Multiple ultradian rhythms of metabolism, body temperature and activity in Djungarian hamsters.

Djungarian hamsters (Phodopus sungorus) living at constant 15 °C Ta in short photoperiod (8:16 h L:D) showed pronounced ultradian rhythms (URs) of metabolic rate (MR), body temperature (Tb) and locomotor activity. The ultradian patterns differed between individuals and varied over time. The period length of URs for MR, Tb and activity was similar although not identical. Wavelet analysis showed that three different URs are existing in parallel, URs of small amplitude and short duration (URsmall), URs of medium amplitude and medium duration (URmedium) and URs of large amplitude (URlarge), superimposed on each other. URlarge were accompanied by an increase in locomotor activity, whereas URsmall and URmedium were of metabolic origin with lacking or delayed responses of activity. An energetic challenge to cold which raised total energy requirements by about 50% did not accelerate the period length of URs, but extended the amplitude of URsmall and URmedium. URlarge corresponds with the URs of activity, feeding and drinking, sleep and arousal as described in previous studies, which are related to midbrain dopaminergic signalling and hypothalamic ultradian signalling. The cause and control of URmedium and URsmall is unknown. Their periods are similar to periods of central and peripheral endocrine ultradian signalling, suggesting a link with URs of metabolism.

Silencing of ultradian rhythms and metabolic depression during spontaneous daily torpor in Djungarian hamsters.

Ultradian rhythms of metabolism, body temperature and activity are attenuated or disappear completely during torpor in Djungarian hamsters, for all three ultradian periodicities (URsmall, URmedium and URlarge). URsmall and URmedium disappear during entrance into torpor, whereas URlarge disappear later or continue with a low amplitude. This suggests a tight functional link between torpor and the expression of ultradian rhythms, i.e. torpor is achieved by suppression of metabolic rate as well as silencing of ultradian rhythms. Spontaneous torpor is often initiated after an ultradian burst of activity and metabolic rate, beginning with a period of motionless rest and accompanied by a decrease of metabolic rate and body temperature. To extend previous findings on the potential role of the adrenergic system on torpor induction we analysed the influence of the ß3-adrenergic agonist Mirabegron on torpor in Djungarian hamsters, as compared to the influence of the ß-adrenergic antagonist Propranolol. Hamsters were implanted with 10 day release pellets of Mirabegron (0.06 mg day-1) or Propranolol (0.3 mg day-1). Mirabegron transiently supressed and accelerated ultradian rhythms but had no effect on torpor behaviour. Propranolol did not affect torpor behaviour nor the expression of ultradian rhythms with the dosage applied during this study.

Association between Ultradian Rhythms of Body Temperature in Small Mammals and Earth's Crust Stress.

Association was assessed between the data harvested by a long-baseline laser interference deformograph and the dynamics of body temperature (BT) in hamsters deprived of natural daily light-darkness changes. The power spectral data revealed the positive correlation between simultaneous time series of hamster BT and the Earth's crust deformation (ECD). The superposed epoch analysis established an association between abrupt upstrokes of hamster BT and ECD increments. Thus, the direct relationships between BT dynamics (reflecting predominance of sympathetic part of autonomic nervous system) and ECD (according to long-baseline laser interference deformography) were established. The study observed synchronization of the free-running circadian rhythm of hamster BT with the tidal stress in Earth's lithosphere. Further studies are needed to find the physical factor underlying the revealed relationships.

Circadian and ultradian rhythms: Clinical implications.

The hypothalamic-pituitary-adrenal axis is an extremely dynamic system with a combination of both circadian and ultradian oscillations. This state of 'continuous dynamic equilibration' provides a platform that is able to anticipate events, is sensitive in its response to stressors, remains robust during perturbations of both the internal and external environments and shows plasticity to adapt to a changed environment. In this review, we describe these oscillations of glucocorticoid (GC) hormones and why they are so important for GC-dependent gene activation in the brain and liver, and their consequent effects on the regulation of synaptic and memory function as well as appetite control and metabolic regulation. Abnormalities of mood, appetite and metabolic regulation are well-known consequences of GC therapy, and we suggest that the pattern of GC treatment and hormone replacement should be a much higher priority for endocrinologists and the pharmaceutical industry. One of the major impediments to our research on the importance of these cortisol rhythms in our patients has been our inability to measure repeated levels of hormones across the day in patients in their home or work surroundings. We describe how new wearable methodologies now allow the measurement of 24-h cortisol profiles - including during sleep - and will enable us to define physiological normality and allow us both to develop better diagnostic tests and inform, at an individual patient level, how to improve replacement therapy.

