Margaret J. Bradbury

Feast and Famine: Critical Role of Glucocorticoids with Insulin in Daily Energy Flow

The hypothesis proposed in this review is that normal diurnal rhythms in the hypothalamic-pituitary-adrenal (HPA) axis are highly regulated by activity in medial hypothalamic nuclei to effect an interaction between corticosteroids and insulin such that optimal metabolism results in response to changes in the fed or fasted state of the animal. There are marked diurnal rhythms in function of the HPA axis under both basal and stress conditions. The HPA axis controls corticosteroid output from the adrenal and, in turn, forward elements of this axis are inhibited by feedback from circulating plasma corticosteroid levels. Basal activity in the HPA axis of mammals fed ad lib peaks about 2 h before the peak of the diurnal feeding rhythm, and is controlled by input from the suprachiasmatic nuclei. The rhythm in stress responsiveness is lowest at the time of the basal peak and highest at the time of the basal trough in the HPA axis activity. There are also diurnal rhythms in corticosteroid feedback sensitivity of basal and stress-induced ACTH secretion which peak at the time of the basal trough. These rhythms are all overridden when feeding, and thus insulin secretion, is disrupted. Corticosteroids interact with insulin on food intake and body composition, and corticosteroids also increase insulin secretion. Corticosteroids stimulate feeding at low doses but inhibit it at high doses; however, it is the high levels of insulin, induced by high levels of corticosteroids, that may inhibit feeding. The effects of corticosteroids on liver, fat, and muscle cell metabolism, with emphasis on their interactions with insulin, are briefly reviewed. Corticosteroids both synergize with and antagonize the effects of insulin. The effects of stress hormones, and their interactions with insulin on lipid and protein metabolism, followed by some of the metabolic effects of injury stress, with or without nutritional support, are evaluated. In the presence of elevated insulin stimulated by glucocorticoids and nutrition, stress causes less severe catabolic effects. In the central nervous system, regulation of function in the HPA axis is clearly affected by the activity of medial hypothalamic nuclei that also alter feeding, metabolism, and obesity in rats. Lesions of the arcuate (ARC) and ventromedial (VMN) paraventricular (PVN) nuclei result in obesity and hyperactivity in the HPA axis. Moreover, adrenalectomy inhibits or prevents development of the lesion-induced obesity. There are interactions among these nuclei; one mode of communication is via inputs of neuropeptide Y (NPY) cells in the ARC to the VMN, dorsomedial nuclei, and PVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Corticosteroids and the Control of Function in the Hypothalamo-Pituitary-Adrenal (HPA) Axisa

The central nervous system (CNS) through its control of secretion of releasing and inhibiting factors from the neuroendocrine hypothalamus into the hypothalamohypophysial portal system regulates hormonal synthesis and secretion from the anterior pituitary, and, via its control of the pituitary hormones, regulates activity of peripheral target endocrine glands. In order to control activity of the peripheral endocrine glands, the CNS must receive information about their function. This information is conveyed by neurons in the CNS that contain receptors for a specific target hormone. The degree to which hormone receptors are occupied by the appropriate target hormone determines input to, and/or the activity of the hypothalamic neuroendocrine cells. Occupancy of the receptors is determined primarily by the concentration of free, unbound hormone in the circulation which is free to diffuse or be transported across the blood-brain barrier to receptor sites in the CNS. The characteristics of function in the HPA axis pose severe demands if there is to be tight feedback regulation by adrenal corticosteroids. There is a challenging range of circulating corticosteroid levels that is associated with the normal physiology of the adrenocortical system. The circadian rhythm in basal activity of the HPA axis results in total circulating corticosteroid concentrations that may be <lo nM at the nadir of the rhythm and about 700 nM at the peak of the rhythm in both man and rats. Of the total corticosteroid concentration, 99-95% is tightly bound in the circulation to transcortin and is unavailable for diffusion to brain sites. The amount of steroid bound depends on the concentrations of both steroid and transcortin in the circulation. Furthermore, at all times of day, the system can be stimulated by stressors to cause corticosteroid values that may exceed 1 pM. Nonetheless, the secretion of corticotropin-releasing factor (CRF) and vasopressin (AVP), the major hypothalamic neuropeptides that regulate ACTH synthesis in and secretion from the corticotropes of the anterior pituitary, appears to be under tight corticosteroid feedback control during the basal circadian trough and peak as well as during CRF/AVP responses to stressors. Although ACTH-secreting cells in the anterior pituitary respond to corticosteroids in vitro, with inhibition of ACTH synthesis and secretion, it seems likely that this effect is primarily a coarse gain feedback that is involved during high level corticosteroid secretion.’” The tight feedback control exerted on activity of the HPA system by the CNS appears to occur because of two corticosteroid receptors of different affinities that C-D. WALKEiR, MARGARET J. BRADBURY, S. SUEMARU,

