Diurnal rhythmicity of the canonical clock genes Per1, Per2 and Bmal1 in the rat adrenal gland is unaltered after hypophysectomy

PER1 Oscillation in Rat Parathyroid Hormone and Calcitonin Producing Cells

Many endocrine glands exhibit circadian rhythmicity, but the interplay between the central circadian clock in the suprachiasmatic nucleus (SCN), the peripheral endocrine clock, and hormones is sparsely understood. We therefore studied the cellular localizations of the clock protein PER1, parathyroid hormone (PTH) and calcitonin (CT) in the parathyroid and thyroid glands, respectively. Thyroid glands, including the parathyroids, were dissected at different time-points from rats housed in 12 h:12 h light-darkness cycles, and were double-immunostained for PER1 and PTH or CT. Sera were analyzed for PTH, CT, phosphate, and calcium. In both glands, PER1 expression peaked late at night, while limited staining was seen during the daytime. High-resolution microscopy revealed cytosolic PER1 at zeitgeber time (ZT)12, and nucleic staining at ZT24 in both PTH and CT cells. PTH peaked at Z12-ZT16, while neither CT staining nor serum CT oscillated during the daily cycle. Serum PTH was significantly higher at ZT12 than ZT24, but only phosphate was found to exhibit significant diurnal oscillation. The staining of the calcium-sensitive receptor (CSR) did not demonstrate circadian oscillation. In conclusion, PER1 expression peaked late at night/early in the morning in hormone-producing cells of both the thyroid and parathyroid glands. In the parathyroids, this was preceded by a PTH peak, while neither CT nor CRS were found to oscillate.

Dysregulation of circadian clock gene expression patterns in a treatment-resistant animal model of depression

Circadian rhythm (CR) disturbances are among the most commonly observed symptoms during major depressive disorder, mostly in the form of disrupted sleeping patterns. However, several other measurable parameters, such as plasma hormone rhythms and differential expression of circadian clock genes (ccgs), are also present, often referred to as circadian phase markers. In the recent years, CR disturbances have been recognized as an essential aspect of depression; however, most of the known animal models of depression have yet to be evaluated for their eligibility to model CR disturbances. In this study, we investigate the potential of adrenocorticotropic hormone (ACTH)-treated animals as a disease model for research in CR disturbances in treatment-resistant depression. For this purpose, we evaluate the changes in several circadian phase markers, including plasma concentrations of corticosterone, ACTH, and melatonin, as well as gene expression patterns of 13 selected ccgs at 3 different time points, in both peripheral and central tissues. We observed no impact on plasma corticosterone and melatonin concentrations in the ACTH rats compared to vehicle. However, the expression pattern of several ccgs was affected in the ACTH rats compared to vehicle. In the hippocampus, 10 ccgs were affected by ACTH treatment, whereas in the adrenal glands, 5 ccgs were affected and in the prefrontal cortex, hypothalamus and liver 4 ccgs were regulated. In the blood, only 1 gene was affected. Individual tissues showed changes in different ccgs, but the expression of Bmal1, Per1, and Per2 were most generally affected. Collectively, the results presented here indicate that the ACTH animal model displays dysregulation of a number of phase markers suggesting the model may be appropriate for future studies into CR disturbances.

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

The Lack of Light-Dark and Feeding-Fasting Cycles Alters Temporal Events in the Goldfish (Carassius auratus) Stress Axis

Vertebrates possess circadian clocks, driven by transcriptional-translational loops of clock genes, to orchestrate anticipatory physiological adaptations to cyclic environmental changes. This work aims to investigate how the absence of a light-dark cycle and a feeding schedule impacts the oscillators in the hypothalamus-pituitary-interrenal axis of goldfish. Fish were maintained under 12L:12D feeding at ZT 2; 12L:12D feeding at random times; and constant darkness feeding at ZT 2. After 30 days, fish were sampled to measure daily variations in plasma cortisol and clock gene expression in the hypothalamus-pituitary-interrenal (HPI) axis. Clock gene rhythms in the HPI were synchronic in the presence of a light-dark cycle but were lost in its absence, while in randomly fed fish, only the interrenal clock was disrupted. The highest cortisol levels were found in the randomly fed group, suggesting that uncertainty of food availability could be as stressful as the absence of a light-dark cycle. Cortisol daily rhythms seem to depend on central clocks, as a disruption in the adrenal clock did not impede rhythmic cortisol release, although it could sensitize the tissue to stress.

Adrenergic Signaling in Circadian Control of Immunity

Circadian rhythms govern a multitude of physiologic processes, both on a cell-intrinsic level and systemically, through the coordinated function of multi-organ biosystems. One such system-the adrenergic system-relies on the catecholamine neurotransmitters, adrenaline and noradrenaline, to carry out a range of biological functions. Production of these catecholamines is under dual regulation by both neural components of the sympathetic nervous system and hormonal mechanisms involving the hypothalamus-pituitary-adrenal axis. Importantly, both neural and hormonal arms receive input from the body's central clock, giving rise to the observed rhythmic variations in catecholamine levels in blood and peripheral tissues. Oscillations in catecholamine signals have the potential to influence various cellular targets expressing adrenergic receptors, including cells of the immune system. This review will focus on ways in which the body's central master clock regulates the adrenergic system to generate circadian rhythms in adrenaline and noradrenaline, and will summarize the existing literature linking circadian control of the adrenergic system to immunologic outcomes. A better understanding of the complex, multi-system pathways involved in the control of adrenergic signals may provide immunologists with new insight into mechanisms of immune regulation and precipitate the discovery of new therapeutics.

Expression of clock-related genes in benign and malignant adrenal tumors

Although the effect of the central clock system on adrenal function has been extensively studied, the role of the peripheral clock system in adrenal tumorigenesis remains largely unexplored. In this study we investigated the expression of clock-related genes in normal adrenocortical tissue and adrenocortical tumors. Twenty-seven fresh frozen human adrenal tissues including 13 cortisol secreting adenomas (CSA), seven aldosterone producing adenomas (APA), and seven adrenocortical carcinomas (ACC) were collected. CLOCK, BMAL1, PER1, CRY1, Rev-ERB, and RORα mRNA and protein expression were determined by qPCR and immunoblotting in pathological tissues and compared with the adjacent normal adrenal tissues. A significant downregulation of PER1, CRY1, and Rev-ERB compared with their normal tissue was demonstrated in CSA. All clock-related genes were overexpressed in APA compared with their normal tissue, albeit not significantly. A significant upregulation of CRY1 and PER1 and downregulation of BMAL1, RORα, and Rev-ERB compared with normal adrenal tissue was observed in ACC. BMAL1 and PER1 were significantly downregulated in APA compared with CSA. CLOCK, CRY1, and PER1 were upregulated, whereas BMAL1, RORα, and Rev-ERB were downregulated in ACC compared with CSA. Our study demonstrated the expression of CLOCK, BMAL1, PER1, CRY1, Rev-ERB, and RORα in normal and pathological human adrenal tissues. Adrenal tumors exhibited altered expression of these genes compared with normal tissue, with specific differences between benign and malignant lesions and between benign tumors arising from glomerulosa vs fasciculata zone. Further studies should clarify whether these alterations could be implicated in adrenocortical tumorigenesis.

