Rapid resetting of the mammalian circadian clock

The suprachiasmatic nuclei (SCN) contain the principal circadian clock governing overt daily rhythms of physiology and behavior. The endogenous circadian cycle is entrained to the light/dark via direct glutamatergic retinal afferents to the SCN. To understand the molecular basis of entrainment, it is first necessary to define how rapidly the clock is reset by a light pulse. We used a two-pulse paradigm, in combination with cellular and behavioral analyses of SCN function, to explore the speed of resetting of the circadian oscillator in Syrian hamster and mouse. Analysis of c-fos induction and cAMP response element-binding protein phosphorylation in the retinorecipient SCN demonstrated that the SCN are able to resolve and respond to light pulses presented 1 or 2 hr apart. Analysis of the phase shifts of the circadian wheel-running activity rhythm of hamsters presented with single or double pulses demonstrated that resetting of the oscillator occurred within 2 hr. This was the case for both delaying and advancing phase shifts. Examination of delaying shifts in the mouse showed resetting within 2 hr and in addition showed that resetting is not completed within 1 hr of a light pulse. These results establish the temporal window within which to define the primary molecular mechanisms of circadian resetting in the mammal.     

Atherosclerosis in the human brachial artery

The hypothalamic suprachiasmatic nucleus (SCN) of the mammal is the circadian pacemaker responsible for generation of circadian rhythms. Several immediate-early genes are expressed in the SCN by light stimuli which induce phase shifts of animal activity rhythms. In the present study, we investigated whether Homer, a PDZ-like protein which is rapidly induced following synaptic activation, mRNA expression is regulated by light in rat SCN. Homer mRNA expression in the SCN of rat killed at 4 h after onset of the light and dark phases was very low. One hour light stimuli during the subjective night dramatically induced Homer mRNA expression in the ventrolateral portion of the SCN, whereas light stimuli during the subjective light phase did not. This finding implies that Homer may be involved in the photic entrainment of the circadian clock.     

Hierarchically coupled ultradian oscillators generating robust circadian rhythms

Ensembles of mutually coupled ultradian cellular oscillators have been proposed by a number of authors to explain the generation of circadian rhythms in mammals. Most mathematical models using many coupled oscillators predict that the output period should vary as the square root of the number of participating units, thus being inconsistent with the well-established experimental result that ablation of substantial parts of the suprachiasmatic nuclei (SCN), the main circadian pacemaker in mammals, does not eliminate the overt circadian functions, which show no changes in the phases or periods of the rhythms. From these observations, we have developed a theoretical model that exhibits the robustness of the circadian clock to changes in the number of cells in the SCN, and that is readily adaptable to include the successful features of other known models of circadian regulation, such as the phase response curves and light resetting of the phase.     

The role of the intergeniculate leaflet in entrainment of circadian rhythms to a skeleton photoperiod

Mammalian circadian rhythms are synchronized to environmental light/dark (LD) cycles via daily phase resetting of the circadian clock in the suprachiasmatic nucleus (SCN). Photic information is transmitted to the SCN directly from the retina via the retinohypothalamic tract (RHT) and indirectly from the retinorecipient intergeniculate leaflet (IGL) via the geniculohypothalamic tract (GHT). The RHT is thought to be both necessary and sufficient for photic entrainment to standard laboratory light/dark cycles. An obligatory role for the IGL-GHT in photic entrainment has not been demonstrated. Here we show that the IGL is necessary for entrainment of circadian rhythms to a skeleton photoperiod (SPP), an ecologically relevant lighting schedule congruous with light sampling behavior in nocturnal rodents. Rats with bilateral electrolytic IGL lesions entrained normally to lighting cycles consisting of 12 hr of light followed by 12 hr of darkness, but exhibited free-running rhythms when housed under an SPP consisting of two 1 hr light pulses given at times corresponding to dusk and dawn. Despite IGL lesions and other damage to the visual system, the SCN displayed normal sensitivity to the entraining light, as assessed by light-induced Fos immunoreactivity. In addition, all IGL-lesioned, free-running rats showed masking of the body temperature rhythm during the SPP light pulses. These results show that the integrity of the IGL is necessary for entrainment of circadian rhythms to a lighting schedule like that experienced by nocturnal rodents in the natural environment.     

Reduction of arthritis and pneumonitis in motheaten mice by soluble tumor necrosis factor receptor

A variety of observations from several rodent species suggest that a serotonin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker may play a role in resetting or entrainment of circadian rhythms by non-photic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possible to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four serotonergic drugs and were then locked in a novel wheel for 3 h during the mid-rest phase, when novelty-induced activity produces maximal phase advance shifts. Drugs tested at several doses were metergoline (5-HT1/2 antagonist; i.p.), (+)-WAY100135 (5-HT1A postsynaptic antagonist, which may also reduce 5-HT release by an agonist effect at 5-HT1A raphe autoreceptors; i.p.), NAN-190 (5-HT1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT1A raphe autoreceptors; i.p.) and ritanserin (5-HT2/7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. These results do not support a hypothesis that 5-HT release or activity at 5HT1, 2 and 7 receptors in the SCN is necessary for the production of activity-induced phase shifts in hamsters.     

