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.

Corticosteroid inhibition of ACTH secretion

Corticosteroid feedback inhibits the brain-hypothalamo-pituitary units of the adrenocortical system. Naturally occurring corticosteroids may have their primary actions in vivo at brain and hypothalamic sites of feedback, whereas synthetic glucocorticoids that do not bind to transcortin may act primarily on corticotropes and regions of brain outside the blood-brain barrier. There appear to be three major time frames of corticosteroid action: fast, intermediate and slow. These time frames probably are the consequence of three separate mechanisms of corticosteroid action at feedback-sensitive sites. The rapidity of occurrence of fast feedback is not compatible with a nuclear site of corticosteroid action, and protein synthesis is not required. The action of CRF on ACTH release may be inhibited by a rapid effect of corticosteroids at the cell membrane. Since stimulated, but not basal, ACTH and CRF release are inhibited in vitro, the corticosteroids may inhibit some event in stimulus-secretion coupling (e.g., cAMP production). Intermediate feedback also decreases ACTH release in response to stimulation of the corticotrope, but does not affect ACTH synthesis; CRF synthesis and release both appear to be affected by the intermediate corticosteroid action. The mechanism of intermediate feedback requires the presence of a protein whose synthesis is corticosteroid-dependent; however, the role of this protein is unknown. Intermediate feedback, like fast feedback, apparently does not involve inhibition of total ACTH stores or the releasable pool of ACTH since basal secretion of ACTH is also not inhibited in vitro within this time domain. On the other hand, slow feedback apparently involves the classical genomic steroid mechanism of action; slow feedback reduces pituitary ACTH content by decreasing levels of mRNA encoding for POMC, the ACTH precursor molecule. Slow feedback, therefore, inhibits basal as well as stimulus induced ACTH secretion. Corticosteroid-induced inhibition of basal ACTH secretion has been shown to occur within 2 h in vivo but not in vitro. The time course and sensitivity of this feedback effect is different than that demonstrated for stimulus induced secretion. This difference suggests that basal secretion is activated by different pathways to (CRF and) ACTH secretion. There is some evidence that suggests that whereas comparator elements are not reset during stress, a comparator element is reset during the course of the circadian rhythm so that different basal levels of steroid are achieved.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Leptin-independent hyperphagia and type 2 diabetes in mice with a mutated serotonin 5-HT2C receptor gene

Brain serotonin and leptin signaling contribute substantially to the regulation of feeding and energy expenditure. Here we show that young adult mice with a targeted mutation of the serotonin 5-HT2C receptor gene consume more food despite normal responses to exogenous leptin administration. Chronic hyperphagia leads to a 'middle-aged'-onset obesity associated with a partial leptin resistance of late onset. In addition, older mice develop insulin resistance and impaired glucose tolerance. Mutant mice also responded more to high-fat feeding, leading to hyperglycemia without hyperlipidemia. These findings demonstrate a dissociation of serotonin and leptin signaling in the regulation of feeding and indicate that a perturbation of brain serotonin systems can predispose to type 2 diabetes.

Stress-related cortisol secretion in men: relationships with abdominal obesity and endocrine, metabolic and hemodynamic abnormalities

