Interplay of neuronal networks modulates mammalian circadian rhythms

  The suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-hour rhythms in physiology and behavior. The SCN neurons function as cell-autonomous oscillators, and the production of a coherent SCN rhythm is dependent upon synchronization among single cells. We investigated how changes in phase-synchronization between individual cells effect the ability of the SCN to phase-shift its rhythm. Empirical and modelling studies revealed larger phase-shifts in synchronized SCN than in desynchronized SCN. The major external stimulus affecting the SCN is light. We explored the ability of melanopsin and rod- and cone photoreceptors to mediate the effects of light on SCN discharge, and found that melanopsin and cones are able to mediate light responses of the SCN. Studies performed in nocturnal species have indicated that the SCN’s rhythmicity is also influenced by the animal’s own behavioral activity. We assessed the effect behavioral activity on the amplitude of the circadian rhythm in SCN electrical discharge rate in the day-active Arvicanthis ansorgei. The results showed acute enhancements of SCN discharge during episodes of behavioral activity. The studies described in this thesis indicate that the SCN is part of a brain network that includes the retina and areas involved in behavioral activity and sleep.  LUMC / Geneeskund

Distinct contribution of cone photoreceptor subtypes to the mammalian biological clock

Ambient light detection is important for the synchronization of the circadian clock to the external solar cycle. Light signals are sent to the suprachiasmatic nuclei (SCN), the site of the major circadian pacemaker. It has been assumed that cone photoreceptors con-tribute minimally to synchronization. Here, however, we find that cone photoreceptors are sufficient for mediating entrainment and transmitting photic information to the SCN, as evaluated in mice that have only cones as functional photoreceptors. Using in vivo electrophysiological recordings in the SCN of freely moving cone-only mice, we observed light responses in SCN neuronal activity in response to 60-s pulses of both ultraviolet (UV) (lambda(max) 365 nm) and green (lambda(max) 505 nm) light. Higher irradiances of UV light led to irradiance-dependent enhancements in SCN neuronal activity, whereas higher irradiances of green light led to a reduction in the sustained response with only the transient response remain-ing. Responses in SCN neuronal activity decayed with a half-max time of similar to 9 min for UV light and less than a minute for green light, indicating differential input between short-wavelength-sensitive and mid-wavelength-sensitive cones for the SCN responsiveness. Furthermore, we show that UV light is more effective for photo-entrainment than green light. Based on the lack of a full sustained response in cone-only mice, we confirmed that rapidly alternating light levels, rather than slowly alternating light, caused substantial phase shifts. Together, our data provide strong evidence that cone types contribute to photoentrainment and differentially affect the electrical activity levels of the SCN.Circadian clocks in health and diseas

Human Embryonic and Fetal Mesenchymal Stem Cells Differentiate toward Three Different Cardiac Lineages in Contrast to Their Adult Counterparts

Mesenchymal stem cells (MSCs) show unexplained differences in differentiation potential. In this study, differentiation of human (h) MSCs derived from embryonic, fetal and adult sources toward cardiomyocytes, endothelial and smooth muscle cells was investigated. Labeled hMSCs derived from embryonic stem cells (hESC-MSCs), fetal umbilical cord, bone marrow, amniotic membrane and adult bone marrow and adipose tissue were co-cultured with neonatal rat cardiomyocytes (nrCMCs) or cardiac fibroblasts (nrCFBs) for 10 days, and also cultured under angiogenic conditions. Cardiomyogenesis was assessed by human-specific immunocytological analysis, whole-cell current-clamp recordings, human-specific qRT-PCR and optical mapping. After co-culture with nrCMCs, significantly more hESC-MSCs than fetal hMSCs stained positive for α-actinin, whereas adult hMSCs stained negative. Furthermore, functional cardiomyogenic differentiation, based on action potential recordings, was shown to occur, but not in adult hMSCs. Of all sources, hESC-MSCs expressed most cardiac-specific genes. hESC-MSCs and fetal hMSCs contained significantly higher basal levels of connexin43 than adult hMSCs and co-culture with nrCMCs increased expression. After co-culture with nrCFBs, hESC-MSCs and fetal hMSCs did not express α-actinin and connexin43 expression was decreased. Conduction velocity (CV) in co-cultures of nrCMCs and hESC-MSCs was significantly higher than in co-cultures with fetal or adult hMSCs. In angiogenesis bioassays, only hESC-MSCs and fetal hMSCs were able to form capillary-like structures, which stained for smooth muscle and endothelial cell markers.Human embryonic and fetal MSCs differentiate toward three different cardiac lineages, in contrast to adult MSCs. Cardiomyogenesis is determined by stimuli from the cellular microenvironment, where connexin43 may play an important role

Interplay of neuronal networks modulates mammalian circadian rhythms

  The suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-hour rhythms in physiology and behavior. The SCN neurons function as cell-autonomous oscillators, and the production of a coherent SCN rhythm is dependent upon synchronization among single cells. We investigated how changes in phase-synchronization between individual cells effect the ability of the SCN to phase-shift its rhythm. Empirical and modelling studies revealed larger phase-shifts in synchronized SCN than in desynchronized SCN. The major external stimulus affecting the SCN is light. We explored the ability of melanopsin and rod- and cone photoreceptors to mediate the effects of light on SCN discharge, and found that melanopsin and cones are able to mediate light responses of the SCN. Studies performed in nocturnal species have indicated that the SCN’s rhythmicity is also influenced by the animal’s own behavioral activity. We assessed the effect behavioral activity on the amplitude of the circadian rhythm in SCN electrical discharge rate in the day-active Arvicanthis ansorgei. The results showed acute enhancements of SCN discharge during episodes of behavioral activity. The studies described in this thesis indicate that the SCN is part of a brain network that includes the retina and areas involved in behavioral activity and sleep.  </div

Synchronization of biological clock neurons by light and peripheral feedback systems promotes circadian rhythms and health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-hour rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-hour rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders

The suprachiasmatic nuclei as a seasonal clock

Circadian clocks in health and diseas

The Proportion of Light-Responsive Neurons Determines the Limit Cycle Properties of the Suprachiasmatic Nucleus

Circadian clocks in health and diseas

Irradiance encoding in the suprachiasmatic nuclei by rod and cone photoreceptors.

Light information is transmitted to the central clock of the suprachiasmatic nuclei (SCN) for daily synchronization to the external solar cycle. Essential for synchronization is the capacity of SCN neurons to respond in a sustained and irradiance-dependent manner to light. Melanopsin has been considered to mediate this photosensory task of irradiance detection. By contrast, the contribution of the classical photoreceptors in irradiance encoding is less clear. Here we investigate the role of classical photoreceptors by in vivo electrophysiological responses in freely moving animals to specific wavelengths of light (UV, λmax 365 nm; blue, λmax 467 nm; and green, λmax 505 nm) in both melanopsin-deficient (Opn4(-/-)) mice and mice lacking rods and cones (rd/rd cl). Short- and long-wavelength light induced sustained irradiance-dependent responses in congenic wild-type mice (+19.6%). Unexpectedly, sustained responses to light persisted in Opn4(-/-) mice (+18.4%). These results provide unambiguous evidence that classical photoreceptors can transmit irradiance information to the SCN. In addition, at light intensities that would stimulate rod and cone photoreceptors, the SCN of rd/rd cl mice showed greatly reduced sustained responses to light (+7.8%). Collectively, our data demonstrate a role for classical photoreceptors in illuminance detection by the SCN