Regulation of pituitary corticotropin releasing hormone (CRH) receptor mRNA and CRH binding during adrenalectomy: role of glucocorticoids and hypothalamic factors

The Hypothalamic-Pituitary-Adrenal Axis: Development, Programming Actions of Hormones, and Maternal-Fetal Interactions

The hypothalamic-pituitary-adrenal axis is a complex system of neuroendocrine pathways and feedback loops that function to maintain physiological homeostasis. Abnormal development of the hypothalamic-pituitary-adrenal (HPA) axis can further result in long-term alterations in neuropeptide and neurotransmitter synthesis in the central nervous system, as well as glucocorticoid hormone synthesis in the periphery. Together, these changes can potentially lead to a disruption in neuroendocrine, behavioral, autonomic, and metabolic functions in adulthood. In this review, we will discuss the regulation of the HPA axis and its development. We will also examine the maternal-fetal hypothalamic-pituitary-adrenal axis and disruption of the normal fetal environment which becomes a major risk factor for many neurodevelopmental pathologies in adulthood, such as major depressive disorder, anxiety, schizophrenia, and others.

Sleep in pituitary insufficient patients compared to patients with depression and healthy controls at baseline and after challenge with CRH

Sleep disturbances are prevalent in both patients with pituitary insufficiency and with depression. The role of corticotropin releasing hormone (CRH), involved in sleep regulation, has not been fully clarified. Pituitary insufficiency is an ideal model for studying sleep-endocrine effects since no consecutive hormone releases and feedback effects occur after hormone administration. 11 male patients with a chronic insufficiency of the anterior pituitary gland (PI) and under stable hormonal substitution were studied during three consecutive nights in the sleep laboratory. The first night served for adapting to laboratory setting, during the second night placebo was administered and during the third night 4 × 50 μg CRH were injected in pulsatile fashion. Sleep parameters were additionally compared with those of 15 healthy male controls (C) and 15 male patients with depression (D). CRH administration was associated with a numerical increase of wake time (115 ± 15 to 131 ± 13 min) and a decrease of REM sleep (89 ± 8 to 80 ± 8 min), REM latency (69 ± 14 to 55 ± 9 min) and slow wave sleep (66 ± 16 to 57 ± 15 min). Yet, none of these changes reached statistical significance. PI showed a worse sleep profile as compared to both control groups, e.g. indicated by a significantly lower sleep efficiency index (PI:0.80 ± 0.03 vs. C:0.94 ± 0.01 vs. D:0.87 ± 0.03). In conclusion sleep-EEG changes after CRH in PI patients resemble those found in in part in patients with depression. Sleep in anterior pituitary insufficiency was impaired despite full hormonal substitution possibly suggesting an alteration of the receptor organisation of brain structures involved in sleep regulation.

Distribution of Corticotrophin-Releasing Factor Type 1 Receptor-Like Immunoreactivity in the Rat Pituitary

Corticotrophin-releasing factor (CRF) regulates the hypothalamic-pituitary-adrenal axis response to stress through its type 1 receptor (CRF₁ ) in the corticotrophs of the anterior pituitary. Although CRF₁ mRNA expression has been confirmed in the rat pituitary, the distribution pattern of CRF₁ protein in the pituitary has not been reported. Therefore, we generated an antiserum against the amino acid fragment corresponding to the 177-188 sequence of the first extracellular loop of the rat CRF₁ . Using the antiserum, CRF₁ -like immunoreactivity (CRF₁ -LI) was detected in the anterior lobe cells of the rat pituitary where some of them expressed intense signals. CRF₁ -LI also appeared in the intermediate lobe cells and on the fibre-like elements of the posterior lobe of the pituitary. Dual immunofluorescence labelling showed that corticotrophs exhibited the highest percentage of CRF₁ (male: 27.1 ± 3.0%, female: 18.0 ± 3.0%), followed by lactotrophs (male: 6.7 ± 3.0%, female: 12.1 ± 1.3%), gonadotrophs (male: 2.6 ± 1.0%, female: 7.5 ± 0.5%), thyrotrophs (male: 2.9 ± 0.1%, female: 5.3 ± 1.2%) and somatotrophs (male: 1.1 ± 0.3%, female: 1.2 ± 0.5%). The percentage of CRF₁ -LI-positive cells that were corticotrophs was significantly higher in male rats than in female rats, whereas CRF₁ -LI-positive lactotrophs and gonadotrophs were significantly higher in female rats than in male rats. Almost all of the melanotrophs were positive for CRF₁ in the intermediate lobe (98.9 ± 0.2%). CRF₁ -LI and the percentage of CRF₁ -LI in corticotrophs were decreased in the anterior pituitary, and the distribution patterns were altered from a diffuse to punctate one by adrenalectomy; the changes were restored by treatment with dexamethasone (100 μg/kg bw). These results suggest that CRF₁ is involved in the modulation of the functions of the pituitary; moreover, protein expression and the distribution patterns of CRF₁ are regulated by glucocorticoids in the rat anterior pituitary.

