Corticosterone modulation of ACTH secretogogue gene expression in the paraventricular nucleus

Energy Homeostasis-Associated (Enho) mRNA Expression and Energy Homeostasis in the Acute Stress Versus Chronic Unpredictable Mild Stress Rat Models

The energy homeostasis-associated (Enho) gene, the transcript for the Adropin peptide, is usually linked to energy homeostasis, adiposity, glycemia, and insulin resistance. Studies on Enho expression in stressful conditions are lacking. This work aimed to investigate Enho mRNA expression and energy homeostasis in acute stress (AS) versus chronic unpredictable mild stress (CUMS) rat models. A total of thirty male Wistar rats (180-220 g) were fed a balanced diet with free access to water. Rats were divided into three equal groups (n = 10): (a) the normal control (NC) group; (b) the AS group, where one episode of stress for 2 h was applied; and (c) the CUMS group, in which rats were exposed to a variable program of mild stressors for 4 weeks. Energy homeostasis was analyzed by the PhenoMaster system for the automatic measuring of food intake (FI), respiratory O₂ volume (VO₂), CO₂ volume (VCO₂), respiratory quotient (RQ), and total energy expenditure (TEE). Finally, liver, whole brain, and adipose (WAT) tissue samples were collected, total RNA was prepared, and RT-PCR analysis of the Enho gene was performed. The CUMS group showed higher VO₂ consumption and VCO₂ production, and a higher RQ than the AS group. Furthermore, the TEE and FI were higher in the CUMS group compared to the AS group. Enho gene expression in the liver, brain, and WAT was significantly higher in the CUMS group than in the AS and NC groups. We can conclude that in the chew-fed AS rats, hypophagia was evident, with a shift in the RQ toward fat utilization, with no changes in body weight despite the increase in Enho mRNA expression in all studied tissues. In the CUMS group, the marked rise in Enho mRNA expression may have contributed to weight loss despite increased FI and TEE.

AVP-eGFP was significantly upregulated by hypovolemia in the parvocellular division of the paraventricular nucleus in the transgenic rats

Arginine vasopressin (AVP) is produced in the paraventricular (PVN) and supraoptic nuclei (SON). Peripheral AVP, which is secreted from the posterior pituitary, is produced in the magnocellular division of the PVN (mPVN) and SON. In addition, AVP is produced in the parvocellular division of the PVN (pPVN), where corticotrophin-releasing factor (CRF) is synthesized. These peptides synergistically modulate the hypothalamic-pituitary-adrenal (HPA) axis. Previous studies have revealed that the HPA axis was activated by hypovolemia. However, the detailed dynamics of AVP in the pPVN under hypovolemic state has not been elucidated. Here, we evaluated the effects of hypovolemia and hyperosmolality on the hypothalamus, using AVP-enhanced green fluorescent protein (eGFP) transgenic rats. Polyethylene glycol (PEG) or 3% hypertonic saline (HTN) was intraperitoneally administered to develop hypovolemia or hyperosmolality. AVP-eGFP intensity was robustly upregulated at 3 and 6 h after intraperitoneal administration of PEG or HTN in the mPVN. While in the pPVN, eGFP intensity was significantly increased at 6 h after intraperitoneal administration of PEG with significant induction of Fos-immunoreactive (-ir) neurons. Consistently, eGFP mRNA, AVP hnRNA, and CRF mRNA in the pPVN and plasma AVP and corticosterone were significantly increased at 6 h after intraperitoneal administration of PEG. The results suggest that AVP and CRF syntheses in the pPVN were activated by hypovolemia, resulting in the activation of the HPA axis.

Noise stress changes mRNA expressions of corticotropin-releasing hormone, its receptors in amygdala, and anxiety-related behaviors

Noise is a psychological, environmental stressor that activates limbic sites in the brain. Limbic sites such as the amygdala and the amygdaloid corticotropin-releasing hormone (CRH) system play an important role in integrating stress response. We investigated the association between noise exposures, CRH-related molecules in the amygdala, and behavioral alterations. In total 54 Sprague-Dawley rats were divided into the following three groups: Control (CON), acute noise exposure (ANE), and chronic noise exposure (CNE). The ANE group was exposed to 100 dB white noise only once in 4 h and the CNE group was exposed to the same for 4 h per day for 30 days. Expression profiles of CRH and its receptors CRH-R1 and CRH-R2 were analyzed by quantitative real-time polymerase chain reaction (qPCR). The same stress procedure was applied to the ANE and CNE groups for behavior testing. The anxiety responses of the animals after acute and chronic stress exposure were measured in the defensive withdrawal test. CNE upregulated CRH and CRH-R1 mRNA levels but downregulated CRH-R2 mRNA levels. ANE led to a decrease in both CRH-R1 and CRH-R2 expression. In the defensive withdrawal test, while the ANE increased, CNE reduced anxiety-like behaviors. The present study shows that the exposure of rats to white noise (100 dB) leads to behavioral alterations and molecule-specific changes in the CRH system. Behavioral alterations can be related to these molecular changes in the amygdala.

