Dynamic interactions between coupled oscillators within the hamster circadian pacemaker

Synchronization, clustering, and weak chimeras in a densely coupled transcription-based oscillator model for split circadian rhythms

The synchronization dynamics for the circadian gene expression in the suprachiasmatic nucleus is investigated using a transcriptional circadian clock gene oscillator model. With global coupling in constant dark (DD) conditions, the model exhibits a one-cluster phase synchronized state, in dim light (dim LL), bistability between one- and two-cluster states and in bright LL, a two-cluster state. The two-cluster phase synchronized state, where some oscillator pairs synchronize in-phase, and some anti-phase, can explain the splitting of the circadian clock, i.e., generation of two bouts of daily activities with certain species, e.g., with hamsters. The one- and two-cluster states can be reached by transferring the animal from DD or bright LL to dim LL, i.e., the circadian synchrony has a memory effect. The stability of the one- and two-cluster states was interpreted analytically by extracting phase models from the ordinary differential equation models. In a modular network with two strongly coupled oscillator populations with weak intragroup coupling, with appropriate initial conditions, one group is synchronized to the one-cluster state and the other group to the two-cluster state, resulting in a weak-chimera state. Computational modeling suggests that the daily rhythms in sleep-wake depend on light intensity acting on bilateral networks of suprachiasmatic nucleus (SCN) oscillators. Addition of a network heterogeneity (coupling between the left and right SCN) allowed the system to exhibit chimera states. The simulations can guide experiments in the circadian rhythm research to explore the effect of light intensity on the complexities of circadian desynchronization.

Sex differences in daily timekeeping and circadian clock circuits

The circadian system regulates behavior and physiology in many ways important for health. Circadian rhythms are expressed by nearly every cell in the body, and this large system is coordinated by a central clock in the suprachiasmatic nucleus (SCN). Sex differences in daily rhythms are evident in humans and understanding how circadian function is modulated by biological sex is an important goal. This review highlights work examining effects of sex and gonadal hormones on daily rhythms, with a focus on behavior and SCN circuitry in animal models commonly used in pre-clinical studies. Many questions remain in this area of the field, which would benefit from further work investigating this topic.

Naturalistic Intensities of Light at Night: A Review of the Potent Effects of Very Dim Light on Circadian Responses and Considerations for Translational Research

In this review, we discuss the remarkable potency and potential applications of a form of light that is often overlooked in a circadian context: naturalistic levels of dim light at night (nLAN), equivalent to intensities produced by the moon and stars. It is often assumed that such low levels of light do not produce circadian responses typically associated with brighter light levels. A solid understanding of the impacts of very low light levels is complicated further by the broad use of the somewhat ambiguous term “dim light,” which has been used to describe light levels ranging seven orders of magnitude. Here, we lay out the argument that nLAN exerts potent circadian effects on numerous mammalian species, and that given conservation of anatomy and function, the efficacy of light in this range in humans warrants further investigation. We also provide recommendations for the field of chronobiological research, including minimum requirements for the measurement and reporting of light, standardization of terminology (specifically as it pertains to “dim” light), and ideas for reconsidering old data and designing new studies.

Metabolic Lateralization in the Hypothalamus of Male Rats Related to Reproductive and Satiety States

The hypothalamus is the main regulatory center of many homeostatic processes, such as reproduction, food intake, and sleep-wake behavior. Recent findings show that there is a strongly interdependent side-linked localization of hypothalamic functions between the left and right hemispheres. The goal of the present study was to trace functional asymmetry of the hypothalamus related to the regulation of food intake and reproduction, in male rodents. Subjects were examined through measurements of mitochondrial metabolism ex vivo. Impact of gonadectomy and scheduled feeding was tested on the modulation of hypothalamic metabolic asymmetry. Results show that in male rats, functional lateralization of the hypothalamus can be attributed to the satiety state rather than to reproductive control. Fasting caused left-sided metabolic dominance, while satiety was linked to the right hemisphere; trends and direction in sided dominance gradually followed the changes in satiety state. Our findings revealed satiety state-dependent metabolic differences between the two hypothalamic hemispheres. It is therefore concluded that, at least in male rats, the hypothalamic hemispheres control the satiety state-related functions in an asymmetric manner.

