Corticotrope response to removal of releasing factors and corticosteroids in vivo

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.

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

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.

Mice selected for extremes in stress reactivity reveal key endophenotypes of major depression: a translational approach

Clear evidence has linked dysregulated hypothalamus-pituitary-adrenocortical (HPA) axis function to the aetiology and pathophysiology of major depression (MD), as observed in the majority of patients. Increased stress reactivity and hyperactivity of the HPA axis seem characteristic for psychotic/melancholic depression, while the atypical subtype of depression has been connected with the opposing phenotypes. However, the underlying molecular-genetic mechanisms are poorly understood. In the present study, mouse lines selectively bred for extremes in stress reactivity (SR), i.e. presenting high (HR) or low (LR) corticosterone secretion in response to stressors, were used to characterise the molecular alterations on all levels of the HPA axis. Results were contrasted with clinical phenotypes of MD patients from the Munich Antidepressant Response Signature project, stratified according to their cortisol response in the Dex/CRH test. Distinct differences between HR and LR mice were found in the expression of HPA axis-related genes in the adrenals, pituitary and selected brain areas. Moreover, HR animals presented an enhanced adrenal sensitivity, increased stress-induced neuronal activation in the PVN and an overshooting Dex/CRH test response, whereas LR animals showed a blunted response in these paradigms. Interestingly, analogous neuroendocrine, morphometric, psychopathological and behavioural differences were observed between the respective high and low HPA axis responder groups of MD patients. Our findings suggests that (i) the SR mouse model can serve as a valuable tool to elucidate HPA axis-related mechanisms underlying affective disorders and (ii) a stratification of MD patients according to their HPA axis-related neuroendocrine function should be considered for clinical research and treatment.

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.

The potential role for corticosterone in the induction of cleft palate in mice after treatment with a selective NK-1 receptor antagonist, casopitant (GW679769B)

BACKGROUND

Casopitant is a potent and selective NK-1 receptor antagonist that has shown clinical efficacy in the prevention of chemotherapy-induced and postoperative-induced nausea and vomiting.

METHODS

In an embryo-fetal development study, pregnant mice were given vehicle (sterile water) or doses of 30, 100, or 300 mg/kg/day casopitant on Gestation Day (GD) 6 to 15. Fetuses were evaluated for external, visceral, and skeletal abnormalities on GD 18. In a follow-on study to investigate casopitant-induced hormonal changes during the developmental period for palate formation, pregnant mice were given vehicle (sterile water) or 300 mg/kg/day casopitant once daily on GD 6 to 13. Blood was collected on GD 13 at various time-points for measurement of plasma adrenocorticotropic hormone and corticosterone (CRT) concentrations.

RESULTS

There was no evidence of developmental toxicity in mice at 30 or 100 mg/kg/day but 9% of fetuses at 300 mg/kg/day had cleft palate. Mice are sensitive to glucocorticoid-induced cleft palates, and NK-1 antagonists are known to modulate the hypothalamic-pituitary-adrenal axis leading to increases in corticosterone. On GD 13, mean plasma adrenocorticotropic hormone levels at 300 mg/kg/day were elevated by approximately twofold from vehicle-treated levels at 1 hr post-dose and mean plasma CRT levels were elevated by 3, 5, and 10-fold at 0.5, 1, and 2 hr post-dose, respectively.

CONCLUSIONS

The increased level of CRT was in the range previously shown in the literature to cause cleft palates in mice and was likely the underlying mechanism behind casopitant-induced cleft palate in mice.

Glucocorticoids, chronic stress, and obesity

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

Deficient hippocampal c-fos expression results in reduced anxiety and altered response to chronic stress in female mice

Stress response is an important neuroendocrine function. Overt or prolonged stress hormone secretion can lead to various disease states. The hippocampus plays an important role in the negative feedback onto the major player in the stress response, the hypothalamo-pituitary-adrenal axis. The transcription factor c-Fos is activated in the hippocampus following a number of stressors, including restraint stress. To determine whether c-fos modulates stress response, we previously generated mutant mice carrying a hippocampal mutation of the c-fos gene. In the current study, we found that female mutant mice display lower anxiety-like behavior than female wild-type mice in the elevated plus maze, whereas male mice are apparently normal. While both male and female mutant mice exhibit normal diurnal glucocorticoid (CORT) production and normal responses to acute restraint stress, female mutant mice habituated faster than female wild-type mice in response to chronic restraint stress. These findings suggest that hippocampal c-fos plays a role in gender-dependent response to stress.

