Rapid reset of the corticosterone rhythm by food presentation in rats under acircadian restricted feeding schedule

Corticosterone levels were determined in the 7-week-old male rat maintained under different feeding and lighting schedules. At 4 weeks of age, the animals were kept either under a natural photoperiod (LD) or were subjected to continuous illumination (LL). Access to food was either ad libitum or restricted to an 8 hr span per 24 hr (circadian) or 32 hr (acircadian).

The food signal seemed able to synchronize the corticosterone rhythm to its own circadian periodicity, irrespective of the lighting regimen. No synchronization was observed in serially sampled LL or LD rats under an acircadian feeding schedule. Instead, the group acrophase appeared 24 hr subsequent to food presentation. Regarding individual patterns, many rats showed an acrophase or a peak also at that time. We speculate that an endogenous circadian mechanism was reset by the food signal, whenever it appeared.     

The internal synchronization of five circadian functions of the rabbit

Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL).

(1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66±8 (S.D.) %), 'water intake' (64±15 (S.D.) %) and 'urine excretion' (58±10 (S.D.) %). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 5S±13 (S.D.) % of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours.

(2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with-: = 24.8±0.5 (S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber.

(3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters.

(4) During LL the level of locomotor activity was reduced for 13±16 (S.D.) %, the events of food intake were increased for 17±12 (S.D.) %.

(5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days.

(6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.     

Tricyclic Imipramine Modification of the Circadian Rhythms of Hypothalamic Serotonin, its Precursors and Acid Catabolite in Individually Housed Rats

Circadian rhythms of serotonin (5HT), its precursors tryptophan (TP) and 5-hydroxy-tryptophan (5HTP) and its acid catabolite 5-hydroxy-indoleacetic acid (5HIAA), were determined in the hypothalamus of control rats and rats which had been treated continuously with subcutaneous imipramine (10 mg/kg/day) for 2 weeks.

Rats were individually housed and entrained to LD12:12. Controls showed the 5HT and TP peaks in the light and dark periods respectively, as reported in the literature, but no inverted correlation (antiphase) between SHT and 5HIAA rhythms.

Imipramine significantly modified circadian rhythm characteristics: the 5HT acrophase was advanced, that of TP and 5HIAA was delayed. Imipramine also significantly increased hypothalamic SHT and TP concentrations.     

Circannual variation of intestinal cell proliferation in BDF1 male mice on three lighting regimens

BDF, male mice were studied over a 24-hr span in winter, spring, summer and fall. For three weeks prior to study, one-third of the animals were kept under a lighting regimen of 8 hr light alternating with 16 hr of darkness (LD 8:16), one-third on a lighting regimen of LD 12:12 and a remainder on a lighting regimen of LD 16:8. During each study, subgroups of animals on all three lighting regimens were killed at 4-hr intervals over a 24-hr span. Twenty minutes prior to being killed, the animals received 5yCi of [³Hthymidine/0.2 ml/20 gm of body weight intraperitoneally. The thymidine uptake in the DNA of the colon and of the small intestine were studied as an index of cell proliferation. A circadian rhythm in [³H]-thymiduie uptake in the colon was found and validated by cosinor analysis. This rhythm was similar in acrophase and amplitude in the animals kept on LD 8:16 and LD 12:12. Also in the mice on LD 16:8, there was a statistically significant circadian rhythm of ('HJ-thymidtne uptake in the DNA of the colon during all four seasons. The acrophases of this rhythm, however, varied widely suggesting free running. A circadian rhythm of pHJ-thymidine uptake in small intestine was less consistent. In animals on all three lighting regimens, however, a circannual variation of f'HJ-thymidine uptake in DNA in colon and small intestine was found with the highest uptake during summer. This study indicates that a lighting regimen of LD 16:8 does not reliably synchronize the circadian rhythm of [³H]-thymidine uptake in the colon. It further shows a circannual rhythm of this function in the colon and in the small intestine which persists under three lighting regimens (LD 8:16, 12:12 and 16:8) maintained for three to four weeks prior to being killed.     

