Photoperiodic entraiment of circadian clock in suprachiasmatic nucleus

Most of physiological processes run in the organisms persistently, they begin in a definite rhythm again and again. The greatest attention is paid to the rhythms, whose period is equal to one day - they are called circadian rhythms. In case of mammals, these circadian rhythms are under control of the central circadian clock that resides in the suprachiasmatic nucleus, a part of the anterior hypothalamus. The mechanism of rhythm generation is based on interacting transcriptional-translational feedback loops that control expression of the clock genes in every single cell. Clock-controlled genes transmit these rhythms into the whole organism where they drive many physiological processes. Clock genes are expressed also in the peripheral oscillators (for example in liver, lungs, heart) and are under direct control of the central oscillator. Circadian clock needs to be entrained everyday to the external time to function precisely. The main entraining cue is the light part of the day. The length of the light part of the day, i.e. photoperiod, changes during the year rapidly in our latitudes and the central oscillator has to adapt to the changes all the time. The length of the photoperiod is encoded directly in the central oscillator by the transcriptional-translational relations among the clock genes and..

Photoperiodic entraiment of circadian clock in suprachiasmatic nucleus

Most of physiological processes run in the organisms persistently, they begin in a definite rhythm again and again. The greatest attention is paid to the rhythms, whose period is equal to one day - they are called circadian rhythms. In case of mammals, these circadian rhythms are under control of the central circadian clock that resides in the suprachiasmatic nucleus, a part of the anterior hypothalamus. The mechanism of rhythm generation is based on interacting transcriptional-translational feedback loops that control expression of the clock genes in every single cell. Clock-controlled genes transmit these rhythms into the whole organism where they drive many physiological processes. Clock genes are expressed also in the peripheral oscillators (for example in liver, lungs, heart) and are under direct control of the central oscillator. Circadian clock needs to be entrained everyday to the external time to function precisely. The main entraining cue is the light part of the day. The length of the light part of the day, i.e. photoperiod, changes during the year rapidly in our latitudes and the central oscillator has to adapt to the changes all the time. The length of the photoperiod is encoded directly in the central oscillator by the transcriptional-translational relations among the clock genes and..

Photoperiodic modulation of the central circadian clock in the suprachismatic nucleus and in the peripheral clock in the liver

Most physiological processes in mammals follow daily oscillations. These circadian rhythms are driven by central oscillator located in the suprachiasmatic nucleus (SCN) of hypothalamus. The SCN coordinates rhythmical activity of the subsidiary peripheral oscillators distributed in many different tissues. In gastrointestinal system, the peripheral clocks and metabolism are closely linked. The mechanism of circadian oscillations is based on transcriptional-translational feedback loops, which drive rhythmic expression of the clock genes. The entrainment with external conditions is essential for proper function of the circadian clock. While the SCN is driven mainly by the light-dark cycle, synchronization of the peripheral clocks depend on many factors, such as feeding and fasting. The length of the light part of the day, i.e. photoperiod, changes throughout the year rapidly and circadian system has to adapt to the changes all the time. However, a mechanism of adjustment to the change in the photoperiod has not been fully understood. The aim of this work was to elucidate the effect of change in the photoperiod on the central SCN clock and on the peripheral clock in the liver. Firstly, we focused on dynamics of adjustment of these clocks to the change from a long photoperiod, with 18 hours of light, to..

Cardian clock in the suprachiasmatic nucleus of the Hypothalamus and the mechanism of its synchronization with external environment

Department of Genetics and MicrobiologyKatedra genetiky a mikrobiologieFaculty of SciencePřírodovědecká fakult

Early Chronotype and Tissue-Specific Alterations of Circadian Clock Function in Spontaneously Hypertensive Rats

<div><p>Malfunction of the circadian timing system may result in cardiovascular and metabolic diseases, and conversely, these diseases can impair the circadian system. The aim of this study was to reveal whether the functional state of the circadian system of spontaneously hypertensive rats (SHR) differs from that of control Wistar rat. This study is the first to analyze the function of the circadian system of SHR in its complexity, i.e., of the central clock in the suprachiasmatic nuclei (SCN) as well as of the peripheral clocks. The functional properties of the SCN clock were estimated by behavioral output rhythm in locomotor activity and daily profiles of clock gene expression in the SCN determined by <em>in situ</em> hybridization. The function of the peripheral clocks was assessed by daily profiles of clock gene expression in the liver and colon by RT-PCR and <em>in vitro</em> using real time recording of <em>Bmal1-dLuc</em> reporter. The potential impact of the SHR phenotype on circadian control of the metabolic pathways was estimated by daily profiles of metabolism-relevant gene expression in the liver and colon. The results revealed that SHR exhibited an early chronotype, because the central SCN clock was phase advanced relative to light/dark cycle and the SCN driven output rhythm ran faster compared to Wistar rats. Moreover, the output rhythm was dampened. The SHR peripheral clock reacted to the dampened SCN output with tissue-specific consequences. In the colon of SHR the clock function was severely altered, whereas the differences are only marginal in the liver. These changes may likely result in a mutual desynchrony of circadian oscillators within the circadian system of SHR, thereby potentially contributing to metabolic pathology of the strain. The SHR may thus serve as a valuable model of human circadian disorders originating in poor synchrony of the circadian system with external light/dark regime.</p> </div

Clock- and metabolism-related gene expression profiles in the liver (left) and colon (right) of Wistar rats (full line) and SHR (dashed line).

<p>The rats were maintained and sampled as described in Fig. 2. Daily profiles of relative expression of <i>Dbp</i>, <i>Wee1</i>, <i>E4bp4</i>, <i>Nampt</i>, <i>Ppara</i>, <i>Pparg</i>, <i>Pgc1α</i>, <i>Hdac3</i>, <i>Hif1a</i> and <i>Ppp1r3c</i> mRNA levels were determined by RT-PCR. Time is expressed as circadian time (h) with CT0 corresponding to lights-on on the previous LD cycle (for details, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046951#s2" target="_blank">Methods</a>). Data were fitted by cosine curves and each point represents the mean ± S.E.M. of 3 to 5 animals.</p

Comparison of acrophases of the circadian rhythms in clock gene expression in the SCN, liver (LIV) and colon (COL).

<p>The acrophases in the Wistar rats (full circle) and SHR (open circle) were determined on the basis of cosine fits of <i>Per1</i>, <i>Per2</i>, <i>Rev-erbα</i> and <i>Bmal1</i> expression profiles shown in Fig. 2 and 3. Error bars represent S.D.</p

Circadian rhythms of fibroblasts from Wistar rats and SHR.

<p>A) Recordings of bioluminescence of spontaneously immortalized fibroblasts from Wistar rats (dotted lines) and SHR (full lines) transfected with Bmal1-dLuc circadian reporter. The fibroblasts were synchronized with simple medium exchange (DMEM), 10 µM Forskolin (FOR) or 50% horse serum (HS). B) Average periods (± S.D.) of the circadian rhythm of spontaneously immortalized fibroblasts from Wistar rats (full column) and SHR (open column) synchronized as described in A. No significant differences between the periods of circadian rhythms in both rat strains were detected.</p