When brain clocks lose track of time: cause or consequence of neuropsychiatric disorders

Circadian rhythms and mood: opportunities for multi-level analyses in genomics and neuroscience: circadian rhythm dysregulation in mood disorders provides clues to the brain's organizing principles, and a touchstone for genomics and neuroscience

In the healthy state, both circadian rhythm and mood are stable against perturbations, yet they are capable of adjusting to altered internal cues or ongoing changes in external conditions. The dual demands of stability and flexibility are met by the collective properties of complex neural networks. Disruption of this balance underlies both circadian rhythm abnormality and mood disorders. However, we do not fully understand the network properties that govern the crosstalk between the circadian system and mood regulation. This puzzle reflects a challenge at the center of neurobiology, and its solution requires the successful integration of existing data across all levels of neural organization, from molecules, cells, circuits, network dynamics, to integrated mental function. This essay discusses several open questions confronting the cross-level synthesis, and proposes that circadian regulation, and its role in mood, stands as a uniquely tractable system to study the causal mechanisms of neural adaptation.

Reprogramming of the circadian clock by nutritional challenge

Circadian rhythms and cellular metabolism are intimately linked. Here, we reveal that a high-fat diet (HFD) generates a profound reorganization of specific metabolic pathways, leading to widespread remodeling of the liver clock. Strikingly, in addition to disrupting the normal circadian cycle, HFD causes an unexpectedly large-scale genesis of de novo oscillating transcripts, resulting in reorganization of the coordinated oscillations between coherent transcripts and metabolites. The mechanisms underlying this reprogramming involve both the impairment of CLOCK:BMAL1 chromatin recruitment and a pronounced cyclic activation of surrogate pathways through the transcriptional regulator PPARγ. Finally, we demonstrate that it is specifically the nutritional challenge, and not the development of obesity, that causes the reprogramming of the clock and that the effects of the diet on the clock are reversible.

Recent advances in sleep-wake cycle and biological rhythms in bipolar disorder

The cyclical nature of periodic switches in energy, motor activation and sleep-wake cycles in bipolar disorder suggests a strong underlying relationship with disturbances in chronobiology. Current research is refining our understanding of the various patterns of sleep-wake and biological rhythms alterations at early and later stages of this illness, as well as across its depressive/fatigue, manic/hypomanic and euthymic phases. This research focuses on early detection and subsequent monitoring to predict and better manage recurrent episodes. Sleep-wake cycle and biological rhythms disturbances are also well known to affect other key aspects of physical health (notably metabolic functions), cognitive performance and elevated risks for suicide. Increasing evidence now supports the integration of behavioural or pharmacological therapeutic strategies that target the sleep-wake and circadian systems in the ongoing treatment of various phases of bipolar disorder.

State variation in the cortisol awakening response

The cortisol awakening response (CAR) is a much studied but poorly understood aspect of the circadian pattern of cortisol secretion. A Scopus search of "cortisol" and "awakening" reveals 666 publications in this area since 1997 when it was first identified by Pruessner and colleagues as a "reliable biomarker of adrenocortical activity". The primary focus of the majority of these studies is centered on its utility as a biomarker associated with a range of psychosocial, physical and mental health variables. Such studies typically examine differences in the CAR (studied on 1 or 2 days) between healthy participants and other comparator groups of interest. Fewer studies (25 in our estimation) have examined correlates of day-to-day variation in the CAR in healthy participants, informing its role and regulation within the healthy circadian pattern of cortisol secretion. This is the first review to examine these studies which, although limited in number, offer a relatively coherent emerging story about state factors that influence the CAR and the impact of the CAR on daily functioning. Greater understanding of these issues helps illuminate the utility of the CAR as a promising biomarker in psychophysiological and epidemiological research. The review also highlights areas that require greater clarification and points to potentially fruitful areas of further research.