Regulation of the hypothalamic-pituitary-adrenal axis by neuropeptides

The major endocrine response to stress occurs via activation of the hypothalamic-pituitary-adrenal (HPA) axis, leading ultimately to increases in circulating glucocorticoids, which are essential for the metabolic adaptation to stress. The major players in the HPA axis are the hypothalamic neuropeptide, corticotropin releasing hormone (CRH), the pituitary hormone adrenocorticotropic hormone, and the negative feedback effects of adrenal glucocorticoids. In addition, a number of other neuropeptides, including vasopressin (VP), angiotensin II, oxytocin, pituitary adenylate cyclase activating peptide, orexin and cholecystokinin, and nesfatin can affect HPA axis activity by influencing the expression and secretion of CRH, and also by modulating pituitary corticotroph function or adrenal steroidogenesis. Of these peptides, VP co-secreted with CRH from axonal terminals in the external zone of the median eminence plays a prominent role by potentiating the stimulatory effect of CRH and by increasing the number of pituitary corticotrophs during chronic challenge. Although the precise role and significance of many of these neuropeptides in regulating HPA axis activity requires further investigation, it is likely that they are part of a multifactorial system mediating the fine tuning of HPA axis activity during adaptation to a variety of physiological and stressful conditions.

Gonadal steroid hormones and the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis represents a complex neuroendocrine feedback loop controlling the secretion of adrenal glucocorticoid hormones. Central to its function is the paraventricular nucleus of the hypothalamus (PVN) where neurons expressing corticotropin releasing factor reside. These HPA motor neurons are a primary site of integration leading to graded endocrine responses to physical and psychological stressors. An important regulatory factor that must be considered, prior to generating an appropriate response is the animal's reproductive status. Thus, PVN neurons express androgen and estrogen receptors and receive input from sites that also express these receptors. Consequently, changes in reproduction and gonadal steroid levels modulate the stress response and this underlies sex differences in HPA axis function. This review examines the make up of the HPA axis and hypothalamo-pituitary-gonadal (HPG) axis and the interactions between the two that should be considered when exploring normal and pathological responses to environmental stressors.

miR-449a contributes to glucocorticoid-induced CRF-R1 downregulation in the pituitary during stress

The hypothalamic-pituitary-adrenal axis is controlled by the feedback of glucocorticoids on the hypothalamus and pituitary. Stress increases CRF, ACTH, and glucocorticoid secretion. The expression of not only CRF mRNA in the hypothalamus and proopiomelanocortin mRNA in corticotrophs, but also CRF type 1 receptor (CRF-R1) mRNA and protein on corticotrophs are downregulated through glucocorticoids. However, the mechanisms underlying the glucocorticoid-induced CRF-R1 downregulation are not fully understood. Short RNA molecules, called microRNAs (miRNAs), are posttranscriptional regulators that usually induce translational repression or gene silencing via binding to complementary sequences within target mRNAs. We hypothesized that glucocorticoids may induce the expression of miRNAs in the pituitary, which are involved in glucocorticoid-induced downregulation of CRF-R1. We found 3 miRNAs with sequences predicted to bind to the CRF-R1 3' untranslated region (3'-UTR) by database search. Expression of 1 of these miRNAs (miR-449a) was significantly higher in the anterior pituitary of restrained rats than in that of unrestrained control rats. Expression of miR-449a was evident in many anterior pituitary cells, including corticotrophs. Although overexpression of miR-449a decreased CRF-R1 mRNA and CRF-R1 protein expression, knockdown of miR-449a attenuated dexamethasone-induced suppression of CRF-R1 mRNA and CRF-R1 protein expression in the monolayer-cultured pituitary cells. Notably, luciferase activity was significantly lower in cells cotransfected with a luciferase vector containing the CRF-R1 3'-UTR and a miR-449a vector. miR-449a expression was significantly increased by dexamethasone. Adrenalectomy attenuated restraint-induced increase in miR-449a expression in the pituitary. These results indicated that miR-449a plays an important role in stress-induced, glucocorticoid-mediated downregulation of CRF-R1 expression.

