Quantitative analysis of the age-related fragmentation of hamster 24-h activity rhythms

Regulation of Rest, Rather Than Activity, Underlies Day-Night Activity Differences in Mice

The suprachiasmatic nucleus (SCN), which serves as the central pacemaker in mammals, regulates the 24-h rhythm in behavioral activity. However, it is currently unclear whether and how bouts of activity and rest are regulated within the 24-h cycle (i.e., over ultradian time scales). Therefore, we used passive infrared sensors to measure temporal behavior in mice housed under either a light–dark (LD) cycle or continuous darkness (DD). We found that a probabilistic Markov model captures the ultradian changes in the behavioral state over a 24-h cycle. In this model, the animal’s behavioral state in the next time interval is determined solely by the animal’s current behavioral state and by the “toss” of a proverbial “biased coin.” We found that the bias of this “coin” is regulated by light input and by the phase of the clock. Moreover, the bias of this “coin” for an animal is related to the average length of rest and activity bouts in that animal. In LD conditions, the average length of rest bouts was greater during the day compared to during the night, whereas the average length of activity bouts was greater during the night compared to during the day. Importantly, we also found that day-night changes in the rest bout lengths were significantly greater than day-night changes in the activity bout lengths. Finally, in DD conditions, the activity and rest bouts also differed between subjective night and subjective day, albeit to a lesser extent compared to LD conditions. The ultradian regulation represented by the model does not result in ultradian rhythms, although some weak ultradian rhythms are present in the data. The persistent differences in bout length over the circadian cycle following loss of the external LD cycle indicate that the central pacemaker plays a role in regulating rest and activity bouts on an ultradian time scale.

Olfactory Bulbectomy Modifies Photic Entrainment and Circadian Rhythms of Body Temperature and Locomotor Activity in a Nocturnal Primate

Studies on rodents have emphasized that removal of the olfactory bulbs modulates circadian rhythmicity. Using telemetric recordings of both body temperature (Tb) and locomotor activity (LA) in a male nocturnal primate, the gray mouse lemur, the authors investigated the effects of olfactory bulbectomy on (1) the circadian periods of Tb and LA in constant dim light condition, and (2) photic reentrainment rates of circadian rhythms following 6-h phase shifts of entrained light-dark cycle (LD 12:12). Under free-running condition, bulb-ectomized males had significantly shorter circadian periods of Tb and LA rhythms than those of control males. However, the profiles of Tb rhythms, characterized by a phase of hypothermia at the beginning of the subjective day, and Tb parameters were not modified by olfactory bulbectomy. Under a light-dark cycle, olfactory bulbectomy significantly modified the expression of daily hypothermia, especially by an increase in the latency to reach minimal daily Tb, suggesting a delayed response to induction of daily hypothermia by light onset. Re-entrainment rates following both a 6-h phase advance and a 6-h phase delay of entrained LD were also delayed in bulbectomized males. Olfactory bulbectomy led to significant fragmentation of locomotor activity and increased locomotor activity levels during the resting period. The shortening of circadian periods in bulbectomized males could partly explain the delayed responses to photic stimuli since in control males, the longer the circadian period, the better the response to light entrainment. This experiment shows for the 1st time that olfactory bulbs can markedly modify the circadian system in a primate.

Aging and Circadian Rhythms

Aging is associated with numerous changes, including changes in sleep timing, duration, and quality. The circadian timing system interacts with a sleep-wake homeostatic system to regulate human sleep, including sleep timing and structure. This article reviews key features of the human circadian timing system, age-related changes in the circadian timing system, and how those changes may contribute to the observed alterations in sleep.

Neuroprotection by radical avoidance: search for suitable agents

Neurodegeneration is frequently associated with damage by free radicals. However, increases in reactive oxygen and nitrogen species, which may ultimately lead to neuronal cell death, do not necessarily reflect its primary cause, but can be a consequence of otherwise induced cellular dysfunction. Detrimental processes which promote free radical formation are initiated, e.g., by disturbances in calcium homeostasis, mitochondrial malfunction, and an age-related decline in the circadian oscillator system. Free radicals generated at high rates under pathophysiological conditions are insufficiently detoxified by scavengers. Interventions at the primary causes of dysfunction, which avoid secondary rises in radical formation, may be more efficient. The aim of such approaches should be to prevent calcium overload, to reduce mitochondrial electron dissipation, to support electron transport capacity, and to avoid circadian perturbations. L-theanine and several amphiphilic nitrones are capable of counteracting excitotoxicity and/or mitochondrial radical formation. Resveratrol seems to promote mitochondrial biogenesis. Mitochondrial effects of leptin include attenuation of electron leakage. Melatonin combines all the requirements mentioned, additionally regulates anti- and pro-oxidant enzymes and is, with few exceptions, very well tolerated. In this review, the perspectives, problems and limits of drugs are compared which may be suitable for reducing the formation of free radicals.