Abdominal obesity has been suggested to be associated with perturbations of the regulation of the hypothalamic-pituitary-adrenal (HPA) axis. In a population of 51-yr-old men (n = 284) salivary cortisol concentrations were determined on repeated (n = 7) occasions over a random working day, and perceived stress was reported in parallel. Cortisol values were then related to reported stress (stress-related cortisol). A standardized lunch was used as a physiological challenge. A low dose (0.5 mg) dexamethasone suppression test was also performed as well as determinations of testosterone and insulin-like growth factor I (IGF-I). Body mass index [weight (kilograms)/height (meters)2]; waist/hip circumference ratio (WHR); sagittal trunk recumbent diameter (D); fasting insulin; blood glucose; triglycerides; and total, low density (LDL), and high density (HDL) lipoprotein cholesterol were also determined. Cortisol concentrations were highest in the morning, and lunch was followed by a peak (P = 0.044). Two types of diurnal cortisol curves were identified, one characterized by a high variability with high morning values, and another with low variability and low morning values. Both correlated strongly with suppression of salivary cortisol by dexamethasone (P < 0.001). Stress-related cortisol secretion was associated with D (P = 0.051), low IGF-I (P = 0.006), and diastolic blood pressure (P = 0.078). When the type of diurnal cortisol curve was taken into consideration by statistical weighting, stress-related cortisol secretion in subjects with high variability showed associations with testosterone (P < 0.001), D, total and LDL cholesterol, diastolic blood pressure (P < 0.001), fasting insulin (P = 0.039), and glucose (P = 0.030) as well as, negatively, triglycerides (P < 0.001). When weighted for a low variability of diurnal cortisol secretion, stress-related cortisol secretion showed strong negative relationships with IGF-I, testosterone, and HDL. Furthermore, strong, consistent relationships (all P < 0.001) were found with obesity factors (body mass index, WHR, and D), and with metabolic (insulin, glucose, triglycerides, and total and LDL cholesterol) as well as hemodynamic variables (systolic and diastolic blood pressure and heart rate). These results clearly show interactions between diurnal cortisol secretion related to perceived stress and anthropometric, endocrine, metabolic, and hemodynamic variables. This seems to occur with apparently normal regulation of the HPA axis (high morning peaks and variability as well as dexamethasone suppression of cortisol), where other endocrine variables are not affected. With a low diurnal cortisol variation and blunted dexamethasone suppression, indicating abnormal regulation of the HPA axis, perceived stress-dependent cortisol values were strongly related to perturbations of other endocrine axes as well as abdominal obesity with metabolic and hemodynamic abnormalities. Perturbations of the regulations of the HPA axis such as those described in combination with low dexamethasone suppressibility are known to follow long term overactivation of the axis by factors such as environmental stress.

Stress update Adaptation of the hypothalamic-pituitary-adrenal axis to chronic stress

The hypothalamic-pituitary-adrenal (HPA) axis exhibits a circadian rhythm, activation by stress, and inhibition by corticosteroids. Activity in the HPA axis is very sensitive to inhibition by corticosteroids when they are administered exogenously. When stress-induced corticosteroid secretion occurs, however, normal activity in the HPA is not inhibited and may even be augmented. Experiments in rats have shown that stress also induces facilitation of subsequent activity in the HPA axis that appears to balance the inhibitory effects of corticosterone and thus maintains responsiveness to new, acute stresses in chronically stressed rats. Stress-induced facilitation of HPA axis activity may be mediated by a parallel stress-induced (CRH-dependent) increase in the capacity of brain noradrenergic cell groups to respond to acute stress. A continually responsive HPA axis, even under conditions of chronic stress, appears to be important for survival. Stress-induced increases in glucocorticoid secretion to levels sufficient to occupy glucocorticoid receptors enable appropriate thermoregulatory and cardiovascular responses to acute stress. There is, however, an overall metabolic cost to the animal of maintaining continued activity in the HPA axis during chronic stress.

The pituitary-adrenocortical system of neonatal rats is responsive to stress throughout development in a time-dependent and stressor-specific fashion

