Short-term but not long-term adrenalectomy modulates amplitude and frequency of the CRH41 episodic release in push-pull cannulated median eminence of free-moving rats

Fast dynamics in the HPA axis: Insight from mathematical and experimental studies

The activity of the hypothalamic-pituitary-adrenal (HPA) axis is characterised by complex dynamics spanning several timescales. This ranges from slow circadian rhythms in blood hormone concentration to faster ultradian pulses of hormone secretion and even more rapid oscillations in electrical and calcium activity in neuroendocrine cells of the hypothalamus and pituitary gland. Here, we focus on the system's oscillations on the short timescale. We highlight some of the mathematical modelling and experimental work that has been carried out to characterise the mechanisms regulating this highly dynamic mode of neuroendocrine signalling and discuss some future directions that may be explored to enhance understanding of HPA function.

Rhythmicity matters: Circadian and ultradian patterns of HPA axis activity

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

Role of glucocorticoid negative feedback in the regulation of HPA axis pulsatility

The hypothalamic-pituitary-adrenal (HPA) axis is the major neuroendocrine axis regulating homeostasis in mammals. Glucocorticoid hormones are rapidly synthesized and secreted from the adrenal gland in response to stress. In addition, under basal conditions glucocorticoids are released rhythmically with both a circadian and an ultradian (pulsatile) pattern. These rhythms are important not only for normal function of glucocorticoid target organs, but also for the HPA axis responses to stress. Several studies have shown that disruption of glucocorticoid rhythms is associated with disease both in humans and in rodents. In this review, we will discuss our knowledge of the negative feedback mechanisms that regulate basal ultradian synthesis and secretion of glucocorticoids, including the role of glucocorticoid and mineralocorticoid receptors and their chaperone protein FKBP51. Moreover, in light of recent findings, we will also discuss the importance of intra-adrenal glucocorticoid receptor signaling in regulating glucocorticoid synthesis.

Dynamic responses of the adrenal steroidogenic regulatory network

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.

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.

Dynamics of adrenal glucocorticoid steroidogenesis in health and disease

The activity of the hypothalamic-pituitary-adrenal (HPA) axis is characterized by an ultradian (pulsatile) pattern of hormone secretion. Pulsatility of glucocorticoids has been found critical for optimal transcriptional, neuroendocrine and behavioral responses. This review will focus on the mechanisms underlying the origin of the glucocorticoid ultradian rhythm. Our recent research shows that the ultradian rhythm of glucocorticoids depends on highly dynamic processes within adrenocortical steroidogenic cells. Furthermore, we have evidence that disruption of these dynamics leads to abnormal glucocorticoid secretion observed in disease and critical illness in both humans and rats.

Encoding and decoding mechanisms of pulsatile hormone secretion

Ultradian pulsatile hormone secretion underlies the activity of most neuroendocrine systems, including the hypothalamic-pituitary adrenal (HPA) and gonadal (HPG) axes, and this pulsatile mode of signalling permits the encoding of information through both amplitude and frequency modulation. In the HPA axis, glucocorticoid pulse amplitude increases in anticipation of waking, and, in the HPG axis, changing gonadotrophin-releasing hormone pulse frequency is the primary means by which the body alters its reproductive status during development (i.e. puberty). The prevalence of hormone pulsatility raises two crucial questions: how are ultradian pulses encoded (or generated) by these systems, and how are these pulses decoded (or interpreted) at their target sites? We have looked at mechanisms within the HPA axis responsible for encoding the pulsatile mode of glucocorticoid signalling that we observe in vivo. We review evidence regarding the 'hypothalamic pulse generator' hypothesis, and describe an alternative model for pulse generation, which involves steroid feedback-dependent endogenous rhythmic activity throughout the HPA axis. We consider the decoding of hormone pulsatility by taking the HPG axis as a model system and focussing on molecular mechanisms of frequency decoding by pituitary gonadotrophs.

Steroid synthesis inhibition with ketoconazole and its effect upon the regulation of the hypothalamus-pituitary-adrenal system in healthy humans

Steroid synthesis inhibitors are commonly used in the treatment of patients with Cushing's disease, but may also improve psychopathology in hypercortisolemic depressed patients. Since glucocorticoids exert a negative feedback at pituitary and supra-pituitary levels, the inhibition of steroid synthesis may lead to increased expression of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP). We studied the effect of treatment with 800 mg ketoconazole (3 weeks) upon the concentrations of basal plasma cortisol in the evening, corticosteroid-binding globulin (CBG), dehydroepiandrosterone-sulfate (DHEA-S), and ACTH as well as the concentrations of cortisol, CRH, and AVP in cerebrospinal fluid (CSF) at 8.30 h in 10 healthy, male volunteers. While we found cortisol plasma concentrations to be unchanged, we noted a significant increase in ACTH (post: 45.1+/-43.5; pre: 14.2+/-5.2 pmol/l; F(1,8)=9.78, p<0.02) and CBG concentrations (post: 38.8+/-4.3; pre: 31.9+/-4.2 microg/l), but DHEA-S plasma concentrations declined (post: 1.75+/-1.83; pre: 2.75+/-2.80 mg/l; F(1,8)=7.9, p<0.03). CRH concentrations in CSF were unchanged after treatment (post: 62.5+/-15.9; pre: 63.7+/-13.9 pg/ml), while there was a trend for AVP concentrations to rise during treatment (post: 2.52+/-1.18; pre: 1.92+/-0.96 pg/ml; paired t=-1.9, p<0.1). Cortisol CSF concentrations declined in the elderly (pre: 52.5+/-23.2; post: 26.7+/-4.6 nmol/l), but not in the young subgroup (pre: 15.6+/-11.3; post: 27.7+/-9.4 nmol/l). We thus conclude that the treatment of healthy controls with steroid-synthesis inhibitors does not lead to a major increase in CRH secretion.