Circannual Variation of Intestinal Cell Proliferation in Bdf1 Male Mice on Three Lighting Regimens

BDF, male mice were studied over a 24-hr span in winter, spring, summer and fall. For three weeks prior to study, one-third of the animals were kept under a lighting regimen of 8 hr light alternating with 16 hr of darkness (LD 8:16), one-third on a lighting regimen of LD 12:12 and a remainder on a lighting regimen of LD 16:8. During each study, subgroups of animals on all three lighting regimens were killed at 4-hr intervals over a 24-hr span. Twenty minutes prior to being killed, the animals received 5yCi of [³Hthymidine/0.2 ml/20 gm of body weight intraperitoneally. The thymidine uptake in the DNA of the colon and of the small intestine were studied as an index of cell proliferation. A circadian rhythm in [³H]-thymiduie uptake in the colon was found and validated by cosinor analysis. This rhythm was similar in acrophase and amplitude in the animals kept on LD 8:16 and LD 12:12. Also in the mice on LD 16:8, there was a statistically significant circadian rhythm of ('HJ-thymidtne uptake in the DNA of the colon during all four seasons. The acrophases of this rhythm, however, varied widely suggesting free running. A circadian rhythm of pHJ-thymidine uptake in small intestine was less consistent. In animals on all three lighting regimens, however, a circannual variation of f'HJ-thymidine uptake in DNA in colon and small intestine was found with the highest uptake during summer. This study indicates that a lighting regimen of LD 16:8 does not reliably synchronize the circadian rhythm of [³H]-thymidine uptake in the colon. It further shows a circannual rhythm of this function in the colon and in the small intestine which persists under three lighting regimens (LD 8:16, 12:12 and 16:8) maintained for three to four weeks prior to being killed.     

Comparison of the circadian rhythm in cell proliferation in corneal epithelium of male rats studied under normal and hypobaric (hypoxic) conditions

Groups of 20-45-day-old rats maintained on a light (0600-1800)/dark (1800-0600) regimen with food and water available ad libitum were studied for the effect of hypoxic hypoxia on the circadian rhythm of corneal epithelial mitosis and thymidine incorporation. In experiments conducted during the months of September and November, hypoxic hypoxia was accomplished by the exposure of rats to a simulated altitude of 7500 m in one series of experiments, or to a gaseous mixture of 8% oxygen and 92% nitrogen at sea level atmospheric pressure (760 mmHg) in another series of experiments. Controls were included as well. Statistically significant (P less than 0.05) circadian rhythmicity in the corneal mitotic index was substantiated in the control animals with mesor (M) = 12.4%, amplitude (A) = 9.6% and acrophase (phi) of 0911. In the hypoxic hypoxia situation, the mesor and amplitude were depressed to 8.6 and 5.9%, respectively. In control groups, thymidine incorporation was circadian rhythmic with M = 38.5 and A = 11.3 cpm/microns DNA and acrophase of 2255. In the hypoxic hypoxia situation, the mesor was similar to the controls; whereas the amplitude was suppressed to 6.1% and acrophase was phase advanced by about 7 hr. Changes in the circadian rhythm of corneal mitosis and in thymidine incorporation under hypoxic hypoxia can be explained by programmed-in-time energy requirements during the corneal cell regeneration cycle.     

Circadian Rhythm of Blood Ethanol Clearance Rates in Rats: Response to Reversal of the L/D Regimen and to Continuous Darkness and Continuous Illumination

Phase relationships of the circadian rhythms of blood ethanol clearance (metabolic) rates and body temperature were studied in rats successively exposed to 4 illumination regimens: LD (light from 0800-2000 hr), DL (light from 2000-0800 hr), constant darkness (DD) and, lastly, constant light (LL). After a 4-wk standardization to each regimen, body temperatures were taken at 9 × 4-hr intervals to establish baseline circadian profiles. One week later, groups (N = 8) received 1.5 g/kg ethanol (i.p.) at 6 equally spaced timepoints during a 24-hr span, when temperatures were again measured. Ethanol clearance rates were estimated from decreasing blood ethanol levels sampled every 20 min from 60-200 min after dosing, and the resultant elimination curves were subjected to cosinor analysis. These studies show for the first time that the high amplitude circadian rhythm in ethanol metabolism persists under constant conditions of illumination (DD and LL), demonstrating that it may well be a truly internal circadian rhythm and not a response to exogenous cues of the light/dark cycle. During both LD and DL, maximal and minimal ethanol clearance rates fell near the end of the dark and light phases, respectively, and followed circadian peak and trough control temperatures by approximately 6 hr. A fixed internal phase relationship between the core body temperature and the circadian rhythm in ethanol metabolism is demonstrated, thus establishing the rhythm in body temperature as a suitable and convenient internal marker rhythm for studies of the metabolism of low-to-moderate ethanol doses. These studies demonstrate that the phase relationships of blood ethanol clearance rate and body temperature can be manipulated by the illumination regimen selected, an observation of both basic and practical importance.     