Molecular biology of the 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase gene family

The 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) isoenzymes are responsible for the oxidation and isomerization of Delta(5)-3beta-hydroxysteroid precursors into Delta(4)-ketosteroids, thus catalyzing an essential step in the formation of all classes of active steroid hormones. In humans, expression of the type I isoenzyme accounts for the 3beta-HSD activity found in placenta and peripheral tissues, whereas the type II 3beta-HSD isoenzyme is predominantly expressed in the adrenal gland, ovary, and testis, and its deficiency is responsible for a rare form of congenital adrenal hyperplasia. Phylogeny analyses of the 3beta-HSD gene family strongly suggest that the need for different 3beta-HSD genes occurred very late in mammals, with subsequent evolution in a similar manner in other lineages. Therefore, to a large extent, the 3beta-HSD gene family should have evolved to facilitate differential patterns of tissue- and cell-specific expression and regulation involving multiple signal transduction pathways, which are activated by several growth factors, steroids, and cytokines. Recent studies indicate that HSD3B2 gene regulation involves the orphan nuclear receptors steroidogenic factor-1 and dosage-sensitive sex reversal adrenal hypoplasia congenita critical region on the X chromosome gene 1 (DAX-1). Other findings suggest a potential regulatory role for STAT5 and STAT6 in transcriptional activation of HSD3B2 promoter. It was shown that epidermal growth factor (EGF) requires intact STAT5; on the other hand IL-4 induces HSD3B1 gene expression, along with IL-13, through STAT 6 activation. However, evidence suggests that multiple signal transduction pathways are involved in IL-4 mediated HSD3B1 gene expression. Indeed, a better understanding of the transcriptional factors responsible for the fine control of 3beta-HSD gene expression may provide insight into mechanisms involved in the functional cooperation between STATs and nuclear receptors as well as their potential interaction with other signaling transduction pathways such as GATA proteins. Finally, the elucidation of the molecular basis of 3beta-HSD deficiency has highlighted the fact that mutations in the HSD3B2 gene can result in a wide spectrum of molecular repercussions, which are associated with the different phenotypic manifestations of classical 3beta-HSD deficiency and also provide valuable information concerning the structure-function relationships of the 3beta-HSD superfamily. Furthermore, several recent studies using type I and type II purified enzymes have elegantly further characterized structure-function relationships responsible for kinetic differences and coenzyme specificity.

Psychoneuroendocrinological links between chronic stress and depression

OBJECTIVE

The goal of this report is to develop a comprehensive model, which integrates psychosocial and neurobiological aspects, for better understanding the link between chronic stress and mood disorders.

METHOD

A selective review of the relevant bibliography was conducted. The significant data were integrated with clinical and preclinical findings, particularly focusing on the effect of the hypothalamo-pituitary-adrenal (HPA) activity on the serotonergic neurotransmission in the CNS.

RESULTS

The reviewed data shows that chronic application of stress responses may lead to alterations in the regulation of the HPA system, and the resulting hypercortisolism may be reflected in various psychoneuroendocrinological processes, such as the observed in the serotonergic system, which was implicated in the origin and development of depression.

CONCLUSIONS

The analysis of the interactions between the different components of this process, suggests that normalization of the HPA system, either directly through psychopharmacologic strategies, or indirectly through psychotherapeutic approaches oriented to improve the cognitive appraisal of stressful situations, may provide us with more effective diagnostic, preventive, and therapeutic methods in the treatment of widespread anxiety and mood disorders.

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.

Pituitary-adrenal axis regulation in CRH-deficient mice

Corticotropin-releasing hormone (CRH)-deficient (knockout (KO)) mice demonstrate severely impaired adrenal responses to restraint, ether, and fasting, and lack the normal diurnal glucocorticoid (GC) rhythm. Here, we summarize recent studies determining the role of CRH in augmenting plasma adrenocorticotrophic hormone (ACTH) concentration after glucocorticoid withdrawal and pituitary-adrenal axis stimulation in the context of inflammation. Even though GC insufficient, basal pituitary proopiomelanocortin (POMC) mRNA, ACTH peptide content within the pituitary, and plasma ACTH concentrations are not elevated in CRH KO mice. POMC mRNA content in CRH KO mice increases following adrenalectomy, and this increase is reversed by GC, but not aldosterone, replacement. In marked contrast to the increase in POMC mRNA, plasma ACTH does not increase in the CRH KO mice following adrenalectomy. Administration of CRH to adrenalectomized CRH KO mice results in acute, robust ACTH secretion. Thus, loss of GC feedback can increase POMC gene expression in the pituitary, but CRH action is essential for increased secretion of ACTH into the circulation. While GC secretion is impaired in CRH KO mice after most stimuli, we have found near-normal GC responses to inflammation and systemic immune challenge. Studies in mice with CRH and IL-6 deficiency reveal that IL-6 is essential for activation of the pituitary-adrenal axis during inflammatory and other stressors in the absence of CRH.

