Adrenal sensitivity to adrenocorticotropin varies diurnally

The interplay between the PI3K/AKT pathway and circadian clock in physiologic and cancer-related pathologic conditions

The circadian clock is responsible for the regulation of different cellular processes, and its disturbance has been linked to the development of different diseases, such as cancer. The main molecular mechanism for this issue has been linked to the crosstalk between core clock regulators and intracellular pathways responsible for cell survival. The PI3K/AKT signalling pathway is one of the most known intracellular pathways in the case of cancer initiation and progression. This pathway regulates different aspects of cell survival including proliferation, apoptosis, metabolism, and response to environmental stimuli. Accumulating evidence indicates that there is a link between the PI3K/AKT pathway activity and circadian rhythm in physiologic and cancer-related pathogenesis. Different classes of PI3Ks and AKT isoforms are involved in regulating circadian clock components in a transcriptional and functional manner. Reversely, core clock components induce a rhythmic fashion in PI3K and AKT activity in physiologic and pathogenic conditions. The aim of this review is to re-examine the interplay between this pathway and circadian clock components in normal condition and cancer pathogenesis, which provides a better understanding of how circadian rhythms may be involved in cancer progression.

Role of Omega-3 Fatty Acids as Non-Photic Zeitgebers and Circadian Clock Synchronizers

Omega-3 fatty acids (ω-3 FAs) are well-known for their actions on immune/inflammatory and neurological pathways, functions that are also under circadian clock regulation. The daily photoperiod represents the primary circadian synchronizer ('zeitgeber'), although diverse studies have pointed towards an influence of dietary FAs on the biological clock. A comprehensive literature review was conducted following predefined selection criteria with the aim of updating the evidence on the molecular mechanisms behind circadian rhythm regulation by ω-3 FAs. We collected preclinical and clinical studies, systematic reviews, and metanalyses focused on the effect of ω-3 FAs on circadian rhythms. Twenty animal (conducted on rodents and piglets) and human trials and one observational study providing evidence on the regulation of neurological, inflammatory/immune, metabolic, reproductive, cardiovascular, and biochemical processes by ω-3 FAs via clock genes were discussed. The evidence suggests that ω-3 FAs may serve as non-photic zeitgebers and prove therapeutically beneficial for circadian disruption-related pathologies. Future work should focus on the role of clock genes as a target for the therapeutic use of ω-3 FAs in inflammatory and neurological disorders, as well as on the bidirectional association between the molecular clock and ω-3 FAs.

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.

Sex Differences in Circadian Rhythms

Sex as a biological variable is the focus of much literature and has been emphasized by the National Institutes of Health, in part, to remedy a long history of male-dominated studies in preclinical and clinical research. We propose that time-of-day is also a crucial biological variable in biomedical research. In common with sex differences, time-of-day should be considered in analyses and reported to improve reproducibility of studies and to provide the appropriate context to the conclusions. Endogenous circadian rhythms are present in virtually all living organisms, including bacteria, plants, invertebrates, and vertebrates. Virtually all physiological and behavioral processes display daily fluctuations in optimal performance that are driven by these endogenous circadian clocks; importantly, many of those circadian rhythms also show sex differences. In this review, we describe some of the documented sex differences in circadian rhythms.

Molecular regulations of circadian rhythm and implications for physiology and diseases

The term “circadian rhythms” describes endogenous oscillations with ca. 24-h period associated with the earth’s daily rotation and light/dark cycle. Such rhythms reflect the existence of an intrinsic circadian clock that temporally orchestrates physiological processes to adapt the internal environment with the external cues. At the molecular level, the circadian clock consists of multiple sets of transcription factors resulting in autoregulatory transcription-translation feedback loops. Notably, in addition to their primary role as generator of circadian rhythm, the biological clock plays a key role in controlling physiological functions of almost all tissues and organs. It regulates several intracellular signaling pathways, ranging from cell proliferation, DNA damage repair and response, angiogenesis, metabolic and redox homeostasis, to inflammatory and immune response. In this review, we summarize findings showing the crosstalk between the circadian molecular clock and some key intracellular pathways, describing a scenario wherein their reciprocal regulation impinges upon several aspects of mammalian physiology. Moreover, based on evidence indicating that circadian rhythms can be challenged by environmental factors, social behaviors, as well as pre-existing pathological conditions, we discuss implications of circadian misalignment in human pathologies, such as cancer and inflammatory diseases. Accordingly, disruption of circadian rhythm has been reported to affect several physiological processes that are relevant to human diseases. Expanding our understanding of this field represents an intriguing and transversal medicine challenge in order to establish a circadian precision medicine.

Changes in adrenal functioning induced by chronic psychosocial stress in male mice: A time course study

BACKGROUND AND AIM

Chronic subordinate colony housing (CSC, 19 days), an established and preclinically-validated mouse model for posttraumatic stress disorder (PTSD), causes evening hypocorticism and a reduced adrenal in vitro ACTH (adrenocorticotropic hormone) sensitivity despite pronounced adrenal hyperplasia. However, until now it remains unclear at what time point during CSC exposure evening hypocorticism and adrenal in vitro ACTH insensitivity develop and whether the repeated change of dominant aggressor mice plays an important role in this context. It is, therefore, the aim of the current study, to explore the detailed time course of these stress-induced adrenal changes.

METHODS

Adrenal weight, plasma corticosterone (CORT) and ACTH were assessed in the morning of days 8 (right before exposure to the 2nd aggressor), 9 (24 h after exposure to the 2nd aggressor), 15 (right before exposure to the 3rd aggressor), 16 (24 h after exposure to the 3rd aggressor) and 20 or in the evening of days 8 (10 h after exposure to the 2nd aggressor), 9 (34 h after exposure to the 2nd aggressor), 15 (10 h after exposure to the 3rd aggressor), 16 (34 h after exposure to the 3rd aggressor) and 20 of CSC exposure. Moreover, we in vitro cultured adrenal explants of all mice euthanized in the morning of days 8, 9, 15, 16 and 20 either in the presence or absence of ACTH to subsequently assess CORT concentration in the supernatants.

RESULTS

Our results indicate that while adrenal mass was increased at all time points assessed, plasma morning CORT only transiently increased in response to the 2nd (on day 8) but not 3rd (on day 15) dominant aggressor mouse. Moreover, although mild signs of adrenal in vitro ACTH insensitivity developed already after one week of CSC exposure, moderate and severe adrenal in vitro ACTH insensitivity required two and three weeks of chronic subordination, respectively.