The timing of the shrew: continuous melatonin treatment maintains youthful rhythmic activity in aging Crocidura russula

BACKGROUND

Laboratory conditions nullify the extrinsic factors that determine the wild expected lifespan and release the intrinsic or potential lifespan. Thus, wild animals reared in a laboratory often show an increased lifespan, and consequently an increased senescence phase. Senescence is associated with a broad suite of physiological changes, including a decreased responsiveness of the circadian system. The time-keeping hormone melatonin, an important chemical player in this system, is suspected to have an anti-aging role. The Greater White-toothed shrew Crocidura russula is an ideal study model to address questions related to aging and associated changes in biological functions: its lifespan is short and is substantially increased in captivity; daily and seasonal rhythms, while very marked the first year of life, are dramatically altered during the senescence process which starts during the second year. Here we report on an investigation of the effects of melatonin administration on locomotor activity of aging shrews.

METHODOLOGY/PRINCIPAL FINDINGS

1) The diel fluctuations of melatonin levels in young, adult and aging shrews were quantified in the pineal gland and plasma. In both, a marked diel rhythm (low diurnal concentration; high nocturnal concentration) was present in young animals but then decreased in adults, and, as a result of a loss in the nocturnal production, was absent in old animals. 2) Daily locomotor activity rhythm was monitored in pre-senescent animals that had received either a subcutaneous melatonin implant, an empty implant or no implant at all. In non-implanted and sham-implanted shrews, the rhythm was well marked in adults. A marked degradation in both period and amplitude, however, started after the age of 14-16 months. This pattern was considerably delayed in melatonin-implanted shrews who maintained the daily rhythm for significantly longer.

CONCLUSIONS

This is the first long term study (>500 days observation of the same individuals) that investigates the effects of continuous melatonin delivery. As such, it sheds new light on the putative anti-aging role of melatonin by demonstrating that continuous melatonin administration delays the onset of senescence. In addition, the shrew appears to be a promising mammalian model for elucidating the precise relationships between melatonin and aging.

Neuropeptide Y-deficient mice show altered circadian response to simulated natural photoperiod

Circadian rhythms, endogenously generated by the suprachiasmatic nucleus (SCN), can be synchronized to a variety of photic and non-photic environmental stimuli. Neuropeptide Y (NPY) is produced in the intergeniculate leaflet (IGL) and known to mediate both photic and non-photic influences on the SCN. We recently found that npy-/- mice were slower to shift their locomotor activity onset to the new time of light offset when photoperiod was abruptly changed from light/dark (LD) cycle 18:6 to LD 6:18. In the present study, we measured the locomotor response of npy-/- mice to gradual changes in photoperiod (4 min a day) for 141 days (LD 16:8 changing to LD 8:16), mimicking external LD cycles in nature. When the photoperiod approached LD 8:16, npy-/- mice showed a significantly delayed onset of activity compared to wild-type mice. Activity patterns disintegrated into multiple bouts and intensity of activity decreased as the photoperiod changed and these changes were more pronounced in npy-/- mice. Our results lend further support to the idea that NPY is involved in circadian entrainment responses to seasonal photoperiod changes.

Effects of ageing on the fine distribution of the circadian CLOCK protein in reticular formation neurons

Many biochemical, physiological and behavioural processes, from bacteria to human, exhibit roughly 24 h cyclic oscillations defined as circadian rhythms. However, during ageing, numerous aspects of the circadian biology undergo alterations; in particular, the sleep pattern changes, with more frequent awakenings and shorter sleep time. The basic mechanism of the circadian clock relies on intracellular molecular pathways involving interlocking transcriptional/translational feedback loops, and CLOCK protein, a transcription factor, is essential for normal circadian rhythms. In this study, the fine distribution of CLOCK protein has been analysed, in adult and old rats, at different phases of the daily cycle in the neurons of the medullary reticular formation, involved in the control of the sleep-wake cycle. The results demonstrate quali-quantitative modifications of CLOCK protein in the neurons of old animals, suggesting that such a deregulation of the intracellular clock mechanism may play some role in the degeneration of the sleep-wake circadian cycle.