Light responsiveness of the suprachiasmatic nucleus: long-term multiunit and single-unit recordings in freely moving rats

The suprachiasmatic nuclei (SCN) of the hypothalamus contain a pacemaker that generates circadian rhythms in many functions. Light is the most important stimulus that synchronizes the circadian pacemaker to the environmental cycle. In this paper we have characterized the baseline neuronal firing patterns of the SCN as well as their response to light in freely moving rats. Multiunit and single-unit recordings showed that SCN neurons increase discharge during daytime and decrease discharge at night. Discharge levels of individual neurons that were followed throughout the circadian cycle appeared in phase with the population and were characterized by low discharge rates (often below 1 Hz), with a twofold increase during the day. The effect of light on the multiunit response was dependent on the duration of light exposure and on light intensity, with light thresholds of approximately 0.1 lux. The light response level showed a strong dependency on time of day, with large responsiveness at night and low responsiveness during day. At both phases of the circadian cycle, the response level could be raised by an increase in light intensity. Single-unit measurements revealed that the time-dependent light response of SCN neurons was present also at the level of single units. The results show that the basic light response characteristics that were observed at the multiunit level result from an integrated response of similarly behaving single units. Research at the single-unit level is therefore a useful approach for investigating the basic principles of photic entrainment.     

GABAB receptor stimulation phase-shifts the mammalian circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) generates 24-h rhythms in vitro. Here we show that the GABAB agonist baclofen resets the SCN pacemaker in vitro in a phase-dependent manner: advances were induced at zeitgeber time (ZT) 6, and delays were induced at ZT 22. Both effects were blocked the GABAB antagonist, 2-hydroxysaclofen, while the GABAA antagonist, bicuculline was ineffective. Thus, the SCN pacemaker is sensitive to resetting by GABAB stimulation.     

Melatonin entrains the restored circadian activity rhythms of syrian hamsters bearing fetal suprachiasmatic nucleus grafts

A circadian pacemaker consists of at least three essential features: the ability to generate circadian oscillations, an output signal, and the ability to be entrained by external signals. In rodents, ablation of the suprachiasmatic nucleus (SCN) results in the loss of circadian rhythms in activity. Rhythmicity can be restored by transplanting fetal SCN into the brain of the lesioned animal, demonstrating the first two of the essential pacemaker features within the grafts. External signals, such as the light/dark cycle, have not, however, been shown to entrain the restored rhythms. Melatonin injections are an effective entraining stimulus in fetal and neonatal Syrian hamsters of the same developmental ages used to provide donor tissue for transplantation. Therefore, melatonin was used to test the hypothesis that SCN grafts contain an entrainable pacemaker. Daily injections of melatonin were given to SCN-lesioned hosts beginning on the day after transplantation of fetal SCN. Two groups that received melatonin at different times of day 12 hr apart each showed significantly clustered phases but with average phases that differed by 8.67 hr. Thus melatonin was able to entrain the restored circadian activity rhythms. In contrast to these initial injections, injections given 6 weeks after transplantation were unable to entrain or phase shift the rhythms. The results demonstrate that SCN grafts contain an entrainable circadian pacemaker. In addition, the results also indicate that the fetal SCN is directly sensitive to melatonin and, as with intact hamsters, sensitivity to melatonin is lost during SCN development.     

Phase control of ultradian feeding rhythms in the common vole (Microtus arvalis): the roles of light and the circadian system

