Temporal profile of circadian clock gene expression in a transplanted suprachiasmatic nucleus and peripheral tissues

The mammalian hypothalamic suprachiasmatic nucleus (SCN) is the master oscillator that regulates the circadian rhythms of the peripheral oscillators. Previous studies have demonstrated that the transplantation of embryonic SCN tissues into SCN-lesioned arrhythmic mice restores the behavioral circadian rhythms of these animals. In our present study, we examined the clock gene expression profiles in a transplanted SCN and peripheral tissues, and also analysed the circadian rhythm of the locomotor activity in SCN-grafted mice. These experiments were undertaken to elucidate whether the transplanted SCN generates a dynamic circadian oscillation and maintains the phase relationships that can be detected in intact mice. The grafted SCN indeed showed dynamic circadian expression rhythms of clock genes such as mPeriod1 (mPer1) and mPeriod2 (mPer2). Furthermore, the phase differences between the expression rhythms of these genes in the grafted SCN and the locomotor activity rhythms of the transplanted animals were found to be very similar to those in intact animals. Moreover, in the liver, kidney and skeletal muscles of the transplanted animals, the phase angles between the circadian rhythm of the grafted SCN and that of the peripheral tissues were maintained as in intact animals. However, in the SCN-grafted animals, the amplitudes of the mPer1 and mPer2 rhythms were attenuated in the peripheral tissues. Our current findings therefore indicate that a transplanted SCN has the capacity to generate a dynamic intrinsic circadian oscillation, and can also lock the normal phase angles among the SCN, locomotor activity and peripheral oscillators in a similar manner as in intact control animals.     

The circadian pacemaker in the cultured suprachiasmatic nucleus from pup mice is highly sensitive to external perturbation

The circadian clock in the suprachiasmatic nucleus of the hypothalamus (SCN) entrains to non-photic maternal rhythms in the fetal and neonatal periods of rodents but this capacity disappears in later life. In order to understand the mechanism behind the non-photic entrainment in the early postnatal period, the phase response of the clock gene ( Bmal1 ) expression rhythm to external stimuli was examined in cultured SCN harvested at postnatal day 6. The SCN was obtained from transgenic mice carrying a bioluminescence reporter for Bmal1 expression. Phase-dependent phase shifts of circadian rhythm were detected in the pup as well as in the adult for culture medium exchange but the amount of phase shift was significantly larger in the pup than in the adult SCN. Half of the pup SCNs did not show integrated circadian rhythmicities in the first few days in culture. In pups, the circadian period of Bmal1 expression rhythm was shorter and the amplitude of circadian rhythm was much lower than in adults. It is concluded that the responsiveness of cultured SCN to medium exchange is much larger in pups than in adult mice. Immaturity of the structural organization in the circadian system seems to underlie the high responsiveness of the pup SCN.     

Daily meal timing is not necessary for resetting the main circadian clock by calorie restriction

In rodents, entrainment and/or resetting by feeding of the central circadian clock, the suprachiasmatic nucleus (SCN), is more efficient when food cues arise from a timed calorie restriction. Because timed calorie restriction is associated with a single meal each day at the same time, its resetting properties on the SCN possibly depend on a combination of meal time-giving cues and hypocaloric conditions per se. To exclude any effect of daily meal timing in resetting by calorie restriction, the present study employed a model of ultradian feeding schedules, divided into six meals with different durations of food access (6 × 8-min versus 6 × 12-min meal schedule) every 4 h over the 24-h cycle. The effects of such an ultradian calorie restriction were evaluated on the rhythms of wheel-running activity (WRA) and body temperature (Tb) in rats. The results indicate that daily/circadian rhythms of WRA and Tb were shifted by a hypocaloric feeding distributed in six ultradian short meals (i.e. 6 × 8-min meal schedule), showing both phase advances and delays. The magnitude of phase shifts was positively correlated with body weight loss and level of day-time behavioural activity. By contrast, rats fed daily with six ultradian meals long enough (i.e. 6 × 12-min meal schedule) to prevent body weight loss, showed only small, if any, phase shifts in WRA and Tb rhythms. The results obtained reveal the potency of calorie restriction to reset the SCN clock without synchronisation to daily meal timing, highlighting functional links between metabolism, calorie restriction and the circadian timing system.     

