Acute systemic inflammation transiently synchronizes clock gene expression in equine peripheral blood

One Health: Circadian Medicine Benefits Both Non-human Animals and Humans Alike

Circadian biology's impact on human physical health and its role in disease development and progression is widely recognized. The forefront of circadian rhythm research now focuses on translational applications to clinical medicine, aiming to enhance disease diagnosis, prognosis, and treatment responses. However, the field of circadian medicine has predominantly concentrated on human healthcare, neglecting its potential for transformative applications in veterinary medicine, thereby overlooking opportunities to improve non-human animal health and welfare. This review consists of three main sections. The first section focuses on the translational potential of circadian medicine into current industry practices of agricultural animals, with a particular emphasis on horses, broiler chickens, and laying hens. The second section delves into the potential applications of circadian medicine in small animal veterinary care, primarily focusing on our companion animals, namely dogs and cats. The final section explores emerging frontiers in circadian medicine, encompassing aquaculture, veterinary hospital care, and non-human animal welfare and concludes with the integration of One Health principles. In summary, circadian medicine represents a highly promising field of medicine that holds the potential to significantly enhance the clinical care and overall health of all animals, extending its impact beyond human healthcare.

Persistence of clock gene expression in peripheral blood in dogs maintained under different photoperiod schedules

Dogs are the common pets adopted by humans, and their circadian behavior and physiology are influenced by human habits. In many families, there is a change of lifestyle with respect to the natural daylight (NDL) cycle. Exposure to constant light disrupts some central and peripheral circadian rhythms. The aim of the present study was to improve the knowledge about the circadian changes of clock components in the peripheral blood in dogs housed under NDL and constant light (LL) conditions. Blood samples were collected on five female Beagle dogs (2 years old, 14 ± 0.5 kg) every 4 hours for a 24-hour period during an NDL (Sunrise 05:05 h - Sunset 20:55  h) and 24-hour period of constant light (LL). Blood samples were stored in a PAX gene Blood RNA Tube, real-time RT-quantitative polymerase chain reaction was performed to determine Clock, Per1-3, and Cry1-2 gene expression. During the NDL, all genes investigated showed robust diurnal daily rhythmicity. During the constant light, only Clock maintained its daily rhythmicity. Clock acrophase was observed close to sunrise (ZT 0) and was statistically different from the other clock genes except for Per3. Per3 daily oscillations were not statistically significant. No differences were observed among the clock genes tested in the amplitude and robustness values. Our results can be considered preliminary data to provide new insights into the adaptation mechanism of the canine peripheral circadian clock. The persistence of Clock gene expression during the LL indicated the presence of an endogenously generated signal in blood. Because peripheral blood is an easily accessible sample in dogs, the analysis of clock gene expression in this tissue could be useful to investigate the adaptive capacity of this species housed in different environmental conditions linked to the owner's lifestyle.

Real-Time Monitoring of Circadian Rhythms in the Eye

The mammalian eye harbors a full circadian system that controls several physiologically relevant functions within this organ. During the last two decades a few laboratories have developed transgenic animal models in which circadian rhythms can be monitored in real time using luciferase activity. The most famous transgenic mouse to record bioluminescence rhythms from different tissues and organs is the PERIOD2::LUCIFERASE (PER2::LUC) mouse developed by the Takahashi laboratory in early 2000. Since then, several studies have used this mouse model to dissect the mammalian circadian system by monitoring the circadian rhythm in the brain, the eye, and in many other peripheral organs and tissues. This chapter describes the methodology to record and analyze bioluminescence rhythms from the retina, retinal pigment epithelium, and cornea of PER2::LUC mice.

