Insulin and the constituent branches of the hepatic vagus interact to modulate hypothalamic and limbic neuropeptide mRNA expression differentially

Insulin and signalling through the vagus nerve act in concert to regulate metabolic homeostasis and ingestive behaviour. Our previous studies using streptozotocin (STZ)-diabetic rats have shown that hepatic branch vagotomy (HV), gastroduodenal branch vagotomy (GV) and capsaicin treatment of the common hepatic branch that selectively destroys afferent fibres (CapV), all promote lard, but not total, caloric intake to levels similar to those achieved with insulin treatment. Because hypothalamic and limbic mRNA expression of neuropeptides linked to energy balance is altered by STZ-diabetes and HV, we examined the role(s) of insulin and the common hepatic and gastroduodenal branches of the vagus nerve and hepatic afferent fibres in the regulation of these neuropeptides in rats with high, steady-state corticosterone levels. STZ-diabetic rats were prepared with osmotic minipumps containing either saline or insulin and were compared with nondiabetic counterparts: half of each group received a vagal manipulation, the other half were sham operated. Five days after surgery, rats were offered the choice of lard and chow to consume for another 5 days, when brains were collected and processed for in situ hybridisation. Paraventricular nucleus corticotrophin-releasing factor (CRF) mRNA was elevated by STZ treatment, an effect prevented by either insulin treatment or GV. By contrast, CRF mRNA expression in the central nucleus of the amygdala and bed nuclei of the stria terminalis was unaffected by STZ treatment, but HV and CapV manipulations elevated expression in the nondiabetic, but not STZ-diabetic groups. Arcuate nucleus neuropeptide Y, but not pro-opiomelanocortin, mRNA expression was elevated by STZ treatment and all vagal manipulations; however, exogenous insulin treatment failed to prevent this, in keeping with their previously documented elevated caloric intake. These results strongly suggest that the gastroduodenal branch and hepatic branch proper, which merge to form the common hepatic branch, differentially interact with prevailing insulin levels to regulate hypothalamic and limbic neuropeptide mRNA expression.

Mapping brain c-Fos immunoreactivity after insulin-induced voluntary lard intake: insulin- and lard-associated patterns

In addition to the inhibitory role of central insulin on food intake, insulin also acts to promote lard intake. We investigated the neural pathways involved in this facet of insulin action. Insulin or saline was infused into either the superior mesenteric or right external jugular veins of streptozotocin-diabetic rodents with elevated steady-state circulating corticosterone concentrations. After postsurgical recovery, rats were offered the choice of chow or lard to eat. Irrespective of the site of venous infusion, insulin increased lard and decreased chow intake. After 4 days, lard was removed for 8 h. On return for 1 h, only insulin infused into the superior mesenteric vein resulted in lard intake. This facilitated distinction between the effects of circulating insulin concentrations (similar in the two insulin-infused groups) and lard ingestion on the patterns of c-Fos(+) cells in the brain, termed insulin- and lard-associated patterns, respectively. Insulin-associated changes in c-Fos(+) cell numbers were evident in the arcuate nucleus, bed nucleus of the stria terminalis and substantia nigra pars compacta, concomitant with elevated leptin levels and reduced chow intake. Lard-associated changes in c-Fos(+) cell numbers were observed in the nucleus of the tractus solitarius, lateral parabrachial nucleus, central nucleus of the amygdala, ventral tegmental area, nucleus accumbens shell and the prefrontal cortex, and were associated with lower levels of triglycerides and free fatty acids. The anterior paraventricular thalamic nucleus exhibited both patterns. These data collectively fit into a framework for food intake and reward and provide targets for pharmacological manipulation to influence the choice of food intake.

Chronic stress-induced effects of corticosterone on brain: direct and indirect

Acutely, glucocorticoids act to inhibit stress-induced corticotrophin-releasing factor (CRF) and adrenocorticotrophic hormone (ACTH) secretion through their actions in brain and anterior pituitary (canonical feedback). With chronic stress, glucocorticoid feedback inhibition of ACTH secretion changes markedly. Chronically stressed rats characteristically exhibit facilitated ACTH responses to acute, novel stressors. Moreover, in adrenalectomized rats in which corticosterone was replaced, steroid concentrations in the higher range are required for facilitation of ACTH responses to occur after chronic stress or diabetes. Infusion of corticosterone intracerebroventricularly into adrenalectomized rats increases basal ACTH, tends to increase CRF, and allows facilitation of ACTH responses to repeated restraint. Therefore, with chronic stressors, corticosterone seems to act in brain in an excitatory rather than an inhibitory fashion. We believe, under conditions of chronic stress, that there is an indirect glucocorticoid feedback that is mediated through the effects of the steroid +/- insulin on metabolism. Increased energy stores feedback on brain to inhibit hypothalamic CRF and decrease the expression of dopamine-beta-hydroxylase in the locus coeruleus. These changes would be expected to decrease the level of discomfort and anxiety induced by chronic stress. Moreover, central neural actions of glucocorticoids abet the peripheral effects of the steroids by increasing the salience and ingestion of pleasurable foods.

Sucrose intake and corticosterone interact with cold to modulate ingestive behaviour, energy balance, autonomic outflow and neuroendocrine responses during chronic stress

In adrenalectomized (ADX) rats, either corticosterone replacement or increased sucrose intake will restore body weight gain, uncoupling protein-1, fat depot mass, food intake and corticotropin-releasing factor mRNA expression to normal. Here, we tested the potential interactions between sucrose intake and circulating corticosterone on behavioural, metabolic, autonomic and neuroendocrine responses to the stress of cold. Rats were left intact, sham-ADX, or ADX and replaced with pellets that provided normal, basal (30%B) or high stress (100%B) constant circulating concentrations of corticosterone +/- sucrose. More calories were consumed in cold than at room temperature (RT), provided that corticosterone concentrations were elevated above mean daily basal values in cold. Neither increased sucrose nor increased chow ingestion occurred in cold if the rats were ADX and replaced with 30%B. However, sucrose drinking in this group markedly ameliorated other responses to cold. By contrast, ADX30%B rats not drinking sucrose fared poorly, and none of the metabolic or endocrine variables were similar to those in sham-ADX controls. ADX100%B group in cold, resembled intact rats without sucrose; however, this group was metabolically abnormal at RT. We conclude that drinking sucrose lowers stress-induced corticosterone secretion while reducing many responses to cold; elevated corticosterone concentrations in the stress-response range are essential for the normal integrated cold-induced responses to occur.

Corticosterone exerts site-specific and state-dependent effects in prefrontal cortex and amygdala on regulation of adrenocorticotropic hormone, insulin and fat depots

Chronic stress stimulates corticosterone secretion and recruits brain pathways that regulate energy balance (caloric acquisition and deposition) and facilitate hypothalamic-pituitary-adrenal responsiveness to new stressors. We implanted corticosterone or cholesterol bilaterally either near the central nucleus of the amygdala (CeA) or in the prefrontal cortex to determine whether high concentrations of the steroid act at either site, with or without chronic stress. Rats were adrenalectomized and treated systemically with low doses of corticosterone. Half were maintained at room temperature and the other half were exposed to 5 degrees C cold for 5 days before all rats were restrained. There was limited diffusion of corticosterone from brain implants. Corticosterone in prefrontal cortex, but not CeA, decreased plasma insulin and adrenocorticotropic hormone (ACTH) responses to acute restraint in both control and chronically cold stressed rats. Corticosterone implants near CeA decreased the weight of fat depots only in cold; corticosterone implants in prefrontal cortex were ineffective. We conclude that (i) corticosterone inhibits insulin and ACTH secretion by an action in prefrontal cortex but not CeA; (ii) high concentrations of corticosterone secreted during chronic stress alter metabolism through (autonomic) outputs of the CeA and prefrontal cortex in site- and variable-specific fashion; and (iii) the amygdala is a component of a stress-recruited, state-dependent pathway.

Haploinsufficiency of steroidogenic factor-1 in mice disrupts adrenal development leading to an impaired stress response

Adrenal steroids are essential for homeostasis and survival during severe physiological stress. Analysis of a patient heterozygous for the steroidogenic factor-1 (SF-1) gene suggested that reduced expression of this nuclear receptor leads to adrenal failure. We therefore examined SF-1 heterozygous (+/-) mice as a potential model for delineating mechanisms underlying this disease. Here we show that SF-1 +/- mice exhibit adrenal insufficiency resulting from profound defects in adrenal development and organization. However, compensatory mechanisms, such as cellular hypertrophy and increased expression of the rate-limiting steroidogenic protein StAR, help to maintain adrenal function at near normal capacity under basal conditions. In contrast, adrenal deficits in SF-1 heterozygotes are revealed under stressful conditions, demonstrating that normal gene dosage of SF-1 is required for mounting an adequate stress response. Our findings predict that natural variations leading to reduced SF-1 function may underlie some forms of subclinical adrenal insufficiency, which become life threatening during traumatic stress.