Impaired hypothalamic-pituitary-adrenal axis and its feedback regulation in serotonin transporter knockout mice

Our previous studies have demonstrated that mice with reduced or absent serotonin transporter (SERT+/- and SERT-/- mice, respectively) are more sensitive to stress relative to their SERT normal littermates (SERT+/+ mice). The aim of the present study was to test the hypothesis that the hypothalamic-pituitary-adrenal (HPA) axis and its feedback regulation are impaired in these mice. The function and gene expression of several components in the HPA axis and its feedback regulation in SERT+/+, +/( and -/- mice were studied under basal (non-stressed) and stressed conditions. The results showed that (1) under basal conditions, corticotrophin-releasing factor (CRF) mRNA levels in the paraventricular nucleus (PVN) of the hypothalamus was lower in both SERT+/( and (/( mice relative to SERT+/+ mice; (2) an increased response to CRF challenge was found in SERT(/( mice, suggesting that the function of CRF type 1 receptors (CRF R1) in the pituitary is increased. Consistent with these findings, (125)I-sauvagine (a CRF receptor antagonist) binding revealed an increased density of CRF R1 in the pituitary of SERT(/( under basal conditions. These data suggest that CRF R1 in the pituitary of SERT(/( mice is up-regulated. However, in the pituitary of SERT+/( mice, the function of CRF R1 was not changed and the density of CRF R1 was reduced relative to SERT+/+ mice; and (3) the expression of the glucocorticoid receptor (GR) in the hypothalamus, pituitary and adrenal cortex was significantly reduced in SERT+/( and (/( mice in comparison with SERT+/+ mice under basal conditions. Consistent with these findings, the corticosterone response to dexamethasone was blunted in SERT(/( mice relative to SERT+/+ and +/( mice. Furthermore, stress induces a rapid increase of the GR expression in the hypothalamus of SERT+/( and (/( mice relative to their basal levels. Together, the present results demonstrated that the HPA axis and its feedback regulation are altered in SERT knockout mice, which could account for the increased sensitivity to stress in these mice.

Decreased ACTH secretion during prolonged transportation stress is associated with reduced pituitary responsiveness to tropic hormone stimulation in cattle

The present study examined the effect of transportation stress on hypothalamic-pituitary-adrenal axis responsiveness to tropic hormone stimulation and on abundance of corticotropin releasing factor (CRF) receptor R1 (CRFR1) and arginine vasopressin (AVP) receptor V3 (V3) mRNAs in the anterior pituitary (AP) of cattle. Holstein steers were transported for 10 h or used as non-transported controls (NTC). Blood samples were collected at start of transportation and every 1-2h thereafter. To test AP responsiveness to tropic hormones, animals were challenged (i.v.) with CRF (0.5 microg/kg), AVP (1 microg/kg) or CRF plus AVP immediately after end of transportation and blood samples collected every 30 min for 3h. The AP of animals transported for 0, 4 or 10 h were harvested for mRNA analyses. Plasma ACTH in transported animals increased within 1h and remained elevated for 6 and 8h versus NTC and 0 h values, respectively. Plasma concentrations of cortisol increased in response to transportation and remained elevated throughout the transport period. Injection of CRF or AVP to NTC animals increased plasma ACTH, but ACTH secretion in response to CRF or AVP was dramatically reduced in transported animals. ACTH secretion following co-injection of CRF and AVP tended to be less in transported animals, but was almost 100% greater than when secretagogues were administered separately. Despite decreased AP responsiveness to CRF and AVP, AP CRFR1 and V3 mRNAs were increased after 10 h transportation. Results indicate decreased AP responsiveness to CRF and AVP may regulate duration of ACTH secretion in response to transportation stress in cattle.

Plasma leptin and ghrelin in the neonatal rat: interaction of dexamethasone and hypoxia

Ghrelin, leptin, and endogenous glucocorticoids play a role in appetite regulation, energy balance, and growth. The present study assessed the effects of dexamethasone (DEX) on these hormones, and on ACTH and pituitary proopiomelanocortin (POMC) and corticotropin-releasing hormone receptor-1 (CRHR1) mRNA expression, during a common metabolic stress - neonatal hypoxia. Newborn rats were raised in room air (21% O2) or under normobaric hypoxia (12% O2) from birth to postnatal day (PD) 7. DEX was administered on PD3 (0.5 mg/kg), PD4 (0.25 mg/kg), PD5 (0.125 mg/kg), and PD6 (0.05 mg/kg). Pups were studied on PD7 (24 h after the last dose of DEX). DEX significantly increased plasma leptin and ghrelin in normoxic pups, but only increased ghrelin in hypoxic pups. Hypoxia alone resulted in a small increase in plasma leptin. Plasma corticosterone and pituitary POMC mRNA expression were decreased 24 h following the last dose of DEX, whereas plasma ACTH and pituitary CRHR1 mRNA expression had already increased (normoxia and hypoxia). Hypoxia alone increased corticosterone, but had no effect on ACTH or pituitary POMC and CRHR1 mRNA expression. Neonatal DEX treatment, hypoxia, and the combination of both affect hormones involved in energy homeostasis. Pituitary function in the neonate was quickly restored following DEX-induced suppression of the hypothalamic-pituitary-adrenal axis. The changes in ghrelin, leptin, and corticosterone may be beneficial to the hypoxic neonate through the maintenance of appetite and shifts in intermediary metabolism.