CONCLUSION

Together with unaffected plasma ACTH levels at all time points assessed, our data suggest that stress-induced adrenal in vitro ACTH insensitivity develops gradually during times of chronic subordination while subordination to different aggressor mice aggravates its severity. Moreover, a mild form of adrenal ACTH insensitivity seems to allow prevention of morning hypercorticism on day 8 of CSC, despite functional adrenal mass being increased, while a moderate and severe form of adrenal ACTH insensitivity in CSC mice seems to promote HPA axis adaptation to repeated homotypic stressor exposure (i.e. dominant aggressor mice) and basal evening hypocorticism in CSC mice, respectively. Our results might, therefore, be the basis for future clinical studies assessing CORT supplementation as novel treatment regimen for somatic and affective pathologies linked to chronic and/or traumatic stress.

Time after time: circadian clock regulation of intestinal stem cells

Daily fluctuations in animal physiology, known as circadian rhythms, are orchestrated by a conserved molecular timekeeper, known as the circadian clock. The circadian clock forms a transcription-translation feedback loop that has emerged as a central biological regulator of many 24-h processes. Early studies of the intestine discovered that many digestive functions have a daily rhythm and that intestinal cell production was similarly time-dependent. As genetic methods in model organisms have become available, it has become apparent that the circadian clock regulates many basic cellular functions, including growth, proliferation, and differentiation, as well as cell signalling and stem cell self-renewal. Recent connections between circadian rhythms and immune system function, and between circadian rhythms and microbiome dynamics, have also been revealed in the intestine. These processes are highly relevant in understanding intestinal stem cell biology. Here we describe the circadian clock regulation of intestinal stem cells primarily in two model organisms: Drosophila melanogaster and mice. Like all cells in the body, intestinal stem cells are subject to circadian timing, and both cell-intrinsic and cell-extrinsic circadian processes contribute to their function.

Rhythmicity matters: Circadian and ultradian patterns of HPA axis activity

Oscillations are a fundamental feature of neural and endocrine systems. The hypothalamic-pituitary-adrenal (HPA) axis dynamically controls corticosteroid secretion in basal conditions and in response to stress. Across the 24-h day, HPA axis activity oscillates with both an ultradian and circadian rhythm. These rhythms have been shown to be important for regulating metabolism, inflammation, mood, cognition and stress responsiveness. Here we will discuss the neural and endocrine mechanisms driving these rhythms, the physiological importance of these rhythms and health consequences when they are disrupted.

Resetting the Aging Clock: Implications for Managing Age-Related Diseases

Worldwide, individuals are living longer due to medical and scientific advances, increased availability of medical care and changes in public health policies. Consequently, increasing attention has been focused on managing chronic conditions and age-related diseases to ensure healthy aging. The endogenous circadian system regulates molecular, physiological and behavioral rhythms orchestrating functional coordination and processes across tissues and organs. Circadian disruption or desynchronization of circadian oscillators increases disease risk and appears to accelerate aging. Reciprocally, aging weakens circadian function aggravating age-related diseases and pathologies. In this review, we summarize the molecular composition and structural organization of the circadian system in mammals and humans, and evaluate the technological and societal factors contributing to the increasing incidence of circadian disorders. Furthermore, we discuss the adverse effects of circadian dysfunction on aging and longevity and the bidirectional interactions through which aging affects circadian function using examples from mammalian research models and humans. Additionally, we review promising methods for managing healthy aging through behavioral and pharmacological reinforcement of the circadian system. Understanding age-related changes in the circadian clock and minimizing circadian dysfunction may be crucial components to promote healthy aging.

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.

Chronic stress alters adrenal clock function in a sexually dimorphic manner

Glucocorticoid production is gated at the molecular level by the circadian clock in the adrenal gland. Stress influences daily rhythms in behavior and physiology, but it remains unclear how stress affects the function of the adrenal clock itself. Here, we examine the influence of stress on adrenal clock function by tracking PERIOD2::LUCIFERASE (PER2::LUC) rhythms in vitro Relative to non-stressed controls, adrenals from stressed mice displayed marked changes in PER2::LUC rhythms. Interestingly, the effect of stress on adrenal rhythms varied by sex and the type of stress experienced in vivo To investigate the basis of sex differences in the adrenal response to stress, we next stimulated male and female adrenals in vitro with adrenocorticotropic hormone (ACTH). ACTH shifted phase and increased amplitude of adrenal PER2::LUC rhythms. Both phase and amplitude responses were larger in female adrenals than in male adrenals, an observation consistent with previously described sex differences in the physiological response to stress. Lastly, we reversed the sex difference in adrenal clock function using stress and sex hormone manipulations to test its role in driving adrenal responses to ACTH. We find that adrenal responsiveness to ACTH is inversely proportional to the amplitude of adrenal PER2::LUC rhythms. This suggests that larger ACTH responses from female adrenals may be driven by their lower amplitude molecular rhythms. Collectively, these results indicate a reciprocal relationship between stress and the adrenal clock, with stress influencing adrenal clock function and the state of the adrenal clock gating the response to stress in a sexually dimorphic manner.

The Arcuate Nucleus: A Site of Fast Negative Feedback for Corticosterone Secretion in Male Rats

Variations in circulating corticosterone (Cort) are driven by the paraventricular nucleus of the hypothalamus (PVN), mainly via the sympathetic autonomic nervous system (ANS) directly stimulating Cort release from the adrenal gland and via corticotropin-releasing hormone targeting the adenohypophysis to release adrenocorticotropic hormone (ACTH). Cort feeds back through glucocorticoid receptors (GRs). Here we show in male Wistar rats that PVN neurons projecting to the adrenal gland do not express GRs, leaving the question of how the ANS in the PVN gets information about circulating Cort levels to control the adrenal. Since the arcuate nucleus (ARC) shows a less restrictive blood-brain barrier, expresses GRs, and projects to the PVN, we investigated whether the ARC can detect and produce fast adjustments of circulating Cort. In low Cort conditions (morning), local microdialysis in the ARC with type I GR antagonist produced a fast and sustained increase of Cort. This was not observed with a type II antagonist. At the circadian peak levels of Cort (afternoon), a type II GR antagonist, but not a type I antagonist, increased Cort levels but not ACTH levels. Antagonist infusions in the PVN did not modify circulating Cort levels, demonstrating the specificity of the ARC to give Cort negative feedback. Furthermore, type I and II GR agonists in the ARC prevented the increase of Cort after stress, demonstrating the role of the ARC as sensor to modulate Cort release. Our findings show that the ARC may be essential to sense blood levels of Cort and adapt Cort secretion depending on such conditions as stress or time of day.