Disruption of arcuate/paraventricular nucleus connections changes body energy balance and response to acute stress

The mediobasal hypothalamus regulates functions necessary for survival, including body energy balance and adaptation to stress. The purpose of this experiment was to determine the contribution of the arcuate nucleus (ARC) in controlling these two functions by the paraventricular nucleus (PVN). Circular, horizontal cuts (1.0 mm radius) were placed immediately above the anterior ARC to sever afferents to the PVN. In shams the knife was lowered to the same coordinates but was not rotated. Food intake and body weight were monitored twice daily, at the beginning and end of the light cycle, for 1 week. On the final day the animals were restrained for 30 min. Lesioned animals had increased food intake in light and dark periods, higher weight gain per day, and more body fat as compared with shams. There was no difference in caloric efficiency. Unlike shams, lesioned rats had no predictable relationship between plasma insulin and leptin. Plasma ACTH was increased at 0 min in lesioned rats but was decreased 15 and 30 min after restraint as compared with shams. There was no difference in plasma corticosterone. Immunostaining revealed that alpha-melanocortin (alphaMSH) and neuropeptide Y (NPY) accumulated below the cuts, and both were decreased in PVN. Food intake and body weight were correlated negatively to alphaMSH, but not NPY in PVN. There was no difference in proopiomelanocortin (POMC) mRNA, but NPY mRNA was reduced in the ARC of lesioned animals. We conclude that ARC controls body energy balance in unstressed rats, possibly by alphaMSH input to PVN, and that ARC also is necessary for PVN regulation of ACTH.

Bottomed out: metabolic significance of the circadian trough in glucocorticoid concentrations

Mild chronic stressors characteristically increase circadian trough corticosteroid concentrations in rats and man. The elevation in trough concentrations is often accompanied by a reduction in peak concentrations and no change in the daily mean values. Here we point out that elevation of trough glucocorticoids, probably through daily increases of glucocorticoid receptor occupancy, has major metabolic effects that bias organisms toward storage of calories as fat. Thus, chronic mild stress, by overriding the normal mineralocorticoid receptor-mediated corticosteroid feedback regulation of trough CRF and ACTH secretion, facilitates development of the metabolic syndrome.

Hypothalamic-pituitary-adrenal dysfunction in Apoe(-/-) mice: possible role in behavioral and metabolic alterations

Several neurological diseases are frequently accompanied by dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis regulates the secretion of glucocorticoids (GCs), which play important roles in diverse brain functions, including cognition, emotion, and feeding. Under physiological conditions, GCs are adaptive and beneficial; however, prolonged elevations in GC levels may contribute to neurodegeneration and brain dysfunction. In the current study, we demonstrate that apolipoprotein E (apoE) deficiency results in age-dependent dysregulation of the HPA axis through a mechanism affecting primarily the adrenal gland. Apoe(-/-) mice, which develop neurodegenerative alterations as they age, had an age-dependent increase in basal adrenal corticosterone content and abnormally increased plasma corticosterone levels after restraint stress, whereas their plasma and pituitary adrenocorticotropin levels were either unchanged or lower than those in controls. HPA axis dysregulation was associated with behavioral and metabolic alterations. When anxiety levels were assessed in the elevated plus maze, Apoe(-/-) mice showed more anxiety than wild-type controls. Apoe(-/-) mice also showed reduced activity in the open field. Finally, Apoe(-/-) mice showed age-dependent increases in food and water intake, stomach and body weights, and decreases in brown and white adipose tissues. These results support a key role for apoE in the tonic inhibition of steroidogenesis and HPA axis activity and have important implications for the behavioral analysis of Apoe(-/-) mice.

Interactions among chronic cold, corticosterone and puberty on energy intake and deposition

We have shown that chronic cold stress strongly interacts with corticosterone (B) to determine subsequent regulation of the hypothalamo-pituitary-adrenal (HPA) axis responses to novel stress. These studies, using the same 2 sets of rats, show that chronic cold also interacts with B and testosterone on signals of energy balance. The two groups of rats differed in weight by 20% and in age by 2 weeks (44-59 days of age). Adrenalectomized rats, replaced with varying doses of B, were exposed to cold or served as controls. Food intake and body weight during the experiments and hormones, metabolites and fat depots were measured on day 5. B, but not cold, affected food intake in the younger rats; by contrast, cold, but not B, affected food intake in the older rats. Testosterone was higher in older control rats and was markedly depressed by cold; younger rats had lower testosterone that was minimally affected by cold. Weight gain decreased in all rats at room temperature with increasing B, whereas they all lost weight in cold independently of B. Cold stimulated and B inhibited interscapular brown adipose tissue DNA content (reflecting sympathetic stimulation of thermogenesis). B stimulated insulin, whereas cold inhibited leptin and insulin; B also increased white adipose tissue weight gain in controls and inhibited its loss in cold. Leptin was unrelated to white adipose tissue depots in older control rats but was strongly related to these stores in younger rats and in all rats in cold. We conclude that: 1. By decreasing signals that act centrally to inhibit food intake (insulin, leptin and testosterone) cold allows B to stimulate food intake; 2. B inhibits weight gain although it causes accrual of fat; 3. Cold, probably through sympathetic stimulation of white adipose tissue, causes fat loss which is modulated by the inhibitory effect of B on sympathetic outflow; and, 4. The slope of the relationship between fat depot size and leptin becomes flatter in cold, possibly because of increased sympathetic outflow to these depots.

Warning! Nearby construction can profoundly affect your experiments

This is meant to alert people to potentially major effects of construction projects on research results. Because we study the effects of stress on regulation of ACTH and corticosterone secretion and of serotonin receptors and stress on energy balance, we serve as an early warning system when things go awry. Most of our experiments include taking daily, or twice daily, measurements of rat or mouse weights and food intake as well as stress hormone levels. We are highly sensitized to environmental disruption and we've shown previously the effects of construction on stress hormones (1). However, we did not anticipate the change and disruption in energy balance that may occur in response to environmental perturbation. We provide two examples of these, below.

Starvation: early signals, sensors, and sequelae

To identify the sequences of changes in putative signals, reception of these and responses to starvation, we sampled fed and starved rats at 2- to 6-h intervals after removal of food 2 h before dark. Metabolites, hormones, hypothalamic neuropeptide expression, fat depots, and leptin expression were measured. At 2 h, insulin decreased, and FFA and corticosterone (B) increased; by 4 h, leptin and glucose levels decreased. Neuropeptide Y messenger RNA (mRNA) increased 6 h after food removal and thereafter. Adrenal and plasma B did not follow ACTH and were elevated throughout, with a nadir at the dark-light transition. Leptin correlated inversely with adrenal B. Fat stores decreased during the last 12 h. Leptin mRNA in perirenal and sc fat peaked during the dark period, resembling plasma leptin in fed rats. We conclude that 1) within the first 4 h, hormonal and metabolic signals relay starvation-induced information to the hypothalamus; 2) hypothalamic neuropeptide synthesis responds rapidly to the altered metabolic signals; 3) catabolic activity quickly predominates, reinforced by elevated B, not driven by ACTH, but possibly to a minor extent by leptin, and more by adrenal neural activity; and 4) leptin secretion decreases before leptin mRNA or fat depot weight, showing synthesis-independent regulation.

Pituitary-adrenocortical responses to persistent noxious stimuli in the awake rat: endogenous corticosterone does not reduce nociception in the formalin test

Although glucocorticoids inhibit inflammation and are used to treat painful inflammatory rheumatic diseases, the contribution, if any, of endogenous pituitary-adrenocortical activity to the control of pain remains unclear. We report that injection of dilute formalin into the hindpaw not only evokes inflammation and pain-related behavior, but it also increases ACTH and corticosterone to a greater extent than restraint and saline injection alone. This difference was particularly robust during the final periods of pain-related behavior in the formalin test, when the ACTH and corticosterone (B) levels in the restraint/saline control group had returned to normal. These results indicate that formalin-evoked increases in ACTH and B reflect nociceptive input, rather than the stress associated with handling. To test the hypothesis that the formalin-induced increase in corticosterone reduces pain and inflammation, we next evaluated the effect of adrenalectomy (to prevent activation of glucocorticoid receptors) or high-dose dexamethasone (to saturate glucocorticoid receptors) on nociceptive processing in the formalin test. Neither adrenalectomy nor dexamethasone changed behavioral or cardiovascular nociceptive responses. Furthermore, the increases in blood pressure and heart rate produced by formalin may not be mediated by adrenomedullary catecholamine release. In addition, we conclude that the nociceptive component of the formalin stimulus is sufficient to activate the pituitary-adrenocortical system in the awake rat, but that the resulting release of corticosterone does not feed back and reduce nociceptive processing.