Period Gene Expression in the Brain of a Dual-Phasing Rodent, the Octodon degus

Clock gene expression is not only confined to the master circadian clock in the suprachiasmatic nucleus (SCN) but is also found in many other brain regions. The phase relationship between SCN and extra-SCN oscillators may contribute to known differences in chronotypes. The Octodon degus is a diurnal rodent that can shift its activity-phase preference from diurnal to nocturnal when running wheels become available. To understand better the relationship between brain clock gene activity and chronotype, we studied the day-night expression of the Period genes, Per1 and Per2, in the SCN and extra-SCN brain areas in diurnal and nocturnal degus. Since negative masking to light and entrainment to the dark phase are involved in the nocturnalism of this species, we also compare, for the first time, Per expression between entrained (EN) and masked nocturnal (MN) degus. The brains of diurnal, MN, and EN degus housed with wheels were collected during the light (ZT4) and dark (ZT16) phases. Per1 and Per2 mRNA levels were analyzed by in situ hybridization. Within the SCN, signals for Per1 and Per2 were higher at ZT4 irrespective of chronotype. However, outside of the SCN, Per1 expression in the hippocampus of EN degus was out of phase (higher values at ZT16) with SCN values. Although a similar trend was seen in MN animals, this day-night difference in Per1 expression was not significant. Interestingly, daily differences in Per1 expression were not seen in the hippocampus of diurnal degus. For other putative brain areas analyzed (cortices, striatum, arcuate, ventromedial hypothalamus), no differences in Per1 levels were found between chronotypes. Both in diurnal and nocturnal degus, Per2 levels in the hippocampus and in the cingulate and piriform cortices were in phase with their activity rhythms. Thus, diurnal degus showed higher Per2 levels at ZT4, whereas in both types of nocturnal degus, Per2 expression was reversed, peaking at ZT16. Together, the present study supports the hypothesis that the mechanisms underlying activity-phase preference in diurnal and nocturnal mammals reside downstream from the SCN, but our data also indicate that there are fundamental differences between nocturnal masked and entrained degus.

Cortisol and depression: three questions for psychiatry

BACKGROUND

Cortisol plays a multifaceted role in major depression disorder (MDD). Diurnal rhythms are disturbed, there is increased resistance to the feedback action of glucocorticoids, excess cortisol may induce MDD, basal levels may be higher and the post-awakening cortisol surge accentuated in those at risk for MDD. Does this suggest new avenues for studying MDD or its clinical management?

METHOD

The relevant literature was reviewed.

RESULTS

Cortisol contributes to genetic variants for the risk for MDD and the way that environmental events amplify risk. The corticoids' influence begins prenatally, but continues into adulthood. The impact of cortisol at each phase depends not only on its interaction with other factors, such as psychological traits and genetic variants, but also on events that have, or have not, occurred previously.

CONCLUSIONS

This review suggests that the time is now right for serious consideration of the role of cortisol in a clinical context. Estimates of cortisol levels and the shape of the diurnal rhythm might well guide the understanding of subtypes of MDD and yield additional indicators for optimal treatment. Patients with disturbed cortisol rhythms might benefit from restitution of those rhythms; they may be distinct from those with more generally elevated levels, who might benefit from cortisol blockade. Higher levels of cortisol are a risk for subsequent depression. Should manipulation of cortisol or its receptors be considered as a preventive measure for some of those at very high risk of future MDD, or to reduce other cortisol-related consequences such as long-term cognitive decline?

Human brain evolution: transcripts, metabolites and their regulators

What evolutionary events led to the emergence of human cognition? Although the genetic differences separating modern humans from both non-human primates (for example, chimpanzees) and archaic hominins (Neanderthals and Denisovans) are known, linking human-specific mutations to the cognitive phenotype remains a challenge. One strategy is to focus on human-specific changes at the level of intermediate phenotypes, such as gene expression and metabolism, in conjunction with evolutionary changes in gene regulation involving transcription factors, microRNA and proximal regulatory elements. In this Review we show how this strategy has yielded some of the first hints about the mechanisms of human cognition.