Clock genes determination in whole blood in goats housed under a long light cycle

An innate 24 h circadian clock drives various behavioral processes via expression of clock genes that regulate circadian rhythmicity and temporal signals. Elucidating the gene expression in goats may contribute to improving the knowledge of the regulation of circadian rhythms in this species. Five nonpregnant and nonlactating Maltese goats with no evidence of disease were kept in an indoor pen under the natural long photoperiod (05:05-20:56 h) and natural environmental temperature (23°C and 60% RH). They were fed an Alfalfa hay and concentrate mixture provided twice a day; water was available ad libitum. Blood samples were collected every 4 h over a 48 h period into PAX gene Blood RNA Tubes and stored at -80°C until processing. Clock genes (Clock; Cry1; Cry2; Per2; Per3) were determined using real-time quantitative polymerase chain reaction. During the experimental period, locomotor activity was monitored by an actigraphy-based data logger that records a digitally integrated measure of motor activity as a means to assess indices of discomfort during study and stability of the circadian rhythm. All of the tested genes showed daily rhythmicity in their expression in whole blood. Differences in their circadian parameters were observed. Mesor and amplitude were statistically different among the tested gene (Mesor: F_(4.30) = 205.30; p < .0001; amplitude: F_(4.30) = 104.80; p < .0001), with each gene showing its acrophase at a different time of day (F_(4.30) = 81.17; p < .0001), and differences were observed between the two days of monitoring (F_(1.30) = 10.25; p = .003). The application of two-way analysis of variance (ANOVA) on robustness of rhythm values did not show statistical differences among the tested genes (F_(4.30) = 1.83; p = .14) and between the two days of monitoring (F_(1.30) = 1.16; p = .28). Locomotor activity data recording were in accordance with the data reported in literature, indicating the absence of discomfort or alteration of circadian rhythms during the experimental period. Our results support the presence of a cyclic transcription of clock genes in whole blood of healthy goats housed under a long light natural photoperiod and natural environmental conditions.

Circadian and Circannual Regulation in the Horse: Internal Timing in an Elite Athlete

Biological rhythms evolved to provide temporal coordination across all tissues and organs and allow synchronization of physiology with predictable environmental cycles. Most important of these are circadian and circannual rhythms, primarily regulated via photoperiod signals from the retina. Understanding the nature of physiological rhythms in horses is crucially important for equine management. Predominantly, they have been removed from exposure to their natural environmental stimuli; the seasonally changing photoperiod, continuous foraging and feeding activity, social herd interactions, and the continuous low-intensity exercise of a grassland dweller. These have been replaced in many cases with confined indoor housing, regimental feeding and exercise times, social isolation, and exposure to lighting that is often erratic and does not come close to mimicking the spectral composition of sunlight. Man has further altered seasonal timing cues through the use of artificial lighting programs that impact reproductive behavior, breeding efficiency, and the development of youngstock. Understanding how these new environmental cues (some stronger and some weaker) impact the internal physiology of the horse in the context of the natural endogenous rhythms that evolved over millennia is key to helping to improve equine health, welfare, and performance, now and into the future. This review provides an overview of the field, highlights the recent discoveries related to biological timing in horses, and discusses the implications that these findings may have for the production and management of the elite equine athlete.

Circadian rhythms of melatonin and behaviour in juvenile sheep in field conditions: Effects of photoperiod, environment and weaning

Entrainment of circadian rhythms (CR) to the light dark cycle has been well described under controlled, experimental conditions. However, studies in rodents have reported that rhythms in the laboratory are not always reproduced under field conditions. The aim of this study was to characterise the CR of sheep maintained under conditions of standard UK farm animal husbandry and to investigate the effects of environmental challenges presented by season, weaning and changes in housing on CR. Male sheep (n = 9) were kept at pasture, or group housed in barns, under natural photoperiod for one year. CR in locomotor activity were monitored using accelerometry, and 24 h patterns in plasma cortisol and melatonin were measured every 4 h by ELISA. CR was measured before and after weaning, in summer and winter, and at pasture and by barn housing. Cosinor analysis revealed high amplitude, diurnal rhythms in locomotor activity that were disrupted by weaning and by barn housing. Rhythms in winter showed an interrupted night time activity pattern, but only when the sheep were kept at pasture. Cortisol and melatonin secretion followed typical circadian patterns in winter and summer. The CR of the sheep under the field conditions of this study were strikingly robust under basal conditions, but easily disrupted by environmental challenges. Interrupted patterns of activity during the long nights of wintertime, not previously reported for sheep kept in experimental conditions were recorded. Based on these findings, we propose that animals require exposure to more complex environments than the laboratory in order to exhibit their true circadian phenotype.

