Alec J. Davidson

Chronic jet-lag increases mortality in aged mice

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Is the food-entrainable circadian oscillator in the digestive system?

Food‐anticipatory activity (FAA) is the increase in locomotion and core body temperature that precedes a daily scheduled meal. It is driven by a circadian oscillator but is independent of the suprachiasmatic nuclei. Recent results that reveal meal‐entrained clock gene expression in rat and mouse peripheral organs raise the intriguing possibility that the digestive system is the site of the feeding‐entrained oscillator (FEO) that underlies FAA. We tested this possibility by comparing FAA and Per1 rhythmicity in the digestive system of the Peru200a1‐luciferase transgenic rat. First, rats were entrained to daytime restricted feeding (RF, 10u2003days), then fed ad libitum (AL, 10u2003days), then food deprived (FD, 2u2003days). As expected FAA was evident during RF and disappeared during subsequent AL feeding, but returned at the correct phase during deprivation. The phase of Per1 in liver, stomach and colon shifted from a nocturnal to a diurnal peak during RF, but shifted back to nocturnal phase during the subsequent AL and remained nocturnal during food deprivation periods. Second, rats were entrained to two daily meals at zeitgeber time (ZT) 0400 and ZT 1600. FAA to both meals emerged after about 10u2003days of dual RF. However, all tissues studied (all five liver lobes, esophagus, antral stomach, body of stomach, colon) showed entrainment consistent with only the night‐time meal. These two results are inconsistent with the hypothesis that FAA arises as an output of rhythms in the gastrointestinal (GI) system. The results also highlight an interesting diversity among peripheral oscillators in their ability to entrain to meals and the direction of the phase shift after RF ends.

Birds of a feather clock together--sometimes: social synchronization of circadian rhythms.

Biological systems use internal circadian clocks to efficiently organize physiological and behavioral activity within the 24-hour time domain. In the absence of time cues, circadian periods vary slightly from 24 hours, but in nature, ambient light serves as the most salient synchronizer for these rhythms, fine-tuning them to exactly 24 hours each day. For some species, social cues can serve to synchronize circadian rhythms in the absence of other time cues or to amplify ambiguous light cues. This has been demonstrated to various degrees in fruit flies, degus, birds, fish, bats, beavers and humans; however, studies in rats and hamsters have shown that social cues are less salient time cues for these species. Social influences on circadian timing might function to tightly organize the social group, thereby decreasing the chances of predation and increasing the likelihood of mating.

Resetting of central and peripheral circadian oscillators in aged rats.

The mammalian circadian timing system is affected by aging. Analysis of the suprachiasmatic nucleus (SCN) and of other circadian oscillators reveals age-related changes which are most profound in extra-SCN tissues. Some extra-SCN oscillators appear to stop oscillating in vivo or display altered phase relationships. To determine whether the dynamic behavior of circadian oscillators is also affected by aging we studied the resetting behavior of the Period1 transcriptional rhythm of peripheral and central oscillators in response to a 6h advance or delay in the light schedule. We employed a transgenic rat with a luciferase reporter to allow for real-time measurements of transcriptional rhythmicity. While phase resetting in the SCN following an advance or a delay of the light cycle appears nearly normal in 2-year-old rats, resynchronization of the liver was seriously disrupted. In addition, the arcuate nucleus and pineal gland exhibited faster resetting in aged rats relative to 4-8-month-old controls. The consequences of these deficits are unknown, but may contribute to organ and brain diseases in the aged as well as the health problems that are common in older shift-workers.

The Methamphetamine-Sensitive Circadian Oscillator (MASCO) in Mice:

The suprachiasmatic nucleus (SCN) orchestrates synchrony among many peripheral oscillators and is required for circadian rhythms of locomotor activity and many physiological processes. However, the unique effects of methamphetamine (MAP) on circadian behavior suggest the presence of an SCN-independent, methamphetamine-sensitive circadian oscillator (MASCO). Substantial data collected using rat models show that chronic methamphetamine dramatically lengthens circadian period of locomotor activity rhythms and induces rhythms in animals lacking an SCN. However, the anatomical substrate and the molecular components of the MASCO are unknown. The response to MAP is less well studied in mice, a model that would provide the genetic tools to probe the molecular components of this extra-SCN oscillator. The authors tested the effects of chronic MAP on 2 strains of intact and SCN-lesioned mice in constant dark and constant light. Furthermore, they applied various MAP availability schedules to SCN-lesioned mice to confirm the circadian nature of the underlying oscillator. The results indicate that this oscillator has circadian properties. In intact mice, the MASCO interacts with the SCN in a manner that is strain, sex, and dose dependent. In SCN-lesioned mice, it induces robust free-running locomotor rhythmicity, which persists for up to 14 cycles after methamphetamine is withdrawn. In the future, localization of the MASCO and characterization of its underlying molecular mechanism, as well as its interactions with other oscillators in the body, will be essential to a complete understanding of the organization of the mammalian circadian system.

Cardiovascular tissues contain independent circadian clocks.

