The hypothalamic–pituitary–adrenal (HPA) axis is a neuroendocrine system that regulates the circulating levels of vital glucocorticoid hormones. The activity of the HPA axis is characterized not only by a classic circadian rhythm, but also by an ultradian pattern of discrete pulsatile release of glucocorticoids. A number of psychiatric and metabolic diseases are associated with changes in glucocorticoid pulsatility, and it is now clear that glucocorticoid responsive genes respond to these rapid fluctuations in a biologically meaningful way. Theoretical modelling has enabled us to identify and explore potential mechanisms underlying the ultradian activity in this axis, which to date have not been identified successfully. We demonstrate that the combination of delay with feed-forward and feedback loops in the pituitary–adrenal system is sufficient to give rise to ultradian pulsatility in the absence of an ultradian source from a supra-pituitary site. Moreover, our model enables us to predict the different patterns of glucocorticoid release mediated by changes in hypophysial-portal corticotrophin-releasing hormone levels, with results that parallel our experimental in vivo data.

The hypothalamic-pituitary-adrenal (HPA) axis regulates circulating levels of glucocorticoid hormones, and is the major neuroendocrine system in mammals that provides a rapid response and defense against stress. Under basal (i.e., unstressed) conditions, glucocorticoids are released with a pronounced circadian rhythm, characterized by peak levels of glucocorticoids during the active phase, that is daytime in humans and nighttime in nocturnal animals such as mice and rats. When studied in more detail, it becomes clear that the circadian rhythm of the HPA axis is characterized by a pulsatile release of glucocorticoids from the adrenal gland that results in rapid ultradian oscillations of hormone levels both in the blood and within target tissues, including the brain. In this review, we discuss the regulation of these circadian and ultradian HPA rhythms, how these rhythms change in health and disease, and how they affect the physiology and behavior of the organism.

Characterization of a peripheral hormonal system identifies the origin and mechanisms of regulation of glucocorticoid hormone oscillations in rats.

Objectives-To characterize the dynamics of the pituitary-adrenal interaction during the course of coronary artery bypass grafting (CABG) both on and off pump. Since our data pointed to a major change in adrenal responsiveness to ACTH we used a reverse translation approach to investigate the molecular mechanisms underlying this change in a rat model of critical illness. Measurements and Results-Clinical studies: Blood samples were taken for 24 hours from placement of the first venous access. Cortisol and ACTH were measured every 10 and 60 minutes respectively, and corticosteroid binding globulin (CBG) was measured at the beginning and end of the 24 hour period and at the end of operation. There was an initial rise in both levels of ACTH and cortisol to supra-normal values at around the end of surgery. ACTH levels then returned towards pre-operative values. Ultradian pulsatility of both ACTH and cortisol was maintained throughout the peri-operative period in all individuals. The sensitivity of the adrenal gland to ACTH increased markedly at around 8 hours after surgery maintaining very high levels of cortisol in the face of 'basal' levels of ACTH. This sensitivity began to return towards pre-operative values at the end of the 24-hour sampling period.Animal studies: Adult, male Sprague-Dawley rats were either given lipopolysaccharide (LPS) or sterile saline via a jugular vein cannula. Hourly blood samples were subsequently collected for ACTH and corticosterone measurement. Rats were sacrificed 6 hours after the injection and the adrenal glands were collected for measurement of StAR, SF-1 and DAX1 mRNA and protein using RTqPCR and Western immunoblotting, respectively. Adrenal levels of the ACTH receptor (MC2R) mRNA and its accessory protein (MRAP) were also measured by RTqPCR. In response to LPS, rats showed a pattern of ACTH and corticosterone that was similar to patients undergoing CABG. We were also able to demonstrate increased intra-adrenal corticosterone levels and an increase in StAR, SF-1 and MRAP mRNAs and StAR protein, and a reduction in DAX1 and MC2R mRNAs, 6h after LPS injection.Conclusions-Severe inflammatory stimuli activate the HPA axis resulting in increased steroidogenic activity in the adrenal cortex and an elevation of cortisol levels in the blood. Following CABG there is a massive increase in both ACTH and cortisol secretion. Despite a subsequent fall of ACTH to basal levels, cortisol remains elevated and co-ordinated ACTHcortisol pulsatility is maintained. This suggested that there is an increase in adrenal sensitivity to ACTH, which we confirmed in our animal model of immune activation of the HPA axis. Using this model we were able to show that this increased adrenal sensitivity results from changes in the regulation of both stimulatory and inhibitory intra-adrenal signaling pathways. Increased understanding of the dynamics of normal HPA responses to major surgery will provide us with a more rational approach to glucocorticoid therapy in critically ill patients.Gibbison et al.