Emerging roles of the unfolded protein response (UPR) in the nervous system: A link with adaptive behavior to environmental stress?

Stressors elicit a neuroendocrine response leading to increased levels of glucocorticoids, allowing the organism to adapt to environmental changes and maintain homeostasis. Glucocorticoids have a broad effect in the body, modifying the activity of the immune system, metabolism, and behavior through the activation of receptors in the limbic system. Chronic exposition to stressors operates as a risk factor for psychiatric diseases such as depression and posttraumatic stress disorder. Among the cellular alterations observed as a consequence of environmental stress, alterations to organelle function at the level of mitochondria and endoplasmic reticulum (ER) are emerging as possible factors contributing to neuronal dysfunction. ER proteostasis alterations elicit the unfolded protein response (UPR), a conserved signaling network that re-establish protein homeostasis. In addition, in the context of brain function, the UPR has been associated to neurodevelopment, synaptic plasticity and neuronal connectivity. Recent studies suggest a role of the UPR in the adaptive behavior to stress, suggesting a mechanistic link between environmental and cellular stress. Here, we revise recent evidence supporting an evolutionary connection between the neuroendocrine system and the UPR to modulate behavioral adaptive responses.

Rhythmic Release of Corticosterone Induces Circadian Clock Gene Expression in the Cerebellum

Neurons of the cerebellar cortex contain a circadian oscillator, with circadian expression of clock genes being controlled by the master clock of the suprachiasmatic nucleus (SCN). However, the signaling pathway connecting the SCN to the cerebellum is unknown. Glucocorticoids exhibit a prominent SCN-dependent circadian rhythm, and high levels of the glucocorticoid receptor have been reported in the cerebellar cortex; we therefore hypothesized that glucocorticoids may control the rhythmic expression of clock genes in the cerebellar cortex. We here applied a novel methodology by combining the electrolytic lesion of the SCN with implantation of a micropump programmed to release corticosterone in a circadian manner mimicking the endogenous hormone profile. By use of this approach, we were able to restore the corticosterone rhythm in SCN-lesioned male rats. Clock gene expression in the cerebellum was abolished in rats with a lesioned SCN, but exogenous corticosterone restored the daily rhythm in clock gene expression in the cerebellar cortex, as revealed by quantitative real-time PCR and radiochemical in situ hybridization for the detection of the core clock genes Per1, Per2, and Arntl. On the contrary, exogenous hormone did not restore circadian rhythms in body temperature and running activity. RNAscope in situ hybridization further revealed that the glucocorticoid receptor colocalizes with clock gene products in cells of the cerebellar cortex, suggesting that corticosterone exerts its actions by binding directly to receptors in neurons of the cerebellum. However, rhythmic clock gene expression in the cerebellum was also detectable in adrenalectomized rats, indicating that additional control mechanisms exist. These data show that the cerebellar circadian oscillator is influenced by SCN-dependent rhythmic release of corticosterone.

Changes in Clock Genes Expression in Esophagus in Rat Reflux Esophagitis

BACKGROUND

Gastroesophageal reflux disease (GERD) is strongly associated with sleep disturbances. Clock genes harmonize circadian rhythms by their periodic expression and regulate several physiological functions. However, the association between clock genes and GERD is still unknown.

AIMS

We investigated whether reflux esophagitis affects circadian variability of clock genes in the esophagus and other organs using a rat reflux esophagitis model.

METHODS

Reflux esophagitis was induced in 7-week-old male Wistar rats. Sham-operated rats were used as controls. Rats were killed at 09:00 (light period) and 21:00 (dark period) 3 days (acute phase) and 21 days (chronic phase) after induction of esophagitis. The expression levels of clock gene mRNAs such as Per1, Per2, Per3, Cry1, Cry2, Arntl, and Clock in the esophagus were investigated by qPCR. Arntl expression was examined in stomach, small intestine, colon, and liver tissues. Serum melatonin and IL-6 levels were measured by ELISA.

RESULTS

Histological examination of reflux esophagitis mainly revealed epithelial defects with marked inflammatory cell infiltration in the acute phase and mucosal thickening with basal cell hyperplasia in the chronic phase. Circadian variability of clock genes, except Cry1, was present in the normal esophagus and was completely disrupted in reflux esophagitis during the acute phase. The circadian variability of Per2, Per3, and Arntl returned to normal, but disruption of Per1, Cry2, and Clock was present in the chronic phase. Disruption of circadian variability of Arntl was observed in the esophagus, as well as in the stomach, small intestine, and liver tissues in reflux esophagitis during the acute phase. There were no significant differences in serum melatonin and IL-6 levels between control and reflux esophagitis animals in both acute and chronic phases.

CONCLUSIONS

Disruption to circadian variability of clock genes may play a role in the pathogenesis of GERD.

The suprachiasmatic nucleus; a responsive clock regulating homeostasis by daily changing the setpoints of physiological parameters

The suprachiasmatic nucleus (SCN) is responsible for determining circadian variations in physiological setpoints. The SCN achieves such control through projections to different target structures within and outside the hypothalamus. Thus the SCN prepares the physiology of the body every 24 h via hormones and autonomic nervous system (ANS), to coming changes in behavior. Resulting rhythms in hormones and ANS activity transmit a precise message to selective organs, adapting their sensitivity to coming hormones, metabolites or other essentials. Thus the SCN as autonomous clock gives rhythm to physiological processes. However when the body is challenged by infections, low or high temperature, food shortage or excess: physiological setpoints need to be changed. For example, under fasting conditions, setpoints for body temperature and glucose levels are lowered at the beginning of the sleep (inactive) phase. However, starting the active phase, a normal increase in glucose and temperature levels take place to support activities associated with the acquisition of food. Thus, the SCN adjusts physiological setpoints in agreement with time of the day and according to challenges faced by the body. The SCN is enabled to do this by receiving extensive input from brain areas involved in sensing the condition of the body. Therefore, when the body receives stimuli contradicting normal physiology, such as eating or activity during the inactive period, this information reaches the SCN, adapting its output to correct this disbalance. As consequence frequent violations of the SCN message, such as by shift work or night eating, will result in development of disease.

Sleep and circadian rhythm disruption and stress intersect in Alzheimer's disease

Alzheimer's disease (AD) was discovered and the pathological hallmarks were revealed more than a century ago. Subsequently, many remarkable discoveries and breakthroughs provided us with mechanistic insights into the pathogenesis of AD. The identification of the molecular underpinning of the disease not only provided the framework of AD pathogenesis but also targets for therapeutic inventions. Despite all the initial successes, no effective treatment for AD has emerged yet as all the late stages of clinical trials have failed. Many factors ranging from genetic to environmental factors have been critically appraised as the potential causes of AD. In particular, the role of stress on AD has been intensively studied while the relationship between sleep and circadian rhythm disruption (SCRD) and AD have recently emerged. SCRD has always been thought to be a corollary of AD pathologies until recently, multiple lines of evidence converge on the notion that SCRD might be a contributing factor in AD pathogenesis. More importantly, how stress and SCRD intersect and make their concerted contributions to AD phenotypes has not been reviewed. The goal of this literature review is to examine at multiple levels - molecular, cellular (e.g. microglia, gut microbiota) and holistic - how the interaction between stress and SCRD bi-directionally and synergistically exacerbate AD pathologies and cognitive impairment. AD, in turn, worsens stress and SCRD and forms the vicious cycle that perpetuates and amplifies AD.