Simultaneous comparison of cerebral dialysis and push-pull perfusion in the brain of rats: a critical review

Over the last 30 years, studies of the in vivo activity of neurotransmitters and other endogenous factors in the brain have comprised a major effort in the neurosciences. Historically, the technology of push-pull perfusion was utilized as a major approach to investigations in this field. In the last 10 years, cerebral dialysis has been used as an alternative method essentially for the same scientific purpose, since the perfusion technique was viewed as difficult and excessively damaging to tissue. This review considers the representative literature in which both systems have been used to study local neurochemical responses to a drug or other chemical factor, a physiological condition or other situation. In addition, new experiments have been undertaken to compare, in the same animal and at the same time, the utility and properties inherent in the techniques of push-pull perfusion and cerebral dialysis in terms of the profile of a neurotransmitter activity and their local histopathological effects. A miniaturized 33/26 ga push-pull needle and a 24 ga dialysis probe were implanted simultaneously in the left and right caudate nuclei, respectively, in the anesthetized rat. An artificial cerebrospinal fluid (CSF) was perfused simultaneously through both devices at a rate of 10 microliters/min in the push-pull cannula and at 1.0 or 2.0 microliters/min in the dialysis probe. Within a series of 8-10 successive perfusions, excess K+ ions in a concentration of either 30 or 60 mM were incorporated in the CSF and delivered simultaneously to both the push-pull cannula and dialysis probe. Samples of perfusate and dialysate were assayed chromatographically by coulometric HPLC detector and quantitated in terms of the pg/min efflux of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). The results showed that the resting level of DA was almost undetectable in dialysate samples from either structure; in push-pull perfusates the recovery of DA ranged between 7.0 to 10.0 pg/min, which was increased threefold by excess K+ ions. The recovery of DA and the three metabolites in samples of push-pull perfusate was two to four times that in samples of dialysate during the condition of excess K+ ions. Post-mortem histological analysis of the sites of perfusion and dialysis revealed little or no differences in the cytological damage induced by either the perfusion needle or dialysis probe. Finally, the advantages and limitations of each of these two experimental approaches to in vivo analysis of neurotransmitter efflux are reviewed in relation to the selection of an open or closed system for the on-line study of in vivo neurochemical events.

Immunoautoradiographic and in situ hybridization analysis of corticotropin-releasing hormone biosynthesis in the hypothalamic paraventricular nucleus

Corticotropin-releasing hormone (CRH) gene transcription and mRNA expression are keyed to stimuli activating the pituitary-adrenocortical axis, suggesting a connection between neuronal activation and synthesis of active peptide. However, the relationship between CRH mRNA and levels of CRH peptide remains to be definitively established. The present report characterizes an immunoautoradiographic (IAR) strategy to assess CRH peptide expression in an anatomical context. Non-fixed tissue sections through the rat hypothalamus were reacted with a primary antibody against rat CRH, followed by incubation with [35S] or [125I] labeled secondary antibody. Autoradiography performed on reacted sections revealed that CRH immunoreactivity could be detected in CRH-containing regions of the hypothalamus and amygdala. Generation of CRH signal was blocked by preabsorption of primary antibody with CRH peptide, demonstrating antibody specificity. IAR performed on nitrocellulose blotted with synthetic CRH peptide revealed a linear relationship between peptide quantity and intensity of autoradiographic signal, verifying that this method is appropriate for semi-quantitative analysis of CRH peptide regulation. Assessment of CRH peptide regulation revealed a significant increase in CRH content in adrenalectomized rats (ADX) relative to sham-adrenalectomized (SHAM) controls (196%). In situ hybridization performed on adjacent sections revealed a similar increase in CRH mRNA expression in ADX rats (256%), and a significant correlation between CRH peptide and mRNA measures (r = 0.68). No ADX induced changes were seen in median eminence, dorsomedial hypothalamus or central amygdaloid nucleus. The results of this study indicate that CRH biosynthesis appears to be driven by amount of available mRNA, rather than changes in translational efficacy. In addition, the IAR technique appears ideally suited to allow concomitant assessment of mRNA and protein expression within defined populations of CNS neurons.

Early hypothalamic activation of combined Fos and CRH41 immunoreactivity and of CRH41 release in push-pull cannulated rats after systemic endotoxin challenge

We previously showed that intra-arterial endotoxin infusion (lipopolysaccharide [LPS]: 25 micrograms.kg-1) induced an early (15 min) and sustained (480 min) rise in plasma ACTH associated with delayed (60-120 min) increases in plasma concentrations of TNF alpha, IL-6, and IL-1 beta. In the present study, we followed the post-LPS time-course of immunocytochemical expression of Fos-like activity in CRH41 neurons whose immunolabeling was enhanced by icv colchicine pretreatment 48 h before the LPS, and CRH41 release in the push-pull cannulated median eminence of free-moving rats, in parallel with the ACTH response. The earliest Fos-like activity in IR-CHR41 neurons was detected 30 min post-LPS. Colchicine strongly inhibited the LPS-induced activation of Fos expression in single-labeled paraventricular neurons. CRH41 release in the median eminence displayed a biphasic stimulation pattern, with a first peak (+60%) at 15 min together with the ACTH surge, followed by a second rise beginning at 45 min and lasting more than 2 h. Thus, the early stage of the ACTH surge following a nonlethal endotoxin challenge (< 60 min) already involves the activation of CRH41-producing neurons.