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.

Sexual Dimorphism in Circadian Physiology Is Altered in LXRα Deficient Mice

The mammalian circadian timing system coordinates key molecular, cellular and physiological processes along the 24-h cycle. Accumulating evidence suggests that many clock-controlled processes display a sexual dimorphism. In mammals this is well exemplified by the difference between the male and female circadian patterns of glucocorticoid hormone secretion and clock gene expression. Here we show that the non-circadian nuclear receptor and metabolic sensor Liver X Receptor alpha (LXRα) which is known to regulate glucocorticoid production in mice modulates the sex specific circadian pattern of plasma corticosterone. Lxrα^-/- males display a blunted corticosterone profile while females show higher amplitude as compared to wild type animals. Wild type males are significantly slower than females to resynchronize their locomotor activity rhythm after an 8 h phase advance but this difference is abrogated in Lxrα^-/- males which display a female-like phenotype. We also show that circadian expression patterns of liver 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and Phosphoenolpyruvate carboxykinase (Pepck) differ between sexes and are differentially altered in Lxrα^-/- animals. These changes are associated with a damped profile of plasma glucose oscillation in males but not in females. Sex specific alteration of the insulin and leptin circadian profiles were observed in Lxα^-/- females and could be explained by the change in corticosterone profile. Together this data indicates that LXRα is a determinant of sexually dimorphic circadian patterns of key physiological parameters. The discovery of this unanticipated role for LXRα in circadian physiology underscores the importance of addressing sex differences in chronobiology studies and future LXRα targeted therapies.

Pregnancy-induced adaptations of the central circadian clock and maternal glucocorticoids

Maternal physiological adaptations, such as changes to the hypothalamic-pituitary-adrenal (HPA) axis, are central to pregnancy success. Circadian variation of the HPA axis is dependent on clock gene rhythms in the hypothalamus, but it is not known whether pregnancy-induced changes in maternal glucocorticoid levels are mediated via this central clock. We hypothesized that hypothalamic expression of clock genes changes across mouse pregnancy and this is linked to altered HPA activity. The anterior hypothalamus and maternal plasma were collected from C57Bl/6J mice prior to pregnancy and on days 6, 10, 14 and 18 of gestation (term=d19), across a 24-h period (0800, 1200, 1600, 2000, 0000, 0400 h). Hypothalamic expression of clock genes and Crh was determined by qPCR, plasma ACTH concentration measured by Milliplex assay and plasma corticosterone concentration by LC-MS/MS. Expression of all clock genes varied markedly across gestation, most notably at mid-gestation when levels of each gene were elevated. The pregnancy-induced increase in maternal corticosterone levels (by up to 14-fold on day 14) was not accompanied by a parallel shift in plasma ACTH (28% lower on day 14 compared with non-pregnant levels). Moreover, while circadian rhythmicity in corticosterone was maintained up to day 14 of gestation, this was effectively lost by day 18. Overall, our data show that the central circadian clock undergoes marked adaptations throughout mouse pregnancy, changes that are likely to contribute to maternal physiological adaptations. Importantly, however, neither hypothalamic clock genes nor plasma ACTH levels appear to drive the marked increase in maternal corticosterone after mid-gestation.

Preanalytical Confounding Factors in the Analysis of Cerebrospinal Fluid Biomarkers for Alzheimer's Disease: The Issue of Diurnal Variation

Given the growing use of cerebrospinal fluid (CSF) beta-amyloid (Aβ) and tau as biomarkers for early diagnosis of Alzheimer’s disease (AD), it is essential that the diagnostic procedures are standardized and the results comparable across different laboratories. Preanalytical factors are reported to be the cause of at least 50% of the total variability. Among them, diurnal variability is a key issue and may have an impact on the comparability of the values obtained. The available studies on this issue are not conclusive so far. Fluctuations of CSF biomarkers in young healthy volunteers have been previously reported, while subsequent studies have not confirmed those observations in older subjects, the ones most likely to receive this test. The observed differences in circadian rhythms need to be further assessed not only in classical CSF biomarkers but also in novel forthcoming biomarkers. In this review, the existing data on the issue of diurnal variations of CSF classical biomarkers for AD will be analyzed, also evaluating the available data on new possible biomarkers.

Altered entrainment to the day/night cycle attenuates the daily rise in circulating corticosterone in the mouse

The suprachiasmatic nucleus (SCN) is a circadian oscillator entrained to the day/night cycle via input from the retina. Serotonin (5-HT) afferents to the SCN modulate retinal signals via activation of 5-HT1B receptors, decreasing responsiveness to light. Consequently, 5-HT1B receptor knockout (KO) mice entrain to the day/night cycle with delayed activity onsets. Since circulating corticosterone levels exhibit a robust daily rhythm peaking around activity onset, we asked whether delayed entrainment of activity onsets affects rhythmic corticosterone secretion. Wheel-running activity and plasma corticosterone were monitored in mice housed under several different lighting regimens. Both duration of the light:dark cycle (T cycle) and the duration of light within that cycle was altered. 5-HT1B KO mice that entrained to a 9.5L:13.5D (short day in a T = 23 h) cycle with activity onsets delayed more than 4 h after light offset exhibited a corticosterone rhythm in phase with activity rhythms but reduced 50% in amplitude compared to animals that initiated daily activity <4 h after light offset. Wild type mice in 8L:14D (short day in a T = 22 h) conditions with highly delayed activity onsets also exhibited a 50% reduction in peak plasma corticosterone levels. Exogenous adrenocorticotropin (ACTH) stimulation in animals exhibiting highly delayed entrainment suggested that the endogenous rhythm of adrenal responsiveness to ACTH remained aligned with SCN-driven behavioral activity. Circadian clock gene expression in the adrenal cortex of these same animals suggested that the adrenal circadian clock was also aligned with SCN-driven behavior. Under T cycles <24 h, altered circadian entrainment to short day (winter-like) conditions, manifest as long delays in activity onset after light offset, severely reduces the amplitude of the diurnal rhythm of plasma corticosterone. Such a pronounced reduction in the glucocorticoid rhythm may alter rhythmic gene expression in the central nervous system and in peripheral organs contributing to an array of potential pathophysiologies.