Chronic cold in adrenalectomized, corticosterone (B)-treated rats: facilitated corticotropin responses to acute restraint emerge as B increases

Small elevations in corticosterone (B) administered exogenously exert potent inhibitory effects on both basal and stress-induced ACTH secretion. However, under conditions of chronic stress with chronic elevations in B, the hypothalamo-pituitary-adrenal system appears to balance the negative feedback signal of B with central neural facilitation so that the system remains fully responsive to acute stressors. In these studies, we tested whether: 1) circulating B concentrations affect responses to acute restraint in rats exposed to 5 days at 5-7 C (cold), compared with room temperature (control); and 2) facilitated ACTH secretion can be explained by increased CRF or vasopressin messenger RNA (mRNA) levels in the hypothalamic parvocellular paraventricular nuclei (PVN). Rats were adrenalectomized and supplied with B in doses that fixed plasma B at constant levels between approximately 2 and 20 microg/dl; rats were placed in cold or remained as controls. Increasing concentrations of fixed B decreased basal ACTH similarly in both groups. By contrast, as B levels increased, ACTH responses to restraint also increased in cold vs. control rats. Semiquantitative analysis of CRF mRNA by in situ hybridization revealed decreases of similar magnitude in both groups with increasing fixed B. Vasopressin mRNA levels also decreased with increasing fixed B in both groups, but with slightly less sensitivity to inhibition by B in cold exposed rats. Taken together, the decreases in mRNA for these major ACTH neuropeptide secretogogues in the parvocellular PVN are unlikely to explain facilitated ACTH responses in chronically stressed rats. We conclude that a brain site is stimulated by B that is proximal to the PVN; feedforward, positive effects of B are thus implicated in mediation of prior stress-induced facilitation of acute hypothalamo-pituitary-adrenal responses to stress.

Dopamine-beta-hydroxylase activity is necessary for hypothalamo-pituitary-adrenal (HPA) responses to ether, and stress-induced facilitation of subsequent HPA responses to acute ether emerges as HPA responses are inhibited by increasing corticosterone (B)

To determine a role of norepinephrine (NE) in stress-induced HPA function, young male rats were treated with diethyldithiocarbamide (DDC) which inhibits dopamine-beta-hydroxylase, the enzyme that synthesizes NE from dopamine (DA). DDC injected 5 h prior to ether stress stimulated ACTH and corticosterone (B) during this time, and there was no further HPA response to ether. To control for elevated B feedback in DDC effects on HPA responses to ether, rats were adrenalectomized (Adx) and replaced with no (0% B), moderate (40% B) and high (80% B) levels of steroid 5 d prior to DDC or saline with ether stress 5 h later; Sham-Adx rats were included. In Adx rats increasing B inhibited thymus weight, median eminence CRF content, pituitary and plasma ACTH. In saline-treated rats, ether 5 h later caused increased CRF content and plasma ACTH in Sham-Adx and Adx, 0% B, increased ACTH in Adx, 40% B, and no response in Adx, 80% B. B treatment did not alter catecholamine content, and DDC treatment reduced NE content in the paraventricular nuclei by 50-60% in all groups. 5 h after DDC, pituitary ACTH was decreased in all rats with B and plasma ACTH was increased in sham-Adx and Adx, 40% B; thus DDC caused significant, prolonged stress which should facilitate subsequent HPA responses to acute stress. There was no HPA response to ether in Sham-Adx, Adx, 0% or 40% B groups, but there was a marked ACTH response to ether in the Adx, 80% B group treated with DDC. We conclude that: 1) the HPA response to ether stress is probably mediated by catecholamines; 2) DDC does not stimulate responses in the HPA axis in the absence of B; and, 3) facilitation of HPA responses to acute stress depends on increased steady-state B signals. Facilitated responses are probably not mediated by catecholamines. The consequence of facilitation is that under conditions of chronic stress and elevated B concentrations, as in depression or anorexia nervosa in man, or adjuvent-induced arthritis in rats, the HPA axis is continually responsive to new stimuli.

A hypercaloric load induces thermogenesis but inhibits stress responses in the SNS and HPA system

Caloric overingestion generates a sympathetic nervous system (SNS)-mediated increase in brown adipose tissue (BAT) thermogenesis; its effect on the hypothalamo-pituitary-adrenal (HPA) axis is unknown. To determine whether metabolic activation affects the HPA axis, male rats were provided palatable sucrose ad libitum. After 5 or 10 days of sucrose ingestion, BAT and basal and restraint-induced HPA variables were measured. Some rats were instrumented with temperature probes. BAT temperature and HPA axis responses to restraint were measured. Although caloric intake increased > or = 18%, body weight gain did not change after sucrose ingestion; DNA, protein, and uncoupling protein increased in BAT depots, and white adipose tissues were heavier after both 5 and 10 days. During days 5-10, the BAT-core temperature difference was +0.30 degrees C in sucrose rats and -0.46 degrees C in controls (P < 0.05); this, together with the biochemical changes, shows persistent activation of BAT by excess calories. Basal HPA measures were not altered. The sucrose group exhibited smaller BAT temperature and HPA responses to restraint on day 10; there was no HPA difference on day 5. We conclude that calorically mediated increases in BAT thermogenesis are independent of basal HPA activity; however, both systems respond concordantly to restraint stress. The diminished response to restraint in both systems in sucrose-fed rats may result from signals indicating increased energy stores.

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.

The neural network that regulates energy balance is responsive to glucocorticoids and insulin and also regulates HPA axis responsivity at a site proximal to CRF neurons

The structure of a large neural system that responds to and regulates energy balance and that encompasses that PVN and activity of the HPA axis has begun to emerge from these experiments (Fig. 6). Several large loops have been delineated within this context of the maintenance of energy balance. Corticosteroids stimulate both feeding and insulin secretion. The actions of corticosteroids in the periphery are catabolic, causing mobilization of energy stores; their actions in the central nervous system are stimulatory to energy acquisition (food intake). By contrast, the action of insulin in the periphery is anabolic, causing energy storage; its action in the central nervous system is inhibitory to energy acquisition (food intake). At the level of the CNS, insulin inhibits and corticosteroids stimulate expression of NPY mRNA in the arcuate nuclei, and these actions may explain, in part, the reciprocal actions of the hormones on energy acquisition. Thus over the long term, stimulation of insulin secretion by corticosteroids tends to supply an automatic brake on the effects of corticosteroids on feeding. The neural system that controls energy balance and responds to the reciprocal signals of corticosterone and insulin also regulates responsivity to restraint stress in the HPA axis. The low-amplitude ACTH responses to restraint, corticosteroid feedback, and prior stress-induced facilitation that are observed under conditions of relative fasting in the PM can be produced in the AM by a 14-h, overnight fast. By contrast, NPY injected ivt stimulates identical ACTH responses in the AM in fed rats and in rats fasted overnight, suggesting that NPY acts to stimulate CRF secretion at a site closer to the PVN than the stress of restraint, which is filtered through the neural energy balance system. In the periphery, corticosteroids and insulin also have reciprocal effects on energy storage; effects that are opposite those exerted in the CNS on energy acquisition. Thus, together, the two hormones may be construed as a bihormonal system that regulates overall energy balance. Although under normal conditions this system is well designed to accomplish energy balance, and provides a mechanism by which total energy stores may be increased appropriately (e.g., prior to hibernation or migration), it seems probable that under conditions of chronic stress, this regulatory system may be maladaptive. Chronic stress and glucocorticoid treatment cause increases in mean daily concentrations of both corticosteroids and insulin. Increases in the absolute levels of both hormones, with the normal ratio between them maintained, results in remodeling of body energy stores-away from muscle stores and toward fat stores, particularly abdominal fat stores. It seems quite likely that some conditions of abdominal obesity in man may be explained, at least in part, by increased activity in the HPA axis. Because abdominal obesity is associated with cardiovascular diseases, these responses, when they persist, are clearly maladaptive. Exploration of the role and control of the HPA axis in and by the larger neural network that regulates energy balance has to date been instructive. Clearly this work has just begun and is primarily still at the level of phenomenology. However, once the phenomenology is understood, mechanistic work can be performed that will flesh out our understanding of this very large and physiologically essential system.

