Temperature rhythm reentrains faster than locomotor rhythm after a light phase shift

Chronophysiology of domestic animals

This review highlights recent findings on biological rhythms and discusses their implications for the management and production of domestic animals. Biological rhythms provide temporal coordination between organs and tissues in order to anticipate environmental changes, orchestrating biochemical, physiological and behavioural processes as the right process may occur at the right time. This allows animals to adapt their internal physiological functions, such as sleep-wake cycles, body temperature, hormone secretion, food intake and regulation of physical performance to environmental stimuli that constantly change. The study and evaluation of biological rhythms of various physiological parameters allows the assessment of the welfare status of animals. Alteration of biological rhythms represents an imbalance of the state of homeostasis that can be found in different management conditions.

Lack of M4 muscarinic receptors in the striatum, thalamus and intergeniculate leaflet alters the biological rhythm of locomotor activity in mice

The deletion of M₄ muscarinic receptors (MRs) changes biological rhythm parameters in females. Here, we searched for the mechanisms responsible for these changes. We performed biological rhythm analysis in two experiments: in experiment 1, the mice [C57Bl/6NTac (WT) and M₄ MR -/- mice (KO)] were first exposed to a standard LD regime (12/12-h light/dark cycle) for 8 days and then subsequently exposed to constant darkness (for 24 h/day, DD regime) for another 16 days. In experiment 2, the mice (after the standard LD regime) were exposed to the DD regime and to one light pulse (zeitgeber time 14) on day 9. We also detected M₁ MRs in brain areas implicated in locomotor biological rhythm regulation. In experiment 1, the biological rhythm activity curves differed: the period (τ, duration of diurnal cycle) was shorter in the DD regime. Moreover, the day mean, mesor (midline value), night mean and their difference were higher in KO animals. The time in which the maximal slope occurred was lower in the DD regime than in the LD regime in both WT and KO but was lower in KO than in WT mice. In experiment 2, there were no differences in biological rhythm parameters between WT and KO mice. The densities of M₁ MRs in the majority of areas implicated in locomotor biological rhythm were low. A significant amount of M₁ MR was found in the striatum. These results suggest that although core clock output is changed by M₄ MR deletion, the structures involved in biological rhythm regulation in WT and KO animals are likely the same, and the most important areas are the striatum, thalamus and intergeniculate leaflet.

Antidepressants with different mechanisms of action show different chronopharmacological profiles in the tail suspension test in mice

The circadian system regulates sleep/wake cycles, metabolism, mood, and other functions. It also influences medication efficacy. In this study, we studied the chronopharmacological profiles of antidepressants with various modes of action. We also investigated the effects of dosing time on the pharmacological activity of several antidepressants acting on serotonergic, noradrenergic, and/or dopaminergic neurons. C57BL/6 mice were intraperitoneally administered fluoxetine, imipramine, venlafaxine, or bupropion at 08:00 h (morning), 14:00 h (mid-day), 20:00 h (evening), or 02:00 h (mid-night). Antidepressant activity was evaluated by the tail suspension test. All antidepressants reduced immobility, and their activities varied according to the dosing time. Fluoxetine and imipramine induced relatively strong rhythms with high amplitudes. Their maximal effects were observed in the morning and evening, respectively. Venlafaxine and bupropion induced weak rhythms with maximal effects in the evening and dawn, respectively. These results suggest that the antidepressant activity is associated with circadian fluctuation, and antidepressants with different modes of action have different chronopharmacological profiles. They affect locomotor activity in animals placed in novel (unfamiliar) environments. Fluoxetine, imipramine, and venlafaxine reduced locomotor activity, whereas bupropion increased it. The effects on locomotor activity also vary with circadian rhythm, and the tested drugs showed a maximal effect during the light phase. The peak time was different from that in TST. Plasma and brain levels of all drugs were slightly higher in the morning than in the evening. The dosing time dependency of the antidepressant activity did not correlate with the sedative/stimulatory activity or tissue drug level. Therefore, these latter two factors may have only a small impact on circadian antidepressant activity fluctuations. The relative activity of the serotonergic, noradrenergic, and dopaminergic systems may determine the chronopharmacological profiles of each drug. These results suggest the possibility that drug therapy be optimized by considering the dosing time when the antidepressant activity is high and other pharmacological activities leading to adverse effects are low. Further studies using animal models of depression and in clinical settings are necessary to confirm the effects of dosing time on depressed subjects.