The responsiveness of the neonatal hypothalamus-pituitary-adrenal (HPA) axis to stress has been thought to be impaired or diminished during the first 2 weeks of life. Although we previously found full responsiveness of the hypothalamus-pituitary unit to adrenalectomy in young rats [days (d) 5-10], we failed to measure a significant increase in ACTH 10 min after ether administration until d14 of age. These studies were, therefore, designed to test the functional activation of the HPA axis after a single or repeated exposures to stress. Both qualitative (time-course, stressor-specific, circadian) and quantitative changes in the ACTH and corticosterone (B) responses to various stressors were tested during the first 10 days of life. Exposure to 3 min of ether vapor increased ACTH and B secretion (P less than 0.05-0.01) in 1-, 5-, and 10-d-old rats, with an increasing amplitude of both ACTH and B responses as a function of age. Peak secretion of ACTH occurred 5 min after the onset of stress (122 +/- 3.8 to 359 +/- 54 pg/ml on d1-10), while the time of maximal B increased as a function of age. Other stressors, such as maternal separation (12 h), cold (4 C; 60 min), or histamine injection (4 mg/kg BW, ip), provoked significant and stressor-specific ACTH and B responses in 10-d old rats. Histamine administration increased ACTH secretion above that of vehicle-injected rats, with a peak of secretion 15 min after drug injection (272 +/- 29 vs. 127 +/- 8 pg/ml; P less than 0.01). Histamine-induced B secretion peaked at 60 min (3.7 +/- 0.5 micrograms/dl). In contrast to early responses observed after ether, separation, or histamine stress, cold stress in 10-d-old pups caused a large ACTH and B release 4 h after the onset of cold compared to that in maternally deprived pups [ACTH: cold, 457 +/- 61 pg/ml; separated, 150 +/- 14 (P less than 0.01); B: cold, 3.3 +/- 0.4 micrograms/dl; separated, 1.8 +/- 0.2 (P less than 0.05)]. We did not detect morning-evening (AM-PM) differences in either the pattern or the magnitude of the ACTH or B response to maternal separation or cold stress. Suppression of cold-induced ACTH release by B injection (1 mg/kg BW) 2 h before stress was observed until 4 h after stress in the AM and PM, whereas when given after cold, B was less effective in the PM than in the AM at preventing the rise in ACTH levels observed at 4 h.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Stress-related cortisol secretion in men: relationships with abdominal obesity and endocrine, metabolic and hemodynamic abnormalities

Abdominal obesity has been suggested to be associated with perturbations of the regulation of the hypothalamic-pituitary-adrenal (HPA) axis. In a population of 51-yr-old men (n = 284) salivary cortisol concentrations were determined on repeated (n = 7) occasions over a random working day, and perceived stress was reported in parallel. Cortisol values were then related to reported stress (stress-related cortisol). A standardized lunch was used as a physiological challenge. A low dose (0.5 mg) dexamethasone suppression test was also performed as well as determinations of testosterone and insulin-like growth factor I (IGF-I). Body mass index [weight (kilograms)/height (meters)2]; waist/hip circumference ratio (WHR); sagittal trunk recumbent diameter (D); fasting insulin; blood glucose; triglycerides; and total, low density (LDL), and high density (HDL) lipoprotein cholesterol were also determined. Cortisol concentrations were highest in the morning, and lunch was followed by a peak (P = 0.044). Two types of diurnal cortisol curves were identified, one characterized by a high variability with high morning values, and another with low variability and low morning values. Both correlated strongly with suppression of salivary cortisol by dexamethasone (P < 0.001). Stress-related cortisol secretion was associated with D (P = 0.051), low IGF-I (P = 0.006), and diastolic blood pressure (P = 0.078). When the type of diurnal cortisol curve was taken into consideration by statistical weighting, stress-related cortisol secretion in subjects with high variability showed associations with testosterone (P < 0.001), D, total and LDL cholesterol, diastolic blood pressure (P < 0.001), fasting insulin (P = 0.039), and glucose (P = 0.030) as well as, negatively, triglycerides (P < 0.001). When weighted for a low variability of diurnal cortisol secretion, stress-related cortisol secretion showed strong negative relationships with IGF-I, testosterone, and HDL. Furthermore, strong, consistent relationships (all P < 0.001) were found with obesity factors (body mass index, WHR, and D), and with metabolic (insulin, glucose, triglycerides, and total and LDL cholesterol) as well as hemodynamic variables (systolic and diastolic blood pressure and heart rate). These results clearly show interactions between diurnal cortisol secretion related to perceived stress and anthropometric, endocrine, metabolic, and hemodynamic variables. This seems to occur with apparently normal regulation of the HPA axis (high morning peaks and variability as well as dexamethasone suppression of cortisol), where other endocrine variables are not affected. With a low diurnal cortisol variation and blunted dexamethasone suppression, indicating abnormal regulation of the HPA axis, perceived stress-dependent cortisol values were strongly related to perturbations of other endocrine axes as well as abdominal obesity with metabolic and hemodynamic abnormalities. Perturbations of the regulations of the HPA axis such as those described in combination with low dexamethasone suppressibility are known to follow long term overactivation of the axis by factors such as environmental stress.