Chronic and acute effects of stress on energy balance: are there appropriate animal models?

Stress activates multiple neural and endocrine systems to allow an animal to respond to and survive in a threatening environment. The corticotropin-releasing factor system is a primary initiator of this integrated response, which includes activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. The energetic response to acute stress is determined by the nature and severity of the stressor, but a typical response to an acute stressor is inhibition of food intake, increased heat production, and increased activity with sustained changes in body weight, behavior, and HPA reactivity. The effect of chronic psychological stress is more variable. In humans, chronic stress may cause weight gain in restrained eaters who show increased HPA reactivity to acute stress. This phenotype is difficult to replicate in rodent models where chronic psychological stress is more likely to cause weight loss than weight gain. An exception may be hamsters subjected to repeated bouts of social defeat or foot shock, but the data are limited. Recent reports on the food intake and body composition of subordinate members of group-housed female monkeys indicate that these animals have a similar phenotype to human stress-induced eaters, but there are a limited number of investigators with access to the model. Few stress experiments focus on energy balance, but more information on the phenotype of both humans and animal models during and after exposure to acute or chronic stress may provide novel insight into mechanisms that normally control body weight.

Regulation of vertebrate corticotropin-releasing factor genes

Developmental, physiological, and behavioral adjustments in response to environmental change are crucial for animal survival. In vertebrates, the neuroendocrine stress system, comprised of the hypothalamus, pituitary, and adrenal/interrenal glands (HPA/HPI axis) plays a central role in adaptive stress responses. Corticotropin-releasing factor (CRF) is the primary hypothalamic neurohormone regulating the HPA/HPI axis. CRF also functions as a neurotransmitter/neuromodulator in the limbic system and brain stem to coordinate endocrine, behavioral, and autonomic responses to stressors. Glucocorticoids, the end products of the HPA/HPI axis, cause feedback regulation at multiple levels of the stress axis, exerting direct and indirect actions on CRF neurons. The spatial expression patterns of CRF, and stressor-dependent CRF gene activation in the central nervous system (CNS) are evolutionarily conserved. This suggests conservation of the gene regulatory mechanisms that underlie tissue-specific and stressor-dependent CRF expression. Comparative genomic analysis showed that the proximal promoter regions of vertebrate CRF genes are highly conserved. Several cis regulatory elements and trans acting factors have been implicated in stressor-dependent CRF gene activation, including cyclic AMP response element binding protein (CREB), activator protein 1 (AP-1/Fos/Jun), and nerve growth factor induced gene B (NGFI-B). Glucocorticoids, acting through the glucocorticoid and mineralocorticoid receptors, either repress or promote CRF expression depending on physiological state and CNS region. In this review, we take a comparative/evolutionary approach to understand the physiological regulation of CRF gene expression. We also discuss evolutionarily conserved molecular mechanisms that operate at the level of CRF gene transcription.

Glucocorticoids, chronic stress, and obesity

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Glucocorticoid regulation of peptide genes in neuroendocrine CRH neurons: a complexity beyond negative feedback

This review will examine our current knowledge of a fundamental property of CRH neuroendocrine neurons: how the major endpoint of the HPA axis--adrenal glucocorticoids--interacts with the mechanisms controlling the expression of the genes that encode ACTH secretogogues. A great deal of work over the past 25 years has led to the notion that this question has an ostensibly simple answer: glucocorticoids inhibit peptide gene expression using "negative feedback" at the CRH neuron and elsewhere. However, closely examining how glucocorticoids act in different physiological circumstances reveals a much more complex set of answers, particularly if we consider how the processes that control peptide synthesis and release are coupled. Out of this examination emerges a more flexible and complex framework for examining the integrative mechanisms controlling the CRH neuron. Although we will mostly focus on the Crh gene, relevant aspects of the vasopressin (Avp) and pro-enkephalin (pEnk) gene regulatory mechanisms will also be discussed.