Circadian Rhythm in Gamma Glutamyltranspeptidase and Leucine Aminopeptidase Urinary Activity in Rats

Urinary gamma glutamyltranspeptidase (GGT) and leucine aminopeptidase (LAP), renal tubular brush border enzymes, have been shown to be sensitive indicators of renal tubular functions. This study documents circadian rhythms in the urinary activity of GGT and LAP, statistically validated and quantified by the cosinor method, in 15 male Wistar rats standardized to a LD 12:12 illumination schedule (light from 0800 hr to 2000 hr) and fed ad libitum. The acrophase of the circadian rhythms in urinary GGT and LAP activity occurred at the end of the rest span of the animals: between 1730 and 1915 for GGT (depending on the mode of expression of the activity) and between 1700 and 1910 for LAP. Of striking resemblance in their timing, both these rhythms were also of large amplitude (about 50% of the mesor for urinary GGT activity and about 45% for LAP one). The circadian acrophases of urinary GGT and LAP activity led in timing the circadian rhythms in urine volume and creatinine excretion by about 13hr. Such findings consistent with the circadian variations found by other investigators in GGT in kidney homogenates or in LAP in human urine thus reflect a periodicity in renal tubular function. The reasons for these circadian variations, still unknown at this time, are discussed. The influence recently demonstrated of the hormonal context on protein and enzyme synthesis at the tubule, and its phase relations to urinary enzyme excretion emphasize how much the circadian rhythm in urinary GGT and LAP activity is well included in the murine time structure. Therefore it should be of interest to consider the circadian rhythm in urinary GGT and LAP release as a marker rhythm of predictive value as to the side effects of nephrotoxic drugs.     

Circadian Rhythm in Gamma Glutamyltranspeptidase and Leucine Aminopeptidase Urinary Activity in Rats

Urinary gamma glutamyltranspeptidase (GGT) and leucine aminopeptidase (LAP), renal tubular brush border enzymes, have been shown to be sensitive indicators of renal tubular functions. This study documents circadian rhythms in the urinary activity of GGT and LAP, statistically validated and quantified by the cosinor method, in 15 male Wistar rats standardized to a LD 12:12 illumination schedule (light from 0800 hr to 2000 hr) and fed ad libitum. The acrophase of the circadian rhythms in urinary GGT and LAP activity occurred at the end of the rest span of the animals: between 1730 and 1915 for GGT (depending on the mode of expression of the activity) and between 1700 and 1910 for LAP. Of striking resemblance in their timing, both these rhythms were also of large amplitude (about 50% of the mesor for urinary GGT activity and about 45% for LAP one). The circadian acrophases of urinary GGT and LAP activity led in timing the circadian rhythms in urine volume and creatinine excretion by about 13hr. Such findings consistent with the circadian variations found by other investigators in GGT in kidney homogenates or in LAP in human urine thus reflect a periodicity in renal tubular function. The reasons for these circadian variations, still unknown at this time, are discussed. The influence recently demonstrated of the hormonal context on protein and enzyme synthesis at the tubule, and its phase relations to urinary enzyme excretion emphasize how much the circadian rhythm in urinary GGT and LAP activity is well included in the murine time structure. Therefore it should be of interest to consider the circadian rhythm in urinary GGT and LAP release as a marker rhythm of predictive value as to the side effects of nephrotoxic drugs.     