The touchscreen operant platform for testing learning and memory in rats and mice

An increasingly popular method of assessing cognitive functions in rodents is the automated touchscreen platform, on which a number of different cognitive tests can be run in a manner very similar to touchscreen methods currently used to test human subjects. This methodology is low stress (using appetitive rather than aversive reinforcement), has high translational potential and lends itself to a high degree of standardization and throughput. Applications include the study of cognition in rodent models of psychiatric and neurodegenerative diseases (e.g., Alzheimer's disease, schizophrenia, Huntington's disease, frontotemporal dementia), as well as the characterization of the role of select brain regions, neurotransmitter systems and genes in rodents. This protocol describes how to perform four touchscreen assays of learning and memory: visual discrimination, object-location paired-associates learning, visuomotor conditional learning and autoshaping. It is accompanied by two further protocols (also published in this issue) that use the touchscreen platform to assess executive function, working memory and pattern separation.

The touchscreen operant platform for testing working memory and pattern separation in rats and mice

The automated touchscreen operant chamber for rats and mice allows for the assessment of multiple cognitive domains within the same testing environment. This protocol presents the location discrimination (LD) task and the trial-unique delayed nonmatching-to-location (TUNL) task, which both assess memory for location. During these tasks, animals are trained to a predefined criterion during ∼20-40 daily sessions. In LD sessions, touching the same location on the screen is rewarded on consecutive trials, followed by a reversal of location-reward contingencies. TUNL, a working memory task, requires animals to 'nonmatch' to a sample location after a delay. In both the LD and TUNL tasks, spatial similarity can be varied, allowing assessment of pattern separation ability, a function that is thought to be performed by the dentate gyrus (DG). These tasks are therefore particularly useful in animal models of hippocampal, and specifically DG, function, but they additionally permit discernment of changes in pattern separation from those in working memory.

Chronic phase advance alters circadian physiological rhythms and peripheral molecular clocks

Shifting the onset of light, acutely or chronically, can profoundly affect responses to infection, tumor progression, development of metabolic disease, and mortality in mammals. To date, the majority of phase-shifting studies have focused on acute exposure to a shift in the timing of the light cycle, whereas the consequences of chronic phase shifts alone on molecular rhythms in peripheral tissues such as skeletal muscle have not been studied. In this study, we tested the effect of chronic phase advance on the molecular clock mechanism in two phenotypically different skeletal muscles. The phase advance protocol (CPA) involved 6-h phase advances (earlier light onset) every 4 days for 8 wk. Analysis of the molecular clock, via bioluminescence recording, in the soleus and flexor digitorum brevis (FDB) muscles and lung demonstrated that CPA advanced the phase of the rhythm when studied immediately after CPA. However, if the mice were placed into free-running conditions (DD) for 2 wk after CPA, the molecular clock was not phase shifted in the two muscles but was still shifted in the lung. Wheel running behavior remained rhythmic in CPA mice; however, the endogenous period length of the free-running rhythm was significantly shorter than that of control mice. Core body temperature, cage activity, and heart rate remained rhythmic throughout the experiment, although the onset of the rhythms was significantly delayed with CPA. These results provide clues that lifestyles associated with chronic environmental desynchrony, such as shift work, can have disruptive effects on the molecular clock mechanism in peripheral tissues, including both types of skeletal muscle. Whether this can contribute, long term, to increased incidence of insulin resistance/metabolic disease requires further study.