Circadian rhythm in bipolar disorder: A review of the literature

Sleep disturbances and circadian rhythm dysfunction have been widely demonstrated in patients with bipolar disorder (BD). Irregularity of the sleep–wake rhythm, eveningness chronotype, abnormality of melatonin secretion, vulnerability of clock genes, and the irregularity of social time cues have also been well‐documented in BD. Circadian rhythm dysfunction is prominent in BD compared with that in major depressive disorders, implying that circadian rhythm dysfunction is a trait marker of BD. In the clinical course of BD, the circadian rhythm dysfunctions may act as predictors for the first onset of BD and the relapse of mood episodes. Treatments focusing on sleep disturbances and circadian rhythm dysfunction in combination with pharmacological, psychosocial, and chronobiological treatments are believed to be useful for relapse prevention. Further studies are therefore warranted to clarify the relation between circadian rhythm dysfunction and the pathophysiology of BD to develop treatment strategies for achieving recovery in BD patients.     

Acute intracerebral haemorrhage: circadian and circannual patterns of onset

Hypothesis of the circannual and circadian variation in onset of intracerebral haemorrhage (CH) was verified, by means of single cosinor method and chi-square test for goodness of fit, in 161 consecutive patients (94 men and 67 women) admitted into the Institute of Neurosurgery of Ferrara Hospital, Italy, over 9 years. The majority of CH occurred in the morning between 06.00 AM and 12.00 noon (36.7% of cases, p<0.001); when considering the specific anatomical sites, typical supratentorial haemorrhages showed a similar pattern (37.4%, p= 0.01). A similar morning behavior was found when considering subgroups by sex (men 36.2%, women 37.3%), age ≥60 years (42.5%), no presence of hypertension (39.7%), no presence of diabetes mellitus (33.3%) and non-smokers (30.4%). The results by cosinor analysis yielded a circadian rhythmicity both for total sample and, for the men's subgroup, with a morning peak at 11.44 and 11.25, respectively. For women, however, spectral analysis found a significant ultradian cycle, having a period of 12 h (p = 0.01). A circannual periodicity, with a prevalent peak in February, was found for total sample and males subgroups, too. The results of this study confirm that intracerebral haemorrhages present a characteristic circadian and circannual pattern in onset.     

Visualizing jet lag in the mouse suprachiasmatic nucleus and peripheral circadian timing system

Circadian rhythms regulate most physiological processes. Adjustments to circadian time, called phase shifts, are necessary following international travel and on a more frequent basis for individuals who work non-traditional schedules such as rotating shifts. As the disruption that results from frequent phase shifts is deleterious to both animals and humans, we sought to better understand the kinetics of resynchronization of the mouse circadian system to one of the most disruptive phase shifts, a 6-h phase advance. Mice bearing a luciferase reporter gene for mPer2 were subjected to a 6-h advance of the light cycle and molecular rhythms in suprachiasmatic nuclei (SCN), thymus, spleen, lung and esophagus were measured periodically for 2 weeks following the shift. For the SCN, the master pacemaker in the brain, we employed high-resolution imaging of the brain slice to describe the resynchronization of rhythms in single SCN neurons during adjustment to the new light cycle. We observed significant differences in shifting kinetics among mice, among organs such as the spleen and lung, and importantly among neurons in the SCN. The phase distribution among all Period2 -expressing SCN neurons widened on the day following a shift of the light cycle, which was partially due to cells in the ventral SCN exhibiting a larger initial phase shift than cells in the dorsal SCN. There was no clear delineation of ventral and dorsal regions, however, as the SCN appear to have a population of fast-shifting cells whose anatomical distribution is organized in a ventral–dorsal gradient. Full resynchronization of the SCN and peripheral timing system, as measured by a circadian reporter gene, did not occur until after 8 days in the advanced light cycle.     

Measuring seasonal time within the circadian system: regulation of the suprachiasmatic nuclei by photoperiod

Day-length (photoperiod) is the primary environmental signal used to synchronise endogenous rhythms of physiology and behaviour. In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus house the master circadian clock. The SCN incorporate photoperiodic information and therefore measure both daily and seasonal time. Over the past decade, there have been significant advances in the understanding of the molecular basis of circadian clocks. It is now becoming apparent that the core molecular clock mechanism is itself regulated by photoperiod, although there is currently debate as to how this occurs. One recent model proposes that distinct groups of core 'clock genes' are associated with either morning or evening phases of the daily light/dark cycle. However, the validity of associating particular genes to morning and evening has been questioned. This article reviews the evidence for photoperiodic regulation of circadian clock function and then discusses alternative models that may explain the available data.     

