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.     

Olfactory bulbectomy impedes social but not photic reentrainment of circadian rhythms in female Octodon degus

Recent studies demonstrated that nonphotic (social) cues markedly accelerate reentrainment to large phase shifts of the light-dark (LD) cycles in female Octodon degus and that such changes are likely effected by chemosensory stimuli. This experiment investigated the effects of olfactory bulbectomies on (1) socially facilitated reentrainment rates of circadian rhythms following a 6-h phase advance of the LD cycle, (2) photic reentrainment rates of circadian rhythms following a 6-h advance of the LD cycle, (3) photic entrainment, and (4) the circadian period (tau) of activity rhythms in constant darkness (DD). olfactory bulbectomies (BX) blocked socially facilitated reentrainment rates but did not alter reentrainment rates of circadian rhythms to photic cues alone. In addition, BX lowered mean daily locomotor activity levels and decreased the amplitude of the activity rhythm in degus housed in entrained (LD 12:12) conditions but did not alter the phase of activity onset or offset, duration (alpha) of activity, or mean daily core body temperature. Bulbectomies also failed to modify tau of free-running activity rhythms. This experiment confirms that the olfactory bulbs and chemosensory cues are necessary for socially facilitated reentrainment. In contrast to their effects in nocturnal rodents, BX do not produce significant circadian photic changes in diurnal degus. This is the first experiment to determine that chemosensory stimuli modulate the circadian system in a diurnal rodent.     

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)     

Glutamatergic antagonists do not attenuate light-induced fos protein in rat intergeniculate leaflet

Photic information that entrains circadian rhythms is transmitted to the suprachiasmatic nucleus (SCN) from the retina and from the retinorecipient intergeniculate leaflet (IGL). Expression of light-induced Fos protein in SCN neurons is correlated with the effectiveness of such light to induce phase shifts, and is prevented by pretreatment with glutamate receptor antagonists that prevent phase shifts as well. In the present study we demonstrate that treatments with N-methyl-d-aspartate (NMDA) and non-NMDA receptor antagonists prior to light pulses during the subjective night have no effect on light-induced Fos immunoreactivity (Fos-IR) in IGL neurons despite attenuating Fos-IR in the SCN. Transmission of photic information along retinogeniculate and retinohypothalamic pathways appears to be mediated by different mechanisms.     

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.     

Circadian patterns of locomotor activity and body temperature in blind mole-rats, Spalax ehrenbergi

A wide variety of organisms exhibit circadian rhythms, regulated by internal clocks that are entrained primarily by the alternating cycle of light and darkness. There have been few studies of circadian rhythms in fossorial species that inhabit a microenvironment where day-night variations in most environmental parameters are minimized and where exposure to light occurs only infrequently. In this study, daily patterns of locomotor activity and body temperature (Tb) were examined in adult blind mole-rats (Spalax ehrenbergi). These fossorial rodents lack external eyes but possess rudimentary ocular structures that are embedded in the Harderian glands and covered by skin and fur. Most individual mole-rats exhibited circadian rhythms of locomotor activity, but some animals were arrhythmic. Individuals that did exhibit robust rhythms of locomotor activity also showed rhythms of Tb. In most cases, Tb was highest during the phase of intense locomotor activity. Locomotor activity rhythms could be entrained to light:dark cycles, and several mole-rats exhibited entrainment to non-24-h light cycles (T-cycles) with period lengths ranging from T = 23 h to T = 25 h. Some individuals also showed entrainment to daily cycles of ambient temperature. There was considerable interindividual variation in the daily patterns of locomotor activity among mole-rats in virtually all the conditions of environmental lighting and temperature employed in this study. Thus, whereas it appears likely that photic cues have a significant role in the entrainment of circadian rhythms in mole-rats, the amount of variability in rhythm patterns among individuals appears to be much greater than for most species that have been studied.     

Compliance in an anti-hypertension trial: a latent process model for binary longitudinal data

Animal experimental studies have shown that the retinohypothalamic tract (RHT) is an anatomical and functionally distinct retinofugal pathway mediating photic entrainment of circadian rhythms. In the present study, RHT projections were studied in the human brain by our recently developed postmortem tracing technique with neurobiotin as a tracer. Similar patterns of labeling were observed in brains of one control subject without neurological or mental disorders and five patients with Alzheimer's disease. The topography of RHT projections has several characteristics. (1) RHT fibers leave the optic chiasm and enter the hypothalamus medially and laterally at the anterior level of the suprachiasmatic nucleus (SCN). (2) The medial fibers enter the ventral part of the SCN and innervate the ventral SCN over its entire length, but the density decreases gradually from anterior to posterior. Labeled RHT fibers in the SCN make contact mainly with immunocytochemically positive neurotensin or vasoactive intestinal polypeptide neurons and only occasionally with vasopressin-positive neurons located in the ventral part of the SCN. (3) Only few projections to the dorsal part of the SCN and the anteroventral part of the hypothalamus were found. (4) Lateral projections reach the ventral part of the ventromedial SON and the area lateral to the SCN. No projections were observed to other hypothalamic areas. The presence of an RHT in humans suggests that the RHT may serve a function in humans similar to that demonstrated in animals.     

