Regulation of ACTH secretion: variations on a theme of B

Prenatal glucocorticoids exposure and adverse cardiovascular effects in offspring

Glucocorticoids (GCs) are steroid hormones fundamental to the body's normal physiological functions and are pivotal in fetal growth and development. During gestation, the mother's cortisol concentration (active GCs) escalates to accommodate the requirements of fetal organ development and maturation. A natural placental GCs barrier, primarily facilitated by 11β hydroxysteroid dehydrogenase 2, exists between the mother and fetus. This enzyme transforms biologically active cortisol into biologically inactive corticosterone, thereby mitigating fetal GCs exposure. However, during pregnancy, the mother may be vulnerable to adverse factor exposures such as stress, hypoxia, caffeine, and synthetic GCs use. In these instances, maternal serum GCs levels may surge beyond the protective capacity of the placental GCs barrier. Moreover, these adverse factors could directly compromise the placental GCs barrier, resulting in excessive fetal exposure to GCs. It is well-documented that prenatal GCs exposure can detrimentally impact the offspring's cardiovascular system, particularly in relation to blood pressure, vascular function, and heart function. In this review, we succinctly delineate the alterations in GCs levels during pregnancy and the potential mechanisms driving these changes, and also analyze the possible causes of prenatal GCs exposure. Furthermore, we summarize the current advancements in understanding the adverse effects and mechanisms of prenatal GCs exposure on the offspring's cardiovascular system.

Glucocorticoid feedback paradox: a homage to Mary Dallman

As the end product of the hypothalamus-pituitary-adrenal (HPA) axis, the glucocorticoid hormones cortisol and corticosterone coordinate circadian activities, stress-coping, and adaptation to change. For this purpose, the hormone promotes energy metabolism and controls defense reactions in the body and brain. This life-sustaining action exerted by glucocorticoids occurs in concert with the autonomic nervous and immune systems, transmitters, growth factors/cytokines, and neuropeptides. The current contribution will focus on the glucocorticoid feedback paradox in the HPA-axis: the phenomenon that stress responsivity remains resilient if preceded by stress-induced secretion of glucocorticoid hormone, but not if this hormone is previously administered. Furthermore, in animal studies, the mixed progesterone/glucocorticoid antagonist RU486 or mifepristone switches to an apparent partial agonist upon repeated administration. To address these enigmas several interesting phenomena are highlighted. These include the conditional nature of the excitation/inhibition balance in feedback regulation, the role of glucose as a determinant of stress responsivity, and the potential of glucocorticoids in resetting the stress response system. The analysis of the feedback paradox provides also a golden opportunity to review the progress in understanding the role of glucocorticoid hormone in resilience and vulnerability during stress, the science that was burned deeply in Mary Dallman's emotions.

The importance of the circadian trough in glucocorticoid signaling: a variation on B-flat

Glucocorticoid hormones are essential for health, but overexposure may lead to many detrimental effects, including metabolic, psychiatric, and bone disease. These effects may not only be due to increased overall exposure to glucocorticoids, but also to elevated hormone levels at the time of the physiological circadian trough of glucocorticoid levels. The late Mary Dallman developed a model that allows the differentiation between the effects of overall 24-hour glucocorticoid overexposure and the effects of a lack of circadian rhythmicity. For this, she continuously treated rats with a low dose of corticosterone (or "B"), which leads to a constant hormone level, without 24-hour overexposure using subcutaneously implanted pellets. The data from this "B-flat" model suggest that even modest elevations of glucocorticoid signaling during the time of the normal circadian trough of hormone secretion are a substantial contributor to the negative effects of glucocorticoids on health.

Homeostatic Regulation of Glucocorticoid Receptor Activity by Hypoxia-Inducible Factor 1: From Physiology to Clinic

Glucocorticoids (GCs) represent a well-known class of lipophilic steroid hormones biosynthesised, with a circadian rhythm, by the adrenal glands in humans and by the inter-renal tissue in teleost fish (e.g., zebrafish). GCs play a key role in the regulation of numerous physiological processes, including inflammation, glucose, lipid, protein metabolism and stress response. This is achieved through binding to their cognate receptor, GR, which functions as a ligand-activated transcription factor. Due to their potent anti-inflammatory and immune-suppressive action, synthetic GCs are broadly used for treating pathological disorders that are very often linked to hypoxia (e.g., rheumatoid arthritis, inflammatory, allergic, infectious, and autoimmune diseases, among others) as well as to prevent graft rejections and against immune system malignancies. However, due to the presence of adverse effects and GC resistance their therapeutic benefits are limited in patients chronically treated with steroids. For this reason, understanding how to fine-tune GR activity is crucial in the search for novel therapeutic strategies aimed at reducing GC-related side effects and effectively restoring homeostasis. Recent research has uncovered novel mechanisms that inhibit GR function, thereby causing glucocorticoid resistance, and has produced some surprising new findings. In this review we analyse these mechanisms and focus on the crosstalk between GR and HIF signalling. Indeed, its comprehension may provide new routes to develop novel therapeutic targets for effectively treating immune and inflammatory response and to simultaneously facilitate the development of innovative GCs with a better benefits-risk ratio.

HPA Axis in the Pathomechanism of Depression and Schizophrenia: New Therapeutic Strategies Based on Its Participation

The hypothalamic-pituitary-adrenal (HPA) axis is involved in the pathophysiology of many neuropsychiatric disorders. Increased HPA axis activity can be observed during chronic stress, which plays a key role in the pathophysiology of depression. Overactivity of the HPA axis occurs in major depressive disorder (MDD), leading to cognitive dysfunction and reduced mood. There is also a correlation between the HPA axis activation and gut microbiota, which has a significant impact on the development of MDD. It is believed that the gut microbiota can influence the HPA axis function through the activity of cytokines, prostaglandins, or bacterial antigens of various microbial species. The activity of the HPA axis in schizophrenia varies and depends mainly on the severity of the disease. This review summarizes the involvement of the HPA axis in the pathogenesis of neuropsychiatric disorders, focusing on major depression and schizophrenia, and highlights a possible correlation between these conditions. Although many effective antidepressants are available, a large proportion of patients do not respond to initial treatment. This review also discusses new therapeutic strategies that affect the HPA axis, such as glucocorticoid receptor (GR) antagonists, vasopressin V1B receptor antagonists and non-psychoactive CB1 receptor agonists in depression and/or schizophrenia.

The use of social network analysis to describe the effect of immune activation on group dynamics in pigs

The immune system can influence social motivation with potentially dire consequences for group-housed production animals, such as pigs. The aim of this study was to test the effect of a controlled immune activation in group-housed pigs, through an injection with lipopolysaccharide (LPS) and an intervention with ketoprofen on centrality parameters at the individual level. In addition, we wanted to test the effect of time relative to the injection on general network parameters in order to get a better understanding of changes in social network structures at the group level. 52 female pigs (11-12 weeks) were allocated to four treatments, comprising two injections: ketoprofen-LPS (KL), ketoprofen-saline (KS), saline-LPS (SL) and saline-saline (SS). Social behaviour with a focus on damaging behaviour was observed continuously in 10 × 15 min bouts between 0800 am and 1700 pm 1 day before (baseline) and two subsequent days after injection. Activity was scan-sampled every 5 min for 6 h after the last injection in the pen. Saliva samples were taken for cortisol analysis at baseline and at 4, 24, 48, 72 h after the injections. A controlled immune activation affected centrality parameters for ear manipulation networks at the individual level. Lipopolysaccharide-injected pigs had a lower in-degree centrality, thus, received less interactions, 2 days after the challenge. Treatment effects on tail manipulation and fighting networks were not observed at the individual level. For networks of manipulation of other body parts, in-degree centrality was positively correlated with cortisol response at 4 h and lying behaviour in the first 6 h after the challenge in LPS-injected pigs. Thus, the stronger the pigs reacted to the LPS, the more interactions they received in the subsequent days. The time in relation to injection affected general network parameters for ear manipulation and fighting networks at the group level. For ear manipulation networks, in-degree centralisation was higher on the days following injection, thus, certain individuals in the pen received more interactions than the rest of the group compared to baseline. For fighting networks, betweenness decreased on the first day after injection compared to baseline, indicating that network connectivity increased after the challenge. Networks of tail manipulation and manipulation of other body parts did not change on the days after injection at the group level. Social network analysis is a method that can potentially provide important insights into the effects of sickness on social behaviour in group-housed pigs.

