Circadian oscillators,cell cycles,and singularities: light perturbations of the free-running rhythm of cell division inEuglena

Summary The free-running circadian rhythm of cell division in the algal flagellate,Euglena gracilis (Z) was perturbed by 3-h light signals of varying intensities imposed at different circadian times (CT). Light pulses within the range of 700 to 7,500 lux were found to yield the same strong (Type 0) phase response curve (PRC) comprising both advance and delaye phase shifts as great as 15 h. Dark signals generated a PRC of reduced amplitude with very little, if any, phase advance being observed. Light perturbations of lower intensity, however, elicited quite different responses if applied at a quite specific circadian time: A 40- to 400-lux pulse given at approximately CT 0 (late subjective night) induced total arrhythmicity, and the culture reverted to asynchronous, exponential growth. Different degrees of arryhtmicity were induced by the same low-intensity perturbations (I ^*) given slightly before or after this sensitive phase point (T ^*), but if imposed at other circadian times, they generated normal type 0 phase resetting. The demonstration of the existence of this critical pulse (T ^*,I ^*) provides further evidence that the cell division cycle ofEuglena (and presumably other microorganisms) is regulated by a circadian oscillator and, in particular, by one having limit cycle dynamics.Abbreviations LL continuous illumination - DD continuous darkness - LD light-dark cycle - LD x, y, light-dark cycle comprisingx h of light andy h of dark - t period of a LD cycle - CO circadian oscillator - CR circadian rhythm - period of a freerunning circadian rhythm in constant conditions (taken here to be the time between onsets of cell division in a population of cells - _R phase marker, or phase reference point (here, the onset of the division burst) - phase of the rhythm - change in phase (phase shift) - new phase attained after phase shift - CT circadian time (CT 0 indicates the phase point of a free-running rhythm that has been normalized to 24 h which corresponds to that occurring at the onset of light in aLD:12, 12 reference cycle) - PRC phase response curve (plot of phase shift engendered by a perturbation as a function of the circadian time of its application) - T ^*,I ^*) coordinates of an annihilating (light) stimulus given at a critical circadian time (T ^*, corresponding to the singularity point) and having a critical strength (I ^*) - CDC cell division cycle - average generation (doubling) time of a cell population - average step-size, or factorial increase in cell titer (plateau to plateau) after a phased division burst Dedicated to Prof. Colin S. Pittendrigh on only his 65th birthday     

Circadian rhythm of covalent modifications in liver DNA.

32P-postlabeling analysis recently revealed that in addition to 5-methylcytosine, mammalian DNA contains covalently modified nucleotides of unknown structures and functions termed I-compounds whose levels increase with age. I-compound levels, in addition, depend on species, strain, sex, tissue, and diet and are generally lowered by carcinogen exposure. As shown here, levels of several non-polar I-compounds in liver DNA of untreated male C3H mice were elevated 2 to 8.5 times at 1800 h and 2400 h as compared to 0600 h and 1200 h, while polar I-compounds and persistent carcinogen-DNA adducts induced by safrole were unaffected by time of day. In liver DNA of male F-344 rats 4 non-polar I-compounds and 4 polar I-compounds showed significant circadian rhythm at 2000 h compared to 0800 h. This novel circadian variation of DNA structure implies mechanisms precisely regulating I-compound levels in vivo and may conceivably be linked to diurnal differences of DNA synthesis and gene expression.     

Hepatic cell division and tissue repair: a key to survival after liver injury

The survival of patients suffering from severe liver damage depends heavily on the ability of the remaining hepatocytes to regenerate and replace the dead or dying cells; death usually occurs when the regenerating ability of the liver is compromised owing to heavy damage to the liver. The current approach to therapy aims only to block additional liver injury from hepatotoxicants or hepatic disease. It hepatocellular regeneration and tissue repair could be stimulated after hepatic damage by a therapeutically compatible mechanism, then it might be possible to prevent death arising from serious liver injury.     