Transcriptome Profiling of the Lungs Reveals Molecular Clock Genes Expression Changes after Chronic Exposure to Ambient Air Particles

The symptoms of asthma, breathlessness, insomnia, etc. all have relevance to pulmonary rhythmic disturbances. Epidemiology and toxicology studies have demonstrated that exposure to ambient air particles can result in pulmonary dysfunction. However, there are no data directly supporting a link between air pollution and circadian rhythm disorder. In the present study, we found that breathing highly polluted air resulted in changes of the molecular clock genes expression in lung by transcriptome profiling analyses in a rodent model. Compared to those exposed to filtered air, in both pregnant and offspring rats in the unfiltered group, key clock genes (Per1, Per2, Per3, Rev-erbα and Dbp) expression level decreased and Bmal1 expression level increased. In both rat dams and their offspring, after continuous exposure to unfiltered air, we observed significant histologic evidence for both perivascular and peribronchial inflammation, increased tissue and systemic oxidative stress in the lungs. Our results suggest that chronic exposure to particulate matter can induce alterations of clock genes expression, which could be another important pathway for explaining the feedbacks of ambient particle exposure in addition to oxidative stress and inflammation.

Clock gene expression in different synovial cells of patients with rheumatoid arthritis and osteoarthritis

Patients with rheumatoid arthritis (RA) show modulated circadian rhythms of inflammatory cytokines and cortisol, which may be associated with a modified expression of clock genes. The expression of major clock genes was previously studied in synovial tissues and fibroblasts of patients with RA and osteoarthritis (OA). We therefore especially aimed to examine the localization of clock genes at the cellular level in synovial tissue. Furthermore we were interested in studying the expression of the D site of albumin promoter (albumin D-box) binding protein (DBP) at the immunohistochemical level in human samples. Methods used include the in situ expression of the clock genes Brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (Bmal 1), Circadian Locomotor Output Cycles Kaput (Clock), Period 1 and 2 (Per 1 and Per 2), and DBP was examined by immunohistochemistry in synovial tissues of patients with RA or OA. Additionally, expression profiles of different clock genes were determined over 24h by real time PCR in synovial fibroblasts (SFs) after a 2h serum shock or TNF-α. Results show that all clock genes investigated were found to be expressed both in RA and OA synovial tissues. Double staining against cell specific markers revealed that clock proteins were especially seen in macrophages, SFs and B-lymphocytes. Cell counting showed that clock proteins were found in approximately 5-20% of cells. Additionally, preliminary cell culture experiments showed that TNF-α treatment resulted in differential 24h expression profiles between RA and OA samples and also compared to the results obtained from the serum shock experiments. From our study we conclude that the major clock genes, including DBP, are expressed in samples from RA and OA patients, especially in macrophages and synovial fibroblasts, but also in B-lymphocytes. Preliminary experiments suggest that TNF-α seems to be able to modify clock gene expression in synovial fibroblasts.

Time-dependent effects of localized inflammation on peripheral clock gene expression in rats

Many aspects of the immune system, including circulating cytokine levels as well as counts and function of various immune cell types, present circadian rhythms. Notably, the mortality rate of animals subjected to high doses of lipopolysaccharide is dependent on the time of treatment. In addition, the severity of symptoms of various inflammatory conditions follows a daily rhythmic pattern. The mechanisms behind the crosstalk between the circadian and immune systems remain elusive. Here we demonstrate that localized inflammation induced by turpentine oil (TURP) causes a time-dependent induction of interleukin (IL)-6 and has time-, gene- and tissue-specific effects on clock gene expression. More precisely, TURP blunts the peak of Per1 and Per2 expression in the liver while in other tissues, the expression nadir is elevated. In contrast, Rev-erbα expression remains relatively unaffected by TURP treatment. Co-treatment with the anti-inflammatory agent IL-1 receptor antagonist (IL-1Ra) did not alter the response of Per2 to TURP treatment in liver, despite the reduced induction of fever and IL-6 serum levels. This indicates that the TURP-mediated changes of Per2 in the liver might be due to factors other than systemic IL-6 and fever. Accordingly, IL-6 treatment had no effect on clock gene expression in HepG2 liver carcinoma cells. Altogether, we show that localized inflammation causes significant time-dependent changes in peripheral circadian clock gene expression, via a mechanism likely involving mediators independent from IL-6 and fever.