Glucocorticoids and insulin: complex interaction on brown adipose tissue

Glucocorticoids and insulin effect long-term reciprocal changes in food intake and body weight. We tested the interactions of corticosterone and insulin on caloric efficiency, white adipose tissue (WAT) stores, and brown adipose tissue (BAT). Two experiments were performed: 1) adrenalectomized rats were treated with corticosterone with or without streptozotocin-induced diabetes and 2) adrenalectomized, corticosterone-treated, diabetic rats were treated with insulin. By 4-5 days later, > or = 50% of the variance in caloric efficiency, plasma triglycerides, and WAT stores was explained by regression of these variables on corticosterone (catabolic) and insulin (anabolic). When the ratio of the hormones was normal, but concentrations high, overall gain of energy stores decreased and energy was redistributed to fat. Both hormones were anabolic on BAT lipid storage; the hormones played a complex role in the regulation of uncoupling protein (UCP) in BAT. Although corticosterone inhibited and insulin stimulated UCP, these effects were only evident in diabetics and with normoglycemia, respectively. For BAT variables, < or = 50% of the variance was explained by regression on corticosterone and insulin, suggesting that the effects of these hormones are mediated through an intermediate such as sympathetic nervous system input to BAT.

Aldosterone and dexamethasone both stimulate energy acquisition whereas only the glucocorticoid alters energy storage

Corticosteroids stimulate and insulin inhibits energy acquisition (food intake); conversely, corticosteroids inhibit and insulin stimulates energy storage (body weight gain). Thus, together these hormones mediate long-term energy balance. This study tested whether the stimulatory action of corticosteroids on food intake was mediated by association with high affinity mineralocorticoid receptors (MRs) or lower affinity glucocorticoid receptors (GRs). Young male rats were adrenalectomized (ADX) and given vehicle (control) or streptozotocin (diabetic); subgroups of rats were infused with vehicle, aldosterone (Aldo, an MR agonist in vivo), dexamethasone (Dex, a GR agonist in vivo), or Aldo&Dex for the 5 days after ADX. Sham-ADX rats were included. Food intake, body weight gain, and epididymal white adipose and interscapular brown adipose tissue stores were weighed. ADX decreased food intake by approximately 24%, and food intake was not increased by diabetes as it was in sham-ADX rats. In control ADX rats, Dex, but not Aldo, stimulated insulin, and food intake was not significantly affected by either hormone; together, Aldo and Dex restored insulin and food intake to sham-ADX rats. Food intake in diabetic ADX rats was significantly increased by each treatment (ADX < Aldo < Dex < Aldo&Dex = sham). Aldo increased body weight through an increase in fluid volume (estimated by decreased plasma protein concentration); however, fat stores were not different from ADX. Dex reduced body weight in control rats but maintained fat stores; in diabetic rats, body weight and fat stores were less than or similar to ADX. We conclude that: 1) corticosteroids, acting through association with both MRs and GRs, stimulate food intake; 2) insulin counteracts the GR-mediated stimulation of food intake in control rats; and 3) Dex and insulin, which is stimulated by Dex, selectively maintain or increase body fat stores, probably at the expense of protein stores.

Eating disorder and epilepsy in mice lacking 5-HT2c serotonin receptors

Serotonin (5-hydroxytryptamine, 5-HT) is a monoaminergic neurotransmitter that is believed to modulate numerous sensory, motor and behavioural processes in the mammalian nervous system. These diverse responses are elicited through the activation of a large family of receptor subtypes. The complexity of this signalling system and the paucity of selective drugs have made it difficult to define specific roles for 5-HT receptor subtypes, or to determine how serotonergic drugs modulate mood and behaviour. To address these issues, we have generated mutant mice lacking functional 5-HT2C receptors (previously termed 5-HT1C), prominent G-protein-coupled receptors that are widely expressed throughout the brain and spinal cord and which have been proposed to mediate numerous central nervous system (CNS) actions of serotonin. Here we show that 5-HT2C receptor-deficient mice are overweight as a result of abnormal control of feeding behaviour, establishing a role for this receptor in the serotonergic control of appetite. Mutant animals are also prone to spontaneous death from seizures, suggesting that 5-HT2C receptors mediate tonic inhibition of neuronal network excitability.

Ventromedial hypothalamic lesions inhibit corticosteroid feedback regulation of basal ACTH during the trough of the circadian rhythm

We have determined the effects of bilateral electrolytic lesions of the ventromedial hypothalamus (VMH) on activity in the hypothalamo-pituitary-adrenal (HPA) system. Acutely, during the first 5 days, lesions of the anterior-medial VMH caused loss of the diurnal rhythms in food intake and plasma corticosterone (B) levels. Plasma B concentrations were elevated during the time of the normal trough of the basal diurnal rhythm in HPA axis activity and the diurnal rhythm in food intake was abolished, in agreement with the results of others. Consistent with hyperactivity in the HPA axis, lesioned rats had increased adrenal weight, decreased thymus and body weights and decreased plasma transcortin concentrations. To determine how lesions of the VMH provoke these increases in activity of the HPA system, the sensitivity of ACTH in adrenalectomized, lesioned rats to replacement with exogenous B was determined under basal conditions during the trough (morning-AM) and peak (evening-PM) of the diurnal rhythm in HPA axis activity. ACTH in lesioned rats in the AM was insensitive to feedback over the very low range of plasma B of 1-4 micrograms/dl, whereas sham-lesioned controls exhibited the normal, high sensitivity of ACTH to B at this time of day. There was no difference between the sensitivity of ACTH to this low range of B in the PM in VMH- and sham-lesioned rats. Two to 5 weeks after VMH lesions, as found by others, mean daily plasma B levels did not differ from sham-lesioned controls; however, plasma B during the AM was still mildly elevated in these rats. Inhibition of plasma B in the PM by dexamethasone was less effective in lesioned rats. Although HPA system responses to hypoglycemia, corticotropin-releasing factor and ACTH were normal, the lesioned rats exhibited obesity, hyperinsulinemia, hyperglycemia, hypertension and tachycardia, all signs consistent with mild hyperactivity of the PHA axis. Occupancy of type I, high-affinity corticosteroid receptors is known to control basal activity of the HPA system during the trough of the diurnal rhythm and to interact with glucocorticoid receptors to affect basal activity during the peak of the diurnal rhythm and during AM stress. We conclude that VMH lesions disrupt transmission of inhibitory signals, mediated by occupancy of type I corticosteroid receptors, that are initiated by a B feed-back site.

Regulation of activity in the hypothalamo-pituitary-adrenal axis is integral to a larger hypothalamic system that determines caloric flow

We have previously reported that there are diurnal rhythms in the magnitude of ACTH responses to stressors and in the sensitivity of stress-induced ACTH responses to facilitation induced by prior stress and to corticosterone (B) feedback induced by exogenous B. In all cases ACTH was more responsive in the morning than in the evening in nocturnally feeding rats. We have also shown in adrenalectomized rats that an overnight fast reduces ACTH responses to restraint in the morning compared with rats fed ad libitum, and we have shown that calorie-containing gavage during the fast increases the amplitude of ACTH responses to restraint in fasted rats. Therefore, this diurnal rhythm is not associated with B feedback and is associated with calories. In these studies we asked whether young, male intact rats that were deprived of food overnight had: 1) hypothalamo-pituitary-adrenal (HPA) axis responses during the fasting period; 2) altered basal activity in the HPA axis; 3) altered responsivity of ACTH to restraint; and 4) altered sensitivity of restraint-induced ACTH responses to facilitation or B feedback. Our results show that food deprivation: 1) induces marked ACTH and B responses during the fast that mirrors the pattern of food intake in fed rats, with an approximately 3-h lag; 2) results in essentially no change in basal ACTH in the morning; 3) reduces ACTH responsivity to stress in the morning; and 4) reduces ACTH responsivity to prior stress-induced facilitation and exogenous B-induced feedback. We conclude that: 1) the HPA axis serves as a default pathway to feeding when food is not available; 2) the diurnal rhythms in restraint-induced ACTH secretion are determined by food intake; and 3) the HPA axis is integral to a larger hypothalamic system that mediates energy flow.

The diurnal rhythm in adrenocorticotropin responses to restraint in adrenalectomized rats is determined by caloric intake

There is a diurnal rhythm in ACTH responses to stressors that peaks, in nocturnally feeding rats, at the time of lights on, in the morning (AM). To determine whether this rhythm is subordinate to the rhythm in food intake, we tested the effects of removing food during the night or the day on ACTH responses in the AM or evening (PM) to the stimulus of restraint in 5-day-adrenalectomized rats. An overnight fast reduced the ACTH response to restraint with tail blood sampling in the AM to the low magnitude observed in the PM in rats fed ad libitum; by contrast, a fast of equivalent duration imposed during the day had no effect on the ACTH response to the stressor in the PM. Short term fasts did not alter the normal AM-PM rhythm in basal ACTH levels. The fasts did, however, significantly decrease the pituitary ACTH concentration at both times of day, suggesting that lack of food had stimulated ACTH secretion during the preceding 14 h. Providing calories by either gavage or manipulation of food presentation increased ACTH responses to restraint in fasted adrenalectomized rats in both the AM and PM. Although four of four experiments showed that provision of calories to fasted rats resulted in increased ACTH responses to the stimulus of restraint, none of the manipulations of caloric intake fully restored ACTH responses in fasted rats to the high amplitude observed in ad libitum fed rats in the AM. We conclude that 1) unlike the circadian rhythm in basal activity in the hypothalamic-pituitary-adrenalocortical (HPA) system, the diurnal rhythm in ACTH responsiveness to stimuli is tightly coupled to the endogenous rhythm in energy intake; and 2) caloric deprivation per se appears to activate the HPA system at some time during the 14- to 17-h fast, but does not produce the normal facilitation in the AM response to acute restraint that is induced by chronic or prior stimulation of the HPA axis.