Chronic intermittent stress does not differentially alter brain corticosteroid receptor densities in rats prenatally exposed to ethanol

Prenatal ethanol exposure produces hypothalamic-pituitary-adrenal (HPA) hyperresponsiveness to stressors. The present study tested the hypothesis that decreased corticosteroid receptor densities at HPA feedback sites may play a role in deficient feedback inhibition and the resultant HPA hyperresponsiveness that is observed following prenatal ethanol exposure. Brains of adult Sprague-Dawley rats from prenatal ethanol (E), pair-fed (PF) and ad libitum-fed control (C) treatment groups were examined for both mineralocorticoid receptor (MR; Type I) and glucocorticoid receptor (GR; Type II) densities using a cytosolic binding assay. Experiment 1 compared the effects of chronic intermittent stress (Stress Regimen I) and corticosterone (CORT) pellet implants on hippocampal corticosteroid receptor densities in control rats. Experiment 2 determined whether exposure to Stress Regimen I would differentially downregulate and whether adrenalectomy (ADX) would differentially upregulate hippocampal corticosteroid receptors in E compared with PF and C animals. Experiment 3 examined the effects of a modified chronic intermittent stress regimen (Stress Regimen II) on corticosteroid receptor densities at several HPA feedback sites (hippocampus, prefrontal cortex, hypothalamus, and anterior pituitary) in E compared with PF and C animals. CORT pellet implants significantly downregulated hippocampal GR and MR densities in control males and females. Exposure to Stress Regimen I produced downregulation of hippocampal GRs and MRs in males comparable with that produced with CORT pellet implants, and significant downregulation of hippocampal GRs in females across all prenatal treatment groups. This stress regimen also elevated basal plasma CORT levels without concurrent changes in plasma CBG levels, and increased relative adrenal weights in both males and females. In addition, upregulation of hippocampal GRs occurred at 7 days compared with 24 h following ADX in females that had previously been exposed to this stress regimen. Following exposure to Stress Regimen II, both the downregulation of hippocampal corticosteroid receptors and the increase in basal CORT levels in males and females appear to have been abolished by the changes in housing condition during the period of chronic stress. Importantly, prenatal ethanol exposure did not differentially alter GR or MR densities at any feedback site under non-stressed conditions. Exposure to Stress Regimen II, revealed subtle effects of prenatal treatments on hippocampal GRs however it is unlikely that these changes in corticosteroid receptor densities mediated the feedback inhibition deficits observed in E animals. Together, these data demonstrate that: (1) a relatively mild intermittent stress regimen can increase basal CORT levels and downregulate hippocampal corticosteroid receptor densities (2) a seemingly small change in housing conditions during stress appears to eliminate both receptor downregulation and increase in basal CORT levels and (3) decreased corticosteroid receptor densities at HPA feedback sites in the brain do not appear to underlie the HPA hyperresponsiveness observed in E animals.

Animal models of CRH deficiency

Corticotropin-releasing hormone (CRH), the major regulator of hypothalamic-pituitary-adrenal (HPA) axis, was first isolated due to its ability to stimulate the release of adrenocorticotropic hormone from the anterior pituitary. Later, it was also found to have also a wide spectrum of actions within the central nervous system and the periphery. Studies with pharmacological administration of this peptide and/or antagonists and antibody neutralization techniques have yielded important information concerning the physiological relevance of CRH. The development of CRH knockout mice (CRH KO) has been an important tool for addressing the physiologic and pathologic roles of CRH. This review describes the phenotype of CRH-deficient mice, as well as the use of this model to study the roles of CRH on fetal development and postnatal life. The role of CRH in prenatal development and postnatal regulation of the HPA axis, in activation of the reproductive system during stress, and in modulation of the immune function will be discussed. The review concludes with a comparison of CRH KO mice with other models of CRH deficiency.