The circadian clock in cancer development and therapy

Most aspects of mammalian function display circadian rhythms driven by an endogenous clock. The circadian clock is operated by genes and comprises a central clock in the brain that responds to environmental cues and controls subordinate clocks in peripheral tissues via circadian output pathways. The central and peripheral clocks coordinately generate rhythmic gene expression in a tissue-specific manner in vivo to couple diverse physiological and behavioral processes to periodic changes in the environment. However, with the industrialization of the world, activities that disrupt endogenous homeostasis with external circadian cues have increased. This change in lifestyle has been linked to an increased risk of diseases in all aspects of human health, including cancer. Studies in humans and animal models have revealed that cancer development in vivo is closely associated with the loss of circadian homeostasis in energy balance, immune function, and aging, which are supported by cellular functions important for tumor suppression including cell proliferation, senescence, metabolism, and DNA damage response. The clock controls these cellular functions both locally in cells of peripheral tissues and at the organismal level via extracellular signaling. Thus, the hierarchical mammalian circadian clock provides a unique system to study carcinogenesis as a deregulated physiological process in vivo. The asynchrony between host and malignant tissues in cell proliferation and metabolism also provides new and exciting options for novel anticancer therapies.

Long-lasting effects of sepsis on circadian rhythms in the mouse

Daily patterns of activity and physiology are termed circadian rhythms and are driven primarily by an endogenous biological timekeeping system, with the master clock located in the suprachiasmatic nucleus. Previous studies have indicated reciprocal relationships between the circadian and the immune systems, although to date there have been only limited explorations of the long-term modulation of the circadian system by immune challenge, and it is to this question that we addressed ourselves in the current study. Sepsis was induced by peripheral treatment with lipopolysaccharide (5 mg/kg) and circadian rhythms were monitored following recovery. The basic parameters of circadian rhythmicity (free-running period and rhythm amplitude, entrainment to a light/dark cycle) were unaltered in post-septic animals compared to controls. Animals previously treated with LPS showed accelerated re-entrainment to a 6 hour advance of the light/dark cycle, and showed larger phase advances induced by photic stimulation in the late night phase. Photic induction of the immediate early genes c-FOS, EGR-1 and ARC was not altered, and neither was phase-shifting in response to treatment with the 5-HT-1a/7 agonist 8-OH-DPAT. Circadian expression of the clock gene product PER2 was altered in the suprachiasmatic nucleus of post-septic animals, and PER1 and PER2 expression patterns were altered also in the hippocampus. Examination of the suprachiasmatic nucleus 3 months after treatment with LPS showed persistent upregulation of the microglial markers CD-11b and F4/80, but no changes in the expression of various neuropeptides, cytokines, and intracellular signallers. The effects of sepsis on circadian rhythms does not seem to be driven by cell death, as 24 hours after LPS treatment there was no evidence for apoptosis in the suprachiasmatic nucleus as judged by TUNEL and cleaved-caspase 3 staining. Overall these data provide novel insight into how septic shock exerts chronic effects on the mammalian circadian system.

Circadian nature of immune function

The primary physiological role of the circadian system is to synchronize and coordinate organ systems, particularly in response to dynamics in the environment. The immune system is under direct circadian control by systemic cues and molecular clocks within immune cells. The master circadian pacemaker called the suprachiasmatic nucleus (SCN) conveys timing information to the immune system through endocrine and autonomic pathways. These signals promote phase coherence of peripheral clocks in the immune system, and also govern daily variations in immune function. The coordination of immune response may compose an anticipatory state for optimal immune response. Interactions between circadian and immune systems are bidirectional, in that immune factors can modulate phasing of circadian clocks. Circadian disruption, such as environmental desynchronization and/or anomalous molecular clock functions, may lead to lack of system coordination, and particular vulnerabilities to infection and disease may develop.