Gonadal steroid hormones and the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis represents a complex neuroendocrine feedback loop controlling the secretion of adrenal glucocorticoid hormones. Central to its function is the paraventricular nucleus of the hypothalamus (PVN) where neurons expressing corticotropin releasing factor reside. These HPA motor neurons are a primary site of integration leading to graded endocrine responses to physical and psychological stressors. An important regulatory factor that must be considered, prior to generating an appropriate response is the animal's reproductive status. Thus, PVN neurons express androgen and estrogen receptors and receive input from sites that also express these receptors. Consequently, changes in reproduction and gonadal steroid levels modulate the stress response and this underlies sex differences in HPA axis function. This review examines the make up of the HPA axis and hypothalamo-pituitary-gonadal (HPG) axis and the interactions between the two that should be considered when exploring normal and pathological responses to environmental stressors.

Neuroscience of glucose homeostasis

Plasma glucose concentrations are homeostatically regulated and maintained within strict boundaries. Several mechanisms are in place to increase glucose output when glucose levels in the circulation drop as a result of glucose utilization, or to decrease glucose output and increase tissue glucose uptake to prevent hyperglycemia. Although the term homeostasis mostly refers to stable levels, the blood glucose concentrations fluctuate over the day/night cycle, with the highest concentrations occurring just prior to the activity period in anticipation of increased caloric need. In this chapter we describe how the brain, particularly the hypothalamus, is involved in both the daily rhythm of plasma glucose concentrations and acute glucose challenges.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Circadian adaptations to meal timing: neuroendocrine mechanisms

Circadian rhythms of behavior and physiology are generated by central and peripheral circadian oscillators entrained by periodic environmental or physiological stimuli. A master circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) is directly entrained by daily light-dark (LD) cycles, and coordinates the timing of other oscillators by direct and indirect neural, hormonal and behavioral outputs. The daily rhythm of food intake provides stimuli that entrain most peripheral and central oscillators, some of which can drive a daily rhythm of food anticipatory activity if food is restricted to one daily mealtime. The location of food-entrainable oscillators (FEOs) that drive food anticipatory rhythms, and the food-related stimuli that entrain these oscillators, remain to be clarified. Here, we critically examine the role of peripheral metabolic hormones as potential internal entrainment stimuli or outputs for FEOs controlling food anticipatory rhythms in rats and mice. Hormones for which data are available include corticosterone, ghrelin, leptin, insulin, glucagon, and glucagon-like peptide 1. All of these hormones exhibit daily rhythms of synthesis and secretion that are synchronized by meal timing. There is some evidence that ghrelin and leptin modulate the expression of food anticipatory rhythms, but none of the hormones examined so far are necessary for entrainment. Ghrelin and leptin likely modulate food-entrained rhythms by actions in hypothalamic circuits utilizing melanocortin and orexin signaling, although again food-entrained behavioral rhythms can persist in lesion and gene knockout models in which these systems are disabled. Actions of these hormones on circadian oscillators in central reward circuits remain to be evaluated. Food-entrained activity rhythms are likely mediated by a distributed system of circadian oscillators sensitive to multiple feeding related inputs. Metabolic hormones appear to play a modulatory role within this system.

Endocrine rhythms in the brown bear (Ursus arctos): Evidence supporting selection for decreased pineal gland size

Many temperate zone animals adapt to seasonal changes by altering their physiology. This is mediated in large part by endocrine signals that encode day length and regulate energy balance and metabolism. The objectives of this study were to determine if the daily patterns of two important hormones, melatonin and cortisol, varied with day length in captive brown bears (Ursus arctos) under anesthetized and nonanesthetized conditions during the active (March-October) and hibernation periods. Melatonin concentrations varied with time of day and season in nonanesthetized female bears despite exceedingly low nocturnal concentrations (1-4 pg/mL) in the active season. In contrast, melatonin concentrations during hibernation were 7.5-fold greater than those during the summer in anesthetized male bears. Functional assessment of the pineal gland revealed a slight but significant reduction in melatonin following nocturnal light application during hibernation, but no response to beta-adrenergic stimulation was detected in either season. Examination of pineal size in two bear species bears combined with a phylogenetically corrected analysis of pineal glands in 47 other species revealed a strong relationship to brain size. However, pineal gland size of both bear species deviated significantly from the expected pattern. Robust daily plasma cortisol rhythms were observed during the active season but not during hibernation. Cortisol was potently suppressed following injection with a synthetic glucocorticoid. The results suggest that melatonin and cortisol both retain their ability to reflect seasonal changes in day length in brown bears. The exceptionally small pineal gland in bears may be the result of direct or indirect selection.

Forced desynchrony reveals independent contributions of suprachiasmatic oscillators to the daily plasma corticosterone rhythm in male rats

The suprachiasmatic nucleus (SCN) is required for the daily rhythm of plasma glucocorticoids; however, the independent contributions from oscillators within the different subregions of the SCN to the glucocorticoid rhythm remain unclear. Here, we use genetically and neurologically intact, forced desynchronized rats to test the hypothesis that the daily rhythm of the glucocorticoid, corticosterone, is regulated by both light responsive and light-dissociated circadian oscillators in the ventrolateral (vl-) and dorsomedial (dm-) SCN, respectively. We show that when the vlSCN and dmSCN are in maximum phase misalignment, the peak of the plasma corticosterone rhythm is shifted and the amplitude reduced; whereas, the peak of the plasma adrenocorticotropic hormone (ACTH) rhythm is also reduced, the phase is dissociated from that of the corticosterone rhythm. These data support previous studies suggesting an ACTH-independent pathway contributes to the corticosterone rhythm. To determine if either SCN subregion independently regulates corticosterone through the sympathetic nervous system, we compared unilateral adrenalectomized, desynchronized rats that had undergone either transection of the thoracic splanchnic nerve or sham transection to the remaining adrenal. Splanchnicectomy reduced and phase advanced the peak of both the corticosterone and ACTH rhythms. These data suggest that both the vlSCN and dmSCN contribute to the corticosterone rhythm by both reducing plasma ACTH and differentially regulating plasma corticosterone through an ACTH- and sympathetic nervous system-independent pathway.

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.