A mathematical model for the role of dopamine-D2 self-regulation in the production of ultradian rhythms.

Many self-motivated and goal-directed behaviours display highly flexible, approximately 4 hour ultradian (shorter than a day) oscillations. Despite lacking direct correspondence to physical cycles in the environment, these ultradian rhythms may be involved in optimizing functional interactions with the environment and reflect intrinsic neural dynamics. Current evidence supports a role of mesostriatal dopamine (DA) in the expression and propagation of ultradian rhythmicity, however, the biochemical processes underpinning these oscillations remain to be identified. Here, we use a mathematical model to investigate D2 autoreceptor-dependent DA self-regulation as the source of ultradian behavioural rhythms. DA concentration at the midbrain-striatal synapses is governed through a dual-negative feedback-loop structure, which naturally gives rise to rhythmicity. This model shows the propensity of striatal DA to produce an ultradian oscillation characterized by a flexible period that is highly sensitive to parameter variations. Circadian (approximately 24 hour) regulation consolidates the ultradian oscillations and alters their response to the phase-dependent, rapid-resetting effect of a transient excitatory stimulus. Within a circadian framework, the ultradian rhythm orchestrates behavioural activity and enhances responsiveness to an external stimulus. This suggests a role for the circadian-ultradian timekeeping hierarchy in governing organized behaviour and shaping daily experience through coordinating the motivation to engage in recurring, albeit not highly predictable events, such as social interactions.

Beyond day and night: The importance of ultradian rhythms in mouse physiology.

Our circadian world shapes much of metabolic physiology. In mice ∼40% of the light and ∼80% of the dark phase time is characterized by bouts of increased energy expenditure (EE). These ultradian bouts have a higher body temperature (Tb) and thermal conductance and contain virtually all of the physical activity and awake time. Bout status is a better classifier of mouse physiology than photoperiod, with ultradian bouts superimposed on top of the circadian light/dark cycle. We suggest that the primary driver of ultradian bouts is a brain-initiated transition to a higher defended Tb of the active/awake state. Increased energy expenditure from brown adipose tissue, physical activity, and cardiac work combine to raise Tb from the lower defended Tb of the resting/sleeping state. Thus, unlike humans, much of mouse metabolic physiology is episodic with large ultradian increases in EE and Tb that correlate with the active/awake state and are poorly aligned with circadian cycling.

Stress-free blood sampling in minipigs: A novel method for assessing 24-h cortisol profiles and drug effects on diurnal and ultradian rhythms.

We developed a novel, stress-free blood sampling method for minipigs, allowing continuous cortisol monitoring over 24 h. Baseline cortisol levels exhibited both ultradian and diurnal rhythms. During nighttime, smaller ultradian rhythms overlaid a lower baseline cortisol, which increased in sleeping pigs before lights were turned on. Additionally, we developed an analytical tool based on the R package "pracma" to quantify ultradian peak and circadian components of the cortisol profiles. To validate our model, we investigated the effects of Verucerfont, a CRH receptor antagonist, and Venlafaxine, a serotonin-norepinephrine reuptake inhibitor. Verucerfont reduced cortisol levels during the first 9 h without affecting diurnal rhythm. Cortisol peak parameters decreased, with a 31% reduction in overall area under the curve (AUC) and a 38% reduction in ultradian average AUC. Ultradian peaks decreased from 7 to 4.5, with 34% lower amplitude. Venlafaxine maintained plasma concentrations within the targeted human effective range. This method enables us to enhance our understanding of cortisol regulation and provide valuable insights for the impact of investigation drugs on the diurnal and ultradian rhythms of cortisol.

Phase Analysis of Ultradian Rhythms of Body Temperature in Laboratory Mice Maintained under Constant Illumination at Different Longitudinal Locations.

The study examined the rhythmic oscillations of body temperature with the period ranging 100-400 min in three groups of laboratory mice maintained under persistent artificial illumination in Moscow and Ulyanovsk. The difference in the moments of sunrise or sunset in these towns is about 1 h. The greatest rhythmic oscillations of body temperature in examined mice had the periods of 100-400 min. The phase analysis of 100-200-min rhythms revealed their synchronicity with local but not universal time despite the mice had no photic signs indicating alternation of day and night. Of them, the most pronounced were the rhythms with the periods of 121, 143, 151, and 186 min. The present data suggest existence of an external environmental synchronizer of body temperature ultradian rhythms related to local solar time.