In their ultradian (2- to 3-hr) feeding rhythm, common voles show intraindividual synchrony from day to day, as well as interindividual synchrony between members of the population, even at remote distances. This study addresses the question of how resetting of the ultradian rhythm, a prerequisite for such synchronization, is achieved. Common voles were subjected to short light-dark cycles (1 hr darkness with light varying between 0.7 and 2.5 hr); to T cycles (long light-dark cycles in the circadian range--16 hr darkness and 3-13 hr light); to light pulses (15 min) during different circadian and ultradian phases; and to addition of D2O to the drinking water (25%). Short light-dark cycles and D2O were also applied to voles without circadian rhythmicity, after lesions of the suprachiasmatic nuclei. In these experiments, four hypotheses on synchronization of ultradian rhythmicity were tested: (I) synchronization by a direct response to light; (II) synchronization via the circadian system with multiple triggers, here called "cogs," each controlling a single ultradian feeding bout; and (III and IV) synchronization via the circadian system with a single "cog," which resets an ultradian oscillator and either (III) originates directly from the circadian pacemaker, or (IV) is mediated via the overt circadian activity rhythm. Short light-dark cycles failed to entrain ultradian rhythms, either in circadian-rhythmic or in non-circadian-rhythmic voles; light pulses did not cause phase shifts; and in extreme T cycles no stable phase relationship with light could be demonstrated. Thus, Hypothesis I was rejected. Changes in the circadian period (tau) were generated as aftereffects of light pulses, by entrainment in various T cycles, and by the addition of D2O to the drinking water. These changes in tau did not lead to parallel, let alone proportional, changes in the ultradian period. This excluded Hypothesis II. Both in T-cycle experiments and in the D2O experiments with circadian-rhythmic voles, the phase of ultradian feeding bouts was locked to the end of circadian activity rather than to the most prominent marker of the pacemaker, the onset of circadian activity. This was not expected under Hypothesis III, but was consistent with entrainment via activity (Hypothesis IV). On the basis of these experiments, we conclude that the most likely mechanism of ultradian entrainment is that of a light-insensitive ultradian oscillator, reset every dawn by the termination of the activity phase controlled by the circadian pacemaker, which is itself entrained by the light-dark cycle. Neither in circadian-rhythmic nor in non-circadian-rhythmic voles was the period of the feeding rhythm lengthened by administration of D2O. This insensitivity to deuterium is exceptional among biological rhythms.     

Conditioned stimulus control in the rat circadian system depends on clock resetting during conditioning.

The authors examined the ability of a conditioned stimulus (CS; mild air disturbance) previously paired with an entraining light pulse to reset the circadian pacemaker in rats. Rats were entrained to a single 30-min light stimulus delivered every 25 hr or 24 hr (T cycle). Each daily light presentation was paired with the CS. After at least 20 days of stable entrainment to each of the T cycles, the rats were allowed to free run and were then presented with the CS at circadian time 15. CS-induced phase shifts in wheel-running activity rhythms were taken as evidence for conditioning. For the most part, conditioning occurred after CS-light pairings on the 25-hr but not 24-hr T cycle. The results suggest that CS control of the circadian clock phase depends on the effect that the entraining light pulse has on the clock during conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of transplantation of embryonic anterior hypothalamic tissue from spontaneously hypertensive rats to normotensive Wistar rats on circadian rhythms of systolic arterial pressure and heart rate

The anterior hypothalamus (AH) participates in the regulation of arterial pressure. The suprachiasmatic nuclei (SCN) of the AH are a major circadian oscillator necessary for the generation and/or the entrainment of circadian rhythms. Circadian rhythms of systolic arterial pressure (SAP) and heart rate (HR) were investigated in spontaneously hypertensive rats (SHR) and in normotensive Wistar rats (NWI) with intact SCN, grafted with SHR embryonic AH tissue containing the SCN. Prominent circadian rhythms for SAP and HR in both NWI and SHR with acrophases during dark were found. The elevation of the MESOR (midline-estimated statistic of rhythm) of the SAP in normotensive rats grafted with AH embryonic tissue obtained from SHR was accompanied by disappearance of the circadian rhythm of SAP. This result suggests an interaction between the grafted tissue containing the SCN on the one hand, and the host SCN on the other hand. Our data ascribe a role for the SCN in the entrainment of the circadian rhythm of arterial pressure. The circadian rhythm of HR was not eliminated by the SCN graft in spite of the amplitude decrease and the phase delay observed. It seems that the entrainment of the circadian rhythm of HR is probably not crucially dependent on the SCN in rats. The circadian rhythms of SAP and HR in rats were differently affected by the grafts, thus suggesting a multioscillatory system for circadian regulation in rats.     

Serotonergic innervation of the hypothalamic suprachiasmatic nucleus and photic regulation of circadian rhythms

Converging lines of evidence have firmly established that the hypothalamic suprachiasmatic nucleus (SCN) is a light-entrainable circadian oscillator in mammals, critically important for the expression of behavioral and physiological circadian rhythms. Photic information essential for the daily phase resetting of the SCN circadian clock is conveyed directly to the SCN from retinal ganglion cells via the retinohypothalamic tract. The SCN also receives a dense serotonergic innervation arising from the mesencephalic raphe. The terminal fields of retinal and serotonergic afferents within the SCN are co-extensive, and serotonergic agonists can modify the response of the SCN circadian oscillator to light. However, the functional organization and subcellular localization of 5HT receptor subtypes in the SCN are just beginning to be clarified. This information is necessary to understand the role 5HT afferents play in modulating photic input to the SCN. In this paper, we review evidence suggesting that the serotonergic modulation of retinohypothalamic neurotransmission may be achieved via at least two different cellular mechanisms: 1) a postsynaptic mechanism mediated via 5HT1A or 5ht7 receptors located on SCN neurons; and 2) a presynaptic mechanism mediated via 5HT1B receptors located on retinal axon terminals in the SCN. Activation of either of these 5HT receptor mechanisms in the SCN by specific 5HT agonists inhibits the effects of light on circadian function. We hypothesize that 5HT modulation of photic input to the SCN may serve to set the gain of the SCN circadian system to light.     