The Function of the Hypothalamic-Pituitary-Adrenal Axis During Experimental Autoimmune Encephalomyelitis: Involvement of Oxidative Stress Mediators

Multiple sclerosis (MS) is an inflammatory, demyelinating disease with an unknown origin. Previous studies showed the involvement of the hypothalamic–pituitary–adrenal (HPA) axis to susceptibility to autoimmune diseases, including MS, and its best-characterized animal model, experimental autoimmune encephalomyelitis (EAE). During MS/EAE, innate immune cells are activated and release cytokines and other inflammatory mediators, leading to a vicious cycle of inflammation. In response to inflammation, the activated HPA axis modulates immune responses via glucocorticoid activity. Because the mechanisms involving oxidative stress to the HPA axis are relatively unrevealed, in this study, we investigate the inflammatory and oxidative stress status of HPA axis during EAE. Our results reveal an upregulation of Pomc gene expression, followed by POMC and ACTH protein increase at the peak of the EAE in the pituitary. Also, prostaglandins are well-known contributors of HPA axis activation, which increases during EAE at the periphery. The upregulated Tnf expression in the pituitary during the peak of EAE occurred. This leads to the activation of oxidative pathways, followed by upregulation of inducible NO synthase expression. The reactive oxidant/nitrosative species (ROS/RNS), such as superoxide anion and NO, increase their levels at the onset and peak of the disease in the pituitary and adrenal glands, returning to control levels at the end of EAE. The corticotrophs in the pituitary increased in number and volume at the peak of EAE that coincides with high lipid peroxidation levels. The expression of MC2R in the adrenal glands increases at the peak of EAE, where strong induction of superoxide anion and malondialdehyde (MDA), reduced total glutathione (GSH) content, and catalase activity occurred at the peak and end of EAE compared with controls. The results obtained from this study may help in understanding the mechanisms and possible pharmacological modulation in MS and demonstrate an effect of oxidative stress exposure in the HPA activation during the course of EAE.

Vitamin D3 Supplementation in Diarrhea-Predominant Irritable Bowel Syndrome Patients: The Effects on Symptoms Improvement, Serum Corticotropin-Releasing Hormone, and Interleukin-6 - A Randomized Clinical Trial

OBJECTIVES

This study aimed to evaluate whether vitamin D deficiency is associated with the severity of symptoms of irritable bowel syndrome (IBS) patients. Stress and gut inflammation can increase the serum level of corticotropin-releasing hormone (CRH) and interleukin-6 (IL-6), leading to a change in bowel movements. The aim of this study was to evaluate the anti-inflammatory and psychological effects of vitamin D3 supplementation on the symptom improvement of patients with a diarrhea-predominant form of IBS (IBS-D).

METHODS

Eighty-eight IBS-D patients (age: 18-65 years) based on Rome IV criteria who suffered from vitamin D deficiency and/or insufficiency were enrolled in this randomized, placebo-controlled trial from February 2017 to May 2018 at Rasoul-e-Akram Hospital, Tehran, Iran. Participants were randomly divided into two groups. The intervention group received 50,000 IU vitamin D3 weekly and the control group received a placebo for 9 weeks. All patients received Mebeverine 135 mg twice a day besides supplementation. The IBS Severity Score System (IBS-SSS), serum 25(OH) vitamin D3, CRH, and IL-6 were measured before and after interventions.

RESULTS

Seventy-four patients completed the study. The severity of IBS symptoms (p < 0.01) and IL-6 (p = 0.02) decreased significantly in the intervention group as compared to the control group, but there was no significant difference in the serum level of CRH. Also, in the treatment group, IBS-SSS and IL-6 were significantly reduced at the end of the study from baseline (p < 0.01 and p < 0.03, respectively).

CONCLUSION

Our findings indicate that vitamin D3 supplementation can modulate the serum level of CRH and IL-6 and can improve symptoms in IBS-D patients. Vitamin D3 supplementation should be considered in IBS-D patients who suffer from vitamin D deficiency and/or insufficiency.

Chronic variable stress alters hypothalamic-pituitary-adrenal axis function in the female mouse

Chronic stress is often associated with a dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, which can greatly increase risk for a number of stress-related diseases, including neuropsychiatric disorders. Despite a striking sex-bias in the prevalence of many of these disorders, few preclinical studies have examined female subjects. Hence, the present study aimed to explore the effects of chronic stress on the basal and acute stress-induced activity of the HPA axis in the female C57BL/6 mouse. We used a chronic variable stress (CVS) paradigm in these studies, which successfully induces physiological and behavioral changes that are similar to those reported for some patients with mood disorders. Using this model, we found pronounced, time-dependent effects of chronic stress on the HPA axis. CVS-treated females exhibited adrenal hypertrophy, yet their pattern of glucocorticoid secretion in the morning resembled that of controls. CVS-treated and control females had similar morning basal corticosterone (CORT) levels, which were both significantly elevated following a restraint stressor. Although morning basal gene expression of the key HPA-controlling neuropeptides corticotropin releasing hormone (CRH), arginine vasopressin (AVP) and oxytocin (OT) was unaltered within the paraventricular nucleus (PVN) by CVS, CVS altered the PVN OT and AVP mRNA responses to acute restraint. In control females, acute stress decreased AVP, but not OT mRNA; whereas, in CVS females, it decreased OT, but not, AVP mRNA. Unlike the morning pattern of HPA activity, in the evening, CVS-treated females showed increased basal CORT with hypoactive responses of CORT and PVN c-Fos immunoreactivity to restraint stress. Furthermore, CVS elevated evening PVN CRH and OT mRNAs in the PVN, but it did not influence anxiety- or depressive-like behavior after a light/dark box or tail suspension test. Taken together, these findings indicate that CVS is an effective model for HPA axis dysregulation in the female mouse and may be relevant for stress-related diseases.

Evidence for a Coupled Oscillator Model of Endocrine Ultradian Rhythms

Whereas long-period temporal structures in endocrine dynamics have been well studied, endocrine rhythms on the scale of hours are relatively unexplored. The study of these ultradian rhythms (URs) has remained nascent, in part, because a theoretical framework unifying ultradian patterns across systems has not been established. The present overview proposes a conceptual coupled oscillator network model of URs in which oscillating hormonal outputs, or nodes, are connected by edges representing the strength of node-node coupling. We propose that variable-strength coupling exists both within and across classic hormonal axes. Because coupled oscillators synchronize, such a model implies that changes across hormonal systems could be inferred by surveying accessible nodes in the network. This implication would at once simplify the study of URs and open new avenues of exploration into conditions affecting coupling. In support of this proposed framework, we review mammalian evidence for (1) URs of the gut-brain axis and the hypothalamo-pituitary-thyroid, -adrenal, and -gonadal axes, (2) UR coupling within and across these axes; and (3) the relation of these URs to body temperature. URs across these systems exhibit behavior broadly consistent with a coupled oscillator network, maintaining both consistent URs and coupling within and across axes. This model may aid the exploration of mammalian physiology at high temporal resolution and improve the understanding of endocrine system dynamics within individuals.

Perinatal Programming of Circadian Clock-Stress Crosstalk

An intact communication between circadian clocks and the stress system is important for maintaining physiological homeostasis under resting conditions and in response to external stimuli. There is accumulating evidence for a reciprocal interaction between both-from the systemic to the molecular level. Disruption of this interaction by external factors such as shiftwork, jetlag, or chronic stress increases the risk of developing metabolic, immune, or mood disorders. From experiments in rodents, we know that both systems maturate during the perinatal period. During that time, exogenous factors such as stress or alterations in the external photoperiod may critically affect-or program-physiological functions later in life. This developmental programming process has been attributed to maternal stress signals reaching the embryo, which lastingly change gene expression through the induction of epigenetic mechanisms. Despite the well-known function of the adult circadian system in temporal coordination of physiology and behavior, the role of maternal and embryonic circadian clocks during pregnancy and postnatal development is still poorly defined. A better understanding of the circadian-stress crosstalk at different periods of development may help to improve stress resistance and devise preventive and therapeutic strategies against chronic stress-associated disorders.

Hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes: sex differences in regulation of stress responsivity

Gonadal hormones play a key role in the establishment, activation, and regulation of the hypothalamic-pituitary-adrenal (HPA) axis. By influencing the response and sensitivity to releasing factors, neurotransmitters, and hormones, gonadal steroids help orchestrate the gain of the HPA axis to fine-tune the levels of stress hormones in the general circulation. From early life to adulthood, gonadal steroids can differentially affect the HPA axis, resulting in sex differences in the responsivity of this axis. The HPA axis influences many physiological functions making an organism's response to changes in the environment appropriate for its reproductive status. Although the acute HPA response to stressors is a beneficial response, constant activation of this circuitry by chronic or traumatic stressful episodes may lead to a dysregulation of the HPA axis and cause pathology. Compared to males, female mice and rats show a more robust HPA axis response, as a result of circulating estradiol levels which elevate stress hormone levels during non-threatening situations, and during and after stressors. Fluctuating levels of gonadal steroids in females across the estrous cycle are a major factor contributing to sex differences in the robustness of HPA activity in females compared to males. Moreover, gonadal steroids may also contribute to epigenetic and organizational influences on the HPA axis even before puberty. Correspondingly, crosstalk between the hypothalamic-pituitary-gonadal (HPG) and HPA axes could lead to abnormalities of stress responses. In humans, a dysregulated stress response is one of the most common symptoms seen across many neuropsychiatric disorders, and as a result, such interactions may exacerbate peripheral pathologies. In this review, we discuss the HPA and HPG axes and review how gonadal steroids interact with the HPA axis to regulate the stress circuitry during all stages in life.

Developmental origins of cardiovascular disease: Impact of early life stress in humans and rodents

The Developmental Origins of Health and Disease (DOHaD) hypothesizes that environmental insults during childhood programs the individual to develop chronic disease in adulthood. Emerging epidemiological data strongly supports that early life stress (ELS) given by the exposure to adverse childhood experiences is regarded as an independent risk factor capable of predicting future risk of cardiovascular disease. Experimental animal models utilizing chronic behavioral stress during postnatal life, specifically maternal separation (MatSep) provides a suitable tool to elucidate molecular mechanisms by which ELS increases the risk to develop cardiovascular disease, including hypertension. The purpose of this review is to highlight current epidemiological studies linking ELS to the development of cardiovascular disease and to discuss the potential molecular mechanisms identified from animal studies. Overall, this review reveals the need for future investigations to further clarify the molecular mechanisms of ELS in order to develop more personalized therapeutics to mitigate the long-term consequences of chronic behavioral stress including cardiovascular and heart disease in adulthood.

A users guide to HPA axis research

Glucocorticoid hormones (cortisol and corticosterone - CORT) are the effector hormones of the hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine system. CORT is a systemic intercellular signal whose level predictably varies with time of day and dynamically increases with environmental and psychological stressors. This hormonal signal is utilized by virtually every cell and physiological system of the body to optimize performance according to circadian, environmental and physiological demands. Disturbances in normal HPA axis activity profiles are associated with a wide variety of physiological and mental health disorders. Despite numerous studies to date that have identified molecular, cellular and systems-level glucocorticoid actions, new glucocorticoid actions and clinical status associations continue to be revealed at a brisk pace in the scientific literature. However, the breadth of investigators working in this area poses distinct challenges in ensuring common practices across investigators, and a full appreciation for the complexity of a system that is often reduced to a single dependent measure. This Users Guide is intended to provide a fundamental overview of conceptual, technical and practical knowledge that will assist individuals who engage in and evaluate HPA axis research. We begin with examination of the anatomical and hormonal components of the HPA axis and their physiological range of operation. We then examine strategies and best practices for systematic manipulation and accurate measurement of HPA axis activity. We feature use of experimental methods that will assist with better understanding of CORT's physiological actions, especially as those actions impact subsequent brain function. This research approach is instrumental for determining the mechanisms by which alterations of HPA axis function may contribute to pathophysiology.

Animal models of resistance exercise and their application to neuroscience research

BACKGROUND

Numerous studies have demonstrated that participation in regular resistance exercise (e.g., strength training) is associated with improvements in mental health, memory, and cognition. However, less is known about the neurobiological mechanisms mediating these effects. The goal of this mini-review is to describe and evaluate the available animal models of resistance exercise that may prove useful for examining CNS activity.

NEW METHOD

Various models have been developed to examine resistance exercise in laboratory animals.

COMPARISON WITH EXISTING METHODS

Resistance exercise models vary in how the resistance manipulation is applied, either through direct stimulation of the muscle (e.g., in situ models) or through behavior maintained by operant contingencies (e.g., whole organism models). Each model presents distinct advantages and disadvantages for examining central nervous system (CNS) activity, and consideration of these attributes is essential for the future investigation of underlying neurobiological substrates.

RESULTS

Potential neurobiological mechanisms mediating the effects of resistance exercise on pain, anxiety, memory, and drug use have been efficiently and effectively investigated using resistance exercise models that minimize stress and maximize the relative contribution of resistance over aerobic factors.

CONCLUSIONS

Whole organism resistance exercise models that (1) limit the use of potentially stressful stimuli and (2) minimize the contribution of aerobic factors will be critical for examining resistance exercise and CNS function.

Glucocorticoids Inhibit CRH/AVP-Evoked Bursting Activity of Male Murine Anterior Pituitary Corticotrophs

Corticotroph cells from the anterior pituitary are an integral component of the hypothalamic-pituitary-adrenal (HPA) axis, which governs the neuroendocrine response to stress. Corticotrophs are electrically excitable and fire spontaneous single-spike action potentials and also display secretagogue-induced bursting behavior. The HPA axis function is dependent on effective negative feedback in which elevated plasma glucocorticoids result in inhibition at the level of both the pituitary and the hypothalamus. In this study, we have used an electrophysiological approach coupled with mathematical modeling to investigate the regulation of spontaneous and CRH/arginine vasopressin-induced activity of corticotrophs by glucocorticoids. We reveal that pretreatment of corticotrophs with 100 nM corticosterone (CORT; 90 and 150 min) reduces spontaneous activity and prevents a transition from spiking to bursting after CRH/arginine vasopressin stimulation. In addition, previous studies have identified a role for large-conductance calcium- and voltage-activated potassium (BK) channels in the generation of secretagogue-induced bursting in corticotrophs. Using the dynamic clamp technique, we demonstrated that CRH-induced bursting can be switched to spiking by subtracting a fast BK current, whereas the addition of a fast BK current can induce bursting in CORT-treated cells. In addition, recordings from BK knockout mice (BK(-/-)) revealed that CORT can also inhibit excitability through BK-independent mechanisms to control spike frequency. Thus, we have established that glucocorticoids can modulate multiple properties of corticotroph electrical excitability through both BK-dependent and BK-independent mechanisms.