Regulation of basal ACTH secretion by corticosterone is mediated by both type I (MR) and type II (GR) receptors in rat brain

The mechanisms involved in the physiology of the secretion of ACTH are reviewed. The secretion is regulated by the biological consequences of the occupancy of high affinity mineralocorticoid (MR) and lower affinity glucocorticoid receptors (GR) for corticosterone at specific sites of the rat brain. The regulation by this mechanism of basal secretion during the circadian rhythm, the effect of adrenalectomy and of corticosterone replacement is discussed. Experiments with RU486, a specific glucocorticoid antagonist, suggest that occupancy of both MR and GR is required for normal control of ACTH at the time of peak activity. The occupancy of the GR for a few hours per day apparently suffices to maintain steady levels of the products of GR-responsive genes throughout the body.

Glucocorticoid deficiency increases phospholipase A2 activity in rats.

An important mechanism for the antiinflammatory effect of pharmacological doses of glucocorticoids is the inhibition of arachidonic acid release from phospholipids by phospholipase A2 (PLA2). As a corollary, one might predict that low endogenous concentrations of glucocorticoids favor inflammatory disease states. Indeed, clinical and experimental observations revealed an association between glucocorticoid deficiency and disease states caused by immunological and/or inflammatory mechanisms. The purpose of the present investigation was to study the regulation of PLA2 mRNA, protein, and enzyme activity in adrenalectomized (ADX) rats where glucocorticoid concentrations were below physiological levels. The mRNA of group I and II PLA2 were measured by PCR. Group II PLA2 mRNA was increased by 126 +/- 9% in lung tissue of ADX rats, whereas group I PLA2 was increased only by 27 +/- 1.5%. The increase in group II mRNA in ADX rats was reflected by a corresponding increase of group II PLA2 protein (70-100%) in lung, spleen, liver, and kidney. This increase was reversed by the administration of exogenous corticosterone. After ADX, the percentage increase in total PLA2 activity was higher than that of mRNA or PLA2 protein, suggesting that the activity of the enzyme was modulated by inhibitors or activators. The concentration of lipocortin-I, an inhibitor of PLA2 enzyme was strongly correlated with the activity of PLA2 in the tissues (lung, spleen, liver, and kidney). In all these tissues, the concentrations of lipocortin-I declined after ADX. Thus upregulation of PLA2 enzyme and downregulation of lipocortin-I might account for the enhanced inflammatory response in hypoglucocorticoid states.

Lesions of the Hippocampal Efferent Pathway (Fimbria-Fornix) Do Not Alter Sensitivity of Adrenocorticotropin to Feedback Inhibition by Corticosterone in Rats

The hypothalamic-pituitary-adrenal (HPA) axis controls the diurnal and stress-induced release of adrenal corticosteroids into the general blood circulation. In turn, corticosteroids inhibit the HPA axis under basal conditions and during stress through occupation of their receptors (types I and II) in the brain by closing a negative feedback loop. The primary site in the brain at which corticosteroids act to inhibit the HPA axis has not been identified. High concentrations of both types of receptors are found in neurons of the hippocampal formation, a structure which has been reported by some, but not others, to control activity within the HPA axis by serving as a major negative feedback site. In many of these past studies, blood was collected after extensive handling or exposure to ether, conditions which do not favor the detection of basal hormone concentrations. To address these controversies, we tested the feedback sensitivity of the anterior pituitary hormone responsible for corticosteroid production, adrenocorticotropin (ACTH), to corticosterone (B), the main corticosteroid in rats, in total fornix- and, as controls, cortex-lesioned rats. All rats were given vascular catheters to avoid any handling-induced differences in plasma B or ACTH when sampling blood. In some experiments, fornix- and cortex-lesioned rats were adrenalectomized and given 1 of 3 doses of exogenous B provided in a subcutaneous pellet to ensure that plasma B was equal in different lesion groups. We hypothesized that if the hippocampal formation were an important site of B-mediated inhibition of the HPA axis, fornix-lesioned rats would have higher plasma B as a result of increased endogenous secretion in the morning or the evening compared to cortex-lesioned rats in rats with adrenal glands. In addition, we hypothesized that adrenalectomized fornix-lesioned rats given the same low to moderate levels of exogenous constant B would have higher basal and stress-induced ACTH than cortex-lesioned rats. Diurnal plasma B was not affected by fornix lesions in intact rats. Moreover, basal ACTH measured in the morning and the evening and stress-induced ACTH was the same in adrenalectomized fornix- and cortex-lesioned rats with constant exogenous B. We conclude, therefore, that information about occupancy of B receptors in the hippocampus carried by the fornix primarily subserves functions which do not directly regulate activity in the HPA axis.