Exceptional Entrainment of Circadian Activity Rhythms With Manipulations of Rhythm Waveform in Male Syrian Hamsters

The activity/rest rhythm of mammals reflects the output of an endogenous circadian oscillator entrained to the solar day by light. Despite detailed understanding of the neural and molecular bases of mammalian rhythms, we still lack practical tools for achieving rapid and flexible adjustment of clocks to accommodate shift-work, trans-meridian jet travel, or space exploration. Efforts to adapt clocks have focused on resetting the phase of an otherwise unaltered circadian clock. Departing from this tradition, recent work has demonstrated that bifurcation of circadian waveform in mice facilitates entrainment to extremely long and short zeitgeber periods. Here we evaluate the formal nature of entrainment to extreme non-24 h days in male Syrian hamsters. Wheel-running rhythms were first bifurcated into a 24 h rest/activity/rest/activity cycle according to established methods. Thereafter the 24 h lighting cycle was incrementally adjusted over several weeks to 30 h or to 18 h. Almost without exception, wheel-running rhythms of hamsters in gradually lengthened or shortened zeitgebers remained synchronized with the lighting cycle, with greater temporal precision observed in the former condition. Data from animals transferred abruptly from 24 h days to long or short cycles suggested that gradual adaptation facilitates but is not necessary for successful behavioral entrainment. The unprecedented behavioral adaptation following waveform bifurcation reveals a latent plasticity in mammalian circadian systems that can be realized in the absence of pharmacological or genetic manipulations. Oscillator interactions underlying circadian waveform manipulation, thus, represent a tractable target for understanding and enhancing circadian rhythm resetting.

Extraordinary behavioral entrainment following circadian rhythm bifurcation in mice

The mammalian circadian timing system uses light to synchronize endogenously generated rhythms with the environmental day. Entrainment to schedules that deviate significantly from 24 h (T24) has been viewed as unlikely because the circadian pacemaker appears capable only of small, incremental responses to brief light exposures. Challenging this view, we demonstrate that simple manipulations of light alone induce extreme plasticity in the circadian system of mice. Firstly, exposure to dim nocturnal illumination (<0.1 lux), rather than completely dark nights, permits expression of an altered circadian waveform wherein mice in light/dark/light/dark (LDLD) cycles “bifurcate” their rhythms into two rest and activity intervals per 24 h. Secondly, this bifurcated state enables mice to adopt stable activity rhythms under 15 or 30 h days (LDLD T15/T30), well beyond conventional limits of entrainment. Continuation of dim light is unnecessary for T15/30 behavioral entrainment following bifurcation. Finally, neither dim light alone nor a shortened night is sufficient for the extraordinary entrainment observed under bifurcation. Thus, we demonstrate in a non-pharmacological, non-genetic manipulation that the circadian system is far more flexible than previously thought. These findings challenge the current conception of entrainment and its underlying principles, and reveal new potential targets for circadian interventions.

A novel excitatory network for the control of breathing

Breathing must be tightly coordinated with other behaviours such as vocalization, swallowing, and coughing. These behaviours occur after inspiration, during a respiratory phase termed postinspiration. Failure to coordinate postinspiration with inspiration can result in aspiration pneumonia, the leading cause of death in Alzheimer's disease, Parkinson's disease, dementia, and other neurodegenerative diseases. Here we describe an excitatory network that generates the neuronal correlate of postinspiratory activity in mice. Glutamatergic-cholinergic neurons form the basis of this network, and GABA (γ-aminobutyric acid)-mediated inhibition establishes the timing and coordination relative to inspiration. We refer to this network as the postinspiratory complex (PiCo). The PiCo has autonomous rhythm-generating properties and is necessary and sufficient for postinspiratory activity in vivo.The PiCo also shows distinct responses to neuromodulators when compared to other excitatory brainstem networks. On the basis of the discovery of the PiCo, we propose that each of the three phases of breathing is generated by a distinct excitatory network: the pre-Bötzinger complex, which has been linked to inspiration; the PiCo, as described here for the neuronal control of postinspiration; and the lateral parafacial region (pF(L)), which has been associated with active expiration, a respiratory phase that is recruited during high metabolic demand.