Roles of type I and II corticosteroid receptors in regulation of basal activity in the hypothalamo-pituitary-adrenal axis during the diurnal trough and the peak: evidence for a nonadditive effect of combined receptor occupation

Negative feedback regulation of basal activity in the hypothalamo-pituitary-adrenal (HPA) axis requires less corticosterone (B) at the trough (morning) than at the peak (evening) of the diurnal rhythm. It has been hypothesized that in the morning in rats, occupation of the high affinity, type I corticosteroid receptors is sufficient to inhibit adrenalectomy (ADX)-induced increases in plasma ACTH secretion, whereas in the evening, regulation occurs through the occupation of the lower affinity type II corticosteroid receptors. To examine this hypothesis, the sensitivity of ACTH to inhibition by two different doses of B or of dexamethasone (DEX) were compared in ADX rats killed in the morning or the evening (B has a higher affinity for type I receptors in vitro and in vivo; in vivo, DEX has a higher affinity for type II receptors). The requirement for greater concentrations of corticosteroids to inhibit ACTH secretion in the evening was verified. The effect of these treatments on the number of neurons immunoreactive for vasopressin (AVP) and on the expression of AVP messenger RNA (mRNA) in the parvocellular portion of the paraventricular nuclei was also examined. In the morning, plasma concentrations of B equivalent to the IC50 for the reduction of plasma ACTH in the morning reduced the amount of AVP mRNA, but not immunoreactive AVP cell number as compared with ADX rats. DEX reduced plasma ACTH in the morning but did not prevent high levels of expression of AVP mRNA or protein. AVP mRNA was more sensitive to B in the morning than in the evening. Antagonist to the type I receptor (spironolactone) given chronically to ADX rats treated with B increased plasma ACTH secretion at both times of day, even though the plasma B concentrations suggested occupancy of a large proportion of the type II receptors. To test the hypothesis that an interaction between the type I and II receptor is necessary for the control of HPA activity at the peak of the diurnal rhythm, ADX rats were given B or DEX, alone or in combination. DEX reduced evening plasma ACTH only in the presence of very low concentrations of B, suggesting that for full potency, type II receptor occupation requires type I receptor occupation. In summary, these results demonstrate that occupation of type I corticosteroid receptors is capable of controlling basal activity in the HPA axis in the morning and that in the evening, type I receptor occupation potentiates the inhibition of plasma ACTH by occupation of type II receptors.

Streptozotocin-diabetic rats exhibit facilitated adrenocorticotropin responses to acute stress, but normal sensitivity to feedback by corticosteroids

A variety of chronic stress paradigms have been shown to increase basal activity in the hypothalamic-pituitary-adrenocortical axis, resulting in hypercorticoidism. Despite this, chronically stressed rats typically exhibit facilitated ACTH responses to acute novel stress, suggesting that the activity of some central neural component(s) in the axis is facilitated by chronic stress. We have used the chronic stress of streptozotocin (STZ)-induced diabetes in rats to determine diurnal sensitivity of basal and stimulated ACTH secretion to exogenous corticosterone (B) feedback in vivo. Control and STZ-diabetic rats were adrenalectomized or adrenalectomized and implanted with a 30% or 50% B pellet at the time of vehicle/STZ injection. Rats were killed 5 days later, under basal conditions or after 6 min of restraint, in the morning or evening. We show that basal ACTH secretion in both the morning and evening was similarly suppressed by B in STZ-diabetic and control rats. However, stress-induced ACTH secretion was significantly greater in STZ-diabetic compared to control rats throughout the range 3-7 micrograms/dl B, when tested in the morning. Suppression of evening stress-induced ACTH secretion by B was also significantly different in STZ-diabetic rats; however, the IC50 values for the inhibition of ACTH by B did not differ. This result shows that in the evening after stress and under basal conditions in both the morning and evening, sensitivity to B feedback is normal in chronically stressed, STZ-diabetic rats. Despite the observed facilitation of morning stress-induced ACTH secretion in STZ-diabetic rats, there were no differences in hypothalamic CRF content between control and STZ-diabetic tissue. We conclude that 1) the facilitatory input to the paraventricular nucleus functions primarily at the time of the circadian trough to maintain or enhance acute stress responsiveness in chronically stressed, hypercorticoid rats; and 2) the sensitivity of ACTH to inhibition by B is normal in rats chronically stressed by STZ-induced diabetes.

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)

Feedback sensitivity of the rat hypothalamo-pituitary-adrenal axis and its capacity to adjust to exogenous corticosterone

Chronic stress causing elevated morning (AM) corticosterone (B) concentrations of 2-8 micrograms B/dl does not appear to inhibit subsequent activity in the hypothalamic-pituitary-adrenal (HPA) axis, a surprising finding in view of the known depression in AM basal ACTH by only 3 micrograms B/dl in adrenalectomized rats. To distinguish between the possibilities that either intact rats are less sensitive to B feedback than adrenalectomized rats, or that chronic stress facilitates responses in the HPA axis, we elevated basal B levels in young male rats with slow-release B pellets in the absence of stress. Between 4-6 days after implantation of B pellets at three doses that elevated basal AM (diurnal trough) plasma B to approximately 1.2, 4, and 10 micrograms/dl, we studied basal ACTH and B at trough (AM) and peak evening (PM) times of the diurnal cycle, as well as the responses to the stress of restraint and blood collection from the tail at each time of day. We also determined mean daily plasma B, insulin, and glucose from samples collected at six intervals during the day. Adrenal, thymus, and body wts were measured as were transcortin (CBG) and adrenal phenylethanolamine-N-methyl transferase activity. Compared to controls implanted with wax pellets, all doses of B inhibited adrenal wt and AM stress responses and tended to inhibit pituitary ACTH content and adrenal phenylethanolamine-N-methyl transferase activity. Inhibition with the middle dose B pellet was close to maximally effective for these endpoints. Plasma glucose and thymus wt were significantly decreased and insulin was significantly increased in the middle and highest B pellet groups, with significantly greater effects at the highest dose. The gain in body wt and transcortin concentrations were significantly decreased only in the highest dose groups, in which mean daily plasma B was approximately 10 micrograms/dl, a level that clearly overwhelmed the capacity of the adrenocortical system to respond to any stimulus tested. By contrast, rats with low and middle dose B pellets appeared to adjust HPA axis function by decreasing the peak diurnal increase in B, so that 24-h mean B levels did not differ from control, and were maintained at approximately 5 micrograms/dl. Both of these groups also had inhibited ACTH responses to stress applied during the diurnal trough (AM). By contrast, neither group had inhibited ACTH responses to stress applied during the diurnal peak (PM). We conclude that: 1) The HPA axis of intact rats is extremely sensitive to exogenous B.(ABSTRACT TRUNCATED AT 400 WORDS)

Feedback and facilitation in the adrenocortical system: unmasking facilitation by partial inhibition of the glucocorticoid response to prior stress

Previously stressed animals remain responsive to subsequent stressors, despite secreting an adequate corticosteroid signal during the first stress which should act to damp the response to a second stress. We have previously postulated that stress acts to facilitate subsequent responses in the adrenocortical system, and that this facilitation is balanced by the corticosteroid feedback signal. To test this hypothesis directly, we treated young male rats with cyanoketone (CK) to partially block the adrenal capacity to synthesize corticosterone (B). Subsequently, groups of CK- or vehicle (VEH)-treated rats were exposed to the FIRST stress of 30-min restraint with small blood samples collected at 0, 15, and 30 min. The FIRST stress was given to subgroups of rats 12, 9, 6, or 3 h before lights off (12 h) or lights on (24 h). At 12 or 24 h, rats were again restrained with blood samples at 0 ("basal") and 30 min (SECOND stress). Control groups were stressed for the first time when the experimental groups received their SECOND stress. Plasma ACTH and B concentrations were measured. Although in the absence of stress, basal B concentrations were normal in CK-treated compared to VEH-treated rats throughout the day, the B response to the FIRST stress was reduced by 60% in the CK- compared to the VEH-treated group. When the FIRST stress was performed during the time of lights on, "basal" plasma ACTH was elevated in CK groups at 12 h (lights off) compared to levels in both previously stressed VEH groups and unstressed CK controls. There was no difference at this time of day in the magnitude of the ACTH response to the SECOND stress in CK rats compared to that in CK rats receiving their only stress (controls) or that in VEH-treated rats receiving the SECOND stress. When first stress was performed during the time of lights off, "basal" plasma ACTH at 24 h (lights on) in CK and VEH rats were not different compared to levels in their respective unstressed controls. The ACTH response to the SECOND stress at 24 h was elevated in all previously stressed CK groups compared to that in either CK control or VEH groups. At neither time of day were SECOND stress ACTH concentrations in VEH rats different from those in control VEH rats. At 12 h (lights off), but not at 24 h (lights on), "basal" ACTH was significantly elevated in VEH rats above the unstressed VEH control values.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Adrenalectomy in the neonate: adult-like adrenocortical system responses to both removal and replacement of corticosterone