The effect of dexamethasone on clock gene mRNA levels in bovine neutrophils and lymphocytes

Circadian rhythms are driven by oscillating expression of a family of transcription factors called clock genes. In rodents, clock genes drive circadian rhythms in white blood cell function, and glucocorticoids are believed to regulate these rhythms. Little is known about circadian rhythms of cattle white blood cells. The objectives of this study were: (1) to quantify mRNA levels of clock genes in neutrophils and lymphocytes over 24h in healthy steers; and (2) to quantify effects of dexamethasone on clock gene mRNA levels in bovine neutrophils and lymphocytes. We hypothesized that bovine neutrophils and lymphocytes would display 24h variations in clock gene mRNA levels and that those patterns would be disrupted by glucocorticoid treatment. Six Holstein steers were injected with 0 or 0.10mg/kg body weight dexamethasone according to a crossover design. Neutrophils and lymphocytes were collected from jugular blood at 0, 4, 8, 12, 16, 20, and 24h relative to treatment administration. Neutrophil and lymphocyte mRNA levels of the clock genes Clock, Bmal1, Per1, Per2, Cry1, Cry2, Rev-erbα, and CK1ɛ were quantified. For neutrophils, an interaction between treatment and time was found for Clock, Cry1, and CK1ɛ. Time affected Clock, Per1, Cry1, Rev-erbα, and CK1ɛ. For all of those genes except Per1, neutrophils from control steers displayed 24h changes of mRNA levels characteristic of circadian regulated cells. The dexamethasone treatment increased neutrophil mRNA levels of Per1, decreased Clock, Cry1, Cry2, and Rev-erbα, and tended to decrease Bmal1. These results suggest that circadian rhythms have the potential to impact bovine neutrophil function, and that glucocorticoid-induced disruption of neutrophil circadian rhythms may contribute to periparturient immunosuppression. For lymphocytes, an interaction between treatment and time was observed for Per1 and tended to occur for Per2 and Cry2. Although time affected Per1 and Rev-erbα, distinct 24h patterns of lymphocyte clock gene mRNA levels were not evident as they were in neutrophils. Treatment increased Per1 and decreased Cry2, but the magnitude of the treatment effect was small. In summary, 24h patterns in clock gene mRNA levels were observed in bovine neutrophils and to some degree in lymphocytes, and these patterns were disrupted by dexamethasone administration. Although further research is needed, individual variation in white blood cell circadian rhythms and glucocorticoid responsiveness may help to explain individual differences in periparturient disease susceptibility.

Circadian dysfunction in disease

The classic view of circadian timing in mammals emphasizes a light-responsive 'master clock' within the hypothalamus which imparts temporal information to the organism. Recent work indicates that such a unicentric model of the clock is inadequate. Autonomous circadian timers have now been demonstrated in numerous brain regions and peripheral tissues in which molecular-clock machinery drives rhythmic transcriptional cascades in a tissue-specific manner. Clock genes also participate in reciprocal regulatory feedback with key signalling pathways (including many nuclear hormone receptors), thereby rendering the clock responsive to the internal environment of the body. This implies that circadian-clock genes can directly affect previously unforeseen physiological processes, and that amid such a network of body clocks, internal desynchronisation may be a key aspect to circadian dysfunction in humans. Here we consider the implications of decentralised and internally responsive clockwork to disease, with a focus on energy metabolism and the immune response.

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.

Validation of a reliable set of primer pairs for measuring gene expression by real-time quantitative RT-PCR in equine leukocytes

Quantification of gene expression using real-time reverse transcription quantitative PCR (RT-qPCR) is a reliable method to monitor cellular responses to pro-inflammatory stimuli. The main objective of this study was to validate a set of equine primer pairs that can be routinely used to monitor expression of genes that are central to inflammatory and immune responses. This paper describes the steps used to optimize and validate 29 equine primer pairs for RT-qPCR assays using SYBR Green detection. To validate these assays, monocytes were isolated from three horses and stimulated with Escherichia coli LPS. Because four of the 29 genes demonstrated poor amplification efficiency due to weak induction of gene expression by LPS, monocytes were stimulated with alternative agents (PMA and Poly I:C) known to induce gene expression in monocytes. These agents, acting through different pathways than LPS, improved the level of gene expression and yielded good amplification efficiency for these genes. PCR efficiency was based on a standard curve for each gene and the calculated efficiency was approximately 100% for all 29 genes. The PCR efficiencies for the majority of the target genes were equivalent to that of the housekeeping gene (18S rRNA) used in all experiments. Dissociation curve analysis and gel electrophoresis revealed a single product for each gene analyzed. To exemplify utilization of the validated primer pairs in studies of inflammatory cell activation, temporal changes in mRNA expression of a subset of genes were monitored in equine monocytes incubated with LPS. The availability of the 29 validated primer pairs reported herein will allow investigators to elucidate the response of horses to a variety of inflammatory stimuli and will further our understanding of disease pathogenesis in horses.