Photoperiodic and circadian bifurcation theories of depression and mania

Seasonal effects on mood have been observed throughout much of human history.  Seasonal changes in animals and plants are largely mediated through the changing photoperiod (i.e., the photophase or duration of daylight).  We review that in mammals, daylight specifically regulates SCN (suprachiasmatic nucleus) circadian organization and its control of melatonin secretion.  The timing of melatonin secretion interacts with gene transcription in the pituitary pars tuberalis to modulate production of TSH (thyrotropin), hypothalamic T3 (triiodothyronine), and tuberalin peptides which modulate pituitary production of regulatory gonadotropins and other hormones.  Pituitary hormones largely mediate seasonal physiologic and behavioral variations.  As a result of long winter nights or inadequate illumination, we propose that delayed morning offset of nocturnal melatonin secretion, suppressing pars tuberalis function, could be the main cause for winter depression and even cause depressions at other times of year.  Irregularities of circadian sleep timing and thyroid homeostasis contribute to depression.  Bright light and sleep restriction are antidepressant and conversely, sometimes trigger mania.  We propose that internal desynchronization or bifurcation of SCN circadian rhythms may underlie rapid-cycling manic-depressive disorders and perhaps most mania.  Much further research will be needed to add substance to these theories.

Individual differences in circadian waveform of Siberian hamsters under multiple lighting conditions

Because the circadian clock in the mammalian brain derives from a network of interacting cellular oscillators, characterizing the nature and bases of circadian coupling is fundamental to understanding how the pacemaker operates. Various phenomena involving plasticity in circadian waveform have been theorized to reflect changes in oscillator coupling; however, it remains unclear whether these different behavioral paradigms reference a unitary underlying process. To test whether disparate coupling assays index a common mechanism, we examined whether there is covariation among behavioral responses to various lighting conditions that produce changes in circadian waveform. Siberian hamsters, Phodopus sungorus, were transferred from long to short photoperiods to distinguish short photoperiod responders (SP-R) from nonresponders (SP-NR). Short photoperiod chronotyped hamsters were subsequently transferred, along with unselected controls, to 24-h light:dark:light: dark cycles (LDLD) with dim nighttime illumination, a procedure that induces bifurcated entrainment. Under LDLD, SP-R hamsters were more likely to bifurcate their rhythms than were SP-NR hamsters or unselected controls. After transfer from LDLD to constant dim light, SP-R hamsters were also more likely to become arrhythmic compared to SP-NR hamsters and unselected controls. In contrast, short photoperiod chronotype did not influence more transient changes in circadian waveform. The present data reveal a clear relationship in the plasticity of circadian waveform across 3 distinct lighting conditions, suggesting a common mechanism wherein individual differences reflect variation in circadian coupling.

Twice daily melatonin peaks in Siberian but not Syrian hamsters under 24 h light:dark:light:dark cycles

The daily pattern of blood-borne melatonin varies seasonally under the control of a multi-oscillator circadian pacemaker. Here we examine patterns of melatonin secretion and locomotor activity in Siberian and Syrian hamsters entrained to bimodal LDLD8:4:8:4 and LD20:4 lighting schedules that facilitate novel temporal arrangements of component circadian oscillators. Under LDLD, both species robustly bifurcated wheel-running activity in distinct day scotophase (DS) and night scotophase (NS) bouts. Siberian hamsters displayed significant melatonin increases during each scotophase in LDLD, and in the single daily scotophase of LD20:4. The bimodal melatonin secretion pattern persisted in acutely extended 16 h scotophases. Syrian hamsters, in contrast, showed no significant increases in plasma melatonin during either scotophase of LDLD8:4:8:4 or in LD20:4. In this species, detectable levels were observed only when the DS of LDLD was acutely extended to yield 16 h of darkness. Established species differences in the phase lag of nocturnal melatonin secretion relative to activity onset may underlie the above contrast: In non-bifurcated entrainment to 24 h LD cycles, Siberian hamsters show increased melatonin secretion within ≈ 2 h after activity onset, whereas in Syrian hamsters, detectable melatonin secretion phase lags activity onset and the L/D transition by at least 4 h. The present results provide new evidence indicating multi-oscillator regulation of the waveform of melatonin secretion, specifically, the circadian control of the onset, offset and duration of nocturnal secretion.