Glucocorticoid Fast Feedback Inhibition of Stress-Induced ACTH Secretion in the Male Rat: Rate Independence and Stress-State Resistance

Normal glucocorticoid secretion is critical for physiological and mental health. Glucocorticoid secretion is dynamically regulated by glucocorticoid-negative feedback; however, the mechanisms of that feedback process are poorly understood. We assessed the temporal characteristics of glucocorticoid-negative feedback in vivo using a procedure for drug infusions and serial blood collection in unanesthetized rats that produced a minimal disruption of basal ACTH plasma levels. We compared the negative feedback effectiveness present when stress onset coincides with corticosterone's (CORT) rapidly rising phase (30 sec pretreatment), high plateau phase (15 min pretreatment), or restored basal phase (60 min pretreatment) as well as effectiveness when CORT infusion occurs after the onset of stress (5 min poststress onset). CORT treatment prior to stress onset acted remarkably fast (within 30 sec) to suppress stress-induced ACTH secretion. Furthermore, fast feedback induction did not require rapid increases in CORT at the time of stress onset (hormone rate independent), and those feedback actions were relatively long lasting (≥15 min). In contrast, CORT elevation after stress onset produced limited and delayed ACTH suppression (stress state resistance). There was a parallel stress-state resistance for CORT inhibition of stress-induced Crh heteronuclear RNA in the paraventricular nucleus but not Pomc heteronuclear RNA in the anterior pituitary. CORT treatment did not suppress stress-induced prolactin secretion, suggesting that CORT feedback is restricted to the control of hypothalamic-pituitary-adrenal axis elements of a stress response. These temporal, stress-state, and system-level features of in vivo CORT feedback provide an important physiological context for ex vivo studies of molecular and cellular mechanisms of CORT-negative feedback.

Sensitization of the Hypothalamic-Pituitary-Adrenal Axis in a Male Rat Chronic Stress Model

Stress activation of the hypothalamic-pituitary-adrenal (HPA) axis is regulated by rapid glucocorticoid negative feedback. Chronic unpredictable stress animal models recapitulate certain aspects of major depression in humans, which have been attributed to impaired glucocorticoid negative feedback. We tested for an attenuated HPA sensitivity to fast glucocorticoid feedback inhibition in male rats exposed to a chronic variable stress (CVS) paradigm. In vitro, parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus recorded in slices from CVS rats showed an increase in basal excitatory synaptic inputs and a decrease in basal inhibitory synaptic inputs compared with neurons from control rats. There was no difference between control and CVS-treated rats in the rapid glucocorticoid suppression of excitatory synaptic inputs, a fast feedback mechanism. In vivo, CVS-treated rats showed an increase in ACTH secretion at baseline and after both iv CRH and acute stress and no impairment of the corticosterone suppression of the ACTH response, compared with controls. In an in vitro pituitary preparation, an increase in basal ACTH release, a small increase in CRH-induced ACTH release, and no decrement in the glucocorticoid suppression of ACTH release were seen in pituitaries from CVS rats. Thus, CVS does not suppress rapid glucocorticoid negative feedback at the hypothalamus or pituitary, but increases the synaptic excitability of paraventricular nucleus CRH neurons and the CRH sensitivity of the pituitary. Therefore, increased HPA activity in chronically stressed male rats is due to sensitization of the HPA axis, rather than to desensitization to rapid glucocorticoid feedback.

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.

Diurnal Corticosterone Presence and Phase Modulate Clock Gene Expression in the Male Rat Prefrontal Cortex

Mood disorders are associated with dysregulation of prefrontal cortex (PFC) function, circadian rhythms, and diurnal glucocorticoid (corticosterone [CORT]) circulation. Entrainment of clock gene expression in some peripheral tissues depends on CORT. In this study, we characterized over the course of the day the mRNA expression pattern of the core clock genes Per1, Per2, and Bmal1 in the male rat PFC and suprachiasmatic nucleus (SCN) under different diurnal CORT conditions. In experiment 1, rats were left adrenal-intact (sham) or were adrenalectomized (ADX) followed by 10 daily antiphasic (opposite time of day of the endogenous CORT peak) ip injections of either vehicle or 2.5 mg/kg CORT. In experiment 2, all rats received ADX surgery followed by 13 daily injections of vehicle or CORT either antiphasic or in-phase with the endogenous CORT peak. In sham rats clock gene mRNA levels displayed a diurnal pattern of expression in the PFC and the SCN, but the phase differed between the 2 structures. ADX substantially altered clock gene expression patterns in the PFC. This alteration was normalized by in-phase CORT treatment, whereas antiphasic CORT treatment appears to have eliminated a diurnal pattern (Per1 and Bmal1) or dampened/inverted its phase (Per2). There was very little effect of CORT condition on clock gene expression in the SCN. These experiments suggest that an important component of glucocorticoid circadian physiology entails CORT regulation of the molecular clock in the PFC. Consequently, they also point to a possible mechanism that contributes to PFC disrupted function in disorders associated with abnormal CORT circulation.

This can't be stressed enough: The contribution of select environmental toxicants to disruption of the stress circuitry and response

Integration of the hypothalamic-pituitary-adrenal (HPA) axis and the limbic system through glucocorticoid signaling is imperative in initiating and regulating a suitable stress response following real or perceived threats. Dysfunction of these circuits that results in a persistent or inhibited glucocorticoid secretion can severely affect processing of stressful experiences and lead to risk for developing further psychiatric pathology. Exposure to toxic chemicals found in our environment, including pesticides, metals, and industrial compounds, have been shown to have significant impact on neurological health and disease. Indeed, studies have begun to identify the HPA axis and limbic system as potential targets of many of these environmental chemicals, suggesting a possible environmental risk for damage to the stress circuit and response to stressful stimuli. This review will focus on our current understanding of the impact exposure to environmental toxicants, including bisphenol A and lead, has on the synaptic physiology of the HPA axis and limbic system and how this contributes to an alteration in behavior output. Further, this discussion will provide a starting point to continue to couple novel toxicological and neurological approaches to elaborate our understanding of the influence of environmental chemicals on the stress response and pathology.

60 YEARS OF NEUROENDOCRINOLOGY: Glucocorticoid dynamics: insights from mathematical, experimental and clinical studies

A pulsatile pattern of secretion is a characteristic of many hormonal systems, including the glucocorticoid-producing hypothalamic-pituitary-adrenal (HPA) axis. Despite recent evidence supporting its importance for behavioral, neuroendocrine and transcriptional effects of glucocorticoids, there has been a paucity of information regarding the origin of glucocorticoid pulsatility. In this review we discuss the mechanisms regulating pulsatile dynamics of the HPA axis, and how these dynamics become disrupted in disease. Our recent mathematical, experimental and clinical studies show that glucocorticoid pulsatility can be generated and maintained by dynamic processes at the level of the pituitary-adrenal axis, and that an intra-adrenal negative feedback may contribute to these dynamics.

Modulation of central glucocorticoid receptors in short- and long-term experimental hyperthyroidism

Hyperthyroidism is associated with a significant increase in circulating glucocorticoid levels and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. The aim of this study was to examine whether the HPA axis hyperactivity observed in hyperthyroidism may be explained by a disturbed feedback inhibition of endogenous glucocorticoids through two specific intracellular receptors in the brain: the high affinity mineralocorticoid receptor (MR) and the lower affinity glucocorticoid receptor (GR). Cytosolic receptor binding and gene expression was assessed in rats with short (7 days) and long standing (60 days) eu- and hyperthyroidism. Glucocorticoid receptor number and binding affinity (Kd) in the hippocampus were measured using [(3)H2]-dexamethasone radioreceptor assay. In situ hybridization was employed to examine the effects of hyperthyroidism on the GR and MR mRNA levels in the hippocampus and the pituitary. Both short- and long-term hyperthyroid rats showed pronounced reduction in the concentration of cytosolic GR in the hippocampus, without changes in binding affinity or changes in GR expression. In contrast, GR mRNA in the pituitary increased after 7 days and decreased after 60 days of thyroxin treatment. MR mRNA was moderately affected. Hyperthyroidism is associated with significant decreases in hippocampal GR levels supporting the hypothesis that hyperactivity of the HPA axis observed in experimentally induced hyperthyroidism may be attributed, at least in part, to decreased negative feedback at the level of the hippocampus. These findings further support the notion that a central locus is principally responsible for the hyperactivity of the HPA axis observed in hyperthyroidism.