Time-Lag in Feeding Schedule Acts as a Stressor That Alters Circadian Oscillators in Goldfish

The circadian system controls temporal homeostasis in all vertebrates. The light-dark (LD) cycle is the most important zeitgeber (“time giver”) of circadian system, but feeding time also acts as a potent synchronizer in the functional organization of the teleost circadian system. In mammals is well known that food intake during the rest phase promotes circadian desynchrony which has been associated with metabolic diseases. However, the impact of a misalignment of LD and feeding cycles in the entrainment of fish circadian oscillators is largely unknown. The objective of this work was to investigate how a time-lag feeding alters temporal homeostasis and if this could be considered a stressor. To this aim, goldfish maintained under a 12 h light-12 h darkness were fed at mid-photophase (SF6) or mid-scotophase (SF18). Daily rhythms of locomotor activity, clock genes expression in hypothalamus, liver, and head kidney, and circulating cortisol were studied. Results showed that SF6 fish showed daily rhythms of bmal1a and clock1a in all studied tissues, being in antiphase with rhythms of per1 genes, as expected for proper functioning clocks. The 12 h shift in scheduled feeding induced a short phase advance (4–5-h) of the clock genes daily rhythms in the hypothalamus, while in the liver the shift for clock genes expression rhythms was the same that the feeding time shift (∼12 h). In head kidney, acrophases of per genes underwent a 12-h shift in SF18 animals, but only 6 h shift for clock1a. Plasma cortisol levels showed a significant daily rhythm in animals fed at SF6, but not in SF18 fish fed, which displayed higher cortisol values throughout the 24-h. Altogether, results indicate that hypothalamus, liver, and head kidney oscillate in phase in SF6 fish, but these clocks are desynchronized in SF18 fish, which could explain cortisol alterations. These data reinforce the hypothesis that the misalignment of external cues (daily photocycle and feeding time) alters fish temporal homeostasis and it might be considered a stressor for the animals.

Circadian Control of Neuroendocrine Function: Implications for Health and Disease

The circadian timing system orchestrates daily rhythms in physiology and behavior via the suprachiasmatic nucleus (SCN), the master brain clock. Because endocrine secretions have far-reaching influence on the brain and periphery, circadian regulation of hormones is essential for normal functioning and disruptions to circadian timing (e.g., irregular sleep patterns, limited exposure to sunlight, jet lag, nighttime light exposure) have detrimental health consequences. Herein, we provide an overview of circadian timing in three major endocrine axes, the hypothalamo-pituitary-gonadal (HPG), hypothalamo-pituitary-adrenal (HPA) and hypothalamo-pituitary-thyroid (HPT) axes, and then consider the negative health consequences of circadian disruptions in each of these systems. For example, disruptions to HPG axis circadian timing lead to a host of negative reproductive outcomes such as irregular menstrual cycles, low sperm density and increased rates of miscarriages and infertility. Dysregulation of HPA axis timing is associated with obesity and metabolic disease, whereas disruptions to the HPT axis are associated with dysregulated metabolic gene rhythms in the heart. Together, this overview underscores the significance of circadian endocrine rhythms in normal health and disease prevention.

MECHANISMS IN ENDOCRINOLOGY: A sense of time of the glucocorticoid circadian clock: from the ontogeny to the diagnosis of Cushing's syndrome

The circadian rhythm of glucocorticoids has long been recognised within the last 75 years. Since the beginning, researchers have sought to identify basic mechanisms underlying the origin and emergence of the corticosteroid circadian rhythmicity among mammals. Accordingly, Young, Hall and Rosbash, laureates of the 2017 Nobel Prize in Physiology or Medicine, as well as Takahashi's group among others, have characterised the molecular cogwheels of the circadian system, describing interlocking transcription/translation feedback loops essential for normal circadian rhythms. Plasma glucocorticoid circadian variation depends on the expression of intrinsic clock genes within the anatomic components of the hypothalamic-pituitary-adrenal axis, which are organised in a hierarchical manner. This review presents a general overview of the glucocorticoid circadian clock mechanisms, highlighting the ontogeny of the pituitary-adrenal axis diurnal rhythmicity as well as the involvement of circadian rhythm abnormalities in the physiopathology and diagnosis of Cushing's disease.

Transcriptome Profile in Unilateral Adrenalectomy-Induced Compensatory Adrenal Growth in the Rat

Compensatory adrenal growth evoked by unilateral adrenalectomy (hemiadrenalectomy) constitutes one of the most frequently studied in vivo models of adrenocortical enlargement. This type of growth has been quite well characterized for its morphological, biochemical, and morphometric parameters. However, the molecular basis of compensatory adrenal growth is poorly understood. Therefore, the aim of this study was to investigate the rat adrenal transcriptome profile during the time of two previously described adrenocortical proliferation waves at 24 and 72 h after unilateral adrenalectomy. Surgical removal of the left adrenal or a sham operation was accomplished via the classic dorsal approach. As expected, the weight of the remaining right adrenal glands collected at 24 and 72 h after hemiadrenalectomy increased significantly. The transcriptome profile was identified by means of Affymetrix^® Rat Gene 2.1 ST Array. The general profiles of differentially expressed genes were visualized as volcano plots and heatmaps. Detailed analyzes consisted of identifying significantly enriched gene ontological groups relevant to adrenal physiology, by means of DAVID and GOplot bioinformatics tools. The results of our studies showed that compensatory adrenal growth induced by unilateral adrenalectomy exerts a limited influence on the global transcriptome profile of the rat adrenal gland; nevertheless, it leads to significant changes in the expression of key genes regulating the circadian rhythm. Our results confirm also that regulation of compensatory adrenal growth is under complex and multifactorial control with a pivotal role of neural regulatory mechanisms and a supportive role of other components.

Multimodal Regulation of Circadian Glucocorticoid Rhythm by Central and Adrenal Clocks

Adrenal glucocorticoids (GCs) control a wide range of physiological processes, including metabolism, cardiovascular and pulmonary activities, immune and inflammatory responses, and various brain functions. During stress responses, GCs are secreted through activation of the hypothalamic-pituitary-adrenal axis, whereas circulating GC levels in unstressed states follow a robust circadian oscillation with a peak around the onset of the active period of a day. A recent advance in chronobiological research has revealed that multiple regulatory mechanisms, along with classical neuroendocrine regulation, underlie this GC circadian rhythm. The hierarchically organized circadian system, with a central pacemaker in the suprachiasmatic nucleus of the hypothalamus and local oscillators in peripheral tissues, including the adrenal gland, mediates periodicities in physiological processes in mammals. In this review, we primarily focus on our understanding of the circadian regulation of adrenal GC rhythm, with particular attention to the cooperative actions of the suprachiasmatic nucleus central and adrenal local clocks, and the clinical implications of this rhythm in human diseases.

