually housed and unrestrained animals were entrained in cool‐white fluorescent light (WH) for 12 hours daily (LD 12:12), then exposed to 4 days of LD12:12 with blue LED lamps (BL). Both sources averaged 6.1 x 1013 quanta cm−2s−1 at eye level. Performance was measured using the Psychomotor Test System (Georgia State Univ.). Brain temperature (Tbr) and sleep were recorded via biotelemetry. Sleep stages from the last two days of WH and BL were scored in 30‐second epochs. The average fraction of time in sleep over 24‐hours, as well as during L or D, did not differ between WH and BL. Shallow slow‐wave sleep (stages 1 and 2) and deep slow‐wave sleep (stages 3 and 4) also showed no significant differences. The fraction of time in REM sleep over 24‐h was significantly elevated as was REM during D. Acrophases from sine fits showed a statistically significant advance of the phase angle of entrainment in Tbr, in the first two days of BL. Acrophases and average waveforms of sleep stages from the subsequent two days show apparent phase advances and a slight increase in daytime sleep, but did not differ significantly.(Supported by NASA Grant NNJ04HF44G)

A Novel Quantitative Trait Locus on Mouse Chromosome 18, “era1,” Modifies the Entrainment of Circadian Rhythms

The mammalian circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus conveys 24-h rhythmicity to sleep-wake cycles, locomotor activity, and other behavioral and physiological processes. The timing of rhythms relative to the light/dark (LD12:12) cycle is influenced in part by the endogenous circadian period and the time of day specific sensitivity of the clock to light. We now describe a novel circadian rhythm phenotype, and a locus influencing that phenotype, in a segregating population of mice. By crossbreeding 2 genetically distinct nocturnal strains of mice (Cast/Ei and C57BL/6J) and backcrossing the resulting progeny to Cast/Ei, we have produced a novel circadian phenotype, called early runner mice. Early runner mice entrain to a light/dark cycle at an advanced phase, up to 9 hours before dark onset. This phenotype is not significantly correlated with circadian period in constant darkness and is not associated with disruption of molecular circadian rhythms in the SCN, as assessed by analysis of period gene expression. We have identified a genomic region that regulates this phenotype-a major quantitative trait locus on chromosome 18 (near D18Mit184) that we have named era1 for Early Runner Activity locus one. Phase delays caused by light exposure early in the subjective night were of smaller magnitude in backcross offspring that were homozygous Cast/Ei at D18Mit184 than in those that were heterozygous at this locus. Genetic variability in the circadian response to light may, in part, explain the variance in phase angle of entrainment in this segregating mouse population.

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.

Sleep and Circadian Rhythms in Humans

During the past 50 years, converging evidence reveals that the fundamental properties of the human circadian system are shared in common with those of other organisms. Concurrent data from multiple physiological rhythms in humans revealed that under some conditions, rhythms oscillated at different periods within the same individuals and led to the conclusion 30 years ago that the human circadian system was composed of multiple oscillators organized hierarchically; this inference has recently been confirmed using molecular techniques in species ranging from unicellular marine organisms to mammals. Although humans were once thought to be insensitive to the resetting effects of light, light is now recognized as the principal circadian synchronizer in humans, capable of eliciting weak (Type 1) or strong (Type 0) resetting, depending on stimulus strength and timing. Realization that circadian photoreception could be maintained in the absence of sight was first recognized in blind humans, as was the property of adaptation of the sensitivity of circadian photoreception to prior light history. In sighted humans, the intrinsic circadian period is very tightly distributed around approximately 24.2 hours and exhibits aftereffects of prior entrainment. Phase angle of entrainment is dependent on circadian period, at least in young adults. Circadian pacemakers in humans drive daily variations in many physiologic and behavioral variables, including circadian rhythms in alertness and sleep propensity. Under entrained conditions, these rhythms interact with homeostatic regulation of the sleep/wake cycle to determine the ability to sustain vigilance during the day and to sleep at night. Quantitative understanding of the fundamental properties of the multioscillator circadian system in humans and their interaction with sleep/wake homeostasis has many applications to health and disease, including the development of treatments for circadian rhythm and sleep disorders.

Preliminary evidences of circadian fan activity rhythm in &lt;i&gt;Sabella spallanzanii&lt;/i&gt; (Gmelin, 1791) (Polychaeta: Sabellidae)

The fan activity rhythm of Sabella spallanzanii (Gmelin, 1791) and its entrainment capability to light were studied. Animals were tested under constant darkness (DD) followed by two consecutive 24 h light-darkness regimes: a first 11 h light period (LD) and a second 9 h light period, with its phase inverted (DL). An infrared analogical video-camera took shots each 30 s. A number of pictures with open fan were counted every 15 min. In DD a weak free-running periodicity in the circadian range was found, thus reinforcing the matching of the 24 h period under study in both photoperiod regimes. A nocturnal activity was characterised with a consistent anticipation to lightOFF (i.e. entrainment). Moreover, this phase of entrainment differed between DL and LD. The presence of endogenous activity rhythm with a variable phase angle of entrainment is a distinctive feature of circadian pacemakers.