Neonatal rats exhibit a period of diminished responsiveness to stress between days 3-10 of life, which has been shown to be associated with an increased sensitivity to corticosterone (B) inhibitory feedback. In this study we further investigated B feedback potency on regulation of ACTH by examining 1) the time course of changes in pituitary ACTH secretion and content, plasma B and B-binding globulin (CBG) concentrations, and thymus weight after adrenalectomy (ADX) performed on 5-day-old pups, with or without sc 5% B pellet replacement, and 2) the time required for acute (B injection) and the B dose required for constant (B pellet) inhibition of ACTH secretion in 10-day-old ADX neonates. As in adult rats, ADX in neonates caused an immediate (3 h) large increase (13-fold) in plasma ACTH levels compared to that in sham-operated rats, followed by a decrease by 12 and 24 h after surgery and a further and sustained increase during the next 4 days. Pituitary ACTH stores were diminished in ADX rats by 3, 12, and 24 h and increased thereafter. Five percent B pellet replacement abolished ADX-induced changes in plasma and pituitary ACTH until days 4-5, when plasma ACTH was slowly released from B inhibition (circulating B values were similar to ADX values). By day 10 of life, inhibition of plasma ACTH by calculated free B showed an IC50 of 1.09 nM. Plasma CBG concentrations exhibited a clear developmental pattern in sham-operated rats, being lower on days 6-8 than earlier or later. Typical ADX-induced increases in CBG levels were observed from day 3 on after surgery, at the same time as a transient decrease in CBG levels occurred in ADX plus 5% B rats. On day 10 of age, inhibition of CBG by calculated free B demonstrated an IC50 of 1.5 nM. Although no enlargement of the thymus was observed after neonatal ADX, thymus weight was significantly diminished by 12 h after B replacement and in a dose-related manner at 5 days with B pellets containing 5-25% B. The thymus contained mostly type II glucocorticoid receptors, which did not up-regulate 3 h or 5 days after ADX. Acute sc injection of B (10-34 micrograms/g BW) in 10-day-old rats inhibited ADX-induced ACTH secretion within 30 min, and the estimated half-time for the inhibition was 40 min. By 2 h after B injection, plasma ACTH levels were comparable to those in sham-operated animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Decreased sensitivity to glucocorticoid fast feedback in chronically stressed rats

A number of changes in anterior pituitary corticotrophs occur after chronic footshock. These include increased ACTH and beta-endorphin content and a loss of glucocorticoid negative feedback on corticotropin-releasing hormone (CRH)-stimulated ACTH and beta-endorphin secretion, without changes in sensitivity to ovine CRH examined in vitro. The present studies were undertaken to determine whether the in vitro changes were reflected by similar changes in vivo. We developed a fast feedback paradigm using a 5-min swim stress as challenge, with injection of saline or corticosterone immediately prior to swim. Corticosterone reliably decreased ACTH and beta-endorphin responses to swim over the 30-min period studied. This feedback inhibition did not occur in rats that were either exposed to 30 min of chronic footshock for 7 or 14 days or in rats that were treated with corticosterone daily for 14 days in a regimen that has been reported to decrease hippocampal glucocorticoid receptors. By contrast, in rats exposed to the less intense stimulus of 30 min swim for 14 days, the fast feedback action of corticosterone was intact. These results suggest that both fast and delayed feedback corticosterone-inhibitory mechanisms may be blocked by relatively high levels of chronic stress or by chronic treatment with corticosterone, possibly as a consequence of decreased hippocampal glucocorticoid receptor number.

Pharmacological evidence that the inhibition of diurnal adrenocorticotropin secretion by corticosteroids is mediated via type I corticosterone-preferring receptors

These studies were performed to determine pharmacologically the corticosteroid receptor type that mediates the effects of corticosterone (B) on ACTH secretion in adrenalectomized rats. We have compared the effects of treating young male rats at the time of adrenalectomy and throughout the next 5 days with B, dexamethasone (DEX), or aldosterone (ALDO) in doses that elevated plasma levels to concentrations in the range between 0.2-30 nM. Plasma ACTH, corticosteroid-binding globulin (CBG), and thymus weight were measured in the morning or evening, and these steroid-sensitive end points were related to the circulating concentrations of B (total B - CBG-bound B), total DEX, and total ALDO. For the inhibition of ACTH the rank order of potency of the three steroids was B greater than DEX greater than or equal to ALDO in the morning (estimated IC50, 0.7 +/- 0.1, 2.3 +/- 0.5, and 4.9 +/- 1.6 nM for B, DEX, and ALDO, respectively). There was a significant shift to the right in steroid efficacy between morning and evening (estimated IC50 in the evening, 3.9 +/- 0.2 and 9.3 +/- 0.8 nM for B and DEX; ALDO at the concentrations achieved was ineffective). The rightward shift in efficacy may result from the circadian increase in drive to ACTH secretion. The rank order of potency for B and DEX on ACTH and the agreement between the steady state IC50 values achieved for these steroids and the Kd values determined for B and DEX with type I receptors in vitro strongly suggest that feedback control of basal diurnal ACTH by corticosteroids is mediated by association with type I, B-preferring receptors. By contrast, DEX was 3 times more potent than B on CBG (estimated IC50, 1.5 and 4.5 nM, respectively) and tended to be more effective on thymus weight, suggesting that the effects of corticosteroids on these peripheral targets are mediated by association of the steroids with type II glucocorticoid receptors. ALDO coinfused with DEX or B did not alter the inhibitory effects of these on ACTH, suggesting that ALDO does not interfere with these type I, B-preferring receptors in vivo. Because there is little if any evidence for type I corticosteroid receptors in the hypothalamus, these results strongly suggest that the majority of corticosteroid feedback inhibition of basal morning and evening ACTH secretion is mediated transynaptically by the activity of extra-hypothalamic neurons.

The adrenocortical system responds slowly to removal of corticosterone in the absence of concurrent stress

After removal of corticosteroid feedback by surgical or pharmacological adrenalectomy, plasma ACTH increases more rapidly than can be explained by changes in receptor-mediated gene expression. In aminoglutethimide-treated rats, plasma ACTH increased only at doses much higher than those inhibiting plasma corticosterone, suggesting that adrenal enzyme blockers may themselves be stressful. To determine the adrenocortical system response to stressless corticosterone removal, adrenalectomized rats maintained for 5 days on corticosterone in the drinking water were switched to steroid-free fluid (-B) or again given steroid (+B); additional rats were adrenalectomized (ADX). Plasma ACTH did not differ between -B and +B rats until 18-24 h after steroid removal, regardless of whether steroid was withdrawn at the circadian maximum or minimum. Plasma ACTH was similar between -B and ADX rats 0.5-14 days after corticosterone removal, although morning plasma ACTH was more stable in -B rats at 4-7 days. Evening plasma ACTH increased significantly after day 3 in ADX and -B rats. Unlike ADX rats, -B rats did not exhibit pituitary ACTH depletion at 12 and 24 h, but both -B and ADX groups had significantly elevated pituitary ACTH by 6.5 days. We conclude that 1) rapid increases in ACTH secretion after surgical or pharmacological adrenalectomy result from interaction between stress and loss of corticosteroid feedback; 2) no immediate interaction occurs between loss of feedback and circadian stimuli; and 3) the effects of steroid withdrawal may require at least 3 days to be stably expressed.