Time-of-Day Dependent Neuronal Injury After Ischemic Stroke: Implication of Circadian Clock Transcriptional Factor Bmal1 and Survival Kinase AKT

Occurrence of stroke cases displays a time-of-day variation in human. However, the mechanism linking circadian rhythm to the internal response mechanisms against pathophysiological events after ischemic stroke remained largely unknown. To this end, temporal changes in the susceptibility to ischemia/reperfusion (I/R) injury were investigated in mice in which the ischemic stroke induced at four different Zeitgeber time points with 6-h intervals (ZT0, ZT6, ZT12, and ZT18). Besides infarct volume and brain swelling, neuronal survival, apoptosis, ischemia, and circadian rhythm related proteins were examined using immunohistochemistry, Western blot, planar surface immune assay, and liquid chromatography-mass spectrometry tools. Here, we present evidence that midnight (ZT18; 24:00) I/R injury in mice resulted in significantly improved infarct volume, brain swelling, neurological deficit score, neuronal survival, and decreased apoptotic cell death compared with ischemia induced at other time points, which were associated with increased expressions of circadian proteins Bmal1, PerI, and Clock proteins and survival kinases AKT and Erk-1/2. Moreover, ribosomal protein S6, mTOR, and Bad were also significantly increased, while the levels of PRAS40, negative regulator of AKT and mTOR, and phosphorylated p53 were decreased at this time point compared to ZT0 (06:00). Furthermore, detailed proteomic analysis revealed significantly decreased CSKP, HBB-1/2, and HBA levels, while increased GNAZ, NEGR1, IMPCT, and PDE1B at midnight as compared with early morning. Our results indicate that nighttime I/R injury results in less severe neuronal damage, with increased neuronal survival, increased levels of survival kinases and circadian clock proteins, and also alters the circadian-related proteins.

Disturbances of diurnal phase markers, behavior, and clock genes in a rat model of depression; modulatory effects of agomelatine treatment

Major depressive disorder (MDD) is a growing problem worldwide. Though, the etiology remains unresolved, circadian rhythm disturbances are frequently observed in MDD and thus is speculated to play a key role herein. The present study focuses on circadian rhythm disturbances in the chronic mild stress (CMS) animal model of depression and examined whether the atypical antidepressant, agomelatine, which is mediating its action via melatonergic and serotonergic receptors, is capable of resynchronizing the perturbed rhythm. Melatonin is often used as a marker of the circadian phase, but the functional and behavioral output is dictated on a cellular level by the molecular clock, driven by the clock genes. We applied in situ hybridization histochemistry to measure the expression levels of the core clock genes, period (Per) 1 and 2 and bone and muscle ARNT-like protein 1 (Bmal1), in multiple brain regions believed to be implicated in depression. Agomelatine showed an antidepressant-like effect in the sucrose consumption test and an anxiolytic-like profile in the elevated zero maze. We found that CMS increased nighttime melatonin release in rats and that agomelatine attenuated this effect. Stress was shown to have a time and region-specific effect on clock gene expression in the brain. Treatment with agomelatine failed to normalize clock gene expression, and the observed modifying effect on gene expression did not associate with the antidepressant-like effect. This suggests that the antidepressant actions of agomelatine are mainly independent of circadian rhythm synchronization and, in this regard, not superior to traditional antidepressants tested in our model.

Hypophysectomy abolishes rhythms in rat thyroid hormones but not in the thyroid clock

The endocrine body rhythms including the hypothalamic-pituitary-thyroid axis seem to be regulated by the circadian timing system, and daily rhythmicity of circulating thyroid-stimulating hormone (TSH) is well established. The circadian rhythms are generated by endogenous clocks in the central brain oscillator located in the hypothalamic suprachiasmatic nucleus (SCN) as well as multiple peripheral clocks, but information on the existence and function of a thyroid clock is limited. The molecular machinery in all clock cells is composed of a number of clock genes and their gene products are connected by autoregulatory feedback loops. Here, we provide evidence for a thyroid clock in the rat by demonstrating 24-h antiphase oscillations for the mRNA of the canonical clock genes Per1 and Bmal1, which was unaffected by hypophysectomy. By immunostaining, we supported the existence of a core oscillator in the individual thyroid cells by demonstrating a daily cytoplasmatic-nuclear shuttling of PER1 protein. In normal rats, we found a significant daily rhythmicity in the circulating thyroid hormones preceded by a peak in TSH. In hypophysectomised rats, although the thyroid clock was not affected, the oscillations in circulating thyroid hormones were abolished and the levels were markedly lowered. No daily oscillations in the expression of TSH receptor mRNA were observed in neither control rats nor hypophysectomised rats. Our findings indicate that the daily rhythm of thyroid hormone secretion is governed by SCN signalling via the rhythmic TSH secretion rather than by the local thyroid clock, which was still ticking after hypophysectomy.

Restricted Feeding Schedules Modulate in a Different Manner the Expression of Clock Genes in Rat Hypothalamic Nuclei

Food access restriction is associated to changes in gene expression of the circadian clock system. However, there are only a few studies investigating the effects of non-photic synchronizers, such as food entrainment, on the expression of clock genes in the central oscillators. We hypothesized that different feeding restriction patterns could modulate the expression of clock genes in the suprachiasmatic nucleus (SCN) “master” clock and in extra-SCN oscillators such as the paraventricular (PVN) and arcuate (ARC) hypothalamic nuclei. Wistar rats were divided into four groups: Control group (CG; food available ad libitum), Restricted night-fed (RF-n; food access during 2 h at night), Restricted day-fed (RF-d; food access during 2 h at daytime), Day-fed (DF; food access during 12 h at daytime). After 21 days, rats were decapitated between ZT2-ZT3 (0800–0900 h); ZT11-ZT12 (1700–1800 h), or ZT17-18 (2300–2400 h). Plasma corticosterone was measured by radioimmunoassay (RIA). The expression of Clock, Bmal1, Per1, Per2, Per3, Cry1, Cry2, Rev-erbα, and Rorα were assessed in SCN, PVN, and ARC hypothalamic nuclei by RT-PCR and calculated by the 2^{[−DeltaDeltaCT(Cyclethreshold)]}(2^−ΔΔCT) method. Restricted food availability during few h led to decreased body weight in RF-n and RF-d groups compared to controls and DF group. We also observed an anticipatory corticosterone peak before food availability in RF-n and RF-d groups. Furthermore, the pattern of clock gene expression in response to RF-n, RF-d, and DF schedules was affected differently in the SCN, PVN, and ARC hypothalamic nuclei. In conclusion, the master oscillator in SCN as well as the oscillator in PVN and ARC, all brain areas involved in food intake, responds in a tissue-specific manner to feeding restriction.