Transcription-based oscillator model for light-induced splitting as antiphase circadian gene expression in the suprachiasmatic nuclei

Daily locomotor patterns of a variety of organisms have been interpreted as driven by dual circadian oscillators. Yet, in mammals, cellular data have revealed many circadian oscillators in the bilateral suprachiasmatic nucleus (SCN). To test how large numbers of oscillators could respond to environmental cues as a pair of oscillators, the authors developed a computational model composed of 2 groups of oscillators with strong local interactions and with weaker coupling between the 2 groups. Unlike previous models that assumed that light affects the timing or polarity of coupling between a pair of oscillators, this simulation assumed that light increased the transcription rate of a clock gene and consequently altered circadian properties of individual cells. In constant dark, weak local (within each of the 2 groups) and distant (between group) coupling established in-phase oscillations and a typical single bout of daily activity. In constant light, local synchrony developed only if coupling was strong and resulted in antiphase synchrony between the 2 groups and bimodal daily activity reminiscent of split behavior. These numerical simulations thus showed that splitting behavior can develop with increased light intensity without structural changes in the coupling topology or sign. Instead, the authors propose that light changes intrinsic oscillator properties through the increase of maximal transcription rate of a clock gene, so that as light intensity increases, the output of the coupled network transitions from a single bout of activity through irregular beating to 2 bouts and, in bright constant light, arrhythmicity.

Dim nighttime illumination interacts with parametric effects of bright light to increase the stability of circadian rhythm bifurcation in hamsters

The endogenous circadian pacemaker of mammals is synchronized to the environmental day by the ambient cycle of relative light and dark. The present studies assessed the actions of light in a novel circadian entrainment paradigm where activity rhythms are bifurcated following exposure to a 24-h light:dark:light:dark (LDLD) cycle. Bifurcated entrainment under LDLD reflects the temporal dissociation of component oscillators that comprise the circadian system and is facilitated when daily scotophases are dimly lit rather than completely dark. Although bifurcation can be stably maintained in LDLD, it is quickly reversed under constant conditions. Here the authors examine whether dim scotophase illumination acts to maintain bifurcated entrainment under LDLD through potential interactions with the parametric actions of bright light during the two daily photophases. In three experiments, wheel-running rhythms of Syrian hamsters were bifurcated under LDLD with dimly lit scotophases, and after several weeks, dim scotophase illumination was either retained or extinguished. Additionally, "full" and "skeleton" photophases were employed under LDLD cycles with dimly lit or completely dark scotophases to distinguish parametric from nonparametric effects of bright light. Rhythm bifurcation was more stable in full versus skeleton LDLD cycles. Dim light facilitated the maintenance of bifurcated entrainment under full LDLD cycles but did not prevent the loss of rhythm bifurcation in skeleton LDLD cycles. These studies indicate that parametric actions of bright light maintain the bifurcated entrainment state; that dim scotophase illumination increases the stability of the bifurcated state; and that dim light interacts with the parametric effects of bright light to increase the stability of rhythm bifurcation under full LDLD cycles. A further understanding of the novel actions of dim light may lead to new strategies for understanding, preventing, and treating chronobiological disturbances.

Reorganization of suprachiasmatic nucleus networks under 24-h LDLD conditions

The suprachiasmatic nucleus (SCN), locus of the master circadian clock in the brain, is comprised of multioscillator neural networks that are highly plastic in responding to environmental lighting conditions. Under a 24-h light:dark:light:dark (LDLD) cycle, hamsters bifurcate their circadian locomotor activity such that wheel running occurs in each of the 2 daily dark periods with complete inactivity in between. In the present study, we explored the neural underpinning of this behavioral bifurcation. Using calbindin (CalB)- containing cells of the SCN as a regional marker, we characterized PER1 and c-FOS expression in the core and shell SCN subregions. In LD-housed animals, it is known that PER1 and c-FOS in the core and shell region are in phase with each other. In contrast, in behaviorally bifurcated animals housed in LDLD, the core and shell SCN exhibit antiphase rhythms of PER1. Furthermore, cells in the core show high FOS expression in each photophase of the LDLD cycle. The activation of FOS in the core is light driven and disappears rapidly when the photophase is replaced by darkness. The results suggest that bifurcated activity bouts in daytime and nighttime are associated with oscillating groups of cells in the core and shell subregions, respectively, and support the notion that reorganization of SCN networks underlies changes in behavioral responses under different environmental lighting conditions.