Endocrine perturbation increases susceptibility of mice to anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) causes vascular collapse and high lethality in BALB/cJ mice, intermediate lethality in C57BL/6J mice, and no lethality in DBA/2J mice. We found that adrenalectomized (ADX) mice of all three strains had increased susceptibility to LT. The increased susceptibility of ADX-DBA/2J mice was not accompanied by changes in their macrophage sensitivity or cytokine response to LT. DBA/2J mice showed no change in serum corticosteroid levels in response to LT injection, while BALB/cJ mice showed a fivefold increase in serum corticosterone. However, LT inhibited dexamethasone (DEX)-induced glucocorticoid receptor gene activation to similar extents in all three strains. DEX treatment did not rescue ADX mice from LT-mediated mortality. Surprisingly, oral DEX treatment also sensitized adrenally intact DBA/2J mice to LT lethality at all doses tested and also exacerbated LT-mediated pathogenesis and mortality in BALB/cJ mice. Aldosterone did not protect ADX mice from toxin challenge. These results indicate that susceptibility to anthrax LT in mice depends on a fine but easily perturbed balance of endocrine functions. Thus, the potentially detrimental consequences of steroid therapy for anthrax must be considered in treatment protocols for this disease.

Hypothalamic-pituitary-adrenocortical axis regulation

As befits a system essential for survival, neuroendocrine regulation of the hypothalamic--pituitary--adrenocortical (HPA) axis is characterized by tight control as well as plasticity. Stimulus-specific afferents code for specific hypothalamic corticotropin (ACTH) secretagogues, which have combinatorial effects on ACTH secretion, resulting in a glucocorticoid response that is tailored to stimulus intensity. Chronic stress-induced stimulation of HPA activity alters ACTH secretagogue expression and hypothalamic afferent activity to maintain adrenocortical responsiveness. Rigorous control of circadian HPA activity optimizes the balance between beneficial and adverse effects of glucocorticoids (largely mediated by glucocorticoid receptors) by minimizing circadian nadir glucocorticoid secretion (an effect mediated by mineralocorticoid receptors). HPA activity also is controlled by other glucocorticoid-regulated factors, such as immune and metabolic status. Dysregulation of these control mechanisms is likely to contribute to a variety of diseases.

ABCB1 (MDR1)-type P-glycoproteins at the blood-brain barrier modulate the activity of the hypothalamic-pituitary-adrenocortical system: implications for affective disorder

Multidrug-resistance gene 1-type P-glycoproteins (ABCB1-type P-gps) protect the brain against the accumulation of many toxic xenobiotics and drugs. We recently could show that the access of the endogenous glucocorticoids corticosterone and cortisol to the brain are regulated by ABCB1-type P-gps in vivo. ABCB1-type P-gp function, therefore, is likely to exert a profound influence on the regulation of the hypothalamic-pituitary-adrenocortical (HPA) system. Hyperactivity of the HPA system is frequently observed in human affective disorder, and a considerable amount of evidence has been accumulated suggesting that normalization of the HPA system might be the final step necessary for stable remission of the disease. To examine whether blood-brain barrier (BBB) function influences neuroendocrine regulation, we investigated HPA system activity in abcb1ab (-/-) mice under basal conditions and following stress. Abcb1ab (-/-) mice showed consistently lower plasma ACTH levels and lower evening plasma corticosterone levels. CRH mRNA expression in the hypothalamic paraventricular nucleus was decreased and pituitary POMC mRNA expressing cells were significantly reduced in number in abcb1ab (-/-) mutants; however, they showed a normal activation of the HPA system following CRH stimulation. Lower doses of dexamethasone were required to suppress plasma corticosterone levels in mutants. Our data thus provide evidence for a sustained suppression of the HPA system at the hypothalamic level in abcb1ab (-/-) mice, suggesting that BBB function significantly regulates HPA system activity. Whether naturally occurring polymorphisms in the human ABCB1 gene might result in persistent changes in the responsiveness and regulation of the HPA system will be the subject of future investigations, correlating both genetic information with individual characteristics of the neuroendocrine phenotype.