Adrenal sensitivity to adrenocorticotropin varies diurnally

To further characterize diurnal changes in the rhythm in adrenal responsiveness to ACTH, we have measured ACTH distribution volume, MCR, and t 1/2. These do not change between morning and evening in groups of untreated, dexamethasone-pretreated, or hypophysectomized female rats. To characterize the nature of the change in adrenal responsiveness to ACTH, dexamethasone-pretreated rats were infused for 2 h with a variety of doses of ACTH in the morning and evening. The adrenal response to an infusion rate of ACTH that maximally stimulated the adrenals (200 pg/100 g BW.min) was the same in the morning and evening, showing that adrenal capacity does not change. However, infusion of ACTH at lower rates (50-100 pg/100 g BW.min) revealed that the slope of the steroid response curve increased between morning and evening, demonstrating a diurnal change in adrenal sensitivity to ACTH. These results together with previous data showing that the magnitude and time course of the adrenal cAMP response to ACTH changes diurnally strongly suggest that ACTH receptor affinity or coupling with adenylate cyclase changes diurnally. In other experiments, plasma ACTH and corticosterone levels were determined in groups of young and adult male and adult female untreated rats killed at 4-h intervals around the clock. Peak sensitivity to ACTH was found at lights-out, and trough sensitivity was found at lights-on, suggesting that the experimentally demonstrated rhythm occurs normally.

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.

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.

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.

Pituitary-adrenal function in rats chronically exposed to cold

Exposure to cold for 2 weeks was used to assess the effects of a sustained stimulus on pituitary-adrenal function in male rats. The diurnal peak in plasma and adrenal corticosterone was advanced by 4 h during the first 24 h of exposure to cold but returned to its usual time (2000 h) by the next day. Plasma ACTH and corticosterone levels were generally greater at all times during the 24-h cycle in animals exposed to cold for up to 2 weeks, with the greatest increase occurring consistently at the time of peak. When rats exposed to cold for 1 week were returned to a normal 24 C environment, plasma corticosterone tended to increase. Plasma ACTH and plasma and adrenal corticosterone responses to a superimposed acute provocative stimulus (ip saline injection) were faster, greater, and more sustained in rats exposed to cold for 3 or 7 days. Similarly, the compensatory adrenal hypertrophy response to unilateral adrenalectomy was greater in cold-exposed rats. Such animals were also more resistant to pituitary-adrenal suppression by prednisolone. In contrast, there was no change in the sensitivity of the adrenal to exogenous ACTH. The results suggest that chronic exposure to cold causes a sustained activation of central mechanisms that regulate pituitary ACTH secretion as well as extra-pituitary mechanisms that regulate adrenal size; it reduces the effectiveness of negative feedback mechanisms, but does not alter those involved in the regulation of adrenal rhythmicity or adrenal sensitivity to ACTH.

High-fat feeding alters both basal and stress-induced hypothalamic-pituitary-adrenal activity in the rat

High-fat feeding induces insulin resistance and increases the risk for the development of diabetes and coronary artery disease. Glucocorticoids exacerbate this hyperinsulinemic state, rendering an individual at further risk for chronic disease. The present studies were undertaken to determine whether dietary fat-induced increases in corticosterone (B) reflect alterations in the regulatory components of the hypothalamic-pituitary-adrenal (HPA) axis. Adult male rats were maintained on a high-fat (20%) or control (4%) diet for varying periods of time. Marked elevations in light-phase spontaneous basal B levels were evident as early as 7 days after fat diet onset, and B concentrations remained significantly elevated up to 21 days after fat diet onset compared with controls. In contrast, there were no significant effects on any parameters of spontaneous growth hormone secretory profiles, thus providing support for the specificity of the effects on the HPA axis. In a second study, all groups of rats fed the high-fat diet for 1, 9, or 12 wk exhibited significantly elevated levels of plasma adrenocorticotropic hormone, B, fatty acid, and glucose before, during, and/or at 20, 60, and/or 120 min after the termination of a restraint stress. Furthermore 12-wk fat-fed animals showed a significant resistance to insulin compared with normally fed controls. There were no differences in negative feedback efficacy in high-fat-fed rats vs. controls. Taken together, these results suggest that dietary fat intake acts as a background form of chronic stress, elevating basal B levels and enhancing HPA responses to stress.

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.