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.

Fetal sheep pituitary proopiomelanocortin in late gestation: effect of bilateral lesions of the paraventricular nucleus on regional and cellular messenger ribonucleic acid levels

Previous experiments have clearly indicated that the successful completion of ovine gestation is dependent upon fetal adrenocortical maturation and the associated preterm rise in fetal plasma cortisol. The purposes of this study were to: 1) examine pituitary POMC messenger RNA (mRNA) levels during normal fetal development; and 2) examine the effects of bilateral lesion of the fetal paraventricular nucleus (PVN) on levels and spatial distribution of pituitary POMC mRNA. Pituitary glands were collected from intact fetal sheep of four gestational ages [100-107 days gestational age (dga), n = 8; 117-121 dga, n = 9; 126-130 dga, n = 9; 144-147 dga, n = 8]. Lesions of the PVN (PVN Lx; n = 4) or sham lesions (Sham; n = 5) were performed at 118-122 dga. Pituitary glands from PVN Lx and Sham fetuses were collected at 139-142 dga (term approximately 147 dga). POMC mRNA levels were determined by in situ hybridization. POMC transcript levels were determined by both regional analysis (20x magnification) and analysis of individual corticotropes (400x magnification). There was no difference among gestational age groups in superior anterior pituitary (AP) POMC mRNA levels determined by regional or cellular analysis. POMC mRNA levels were significantly greater in the inferior AP at 144-147 dga, compared with other gestational ages, using regional analysis (P = 0.003) or analysis of individual corticotropes (P < 0.01). POMC mRNA levels in the neurointermediate lobe in 126- to 130-dga fetuses were significantly greater than those in younger fetuses (P = 0.005) but not those in 144- to 147-dga fetuses. There was no difference in POMC mRNA levels in the superior AP between PVN Lx and Sham, using regional analysis or analysis of individual corticotropes. In the inferior AP, there was a significant decrease in POMC mRNA levels in PVN Lx, compared with Sham, using both regional analysis (P < 0.01) and cellular analysis (P < 0.01). There was no difference in POMC mRNA levels in the neurointermediate lobe as the result of bilateral PVN Lx. Our findings support that basal AP POMC mRNA levels are heterogenously distributed in the ovine fetal AP, with POMC mRNA levels in the inferior AP being significantly greater than in superior AP, by 144-147 dga. We further found that the higher POMC mRNA levels in the inferior AP reflect significantly higher corticotrope POMC transcripts and not simply a greater density of corticotropes in this AP region. The increase in POMC mRNA levels at 144-147 dga in the inferior AP seems unrelated to the onset of adrenocortical maturation (at approximately 125-130 dga). Finally, we report that increase in corticotrope POMC transcripts during late gestation in the inferior AP requires an intact PVN.

Bilateral adrenal enucleation-induced changes in adenohypophyseal pro-opiomelanocortin (POMC)-related peptides synthesis and secretion: a comparative study with adrenalectomized rats

