Increased sensitivity of the circadian system to temporal changes in the feeding regime of spontaneously hypertensive rats - a potential role for Bmal2 in the liver.

The mammalian timekeeping system generates circadian oscillations that rhythmically drive various functions in the body, including metabolic processes. In the liver, circadian clocks may respond both to actual feeding conditions and to the metabolic state. The temporal restriction of food availability to improper times of day (restricted feeding, RF) leads to the development of food anticipatory activity (FAA) and resets the hepatic clock accordingly. The aim of this study was to assess this response in a rat strain exhibiting complex pathophysiological symptoms involving spontaneous hypertension, an abnormal metabolic state and changes in the circadian system, i.e., in spontaneously hypertensive rats (SHR). The results revealed that SHR were more sensitive to RF compared with control rats, developing earlier and more pronounced FAA. Whereas in control rats, the RF only redistributed the activity profiles into two bouts (one corresponding to FAA and the other corresponding to the dark phase), in SHR the RF completely phase-advanced the locomotor activity according to the time of food presentation. The higher behavioral sensitivity to RF was correlated with larger phase advances of the hepatic clock in response to RF in SHR. Moreover, in contrast to the controls, RF did not suppress the amplitude of the hepatic clock oscillation in SHR. In the colon, no significant differences in response to RF between the two rat strains were detected. The results suggested the possible involvement of the Bmal2 gene in the higher sensitivity of the hepatic clock to RF in SHR because, in contrast to the Wistar rats, the rhythm of Bmal2 expression was advanced similarly to that of Bmal1 under RF. Altogether, the data demonstrate a higher behavioral and circadian responsiveness to RF in the rat strain with a cardiovascular and metabolic pathology and suggest a likely functional role for the Bmal2 gene within the circadian clock.

Early chronotype and tissue-specific alterations of circadian clock function in spontaneously hypertensive rats.

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 in situ 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 in vitro using real time recording of Bmal1-dLuc 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.

Photoperiodic modulation of the hepatic clock by the suprachiasmatic nucleus and feeding regime in mice.

Changes in photoperiod modulate the central circadian clock in the suprachiasmatic nucleus (SCN) as well as the peripheral clocks. Consequently, the SCN-driven output rhythms in activity and feeding are also modulated by the photoperiod. The aim of the present study was to elucidate whether photoperiodic modulation of the hepatic clock is mediated by changes in feeding or by another SCN-driven pathway. Five days after the change from short photoperiod (SP) to long photoperiod (LP), the profiles of Per2 and Rev-erbα expression in the rostral, middle and caudal regions of the SCN were desynchronized and those in the liver were modulated as in mice fully entrained to LP. The SCN profiles were not affected in mice left under SP and subjected to the 6-h night-time feeding regime for 5 days. In the liver, the profiles were shifted to the same phase, but their waveforms were not modulated compared with those under LP. In mice subjected to the change from SP to LP and fed twice daily during the daytime, the profiles in the SCN were not affected, whereas the waveforms and phases of those in the liver were affected. The data demonstrate that the adjustment of gene expression profiles in the rostral, middle and caudal SCN to the change from SP to LP proceeds within 5 days and is not affected by changes in the feeding regime. The results also suggest that the photoperiod-modulated SCN affects waveforms of gene expression profiles in the liver by food-independent signals.

Different mechanisms of adjustment to a change of the photoperiod in the suprachiasmatic and liver circadian clocks.

Changes in photoperiod modulate the circadian system, affecting the function of the central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The aim of the present study was to elucidate the dynamics of adjustment to a change of a long photoperiod with 18 h of light to a short photoperiod with 6 h of light of clock gene expression rhythms in the mouse SCN and in the peripheral clock in the liver, as well as of the locomotor activity rhythm. Three, five, and thirteen days after the photoperiod change, daily profiles of Per1, Per2, and Rev-erbalpha expression in the rostral, middle, and caudal parts of the SCN and of Per2 and Rev-erbalpha in the liver were determined by in situ hybridization and real-time RT-PCR, respectively. The clock gene expression rhythms in the different SCN regions, desynchronized under the long photoperiod, attained synchrony gradually following the transition from long to short days, mostly via advancing the expression decline. The photoperiodic modulation of the SCN was due not only to the degree of synchrony among the SCN regions but also to different waveforms of the rhythms in the individual SCN parts. The locomotor activity rhythm adjusted gradually to short days by advancing the activity onset, and the liver rhythms adjusted by advancing the Rev-erbalpha expression rise and Per2 decline. These data indicate different mechanisms of adjustment to a change of the photoperiod in the central SCN clock and the peripheral liver clock.