An improved sample packing device for rapid freeze-trap electron paramagnetic resonance spectroscopy kinetic measurements

Circadian rhythms are generated by the suprachiasmatic nuclei (SCN) and synchronized (entrained) to environmental light-dark cycles by the retinohypothalamic tract (RHT), a direct pathway from the retina to the suprachiasmatic nuclei. In anophthalmic mice, the optic primordia are resorbed between embryonic days 11.5 and 13, before retinal ganglion cells emerge. Thus the retinohypothalamic tract, which is the primary "zeitgeber" for circadian rhythms in sighted animals, never forms, and there is no retinal or photic input to the circadian system. We have used wheel running activity, a highly consistent and reliable measure of circadian rhythmicity in rodents, to establish the properties of endogenous locomotor rhythms of anophthalmic mice. We have identified three subpopulations of anophthalmic mice: a) rhythmic with strong stable circadian period but significantly increased period length; b) rhythmic with unstable circadian period; and c) arrhythmic. Future correlation of locomotor rhythms with properties of the suprachiasmatic nuclei in these mice will clarify the relationship between generation and properties of circadian rhythms and the neuroanatomical, neurochemical, and molecular organization of the circadian clock.     

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.     

Circadian rhythms in food intake and activity in domestic cats.

Examined the daily patterns of food intake and activity in normal and pontile-lesioned adult cats. Food intake in the dark and light phases of the light–dark (LD) cycles was determined separately by weighing the food, and a percentage nocturnal score was calculated. The measure of activity was infrared photobeam interruptions, with the photobeam placed in front of the cages, over the food bowl. No differences between normal and pontile-lesioned Ss were detected for any of the measures. Ss' food intake was influenced by simulated starlight and moonlight conditions and by the presence of humans. Ss in isolation from humans and human noises exhibited random patterns of activity in constant light and free-running circadian rhythms in constant dark. Idiosyncratic differences in entrainment to LD cycles were found among the Ss. The relevance of this variability is noted for studies of photoperiodic phenomena in this species. (44 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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.     

Pituitary adenylate cyclase-activating polypeptide and melatonin in the suprachiasmatic nucleus: effects on the calcium signal transduction cascade

The suprachiasmatic nucleus (SCN) harbors an endogenous oscillator generating circadian rhythms that are synchronized to the external light/dark cycle by photic information transmitted via the retinohypothalamic tract (RHT). The RHT has recently been shown to contain pituitary adenylate cyclase-activating polypeptide (PACAP) as neurotransmitter/neuromodulator. PACAPergic effects on cAMP-mediated signaling events in the SCN are restricted to distinct time windows and sensitive to melatonin. In neurons isolated from the SCN of neonatal rats we investigated by means of the fura-2 technique whether PACAP and melatonin also influence the intracellular calcium concentration ([Ca2+]i). PACAP elicited increases of [Ca2+]i in 27% of the analyzed neurons, many of which were also responsive to the RHT neurotransmitters glutamate and/or substance P. PACAP-induced changes of [Ca2+]i were independent of cAMP, because they were not mimicked by forskolin or 8-bromo-cAMP. PACAP caused G-protein- and phospholipase C-mediated calcium release from inositol-trisphosphate-sensitive stores and subsequent protein kinase C-mediated calcium influx, demonstrated by treatment with GDP-beta-S, neomycin, U-73122, calcium-free saline, thapsigargin, bisindolylmaleimide, and chelerythrine. The calcium influx was insensitive to antagonists of voltage-gated calcium channels of the L-, N-, P-, Q- and T-type (diltiazem, nifedipine, verapamil, omega-conotoxin, omega-agatoxin, amiloride). Immunocytochemical characterization of the analyzed cells revealed that >50% of the PACAP-sensitive neurons were GABA-immunopositive. Our data demonstrate that in the SCN PACAP affects the [Ca2+]i, suggesting that different signaling pathways (calcium as well as cAMP) are involved in PACAPergic neurotransmission or neuromodulation. Melatonin did not interfere with calcium signaling, indicating that in SCN neurons the hormone primarily affects the cAMP signaling pathway.     