Adrenal-dependent diurnal modulation of conditioned fear extinction learning

Post traumatic stress disorder (PTSD) is associated with altered conditioned fear extinction expression and impaired circadian function including dysregulation of glucocorticoid hormone secretion. We examined in adult male rats the relationship between conditioned fear extinction learning, circadian phase, and endogenous glucocorticoids (CORT). Rats maintained on a 12h light:dark cycle were trained and tested across 3 separate daily sessions (conditioned fear acquisition and 2 extinction sessions) that were administered during either the rats' active or inactive circadian phase. In an initial experiment we found that rats at both circadian phases acquired and extinguished auditory cue conditioned fear to a similar degree in the first extinction session. However, rats trained and tested at zeitgeber time-16 (ZT16) (active phase) showed enhanced extinction memory expression during the second extinction session compared to rats trained and tested at ZT4 (inactive phase). In a follow-up experiment, adrenalectomized (ADX) or sham surgery rats were similarly trained and tested across 3 separate daily sessions at either ZT4 or ZT16. ADX had no effect on conditioned fear acquisition or conditioned fear memory. Sham ADX rats trained and tested at ZT16 exhibited better extinction learning across the two extinction sessions compared to all other groups of rats. These results indicate that conditioned fear extinction learning is modulated by time of day, and this diurnal modulation requires the presence of adrenal hormones. These results support an important role of CORT-dependent circadian processes in regulating conditioned fear extinction learning, which may be capitalized upon to optimize effective treatment of PTSD.

Role of the dorsomedial hypothalamus in glucocorticoid-mediated feedback inhibition of the hypothalamic-pituitary-adrenal axis

Previous studies suggest that multiple corticolimbic and hypothalamic structures are involved in glucocorticoid-mediated feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, including the dorsomedial hypothalamus (DMH), but a potential role of the DMH has not been directly tested. To investigate the role of the DMH in glucocorticoid-mediated negative feedback, adult male Sprague Dawley rats were implanted with jugular cannulae and bilateral guide cannulae directed at the DMH, and finally were either adrenalectomized (ADX) or were subjected to sham-ADX. ADX rats received corticosterone (CORT) replacement in the drinking water (25 μg/mL), which, based on initial studies, restored a rhythm of plasma CORT concentrations in ADX rats that was similar in period and amplitude to the diurnal rhythm of plasma CORT concentrations in sham-ADX rats, but with a significant phase delay. Following recovery from surgery, rats received microinjections of either CORT (10 ng, 0.5 μL, 0.25 μL/min, per side) or vehicle (aCSF containing 0.2% EtOH), bilaterally, directly into the DMH, prior to a 40-min period of restraint stress. In sham-ADX rats, bilateral intra-DMH microinjections of CORT, relative to bilateral intra-DMH microinjections of vehicle, decreased restraint stress-induced elevation of endogenous plasma CORT concentrations 60 min after the onset of intra-DMH injections. Intra-DMH CORT decreased the overall area under the curve for plasma CORT concentrations during the intermediate time frame of glucocorticoid negative feedback, from 0.5 to 2 h following injection. These data are consistent with the hypothesis that the DMH is involved in feedback inhibition of HPA axis activity at the intermediate time frame.

The Effects of Using a Ramp and Elevator to Load and Unload Trailers on the Behavior and Physiology of Piglets

Transport is an inevitable process in the modern U.S. swine industry. The loading process is a novel and potentially stressful experience. This study uses behavior, heart rate and leukocyte counts to compare stress one hour before, during and after loading via ramp or elevator. Piglets were held in a home pen (control (CON)), walked up and down an aisle (handled (HAN)), or walked to a truck and loaded via elevator (ELE) or ramp (RAM). Sitting, feeding and blood parameters did not show a significant treatment by time effect (p > 0.05). Standing behavior did not differ between CON and HAN piglets nor between RAM and ELE piglets (p > 0.05); however, CON and HAN piglets stood more than RAM and ELE piglets during treatment (p < 0.05). After treatment, drinking behavior was increased in RAM piglets (p < 0.05). The heart rate of ELE piglets decreased 6.3% after treatment; whereas the heart rate of RAM piglets remained elevated 2.4% (p < 0.05). In terms of heart rate, loading by elevator appears to be less stressful than loading by ramp.

Glucocorticoid actions on synapses, circuits, and behavior: implications for the energetics of stress

Environmental stimuli that signal real or potential threats to homeostasis lead to glucocorticoid secretion by the hypothalamic-pituitary-adrenocortical (HPA) axis. Glucocorticoids promote energy redistribution and are critical for survival and adaptation. This adaptation requires the integration of multiple systems and engages key limbic-neuroendocrine circuits. Consequently, glucocorticoids have profound effects on synaptic physiology, circuit regulation of stress responsiveness, and, ultimately, behavior. While glucocorticoids initiate adaptive processes that generate energy for coping, prolonged or inappropriate glucocorticoid secretion becomes deleterious. Inappropriate processing of stressful information may lead to energetic drive that does not match environmental demand, resulting in risk factors for pathology. Thus, dysregulation of the HPA axis may promote stress-related illnesses (e.g. depression, PTSD). This review summarizes the latest developments in central glucocorticoid actions on synaptic, neuroendocrine, and behavioral regulation. Additionally, these findings will be discussed in terms of the energetic integration of stress and the importance of context-specific regulation of glucocorticoids.

Absence of glucocorticoids augments stress-induced Mkp1 mRNA expression within the hypothalamic-pituitary-adrenal axis

Stress-induced activation of hypothalamic paraventricular nucleus (PVN) corticotropin-releasing hormone (CRH) neurons trigger CRH release and synthesis. Recent findings have suggested that this process depends on the intracellular activation (phosphorylation) of ERK1/2 within CRH neurons. We have recently shown that the presence of glucocorticoids constrains stress-stimulated phosphorylation of PVN ERK1/2. In some peripheral cell types, dephosphorylation of ERK has been shown to be promoted by direct glucocorticoid upregulation of the MAP kinase phosphatase 1 (Mkp1) gene. In this study, we tested the hypothesis that glucocorticoids regulate Mkp1 mRNA expression in the neural forebrain (medial prefrontal cortex, mPFC, and PVN) and endocrine tissue (anterior pituitary) by subjecting young adult male Sprague-Dawley rats to various glucocorticoid manipulations with or without acute psychological stress (restraint). Restraint led to a rapid increase in Mkp1 mRNA within the mPFC, PVN, and anterior pituitary, and this increase did not require glucocorticoid activity. In contrast to glucocorticoid upregulation of Mkp1 gene expression in the peripheral tissues, we found that the absence of glucocorticoids (as a result of adrenalectomy) augmented basal mPFC and stress-induced PVN and anterior pituitary Mkp1 gene expression. Taken together, this study indicates that the presence of glucocorticoids may constrain Mkp1 gene expression in the neural forebrain and endocrine tissues. This possible constraint may be an indirect consequence of the inhibitory influence of glucocorticoids on stress-induced activation of ERK1/2, a known upstream positive regulator of Mkp1 gene transcription.

Vasopressin indirectly excites dorsal raphe serotonin neurons through activation of the vasopressin1A receptor

The neuropeptide vasopressin (AVP; arginine-vasopressin) is produced in a handful of brain nuclei located in the hypothalamus and extended amygdala and is released both peripherally as a hormone and within the central nervous system as a neurotransmitter. Central projections have been associated with a number of functions including regulation of physiological homeostasis, control of circadian rhythms, and modulation of social behavior. The AVP neurons located in the bed nucleus of the stria terminalis and medial amygdala (i.e., extended amygdala) in particular have been associated with affiliative social behavior in multiple species. It was recently demonstrated that in the mouse AVP projections emanating from extended amygdala neurons innervate a number of forebrain and midbrain brain regions including the dorsal raphe nucleus (DR), the site of origin of most forebrain-projecting serotonin neurons. Based on the presence of AVP fibers in the DR, we hypothesized that AVP would alter the physiology of serotonin neurons via AVP 1A receptor (V1AR) activation. Using whole-cell electrophysiology techniques, we found that AVP increased the frequency and amplitude of excitatory post-synaptic currents (EPSCs) in serotonin neurons of male mice. The indirect stimulation of serotonin neurons was AMPA/kainate receptor dependent and blocked by the sodium channel blocker tetrodotoxin, suggesting an effect of AVP on glutamate neurons. Further, the increase in EPSC frequency induced by AVP was blocked by selective V1AR antagonists. Our data suggest that AVP had an excitatory influence on serotonin neurons. This work highlights a new target (i.e., V1AR) for manipulating serotonin neuron excitability. In light of our data, we propose that some of the diverse effects of AVP on physiology and behavior, including social behavior, may be due to activation of the DR serotonin system.

