Circadian rhythms in mouse blood coagulation

The circadian clock, influencing many biological processes, has been demonstrated to modulate levels of specific coagulation factors, but its impact on the coagulation efficiency is unknown. In a mouse model, the authors evaluated the temporal variations in the initial rate of activated factor X (FXa) and thrombin generation. Upon coagulation activation through the FVIIa-TF pathway (extrinsic activation), both parameters showed rhythmic variations with a significant peak at ZT 12, the light-to-dark transition. In mice subjected to a 6-h delayed light-dark cycle, the peak was shifted as expected. These cyclic oscillations were also observed in constant darkness, thus demonstrating, for the first time, the existence of strong circadian rhythms of the initial rate of either FXa or thrombin generation activity levels. These circadian variations overlapped with those that have been recently described in factor VII (FVII) activity. The peak of FXa generation activity was simulated by the addition of purified human FVII, thus indicating that circadian variations in FVII activity are important determinants of the circadian rhythm of the procoagulant cascade efficiency. These findings help to elucidate the complex control on the coagulation process and might contribute in explaining the temporal variations in the frequency of cardiovascular events observed in humans.     

Circadian rhythms of arterial blood pressure

We measured variations of blood pressure during the day in 46 untreated outpatients (25 hypertensives and 21 normotensives) to assess their circadian patterns. The self-measured blood pressures (autorhythmometry), determined at 08:00, 12:00, 16:00, 20:00 and 23:00 for 40 consecutive days were evaluated chronobiologically from single and population mean cosinors. On the basis of the results, we conclude that it is impossible to propose a common schedule of temporal therapy that will be effective for all hypertensive patients because of the unpredictable circadian behavior of blood pressure in this kind of subjects. Therefore, we suggest that individual circadian patterns of arterial pressure be studied when therapy is being established, so that the best therapeutic results can be achieved.     

Circadian rhythms in phonological and visuospatial storage components of working memory

Working memory is a basic cognitive process that temporarily maintains the information necessary for the performance of many complex tasks such as reading comprehension, learning and reasoning. Working memory includes two storage components: phonological and visuospatial, and a central executive control. The objective of this study was to identify possible circadian rhythms in phonological and visuospatial storage components of working memory using a constant routine protocol. Participants were eight female undergraduate students, aged 17.5±0.93, range = 16 - 19 years old. They were recorded in the laboratory in a constant routine protocol during 30 h. Rectal temperature was recorded every minute; subjective sleepiness and tiredness, as well as phonological and visuospatial working memory tasks, were assessed each hour. There were circadian variations in correct responses in phonological and visuospatial working memory tasks. Cross-correlation analysis showed a 1-h phase delay of the phonological storage component and a 3-h phase delay of the visuospatial storage component with respect to rectal temperature. This result may explain the changes in the performance of many complex tasks during the day.     

Circadian rhythms

The neurobiological substratum of circadian rhythmicity encompasses three levels of integration: firstly, generation of time signals by circadian pacemakers; secondly, entrainment of pacemakers by environmental influences; thirdly, coupling of circadian pacemakers among themselves and with target systems responsible for the expression of overt rhythms. From recent contributions, the notion that circadian organization results from the interaction of independent oscillators and pathways has been strengthened. In addition, recent evidence supports the existence of circadian rhythmicity in single isolated neurons. New information was produced on the gene control of circadian rhythm generation in Drosophila, as well as interesting advances in the understanding of neuronal mechanisms involved in the generation, entrainment and coupling of circadian rhythms in various species.     

Circadian rhythms in cockroaches

Summary The driving oscillator, which mediates circadian locomotor rhythms in cockroaches, appears to reside in the protocerebrum of the brain. The evidence indicates that the optic lobes are crucial elements in this circadian system, and that control of rhythmicity is mediated through electrical, rather than hormonal, channels. Lesions were placed at various sites within the optic lobes in order to localize the areas controlling rhythmicity. It appears that the two innermost synaptic areas (the lobula and the medulla) constitute the crucial optic lobe elements. The outer synaptic area of the optic lobe (the lamina) is not necessary for the expression of rhythmicity, but does function as a coupling through which light cycles, transduced by the compound eyes, entrain the circadian clock.I would like to thank both Dr. Sue Binkley for her helpful comments in the preparation of this report, and Mr. Eli Levine for his assistance in photography. Support for this research was provided by a grant from the National Science Foundation (NS GB-30497).     

Circadian physiological rhythms

Mammals show numerous circadian rhythms, some exogenous but nonetheless useful, some endogenous. Even in the absence of exogenous synchronizers, different physiological functions usually preserve their normal temporal relations, suggesting that a single clock controls most or all of the rhythmic manifestations. The role of adrenal corticosteroids and of temperature as means whereby the clock influences other functions is considered.     

Circadian rhythms of androgen levels in the blood of male hamadryas baboons

Radioimmunoassay was used to investigate circadian rhythms of blood plasma testosterone, 5 alpha-dihydrotestosterone, androstendione and dehydroepiandrosterone in intact animals and in those preadapted for experimental conditions. Blood plasma of monkey demonstrated monophasic circadian rhythms in the levels of androstendione, dehydroepiandrosterone and 5 alpha-dihydrotestosterone, with the maximal values seen early in the morning and minimal values in the evening. Circadian rhythms of testosterone were opposite in character. The adaptation of monkeys for housing and experimental conditions was marked by an increase in testosterone and 5 alpha-dihydrotestosterone levels in the blood as well as by an insignificant elevation of the levels of androgens of adrenal origin, leading to the changes in circadian rhythms of steroid hormones.     

Circadian rhythms and insomnia

Sleep and Biological Rhythms - Circadian rhythms strongly influence when we are sleepy and when we are alert. If the timing of these rhythms is later than normal, it can contribute to sleep onset...     

Circadian locomotor rhythms in individual honeybees

ABSTRACT. The circadian locomotor (walking) rhythms of individual forager honeybees ( Apis mellifera ligustica L.) were measured under a variety of conditions. In constant dark the rhythms exhibited endogenous periodicities that were less than 24 h, whereas under constant light the periods tended to be greater than 24 h. Individual honeybees readily entrained to photoperiods, displaying a diurnal pattern of entrainment with most of the activity occurring in late photophase. Evidence is presented which suggests that foraging behaviour and general locomotor behaviour may be governed by two different circadian clock systems.     

Circadian rhythms in epidermal mitotic activity

Virchows Archiv B Cell Pathology - Circadian variations in mitotic index, mitotic rate (cells entering mitosis per hour) and mitotic duration were investigated in the epidermis of the back and ear...     

Circadian rhythms lit up in Chlamydomonas

Recent work on the circadian clock of the unicellular green alga Chlamydomonas reinhardtii strengthens its standing as a convenient model system for circadian study. It was shown to be amenable to molecular engineering using a luciferase-based real-time reporter for circadian rhythms. Together with the completed draft genomic sequence, the new system opens the door for genome-scale forward and reverse genetic analysis.