The aim of the present study was to elucidate the modulatory effect of transient changes in endogenous glucocorticoids, occurring after bilateral adrenal enucleation (ENUC), on anterior pituitary (AP) proopiomelanocortin (POMC)-derived peptides synthesis and output in rats. For this purpose, adult female rats were either bilaterally ENUC, adrenalectomized (ADX), or sham-operated (SHAM) and killed by decapitation 2, 7, 14, and 21 days after surgery. Trunk blood was collected for measurements of ACTH, beta-endorphin (beta-END) and corticosterone (B) concentrations; APs were quickly dissected for the determination of ACTH, beta-endorphin (beta-END)-like (beta-END-LI) and gamma 3-MSH contents and adrenal glands were removed and submitted to histological study. The results indicate that ENUC and ADX increased AP POMC-related peptides synthesis and release in association with changes in the AP processing of peptides belonging to the N-terminal (gamma 3-MSH), mid (ACTH) and C-terminal (beta-LPH/ENDs) portions of POMC. While ADX abolished plasma B levels, ENUC induced a transient (day 2) decrease in plasma B concentrations which returned to SHAM levels at 7 days after surgery. These data tallied with the histological observations carried out, indicating a time-dependent regenerative process of the adrenal which was completed by three weeks after ENUC. There was a different pattern in plasma ACTH and beta-END levels between ENUC and ADX; maximal plasma peptide levels were found 7-14 days after ENUC, then falling down to SHAM values at 21 days post ENUC. Conversely, there was a constant increment in plasma peptide levels up to 21 days after ADX. At 2 days after both ENUC and ADX all peptides measured in the AP were lower than SHAM values, thus reflecting a rapid corticotrope secretion. Thereafter, 7 or more days after surgery, AP peptide content in ADX rats increased, in a time-related fashion, up to 21 days after surgery. Only beta-END-LI showed a similar AP content to that of the SHAM group, thereafter indicating a preferential cleavage of POMC to beta-END long after ADX (21 days). ENUC rats showed increased AP POMC peptides content throughout the whole time, and it was significantly different from SHAM and ADX values 14 days post-surgery. Interestingly, we found an increment in AP gamma 3-MSH, a peptide which is preferentially synthesized in the intermediate lobe of the rat pituitary, in both ENUC and ADX situations. Our results further indicate that: 1) glucocorticoids, from regenerating adrenal origin, induce a fast negative feedback mechanism on AP secretion, and 2) there might be a delayed inhibitory action of newly synthesized corticosteroids on higher levels of the central nervous system. The lack of glucocorticoids (ADX) clearly corroborates a persistent enhancement of AP POMC-related peptides synthesis and secretion. The differences in AP processing of POMC between ENUC and ADX might be due to qualitative/quantitative changes in hypothalamic ACTH secretagogues output.

Study of ACTH-immunoreactive cells after transplantation of neonatal adrenal glands into adult adrenalectomized rats

The morphological and morphometrical features of the ACTH-immunoreactive cells of the anterior pituitary gland were compared between adult rats with intact adrenals, adult rats without adrenals, and adult rats with regenerated adrenocortical nodules. The removal of the adrenal glands produced a great increase in the number and size of the cytoplasmic processes of the ACTH-immunoreactive cells that sometimes establish contact with those of the neighbouring immunoreactive cells. In addition, in this experimental group, the ACTH-cell percentage and the mean cell, cytoplasmic, and nuclear areas were greatly increased in comparison with those found in control animals. Several regenerated adrenocortical nodules were observed in the small-bowel segment of rats transplanted with neonatal adrenal glands. The percentage and morphometrical values of the pituitary ACTH-reactive cells of this experimental group, although slightly increased relative to the rats with intact adrenals, were greatly decreased in relation to those of the adrenalectomized animals. It is concluded that the neonatal adrenal tissue regenerated after its transplantation into adult rats, a process that modified the percentage, the morphological characteristics, and the morphometrical values of the ACTH-immunoreactive cells of the adrenalectomized rats.

Sex and strain variability in the rat hypothalamo-pituitary-adrenal (HPA) axis function

In the present investigation, we examined the influence of both genetic background and sex factors in the rat hypothalamo-pituitary-adrenal (HPA) axis function under both basal and post adrenalectomy (ADX) conditions. For these purposes adult female and male rats, from Sprague-Dawley (S-D), Fischer (F344/N), Lewis (LEW/N) and Buffalo (BUF) strains, were decapitated in basal condition or several (2, 7 and 14) days after ADX. Plasma stress hormones levels and adrenal corticosterone (B) concentration as well as peptide (ACTH, CRH and vasopressin, AVP) content in different tissues (anterior pituitary, AP; medial basal hypothalamus, MBH), were then evaluated by specific assays. Our results indicate that: a) despite no sex- and strain-related differences in AP ACTH and MBH ACTH secretagogues in basal condition, there exits a clear sexual dimorphism in plasma ACTH levels as well as in both plasma and adrenal B concentrations, with values significantly higher in females than in males, regardless the strain; b) ADX abolished plasma B levels and increased AP ACTH output in a time-dependent fashion up to the 14th day post surgery; c) AP ACTH content decreased 2 days after ADX, except in BUF female rats, thereafter tending to either recover or increase sham values by two weeks post ADX; d) ADX decreased MBH CRH at all periods studied, except in BUF female animals on day 14; e) ADX clearly diminished MBH AVP only in S-D rats, and f) a sexual dimorphism was also found in AP ACTH in 7-day-ADX S-D rats and in 14-day-ADX S-D and F344/N animals; also, a dimorphic pattern in MBH CRH was found in 7-day-ADX S-D as well as in 14-day-ADX F344/N and LEW/N rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Corticotroph, somatotroph and mammotroph cell kinetics in the postnatal infant female rat