The adrenal peripheral clock: glucocorticoid and the circadian timing system

The mammalian circadian timing system is organized in a hierarchy, with the master clock residing in the suprachiasmatic nucleus (SCN) of the hypothalamus and subsidiary peripheral clocks in other brain regions as well as peripheral tissues. Since the local oscillators in most cells contain a similar molecular makeup to that in the central pacemaker, determining the role of the peripheral clocks in the regulation of rhythmic physiology and behavior is an important issue. Glucocorticoids (GCs) are a class of multi-functional adrenal steroid hormones, which exhibit a robust circadian rhythm, with a peak linked with the onset of the daily activity phase. It has long been believed that the production and secretion of GC is primarily governed through the hypothalamus–pituitary–adrenal (HPA) neuroendocrine axis in mammals. Growing evidence, however, strongly supports the notion that the periodicity of GC involves the integrated activity of multiple regulatory mechanisms related to circadian timing system along with the classical HPA neuroendocrine regulation. The adrenal-intrinsic oscillator as well as the central pacemaker plays a pivotal role in its rhythmicity. GC influences numerous biological processes, such as metabolic, cardiovascular, immune and even higher brain functions, and also acts as a resetting signal for the ubiquitous peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior. In this review, we will therefore focus on the recent advances in our understanding of the circadian regulation of adrenal GC and its functional relevance.     

Spatial and temporal expression patterns of Bmal delineate a circadian clock in the nervous system of Branchiostoma lanceolatum

We cloned the homologue of the clock gene Bmal from a cephalochordate, Branchiostoma lanceolatum (syn. amphioxus). Amphioxus possesses a single copy of this gene (amphiBmal) that encodes for a protein of 649 amino acids, which is quite similar to BMALs of other chordates. The gene is expressed by a restricted cell group in the anterior vesicle of the neural tube, and its expression site coincides with that of another clock gene, namely, amphiPer. The expression of amphiBmal shows a rhythmic fluctuation that persists under constant darkness and is, thus, circadian. Similar to the situation in craniates, the peak phases of the amphiBmal and amphiPer expression are offset by 12 hours. Based on these observations and the putative homology between the diencephalon of vertebrates and the anterior vesicle of lancelets, we suggest a homology between the suprachiasmatic nucleus of craniates and the amphiBmal/amphiPer‐expressing cell group of amphioxus. J. Comp. Neurol. 518:1837–1846, 2010. © 2009 Wiley‐Liss, Inc.     

Food-entrainable circadian oscillators in the brain

Circadian rhythms in mammalian behaviour and physiology rely on daily oscillations in the expression of canonical clock genes. Circadian rhythms in clock gene expression are observed in the master circadian clock, the suprachiasmatic nucleus but are also observed in many other brain regions that have diverse roles, including influences on motivational and emotional state, learning, hormone release and feeding. Increasingly, important links between circadian rhythms and metabolism are being uncovered. In particular, restricted feeding (RF) schedules which limit food availability to a single meal each day lead to the induction and entrainment of circadian rhythms in food-anticipatory activities in rodents. Food-anticipatory activities include increases in core body temperature, activity and hormone release in the hours leading up to the predictable mealtime. Crucially, RF schedules and the accompanying food-anticipatory activities are also associated with shifts in the daily oscillation of clock gene expression in diverse brain areas involved in feeding, energy balance, learning and memory, and motivation. Moreover, lesions of specific brain nuclei can affect the way rats will respond to RF, but have generally failed to eliminate all food-anticipatory activities. As a consequence, it is likely that a distributed neural system underlies the generation and regulation of food-anticipatory activities under RF. Thus, in the future, we would suggest that a more comprehensive approach should be taken, one that investigates the interactions between multiple circadian oscillators in the brain and body, and starts to report on potential neural systems rather than individual and discrete brain areas.     

Intraventricular carbachol mimics the phase-shifting effect of light on the circadian rhythm of wheel-running activity

Intraventricular injections of carbachol, a cholinergic agonist, into free-running mice can cause phase dependent phase shifts, in both directions, in the circadian rhythm of wheel-running activity. These effects mimic the action of light in entraining the circadian pacemaker, and suggest that acetylcholine may play a role in photoentrainment.