Plasma cortisol in cattle: Circadian rhythm and exposure to a simulated high altitude of 5,000 m

After a 2 week control period at 400 m, cattle were exposed to 5,000 m simulated altitude for 2 weeks, which was followed by a 2-week post-altitude control period. Plasma cortisol values from blood samples taken every 30 min for a total of 24 h indicated that cortisol was secreted episodically and that a circadian rhythm existed. When cortisol values were grouped into 4, 6-h periods, plasma cortisol was most abundant from 06:00 to 12:00 h with an average of 0.96µ g/100 ml and least abundant from 00:30 to 06:00 h with an average of 0.55µ g/100 ml. Plasma cortisol increased from 0.42 to 3.08µ g/100 ml during the 4 h ascent to 5,000 m and decreased to near normal levels the following day. A rhythmic plasma cortisol pattern was maintained after one day at simulated high altitude.     

Role of adhesive lymphoid cell subpopulation in areactivity to hepatoma 22a]

The role of adhesive fraction of T lymphocytes in nonreactivity of mice to hepatoma 22a was studied. It was shown that the removal of the adhesive fraction from the spleen suspension enriched with T lymphocytes promotes intensification of cell immunity in tumor-tolerant mice.     

Circadian rhythm in Alzheimer disease after trazodone use

A circadian rhythm is a cycle of approximately 24?h, responsible for many physiological adjustments, and ageing of the circadian clock contributes to cognitive decline. Rhythmicity is severely impaired in Alzheimer disease (AD) and few therapeutic attempts succeeded in improving sleep disorders in such context. This study evaluated sleep parameters by actigraphy in 30 AD patients before and after trazodone use for 2 weeks, and we show a significant improvement in relative rhythm amplitude (RRA), compatible with a more stable daytime behavioral pattern. So, trazodone appears to produce a stabilization of the circadian rhythms in individuals with AD.     

Circadian rhythm of active amino acid transport in rat liver

Abstract

The circadian change of the encephalic photosensitivity of quail has been demonstrated. In this study the diurnal variation of the retinal photosensitivity was investigated by the electroretinogram (ERG) to explore the phase relation between the retinal and encephalic systems. The b‐wave, a major component of the ERG, was used as a measure of retinal photosensitivity.

1. In the bird maintained in LD12:12 the b‐wave amplitude of the ERG stayed at a high level for the first 10 h of the light period, and decreased abruptly around the time of light‐off. The decreased level continued until midnight. Thenceforth the b‐wave amplitude recovered progressively to the daytime level before the time of next light on.

2. In the bird maintained in LD16:8, the decrease of the retinal sensitivity in the light phase was initiated prior to the time of expected light‐off, and the onset time of decreasing tendency was advanced by 2 h, with dark adaptation for 30 min given immediately before the ERG measurement. Upon continuous light exposure, the b‐wave amplitude always remained at low level.

3. Periodic changes in retinal sensitivity persisted when the environmental dark phase was temporarily extended for 19 to 30 h.

These observations suggest that the diurnal rhythm of the neural retina in quail might be generated endogenously and appears to control the encephalic photosensitive system.     

Circadian variations in the committed stem cell responsiveness to granulopoiesis inhibitory factor (chalone) in vitro

The effect of inhibitory extracts of human granulocytes (chalone) on the circadian rhythm of myelopoietic colony formation in soft agar (CFUc) was studied over one year. Normally, the number of bone marrow CFUc shows a strong circadian rhythm. Preincubation of the cells with the extracts before plating modulated this rhythm with phase shifts and modifications of amplitude. The effects of the extracts oscillated between significant inhibition and stimulation over a 24 h period. No significant seasonal variations in the inhibitory effects were found.     

不同品种甘蔗茎尖细胞分裂节律研究

对 6个不同甘蔗品种 (粤糖 91 /976、粤糖 86/3 68、桂糖 1 1号、新台糖 1 6、CP80、农林 8号 )茎尖生长点细胞分裂进行切片观察研究 ,结果表明 :甘蔗一天内 (白天 )不同时间都存在细胞分裂 ,而且细胞分裂指数呈规律性变化 ,大部分呈先升后降再上升的变化趋势。除了桂糖 1 1号在 8∶0 0出现细胞分裂高峰之外 ,其它五个品种均在 1 0∶0 0出现细胞分裂高峰期 ,1 2∶0 0～ 1 4∶0 0是细胞分裂的低谷。种在不同生长时期细胞分裂指数变化规律不一样 ,细胞分裂高峰期出现的时期也不一样 ,早熟品种出现的时期早一些 ,晚熟品种则晚一些。甘蔗茎径和各生长时期细胞分裂指数是呈正相关的 ,茎径大 ,细胞分裂指数高 ,相反 ,茎径小 ,细胞分裂指数低。