Hypothalamic-pituitary-adrenal (HPA) axis function in the California mouse (Peromyscus californicus): Changes in baseline activity, reactivity, and fecal excretion of glucocorticoids across the diurnal cycle

The California mouse, Peromyscus californicus, is an increasingly popular animal model in behavioral, neural, and endocrine studies, but little is known about its baseline hypothalamic-pituitary-adrenal (HPA) axis activity or HPA responses to stressors. We characterized plasma corticosterone (CORT) concentrations in P. californicus under baseline conditions across the diurnal cycle, in response to pharmacological manipulation of the HPA axis, and in response to a variety of stressors at different times of day. In addition, we explored the use of fecal samples to monitor adrenocortical activity non-invasively. California mice have very high baseline levels of circulating CORT that change markedly over 24h, but that do not differ between the sexes. This species may be somewhat glucocorticoid-resistant in comparison to other rodents as a relatively high dose of dexamethasone (5mg/kg, s.c.) was required to suppress plasma CORT for 8h post-injection. CORT responses to stressors and ACTH injection differed with time of day, as CORT concentrations were elevated more readily during the morning (inactive period) than in the evening (active period) when compared to time-matched control. Data from (3)H-CORT injection studies show that the time course for excretion of fecal CORT, or glucocorticoid metabolites, differs with time of injection. Mice injected in the evening excreted the majority of fecal radioactivity 2-4h post-injection whereas mice injected during the morning did so at 14-16h post-injection. Unfortunately, the antibody we used does not adequately bind the most prevalent fecal glucocorticoid metabolites and therefore we could not validate its use for fecal assays.

Circadian and ultradian rhythms of free glucocorticoid hormone are highly synchronized between the blood, the subcutaneous tissue, and the brain

Total glucocorticoid hormone levels in plasma of various species, including humans, follow a circadian rhythm that is made up from an underlying series of hormone pulses. In blood most of the glucocorticoid is bound to corticosteroid-binding globulin and albumin, resulting in low levels of free hormone. Although only the free fraction is biologically active, surprisingly little is known about the rhythms of free glucocorticoid hormones. We used single-probe microdialysis to measure directly the free corticosterone levels in the blood of freely behaving rats. Free corticosterone in the blood shows a distinct circadian and ultradian rhythm with a pulse frequency of approximately one pulse per hour together with an increase in hormone levels and pulse height toward the active phase of the light/dark cycle. Similar rhythms were also evident in the subcutaneous tissue, demonstrating that free corticosterone rhythms are transferred from the blood into peripheral target tissues. Furthermore, in a dual-probe microdialysis study, we demonstrated that the circadian and ultradian rhythms of free corticosterone in the blood and the subcutaneous tissue were highly synchronized. Moreover, free corticosterone rhythms were also synchronous between the blood and the hippocampus. These data demonstrate for the first time an ultradian rhythm of free corticosterone in the blood that translates into synchronized rhythms of free glucocorticoid hormone in peripheral and central tissues. The maintenance of ultradian rhythms across tissue barriers in both the periphery and the brain has important implications for research into aberrant biological rhythms in disease and for the development of improved protocols for glucocorticoid therapy.

Photoperiod regulates corticosterone rhythms by altered adrenal sensitivity via melatonin-independent mechanisms in Fischer 344 rats and C57BL/6J mice

Most species living in temperate zones adapt their physiology and behavior to seasonal changes in the environment by using the photoperiod as a primary cue. The mechanisms underlying photoperiodic regulation of stress-related functions are not well understood. In this study, we analyzed the effects of photoperiod on the hypothalamic-pituitary-adrenal axis in photoperiod-sensitive Fischer 344 rats. We first examined how photoperiod affects diurnal variations in plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone. ACTH levels did not exhibit diurnal variations under long- and short-day conditions. On the other hand, corticosterone levels exhibited a clear rhythm under short-day condition with a peak during dark phase. This peak was not observed under long-day condition in which a significant rhythm was not detected. To analyze the mechanisms responsible for the photoperiodic regulation of corticosterone rhythms, ACTH was intraperitoneally injected at the onset of the light or dark phase in dexamethasone-treated rats maintained under long- and short-day conditions. ACTH induced higher corticosterone levels in rats examined at dark onset under short-day condition than those maintained under long-day condition. Next, we asked whether melatonin signals are involved in photoperiodic regulation of corticosterone rhythms, and rats were intraperitoneally injected with melatonin at late afternoon under long-day condition for 3 weeks. However, melatonin injections did not affect the corticosterone rhythms. In addition, photoperiodic changes in the amplitude of corticosterone rhythms were also observed in melatonin-deficient C57BL/6J mice, in which expression profiles of several clock genes and steroidgenesis genes in adrenal gland were modified by the photoperiod. Our data suggest that photoperiod regulates corticosterone rhythms by altered adrenal sensitivity through melatonin-independent mechanisms that may involve the adrenal clock.

Central and peripheral circadian clocks in mammals

The circadian system of mammals is composed of a hierarchy of oscillators that function at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator throughout the body, yet this clock system is adapted to different functional contexts. In the central suprachiasmatic nucleus (SCN) of the hypothalamus, a coupled population of neuronal circadian oscillators acts as a master pacemaker for the organism to drive rhythms in activity and rest, feeding, body temperature, and hormones. Coupling within the SCN network confers robustness to the SCN pacemaker, which in turn provides stability to the overall temporal architecture of the organism. Throughout the majority of the cells in the body, cell-autonomous circadian clocks are intimately enmeshed within metabolic pathways. Thus, an emerging view for the adaptive significance of circadian clocks is their fundamental role in orchestrating metabolism.

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.

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.

The transformation of hormonal stress responses throughout puberty and adolescence

Prepubertal rats display heightened hormonal stress reactivity compared with adults in that levels of ACTH and corticosterone take twice as long (i.e. 40-60 min) to return to baseline following an acute stressor. Despite this substantial change in stress responsiveness, and the critical nature of the adolescence period of development, the maturation of the hormonal stress response from the time of pubertal onset to adulthood has not been thoroughly investigated. To examine this, we measured ACTH, corticosterone, and testosterone in 30-, 40-, 50-, 60-, and 70-day-old (i.e. spanning pubertal and adolescent development) male rats before and after a 30 min session of restraint stress. We found that the adult-like ACTH stress response develops between 50 and 60 days of age, while the corticosterone response changes between 30 and 40 days of age. We also found that adrenal corticosterone concentrations paralleled the plasma corticosterone response following restraint, suggesting that stress-induced adrenal corticosterone synthesis decreases during adolescent development and may, at least in part, contribute to the differential stress response observed before and after puberty. Finally, stress leads to increases in testosterone secretion, but only after 50 days of age. Collectively, these results indicate that shifts in hormonal stress responses occur throughout adolescent maturation and that these responses show distinct developmental profiles.