Photic entrainment of the mammalian rhythm in melatonin production

This review summarizes studies on the photic entrainment of the circadian rhythm in the rat pineal melatonin production, namely of the rhythm in N-acetyltransferase (NAT) activity, and compares the NAT rhythm resetting with preliminary results on the resetting of an intrinsic rhythmicity in the suprachiasmatic nucleus (SCN) of the hypothalamus, namely with the entrainment of the rhythm in the light-induced c-fos gene expression. Phase delaying of the NAT rhythm after various light stimuli proceeds within 1 day with almost no transients, whereas during phase advancing of the rhythm only the morning NAT decline is phase advanced within 1 day and the evening rise phase shifts through transients. A light stimulus encompassing the middle of the night may phase delay the evening NAT rise, phase advance the morning decline, compress the rhythm waveform, and eventually lower its amplitude. Similarly, a long photoperiod compresses the NAT rhythm waveform. The magnitude of phase shifts of the NAT rhythm, as well as their direction, depends on a previous photoperiod. Phase shifts of the evening rise in c-fos gene photoinduction in the SCN and of the morning decline are similar to those of the pineal NAT rhythm after all light stimuli studied so far. The data indicate that the resetting of the rhythm in melatonin production in the rat pineal gland reflects changes in the SCN functional state and suggest that the underlying SCN pacemaking system is complex.     

Immortal time: circadian clock properties of rat suprachiasmatic cell lines

Cell lines derived from the rat suprachiasmatic nucleus (SCN) were screened for circadian clock properties distinctive of the SCN in situ. Immortalized SCN cells generated robust rhythms in uptake of the metabolic marker 2-deoxyglucose and in their content of neurotrophins. The phase relationship between these rhythms in vitro was identical to that exhibited by the SCN in vivo. Transplantation of SCN cell lines, but not mesencephalic or fibroblast lines, restored the circadian activity rhythm in arrhythmic, SCN-lesioned rats. Thus, distinctive oscillator, pacemaker, and clock properties of the SCN are not only retained but also maintained in an appropriate circadian phase relationship by immortalized SCN progenitors.     

Family coping during critical illness

The suprachiasmatic nuclei (SCN) of the anterior hypothalamus contain the master circadian pacemaker in mammals. On the occasion of the 25th anniversary of the discovery of the SCN as the circadian clock, Charles A. Czeisler and Steven M. Reppert organized a meeting to review milestones and recent developments in the study of the SCN. The discovery that the SCN contain tissue necessary for generation of circadian rhythmicity was established by lesion studies published in 1972. The second phase of study demonstrated unequivocally that the SCN contain an autonomous circadian pacemaker. The principal studies in this period showed the presence of metabolic and electrical activity rhythms in the SCN in vivo and progressed to studies showing that the SCN maintain rhythmicity in vitro, demonstrating that the transplanted SCN can restore circadian function following destruction of the host SCN and ultimately showing that single SCN "clock cells" exhibit independent rhythms in firing rate. The third phase of study, aimed at identifying the biochemical and molecular mechanisms responsible for rhythmicity within the SCN, has begun with the identification of circadian mutants (tau mutant hamsters and Clock mutant mice) and the isolation of the Clock gene. This report traces the important steps forward in our understanding of the suprachiasmatic circadian clock by recounting the information presented at the SCN Silver Anniversary Celebration.     

Nonphotic entrainment of circadian activity rhythms in suprachiasmatic nuclei-ablated hamsters.

Examined the role of the suprachiasmatic nuclei (SCN) in nonphobic entrainment. The wheel-running activity of SCN-ablated hamsters was recorded in constant dark (DD) and then under prolonged schedules of 2-hr daily cage changes, restricted food availability, and daily light–dark (LD) cycles. Ss with very large lesions subsuming the SCN and surrounding areas exhibited significant, albeit unstable, circadian activity rhythms in DD. Some Ss with similar ablations also showed entrained rhythms to daily cage change schedules. Ss showed robust rhythms entrained to a daily feeding schedule. No Ss showed entrainment to LD cycles. Competent circadian oscillators evidently exist outside the SCN, at least 0.5 mm or more away, and at least some are nonphotically entrainable. Weaker entrainment in animals with larger lesions suggests that nonphotically entrainable oscillators also exist within the SCN or its immediate vicinity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)