Regression Models for the Estimation of Orcadian Rhythms in the Presence of Effects Due to Masking

The estimation of human circadian rhythms from experimental data is complicated by the presence of “masking” effects associated with the sleep-wake cycle. The observed rhythm may include a component due to masking, as well as the endogenous component linked to a circadian pacemaker. In situations where the relationship between the sleep-wake cycle and the circadian rhythm is not constant, it may be possible to obtain individual estimates of these two components, but methods commonly used for the estimation of circadian rhythms, such as the cosinor analysis, spectral analysis, average waveforms and complex demodulation, have not generally been adapted to identify the modulations that arise from masking. The estimates relate to the observed rhythms, and the amplitudes and acrophases do not necessarily refer to the endogenous rhythm.

In this paper methods are discussed for the separation of circadian and masking effects using regression models that incorporate a sinusoidal circadian variation together with functions of time since sleep and time during sleep. The basic model can be extended to include a time-varying circadian rhythm and estimates are available for the amplitude and phase at a given time, together with their joint confidence intervals and tests for changes in amplitude and acrophase between any two selected times. Modifications of these procedures are discussed to allow for non-sinusoidal circadian rhythms, non-additivity of the circadian and time-since-sleep effects and the breakdown of the usual assumptions concerning the residual errors.

This approach enables systematic masking effects associated with the sleep-wake cycle to be separated from the circadian rhythm, and it has applications to the analysis of data from experiments where the sleep-wake cycle is not synchronized with the circadian rhythm, for example after time-zone transitions or during irregular schedules of work and rest.     

Circadian rhythm of corticosterone in diabetic rats

We proposed that the circadian rhythm of corticosterone in diabetic rats would have a different pattern than that in non-diabetic control rats. To test this hypothesis, 20 male Sprague-Dawley rats were given ad libitum access to a stock diet and housed individually in a light and temperature controlled room. Ten rats were made diabetic by two subcutaneous injections of streptozotocin. Ten rats which were not injected served as controls. Thirteen days after induction of the diabetes, tail blood samples were taken every 4 h for 24 h. Plasma corticosterone levels were significantly higher in diabetic rats than in control rats at 3 time points during the light cycle; however, concentrations were similar during the dark cycle. We speculate that diabetes may cause alterations in the steroid feedback mechanism to the hypothalamus and/or pituitary, resulting in an abnormal circadian rhythm of plasma corticosterone.     

Effect of reserpine on the circadian dynamics of motor activity in rats

Administration of reserpine to rats lowers the amplitude of circadian rhythm of motor activity and shifts its acrophase to earlier hours. Efficiency of drug is higher in animals with a well expressed circadian rhythms. Some rats by the character of initial circadian mobility resemble reserpinized animals. Such initially "depressive" rats are characterized by a lesser emotionality and they reorganize with more difficulty the trajectory of avoidance response in Y-maze.     

Influence of the light regimen on the circadian rhythm of serum lipids in the laboratory rat

Young male rats of a conventional Wistar breed were adapted for several weeks (3-6) to a 12:12 h light:dark (7 a.m. - 7 p.m., 7 p.m. - 7 a.m.) regimen (LD), to inversion of this standard regimen (DL), to continuous darkness (DD) or to continuous light (LL). After adaptation, groups of animals were killed at 3-hour intervals during the day and the basic lipid fractions were determined in their serum. Under the standard light regimen the cosinor test demonstrated a rhythm in all the indicators (triacylglycerols, total cholesterol, phospholipids) except for non-esterified fatty acids. When the regimen was inverted (DL), the peaks of the circadian non-esterified fatty acid and triacylglycerol curves shifted to the antiphase. The acrophases of "free-running" serum lipid rhythm under the DD regimen of the standard regimen rhythms differed in the case of cholesterol and phospholipids. In the case of triacylglycerols and phospholipids there was disagreement between the DD and LL regimen curves. With reference to the localization of the acrophase under the DD and LD regimens it was assumed that the influence of the light regimen on the synchronization of circadian rhythms is small in the case of serum non-esterified fatty acids and triacylglycerols and that it is greater in the case of serum cholesterol and phospholipids.     