Acute cardiovascular events exhibit a circadian rhythm in the frequency of occurrence. The mechanisms underlying these phenomena are not yet fully understood, but they may be due to rhythmicity inherent in the cardiovascular system. We have begun to characterize rhythmicity of the clock gene mPer1 in the rat cardiovascular system. Luciferase activity driven by the mPer1 gene promoter is rhythmic in vitro in heart tissue explants and a wide variety of veins and arteries cultured from the transgenic Per1-luc rat. The tissues showed between 3 and 12 circadian cycles of gene expression in vitro before damping. Whereas peak per1-driven bioluminescence consistently occurred during the late night in the heart and all arteries sampled, the phases of the rhythms in veins varied significantly by anatomical location. Varying the time of the culture procedure relative to the donor animals light:dark cycle revealed that, unlike some other rat tissues such as liver, the phases of in vitro rhythms of arteries, veins, and heart explants were affected by culture time. However, phase relationships among tissues were consistent across culture times; this suggests diversity in circadian regulation among components of the cardiovascular system.

Daily oscillations in liver function: diurnal vs circadian rhythmicity

Abstract: The rodent suprachiasmatic nucleus (SCN), a site in the brain that contains a light‐entrained biological (circadian) clock, has been thought of as the master oscillator, regulating processes as diverse as cell division, reproductive cycles, sleep, and feeding. However, a second circadian system exists that can be entrained by meal feeding and has an influence over metabolism and behavior. Recent advances in the molecular genetics of circadian clocks are revealing clock characteristics such as rhythmic clock gene expression in a variety of non‐neural tissues such as liver. Although little is known regarding the function of these clock genes in the liver, there is a large literature that addresses the capabilities of this organ to keep time. This time‐keeping capability may be an adaptive function allowing for the prediction of mealtime and therefore improved digestion and energy usage. Consequently, an understanding of these rhythms is of great importance. This review summarizes the results of studies on diurnal and circadian rhythmicity in the rodent liver. We hope to lend support to the hypothesis that there are functionally important circadian clocks outside of the brain that are not light‐ or SCN‐dependent. Rather, these clocks are largely responsive to stimuli involved in nutrient intake. The interaction between these two systems may be very important for the ability of organisms to synchronize their internal physiology.

Food-anticipatory activity and liver per1-luc activity in diabetic transgenic rats.

The mammalian Per1 gene is an important component of the core cellular clock mechanism responsible for circadian rhythms. The rodent liver and other tissues rhythmically express Per1 in vitro but typically damp out within a few cycles. In the liver, the peak of this rhythm occurs in the late subjective night in an ad lib-fed rat, but will show a large phase advance in response to restricted availability of food during the day. The relationship between this shift in the liver clock and food-anticipatory activity (FAA), the circadian behavior entrained by daily feeding, is currently unknown. Insulin is released during feeding in mammals and could serve as an entraining signal to the liver. To test the role of insulin in the shift in liver Per1 expression and the generation of FAA, per-luciferase transgenic rats were made diabetic with a single injection of streptozotocine. Following 1 week of restricted feeding and locomotor activity monitoring, liver was collected for per-luc recording. In two separate experiments, FAA emerged and liver Per1 phase-shifted in response to daytime 8-h food restriction. The results rule out insulin as a necessary component of this system.

Interferon-γ Alters Electrical Activity and Clock Gene Expression in Suprachiasmatic Nucleus Neurons

The proinflammatory cytokine interferon (IFN-γ) is an immunomodulatory molecule released by immune cells. It was originally described as an antiviral agent but can also affect functions in the nervous system including circadian activity of the principal mammalian circadian pacemaker, the suprachiasmatic nucleus. IFN-γ and the synergistically acting cytokine tumor necrosis factor-α acutely decrease spontaneous excitatory postsynaptic activity and alter spiking activity in tissue preparations of the SCN. Because IFN-γ can be released chronically during infections, the authors studied the long-term effects of IFN-γ on SCN neurons by treating dispersed rat SCN cultures with IFN-γ over a 4-week period. They analyzed the effect of the treatment on the spontaneous spiking pattern and rhythmic expression of the “clock gene,” Period 1. They found that cytokine-treated cells exhibited a lower average spiking frequency and displayed a more irregular firing pattern when compared with controls. Furthermore, long-term treatment with IFN-γ in cultures obtained from a transgenic Per1-luciferase rat significantly reduced the Per1-luc rhythm amplitude in individual SCN neurons. These results show that IFN-γ can alter the electrical properties and circadian clock gene expression in SCN neurons. The authors hypothesize that IFN-γ can modulate circadian output, which may be associated with sleep and rhythm disturbances observed in certain infections and in aging.

Thermochron iButtons: An inexpensive method for long-term recording of core body temperature in untethered animals

In recent years, radiotelemetry has provided the best method for chronic recording of core body temperature in small laboratory animals. It offers high resolution with respect to both temperature and time, as well as unlimited data collection capacity, and can provide an index of locomotor activity without a running wheel or other apparatus. Disadvantages of this method include the high setup cost, the requirement of a receiver and dedicated PC, and an inability to record from multiple animals housed together. Here we report our initial experiences with an inexpensive yet effective method for studies in which chronic core temperature data are required. Thermochron iButtons (DS1921H-F50; Maxim/Dallas Semiconductor, Dallas, TX; http://www.ibutton. com) (Fig. 1B, inset) sell for less than U.S.