Reduced hypothalamic vasopressin secretion underlies attenuated adrenocorticotropin stress responses in pregnant rats

We sought to explain decreased ACTH secretory responses to stress in pregnant rats by investigating hypothalamic CRH and vasopressin secretion and actions on anterior pituitary corticotrophs. In late pregnancy median eminence, CRH content was reduced (by 12%). Anterior pituitary proopiomelanocortin mRNA expression, measured by in situ hybridization but not radioimmunoassayed ACTH content, was also reduced (by 45% on d 21); CRH receptor (CRHR)1 mRNA expression was unaltered in pregnancy, but V1b receptor mRNA expression was reduced (by 19%). ACTH secretory responses, measured in jugular blood, to CRH (200 ng/kg iv) or vasopressin (1.7 microg/kg, iv) were reduced on d 21 vs. virgins (49% and 44%), but the response to combined CRH and vasopressin injection was intact. Either antalarmin (CRHR1 antagonist; 20 mg/kg ip) or dP(Tyr(Me)2),Arg-NH2(9))AVP (V1a/b antagonist; 10 microg/kg, iv) pretreatment reduced the ACTH secretory response to forced swimming (90 sec) in virgin rats (by 57% and 40%), but only antalarmin was effective in pregnant rats (53% decrease). In vitro, measuring ACTH secretion from acutely dispersed anterior pituitary cells showed increased corticotroph sensitivity in pregnancy to CRH and to CRH augmentation by vasopressin, attributable to increased intracellular cAMP action. Hence, in late pregnancy, reduced anterior pituitary CRHR1 or V1b receptor expression did not impair corticotroph responses to CRH or vasopressin. Rather, diminished secretagogue secretion in vivo accounts for reduced action of stress levels of exogenous CRH or vasopressin alone; the late pregnancy attenuated ACTH secretory response to swim stress is deduced to be due to reduced vasopressin release by parvocellular paraventricular nuclei neurones.

Chronic cortisol suppresses pituitary and hypothalamic peptide message expression in pigtailed macaques

The effects of chronic elevations in circulating glucocorticoids on the expression of peptides and peptide receptors of the hypothalamic-pituitary-adrenal (HPA) axis have been studied extensively in rodents, but they have not been examined in primates. To determine the responses of the HPA axis in primates to elevated cortisol, hypothalamic and pituitary tissue from normal older pigtailed macaques (Macaca nemestrina) that had received daily oral administration of cortisol or placebo for 1 year were studied. Pro-opiomelanocortin in the anterior pituitary and corticotropin-releasing factor (CRF) mRNA expression in the hypothalamic paraventricular nucleus (PVN) were significantly reduced in cortisol-treated monkeys in comparison with controls. CRF receptor 1 (CRF-R1) expression in the anterior pituitary and arginine vasopressin mRNA expression in the PVN were unchanged by chronic cortisol administration. Sustained elevation of circulating glucocorticoids results in suppression of HPA peptide and peptide receptor expression in the PVN and anterior pituitary similar to those found in rodents. Chronic therapeutic administration of glucocorticoids in humans may have unintended consequences for hypothalamic and pituitary function.

Corticotropin releasing hormone receptors: two decades later

Hypothalamic corticotropin releasing hormone (CRH) regulates pituitary ACTH secretion and mediates behavioral and autonomic responses to stress, through interaction with type 1 plasma membrane receptors (CRHR1) located in pituitary corticotrophs and the brain. Although the CHRI are essential for ACTH responses to stress, their number in the pituitary gland does not correlate with corticotroph responsiveness, suggesting that activation of a small number of receptors is sufficient for maximum ACTH production. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent changes in CRH receptors and CRH1 mRNA in the pituitary, with a reduction in receptor binding but normal or elevated expression of CHR1 mRNA levels. Western blot analysis of CRHR1 protein in pituitary membranes from adrenalectomized rats showed unchanged receptor mRNA levels and increased CRHR1 protein, despite binding down-regulation, suggesting that decreased binding is due to homologous desensitization, rather than reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with a reduction in CRHR1 protein, suggesting inhibition of CRH1 mRNA translation. The regulation of CRHR1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'-UTR of the CRHR1 mRNA. It is likely that post-transcriptional regulatory mechanisms that permit rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous changes in physiological demands.