Effects of light on the circadian rhythm of diabetic rats under restricted feeding

The aim of this study was to investigate whether the entrainment of light cue is affected or not in diabetic animals. We found that the individual light/dark (LD) reversal showed a tissue- and gene-specific effect on the circadian phases of peripheral clock genes, which was generally similar between the control and diabetic rats. In the liver and heart, the peak phases of examined clock genes (Bmal1, Rev-erbα, Per1, and Per2) were slightly shifted by 0～4 h in the liver and heart of control and diabetic rats. However, we found that the peak phases of these clock genes were greatly shifted by 8～12 h after the LD reversal for 7 days in the pineal gland of both control and diabetic rats. However, the activity rhythm was greatly different between two groups. After the individual LD reversal, the activity rhythm was completely shifted in the control rats but retained in the diabetic rats. These observations suggested that the behavioral rhythm of diabetic rats may be uncoupled from the master clock after the individual LD reversal. Moreover, we also found that the serum glucose levels of diabetic rats kept equally high throughout the whole day without any shift of peak phase after the individual reversal of LD cycle. While the serum glucose levels of control rats were tightly controlled during the normal and LD reversal conditions. Thus, the impaired insulin secretion induced uncontrollable serum glucose level may result in uncoupled activity rhythm in the diabetic rats after the individual LD reversal.     

Entrainment of circadian rhythms of cholesterol‐LDL,corticosterone and motor activity by feeding schedules in rats

Abstract

The daily variations of locomotor activity, plasma and adrenal corticosterone levels and cholesterol‐LDL were studied in male Wistar rats with food ad libitum and feeding restricted to the first 4 hours of the light phase in LD 12:12..

Under LD 12:12 (light on from 9:00 to 21:00h) rats with food ad libitum were eating and moving during the dark period and the locomotor activity clearly showed a biphasic pattern with three harmonic components. Plasma and adrenal corticosterone levels increased during the light period and reached a maximum value just before the dark period whereas the acrophase of cholesterol‐LDL is found at the beginning of the light phase.

The acrophases of activity, plasma and adrenal corticosterone levels in the restricted feeding schedule rats occurred in the first three hours of lighting and the cholesterol‐LDL acrophase at the beginning of the dark phase.

These results confirm a previous report that the shift of feeding to the light phase seems to cause a concomitant phase‐shift in all the variables measured.     

Harderian gland porphyrin, lysosomal and type II 5'-deiodinase rhythms in Sprague-Dawley and Fischer-344 rats kept under chronic long- or short-photoperiod conditions.

1. Harderian gland porphyrin concentrations were 1.5-fold higher in Fischer-344 male rats than in Sprague-Dawley male rats and there were no differences due to exposure to LD 14:10 (LP) or LD 10:14 (SP) lighting conditions for 8 weeks in either strain. 2. 24-hr periodic regression analyses of porphyrin concentration detected a significant rhythm in both lighting conditions in both strains, with no differences in acrophase due to lighting conditions. 3. In both strains, porphyrin levels were highest in the late phase-early dark period and fell during the early part of the dark period. 4. Acid phosphatase activity did not vary with time (circadian rhythm), strain or photoperiodic lighting condition. 5. Circadian rhythms in beta-glucuronidase, alpha-mannosidase and hexosaminidase activity were present in some instances, but, probably due to the low amplitude to these rhythms, a consistent effect of strain or housing condition was not found. When 24-hr rhythms were observed in either strain, the acrophase occurred during the afternoon-early evening daylight period. 6. A significant effect of strain on mean values of type II 5'-deiodinase activity was noted in Fischer-344 rats. 7. Significant rhythms in type II 5'-deiodinase activity were found in both strains exposed to LD 10:14.     

