Cycles in anilmals

The impact of stress and stress hormones on endogenous clocks and circadian rhythms

In mammals, daily rhythms in physiology and behavior are under control of a circadian pacemaker situated in the suprachiasmatic nucleus (SCN). This master clock receives photic input from the retina and coordinates peripheral oscillators present in other tissues, maintaining all rhythms in the body synchronized to the environmental light-dark cycle. In line with its function as a master clock, the SCN appears to be well protected against unpredictable stressful stimuli. However, available data indicate that stress and stress hormones at certain times of day are capable of shifting peripheral oscillators in, e.g., liver, kidney and heart, which are normally under control of the SCN. Such shifts of peripheral oscillators may represent a temporary change in circadian organization that facilitates adaptation to repeated stress. Alternatively, these shifts of internal rhythms may represent an imbalance between precisely orchestrated physiological and behavioral processes that may have severe consequences for health and well-being.

In vitro circadian rhythms: imaging and electrophysiology

In vitro assays have localized circadian pacemakers to individual cells, revealed genetic determinants of rhythm generation, identified molecular players in cell-cell synchronization and determined physiological events regulated by circadian clocks. Although they allow strict control of experimental conditions and reduce the number of variables compared with in vivo studies, they also lack many of the conditions in which cellular circadian oscillators normally function. The present review highlights methods to study circadian timing in cultured mammalian cells and how they have shaped the hypothesis that all cells are capable of circadian rhythmicity.

Several cytokinetic methods for showing circadian variation in normal murine tissue and in a tumor

Circadian rhythmicity in cell proliferation was studied by several cytokinetic techniques in 8-week-old CD2F1 and Swiss male mice. In separate experiments on four different dates, subgroups of six or seven mice (fed ad libitum and standardized to 8 hours light alternating with 16 hours of darkness) were killed at 3-hour intervals over a period of 24-48 hours. Cosinor analyses were used to determine the parameters of circadian periodicity in the data. Flow cytometric (FCM) analyses of DNA synthesis in tongue and stomach were compared with estimates of tritiated thymidine (3HTdR) incorporation into the DNA in these tissues. Circadian variation in tongue epithelium was reproducible in phasing and in range-of-change (100-140%) in seven 24-hour studies. The proportion of cells in G1 and G2 in tongue epithelium also demonstrated a circadian periodicity. When DNA synthesis in stomach was analyzed by the two methods, the results were entirely different and thus not comparable. The circadian rhythms in 3HTdR uptake agreed with those reported previously, but results of analyses by FCM were less certain, since epithelium of this organ could not be analyzed separately. Circadian rhythms were detected by FCM in G2, S, and G2M in Ehrlich ascites carcinoma (EAC). The mitotic index of this tumor varied with a circadian periodicity similar to that of the FCM-derived rhythm in G2M. This study validates the reliability of older techniques, such as 3HTdR uptake and mitotic indices, and suggests that circadian rhythms can be important in studies of cellular properties using FCM.

Importance of circadian rhythms in animal cell cultures

Circadian rhythms are a characteristic feature of many cell and organ cultures. Such rhythms may be important in the interpretation of data from cells in culture. More examples of circadian rhythms in tissue culture are badly needed to understand this phenomena. However, they will only be expressed under optimum and well-understood conditions.