Modulation of the circadian clock by glucocorticoid receptor isoforms in the H295R cell line

Peripheral clocks are set by different nervous, hormonal and metabolic stimuli, and regulate the circadian expression of several genes. We investigated whether a peripheral clock could be induced in the human adrenocortical cell line H295R and whether glucocorticoid receptor isoforms (GRα and GRß) are involved in this clock system. After synchronization of cells with serum shock, the rhythmic oscillation of clock genes PER1, PER2, REV-ERBα, and ARNTL was confirmed. In addition, H295R cells even without serum shock showed rhythmic expression of PER1, PER2, CRY1 and ARNTL. Glucocorticoid treatment induced a rapid response of PER1, PER2 and CRY1 in a GRα-dependent manner. Continuous glucocorticoid stimulation after 6h caused suppression of REV-ERBα. Administration of a GR antagonist, RU486, disrupted the circadian oscillation of clock genes and prevented the acute changes in PER1, PER2 and CRY1 levels. Overexpression of the GRß isoform alone did not alter the expression of the examined clock genes, but did prevent the GRα-related suppression of REV-ERBα. These alterations occurred independently from ACTH and CRH. Our data demonstrate that a peripheral clock system is present in a human adrenocortical cell line and that periodic oscillations of clock genes are influenced by glucocorticoids, mainly through GRα.

Interaction between circadian rhythms and stress

Life on earth has adapted to the day-night cycle by evolution of internal, so-called circadian clocks that adjust behavior and physiology to the recurring changes in environmental conditions. In mammals, a master pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus receives environmental light information and synchronizes peripheral tissues and central non-SCN clocks to geophysical time. Regulatory systems such as the hypothalamus-pituitary-adrenal (HPA) axis and the autonomic nervous system (ANS), both being important for the regulation of stress responses, receive strong circadian input. In this review, we summarize the interaction of circadian and stress systems and the resulting physiological and pathophysiological consequences. Finally, we critically discuss the relevance of rodent stress studies for humans, addressing complications of translational approaches and offering strategies to optimize animal studies from a chronobiological perspective.

Altered Expression Pattern of Clock Genes in a Rat Model of Depression

BACKGROUND

Abnormalities in circadian rhythms may be causal factors in development of major depressive disorder. The biology underlying a causal relationship between circadian rhythm disturbances and depression is slowly being unraveled. Although there is no direct evidence of dysregulation of clock gene expression in depressive patients, many studies have reported single-nucleotide polymorphisms in clock genes in these patients.

METHODS

In the present study we investigated whether a depression-like state in rats is associated with alternations of the diurnal expression of clock genes. The validated chronic mild stress (CMS) animal model of depression was used to investigate rhythmic expression of three clock genes: period genes 1 and 2 (Per1 and Per2) and Bmal1. Brain and liver tissue was collected from 96 animals after 3.5 weeks of CMS (48 control and 48 depression-like rats) at a 4h sampling interval within 24h. We quantified expression of clock genes on brain sections in the prefrontal cortex, nucleus accumbens, pineal gland, suprachiasmatic nucleus, substantia nigra, amygdala, ventral tegmental area, subfields of the hippocampus, and the lateral habenula using in situ hybridization histochemistry. Expression of clock genes in the liver was monitored by real-time quantitative polymerase chain reaction (PCR).

RESULTS

We found that the effect of CMS on clock gene expression was selective and region specific. Per1 exhibits a robust diurnal rhythm in most regions of interest, whereas Bmal1 and in particular Per2 were susceptible to CMS.

CONCLUSION

The present results suggest that altered expression of investigated clock genes is likely associated with the induction of a depression-like state in the CMS model.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Circadian Clocks, Stress, and Immunity

In mammals, molecular circadian clocks are present in most cells of the body, and this circadian network plays an important role in synchronizing physiological processes and behaviors to the appropriate time of day. The hypothalamic-pituitary-adrenal endocrine axis regulates the response to acute and chronic stress, acting through its final effectors - glucocorticoids - released from the adrenal cortex. Glucocorticoid secretion, characterized by its circadian rhythm, has an important role in synchronizing peripheral clocks and rhythms downstream of the master circadian pacemaker in the suprachiasmatic nucleus. Finally, glucocorticoids are powerfully anti-inflammatory, and recent work has implicated the circadian clock in various aspects and cells of the immune system, suggesting a tight interplay of stress and circadian systems in the regulation of immunity. This mini-review summarizes our current understanding of the role of the circadian clock network in both the HPA axis and the immune system, and discusses their interactions.

Sexual Differentiation of Circadian Clock Function in the Adrenal Gland

Sex differences in glucocorticoid production are associated with increased responsiveness of the adrenal gland in females. However, the adrenal-intrinsic mechanisms that establish sexual dimorphic function remain ill defined. Glucocorticoid production is gated at the molecular level by the circadian clock, which may contribute to sexual dimorphic adrenal function. Here we examine sex differences in the adrenal gland using an optical reporter of circadian clock function. Adrenal glands were cultured from male and female Period2::Luciferase (PER2::LUC) mice to assess clock function in vitro in real time. We confirm that there is a pronounced sex difference in the intrinsic capacity to sustain PER2::LUC rhythms in vitro, with higher amplitude rhythms in adrenal glands collected from males than from females. Changes in adrenal PER2::LUC rhythms over the reproductive life span implicate T as an important factor in driving sex differences in adrenal clock function. By directly manipulating hormone levels in adult mice in vivo, we demonstrate that T increases the amplitude of PER2::LUC rhythms in adrenal glands of both male and female mice. In contrast, we find little evidence that ovarian hormones modify adrenal clock function. Lastly, we find that T in vitro can increase the amplitude of PER2::LUC rhythms in male adrenals but not female adrenals, which suggests the existence of sex differences in the mechanisms of T action in vivo. Collectively these results reveal that activational effects of T alter circadian timekeeping in the adrenal gland, which may have implications for sex differences in stress reactivity and stress-related disorders.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Circadian Clocks and the Interaction between Stress Axis and Adipose Function

Many physiological processes and most endocrine functions show fluctuations over the course of the day. These so-called circadian rhythms are governed by an endogenous network of cellular clocks and serve as an adaptation to daily and, thus, predictable changes in the organism's environment. Circadian clocks have been described in several tissues of the stress axis and in adipose cells where they regulate the rhythmic and stimulated release of stress hormones, such as glucocorticoids, and various adipokine factors. Recent work suggests that both adipose and stress axis clock systems reciprocally influence each other and adrenal-adipose rhythms may be key players in the development and therapy of metabolic disorders. In this review, we summarize our current understanding of adrenal and adipose tissue rhythms and clocks and how they might interact to regulate energy homoeostasis and stress responses under physiological conditions. Potential chronotherapeutic strategies for the treatment of metabolic and stress disorders are discussed.

Adrenal Clocks and the Role of Adrenal Hormones in the Regulation of Circadian Physiology

The mammalian circadian timing system consists of a master pacemaker in the suprachiasmatic nucleus (SCN) and subordinate clocks that disseminate time information to various central and peripheral tissues. While the function of the SCN in circadian rhythm regulation has been extensively studied, we still have limited understanding of how peripheral tissue clock function contributes to the regulation of physiological processes. The adrenal gland plays a special role in this context as adrenal hormones show strong circadian secretion rhythms affecting downstream physiological processes. At the same time, they have been shown to affect clock gene expression in various other tissues, thus mediating systemic entrainment to external zeitgebers and promoting internal circadian alignment. In this review, we discuss the function of circadian clocks in the adrenal gland, how they are reset by the SCN and may further relay time-of-day information to other tissues. Focusing on glucocorticoids, we conclude by outlining the impact of adrenal rhythm disruption on neuropsychiatric, metabolic, immune, and malignant disorders.