Chronobiological serial sections for core temperature monitoring after transplantation of the murine pancreas

In order to study any early sign of rejection of pancreas transplantation, rhythmometry was carried out on female adult inbred Lewis rats. Animals previously kept in continuous light, more or less synchronized in frequency by cyclic human activities, were transferred to a regimen of light (L) and darkness (D) alternating at 12-h intervals in single cages at the room temperature of 24 +/- 1 degrees C, with food and water ad libitum. At this time under ether anesthesia, temperature transensors were implanted in healthy rats and rats rendered diabetic by the administration of streptozotocin. Some of the diabetic rats were left untreated; casual blood sampling showed gross hyperglycemia. Other rats were treated by pancreatic grafts from ethionine-prepared donors, either by isografts (in rats of the Lewis strain) or by allografts (of the pancreas from inbred Fischer rats transplanted to Lewis rats). Intraperitoneal temperature was telemetered at 10-min intervals for 3 weeks following transplantation. Urine volumes were determined from rats housed in metabolic cages. Data were analyzed rhythmometrically. Chronobiological serial sections and single cosinors served this purpose. Following sensor implantation and transfer to an LD 12:12 regimen, the adjustment of the thermal acrophase consistently near the middle of the daily dark span occurred within approximately 7 days in healthy rats and in streptozotocin-diabetic rats cured by isograft. Thermal acrophase adjustment was slower for animals rendered diabetic by streptozotocin and left untreated or for animals thus rendered diabetic which had rejected the pancreatic allograft (as documented by hyperglycemia in casually sampled blood). The eventual synchronization of the circadian temperature rhythm of allografted rats differed from one rat to the other and, for some allografted animals, from the consistent synchronization of the circadian rhythm in telemetered intraperitoneal temperature of diabetic and non-diabetic Lewis rats. The acrophase of the circadian rhythm in urine volume of healthy rats or of a rat with a pancreatic isograft (which cured a prior streptozotocin-induced diabetes) differed with statistical significance from those of rats with untreated diabetes, some in this state after the rejection of a pancreatic allograft. Both urine volume and core temperature are ready marker rhythms, not only for rats but also for human beings. Both variables can be self-monitored by the cooperation of instructed but not necessarily extensively educated patients. Temperature, in particular, can also be monitored with automatic devices and alterations of certain of its rhythm characteristics may signal changes preceding fever. The use of such admittedly unspecific yet eminently practical and possibly informative marker rhythmometry awaits clinical testing.     

Entraining effects of variations in light spectral composition on the rest-activity rhythm of a nocturnal rodent

The ability to predict and adjust physiology and behavior to recurring environmental events has been necessary for survival on Earth. Recent discoveries revealed that not only changes in irradiance but also light spectral composition can stimulate the suprachiasmatic nucleus (SCN), ensuring the body’s synchronization to the environment. Therefore, using a lighting system that modulates spectral composition during the day using combined red-green-blue (RGB) lights, we evaluated the effect of variations in light spectral composition on the rest-activity rhythm of rodents. Male Wistar rats (n = 17) were gestated and raised under different lighting conditions and exposed to a long photoperiod (16 h light: 8 h dark). The difference between groups was the presence of variations in light spectral composition during the day (RGB-v) to simulate daily changes in natural light, or not (RGB-f). After weaning, spontaneous motor activity was recorded continuously for rhythm evaluation. Our results indicated that animals under RGB-v did not present a reactive peak of activity after the beginning of the light phase, suggesting that this group successfully detected the variations aimed at mimicking daily alterations of natural light. Furthermore, RGB-v animals exhibited an earlier activity acrophase in comparison to animals under RGB-f (RGB-v = 12:16 – “hh:mm”, RGB-f = 13:02; p < 0.001), which might have been due to the capability to predict the beginning of the dark phase when exposed to variations in light spectrum. However, this earlier activity acrophase can be also explained by the blue-light peak that occurred in RGB-v. The spectral and waveform analysis of daily patterns of motor activity revealed that rats in the RGB-v group were better entrained to a circadian rhythm throughout the experiment. RGB-v showed higher interdaily stability (IS) values (29.75 ± 6.5, n = 9) than did RGB-f (t₍₁₅₎ = 2.74, p = 0.015). Besides, the highest power content (PC) on the first harmonic (circadian) was reached earlier in the RGB-v group. The circadianity index (CI) of the whole period was higher in the RGB-v group (t₍₁₅₎ = 3.47, p = 0.003). Thus, we could consider that locomotor activity rhythm was entrained to the light-dark cycle in the RGB-v rats earlier compared to the RGB-f rats. Our results provide additional evidence for the effect of variations in light spectral composition on the rest-activity pattern of nocturnal rodents. This suggests that these animals might predict the arrival of the activity phase by its advanced acrophase when exposed to RGB-v, demonstrating a better synchronization to a 24-h rhythm.     