Effects of experimentally induced hyperthyroidism on central hypothalamic-pituitary-adrenal axis function in rats: in vitro and in situ studies

Hyperthyroidism is associated with hypercorticosteronemia, although the locus that is principally responsible for the hypercorticosteronism remains unclear. The purpose of this study was to assess the effects of hyperthyroidism on the functional integrity of the hypothalamic-pituitary-adrenal (HPA) axis, to identify the locus in the HPA axis that is principally affected, and address the time-dependent effects of alterations in thyroid status. The functional integrity of each component of the HPA axis was examined in vitro and in situ in sham-thyroidectomized male Sprague-Dawley rats given placebo or in thyroidectomized rats given pharmacological dose (50 μg) of thyroxin for 7 or 60 days. Basal plasma corticosterone and corticosterone binding globulin (CBG) concentrations were significantly increased in short- and long-term hyperthyroid rats, and by 60 days. Basal plasma ACTH levels were similar to controls. Both hypothalamic CRH content and the magnitude of KCL- and arginine vasopressin (AVP)-induced CRH release from hypothalamic culture were increased in long-term hyperthyroid rats. There was a significant increase in the content of both ACTH and β-endorphin in the anterior pituitaries of both short- and long-term hyperthyroid animals. Short-term hyperthyroid rats showed a significant increase in basal POMC mRNA expression in the anterior pituitary, and chronically hyperthyroid animals showed increased stress-induced POMC mRNA expression. Adrenal cultures taken from short-term hyperthyroid rats responded to exogenous ACTH with an exaggerated corticosterone response, while those taken from 60-day hyperthyroid animals showed responses similar to controls. The findings show that hyperthyroidism is associated with hypercorticosteronemia and HPA axis dysfunction that becomes more pronounced as the duration of hyperthyroidism increases. The evidence suggests that experimentally induced hyperthyroidism is associated with central hyperactivity of the HPA axis.

Is it really a matter of simple dualism? Corticotropin-releasing factor receptors in body and mental health

Physiological responses to stress coordinated by the hypothalamo-pituitary-adrenal axis are concerned with maintaining homeostasis in the presence of real or perceived challenges. Regulators of this axis are corticotrophin releasing factor (CRF) and CRF related neuropeptides, including urocortins 1, 2, and 3. They mediate their actions by binding to CRF receptors (CRFR) 1 and 2, which are located in several stress-related brain regions. The prevailing theory has been that the initiation of and the recovery from an elicited stress response is coordinated by two elements, viz. the (mainly) opposing, but well balanced actions of CRFR1 and CRFR2. Such a dualistic view suggests that CRF/CRFR1 controls the initiation of, and urocortins/CRFR2 mediate the recovery from stress to maintain body and mental health. Consequently, failed adaptation to stress can lead to neuropathology, including anxiety and depression. Recent literature, however, challenges such dualistic and complementary actions of CRFR1 and CRFR2, and suggests that stress recruits CRF system components in a brain area and neuron specific manner to promote adaptation as conditions dictate.

Effects of short- and long-duration hypothyroidism on function of the rat hypothalamic-pituitary-adrenal axis

The effects of hypothyroidism on the functional integrity of the hypothalamic-pituitary-adrenal (HPA) axis were investigated in adult male rats. HPA axis function was examined in vivo in sham-thyroidectomized male Sprague-Dawley rats or in thyroidectomized rats for 7 (short-term hypothyroidism) or 60 (long-term hypothyroidism) days. Peripheral ACTH and corticosterone responses to insulin-induced hypoglycemia and interleukin (IL)-1α stimulation were used to indirectly assess the hypothalamic CRH neuron. Hypothyroidism resulted in exaggerated ACTH responses to both hypoglycemic stress and IL-1α administration. The adrenal cortex of hypothyroid animals showed a significant reduction in adrenal reserves, as assessed by its response to low-dose ACTH, following suppression of the HPA axis with dexamethasone. Hypothyroid rats were also associated with significant decreases in cerebrospinal fluid corticosterone concentrations and decreased adrenal weights. The findings suggest that experimentally induced hypothyroidism is associated with a mild, yet significant, adrenal insufficiency, which involves abnormalities in all components of the HPA axis.

Intact catecholamine inputs to the forebrain are required for appropriate regulation of corticotrophin-releasing hormone and vasopressin gene expression by corticosterone in the rat paraventricular nucleus

Corticotrophin-releasing hormone (CRH) neuroendocrine neurones in the paraventricular nucleus of the hypothalamus (PVH) drive adrenocorticotrophic hormone (ACTH) and thereby glucocorticoid release from pituitary corticotrophs and the adrenal cortex, respectively. Glucocorticoids suppress the ability of neuroendocrine corticotrophin-releasing hormone (CRH) neurones to synthesise and release ACTH secretogogues. Despite the importance of glucocorticoids as regulatory signals to CRH neurones in the extended time domain, how and where they act in this capacity is still not fully understood. Ascending catecholamine projections encode important cardiovascular, metabolic and other visceral information to the rat PVH and surrounding hypothalamus. These afferents have previously been implicated as targets for glucocorticoid action, including a role in the feedback regulation of PVH neuroendocrine neurones. To determine the contribution of these neurones to the long-term actions of corticosterone on CRH and vasopressin (AVP) gene expression in the PVH, we used an immunocytotoxin (a conjugate of the cytotoxin saporin and an antibody against dopamine-β-hydroxylase) that specifically ablates adrenergic and noradrenergic neurones. Lesions were administered to intact animals and to adrenalectomised animals with either no corticosterone or corticosterone replacement that provided levels above those required to normalise Crh expression. The ability of elevated levels of corticosterone to suppress Crh expression was abolished in animals lacking catecholaminergic innervation of the PVH. No effect was seen in the absence of corticosterone or in animals with intact adrenals. Furthermore, Avp expression, which is increased in CRH neurones following adrenalectomy, was suppressed in adrenalectomised catecholaminergic-lesioned animals. Interactions between corticosterone and catecholaminergic projections to the hypothalamus therefore make significant contributions to the regulation of Crh and Avp expression. However, the importance of catecholamine inputs is only apparent when circulating corticosterone concentrations are maintained either below or above those required to maintain the activity of the hypothalamic-pituitary-adrenal axis that is seen in intact animals.

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.

Different stressors produce excitation or inhibition of mesolimbic dopamine neuron activity: response alteration by stress pre-exposure

Stressors can exert a wide variety of responses, ranging from adaptive responses to pathological changes; moreover, recent studies suggest that mild stressors can attenuate the response of a system to major stressful events. We have previously shown that 2-week exposure to cold, a comparatively mild inescapable stressor, induced a pronounced reduction in ventral tegmental area (VTA) dopamine (DA) neuron activity, whereas restraint stress increases DA neuron activity. However, it is not known if these stressors differentially impact the VTA in a region-specific manner, if they differentially impact behavioral responses, or whether the effects of such different stressors are additive or antagonistic with regard to their impact on DA neuron firing. To address these questions, single-unit extracellular recordings were performed in anesthetized control rats and rats exposed to chronic cold, and tested after delivery of a 2-h restraint session. Chronic cold stress strongly attenuated the number of DA neurons firing in the VTA, and this effect occurred primarily in the medial and central VTA regions that preferentially project to reward-related ventral striatal regions. Chronic cold exposure also prevented the pronounced increase in DA neuron population activity without affecting the behavioral sensitization to amphetamine produced by restraint stress. Taken together, these data show that a prolonged inescapable mild stressor can induce plastic changes that attenuate the DA system response to acute stress.