ACTH-dependent ultradian rhythm of corticosterone secretion

The activity of the hypothalamic-pituitary-adrenal axis is characterized by an ultradian pulsatile pattern of glucocorticoid secretion. Despite increasing evidence for the importance of pulsatility in regulating glucocorticoid-responsive gene transcription, little is known about the mechanism underlying the pulsatility of glucocorticoid synthesis and release. We tested the hypothesis that pulsatile ACTH release is critical for optimal adrenocortical function. Hypothalamic-pituitary-adrenal activity was suppressed by oral methylprednisolone, and ACTH (4 ng/h) was infused for 24h either as a constant infusion or in 5-min pulses at hourly intervals. Control methylprednisolone-treated rats had very low plasma corticosterone (CORT) levels with undetectable pulses and also had steroidogenic acute regulatory protein (StAR) and cytochrome P450 side-chain cleavage (P450scc) heteronuclear RNA levels reduced to approximately 50% of that seen in untreated animals. Pulsatile but not constant ACTH infusion restored pulsatile CORT secretion, and this was accompanied by parallel rises in StAR and P450scc heteronuclear RNA levels during the rising phase of the CORT pulse, which then fell during the falling phase. The pulsatile pattern of StAR and P450scc was paralleled by pulsatile transcription of the melanocortin 2 receptor accessory protein. Pulsatile ACTH activation of the adrenal cortex not only is critical for the secretion of CORT but also induces episodic transcription of the rate-limiting enzymes necessary for physiological steroidogenic responses. Because constant infusion of identical amounts of ACTH did not activate CORT secretion, pulsatility of ACTH provides a more effective signaling system for the activation of adrenocortical activity.

The cortisol awakening response: more than a measure of HPA axis function

In most healthy people morning awakening is associated with a burst of cortisol secretion: the cortisol awakening response (CAR). It is argued that the CAR is subject to a range physiological regulatory influences that facilitate this rapid increase in cortisol secretion. Evidence is presented for reduced adrenal sensitivity to rising levels of ACTH in the pre-awakening period, mediated by an extra-pituitary pathway to the adrenal from the suprachiasmatic nucleus (SCN). A role for the hippocampus in this pre-awakening regulation of cortisol secretion is considered. Attainment of consciousness is associated with 'flip-flop' switching of regional brain activation, which, it is argued, initiates a combination of processes: (1) activation of the hypothalamic pituitary adrenal (HPA) axis; (2) release of pre-awakening reduced adrenal sensitivity to ACTH; (3) increased post-awakening adrenal sensitivity to ACTH in response to light, mediated by a SCN extra-pituitary pathway. An association between the CAR and the ending of sleep inertia is discussed.

Recent findings on the organization of central nervous system structures involved in the innervation of endocrine glands and other organs; observations obtained by the transneuronal viral double-labeling technique

This review summarizes the data obtained with the aid of the recently introduced dual viral tracing technique, which uses isogenic recombinants of pseudorabies virus that express unique reporter gene. This approach made possible to explore simultaneously neural circuits of two organs. The results of these studies indicate: (1) there are neurons innervating exclusively a given organ; (2) left-sided predominance in the supraspinal innervation of the endocrine glands (adrenal, ovary) studied, so far; (3) viral co-infection of neurons, i.e., special neuronal populations coexist in different brain areas that are transsynaptically connected with both paired endocrine and non-endocrine organs, endocrine glands and non-endocrine organs, and organs of bodily systems other than the endocrine one. The number of common neurons seems to be related to the need of coordinating action of different systems. The data on co-infection of neurons suggest that the central nervous system has the capacity to coordinate different organ functions via common brain neurons providing supraspinal innervation of the organs.

Effects of nocturnal light on (clock) gene expression in peripheral organs: a role for the autonomic innervation of the liver

Background

The biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), controls the daily rhythms in physiology and behavior. Early studies demonstrated that light exposure not only affects the phase of the SCN but also the functional activity of peripheral organs. More recently it was shown that the same light stimulus induces immediate changes in clock gene expression in the pineal and adrenal, suggesting a role of peripheral clocks in the organ-specific output. In the present study, we further investigated the immediate effect of nocturnal light exposure on clock genes and metabolism-related genes in different organs of the rat. In addition, we investigated the role of the autonomic nervous system as a possible output pathway of the SCN to modify the activity of the liver after light exposure.

Methodology and Principal Findings

First, we demonstrated that light, applied at different circadian times, affects clock gene expression in a different manner, depending on the time of day and the organ. However, the changes in clock gene expression did not correlate in a consistent manner with those of the output genes (i.e., genes involved in the functional output of an organ). Then, by selectively removing the autonomic innervation to the liver, we demonstrated that light affects liver gene expression not only via the hormonal pathway but also via the autonomic input.

Conclusion

Nocturnal light immediately affects peripheral clock gene expression but without a clear correlation with organ-specific output genes, raising the question whether the peripheral clock plays a “decisive” role in the immediate (functional) response of an organ to nocturnal light exposure. Interestingly, the autonomic innervation of the liver is essential to transmit the light information from the SCN, indicating that the autonomic nervous system is an important gateway for the SCN to cause an immediate resetting of peripheral physiology after phase-shift inducing light exposures.

Effects of prenatal ethanol exposure on hypothalamic-pituitary-adrenal function across the estrous cycle

BACKGROUND

Rats prenatally exposed to ethanol (E) typically show increased hypothalamic-pituitary-adrenal (HPA) responses to stressors in adulthood. Importantly, prenatal ethanol may differentially alter stress responsiveness in male and female offspring, suggesting a role for the gonadal hormones in mediating the effects of ethanol on HPA activity. We investigated the role of ethanol-induced changes in hypothalamic-pituitary-gonadal (HPG) activity in the differential HPA regulation observed in E compared to control females across the estrous cycle.

METHODS

Peripheral hormones and changes in central neuropeptide mRNA levels were measured across the estrous cycle in adult female offspring from E, pair-fed (PF) and ad libitum-fed control (C) dams.

RESULTS

Ethanol females showed normal estrous cyclicity (vaginal smears) but delayed sexual maturation (vaginal opening). Both HPG and HPA activity were differentially altered in E (and in some cases, PF) compared to control females as a function of estrous cycle stage. In relation to HPG activity, E and PF females had higher basal and stress estradiol (E(2)) levels in proestrus compared to other phases of the cycle, and decreased GnRH mRNA levels compared to C females in diestrus. Further, E females had greater variation in LH than PF and C females across the cycle, and in proestrus, only E females showed a significant LH increase following stress. In relation to HPA activity, both basal and stress CORT levels and overall ACTH levels were greater in E than in C females in proestrus. Furthermore, AVP mRNA levels were increased overall in E compared to PF and C females.

CONCLUSIONS

These data demonstrate ethanol-induced changes in both HPG and HPA activity that are estrous phase-specific, and support the possibility that changes in HPA activity in E females may reflect differential sensitivity to ovarian steroids. E females appear to have an increased HPA sensitivity to E(2), and a possible shift toward AVP regulation of HPA activity. That PF were similar to E females on some measures suggests that nutritional effects of diet or food restriction played a role in mediating at least some of the changes observed.