Correlation between the stress-induced transient increase in corticotropin-releasing hormone content of the median eminence of the hypothalamus and adrenocorticotropic hormone secretion

Intravenous angiotensin II and ether stress were found to produce a rapid, transient increase in the corticotropin-releasing hormone (CRH) content of the median eminence as measured by a radioimmunoassay employing an antibody against rat CRH(1-41). This confirms previous reports of transient increases in CRH measured by bioassay. The increase did not occur in the paraventricular region or in other parts of the brain. It occurred along with an increase in plasma adrenocorticotropic hormone (ACTH) when a second ether stress was administered 1 h after the first, and it also occurred when rats that had been adrenalectomized for 5 days were exposed to ether. The increases in CHR and the ACTH responses to ether were reduced or abolished by dexamethasone and pentobarbital. Four days after semicircular knife cuts in the posterior hypothalamus, resting CRH in the median eminence was increased but there was no further rise after ether stress. Plasma ACTH was normal at rest after the cuts, but the increase produced by ether was reduced. The ACTH responses to angiotensin II and immobilization were also reduced. Because the posterior knife cuts reduced hypothalamic catecholamine content, the effects of reducing hypothalamic norepinephrine and epinephrine by administration of the dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate (DDC) were tested. Five hours after DDC, plasma ACTH was elevated but there was no further increase with ether stress. The median eminence CRH content was normal but failed to increase after exposure to ether.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian variations in plasma corticosterone permit normal termination of adrenocorticotropin responses to stress

We previously reported that adrenalectomized rats given constant corticosterone via a sc pellet (B-PELLET) hypersecrete ACTH in response to stress. Although lacking a feedback signal, B-PELLET rats do not secrete ACTH indefinitely after stress; plasma ACTH levels in these animals returned to those in sham-operated (SHAM) rats within 1-4 h after 2-min restraint. To distinguish between the requirement for circadian or stress-induced increases in corticosterone, we compared changes in ACTH and corticosterone levels after stress in SHAM and B-PELLET rats with those in cyanoketone-treated rats (CK) and adrenalectomized rats given corticosterone in their drinking fluid (B-WATER). B-WATER rats exhibited sustained increases in plasma corticosterone after lights-off, correlating with the nocturnal feeding period. Morning plasma corticosterone levels in B-WATER rats were constant and even lower than those in B-PELLET rats; however, B-WATER rats did not differ from SHAM rats in their ACTH response to ip injection. CK rats, which have an approximately normal circadian corticosterone rhythm but do not have significant corticosterone responses to acute stimuli, also exhibited plasma ACTH levels similar to those of SHAM rats at all times after 5-min restraint. Compared with SHAM and B-WATER rats in the same experiment, B-PELLET rats tended to hypersecrete ACTH 60 min after 5 min of restraint, but only had significantly elevated plasma ACTH relative to both groups 45 min after 10 min of restraint. We conclude that circadian, rather than stress-induced, increases in corticosterone may be sufficient for normal termination of ACTH responses to stress.

Constant corticosterone replacement normalizes basal adrenocorticotropin (ACTH) but permits sustained ACTH hypersecretion after stress in adrenalectomized rats

To characterize further the effects of providing a constant corticosterone signal after bilateral adrenalectomy, we have compared the effects of bilateral adrenalectomy with no replacement (ADX) and with replacement with a corticosterone pellet implanted sc at surgery (B-PELLET) to those of sham-adrenalectomy (SHAM) on pituitary and plasma ACTH concentrations during the first 3 postoperative days. In ADX rats, plasma ACTH concentrations were elevated at all times compared to those in the SHAM group; pituitary ACTH content decreased during the first 12 h, then increased and was not different from that in the SHAM group thereafter. Replacement of corticosterone at the time of adrenal surgery in B-PELLET rats resulted in no differences in pituitary and plasma ACTH concentrations from SHAM values, suggesting that immediate steroid replacement prevents the major adrenalectomy-induced changes in central regulatory components governing basal activity of the adrenocortical system. Although B-PELLET rats had normal basal morning ACTH concentrations 5 days after surgery, they exhibited augmented and sustained ACTH responses to five different ACTH-releasing stimuli (injection, restraint, chlorpromazine, and, under pentobarbital anesthesia, morphine or sham adrenalectomy). The circulating corticosterone concentrations were maintained at relatively constant, low levels (3-6 micrograms/dl). Because these concentrations appear to restore basal morning ACTH concentrations to normal, but do not restore the ACTH response to stress to normal, we conclude that a different corticosterone signal is required to normalize stress-induced ACTH responses.

Characterization of corticosterone feedback regulation of ACTH secretion

Adrenalectomy-induced increases in ACTH secretion in rats are returned to normal by an action of corticosterone on the brain, not on the pituitary. Five days after adrenalectomy with constant steroid replacement, the concentration of free corticosterone in plasma which reduces plasma ACTH by 50% is approximately 0.8 nM. By contrast, the concentration of free plasma corticosterone required for 50% reduction of thymus wet weight or plasma transcortin concentration (both targets for glucocorticoid action) is about 4.5 nM. These results suggested that the inhibition of ACTH by corticosterone might be mediated by association of the steroid with high affinity, type I corticosteroid receptors, whereas the inhibition of thymus weight and transcortin might be mediated by association of the steroid with lower affinity, type II receptors. The results of studies comparing the ability of corticosterone, dexamethasone and aldosterone to inhibit adrenalectomy-induced ACTH secretion support the hypothesis that basal ACTH secretion in rats is mediated by association of corticosterone with type I receptors.

Pancreatic acinar cell amylase gene expression: selective effects of adrenalectomy and corticosterone replacement

To examine the role of glucocorticoids in the regulation of the acinar pancreas, adult male rats were adrenalectomized (Adx) and replaced with no corticosterone (B), normal B, or high B. Plasma B concentration, body weight gain, and thymus weight were used as independent measures of treatment efficacy. Compared to controls, Adx animals had a 75 +/- 0.5% (n = 30) reduction in pancreatic amylase content; a 50% decrease occurred within 1 day and the maximal 75% decrease was observed after 5 days. In Adx animals, amylase content was normalized by normal B replacement and was increased to 235 +/- 39% (n = 30) of control by high B replacement. Furthermore, in all Adx rats, pancreatic content of amylase and plasma B concentration was significantly correlated (r = 0.81, n = 30). The effect of adrenalectomy was selective for amylase; contents of ribonuclease, chymotrypsin, and elastase were not altered. However, the effects of high B replacement were not selective, and increased the content of all digestive enzymes. To determine whether the changes in enzyme content were associated with changes in messenger RNA (mRNA), pancreatic RNA was probed with 32P-labeled complementary DNAs for amylase, ribonuclease, and chymotrypsin. After adrenalectomy and B replacement there was a significant correlation only between amylase mRNA (r = 0.87, n = 13) and plasma B concentration. These data indicate that physiological levels of B have a selective effect on pancreatic amylase gene expression. In contrast, high levels of B have the separate, nonselective effect of increasing the content of all digestive enzymes without increasing corresponding mRNA levels.

Plasma adrenocorticotropin is more sensitive than transcortin production or thymus weight to inhibition by corticosterone in rats

After adrenalectomy, ACTH, corticosterone-binding globulin (CBG), and thymus wet weight increase in rats as a consequence of the removal of corticosterone (B) and decrease again in response to replacement with glucocorticoids. We have studied the effect of replacing adrenalectomized male rats with a variety of different concentrations of B. Plasma concentrations of ACTH and CBG and thymus wet weights were related to the measured concentration of free ultrafilterable B in plasma. In other experiments, the clearance of [125I]CBG was determined in adrenalectomized rats with and without B replacement, and the time required for the changes in plasma CBG concentrations after removal and/or replacement of B in adrenalectomized rats was determined. Five to 7 days after adrenalectomy and institution of a relatively constant B replacement signal, plasma CBG concentrations were highly correlated with circulating B concentrations (r2 = 0.745; P less than 0.001). The effect of B was on the CBG production rate, since clearance did not change. Because CBG concentrations decrease as total B concentrations increase, there is an amplification of free B concentrations with increasing total B. The relationships of plasma ACTH and CBG and thymus wet weight to circulating free B levels showed that 50% inhibition of ACTH was achieved at a free B concentration of 0.8 +/- 0.05 nM, whereas 50% inhibition of CBG and thymus wet weight were achieved at free B concentrations of 4.6 +/- 0.9 and 4.4 +/- 0.6 nM, respectively. These inhibition values correlate well with the known Kd values for the high affinity type I B receptor (0.5 nM) and for the lower affinity type II glucocorticoid receptor (2.5-5 nM), respectively, suggesting that ACTH secretion, CBG production, and thymus wet weight are regulated by the association of B with these receptor types.