Single administration of interleukin-1 increased corticotropin releasing hormone and corticotropin releasing hormone-receptor mRNA in the hypothalamic paraventricular nucleus which paralleled long-lasting (weeks) sensitization to emotional stressors

Single exposure to the proinflammatory cytokine interleukin-1 induces sensitization of the adrenocorticotropin hormone and corticosterone responses to stressors weeks later (hypothalamus-pituitary-adrenal sensitization). Hypothalamus-pituitary-adrenal responses are controlled by corticotropin-releasing hormone and arginine-vasopressin secreted from parvocellular corticotropin-releasing hormone neurons of the hypothalamic paraventricular nucleus and may involve autoexcitatory feedback mechanisms. Therefore, we studied the temporal relationship between resting levels of corticotropin-releasing hormone, corticotropin-releasing hormone-R1 and arginine-vasopressin receptor (V1a, V1b) mRNAs in the paraventricular nucleus and the development of hypothalamus-pituitary-adrenal sensitization to an emotional stressor (novelty). The adrenocorticotropin hormone precursor molecule proopiomelanocortin hnRNA in the pituitary gland served as an index for acute activation. Single administration of interleukin-1 induced sensitization of the hypothalamus-pituitary-adrenal to novelty from 3 to 22 days later, but not after 42 days. Single administration of interleukin-1 induced biphasic increases in corticotropin-releasing hormone and corticotropin-releasing hormone-R1 mRNAs in the paraventricular nucleus: an early peak within 24 h, followed by a delayed (>7 days) increase that peaked after 22 days. Hypothalamic V1a and V1b mRNA levels were unaffected. In contrast, in the pituitary gland, there was an early decrease in corticotropin-releasing hormone-R1 mRNA (from 10.5 to 3 h after interleukin-1) and V1b receptor mRNA (3 to 6 h), which returned to control levels from 24 h onwards. Thus, interleukin-1-induced long-lasting hypothalamus-pituitary-adrenal sensitizations associated with prolonged activation of corticotropin-releasing hormone and corticotropin-releasing hormone-R1 mRNA expression in the paraventricular nucleus, but not with changes in the expression of proopiomelanocortin hnRNA or V1b receptor or corticotropin-releasing hormone R1 mRNAs in the pituitary gland. We propose that transient exposure to immune events can induce long-lasting hypothalamus-pituitary-adrenal sensitization, which at least in part involves long-term hypothalamic adaptations that enhance central corticotropin-releasing hormone signaling.

The role of hypothalamic input on corticotroph maturation in fetal sheep

Corticotropin-releasing hormone receptor type 1 (CRH-R1) expression and vasopressin type 1b (V1b) receptor protein decrease in late-gestation fetal sheep. Because hypothalamo-pituitary disconnection (HPD) has been demonstrated to prevent the morphological maturation of corticotrophs, we hypothesized that hypothalamic input is necessary for the maturational changes in CRH-R1 and V1b receptor levels. We measured CRH-R1 and V1b receptor expression in the anterior pituitaries of fetuses at 140 days gestational age (dGA) that underwent HPD or sham surgery at 120 dGA. CRH-R1 mRNA decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. However, CRH-R1 protein levels were elevated in HPD fetuses compared with sham and were not different from 120 dGA values. V1b protein levels decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. We conclude that hypothalamic input to the pituitary is necessary for the decrease in CRH-R1 receptor protein levels in late-gestation fetal sheep. However, hypothalamic input is not necessary for the decrease in V1b receptor expression seen in late gestation.