The aim of this work was to detect if hypothalamic-pituitary maturation was accompanied by significant proliferation changes in differentiated pituitary cell pools. For this purpose, pituitary corticotroph (Ct), mammotroph (Mt) and somatotroph (St) proliferation activities were scanned in intact female rats during the postnatal (P) period (1-35 postnatal days). The techniques of tritiated thymidine labelling, immunostaining and autoradiography were combined to visualize DNA synthesis of hormone containing cells. Immunoreactive cell densities were measured using image analysis, and double labelled cells were counted. Corticotroph proliferation activity increased significantly on day P12, followed by an increase in the Ct proportion on days P13-14. This is the first observation of a spontaneous change of corticotroph proliferation at the end of the stress nonresponsive period. The mammotroph density and proliferation rate increased gradually during postnatal maturation, until the Mt pool overran other cell types of the female hypophysis on day 35. The somatotroph pool was the most numerous until day P20; the proliferation rate remained constant while St proportions increased reaching a plateau between days P13 and 20, then decreased to the adult level. Each cell type examined showed a characteristic, individual density and proliferation pattern.

The effects of chronic glucocorticoid excess, adrenalectomy and stress on neuropeptide Y in individual rat hypothalamic nuclei

Several lines of evidence indicate a role for neuropeptide Y (NPY) in the modulation of the corticotroph axis. In two separate studies reported here, the concentrations of NPY and noradrenaline (NA), as well as corticotropin-releasing factor (CRF) and arginine vasopressin (AVP), were measured in extracts of individual rat hypothalamic nuclei after various manipulations producing either a state of chronic glucocorticoid excess or depletion, and also following repeated restraint stress. Alterations induced in the activity of hypothalamic neurones were inferred from the respective changes in these concentrations. 12 days after bilateral adrenalectomy (ADX), NPY levels were decreased by 24% in the arcuate nucleus (ARC) and 23% in the paraventricular nucleus (PVN, p < 0.05 vs controls). Forced immobilization of the animals for 4 h each day for 9 consecutive days (repeated stress) also decreased NPY content of the ARC by 25% (p < 0.01 vs controls), an effect blocked by the administration of glucocorticoids. NA levels in both hypothalamic nuclei were unaffected by repeated stress or ADX. Administration of glucocorticoids in the first of these studies induced decreases in NA levels by 15% and 25% in the ARC and PVN respectively (p < 0.05 vs controls). However, in subsequent experiments no significant effect of glucocorticoids on NA was observed. Our results demonstrate that the activity of the hypothalamic NPY-ergic neurones is modulated by glucocorticoids and by chronic stress. They also suggest that brainstem catecholaminergic and hypothalamic NPY-ergic neurones are differentially affected by altered glucocorticoid concentrations or by chronic stress, possibly in a stimulus-specific way.

Animals, care, and nursing research

These multiple measures of health were positive and encouraging. It was possible to integrate nursing actions into protocols for this long-term study. It is the author's belief that by strengthening the humane care and handling of animals the study results can more accurately reflect the discipline of nursing. Through such efforts, discipline-specific findings could be strengthened when an animal model is used, particularly in long-term or survival nursing research.

Is vasopressin preferentially released from corticotropin-releasing factor and vasopressin containing nerve terminals in the median eminence of adrenalectomized rats?

The effects of adrenalectomy (ADX) on the amounts of immunoreactive corticotropin-releasing factor (CRFi) and arginine vasopressin (AVPi) that are stored in the zona externa of the median eminence (ZEME) were investigated by means of quantitative immunocytochemistry. Although ADX of male Wistar rats for 1 week or 4 weeks did not affect CRFi in the ZEME as compared to sham-operated or intact controls, AVPi showed a progressive accumulation. The ratio of AVPi over CRFi in the ZEME had already increased 1 day after ADX. However, it should be noted that the exact changes in CRFi and AVPi as measured by radioimmunoassay and/or quantitative immunocytochemistry were dependent on the substrain of rats used. The secretion rate of CRFi and AVPi was estimated in 1 week and 4 week ADX rats, by measuring the disappearance rate of CRFi and AVPi from the ZEME after blockade of fast axonal transport, by a low non-toxic dose of colchicine (5 micrograms per rat). In contrast to intact rats, where this dose of colchicine did not affect CRFi or AVPi in the ZEME, ADX rats showed a progressive depletion of the CRFi and AVPi stores as measured 2.5 and 5 h later. In 1 week ADX rats, CRFi and AVPi both disappeared at a rate of 7% to 8% of their stores per hour. In contrast, after 4 weeks of ADX the fractional disappearance rates of CRFi and AVPi were different and were 3% and 8% of the content per hour, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Dysregulation of the Hypothalamus-Pituitary-Adrenal Axis in Male and Female, Genetically Obese (ob/ob) Mice