Clamped Corticosterone (B) Reveals the Effect of Endogenous B on Both Facilitated Responsivity to Acute Restraint and Metabolic Responses to Chronic Stress

To determine the effects of both corticosterone (B) and chronic stressors on acute ACTH responses to restraint, young male rats were exposed to streptozotocin-induced diabetes, cold (5-7 degreesC) or intracerebroventricular (icv) neuropeptide Y (NPY) for 5 d and then exposed to restraint within 2 h after lights on. Two groups of rats were studied: intact and adrenalectomized replaced with B pellets that maintained plasma B in the normal mean 24-h range of intact rats. In addition to ACTH and B responses to restraint on d 5, body weight, food intake, fat depots, glucose and other hormones were measured to determine the role of stress-induced elevations in B on energy balance. ACTH responses to restraint were normal in intact rats subjected to diabetes or cold. By contrast, there was no ACTH or B response to restraint in NPY-infused intact rats. All 3 groups of chronically stimulated adrenalectomized rats with clamped B had facilitated ACTH responses to restraint compared to their treatment controls. Overall food intake increased in all groups of stressed rats; however, augmented intake occurred only during the light in intact rats and equally in the light and dark in B-clamped rats. White adipose depot weights were decreased by both diabetes and cold and increased by NPY in intact rats; the decreases with cold and increases with NPY were both blunted and changes in fat stores were not significant in adrenalectomized, B-clamped rats. We conclude that: 1. diabetes- and cold-induced facilitation of restraint-induced afferent input to hypothalamic control of the hypothalamo-pituitary-adrenal (HPA) axis is opposed in intact rats by the elevated feedback signal of B secretion; 2. NPY does not induce facilitation of afferent stress pathways; 3. chronic stimulation of the HPA axis induces acute hyperresponsiveness of hypothalamic neurons to restraint provided that the afferent input of this acute stimulus is not prevented by B feedback; 4. stimulus-induced elevations in B secretion result in day-time feeding; 5. insensitivity of both caloric efficiency and white fat stores to chronic stress in adrenalectomized, B-clamped rats results from loss of normally variable B levels.

Binding Kinetics Redefine the Antagonist Pharmacology of the Corticotropin-Releasing Factor Type 1 Receptor

Corticotropin-releasing factor (CRF) receptor antagonists are under preclinical and clinical investigation for stress-related disorders. In this study the impact of receptor-ligand binding kinetics on CRF1 receptor antagonist pharmacology was investigated by measuring the association rate constant (k1), dissociation rate constant (k−1), and kinetically derived affinity at 37°C. Three aspects of antagonist pharmacology were reevaluated: comparative binding activity of advanced compounds, in vivo efficacy, and structure-activity relationships. Twelve lead compounds, with little previously noted difference of affinity, varied substantially in their kinetic binding activity with a 510-fold range of kinetically derived affinity (k−1/k1), 170-fold range of k−1, and 13-fold range of k1. The k−1 values indicated previous affinity measurements were not close to equilibrium, resulting in compression of the measured affinity range. Dissociation was exceptionally slow for three ligands (k−1 t1/2 of 1.6–7.2 h at 37°C). Differences of binding behavior were consistent with in vivo pharmacodynamics (suppression of adrenocorticotropin in adrenalectomized rats). Ligand concentration-effect relationships correlated with their kinetically derived affinity. Two ligands that dissociated slowly (53 and 130 min) produced prolonged suppression, whereas only transient suppression was observed with a more rapidly dissociating ligand (16 min). Investigating the structure-activity relationship indicated exceptionally low values of k1, approaching 100,000-fold less than the diffusion-limited rate. Retrospective interpretation of medicinal chemistry indicates optimizing specific elements of chemical structure overcame kinetic barriers in the association pathway, for example, constraint of the pendant aromatic orthogonal to the ligand core. Collectively, these findings demonstrate receptor binding kinetics provide new dimensions for understanding and potentially advancing the pharmacology of CRF1 receptor antagonists.