The hypothalamic-pituitary-adrenal axis is a dynamic system regulating glucocorticoid hormone synthesis in the adrenal glands. Many key factors within the adrenal steroidogenic pathway have been identified and studied, but little is known about how these factors function collectively as a dynamic network of interacting components. To investigate this, we developed a mathematical model of the adrenal steroidogenic regulatory network that accounts for key regulatory processes occurring at different timescales. We used our model to predict the time evolution of steroidogenesis in response to physiological adrenocorticotropic hormone (ACTH) perturbations, ranging from basal pulses to larger stress-like stimulations (e.g., inflammatory stress). Testing these predictions experimentally in the rat, our results show that the steroidogenic regulatory network architecture is sufficient to respond to both small and large ACTH perturbations, but coupling this regulatory network with the immune pathway is necessary to explain the dissociated dynamics between ACTH and glucocorticoids observed under conditions of inflammatory stress. T he hypothalamic-pituitary-adrenal (HPA) axis is a stressresponsive neuroendocrine system that controls circulating levels of the vital glucocorticoid (CORT) hormones corticosterone (in rodents) and cortisol (in humans). These are steroids synthesized by the adrenal gland in response to stimulation by adrenocorticotropic hormone (ACTH), which is secreted by the anterior pituitary in response to corticotrophin-releasing hormone (CRH) and arginine vasopressin released from hypothalamic paraventricular neurons. These neurons receive circadian inputs from the suprachiasmatic nucleus and are activated in response to stress. Via the bloodstream, CORT accesses target tissues where it mediates metabolic, cognitive, and immune responses. CORT also regulates its own production through negative feedback inhibition of ACTH and CRH secretion from the pituitary and hypothalamus, respectively. To mount an effective response to stress, CORT must be secreted rapidly by the adrenal glands. However, because of its lipophilic nature, CORT cannot be prestored in vesicles and must therefore be rapidly synthesized de novo in response to ACTH stimulation.Under basal, unstressed conditions, ACTH and CORT exhibit ultradian oscillations. Although there is some evidence for pulsatility of CRH (1, 2), our recent work suggests that ACTH and CORT pulsatility is predominantly generated by a subhypothalamic oscillator within the pituitary-adrenal system (3, 4). The amplitude of these pulses varies in a circadian manner with larger pulses occurring at the start of the active phase (morning in humans, evening in rodents). Under normal physiological conditions, CORT secretion is tightly correlated with ACTH (5). However, there are a number of conditions where a dynamic dissociation between these hormones occurs (reviewed in ref. 6). For example, there is evidence that proinflammatory cytokines released during inflammation can poten...

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The hypothalamic–pituitary–adrenal axis is a vital neuroendocrine system that regulates the secretion of glucocorticoid hormones from the adrenal glands. This system is characterized by a dynamic ultradian hormonal oscillation, and in addition is highly responsive to stressful stimuli. We have recently shown that a primary mechanism generating this ultradian rhythm is a systems-level interaction where adrenocorticotrophin hormone (ACTH) released from the pituitary stimulates the secretion of adrenal glucocorticoids, which in turn feedback at the level of the pituitary to rapidly inhibit ACTH secretion. In this study, we combine experimental physiology and mathematical modelling to investigate intra-adrenal mechanisms regulating glucocorticoid synthesis. Our modelling results suggest that glucocorticoids can inhibit their own synthesis through a very rapid (within minutes), presumably non-genomic, intra-adrenal pathway. We present further evidence for the existence of a short time delay in this intra-adrenal inhibition, and also that at the initiation of each ACTH stimulus, this local feedback mechanism is rapidly antagonized, presumably via activation of the specific ACTH receptor (MC2R) signalling pathway. This mechanism of intra-adrenal inhibition enables the gland to rapidly release glucocorticoids while at the same time preventing uncontrolled release of glucocorticoids in response to large surges in ACTH associated with stress.

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.