Internal desynchronization facilitates seizures

PURPOSE

The occurrence of spike-wave discharges (SWDs) in WAG/Rij rats is modulated by the circadian timing system and is shaped by the presence of a light-dark cycle, motor activity, and state of vigilance. Here it is investigated whether the response to a phase shift is different between the SWDs and general motor activity rhythm. The process of reentrainment of both rhythms and its effect on number of absences was compared after a phase shift in the light-dark cycle, a condition known to induce internal desynchronization in the circadian timing system.

METHODS

Chronic electroencephalographic and motor activity recordings were made in adult WAG/Rij rats, kept in the 12:12 h light-dark cycle. After four baseline days, rats were exposed to an 8-h phase delay by shifting the light onset. Recordings were continuously made for another 10 consecutive days.

KEY FINDINGS

An immediate effect of the phase shift on both rhythms was observed: the acrophases were 7.5 h advanced. Next, they gradually returned to the baseline level, however, with a different speed. The more robust motor activity rhythm stabilizes first, whereas the weaker rhythm of SWDs adapted more slowly. The phase shift caused a prolonged aggravation of epileptic activity, observed mostly during the light phase.

SIGNIFICANCE

Different speed and character of reentrainment suggests that the occurrence of seizures and motor activity are controlled by distinct circadian oscillators. The prolonged increase in absences after the phase shift has immediate practical consequences.

Impairment of heme biosynthesis induces short circadian period in body temperature rhythms in mice

It has been demonstrated that the function of mammalian clock gene transcripts is controlled by the binding of heme in vitro. To examine the effects of heme on biological rhythms in vivo, we measured locomotor activity (LA) and core body temperature (Tb) in a mouse model of porphyria with impaired heme biosynthesis by feeding mice a griseofulvin (GF)-containing diet. Mice fed with a 2.0% GF-containing diet (GF2.0) transiently exhibited phase advance or phase advance-like phenomenon by 1–3 h in terms of the biological rhythms of Tbor LA, respectively (both, P < 0.05) while mice were kept under conditions of a light/dark cycle (12 h:12 h). We also observed a transient, ∼0.3 h shortening of the period of circadian Tbrhythms in mice kept under conditions of constant darkness ( P < 0.01). Interestingly, the observed duration of abnormal circadian rhythms in GF2.0 mice lasted between 1 and 3 wk after the onset of GF ingestion; this finding correlated well with the extent of impairment of heme biosynthesis. When we examined the effects of therapeutic agents for acute porphyria, heme, and hypertonic glucose on the pathological status of GF2.0 mice, it was found that the intraperitoneal administration of heme (10 mg·kg−1·day−1) or glucose (9 g·kg−1·day−1) for 7 days partially reversed (50%) increases in urinary δ-aminolevulinic acids levels associated with acute porphyria. Treatment with heme, but not with glucose, suppressed the phase advance (-like phenomenon) in the diurnal rhythms ( P < 0.05) and restored the decrease of heme ( P < 0.01) in GF2.0 mice. These results suggest that impairments of heme biosynthesis, in particular a decrease in heme, may affect phase and period of circadian rhythms in animals.

Scheduled Food Hastens Re-Entrainment More Than Melatonin Does after a 6-h Phase Advance of the Light-Dark Cycle in Rats

Circadian desynchrony occurs when individuals are exposed to abrupt phase shifts of the light-dark cycle, as in jet lag. For reducing symptoms and for speeding up resynchronization, several strategies have been suggested, including scheduled exercise, exposure to bright light, drugs, and especially exogenous melatonin administration. Restricted feeding schedules have shown to be powerful entraining signals for metabolic and hormonal daily cycles, as well as for clock genes in tissues and organs of the periphery. This study explored in a rat model of jet lag the contribution of exogenous melatonin or scheduled feeding on the re-entrainment speed of spontaneous general activity and core temperature after a 6-h phase advance of the light-dark cycle. In a first phase, the treatment was scheduled for 5 days prior to the phase shift, while in a second stage, the treatment was simultaneous with the phase advance of the light-dark cycle. Melatonin administration and especially scheduled feeding simultaneous with the phase shift improved significantly the re-entrainment speed. The evaluation of the free-running activity and temperature following the 5-day treatment proved that both exogenous melatonin and specially scheduled feeding accelerated re-entrainment of the SCN-driven general activity and core temperature, respectively, with 7, 5 days ( p < 0.01) and 3, 3 days ( p < 0.001). The present results show the relevance of feeding schedules as entraining signals for the circadian system and highlight the importance of using them as a strategy for preventing internal desynchrony.