Effects of prenatal ethanol exposure on regulation of basal hypothalamic-pituitary-adrenal activity and hippocampal 5-HT1A receptor mRNA levels in female rats across the estrous cycle

Prenatal ethanol exposure, like other early adverse experiences, is known to alter hypothalamic-pituitary-adrenal (HPA) activity in adulthood. The present study examined the modulatory effects of the gonadal hormones on basal HPA regulation and serotonin Type 1A receptor (5-HT(1A)) mRNA levels in adult female rats prenatally exposed to ethanol (E) compared to that in females from pair-fed (PF) and ad libitum-fed control (C) conditions. We demonstrate, for the first time, long-lasting consequences of prenatal ethanol exposure for basal corticosterone (CORT) regulation and basal levels of hippocampal mineralocorticoid (MR), glucocorticoid (GR) and serotonin Type 1A (5-HT(1A)) receptor mRNA, as a function of estrous cycle stage: (1) basal CORT levels were higher in E compared to C females in proestrus but lower in E and PF compared to C females in estrus; (2) there were no differences among groups in basal levels of adrenocorticotropin (ACTH), estradiol or progesterone; (3) hippocampal MR mRNA levels were decreased in E compared to PF and C females across the estrus cycle, with the greatest effects in proestrus, whereas E (but not PF or C) females had higher hippocampal GR mRNA levels in proestrus than in estrous and diestrus; (4) 5-HT(1A) mRNA levels were increased in E compared to PF and C females in diestrus. That alterations were revealed as a function of estrous cycle stage suggests a role for the ovarian steroids in mediating the adverse effects of ethanol. Furthermore, it appears that ethanol-induced nutritional effects may play a role in mediating at least some of the effects observed. The resetting of HPA activity by early environmental events could be one mechanism linking early life experiences with long-term health consequences. Thus, changes in basal CORT levels, a shift in the MR/GR balance and alterations in 5-HT(1A) receptor mRNA could have important clinical implications for understanding the secondary disabilities, such as an increased incidence of depression, in children with FASD.

Dissociation of ACTH and glucocorticoids

It is increasingly clear that significant differential regulation of pituitary and adrenal gland activation exists, leading to a dissociation of plasma adrenocorticotropic hormone and corticosteroid secretion during fetal, postnatal and adult life. An increasing number of preclinical and clinical studies report dissociation of adrenocorticotropic hormone and cortisol levels in critical illness, inflammation and mental disorders. Mechanisms involve an altered adrenal sensitivity, aberrant receptor expression or modulation of adrenal function by cytokines, vasoactive factors or neuropeptides. The degree of dissociation has been associated with the level of complications of sepsis, surgery, malignant disease and depression. The separation of adrenocorticotropic hormone and corticosteroid secretion is of clinical relevance and should be incorporated into our view on endocrine stress regulation.

Pituitary-dependent hyperadrenocorticism in a cockatoo

CASE DESCRIPTION

A 13-year-old female intact Moluccan cockatoo (Cacatua moluccensis) was evaluated because of coelomic distention, presumed to be secondary to an abdominal hernia. The patient also had a history of rapid weight gain and polyuria and polydipsia.

CLINICAL FINDINGS

Ultrasonography was used to confirm the existence of a pseudohernia that appeared to contain the small intestines, pancreas, and reproductive tract. Results of plasma biochemical analysis revealed hyperglycemia, hypophosphatemia, and high nonfasting bile acid concentrations and aspartate aminotransferase activity. A CBC revealed a relative heterophilia with a concomitant lymphopenia and mild monocytosis. Histologic evaluation of a liver biopsy specimen indicated chronic hepatic lipidosis. Despite a strong clinical suspicion of hyperadrenocorticism, ACTH stimulation test results were equivocal.

TREATMENT AND OUTCOME

The pseudohernia was strengthened with a prolene mesh. Despite ongoing medical and surgical care, the patient developed complications associated with the herniorrhaphy and was euthanatized. The clinical suspicion of hyperadrenocorticism was confirmed on the basis of histologic evaluation of the pituitary gland by use of special stains.

CLINICAL RELEVANCE

To our knowledge, pituitary-dependent hyperadrenocorticism has not been previously confirmed in Psittaciformes. The condition should be considered in birds with clinical signs consistent with those observed in mammals. For the cockatoo of this report, ACTH stimulation test results were equivocal and additional diagnostic tests should be developed for avian patients.

Effects of Prenatal Ethanol Exposure on Basal Limbic?Hypothalamic?Pituitary?Adrenal Regulation: Role of Corticosterone

BACKGROUND

Rats prenatally exposed to ethanol (E) exhibit hypothalamic-pituitary-adrenal (HPA) hyperresponsiveness and changes in central HPA regulation following exposure to stressors. Whether ethanol-induced alterations in basal HPA regulation play a role in mediating HPA hyperresponsiveness remains unclear. We utilized adrenalectomy (ADX), with or without corticosterone (CORT) replacement, to investigate basal HPA function and the role of CORT in mediating ethanol-induced alterations.

METHODS

Adult males and females from prenatal E, pair-fed (PF), and ad lib-fed control (C) groups were terminated at the circadian peak, 7 days following sham surgery or ADX, with or without CORT replacement. Plasma levels of CORT and adrenocorticotropin (ACTH), and mRNA levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in the paraventricular nucleus, CRH Type 1 receptor (CRH-R1) and pro-opiomelanocortin (POMC) in the anterior pituitary, and mineralocorticoid (MR) and glucocorticoid (GR) receptors in the dorsal hippocampus were determined.

RESULTS

Adrenalectomy resulted in significantly greater plasma ACTH elevations in E and PF males, and parallel CRH mRNA elevations in both E and PF males and females compared with their C counterparts. In contrast, pituitary CRH-R1 mRNA levels were lower in E compared with C males, with no differences in POMC. In addition, in response to ADX, E females showed a greater MR mRNA response, and E males showed a greater GR mRNA response compared with their C counterparts, and CORT replacement was ineffective in normalizing ADX-induced alterations in ACTH levels in E and PF females, hippocampal MR mRNA levels in E males, and AVP mRNA levels in PF males and females.

CONCLUSIONS

Together, these data indicate that the prenatal ethanol exposure induces HPA dysregulation under basal conditions at multiple levels of the axis, resulting in alterations in both HPA drive and feedback regulation and/or in the balance between drive and feedback. While some effects may be nutritionally mediated, it appears that the mechanisms underlying basal HPA dysregulation may differ between E and PF animals rather than occurring along a continuum of effects on the same pathway. Altered basal HPA tone may play a role in mediating the HPA hyperresponsiveness to stressors observed in E offspring.