Constant light disrupts the circadian rhythm of steroidogenic proteins in the rat adrenal gland

The circadian rhythm of corticosterone (CORT) secretion from the adrenal cortex is regulated by the suprachiasmatic nucleus (SCN), which is entrained to the light-dark cycle. Since the circadian CORT rhythm is associated with circadian expression of the steroidogenic acute regulatory (StAR) protein, we investigated the 24h pattern of hormonal secretion (ACTH and CORT), steroidogenic gene expression (StAR, SF-1, DAX1 and Nurr77) and the expression of genes involved in ACTH signalling (MC2R and MRAP) in rats entrained to a normal light-dark cycle. We found that circadian changes in ACTH and CORT were associated with the circadian expression of all gene targets; with SF-1, Nurr77 and MRAP peaking in the evening, and DAX1 and MC2R peaking in the morning. Since disruption of normal SCN activity by exposure to constant light abolishes the circadian rhythm of CORT in the rat, we also investigated whether the AM-PM variation of our target genes was also disrupted in rats exposed to constant light conditions for 5weeks. We found that the disruption of the AM-PM variation of ACTH and CORT secretion in rats exposed to constant light was accompanied by a loss of AM-PM variation in StAR, SF-1 and DAX1, and a reversed AM-PM variation in Nurr77, MC2R and MRAP. Our data suggest that circadian expression of StAR is regulated by the circadian expression of nuclear receptors and proteins involved in both ACTH signalling and StAR transcription. We propose that ACTH regulates the secretion of CORT via the circadian control of steroidogenic gene pathways that become dysregulated under the influence of constant light.

Altered rhythm of adrenal clock genes, StAR and serum corticosterone in VIP receptor 2-deficient mice

The circadian time-keeping system consists of clocks in the suprachiasmatic nucleus (SCN) and in peripheral organs including an adrenal clock linked to the rhythmic corticosteroid production by regulating steroidogenic acute regulatory protein (StAR). Clock cells contain an autonomous molecular oscillator based on a group of clock genes and their protein products. Mice lacking the VPAC2 receptor display disrupted circadian rhythm of physiology and behaviour, and therefore, we using real-time RT-PCR quantified (1) the mRNAs for the clock genes Per1 and Bmal1 in the adrenal gland and SCN, (2) the adrenal Star mRNA and (3) the serum corticosterone concentration both during a light/dark (L/D) cycle and at constant darkness in wild type (WT) and VPAC2 receptor-deficient mice (VPAC2-KO). We also examined if PER1 and StAR were co-localised in the adrenal steroidogenic cells. Per1 and Bmal1 mRNA showed a 24-h rhythmic expression in the adrenal of WT mice under L/D and dark conditions. During a L/D cycle, the adrenal clock gene rhythm in VPAC2-KO mice was phase-advanced by approximately 6 h compared to WT mice and became arrhythmic in constant darkness. A significant 24-h rhythmic variation in the adrenal Star mRNA expression and circulating corticosterone concentration was similarly phase-advanced during the L/D cycle. The loss of adrenal clock gene rhythm in the VPAC2 receptor knockout mice after transfer into constant darkness was accompanied by disappearance of rhythmicity in Star mRNA expression and serum corticosterone concentration. Double immunohistochemistry showed that the PER1 protein and StAR were co-localised in the same steroidogenic cells. Circulating corticosterone plays a role in the circadian timing system and the misaligned corticosterone rhythm in the VPAC2 receptor knockout mice could be involved in their abnormal rhythms of physiology.

Expression of the clock genes Per1 and Bmal1 during follicle development in the rat ovary. Effects of gonadotropin stimulation and hypophysectomy

Daily oscillations of clock genes have recently been demonstrated in the ovaries of several species. Clock gene knockout or mutant mice demonstrate a variety of reproductive defects. Accumulating evidence suggests that these rhythms act to synchronise the expression of specific ovarian genes to hypothalamo-pituitary signals and that they are regulated by one or both of the gonadotropins. The aim of this study has been to examine the spatio-temporal expression of the clock genes Per1 and Bmal1 during gonadotropin-independent and gonadotropin-dependent follicle development in the rat ovary. We have examined the ovaries of prepubertal rats, of prepubertal rats stimulated with equine chorionic gonadotropin (eCG)/human chorionic gonadotropin (hCG) and of hypophysectomised adult animals. Using quantitative reverse transcription with the polymerase chain reaction, in situ hybridisation histochemistry and immunohistochemistry, we have demonstrated that the expression of the two clock genes is low and arrhythmic in ovarian cells during early gonadotropin-independent follicle development in prepubertal animals and in hypophysectomised animals. We have also demonstrated that the expression of the clock genes becomes rhythmic following eCG stimulation in the theca interna cells and the secondary interstitial cells and that, following additional hCG stimulation, the expression of the clock genes also becomes rhythmic in the granulosa cells of preovulatory follicles. These findings link the initiation of clock gene rhythms in the rat ovary to the luteinising hormone receptor and suggest a functional link to androgen and progesterone production. In hypophysectomised animals, rhythmic clock gene expression is also observed in the corpora lutea and in secondary interstitial cells demonstrating that, in these compartments, entrainment of clock gene rhythms is gonadotropin-independent.

The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers

Approximately 30% of the general population suffers from insomnia. Given that insomnia causes many problems, amelioration of the symptoms is crucial. Recently, we found that a non-essential amino acid, glycine subjectively and objectively improves sleep quality in humans who have difficulty sleeping. We evaluated the effects of glycine on daytime sleepiness, fatigue, and performances in sleep-restricted healthy subjects. Sleep was restricted to 25% less than the usual sleep time for three consecutive nights. Before bedtime, 3 g of glycine or placebo were ingested, sleepiness, and fatigue were evaluated using the visual analog scale (VAS) and a questionnaire, and performance were estimated by personal computer (PC) performance test program on the following day. In subjects given glycine, the VAS data showed a significant reduction in fatigue and a tendency toward reduced sleepiness. These observations were also found via the questionnaire, indicating that glycine improves daytime sleepiness and fatigue induced by acute sleep restriction. PC performance test revealed significant improvement in psychomotor vigilance test. We also measured plasma melatonin and the expression of circadian-modulated genes expression in the rat suprachiasmatic nucleus (SCN) to evaluate the effects of glycine on circadian rhythms. Glycine did not show significant effects on plasma melatonin concentrations during either the dark or light period. Moreover, the expression levels of clock genes such as Bmal1 and Per2 remained unchanged. However, we observed a glycine-induced increase in the neuropeptides arginine vasopressin and vasoactive intestinal polypeptide in the light period. Although no alterations in the circadian clock itself were observed, our results indicate that glycine modulated SCN function. Thus, glycine modulates certain neuropeptides in the SCN and this phenomenon may indirectly contribute to improving the occasional sleepiness and fatigue induced by sleep restriction.