Neuroimmunology of the circadian clock

Circadian timekeeping is a ubiquitous feature of all eukaryotes which allows for the imposition of a biologically appropriate temporal architecture on an animal's physiology, behavior and metabolism. There is growing evidence that in mammals the processes of circadian timing are under the influence of the immune system. Such a role for the neuroimmune regulation of the circadian clock has inferences for phenomena such as sickness behavior. Conversely, there is also accumulating evidence for a circadian influence on immune function, raising the likelihood that there is a bidirectional communication between the circadian and immune systems. In this review, we examine the evidence for these interactions, including circadian rhythmicity in models of disease and immune challenge, distribution of cytokines and their receptors in the suprachiasmatic nucleus of the hypothalamus, the site of the master circadian pacemaker, and the evidence for endogenous circadian timekeeping in immune cells.

Validation of quantitative polymerase chain reaction assays for measuring cytokine expression in equine macrophages

The study of the equine immune system and inflammatory responses, by measuring cytokine expression, can provide important insight into disease pathogenesis in the horse. A set of quantitative real-time polymerase chain reaction (QPCR) assays for the equine cytokines IL-1alpha, IL-1beta, IL-6, IL-8 and TNF-alpha were validated using QPCR primers and probes which were generated for the equine IL-1alpha, IL-1beta, IL-6, IL-8, TNF-alpha and 18S genes. Amplification efficiency, intra-assay and inter-assay variation were determined using 10-fold dilutions of plasmid for each gene. Under these conditions the amplification efficiencies of the primers and probes ranged from 99% to 101%. The mean coefficient of variation (CV) across five sets of plasmid DNA for both intra-assay and inter-assay variation was 0.63% (range 0.2% to 1.8%). Amplification efficiency was also determined using 2-fold dilutions of cDNA and under these conditions amplification efficiency ranged from 83% to 95%. The specificity of amplification was confirmed by DNA sequencing of reaction products. The QPCR assays were also evaluated using three sets of cDNA from equine monocyte derived macrophages (EMDM) stimulated for 1 h with lipopolysaccharide (LPS). The general trend was the same for all three samples with IL-1alpha showing the greatest induction and IL-6 the lowest induction. The range of cytokine induction was greater than has previously been reported with values ranging from 12-fold to 30,000-fold. We present a set of QPCR primers and probes that are suitable for quantitation of expression of a set of equine cytokines. The primers and probes have been rigorously analyzed, and we demonstrate that they are specific for the desired genes, have a high amplification efficiency and the assays are highly reproducible.

Neuroendocrine-immune correlates of circadian physiology: studies in experimental models of arthritis, ethanol feeding, aging, social isolation, and calorie restriction

Virtually all neuroendocrine and immunological variables investigated in animals and humans display biological periodicity. Circadian rhythmicity is revealed for every hormone in circulation as well as for circulating immune cells, lymphocyte metabolism and transformability, cytokines, receptors, and adhesion molecules. Clock genes, notably the three Period (Per1/Per2/Per3) genes and two Cryptochrome (Cry1/Cry2) genes, are present in immune and endocrine cells and are expressed in a circadian manner in human cells. This review discusses the circadian disruption of hormone release and immune-related mechanisms in several animal models in which circulating cytokines are modified including rat adjuvant arthritis, social isolation in rats and rabbits and alcoholism, the aging process and calorie restriction in rats. In every case the experimental manipulation used perturbed the temporal organization by affecting the shape and amplitude of a rhythm or by modifying the intrinsic oscillatory mechanism itself.