Hypothalamic?Pituitary?Adrenal Responses to 5-HT1Aand 5-HT2A/CAgonists Are Differentially Altered in Female and Male Rats Prenatally Exposed to Ethanol

BACKGROUND

Prenatal ethanol exposure alters the development of the hypothalamic-pituitary-adrenal (HPA) axis, resulting in HPA hyper-responsiveness to stressors in adulthood. Prenatal ethanol exposure also alters the development and activity of the serotoninergic (5-HT) system. We have previously shown that 5-HT(1A) and 5-HT(2A/C) receptor-mediated behavioral and physiological function are altered in fetal ethanol-exposed offspring. As there are extensive interactions between the HPA axis and the 5-HT system, the present study tested the hypothesis that prenatal ethanol exposure would alter 5-HT(1A) and 5-HT(2A/C) receptor-mediated HPA function.

METHODS

The 5-HT(1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 mg/kg), and the 5-HT(2A/C) agonist, (+)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI; 0.3 mg/kg), or vehicle (1 mL/kg) were administered to adult female and male offspring from prenatal ethanol-exposed (E), pair-fed control (PF), and ad libitum-fed control (C) dams. The plasma concentration of adrenocorticotropin (ACTH) and corticosterone (CORT) were determined at 0, 15, 30, 60, and 120 minutes postinjection. In addition, corticotropin releasing hormone (CRH) mRNA expression in the paraventricular nucleus of the hypothalamus, and 5-HT(1A) and 5-HT(2A/C) receptor mRNA expression in the hippocampus and prefrontal cortex, respectively, were determined by in situ hybridization.

RESULTS

Ethanol-exposed females showed a blunted ACTH response to 8-OH-DPAT at 15 and 30 minutes, and conversely, an increased ACTH response to DOI at all time points postinjection, compared with PF and C females. Differences among E, PF, and C males failed to reach significance. Centrally, however, DOI resulted in a trend toward lower CRH mRNA levels in E and PF compared with C females, but higher CRH mRNA levels in E compared with control males. There were no differences among prenatal groups in 5-HT(2A) receptor expression in the prefrontal cortex following either 8-OH-DPAT or DOI treatment. However, following 8-OH-DPAT, hippocampal 5-HT(1A) receptor expression was higher in E than in PF females in CA1, with a trend toward higher expression in E than in C females in CA2, whereas following DOI, a prenatal group by subfield interaction suggests lower 5-HT(1A) mRNA levels in E and PF compared with C females in CA1 and the dentate gyrus.

CONCLUSIONS

These data are the first to demonstrate that prenatal ethanol exposure has differential long-term effects on 5-HT(1A)-mediated and 5-HT(2A)-mediated neuroendocrine function in females and males, and suggest a sex-specific ethanol-induced alteration in the interaction between the HPA axis and the serotonin system.

Molecular aspects of adrenal regulation for circadian glucocorticoid synthesis by chronic voluntary exercise

Chronic voluntary running of mice is known to increase the circadian peak of plasma corticosterone without change in the level of adrenocorticotropic hormone (ACTH). In order to investigate how chronic exercise modulates the circadian HPA axis, we used two weeks of voluntary wheel running of mice and confirmed the significant increase of the circadian peak of plasma corticosterone without alteration in ACTH level. To elucidate the mechanisms of exercise modulation on corticosterone synthesis, we first examined the levels of transcripts involved in corticosterone synthesis of the adrenal gland. Among them, only steroidogenic acute regulatory protein (StAR), the rate-limiting factor that transfers substrate cholesterol into inner mitochondrial membrane, showed significantly higher expression in the exercise group. Since the splanchnic nerve input to the adrenal gland has been reported as a factor involved in the direct modulation of corticosterone synthesis, we next examined the expression levels of enzymes for the catecholamine synthesis as indices of sympatho-adrenomedullary activity. We found that the only rate-limiting enzyme, tyrosine hydroxylase (TH), was significantly higher in the adrenals of exercise group. In addition to the increment of StAR and TH mRNA in response to the chronic exercise, surprisingly, we found only these factors showed the circadian variation in its expression levels that was correlated to the circadian rhythm of corticosterone. Chronic exercise seems to alter the circadian corticosterone synthesis, at least partially via altering the levels of circadian-regulated transcripts, StAR and TH of the adrenal gland.

Effect of Chronic Ethanol Consumption on the Response of Parathyroid Hormone to Hypocalcemia in the Pregnant Rat

BACKGROUND

Chronic alcohol (ethanol) consumption during pregnancy results in maternal/fetal hypocalcemia, which may underlie some of ethanol's adverse effects on maternal and fetal bone, and fetal/neonatal health. Ethanol appears to alter the relationship between parathyroid hormone (PTH) and blood calcium (Ca) level, and PTH does not increase in response to ethanol-induced hypocalcemia. However, it is not known whether ethanol actually prevents PTH from responding, or whether the ability to regulate blood Ca is intact, but ethanol lowers the level of Ca maintained. The objective of this study was to determine whether chronic ethanol consumption impairs the ability of the pregnant female to increase PTH in response to acute hypocalcemia.

METHODS

Rats were fed isocaloric diets with ethanol (36% ethanol-derived calories, E group) or without ethanol [pair-fed (PF) and control (C) groups], before and throughout 21 days of gestation. On day 21 gestation, rats received an intraperitoneal injection of ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) (300 or 500 mumol/kg body weight) or saline (saline group), or no injection (baseline group). Blood was collected from the baseline group, and at 30 or 60 minutes postinjection (saline and EGTA groups), and analyzed for ionized Ca (iCa), pH, and PTH.

RESULTS

Consistent with previous studies, ethanol consumption decreased blood iCa levels at baseline, but PTH levels did not differ among groups. Administration of EGTA significantly decreased blood iCa levels by 30 minutes, but ethanol did not prevent PTH from increasing in response to the hypocalcemia. In all diet groups, PTH levels were significantly increased by 30 minutes. Ethanol did, however, appear to decrease the maximum PTH level achievable in blood.

CONCLUSIONS

These data suggest that chronic ethanol consumption does not impair the ability of the pregnant rat to raise serum PTH levels in response to acute hypocalcemia, but ethanol's effect on maximal PTH secretion could impair the ability of the pregnant female to sustain high PTH levels in response to chronic hypocalcemia.