Melanocortin-3 receptors and metabolic homeostasis

Attenuated activity of the central nervous melanocortin system causes obesity and insulin resistance. Obese rodents treated with melanocortins exhibit improvements in obesity and metabolic homeostasis that are not mutually dependent, suggesting metabolic actions that are independent of weight changes. These responses are generally thought to involve G-protein-coupled receptors expressed in the brain. Melanocortin-4 receptors (MC4Rs) regulate satiety and autonomic nervous system and thyroid function. MC3Rs are expressed in hypothalamic and limbic regions involved in controlling ingestive behaviors and autonomic function. Mc3r-/- mice exhibit increased adiposity and an accelerated diet-induced obesity. While this phenotype is not dependent on hyperphagia, data on the regulation of food intake by MC3Rs are inconsistent. Recent investigations by our laboratory suggest a unique combination of behavioral and metabolic disorders in Mc3r-/- mice. MC3Rs are critical for the expression of the anticipatory response and metabolic homeostasis when food intake occurs outside the normal voluntary rhythms driven by photoperiod. Using a Cre-Lox strategy, we can now investigate MC3Rs expressed in different brain regions and organ systems in the periphery. While focusing on the functions of neural MC3Rs, early results suggest an additional layer of complexity with central and peripheral MC3Rs involved in the defense of body weight.

Human-specific transcriptional networks in the brain

Understanding human-specific patterns of brain gene expression and regulation can provide key insights into human brain evolution and speciation. Here, we use next-generation sequencing, and Illumina and Affymetrix microarray platforms, to compare the transcriptome of human, chimpanzee, and macaque telencephalon. Our analysis reveals a predominance of genes differentially expressed within human frontal lobe and a striking increase in transcriptional complexity specific to the human lineage in the frontal lobe. In contrast, caudate nucleus gene expression is highly conserved. We also identify gene coexpression signatures related to either neuronal processes or neuropsychiatric diseases, including a human-specific module with CLOCK as its hub gene and another module enriched for neuronal morphological processes and genes coexpressed with FOXP2, a gene important for language evolution. These data demonstrate that transcriptional networks have undergone evolutionary remodeling even within a given brain region, providing a window through which to view the foundation of uniquely human cognitive capacities.

A new theoretical approach to the functional meaning of sleep and dreaming in humans based on the maintenance of 'predictive psychic homeostasis'

Different theories have been put forward during the last decade to explain the functional meaning of sleep and dreaming in humans. In the present paper, a new theory is presented which, while taking advantage of these earlier theories, introduces the following new and original aspects:   • Circadian rhythms relevant to various organs of the body affect the reciprocal interactions which operate to maintain constancy of the internal milieu and thereby also affect the sleep/wakefulness cycle. Particular attention is given to the constancy of natraemia and osmolarity and to the permissive role that the evolution of renal function has had for the evolution of the central nervous system and its integrative actions. • The resetting of neuro-endocrine controls at the onset of wakefulness leads to the acquisition of new information and its integration within previously stored memories. This point is dealt with in relation to Moore-Ede's proposal for the existence of a 'predictive homeostasis'. • The concept of 'psychic homeostasis' is introduced and is considered as one of the most important states since it is aimed at the well-being, or eudemonia, of the human psyche. Sleep and dreaming in humans are discussed as important functions for the maintenance of a newly proposed composite state: that of 'predictive psychic homeostasis'. On the basis of these assumptions, and in accordance with the available neurobiological data, the present paper puts forward the novel hypothesis that sleep and dreaming play important functions in humans by compensating for psychic allostatic overloads. Hence, both consolatory dreams and disturbing nightmares can be part of the vis medicatrix naturae, the natural healing power, in this case, the state of eudemonia.

Are stress responses to geomagnetic storms mediated by the cryptochrome compass system?