Decreased wheel-running activity in hamsters post myocardial infarction

Reduced exercise capacity is a key symptom and an independent determinant of mortality in patients with heart failure. We analyzed the running activity of hamsters with cardiac dysfunction after myocardial infarction. In 39 male Syrian hamsters aged 10 to 12 weeks, a myocardial infarction (MI) was produced by permanent ligation of the left coronary artery. Spontaneous running activity in a wheel was monitored daily. After four weeks, left ventricular (LV) hemodynamics (catheter tip manometry) were measured at baseline and during inotropic stimulation (isoprenaline 0.03, 0.1 and 0.3 microg/kg/min i.v.). LV infarct size was quantified using planimetry. Four weeks post MI, daily running distance was reduced stepwise in animals with small (4-15% of LV: 9.8 +/- 3.4 km/d) and large (> 15% of LV: 7.5 +/- 3.5 km/d) MI, compared to sham-operated hamsters (11.5 +/- 1.5 km/d). Similar reductions were observed in maximum speed and distance of longest running period. MI size influenced daily running distance, maximum speed, and longest running period (linear correlations, all p < 0.05). MI size also impaired LV systolic and diastolic function under isoprenaline stimulation. The results suggest that myocardial infarction reduces running capacity and isoprenaline stimulated LV function in hamsters, mimicking impaired exercise performance in patients with heart failure. Analysis of running activity in hamsters with myocardial infarction offers a unique opportunity for non-invasive and serial functional assessment of heart failure in the experimental setting.

Temporal precision in the mammalian circadian system: a reliable clock from less reliable neurons

The mammalian SCN contains a biological clock that drives remarkably precise circadian rhythms in vivo and in vitro. This study asks whether the cycle-to-cycle variability of behavioral rhythms in mice can be attributed to precision of individual circadian pacemakers within the SCN or their interactions. The authors measured the standard deviation of the cycle-to-cycle period from 7-day recordings of running wheel activity, Period1 gene expression in cultured SCN explants, and firing rate patterns of dispersed SCN neurons. Period variability of the intact tissue and animal was lower than single neurons. The median variability of running wheel and Period1 rhythms was less than 40 min per cycle compared to 2.1 h in firing rate rhythms of dispersed SCN neurons. The most precise SCN neuron, with a period deviation of 1.1 h, was 10 times noisier than the most accurate SCN explant (0.1 h) or mouse (0.1 h) but comparable to the least stable explant (2.1 h) and mouse (1.1 h). This variability correlated with intrinsic period in mice and SCN explants but not with single cells. Precision was unrelated to the amplitude of rhythms and did not change significantly with age up to 1 year after birth. Analysis of the serial correlation of cycle-to-cycle period revealed that approximately half of this variability is attributable to noise outside the pacemaker. These results indicate that cell-cell interactions within the SCN reduce pacemaker noise to determine the precision of circadian rhythms in the tissue and in behavior.

Effects of age on circadian rhythms are similar in wild-type and heterozygous Clock mutant mice

The amplitudes of many circadian rhythms, at the behavioral, physiological, cellular, and biochemical levels, decrease with advanced age. Previous studies suggest that the amplitude of the central circadian pacemaker is decreased in old animals. Recently, it has been reported that expression of several circadian clock genes, including Clock, is lower in the master circadian pacemaker of old rodents. To test the hypothesis that decreased activity of a circadian clock gene renders animals more susceptible to the effects of aging, we analyzed the circadian rhythm of locomotor activity in young and old wild-type and heterozygous Clock mutant mice. We found that the effects of age and the Clock mutation were additive. These results indicate that age-related changes in circadian rhythmicity occur equally in wild-type and heterozygous Clock mutants, suggesting that the Clock mutation does not render mice more susceptible to the effects of age on the circadian pacemaker.

Age-related changes in circadian responses to dark pulses

Aging involves many alterations in circadian rhythms, including a loss of sensitivity to both photic and nonphotic time signals. This study investigated the sensitivity of young and old hamsters to the phase advancing effect of a 6-h dark pulse on the locomotor activity rhythm. Each hamster was tested four times during a period of approximately 9 mo; periods of exposure to a 14-h photoperiod were alternated with the periods of exposure to constant light (20-80 lx), during which the dark pulses were administered. There was no significant difference in the phase shifts exhibited by the young (4-10 mo) and old hamsters (19-25 mo) or in the amount of wheel running activity displayed during each dark pulse. However, young hamsters had a significantly greater propensity to exhibit split rhythms immediately after the dark pulses. These results suggest that, although aging does not reduce the sensitivity of the circadian pacemaker to this nonphotic signal, it alters one property of the pacemaker, i.e., the flexibility of the coupling of its component oscillators.