The autosomal, recessive obesity of ob/ob mice is associated with hypercorticosteronemia and amelioration of most symptoms of obesity following adrenalectomy. Increased adrenocorticotropic hormone (ACTH) secretion has been hypothesized on the basis of several reports of higher pituitary ACTH content in ob/ob mice compared to lean littermates. However, the only measurement of ACTH blood concentration found lower levels in ob/ob mice than in leans suggesting that hypercorticosteronemia might result solely from an enhanced adrenal response to ACTH and also suggesting that the ob/ob's elevated pituitary ACTH content might be due to decreased ACTH secretion rather than increased ACTH synthesis. In our study, basal serum ACTH levels were higher in ob/ob males and females compared to sex-matched lean littermates. Anterior pituitary ACTH synthesis was also elevated as indicated by increased content of ACTH and proopiomelanocortin mRNA in obese mice of both sexes; however hypothalamic corticotropin-releasing factor content was not different in lean and obese mice. Basal serum ACTH and corticosterone (CS) levels showed normal circadian rhythm in both phenotypes and sexes, but the circadian increase in CS level was much greater in obese mice than in leans despite equal serum ACTH increases in the two phenotypes. Ether stress at both peak and trough of the circadian rhythm also stimulated much larger serum CS increases in obese mice even though ACTH increases were again equal in the two phenotypes. Taken together, these results strongly indicate that ob/ob mice have increased synthesis and secretion of pituitary ACTH despite the presence of chronically elevated serum CS. This hyperactivity of the hypothalamo-pituitary-adrenal axis appears to be most pronounced in ob/ob females since pituitary ACTH content was equal in obese males and females despite much higher circulating CS levels in the females. Furthermore, the results also indicate an enhanced response to ACTH by the adrenal cortex of the obese mouse. Thus, ob/ob mice exhibit abnormal hypothalamo-pituitary-adrenal axis function with hyperactivity occurring at the level of pituitary ACTH synthesis/secretion as well as at the level of adrenocortical response to ACTH.

Identification of DNA elements cooperatively activating proopiomelanocortin gene expression in the pituitary glands of transgenic mice

The proopiomelanocortin (POMC) gene is highly expressed in adult mouse pituitary anterior lobe corticotrophs and intermediate lobe melanotrophs. To identify the DNA elements important for this tissue-specific expression, we analyzed a series of POMC reporter genes in transgenic mice. A DNA fragment containing rat POMC 5'-flanking sequences from -323 to -34 recapitulated both basal pituitary cell-specific and hormonally stimulated expression in adult mice when fused to a heterologous thymidine kinase promoter. Developmental onset of the reporter gene expression lagged by 1 day but otherwise closely paralleled the normal ontogeny of murine POMC gene expression, including corticotroph activation at embryonic day 14.5 (E14.5) followed by melanotroph activation at E15.5 to E16.5. AtT20 corticotroph nuclear protein extracts interacted with three specific regions of the functional POMC promoter in DNase I protection assays. The positions of these protected sites were -107 to -160 (site 1), -182 to -218 (site 2), and -249 to -281 (site 3). Individual deletions of these footprinted sites did not alter transgene expression; however, the simultaneous deletion of sites 2 and 3 prevented transgene expression in both corticotrophs and melanotrophs. Electrophoretic mobility shift and Southwestern (DNA-protein) assays demonstrated that multiple AtT20 nuclear proteins bound to these footprinted sites. We conclude that the sequences between -323 and -34 of the rat POMC gene promoter are both necessary and sufficient for correct spatial, temporal, and hormonally regulated expression in the pituitary gland. Our data suggest that the three footprinted sites within the promoter are functionally interchangeable and act in combination with promoter elements between -114 and -34. The inability of any reporter gene construction to dissociate basal and hormonally stimulated expression suggests that these DNA elements are involved in both of these two characteristics of POMC gene expression in vivo.