Amplitude of One-Minute Fluctuations of Secondary Cosmic Rays as a Marker of Environmental Factor Determining Ultradian Rhythms in Body Temperature of Laboratory Rats.

The study examined association between oscillations of body temperature of laboratory Wistar rats maintained under constant illumination with the amplitude of fluctuations of secondary cosmic rays reported by neutron count rate provided by neutron monitors and geomagnetic undulations. In contrast to geomagnetic undulations, neutron count rate variations and body temperature oscillations in rats assessed by spectrum analysis of the corresponding step functions at 1-min intervals demonstrated almost permanent variations with the periods ranging from 100 to 400 min. Under conditions of constant illumination inducing changes in the period of circadian rhythm and predominance of the ultradian rhythms, an association between neutron count rate fluctuations and body temperature oscillations was observed perpetually during the day- and nighttime.

Possible External Factors Determining Ultradian (4-20-min) Rhythms of Body Temperature.

The study examined the effect of passive magnetic shielding on the magnitude of rhythmic oscillations of body temperature (BT) with 4-20 min periods in mice and their correlation with similar oscillations in unshielded control group. A magnetic permalloy screen that 35-fold attenuates the total geomagnetic field and decreased the undulations of magnetic field with the periods of few minutes by 5 times, produced no effect on the mean amplitude of BT oscillations within the same period range, their spectral power, and the cross-spectral density of examined rhythms in comparison with the control (unshielded) mice. Thus, either the mice possess a very sensitive magnetic sensory system or there exists an external non-magnetic factor affecting rhythmicity of BT. The study advanced intensity of thermal neutron radiation near the Earth's surface known to reflect the flow of accelerated particles generated by the secondary cosmic rays as the external factor, which strongly correlates with BT rhythms revealed by cross-spectrum analysis.

Changes in Body Temperature of Small Mammals and Birds in a Few Minutes Range as Reflection of Environmental Influences.

The study examined the changes in intraperitoneal body temperature of laboratory mice, Jungar hamsters, European greenfinch Chloris chloris, and starlings. In a few minutes range, these changes significantly correlated not only between the animals of the same species, but also between the different classes such as birds and mammals, which were isolated from each other and maintained under different illumination regimen. This phenomenon indicates some external influence(s) on the central mechanisms of the thermal control system not related to illumination regiment. In 80% cases, the phases of most pronounced rhythms of body temperature oscillating with the periods of 8-9 and 12-13 min coincided with those of geomagnetic field within the accuracy of ±1 min. However, the amplitude of body temperature oscillations did not depend on the amplitude of geomagnetic field (GMF) oscillations. Synchronicity of the changes in body temperature and GMF was observed at the amplitude of GMF oscillation of 0.4 nT, which is extremely low value. In contrast, there was no reaction of body temperature to greater (6-10 nT) but irregular and abrupt perturbations of GMF.

Peroxiredoxins couple metabolism and cell division in an ultradian cycle.

Redox cycles have been reported in ultradian, circadian and cell cycle-synchronized systems. Redox cycles persist in the absence of transcription and cyclin-CDK activity, indicating that cells harbor multiple coupled oscillators. Nonetheless, the causal relationships and molecular mechanisms by which redox cycles are embedded within ultradian, circadian or cell division cycles remain largely elusive. Yeast harbor an ultradian oscillator, the yeast metabolic cycle (YMC), which comprises metabolic/redox cycles, transcriptional cycles and synchronized cell division. Here, we reveal the existence of robust cycling of H2O2 and peroxiredoxin oxidation during the YMC and show that peroxiredoxin inactivation disrupts metabolic cycling and abolishes coupling with cell division. We find that thiol-disulfide oxidants and reductants predictably modulate the switching between different YMC metabolic states, which in turn predictably perturbs cell cycle entry and exit. We propose that oscillatory H2O2-dependent protein thiol oxidation is a key regulator of metabolic cycling and its coordination with cell division.

Budgerigars have complex sleep structure similar to that of mammals.