[Biological rhythms for anaesthesia and intensive care]

Knowledge of biological rhythms has led to better understanding of the time-of-day dependent effects of anaesthetic drugs. These chronopharmacological effects are currently explained by the biological rhythms modulating the pharmacokinetic, toxic and pharmacodynamic parameters of these substances. Such effect has been described for general anesthetics, local anaesthetics, analgesics as well as for antibiotics. But recent data also highlight that general anaesthetics, probably part of their brain effects, also alter the regulation of biological rhythms, including the sleep-wake or the endogenous circadian temperature rhythms. This desynchronization of biological rhythms can led to disturbance of the circadian secretion of many substances, including hormones. Finally, biological rhythms have been also described with regard to physiology of pain and cardiovascular physiopathology. The concept of biological rhythm should be present in mind not only for the clinical management of patients but also for setting studies in the field of anaesthesia, pain and intensive care.

Chronobiology and the horse: recent revelations and future directions

The circadian system provides animals with a means to adapt their internal physiology to the constantly changing environmental stimuli that exist on a rotating planet. Light information is translated into molecular timing mechanisms within pacemaker cells of the mammalian hypothalamic suprachiasmatic nucleus (SCN) via transcriptional-translational feedback loops. Humoral and neural outputs from this 'master' clock result in circadian rhythms of physiology and behaviour. The larger circadian system involves SCN synchronisation of cellular clocks throughout the organism such that individual organs can adapt their specific function to the time of day. In the short history of this scientific field, the vast majority of mammalian chronobiological research has been conducted using small laboratory animals. This review examines what these studies have revealed, discusses how recent chronobiological findings in the horse compare to what is known and highlights how the principles of circadian biology are applicable to equine husbandry and veterinary care.

Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light/dark cycle in the horse

Background

Rapid displacement across multiple time zones results in a conflict between the new cycle of light and dark and the previously entrained program of the internal circadian clock, a phenomenon known as jet lag. In humans, jet lag is often characterized by malaise, appetite loss, fatigue, disturbed sleep and performance deficit, the consequences of which are of particular concern to athletes hoping to perform optimally at an international destination. As a species renowned for its capacity for athletic performance, the consequences of jet lag are also relevant for the horse. However, the duration and severity of jet lag related circadian disruption is presently unknown in this species. We investigated the rates of re-entrainment of serum melatonin and core body temperature (BT) rhythms following an abrupt 6-h phase advance of the LD cycle in the horse.

Methods

Six healthy, 2 yr old mares entrained to a 12 h light/12 h dark (LD 12:12) natural photoperiod were housed in a light-proofed barn under a lighting schedule that mimicked the external LD cycle. Following baseline sampling on Day 0, an advance shift of the LD cycle was accomplished by ending the subsequent dark period 6 h early. Blood sampling for serum melatonin analysis and BT readings were taken at 3-h intervals for 24 h on alternate days for 11 days. Disturbances to the subsequent melatonin and BT 24-h rhythms were assessed using repeated measures ANOVA and analysis of Cosine curve fitting parameters.

Results

We demonstrate that the equine melatonin rhythm re-entrains rapidly to a 6-h phase advance of an LD12:12 photocycle. The phase shift in melatonin was fully complete on the first day of the new schedule and rhythm phase and waveform were stable thereafter. In comparison, the advance in the BT rhythm was achieved by the third day, however BT rhythm waveform, especially its mesor, was altered for many days following the LD shift.

Conclusion

Aside from the temperature rhythm disruption, rapid resynchronization of the melatonin rhythm suggests that the central circadian pacemaker of the horse may possess a particularly robust entrainment response. The consequences for athletic performance remain unknown.