Temporally sensitive trophic responsiveness of the adrenalectomized rat anterior pituitary to dexamethasone challenge: relationship between mitotic activity and apoptotic sensitivity

Depending on timing and dose, exogenous glucocorticoids induce a wave of apoptosis in the adult rat anterior pituitary, a response that is enhanced by adrenalectomy. In this study, we show that the size of the glucocorticoid-sensitive apoptotic population progressively increases during the week following surgical adrenalectomy, plateaus for a further week, then spontaneously declines to levels seen in intact animals by 4 wk. Mitotic activity, in contrast, rises rapidly post adrenalectomy but returns to baseline within 2 wk. Increased mitotic activity precedes the increase in the population of cells that undergo glucocorticoid-induced apoptosis and the subsequent decline in mitotic activity precedes the decline in apoptotic sensitivity despite persistent elevation of hypothalamic CRH and pituitary proopiomelanocortin transcripts. If glucocorticoid exposure is delayed until 4 wk post adrenalectomy when the apoptotic response has returned to baseline, glucocorticoid withdrawal, by transiently increasing mitotic activity, again primes the formation of an expanded glucocorticoid-sensitive apoptotic cell population. These data suggest that apoptotic sensitivity is largely confined to cells that have recently entered the cell cycle. This observation is further corroborated by demonstrating an abrupt glucocorticoid-induced step-down in the bromodeoxyuridine-labeling index to basal levels in rats given daily injections of bromodeoxyuridine during the week following adrenalectomy.

Dexamethasone attenuates by colchicine induced Fos expression in the rat deep cerebellar and vestibular nuclei

1. The intent of the present study was to find out whether dexamethasone pretreatment may affect the induction of Fos protein in cell nuclei of the cerebellar vestibular neuronal complex (CVNC) elicited by central administration of colchicine. Specifically, the rate of the dexamethasone-sensitive cell population was analyzed and compared at different levels of the CVNC using a light microscopic avidin-biotin peroxidase immunohistochemistry. 2. Male Wistar rats were pretreated with dexamethasone 3 days prior (2.5 mg/kg/day, s.c.) and 24 h after an intracerebroventricular delivery of colchicine (60 microg/10 microL). Animals were sacrificed 48 h after colchicine treatment by a transcardial perfusion with fixative. 3. Dexamethasone in itself had no effect on the activity of cells of the CVNC. However, in colchicine treated animals, which exhibited a large number of Fos-positive cells over the entire CVNC, the dexamethasone elicited a substantial reduction in the number of the Fos-immunoreactive cells over the CVNC. Distinct dexamethasone dependent reduction (50-90%) of Fos-immunoreactivity was observed in each of the deep cerebellar nuclei. On the other hand, less number of dexamethasone-sensitive cells were recognized in the vestibular structures. From these, maximal Fos-inhibition by dexamethasone was recognized in the medial vestibular nucleus, however, even in this case the number of suppressed cells did not exceed 50%. 4. The results provide for the first time evidence about the dexamethasone dependent reduction of Fos-immunoreactivity in the cells of the CVNC in response to stimulation elicited by colchicine. The data also indicate that the glucocorticoids might be involved in the regulation of some functions of the CVNC under stress conditions.

Multiple sites of control of type-1 corticotropin releasing hormone receptor levels in the pituitary

Hypothalamic corticotropin releasing hormone (CRH) stimulates pituitary ACTH secretion through interaction with type 1 CRH receptors (CRH-R1), the number of which varies during alterations of the hypothalamic-pituitary-adrenal (HPA) axis. CRH-R1 are essential for ACTH responses to stress but CRH receptor content in the pituitary does not correlate with corticotroph responsiveness. This indicates that a small number of receptors is sufficient for full ACTH responses probably through post-receptor interaction with vasopressin (VP) signaling. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent levels of CRH receptors and CRH-R1 mRNA in the pituitary, with binding reductions but normal or elevated CRH-R1 mRNA levels during alterations of the HPA axis. Western blot analysis of CRH-R1 protein in pituitary membranes from adrenalectomized rats show unchanged CRH-R1 mRNA levels, but reduced CRH binding associated with significant increases in CRH-R1 protein, suggesting that the decrease in binding is due to homologous desensitization and not to reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with reduction in CRH-R1 protein suggesting inhibition of CRH-R1 mRNA translation. Regulation of CRH-R1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'UTR of the CRH-R1 mRNA. Post-transcriptional regulatory mechanisms allowing rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous change in physiological demand.