Birds and mammals share specialized forms of sleep including slow wave sleep (SWS) and rapid eye movement sleep (REM), raising the question of why and how specialized sleep evolved. Extensive prior studies concluded that avian sleep lacked many features characteristic of mammalian sleep, and therefore that specialized sleep must have evolved independently in birds and mammals. This has been challenged by evidence of more complex sleep in multiple songbird species. To extend this analysis beyond songbirds, we examined a species of parrot, the sister taxon to songbirds. We implanted adult budgerigars (Melopsittacus undulatus) with electroencephalogram (EEG) and electrooculogram (EOG) electrodes to evaluate sleep architecture, and video monitored birds during sleep. Sleep was scored with manual and automated techniques, including automated detection of slow waves and eye movements. This can help define a new standard for how to score sleep in birds. Budgerigars exhibited consolidated sleep, a pattern also observed in songbirds, and many mammalian species, including humans. We found that REM constituted 26.5% of total sleep, comparable to humans and an order of magnitude greater than previously reported. Although we observed no spindles, we found a clear state of intermediate sleep (IS) similar to non-REM (NREM) stage 2. Across the night, SWS decreased and REM increased, as observed in mammals and songbirds. Slow wave activity (SWA) fluctuated with a 29-min ultradian rhythm, indicating a tendency to move systematically through sleep states as observed in other species with consolidated sleep. These results are at variance with numerous older sleep studies, including for budgerigars. Here, we demonstrated that lighting conditions used in the prior budgerigar study-and commonly used in older bird studies-dramatically disrupted budgerigar sleep structure, explaining the prior results. Thus, it is likely that more complex sleep has been overlooked in a broad range of bird species. The similarities in sleep architecture observed in mammals, songbirds, and now budgerigars, alongside recent work in reptiles and basal birds, provide support for the hypothesis that a common amniote ancestor possessed the precursors that gave rise to REM and SWS at one or more loci in the parallel evolution of sleep in higher vertebrates. We discuss this hypothesis in terms of the common plan of forebrain organization shared by reptiles, birds, and mammals.

Ultradian rhythms in heart rate variability and distal body temperature anticipate onset of the luteinizing hormone surge.

The menstrual cycle is characterized by predictable patterns of physiological change across timescales. Although patterns of reproductive hormones across the menstrual cycle, particularly ultradian rhythms, are well described, monitoring these measures repeatedly to predict the preovulatory luteinizing hormone (LH) surge is not practical. In the present study, we explored whether non-invasive measures coupled to the reproductive system: high frequency distal body temperature (DBT), sleeping heart rate (HR), sleeping heart rate variability (HRV), and sleep timing, could be used to anticipate the preovulatory LH surge in women. To test this possibility, we used signal processing to examine these measures in 45 premenopausal and 10 perimenopausal cycles alongside dates of supra-surge threshold LH and menstruation. Additionally, urinary estradiol and progesterone metabolites were measured daily surrounding the LH surge in 20 cycles. Wavelet analysis revealed a consistent pattern of DBT and HRV ultradian rhythm (2-5 h) power that uniquely enabled anticipation of the LH surge at least 2 days prior to its onset in 100% of individuals. Together, the present findings reveal fluctuations in distal body temperature and heart rate variability that consistently anticipate the LH surge, suggesting that automated ultradian rhythm monitoring may provide a novel and convenient method for non-invasive fertility assessment.

Body Temperature Dynamics in Small Mammals and Birds in 10-120-min Period Range.

The dynamics of intraperitoneal body temperature was analyzed in males of C57BL/6 mice and common greenfinches (Chloris chloris). Despite the membership in different classes, these mammals demonstrated the identical set of harmonics in body temperature spectra. The study revealed synchronicity of body temperature oscillations in distantly isolated animals. The data suggest that body temperature oscillations in 10-120-min (circahoralian) period range reflect the effect of an external environmental biotropic factor on temperature control in small mammals and birds.

Effect of Surgical Removal of Testicles and Adrenal Glands on Ultradian Rhythms of Body Temperature in Male Wistar Rats under Conditions of Constant Illumination.

We have studied body temperature dynamics of mature male Wistar rats maintained under constant illumination after surgical removal of the testicles and adrenal glands. In gonadectomized animals, pronounced increases in body temperature (>0.9°C) were observed every 4-6 h; during the periods 03.35-04.30, 07.35-08.30, 11.35-12.30, 15.35-16.30, 19.35-20.30, and 23.35-00.30, they were recorded 1.5-fold more often than during the rest periods. These results do not significantly differ from the parameters of the control group. Combined removal of the testicles and adrenal glands led to shortening of main period of temperature oscillations to 4-4.5 h and a decrease in its amplitude; pronounced increase in body temperature (>0.5°C) was observed 2.1 times more often during the periods 03.35-04.30, 07.35-08.30, 11.35-12.30, 15.35-16.30, 19.35-20.30, and 23.35-00.30 than in other time intervals. Thus, the removal of the testicles and adrenal glands does not violate the 4-h intraday rhythm of body temperature.