The suprachiasmatic nucleus: cellular approach to clock functioning

The hypothalamic suprachiasmatic nucleus (SCN), the circadian clock in mammals, generates and maintains a variety of daily rhythms. The present review is an attempt to synthesis experimental data on the anatomical organisation and cellular activities within SCN. The clock exhibits an endogenous rhythmic activity and can also be entrained by environmental synchronisers such as the light/dark cycle. It can be also influenced by internal signals such as the rhythmic secretion of melatonin which is under control of SCN activity. This tiny structure contains a variety of peptides organised in a specific distribution. It receives three main inputs from the retina (glutamate), the intergeniculate leaflet (NPY) and the dorsal raphe (serotonin). VIP containing cells located in the ventral part of SCN receive all these afferences and innervate the whole structure. VIP, PHI and GRP are likely implicated in the entrainment of the clock. The vasopressin (VP) cells exhibiting an endogenous rhythmic synthesis are considered as an output of the clock. The specific induction of immediate early genes (c-fos, jun B) within SCN by light pulses during the subjective night suggests the participation of these genes in the process of cellular entrainment by the photic input. The demonstration of a rhythmic astrocytic activity within SCN suggests an active involvement of this cellular population in the functioning of the clock facilitating or not neuronal communication. Cellular disturbances such as a decrease in VIP or VP cell population, reduction in the amplitude of functional cellular rhythms, astrocytic proliferation could explain some pathologies observed with ageing.     

Photic responses of suprachiasmatic area neurons in diurnal degus (Octodon degus) and nocturnal rats (Rattus norvegicus)

Photic sensitivity of cells in the suprachiasmatic nuclei (SCN), the principal pacemaker of the mammalian circadian system, has been documented in several species. In nocturnal rodents, the majority of photically responsive SCN cells are activated by retinal illumination. One report identified mostly photic suppressions among SCN cells in a diurnal rodent, studied under somewhat different conditions. We examined photic sensitivity of SCN cells in a predominantly diurnal rodent, the degu, studied in vivo under identical conditions to rats, and found that a large majority of photic SCN cells were suppressed by light. In both rats and degus, SCN cells were more responsive to light during the subjective night than during the subjective day. Light-responsive cells did not show a daily rhythm in baseline firing rates in either species, but rat SCN cells that did not respond to light were more active spontaneously during the subjective day. Light-unresponsive SCN cells in degus did not show a similar pattern. There are substantial differences in the neurophysiological activity and photic responsiveness of SCN cells in diurnal degus and nocturnal rats.     

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.     

The suprachiasmatic nucleus and intergeniculate leaflet of Arvicanthis niloticus, a diurnal murid rodent from East Africa

Little is known about the neural substrates controlling circadian rhythms in day-active compared to night-active mammals primarily because of the lack of a suitable diurnal rodent with which to address the issue. The murid rodent, Arvicanthis niloticus, was recently shown to exhibit a predominantly diurnal pattern of activity and body temperature, and may be suitable for research on the neural mechanisms underlying circadian rhythms. This paper describes, in A. niloticus, the anatomy of two neural structures that play important roles in the control of circadian rhythms, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL). Immunohistochemical techniques were used to examine the distribution of neuroactive peptides in the SCN and IGL, and retinal projections to these structures were traced with anterograde transport of the beta subunit of cholera toxin. In A. niloticus, distinct subdivisions of the SCN contained cell bodies with immunoreactive (IR) vasopressin, vasoactive intestinal polypeptide, gastrin-releasing peptide, and corticotropin-releasing factor. The SCN did not contain cell bodies with met-enkephalin-IR and substance P-IR, but did contain fibers with substance P-IR and neuropeptide Y-IR. Retinal fibers were present throughout the SCN, but were most densely concentrated along its ventral edge, particularly in the contralateral SCN. Retinal fibers also extended to a variety of hypothalamic regions outside the SCN, including the supraoptic nucleus and the subparaventricular region. The IGL contained cells with neuropeptide Y-IR and enkephalin-IR cells. Retinal fibers projected to both the ipsilateral and contralateral IGL. The anatomy of the SCN and IGL were compared and contrasted with that previously described for other nocturnal and diurnal species.     

A differential response of two putative mammalian circadian regulators, mper1 and mper2, to light

A mouse gene, mper1, having all the properties expected of a circadian clock gene, was reported recently. This gene is expressed in a circadian pattern in the suprachiasmatic nucleus (SCN). mper1 maintains this pattern of circadian expression in constant darkness and can be entrained to a new light/dark cycle. Here we report the isolation of a second mammalian gene, mper2, which also has these properties and greater homology to Drosophila period. Expression of mper1 and mper2 is overlapping but asynchronous by 4 hr. mper1, unlike period and mper2, is expressed rapidly after exposure to light at CT22. It appears that mper1 is the pacemaker component which responds to light and thus mediates photic entrainment.