Reset of feedback in the adrenocortical system: an apparent shift in sensitivity of adrenocorticotropin to inhibition by corticosterone between morning and evening

There is evidence in man and rats that higher circulating levels of glucocorticoids are required to normalize basal unstimulated ACTH levels at the peak of the circadian rhythm than at the trough. To explore this phenomenon, we tested the inhibitory effect of constant levels of corticosterone on plasma ACTH in the morning (AM) and evening (PM) in young male rats implanted with fused pellets of corticosterone-cholesterol at the time of adrenalectomy (ADX+B) and studied 5 days later. There was a marked shift of the plasma corticosterone-ACTH inhibition curve to the right between AM and PM, demonstrating that the efficacy of corticosterone feedback inhibition of ACTH is less in the PM. Comparison of plasma ACTH and corticosterone levels during 24 h in sham-adrenalectomized rats (SHAM-ADX), adrenalectomized rats (ADX), and ADX+B revealed constantly low ACTH in SHAM-ADX, constantly high ACTH in ADX, and biphasic ACTH levels in ADX+B. Corticosterone levels were biphasic in SHAM-ADX and were constant in the other two groups. These results again showed a shift in corticosterone feedback efficacy as a function of the time of day and also suggested that basal ACTH secretion is maintained in the low normal range in intact rats because of the marked diurnal rhythm in corticosterone. The sensitivity of the pituitary ACTH response to exogenous CRF did not change between AM and PM in either intact or ADX+B showing that the shift in feedback sensitivity to corticosterone does not reside in the pituitary. The response of the entire adrenocortical system to histamine stress was shown to be equivalent in both the AM and PM, suggesting that feedback sensitivity of the entire system to corticosterone does not change as a function of the time of day. We conclude from these results that there is an apparent diurnal change in ACTH sensitivity to corticosterone feedback that can be defined operationally as reset. We believe that the site of feedback being tested shifts solely from the pituitary in the AM (at the nadir of the rhythm) to the brain and the pituitary in the PM (at the peak of the rhythm). The lack of the normally high transients of corticosterone that occur in SHAM-ADX rats results in increased brain drive of the pituitary in ADX+B.

Corticosterone: narrow range required for normal body and thymus weight and ACTH

ACTH secretion appears to be under fairly tight negative feedback control by corticosteroids secreted from the adrenal cortex. In these studies we determined the circulating levels of a constant corticosterone signal that best restored body weight gain, thymus weight and ACTH levels to normal in bilaterally adrenalectomized rats given saline to drink. Young male rats were treated at the time of adrenalectomy with subcutaneously implanted pellets of wax or various ratios of corticosterone-cholesterol. Sham-adrenalectomized rats and adrenalectomized rats given corticosterone in the drinking fluid served as comparison groups. Rats were killed 3, 7, or 14 days after adrenalectomy. There was no difference in levels of plasma corticosterone in the morning and in the evening in pellet-implanted rats in contrast to the diurnal variation in the reference groups. Circulating corticosterone levels that best restored body weight, thymus weight, and resting and stress-induced ACTH levels to normal ranged between 4.5 and 7.4 micrograms/dl. Plasma corticosterone levels of 8-11 micrograms/dl were excessive and levels of 2-4 micrograms/dl were not adequate. We conclude that there is a very narrow range of plasma corticosterone compatible with normal growth rate, thymus mass and ACTH secretion. These results reveal the necessity for strict negative feedback regulation of ACTH secretion by corticosteroids.

Closed-loop feedback control of the nyctohemeral rise in adrenocortical system function

We tested the hypothesis that the increase in adrenocortical system activity is controlled by an error signal generated by the difference between a central reference, or set point, value and some aspect of circulating corticosterone levels in rats maintained on a 12-h light, 12-h dark cycle. The test rats were treated with cyanoketone, an inhibitor that blocks conversion of pregnenolone to progesterone; control rats were treated with vehicle. Test and control rats were fitted with arterial cannulas and, after recovery, continuously collected blood was sampled for plasma corticosterone at 5-min intervals over 2-h periods at five times during the day. Samples were collected at the end of each period for plasma adrenocorticotropin (ACTH). Corticosterone and ACTH levels were similar in the two groups of rats at the nadir (0-2 h after lights-on) and at the peak (0-2 h after lights-off) of the diurnal rhythm. During the time of the maximal diurnal rise in corticosterone (8-10 and 10-12 h after lights-on), test rats had lower mean corticosterone and higher ACTH levels than controls. Infusion of 8 micrograms corticosterone during the 2 h before lights-out did not significantly elevate mean corticosterone levels but decreased end ACTH levels in test rats, whereas similar infusions in control rats resulted in decreased mean corticosterone levels and no change in end ACTH. We conclude that the nyctohemeral rise in ACTH is driven by an error signal resulting from the difference between a central set point value and some aspect of plasma corticosterone levels.

Relationships among adrenal weight, corticosterone, and stimulated adrenocorticotropin levels in rats

Based on results of previous studies, we tested the hypothesis that increased adrenal weight in the absence of increased corticosterone secretion would inhibit the magnitude of the ACTH response to ether vapor. Adrenal hypertrophy was induced by treatment of rats for 3 days with aminoglutethimide, cyanoketone, or metyrapone, three agents that act to inhibit enzymes required for corticosterone synthesis. On the fourth day, resting plasma ACTH and corticosterone levels were normal; however, the logarithm of the ACTH response to ether was directly related to total adrenal weight. This result did not support the hypothesis. Unilateral adrenalectomy and treatment with cyanoketone resulted in total adrenal weight equivalent to that of control rats bearing two adrenals. Resting ACTH levels were normal, but stimulated ACTH was significantly greater in these rats than in controls or in rats with two adrenals, suggesting that there is an interaction between the effects of unilateral adrenalectomy and adrenal enzyme inhibition. As anticipated, adrenal hypertrophy with increased corticosterone production caused by ACTH infusion resulted in a significant negative relationship between resting and stimulated ACTH levels and adrenal weight. We conclude that when adrenals are enlarged by means that prevent excessive corticosterone secretion, there is a mechanism associated with the increase in adrenal weight that correlates directly with the magnitude of stimulated ACTH secretion. We have reexamined the results of our previous results in the light of these experiments.

Drug-induced adrenal hypertrophy provides evidence for reset in the adrenocortical system

These studies were performed to determine how bilateral adrenal hypertrophy persists despite normal resting ACTH levels in male rats for weeks after a 3-day course of treatment with cyanoketone (CK). This drug blocks corticosterone synthesis by binding to the enzyme 3 beta-ol-dehydrogenase-delta 5, delta 4-isomerase (EC 1.1.1.51). Although plasma ACTH levels were significantly elevated at all times during the course of treatment with CK, ACTH and corticosterone levels of CK-treated rats were normal thereafter compared to those in control rats killed in the morning. Despite these normal ACTH levels, adrenal weight remained elevated (by 100%) for at least 14 days. We determined that increased ACTH secretion was required to initiate and maintain the adrenal growth response to CK. Rats pretreated with hypophysectomy or dexamethasone (1 mg/kg) did not respond with increases in adrenal weight after CK treatment. Sustained elevations in ACTH during CK treatment were required for increased adrenal weight, because rats treated with dexamethasone 3 h after CK injection did not have enlarged adrenals at 24 h. Increased adrenal weight was not sustained after removal of supranormal levels of ACTH achieved by infusion, suggesting that after adrenal hypertrophy is achieved by ACTH, elevated ACTH levels are also required to maintain adrenal enlargement. Finally, ACTH and corticosterone levels were measured in the evening (at the peak of the diurnal rhythm in the adrenocortical system of rats) in CK-treated rats. Evening ACTH levels were significantly elevated in CK-treated rats compared to those in controls 7, 10, and 14 days after the onset of 3 days of treatment with CK; however, corticosterone levels were normal. The effects of CK on the adrenocortical system were exerted via the adrenal, since adrenalectomy normalized the amplitude of the diurnal rhythm of ACTH in CK-treated rats compared to that in adrenalectomized controls. We conclude 1) that adrenal hypertrophy after CK is maintained by increased ACTH secretion which occurs daily in the evening; and 2) that the results provide evidence for a daily reset of the corticosteroid feedback sensor in the adrenocortical system. This conclusion arises from the findings that there is a normal rhythm in corticosterone levels in CK-treated rats and that morning ACTH levels are normal although evening ACTH levels are significantly elevated.

Alterations in adrenal growth and corticosteroid content in foetal and neonatal rats developing at high altitude

To study the effects of chronic maternal hypoxia on the growth and functional development of foetal and neonatal adrenal glands, Long-Evans rats were acclimatized to high altitude (3800 m) before mating and were maintained at this height throughout gestation. The body growth of the progeny at high altitude was essentially normal during the perinatal period, but adrenal weight and adrenocortical function showed marked differences from those of control rats maintained at sea level. The adrenal glands were larger in foetuses but smaller in neonates, compared with the adrenal glands of control animals maintained at sea level. Differences in the protein content of the adrenal glands between the two groups paralleled differences in adrenal weight. The concentration and content of corticosterone in the adrenal glands of both foetuses and neonates kept at high altitude were markedly lower than values in animals kept at sea level. The lower adrenal corticosterone content was not reflected in the concentration of the hormone in the peripheral plasma, since this was essentially the same at high altitude and at sea level in both mothers and perinatal animals. The reduction in the adrenal corticosterone content was accompanied by and may have resulted from, a reduction in the concentration of cytochrome P-450 in the adrenal tissue of foetuses maintained at high altitude. Possible explanations for the dichotomous results are discussed.