Ultradian Secretion of Growth Hormone in Mice: Linking Physiology With Changes in Synapse Parameters Using Super-Resolution Microscopy.

Neuroendocrine circuits are orchestrated by the pituitary gland in response to hypothalamic hormone-releasing and inhibiting factors to generate an ultradian and/or circadian rhythm of hormone secretion. However, mechanisms that govern this rhythmicity are not fully understood. It has been shown that synaptic transmission in the rodent hypothalamus undergoes cyclical changes in parallel with rhythmic hormone secretion and a growing body of evidence suggests that rapid rewiring of hypothalamic neurons may be the source of these changes. For decades, structural synaptic studies have been utilizing electron microscopy, which provides the resolution suitable for visualizing synapses. However, the small field of view, limited specificity and manual analysis susceptible to bias fuel the search for a more quantitative approach. Here, we apply the fluorescence super-resolution microscopy approach direct Stochastic Optical Reconstruction Microscopy (dSTORM) to quantify and structurally characterize excitatory and inhibitory synapses that contact growth hormone-releasing-hormone (GHRH) neurons during peak and trough values of growth hormone (GH) concentration in mice. This approach relies on a three-color immunofluorescence staining of GHRH and pre- and post-synaptic markers, and a quantitative analysis with a Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm. With this method we confirm our previous findings, using electron microscopy, of increased excitatory synaptic input to GHRH neurons during peak levels of GH. Additionally, we find a shift in synapse numbers during low GH levels, where more inhibitory synaptic inputs are detected. Lastly, we utilize dSTORM to study novel aspects of synaptic structure. We show that more excitatory (but not inhibitory) pre-synaptic clusters associate with excitatory post-synaptic clusters during peaks of GH secretion and that the numbers of post-synaptic clusters increase during high hormone levels. The results presented here provide an opportunity to highlight dSTORM as a valuable quantitative approach to study synaptic structure in the neuroendocrine circuit. Importantly, our analysis of GH circuitry sheds light on the potential mechanism that drives ultradian changes in synaptic transmission and possibly aids in GH pulse generation in mice.

Ultradian Rhythms in the Hypothalamic Arcuate Nucleus Kisspeptin Neurons and Developmental Processes.

Numerous physiological processes in nature have multiple oscillations within 24 h, that is, ultradian rhythms. Compared to the circadian rhythm, which has a period of approximately one day, these short oscillations range from seconds to hours, and the mechanisms underlying ultradian rhythms remain largely unknown. This review aims to explore and emphasize the implications of ultradian rhythms and their underlying regulations. Reproduction and developmental processes show ultradian rhythms, and these physiological systems can be regulated by short biological rhythms. Specifically, we recently uncovered synchronized calcium oscillations in the organotypic culture of hypothalamic arcuate nucleus (ARN) kisspeptin neurons that regulate reproduction. Synchronized calcium oscillations were dependent on voltage-gated ion channel-mediated action potentials and were repressed by chemogenetic inhibition, suggesting that the network within the ARN and between the kisspeptin population mediates the oscillation. This minireview describes that ultradian rhythms are a general theme that underlies biological features, with special reference to calcium oscillations in the hypothalamic ARN from a developmental perspective. We expect that more attention to these oscillations might provide insight into physiological or developmental mechanisms, since many oscillatory features in nature still remain to be explored.

Salivary cortisol as a non-invasive window on the brain.

The validation of accurate and meaningful assessment of cortisol in saliva samples has proved revolutionary in stress research. Its many advantages have expanded the scope of investigation from traditional laboratory and clinical settings to include multidisciplinary and community-based research. These developments have given rise to a wealth insight into the links between stress and health. Here we highlight the potential of salivary cortisol as both a product and mediator of brain function, instrumental in disturbing brain health. However, the subtleties of salivary cortisol as a measure can be underestimated, leading to misinterpretation of findings. These issues are explored, with a particular emphasis on necessary methodological rigor. Notwithstanding great promise, there is undeniably more to learn so we conclude by making recommendations for future research including use of salivary cortisol in the development of integrative predictive models of stress-related risk factors and resilience across the life course.