Treatment of corticotroph deficiency

Corticotroph deficiency is a crucial element of anterior pituitary failure requiring careful management. Clinicians should always look for corticotroph deficiency in patients with pituitary disease and also consider the diagnosis in patients with unexplained fatigue, especially when associated with weight loss and/or hypotension. The diagnosis is based on the morning (8 a.m.) serum cortisol level, generally completed with a stimulation test, keeping in mind its poor sensitivity. Metopirone or insulin tolerance tests may be needed. Treatment is based on administration of hydrocortisone at the daily dose of about 20mg, fractionated into two or three doses if possible. There are no reliable biological parameters for monitoring therapeutic efficacy. Treatment is adapted as a function of clinical criteria: fatigue, blood pressure, body weight, and skin trophicity. Therapeutic education is a key element for the prevention of acute adrenal failure, which can occur if the treatment is not correctly adapted during episodes of gastrointestinal disease or stress.

Circadian rhythms in the hypothalamo-pituitary-adrenal (HPA) axis

The pronounced daily variation in the release of adrenal hormones has been at the heart of the deciphering and understanding of the circadian timing system. Indeed, the first demonstration of an endocrine day/night rhythm was provided by Pincus (1943), by showing a daily pattern of 17-keto-steroid excretion in the urine of 7 healthy males. Twenty years later the adrenal gland was one of the very first organs to show, in vitro, that circadian rhythmicity was maintained. In the seventies, experimental manipulation of the daily corticosterone rhythm served as evidence for the identification of respectively the light- and food-entrainable oscillator. Another 20 years later the hypothalamo-pituitary-adrenal (HPA)-axis was key in furthering our understanding of the way in which rhythmic signals generated by the central pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) are forwarded to the rest of the brain and to the organism as a whole. To date, the adrenal gland is still of prime importance for understanding how the oscillations of clock genes in peripheral tissues result in functional rhythms of these tissues, whereas it has become even more evident that adrenal glucocorticoids are key in the resetting of the circadian system after a phase-shift. The HPA-axis thus still is an excellent model for studying the transmission of circadian information in the body.

Clocks on top: the role of the circadian clock in the hypothalamic and pituitary regulation of endocrine physiology

Recent strides in circadian biology over the last several decades have allowed researchers new insight into how molecular circadian clocks influence the broader physiology of mammals. Elucidation of transcriptional feedback loops at the heart of endogenous circadian clocks has allowed for a deeper analysis of how timed cellular programs exert effects on multiple endocrine axes. While the full understanding of endogenous clocks is currently incomplete, recent work has re-evaluated prior findings with a new understanding of the involvement of these cellular oscillators, and how they may play a role in constructing rhythmic hormone synthesis, secretion, reception, and metabolism. This review addresses current research into how multiple circadian clocks in the hypothalamus and pituitary receive photic information from oscillators within the hypothalamic suprachiasmatic nucleus (SCN), and how resultant hypophysiotropic and pituitary hormone release is then temporally gated to produce an optimal result at the cognate target tissue. Special emphasis is placed not only on neural communication among the SCN and other hypothalamic nuclei, but also how endogenous clocks within the endocrine hypothalamus and pituitary may modulate local hormone synthesis and secretion in response to SCN cues. Through evaluation of a larger body of research into the impact of circadian biology on endocrinology, we can develop a greater appreciation into the importance of timing in endocrine systems, and how understanding of these endogenous rhythms can aid in constructing appropriate therapeutic treatments for a variety of endocrinopathies.

The Changes They are A-Timed: Metabolism, Endogenous Clocks, and the Timing of Puberty

Childhood obesity has increased dramatically over the last several decades, particularly in industrialized countries, often accompanied by acceleration of pubertal progression and associated reproductive abnormalities (Biro et al., 2006; Rosenfield et al., 2009). The timing of pubertal initiation and progression in mammals is likely influenced by nutritional and metabolic state, leading to the hypothesis that deviations from normal metabolic rate, such as those seen in obesity, may contribute to observed alterations in the rate of pubertal progression. While several recent reviews have addressed the effects of metabolic disorders on reproductive function in general, this review will explore previous and current models of pubertal timing, outlining a potential role of endogenous timing mechanisms such as cellular circadian clocks in the initiation of puberty, and how these clocks might be altered by metabolic factors. Additionally, we will examine recently elucidated neuroendocrine regulators of pubertal progression such as kisspeptin, explore models detailing how the mammalian reproductive axis is silenced during the juvenile period and reactivated at appropriate developmental times, and emphasize how metabolic dysfunction such as childhood obesity may alter timing cues that advance or delay pubertal progression, resulting in diminished reproductive capacity.

When does it start ticking? Ontogenetic development of the mammalian circadian system

Circadian rhythms in physiology and behavior ensure that vital functions are temporally synchronized with cyclic environmental changes. In mammals, the circadian system is conducted by a central circadian rhythm generator that resides in the hypothalamic suprachiasmatic nucleus (SCN) and controls multiple subsidiary circadian oscillators in the periphery. The molecular clockwork in SCN and peripheral oscillators consists of autoregulatory transcriptional/translational feedback loops of clock genes. The adult circadian system is synchronized to the astrophysical day by light whereas the fetal and neonatal circadian system entrains to nonphotic rhythmic maternal signals. This chapter reviews maturation and entrainment of the central circadian rhythm generator in the SCN and of peripheral oscillators during ontogenetic development.

The adrenal peripheral clock: glucocorticoid and the circadian timing system

The mammalian circadian timing system is organized in a hierarchy, with the master clock residing in the suprachiasmatic nucleus (SCN) of the hypothalamus and subsidiary peripheral clocks in other brain regions as well as peripheral tissues. Since the local oscillators in most cells contain a similar molecular makeup to that in the central pacemaker, determining the role of the peripheral clocks in the regulation of rhythmic physiology and behavior is an important issue. Glucocorticoids (GCs) are a class of multi-functional adrenal steroid hormones, which exhibit a robust circadian rhythm, with a peak linked with the onset of the daily activity phase. It has long been believed that the production and secretion of GC is primarily governed through the hypothalamus-pituitary-adrenal (HPA) neuroendocrine axis in mammals. Growing evidence, however, strongly supports the notion that the periodicity of GC involves the integrated activity of multiple regulatory mechanisms related to circadian timing system along with the classical HPA neuroendocrine regulation. The adrenal-intrinsic oscillator as well as the central pacemaker plays a pivotal role in its rhythmicity. GC influences numerous biological processes, such as metabolic, cardiovascular, immune and even higher brain functions, and also acts as a resetting signal for the ubiquitous peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior. In this review, we will therefore focus on the recent advances in our understanding of the circadian regulation of adrenal GC and its functional relevance.