Characterizing dynamic interactions between ultradian glucocorticoid rhythmicity and acute stress using the phase response curve

The hypothalamic-pituitary-adrenal (HPA) axis is a dynamic oscillatory hormone signalling system that regulates the pulsatile secretion of glucocorticoids from the adrenal glands. In addition to regulation of basal levels of glucocorticoids, the HPA axis provides a rapid hormonal response to stress that is vitally important for homeostasis. Recently it has become clear that glucocorticoid pulses encode an important biological signal that regulates receptor signalling both in the central nervous system and in peripheral tissues. It is therefore important to understand how stressful stimuli disrupt the pulsatile dynamics of this system. Using a computational model that incorporates the crucial feed-forward and feedback components of the axis, we provide novel insight into experimental observations that the size of the stress-induced hormonal response is critically dependent on the timing of the stress. Further, we employ the theory of Phase Response Curves to show that an acute stressor acts as a phase-resetting mechanism for the ultradian rhythm of glucocorticoid secretion. Using our model, we demonstrate that the magnitude of an acute stress is a critical factor in determining whether the system resets via a Type 1 or Type 0 mechanism. By fitting our model to our in vivo stress-response data, we show that the glucocorticoid response to an acute noise stress in rats is governed by a Type 0 phase-resetting curve. Our results provide additional evidence for the concept of a deterministic sub-hypothalamic oscillator regulating the ultradian glucocorticoid rhythm, which constitutes a highly responsive peripheral hormone system that interacts dynamically with hypothalamic inputs to regulate the overall hormonal response to stress.

Neural Regulation of the Stress Response: The Many Faces of Feedback

The mammalian stress response is an integrated physiological and psychological reaction to real or perceived adversity. Glucocorticoids (GCs) are an important component of this response, acting to redistribute energy resources to both optimize survival in the face of challenge and restore homeostasis after the immediate threat has subsided. Release of GCs is mediated by the hypothalamo-pituitary-adrenocortical (HPA) axis, driven by a neural signal originating in the paraventricular nucleus (PVN). Stress levels of GCs bind to glucocorticoid receptors (GRs) in multiple body compartments, including brain, and consequently have wide-reaching actions. For this reason, GCs serve a vital function in feedback inhibition of their own secretion. Fast, non-genomic feedback inhibition of the HPA axis is mediated at least in part by GC signaling in the PVN, acting by a cannabinoid-dependent mechanism to rapidly reduce both neural activity and GC release. Delayed feedback termination of the HPA axis response is mediated by forebrain GRs, presumably by genomic mechanisms. GCs also act in the brainstem to attenuate neuropeptidergic excitatory input to the PVN via acceleration of mRNA degradation, providing a mechanism to attenuate future responses to stressors. Thus, rather than having a single defined feedback switch, GCs work through multiple neurocircuits and signaling mechanisms to coordinate HPA axis activity to suit the overall needs of multiple body systems.

Control of hypothalamic-pituitary-adrenal stress axis activity by the intermediate conductance calcium-activated potassium channel, SK4

The anterior pituitary corticotroph is a major control point for the regulation of the hypothalamic-pituitary-adrenal (HPA) axis and the neuroendocrine response to stress. Although corticotrophs are known to be electrically excitable, ion channels controlling the electrical properties of corticotrophs are poorly understood. Here, we exploited a lentiviral transduction system to allow the unequivocal identification of live murine corticotrophs in culture. We demonstrate that corticotrophs display highly heterogeneous spontaneous action-potential firing patterns and their resting membrane potential is modulated by a background sodium conductance. Physiological concentrations of corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) cause a depolarization of corticotrophs, leading to a sustained increase in action potential firing. A major component of the outward potassium conductance was mediated via intermediate conductance calcium-activated (SK4) potassium channels. Inhibition of SK4 channels with TRAM-34 resulted in an increase in corticotroph excitability and exaggerated CRH/AVP-stimulated ACTH secretion in vitro. In accordance with a physiological role for SK4 channels in vivo, restraint stress-induced plasma ACTH and corticosterone concentrations were significantly enhanced in gene-targeted mice lacking SK4 channels (Kcnn4(-/-)). In addition, Kcnn4(-/-) mutant mice displayed enhanced hypothalamic c-fos and nur77 mRNA expression following restraint, suggesting increased neuronal activation. Thus, stress hyperresponsiveness observed in Kcnn4(-/-) mice results from enhanced secretagogue-induced ACTH output from anterior pituitary corticotrophs and may also involve increased hypothalamic drive, thereby suggesting an important role for SK4 channels in HPA axis function.

Tonic, but not phasic corticosterone, constrains stress activatedextracellular-regulated-kinase 1/ 2 immunoreactivity within the hypothalamic paraventricular nucleus

The negative-feedback actions of corticosterone (CORT) depend on both phasic and tonic CORT secretion patterns to regulate hypothalamic-pituitary-adrenal (HPA) axis activity. How these two different CORT secretion pattens influence specific intracellular signal transduction pathway activity within the cellular elements of the HPA axis has not been determined. For example, it is unknown whether CORT has suppressive actions over signal transduction events within medial parvocellular paraventricular nucleus (PVN) corticotrophin-releasing hormone (CRH) neurones, nor whether these suppressive actions are responsible for alterations in PVN transcriptional processes and neurohormone secretion associated with stress. The extracellular-regulated kinase (ERK) is a stress activated intracellular signalling molecule that is potentially subject to glucocorticoid negative-feedback regulation. We tested the ability of CORT to modulate levels of the active (phosphorylated) form of ERK (pERK1/2) in the PVN of rats. Acute psychological stress (restraint) produced a rapid increase in the number of PVN pERK1/2 immunopositive cells within CRH neurones. Absence of tonic CORT via adrenalectomy (ADX) produced no change in basal pERK1/2 cell counts but augmented the increased pERK1/2 cell counts elicited by acute restraint. Treatment of ADX rats with CORT in the drinking water normalised this enhanced pERK1/2 response to stress. By contrast, treatment of ADX rats with a phasic increase in CORT 1 h before restraint had no effect on pERK1/2 cell counts, despite substantially suppressing stress-induced PVN crh gene expression and adrenonocorticotrophic hormone secretion. This tonic CORT inhibition of stress-induced activation of ERK1/2 may involve both alteration of the activity of stress-dependent neural inputs to PVN CRH neurones and alteration within those neurones of stress-dependent intracellular signalling mechanisms associated with ERK activation.

Inhibitory effects of corticosterone in the hypothalamic paraventricular nucleus (PVN) on stress-induced adrenocorticotrophic hormone secretion and gene expression in the PVN and anterior pituitary

Endogenous glucocorticoid negative-feedback influence on the hypothalamic-pituitary-adrenal (HPA) axis depends on glucocorticoid actions exerted on multiple glucocorticoid-sensitive tissues and differential glucocorticoid effects that are expressed within several distinct temporal domains. The relative contribution and underlying molecular mechanisms of action for the effects of location and timing of glucocorticoid exposure on HPA axis activity remain to be determined. In the present study, we examined the effects of acute exposure to corticosterone (CORT) at the level of the paraventricular nucleus (PVN) on the HPA axis response to a subsequent stressor in a short-term (1 h) timeframe. Intra-PVN CORT microinjection 1 h before restraint suppressed the adrenocorticotrophic hormone (ACTH) response and blunted restraint-induced corticotrophin-releasing hormone (CRH) heterogeneous nuclear (hn)RNA expression in the PVN and pro-opiomelanocortin hnRNA expression in the anterior pituitary (AP); however, it had no effect on restraint-induced plasma prolactin levels and c-fos mRNA expression (PVN and AP). This pattern of results suggests that CORT acts locally at the level of the PVN within a short-term timeframe to suppress stress-induced excitation-exocytosis coupling within CRH neurones and CRH gene induction without altering the stress-associated trans-synaptic input and intracellular signal transduction that converges on PVN c-fos gene induction. The present study is the first to demonstrate that an acute infusion of CORT into the PVN is sufficient to suppress the ACTH response to stress initiated 1 h after CORT infusion.

Changes in CRH and ACTH synthesis during experimental and human septic shock

CONTEXT

The mechanisms of septic shock-associated adrenal insufficiency remain unclear. This study aimed at investigating the synthesis of corticotropin-releasing hormone (CRH) and vasopressin (AVP) by parvocellular neurons and the antehypophyseal expression of ACTH in human septic shock and in an experimental model of sepsis.