The paraventricular nucleus of the thalamus alters rhythms in core temperature and energy balance in a state-dependent manner

Exposure to chronic stress facilitates activity within the hypothalamic-pituitary-adrenal (HPA) axis and is associated with enhanced neuronal activity in a discreet set of brain regions, including the posterior division of the paraventricular nucleus of the thalamus (pPVTh). Because HPA function is intimately associated with systems that regulate metabolism, including core temperature and energy balance, we examined the effects of chronic stress on circadian rhythms in temperature, locomotor activity, body weight gain and food intake and adipose depot weights in rats. We also examined the potential role of the pPVTh in mediating these functions using ibotenate lesions of this nucleus. Chronic stress lowered the amplitude of core temperature rhythms, and lesions of the pPVTh blocked this effect in chronically stressed animals, but did not affect the amplitude of temperature rhythms in unstressed controls. In addition, lesions of the pPVTh increased cumulative food intake and overall body weight gain in controls but they increased subcutaneous white adipose depot weight in chronically stressed animals. Thus, the functional paraventricular nucleus of the thalamus appears to inhibit both temperature rhythms and specific white adipose depots only in chronically stressed animals. Together with our previous results, we show that the PVTh affects rhythms in food intake and body weight and is a nexus that differentially regulates core temperature rhythms/HPA activity/specific white adipose depots depending on the stress state of the animal.

Effects of streptozotocin-induced diabetes and insulin treatment on the hypothalamic melanocortin system and muscle uncoupling protein 3 expression in rats

Hypothalamic melanocortins are among several neuropeptides strongly implicated in the control of food intake. Agonists for melanocortin 4 (MC-4) receptors such as alpha-melanocyte-stimulating hormone (alpha-MSH), a product of proopiomelanocortin (POMC), reduce food intake, whereas hypothalamic agouti-related protein (AgRP) is a MC-4 receptor antagonist that increases food intake. To investigate whether reduced melanocortin signaling contributes to hyperphagia induced by uncontrolled diabetes, male Sprague-Dawley rats were studied 7 days after administration of streptozotocin (STZ) or vehicle. In addition, we wished to determine the effect of diabetes on muscle uncoupling protein 3 (UCP-3), a potential regulator of muscle energy metabolism. STZ diabetic rats were markedly hyperglycemic (31.3 +/- 1.0 mmol/l; P < 0.005) compared with nondiabetic controls (9.3 +/- 0.2 mmol/l). Insulin treatment partially corrected the hyperglycemia (18.8 +/- 2.5 mol/l; P < 0.005). Plasma leptin was markedly reduced in STZ diabetic rats (0.4 +/- 0.1 ng/ml; P < 0.005) compared with controls (3.0 +/- 0.4 ng/ml), an effect that was also partially reversed by insulin treatment (1.8 +/- 0.3 ng/ml). Untreated diabetic rats were hyperphagic, consuming 40% more food (48 +/- 1 g/day; P < 0.005) than controls (34 +/- 1 g/day). Hyperphagia was prevented by insulin treatment (32 +/- 2 g/day). In untreated diabetic rats, hypothalamic POMC mRNA expression (measured by in situ hybridization) was reduced by 80% (P < 0.005), whereas AgRP mRNA levels were increased by 60% (P < 0.01), suggesting a marked decrease of hypothalamic melanocortin signaling. The change in POMC, but not in AgRP, mRNA levels was partially reversed by insulin treatment. By comparison, the effects of diabetes to increase hypothalamic neuropeptide Y (NPY) expression and to decrease corticotropin-releasing hormone (CRH) expression were normalized by insulin treatment, whereas the expression of mRNA encoding the long form of the leptin receptor in the arcuate nucleus was unaltered by diabetes or insulin treatment. UCP-3 mRNA expression in gastrocnemius muscle from diabetic rats was increased fourfold (P < 0.005), and the increase was prevented by insulin treatment. The effect of uncontrolled diabetes to decrease POMC, while increasing AgRP gene expression, suggests that reduced hypothalamic melanocortin signaling, along with increased NPY and decreased CRH signaling, could contribute to diabetic hyperphagia. These responses, in concert with increased muscle UCP-3 expression, may also contribute to the catabolic effects of uncontrolled diabetes on fuel metabolism in peripheral tissues.

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.