Behavioral and neurochemical sources of variability of circadian period and phase: studies of circadian rhythms of npy−/− mice

The cycle length or period of the free-running rhythm is a key characteristic of circadian rhythms. In this study we verify prior reports that locomotor activity patterns and running wheel access can alter the circadian period, and we report that these treatments also increase variability of the circadian period between animals. We demonstrate that the loss of a neurochemical, neuropeptide Y (NPY), abolishes these influences and reduces the interindividual variability in clock period. These behavioral and environmental influences, from daily distribution of peak locomotor activity and from access to a running wheel, both act to push the mean circadian period to a value < 24 h. Magnitude of light-induced resetting is altered as well. When photoperiod was abruptly changed from a 18:6-h light-dark cycle (LD18:6) to LD6:18, mice deficient in NPY were slower to respond to the change in photoperiod by redistribution of their activity within the prolonged dark and eventually adopted a delayed phase angle of entrainment compared with controls. These results support the hypothesis that nonphotic influences on circadian period serve a useful function when animals must respond to abruptly changing photoperiods and point to the NPYergic pathway from the intergeniculate leaflet innervating the suprachiasmatic nucleus as a circuit mediating these effects.

Phase Angle Difference Alters Coupling Relations of Functionally Distinct Circadian Oscillators Revealed by Rhythm Splitting

The interactions (i.e., coupling) between multiple oscillators of a circadian system determine basic properties of the integrated pacemaker. Unfortunately, there are few experimental models to investigate the putative interactions of functionally defined oscillators comprising the mammalian circadian pacemaker. Here the authors induce in hamsters a novel circadian entrainment pattern that is characterized by the daily expression of robust wheel-running activity in each scotophase of a 24-h light:dark:light:dark cycle. The daily activity bouts are mediated by 2 circadian oscillators, here designated "daytime" and "nighttime," that have been temporally dissociated under this light regime. To assess the phase dependence of interactions between oscillatory components, the phase relationship of the 2 daily scotophases was manipulated over a 4-h range, and the timing of activity of the daytime and nighttime components under entrained and probe conditions was examined. The average phase angle of entrainment and the day-to-day variability of activity onset of each activity component depended on the phase relationship of the respective scotophases and not on whether the component occurred in the daytime or the nighttime. Short-term denial of wheel access subsequently influenced amount and duration of wheel running but not timing of its onset, suggesting that only the former measures depend on a homeostatic mechanism sensitive to the time elapsed since prior intense running. Replacement of individual photophases with darkness revealed phase attraction between oscillators that was not dependent on the phase relationship of component oscillators but differed for daytime versus nighttime activity components. Entrainment patterns shown here cannot be accounted for by only nonparametric actions of light. Instead, the phase-dependent interactions of oscillators strongly influence entrainment properties, whereas intrinsic functional differences in dissociated oscillators apparently influence their attraction in darkness. This model system may be ideal for identifying genomic and physiological factors that mediate these interactions and thus contribute importantly to system properties of the mammalian circadian clock.

Photic Entrainment Is Altered in the 5-HT1B Receptor Knockout Mouse

The hypothalamic suprachiasmatic nucleus (SCN) is a circadian oscillator that receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain. Activation of presynaptic serotonin 1B (5-HT1B) receptors on retinal terminals in the SCN inhibits retinohypothalamic neurotransmission and light-induced behavioral phase shifts. To assess the role of 5-HT1B receptors in photic entrainment, 5-HT1B receptor knockout (5-HT1B KO) and wild-type (WT) mice were maintained in non-24 h L:D cycles (T cycles). WT mice entrained to T = 21 h and T = 22 h cycles, whereas 5-HT1B KO animals did not. 5-HT1B KO animals did entrain to T = 23 h and T = 26 h cycles, although their phase angle of entrainment was altered compared to WT animals. 5-HT1B KO mice were significantly more phase delayed under T = 23 h conditions and significantly more phase advanced under T = 26 h conditions compared to WT mice. When 5-HT1B KO mice were housed in a T = 23 h short-day photoperiod (9.5L:13.5D), the delayed phase angle of entrainment was more pronounced. Light-induced phase shifts were reduced in 5-HT1B KO mice, consistent with their behavior in T cycles, suggesting an attenuated response to light. Based on previous work, this attenuated response to light might not have been predicted but can be explained by consideration of GABAergic mechanisms within the SCN. Phase-delayed circadian rhythms during the short days of winter are characteristic of patients suffering from seasonal affective disorder, and 5-HT has been implicated in its pathophysiology. The 5-HT1B KO mouse may be useful for investigating the altered entrainment evident during this serious mood disorder.