Effect of repeated lipopolysaccharide administration on tissue cytokine expression and hypothalamic-pituitary-adrenal axis activity in rats

The effects of chronic immune challenge on cytokine expression and hypothalamic-pituitary-adrenal axis (HPA) axis responses to stress were studied in Wistar rats after administration of increasing doses of lipopolysaccharide (LPS). Repeated LPS (R-LPS) decreased body weight and increased adrenal weight and pituitary pro-opiomelanocortin mRNA levels. LPS injection increased plasma adrenocorticotropic hormone (ACTH) and corticosterone but the effect was attenuated in R-LPS. Plasma corticosterone but not ACTH responses to restraint were also reduced in R-LPS. Basal and restraint-stimulated corticotropin releasing hormone (CRH) mRNA levels were lower in R-LPS, but responses to a new LPS injection were similar to controls. In contrast, type 1 CRH receptor (CRH-R1) mRNA responses to both LPS and restraint were blunted in R-LPS. Vasopressin mRNA levels in parvocellular neurones were higher in R-LPS, and increased further after restraint but not after a new LPS injection. Glucocorticoid receptor (GR) levels in the paraventricular nucleus (PVN) increased after a single LPS or R-LPS (24 h after the last injection) but declined after a new injection in R-LPS. Interleukin (IL)-1beta and IL-6 mRNAs increased in the pituitary, spleen and circumventricular organs after single or R-LPS, suggesting that cytokines may contribute to the activation of the HPA axis though pathways from the circumventricular organs as well as paracrine effects in the pituitary. The data show that (i) adaptation of the HPA axis during repeated LPS injection involves increases in vasopressin : CRH expression ratios in parvocellular neurones; (ii) that hypothalamic CRH and vasopressin responses to acute stimulation are independent of CRH-R1 expression in the PVN; and (iii) there is a dissociation between pituitary and adrenal responses to acute stress suggesting a decrease of adrenal sensitivity to ACTH.

Regulation of pituitary corticotropin releasing hormone receptors

Corticotropin releasing hormone (CRH) stimulates pituitary ACTH secretion through type-1 CRH (CRH1) receptors. Stimulation of the hypothalamic pituitary adrenal (HPA) axis as well as increased corticotroph responsiveness during stress and adrenalectomy are associated with marked pituitary CRH binding downregulation. The presence of CRH1 receptors in the pituitary are essential to maintain ACTH secretion. Downregulation of CRH binding is associated with normal or elevated levels of CRH1 receptor mRNA and this may contribute to the maintainence of permissive levels of CRH1 receptors in the pituitary. Injection of either CRH or glucocorticoids in rats in vivo induces CRH binding and CRH1 receptor mRNA downregulation, whereas their simultaneous administration causes only transient CRH1 receptor mRNA loss. Vasopressin increases CRH1 receptor mRNA levels. This suggest that interactions between CRH, vasopressin and glucocorticoids accounts for CRH1 receptor mRNA upregulation during stress. The lack of correlation between CRH binding and CRH1 receptor mRNA indicates that the major sites for pituitary CRH1 receptor regulation are at the post-transcriptional level.

The effect of hypothalamic lesions on hypothalamo-pituitary-adrenal axis activity and inflammation in adjuvant-induced arthritis

Adjuvant-induced arthritis (AA) was induced in control and in hypothalamic lesioned Piebald-Viral-Glaxo (PVG) rats. Following discrete paraventricular nucleus (PVN) lesions plasma corticosterone was increased 14 days after adjuvant injection as in controls, when hind paw inflammation was apparent. PVN lesion did not affect the severity of inflammation.In contrast, following medial basal hypothalamus (MBH) lesions adjuvant did not increase corticosterone levels and the increase in paw volume at day 14 was potentiated. Basal proopiomelanocortin(POMC) mRNA expression in the anterior lobe was unchanged by PVN lesions and decreased by MBH lesions. AA increased POMC mRNA in controls and in both PVN and MBH lesioned rats. After complete MBH lesion, surviving anterior pituitary tissue maintained morning levels of corticosterone.Thus, AA may activate the hypothalamo-pituitary-adrenal axis without the mediation of PVN neurones projecting to the median eminence. However, the loss of the corticosterone response to AA and the increase in severity of inflammation in the MBH lesioned rats suggests a central (non-PVN) component mediates effects of inflammation. Furthermore, the increase in POMC mRNA in the MBH lesioned AA rats suggests that part of this process is not mediated by releasing factors in the hypothalamo-hypophysial portal system, and that extrahypothalamic(peripheral) mediators act on the pituitary during chronic inflammation.

Defective ACTH response to stress in previously stressed rats: dependence on glucocorticoid status

The effect of previous exposure to stress on the pituitary-adrenal response to a further stress was characterized in rats with different glucocorticoid status: sham-operated rats (Sham), adrenalectomized (ADX) rats, and ADX rats supplemented with a low corticosterone (B) dose in the drinking saline (ADX + B). Previous exposure of Sham rats to 1 h of immobilization (Imo) reduced, 2 h later, the ACTH response to a second severe stressor (Imo) but not to a less severe stressor (tail shock). In ADX rats, previous Imo totally suppressed the ACTH response to Imo or to shock. In ADX + B rats the response to shock was blocked and that to Imo tended to be lower. These changes were not explained by depletion of adenohypophysial ACTH stores. After previous Imo, reduced response to corticotropin-releasing factor was observed in Sham and ADX + B, but not in ADX, rats. Taken together, the present results suggest that the reduced ACTH response of previously stressed rats to a second severe stress is observed in the presence and absence of glucocorticoids, but the main site at which such inhibition occurs might be critically dependent on the glucocorticoid status.