Chronic stress and obesity: a new view of "comfort food"

The effects of adrenal corticosteroids on subsequent adrenocorticotropin secretion are complex. Acutely (within hours), glucocorticoids (GCs) directly inhibit further activity in the hypothalamo-pituitary-adrenal axis, but the chronic actions (across days) of these steroids on brain are directly excitatory. Chronically high concentrations of GCs act in three ways that are functionally congruent. (i) GCs increase the expression of corticotropin-releasing factor (CRF) mRNA in the central nucleus of the amygdala, a critical node in the emotional brain. CRF enables recruitment of a chronic stress-response network. (ii) GCs increase the salience of pleasurable or compulsive activities (ingesting sucrose, fat, and drugs, or wheel-running). This motivates ingestion of "comfort food." (iii) GCs act systemically to increase abdominal fat depots. This allows an increased signal of abdominal energy stores to inhibit catecholamines in the brainstem and CRF expression in hypothalamic neurons regulating adrenocorticotropin. Chronic stress, together with high GC concentrations, usually decreases body weight gain in rats; by contrast, in stressed or depressed humans chronic stress induces either increased comfort food intake and body weight gain or decreased intake and body weight loss. Comfort food ingestion that produces abdominal obesity, decreases CRF mRNA in the hypothalamus of rats. Depressed people who overeat have decreased cerebrospinal CRF, catecholamine concentrations, and hypothalamo-pituitary-adrenal activity. We propose that people eat comfort food in an attempt to reduce the activity in the chronic stress-response network with its attendant anxiety. These mechanisms, determined in rats, may explain some of the epidemic of obesity occurring in our society.

A spoonful of sugar: feedback signals of energy stores and corticosterone regulate responses to chronic stress

To begin to understand the effects of chronic stress on food intake and energy stores, the effects of increased activity in the hypothalamo-pituitary-adrenal (HPA) axis and glucocorticoids (GCs) on the body and brain must first be understood. We propose two major systems that are both GC sensitive: a metabolic feedback that is inhibitory and a direct central GC drive. Under basal conditions, the metabolic feedback signal to brain is dominant, although infusion of GC into a lateral brain ventricle blocks the effects of the metabolic feedback. Chronic stress activates GC secretion and brain nuclear GC receptor occupancy, markedly changing the normal relationships between these two major corticosteroid-activated systems. The stressor-induced switch in the relative strengths of these signals determines subsequent brain regulation of stress responses (behavioral, neuroendocrine and autonomic outflows). The metabolic feedback effects of GCs are mimicked by voluntary sucrose ingestion in adrenalectomized rats, and experiments suggest that the metabolic feedback also inhibits the stressor-induced direct GC drive on brain. We speculate that the interaction between peripheral and central GC-sensitive signaling systems may be coupled through the inhibitory actions of endogenous opiatergic inputs on corticotropin-releasing factor (CRF) neurons.

Identification of stimulatory and inhibitory inputs to the hypothalamic-pituitary-adrenal axis during hypoglycaemia or transport in ewes

This study used the novel approach of statistical modelling to investigate the control of hypothalamic-pituitary-adrenal (HPA) axis and quantify temporal relationships between hormones. Two experimental paradigms were chosen, insulin-induced hypoglycaemia and 2 h transport, to assess differences in control between noncognitive and cognitive stimuli. Vasopressin and corticotropin-releasing hormone (CRH) were measured in hypophysial portal plasma, and adrenocorticotropin hormone (ACTH) and cortisol in jugular plasma of conscious sheep, and deconvolution analysis was used to calculate secretory rates, before modelling. During hypoglycaemia, the relationship between plasma glucose and vasopressin or CRH was best described by log10 transforming variables (i.e. a positive power-curve relationship). A negative-feedback relationship with log10 cortisol concentration 2 h previously was detected. Analysis of the "transport" stimulus suggested that the strength of the perceived stimulus decreased over time after accounting for cortisol facilitation and negative-feedback. The time course of vasopressin and CRH responses to each stimulus were different However, at the pituitary level, the data suggested that log10 ACTH secretion rate was related to log10 vasopressin and CRH concentrations with very similar regression coefficients and an identical ratio of actions (2.3 : 1) for both stimuli. Similar magnitude negative-feedback effects of log10 cortisol at -110 min (hypoglycaemia) or -40 min (transport) were detected, and both models contained a stimulatory relationship with cortisol at 0 min (facilitation). At adrenal gland level, cortisol secretory rates were related to simultaneously measured untransformed ACTH concentration but the regression coefficient for the hypoglycaemia model was 2.5-fold greater than for transport. No individual sustained maximum cortisol secretion for longer than 20 min during hypoglycaemia and 40 min during transport. These unique models demonstrate that corticosteroid negative-feedback is a significant control mechanism at both the pituitary and hypothalamus. The amplitude of HPA response may be related to stimulus intensity and corticosteroid negative-feedback, while duration depended on feedback alone.