Circadian and circannual rhythms of hormonal variables in elderly men and women

A group of fourteen men (73 ± 5 yr of age), and eighteen women (77 ± 7 yr of age) institutionalized at the Berceni Clinical Hospital, Bucharest, Romania, were studied over a 24-hr span once during each season (winter, spring, summer and fall). All subjects followed a diurnal activity pattern with rest at night and ate three meals per day with breakfast at about 0830, lunch at about 1300 and dinner at about 1830. The meals were similar, although not identical for all subjects during all seasons. On each day of sampling blood was collected at 4-hr intervals over a 24-hr span. Seventeen hormonal variables were determined by radioimmunoassay. Statistically significant circadian rhythms were detected and quantitated by population mean cosinor analysis in pooled data from all four seasons in both sexes for ACTH, aldosterone, Cortisol, C-peptide, dehydroepiandrosterone-sulfate (DHEA-S), immunoreactive insulin, prolactin, 17-OH progesterone, testosterone, total T₄ and TSH. In women, estradiol and progesterone also were determined and showed a circadian rhythm during all seasons. Total T, and FSH showed circadian rhythm detection by cosinor analysis in the men only; LH showed no consistent circadian rhythm as group phenomenon in men or women.

A circannual rhythm was detected using the circadian means of each subject at each season as input for the population mean cosinor in the women for ACTH, C-peptide, DHEA-S, FSH, LH, progesterone, 17-OH progesterone and TSH. In the men, a circannual rhythm was detected for ACTH, FSH, insulin, LH, testosterone and T₃. There were phase differences between men and women in ACTH, FSH and LH. In those functions in which both the circadian and circannual rhythms were statistically significant, a comparison of the amplitudes showed in the women a higher circannual rather than circadian amplitude for DHEA-S. In 17-OH progesterone, TSH and C-peptide, the circadian amplitude in women was larger. In men, the circannual amplitude of T₃ was larger than the circadian amplitude and in insulin the circadian amplitude was larger than the circannual amplitude. There was no statistically significant difference between the circadian and circannual amplitudes in the women in ACTH and progesterone and in the men in ACTH and testosterone.     

Circadian rhythm of thymosin-alpha 1 in normal and thymectomized mice

Studies by many investigators have demonstrated that the immune system is subject to regular circadian fluctuation. Some rhythms that have been reported include circadian changes in components of the immune system, e.g., lymphocytes, and circadian variation in primary and secondary immune responsiveness. The observation that many of these rhythms are inversely correlated to the glucocorticoid rhythm has led to the suggestion that fluctuations in the immune system may be a result of the glucocorticoid circadian rhythm. This study was designed to see if thymosin-alpha 1 (Tsn-alpha 1), a 28-amino acid polypeptide isolated from bovine thymus that has been reported to influence thymocyte differentiation, might follow a circadian rhythm, and thus play a role in the periodicity of the immune system. In these experiments, groups of 10 C57BL/6 or Swiss Webster mice were sacrificed at 4- or 6-hr intervals over a 24-hr period. Serum Tsn-alpha 1 and corticosterone levels were determined by radioimmunoassay. Results from the first experiment showed that Tsn-alpha 1 undergoes a circadian rhythm (p less than 0.001) with an acrophase (time of peak levels) 1.5 hr after the onset of light, and an amplitude (amount of maximum variation from the 24-hr mean) of 0.493 ng/ml Tsn-alpha 1-like immunoreactivity. These results were confirmed in an experiment in which the animals were placed on a reversed light cycle. In a separate experiment, the Tsn-alpha 1 circadian rhythm persisted in mice thymectomized 6 mo. earlier. In this latter experiment, a significant increase in the amplitude of the corticosterone rhythm in the thymectomized relative to sham-operated controls was also observed. Although these experiments do not imply casuality, it is interesting that the time of peak Tsn-alpha 1 levels can be correlated with the time of optimal immune function.