The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety

Neuroendocrine studies strongly suggest that dysregulation of the hypothalamic pituitary-adrenocortical (HPA) system plays a causal role in the development and course of depression. Whereas the initial mechanism resulting in HPA hyperdrive remains to be elucidated, evidence has emerged that corticosteroid receptor function is impaired in many patients with depression and in many healthy individuals at increased genetic risk for an depressive disorder. Assuming such impaired receptor function, then central secretion of CRH would be enhanced in many brain areas, which would account for a variety of depressive symptoms. As shown in rats and also in transgenic mice with impaired glucocorticoid receptor function, antidepressants enhance the signaling through corticosteroid receptors. This mechanism of action can be amplified through blocking central mechanisms that drive the HPA system. Animal experiments using antisense oligodeoxynucleotides directed against the mRNA of both CRH receptor subtypes identified the CRH1 receptor as the mediator of the anxiogenic effects of CRH. Studies in mouse mutants in which this receptor subtype had been deleted extended these findings as the animals were less anxious than wild-type mice when experimentally stressed. Thus, patients with clinical conditions that are causally related to HPA hyperactivity may profit from treatment with a CRH1 receptor antagonist.

Corticotropin releasing hormone, receptor regulation and the stress response

Corticotropin releasing hormone (CRH) coordinates behavioral, autonomic and hormonal responses to stress, including activation of the hypothalamic-pituitary-adrenal (HPA) axis with stimulation of adrenocorticotropin (ACTH) and glucocorticoids. Differential changes of expression of CRH and vasopressin(VP) in the parvicellular hypothalamic paraventricular nucleus (PVN), as well as regulation of CRH and VP receptors, are critical for the responsiveness of the HPA axis during stress. Pituitary CRH receptor (CRH-R)expression and content is controlled by the coordinated action of CRH, VP and glucocorticoids. Marked changes in hypothalamic and pituitary CRH-R expression support a key regulatory role for CRH in the HPA axis and the integrated stress response.

Interaction between glucocorticoids and corticotropin releasing hormone (CRH) in the regulation of the pituitary CRH receptor in vivo in the rat

Acute stress causes biphasic changes in corticotropin releasing hormone (CRH) receptor mRNA expression with an early decrease followed by an increase. However, in the absence of glucocorticoids in adrenalectomized rats, stress results in prolonged CRH receptor (CRH-R) mRNA loss, suggesting that interactions between glucocorticoids and hypothalamic factors are critical for regulation of CRH receptor mRNA. To address this question, CRH binding, type-1 CRH-R mRNA, POMC mRNA and POMC hnRNA expression were measured by binding autoradiography and in situ hybridization in pituitaries from intact and adrenalectomized rats. CRH-R mRNA decreased by 59% 5 h after injection of corticosterone (10 mg s.c.) and returned to basal levels by 18 h, a time when plasma corticosterone concentrations were still elevated, and CRH binding and POMC hnRNA were significantly reduced. Elevations in plasma corticosterone in the range of acute stress by injection of 2 mg s.c. caused CRH-R mRNA expression to return to near basal values by 6 h, after a 52% and 39% decrease at 2 h and 4 h. More transient changes were seen after a single injection of CRH (1 microg), with a 44% decrease in CRH-R mRNA and a 175% increase in POMC hnRNA by 2 h, returning to basal values by 4 h. The transient effect of CRH was not due to clearance of CRH from the circulation or receptor desensitization since CRH receptor mRNA expression also recovered after injection of a higher dose (10 microg) or repeated injections of CRH which caused sustained increases in plasma CRH and pituitary POMC hnRNA levels. CRH injection in adrenalectomized rats decreased CRH-R mRNA for up to 6 h, suggesting that glucocorticoids are permissive for the recovery of CRH-R mRNA. Supporting this hypothesis, simultaneous injection of corticosterone and CRH restored CRH-R mRNA expression by 4 h, and increased CRH binding 4 h and 6 h after injection. The data show that interaction between CRH and glucocorticoids counteracts individual inhibitory effects of these regulators alone, and that such effects are likely to contribute to the regulatory pattern of pituitary CRH receptors during acute stress.