A controversial body of literature demonstrates associations of geomagnetic storms (GMS) with numerous cardiovascular, psychiatric and behavioural outcomes. Various melatonin hypotheses of GMS have suggested that temporal variation in the geomagnetic field (GMF) may be acting as an additional zeitgeber (a temporal synchronizer) for circadian rhythms, with GMS somehow interfering with the hypothesized system. The cryptochrome genes are known primarily as key components of the circadian pacemaker, ultimately involved in controlling the expression of the hormone melatonin. Cryptochrome is identified as a clear candidate for mediating the effect of GMS on humans, demonstrating the prior existence of several crucial pieces of evidence. A distinct scientific literature demonstrates the widespread use of geomagnetic information for navigation across a range of taxa. One mechanism of magnetoreception is thought to involve a light-dependent retinal molecular system mediated by cryptochrome, acting in a distinct functionality to its established role as a circadian oscillator. There is evidence suggesting that such a magnetosense--or at least the vestiges of it--may exist in humans. This paper argues that cryptochrome is not acting as secondary geomagnetic zeitgeber to influence melatonin synthesis. Instead, it is hypothesized that the cryptochrome compass system is mediating stress responses more broadly across the hypothalamic-pituitary-adrenal (HPA) axis (including alterations to circadian behaviour) in response to changes in the GMF. Two conceptual models are outlined for the existence of such responses--the first as a generalized migrational/dispersal strategy, the second as a stress response to unexpected signals to the magnetosense. It is therefore proposed that GMS lead to disorientation of hormonal systems in animals and humans, thus explaining the effects of GMS on human health and behaviour.

Mood disorders, circadian rhythms, melatonin and melatonin agonists

Recent advances in the understanding of circadian rhythms have led to an interest in the treatment of major depressive disorder with chronobiotic agents. Many tissues have autonomous circadian rhythms, which are orchestrated by the master clock, situated in the suprachiasmatic nucleus (SNC). Melatonin (N-acetyl-5-hydroxytryptamine) is secreted from the pineal gland during darkness. Melatonin acts mainly on MT1 and MT2 receptors, which are present in the SNC, regulating physiological and neuroendocrine functions, including circadian entrainment, referred to as the chronobiotic effet. Circadian rhythms has been shown to be either misaligned or phase shifted or decreased in amplitude in both acute episodes and relapse of major depressive disorder (MDD) and bipolar disorder. Manipulation of circadian rhythms either using physical treatments (such as high intensity light) or behavioral therapy has shown promise in improving symptoms. Pharmacotherapy using melatonin and pure melatonin receptor agonists, while improving sleep, has not been shown to improve symptoms of depression. A novel antidepressant, agomelatine, combines 5HT2c antagonist and melatonin agonist action, and has shown promise in both acute treatment of MDD and in preventing relapse.

Recovery from Adolescent Anorexia Nervosa and Associations with Diurnal Patterns of Salivary Stress Hormones: A Case Report

In the neurodevelopment of adolescent anorexia nervosa (AN), dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is proposed to be a central component. Furthermore, a therapeutic milieu focusing on affect regulation can contribute much to treatment, given the emotional processing difficulties associated with this disorder. Studies of HPA axis function following such specialist treatments for adolescent AN, however, are rare. This study describes the diurnal pattern of HPA axis activation, including the cortisol awakening response (CAR), in a 16-year-old female diagnosed with AN both during illness and at clinical recovery following milieu therapy with a focus on affect regulation. Specialised single-case study statistics were used to assess whether the patient's data were significantly different from the healthy “norm” at illness and recovery. During illness, her measure of affective problems was outside of the normal range and cortisol and DHEA secretory profiles were significantly elevated across the diurnal period. However, at recovery both her affective state and HPA axis function became comparable to healthy controls. This case study suggests that salivary markers of HPA axis function can be feasibly incorporated into the clinical regime within a specialist adolescent AN residential service and could be used by clinicians to monitor prognosis and interventions.