A circadian clock entrained by melatonin is ticking in the rat fetal adrenal

The adrenal gland in the adult is a peripheral circadian clock involved in the coordination of energy intake and expenditure, required for adaptation to the external environment. During fetal life, a peripheral circadian clock is present in the nonhuman primate adrenal gland. Whether this extends to the fetal adrenal gland like the rat is unknown. Here we explored in vivo and in vitro whether the rat fetal adrenal is a peripheral circadian clock entrained by melatonin. We measured the 24-h changes in adrenal content of corticosterone and in the expression of clock genes Per-2 and Bmal-1 and of steroidogenic acute regulatory protein (StAR), Mt1 melatonin receptor, and early growth response protein 1 (Egr-1) expression. In culture, we explored whether oscillatory expression of these genes persisted during 48 h and the effect of a 4-h melatonin pulse on their expression. In vivo, the rat fetal adrenal gland showed circadian expression of Bmal-1 and Per-2 in antiphase (acrophases at 2200 and 1300 h, respectively) as well as of Mt1 and Egr-1. This was accompanied by circadian rhythms of corticosterone content and of StAR expression both peaking at 0600 h. The 24-h oscillatory expression of Bmal-1, Per-2, StAR, Mt1, and Egr-1 persisted during 48 h in culture; however, the antiphase between Per-2 and Bmal-1 was lost. The pulse of melatonin shifted the acrophases of all the genes studied and restored the antiphase between Per-2 and Bmal-1. Thus, in the rat, the fetal adrenal is a strong peripheral clock potentially amenable to regulation by maternal melatonin.

The clock gene Per2 is required for normal platelet formation and function

INTRODUCTION

Apoptotic cell death is a highly regulated genetic program, which has been observed in mature megakaryocytes fragmenting into platelets. The clock gene Per2, a key component of core clock oscillator, was involved in affecting both cell cycle control and apoptosis. Thus, loss of Per2 function may be considered potential influence of platelet formation and function.

METHODS

Per2-null mice and C57BL/6 mice were used in the study. Bleeding time, platelet count, megakaryocyte count, megakaryocyte ploidy, megakaryocyte apoptosis, rate of proplatelet formation, clot retraction, platelet aggregation and secretion were performed to evaluate thrombopoiesis and hemostasis. Quantitative RT-PCR was employed to analyze genes expression in liver, bone marrow and enriched megakaryocytes.

RESULTS

The Per2-null mice had nearly 50% platelet counts in peripheral blood. Per2-null platelets were compromised in their ability to aggregate and secretion, consistent with a marked reduction in the number of dense and a-granules. Megakaryocytes from Per2-null mice showed no significant variation in number but increased in ploidy. Ultrastructural examination of Per2-null megakaryocytes revealed many vacuoles in demarcation membranes and reduction in platelet granules. Megakaryocytes from Per2-null bone marrow decreased the rate of proplatelet formation and impaired apoptosis. Per2-null mice showed increased both in Tpo in livers and its receptors C-mpl in bone marrow, and the megakaryocytes from these mice decreased P53 expression, consequently increased Bcl-xl and Bcl-2 level.

CONCLUSIONS

The clock gene Per2 modulating the apoptosis of megakaryocytes was required for platelet formation and function.

The comparison between circadian oscillators in mouse liver and pituitary gland reveals different integration of feeding and light schedules

The mammalian circadian system is composed of multiple peripheral clocks that are synchronized by a central pacemaker in the suprachiasmatic nuclei of the hypothalamus. This system keeps track of the external world rhythms through entrainment by various time cues, such as the light-dark cycle and the feeding schedule. Alterations of photoperiod and meal time modulate the phase coupling between central and peripheral oscillators. In this study, we used real-time quantitative PCR to assess circadian clock gene expression in the liver and pituitary gland from mice raised under various photoperiods, or under a temporal restricted feeding protocol. Our results revealed unexpected differences between both organs. Whereas the liver oscillator always tracked meal time, the pituitary circadian clockwork showed an intermediate response, in between entrainment by the light regimen and the feeding-fasting rhythm. The same composite response was also observed in the pituitary gland from adrenalectomized mice under daytime restricted feeding, suggesting that circulating glucocorticoids do not inhibit full entrainment of the pituitary clockwork by meal time. Altogether our results reveal further aspects in the complexity of phase entrainment in the circadian system, and suggest that the pituitary may host oscillators able to integrate multiple time cues.

Angiotensin II-regulated transcription regulatory genes in adrenal steroidogenesis

Transcription regulatory genes are crucial modulators of cell physiology and metabolism whose intracellular levels are tightly controlled in response to extracellular stimuli. We previously reported a set of 29 transcription regulatory genes modulated by angiotensin II in H295R human adrenocortical cells and their roles in regulating the expression of the last and unique enzymes of the glucocorticoid and mineralocorticoid biosynthetic pathways, 11β-hydroxylase and aldosterone synthase, respectively, using gene expression reporter assays. To study the effect of this set of transcription regulatory genes on adrenal steroidogenesis, H295R cells were transfected by high-efficiency nucleofection and aldosterone and cortisol were measured in cell culture supernatants under basal and angiotensin II-stimulated conditions. BCL11B, BHLHB2, CITED2, ELL2, HMGA1, MAFF, NFIL3, PER1, SERTAD1, and VDR significantly stimulated aldosterone secretion, while EGR1, FOSB, and ZFP295 decreased aldosterone secretion. BTG2, HMGA1, MITF, NR4A1, and ZFP295 significantly increased cortisol secretion, while BCL11B, NFIL3, PER1, and SIX2 decreased cortisol secretion. We also report the effect of some of these regulators on the expression of endogenous aldosterone synthase and 11β-hydroxylase under basal and angiotensin II-stimulated conditions. In summary, this study reports for the first time the effects of a set of angiotensin II-modulated transcription regulatory genes on aldosterone and cortisol secretion and the expression levels of the last and unique enzymes of the mineralocorticoid and glucocorticoid biosynthetic pathways. Abnormal regulation of mineralocorticoid or glucocorticoid secretion is involved in several pathophysiological conditions. These transcription regulatory genes may be involved in adrenal steroidogenesis pathologies; thus they merit additional study as potential candidates for therapeutic intervention.

Replication of cortisol circadian rhythm: new advances in hydrocortisone replacement therapy

Cortisol has one of the most distinct and fascinating circadian rhythms in human physiology. This is regulated by the central clock located in the suprachiasmatic nucleus of the hypothalamus. It has been suggested that cortisol acts as a secondary messenger between central and peripheral clocks, hence its importance in the synchronization of body circadian rhythms. Conventional immediate-release hydrocortisone, either at twice- or thrice-daily doses, is not capable of replicating physiological cortisol circadian rhythm and patients with adrenal insufficiency or congenital adrenal hyperplasia still suffer from a poor quality of life and increased mortality. Novel treatments for replacement therapy are therefore essential. Proof-of-concept studies using hydrocortisone infusions suggest that the circadian delivery of hydrocortisone may improve biochemical control and life quality in patients lacking cortisol with an impaired cortisol rhythm. Recently oral formulations of modified-release hydrocortisone are being developed and it has been shown that it is possible to replicate cortisol circadian rhythm and also achieve better control of morning androgen levels. These new drug therapies are promising and potentially offer a more effective treatment with less adverse effects. Definite improvements clearly need to be established in future clinical trials.