OBJECTIVE

To test the hypothesis that ACTH secretion is decreased secondarily to alteration of CRH or AVP synthesis, we undertook a neuropathological study of the antehypophyseal system in patients who had died from septic shock and rats with experimental faecal peritonitis.

METHODS

Brains obtained in 9 septic shock patients were compared to 10 nonseptic patients (controls). Parvocellular expression of AVP and CRH mRNA were evaluated by in situ hybridization. Antehypophyseal expression of ACTH, vasopressin V1b and CRH R1 receptors and parvocellular expression of iNOS in the PVN were evaluated by immunohistochemistry. The same experiments were carried out in a fecal peritonitis-induced model of sepsis. Data from septic rats with (n = 6) or without (n = 10) early death were compared to sham-operated (n = 8) animals.

RESULTS

In patients and rats, septic shock was associated with a decreased expression of ACTH, unchanged expression of V1B receptor, CRHR1 and AVP mRNA, and increased expression of parvocellular iNOS compared to controls. Septic shock was also characterized by an increased expression of CRH mRNA in rats but not in patients, who notably had a greater duration of septic shock.

CONCLUSION

The present study suggests that in humans and in rats, septic shock is associated with decreased ACTH synthesis that is not compensated by its two natural secretagogues, AVP and CRH. One underlying mechanism might be increased expression of iNOS in hypothalamic parvocellular neurons.

Sex differences in cortisol response to corticotropin releasing hormone challenge over puberty: Pittsburgh Pediatric Neurobehavioral Studies

OBJECTIVE

Consistent sex differences in regulation of the hypothalamic pituitary adrenocortical (HPA) axis have been shown in animal models and emerge over puberty. However, parallel work in humans is lacking despite implications for elucidating the emergence of sex differences in depression over puberty. We investigated sex differences in HPA response to corticotropin releasing hormone (CRH) challenge over puberty in a carefully screened normative sample.

METHODS

Participants were 68 healthy children (41% girls), ages 6-16, with no personal or family history of psychiatric disorder. Pubertal maturation was determined by Tanner staging. Following 24h of adaptation, 9-10 plasma cortisol samples were collected over 30-40 min pre-infusion baseline, 1 μg/kg CRH infusion, and 90-180 min post-infusion recovery. Thirty-seven participants completed 2+ CRH challenges allowing inclusion of cross-sectional and longitudinal data in all analyses. The influence of gender and pubertal maturation on parameters of cortisol response to CRH challenge was investigated using nonlinear mixed model methodology.

RESULTS

Girls showed increasing total cortisol output following CRH challenge over puberty, while boys showed little change in total cortisol output over puberty. Increased cortisol output in girls was explained by slower reactivity and recovery rates leading to prolonged time to reach peak cortisol and delayed return to baseline over puberty. Girls also showed increasing baseline cortisol over puberty, while boys showed declining baseline over puberty.

CONCLUSION

Results reveal subtle normative sex differences in the influence of pubertal maturation on HPA regulation at the pituitary level. This normative shift may tip the balance towards stress response dysregulation in girls at high risk for depression, and may represent one potential mechanism underlying elevated rates of depression among pubescent girls.

Brain stem catecholamines circuitry: activation by alcohol and role in the hypothalamic-pituitary-adrenal response to this drug

Although the stimulatory effect of alcohol on the rat hypothalamic-pituitary-adrenal (HPA) axis is well known, the mechanisms underlying this influence remain poorly understood. In the present study, we tested the hypothesis that brain catecholamines play an important role in this response. As expected, the acute intragastric administration of alcohol to adult male rats elevated plasma adrenocorticotrophic hormone (ACTH) levels and activated hypothalamic corticotrophin-releasing factor neurones. Novel findings pertain to the effect of alcohol on, and the role played by, brain adrenergic circuits. We first observed that alcohol up-regulated c-fos signals in the locus coeruleus, the main noradrenergic brain cell group; and that it activated (nor)adrenergic medullary cells (A1-A2/C1-C3). Evidence for the role played by these catecholaminergic circuits then came from the observation that blockade of α(1) -, but not β-, adrenergic receptors interfered with alcohol-induced ACTH secretion; and that depletion of catecholaminergic input to the paraventricular nucleus (PVN) by the toxin 6-hydroxydopamine significantly decreased the ACTH response to alcohol. Finally, destruction of the A1-A2/C1-C3 region with the immunotoxin anti-dopamine-B-hydroxylase-saporin interfered with the catecholaminergic input to the PVN. Collectively, our work extends our knowledge of the ability of this drug to up-regulate catecholamine circuitry in the rat brain. It also shows that medullary catecholamine innervation of the hypothalamus plays an important role in modulating the stimulatory effect of alcohol on the HPA axis, an effect exerted through activation of α(1) -adrenergic receptors.

Corticosterone pretreatment suppresses stress-induced hypothalamic-pituitary-adrenal axis activity via multiple actions that vary with time, site of action, and de novo protein synthesis

Glucocorticoid regulation of the hypothalamic-pituitary-adrenal (HPA) axis is believed to depend on multiple actions operative within discrete time domains. However, the underlying cellular and molecular mechanisms for those glucocorticoid actions remain undetermined. Moreover, there is absence of in vivo studies examining whether there are multiple glucocorticoid effects on HPA axis-related function within an intermediate feedback time frame (1-3  h after glucocorticoid elevation), and whether those effects depend on de novo protein synthesis. We examined in rats the effects of protein synthesis inhibition on HPA axis response to restraint (15 min) after 1 and 3  h phasic corticosterone (CORT) pretreatment. We measured HPA axis hormones (ACTH and CORT) and gene expression in the paraventricular nucleus (c-fos and crh genes), as well as gene expression in the anterior and intermediate pituitaries (c-fos and pomc genes). Both CORT pretreatment intervals produced inhibition of stress-induced ACTH secretion, but no inhibition was observed in the presence of protein synthesis inhibition. CORT pretreatment produced inhibitory effects on stress-induced gene expression that varied for each gene depending on the anatomical site, pretreatment time, and protein synthesis dependency. Taken together, the ACTH and gene expression patterns support the presence of multiple independent glucocorticoid actions initiated during the intermediate glucocorticoid negative feedback phase. Moreover, we conclude that those effects are exerted predominantly on the intrinsic anatomical elements of the HPA axis, and some of those effects depend on CORT induction of the expression of one or more regulatory gene products.

HPA axis responsiveness to stress: implications for healthy aging

The major neuroendocrine response mediating stress adaptation is activation of the hypothalamic pituitary adrenal axis, with stimulation of corticotropin releasing hormone (CRH) and vasopressin (VP) from parvocellular neurons of the hypothalamic paraventricular nucleus, leading to stimulation of pituitary ACTH secretion and increases in glucocorticoid secretion from the adrenal cortex. Basal production and transient increases during stress of glucocorticoids and its hypothalamic regulators are essential for neuronal plasticity and normal brain function. While activation of the HPA axis is essential for survival during stress, chronic exposure to stress hormones can predispose to psychological, metabolic and immune alterations. Thus, prompt termination of the stress response is essential to prevent negative effects of inappropriate levels of CRH and glucocorticoids. This review addresses the regulation of HPA axis activity with emphasis on the mechanisms of termination of CRH transcription, which is a critical step in this process. In addition, the actions by which glucocorticoids, CRH and VP can affect the aging process will be discussed.

Social allostasis: anticipatory regulation of the internal milieu

Social regulation of the internal milieu is a fundamental behavioral adaptation. Cephalic capability is reflected by anticipatory behaviors to serve systemic physiological regulation. Homeostatic regulation, a dominant perspective, reflects reactive responses; allostatic regulation, the physiology of change, emphasizes longer-term anticipatory, and feedforward systems. Steroids, such as cortisol, and peptides such as corticotrophin releasing hormone are but one example of such anticipatory regulatory systems. The concept of "allostasis" is in part to take account of anticipatory control amidst diverse forms of adaptation underlying this regulatory adaptation that supports social contact and the internal milieu.