Identification of DNA elements cooperatively activating proopiomelanocortin gene expression in the pituitary glands of transgenic mice

The proopiomelanocortin (POMC) gene is highly expressed in adult mouse pituitary anterior lobe corticotrophs and intermediate lobe melanotrophs. To identify the DNA elements important for this tissue-specific expression, we analyzed a series of POMC reporter genes in transgenic mice. A DNA fragment containing rat POMC 5'-flanking sequences from -323 to -34 recapitulated both basal pituitary cell-specific and hormonally stimulated expression in adult mice when fused to a heterologous thymidine kinase promoter. Developmental onset of the reporter gene expression lagged by 1 day but otherwise closely paralleled the normal ontogeny of murine POMC gene expression, including corticotroph activation at embryonic day 14.5 (E14.5) followed by melanotroph activation at E15.5 to E16.5. AtT20 corticotroph nuclear protein extracts interacted with three specific regions of the functional POMC promoter in DNase I protection assays. The positions of these protected sites were -107 to -160 (site 1), -182 to -218 (site 2), and -249 to -281 (site 3). Individual deletions of these footprinted sites did not alter transgene expression; however, the simultaneous deletion of sites 2 and 3 prevented transgene expression in both corticotrophs and melanotrophs. Electrophoretic mobility shift and Southwestern (DNA-protein) assays demonstrated that multiple AtT20 nuclear proteins bound to these footprinted sites. We conclude that the sequences between -323 and -34 of the rat POMC gene promoter are both necessary and sufficient for correct spatial, temporal, and hormonally regulated expression in the pituitary gland. Our data suggest that the three footprinted sites within the promoter are functionally interchangeable and act in combination with promoter elements between -114 and -34. The inability of any reporter gene construction to dissociate basal and hormonally stimulated expression suggests that these DNA elements are involved in both of these two characteristics of POMC gene expression in vivo.

Ontogeny of expression and glucocorticoid regulation of the arginine vasopressin gene in the rat hypothalamic paraventricular nucleus

Corticotropin-releasing factor and arginine vasopressin (AVP) are the two major hypothalamic factors that regulate anterior pituitary adrenocorticotropin secretion during stress. We have previously reported that the expression of the corticotröpin-releasing factor gene in the hypothalamus and its regulation by glucocorticoids were not mature during the first week of life in the rat, i.e. during the stress non-responsive period. In this report, we studied the ontogeny of expression of the AVP gene in the hypothalamic paraventricular nucleus in rats using in situ hybridization. AVP mRNA was detected as early as day 20 of gestation (E20) both in the parvocellular and the magnocellular portion of the paraventricular nucleus. The levels of expression of the AVP gene increased steadily from E20 to the third day after birth (P3) and remained stable from P3 to P14. Bilateral surgical adrenalectomy induced an increase in AVP mRNA levels in the parvocellular portion of the paraventricular nucleus, but not in the magnocellular portion, in both 7-day-old and 14-day-old rats, suggesting that the glucocorticoid regulation of the AVP synthesizing neurons of the paraventricular nucleus is mature in the developing postnatal rat.

Alteration of pituitary-adrenal responses to adrenalectomy by the immunological targeting of CRF neurons

We previously demonstrated that cellular toxins added to a cytotoxic IgG2a monoclonal antibody to corticotropin releasing factor (CRF-MAb) may specifically penetrate some hypothalamic CRF neurons, after central injection near the paraventricular nuclei. We attempt here to evaluate the consequential effects on the CRF neurons functioning. Such a toxic mix, 4 weeks after its central injection, caused a marked reduction (66%) of the chronic adrenocorticotropic hormone (ACTH) release in response to a bilateral adrenalectomy (7th day). This change was accompanied by a reduction in the CRF concentration (43%) measured in the median eminence. We concluded that specific internalization of toxins, by the way of CRF-MAb, leads to a long-term dysregulation of the CRF synthesis and/or neuronal transport.

Multifactorial regulation of the hypothalamic-pituitary-adrenal axis during development

The hypothalamic-pituitary-adrenal system shows an overall diminished responsiveness throughout ontogeny. Thus, during this period, the sensitivity of the adrenal gland to ACTH is markedly reduced. Furthermore, basal and stress-induced concentrations of corticosterone (CORT), ACTH and hypothalamic secretagogues remain at very low levels. Both structural immaturity and active inhibitory processes appear to underlie this overall hyporesponsiveness. The available data indicate that the characteristic developmental pattern of the HPA system results from multiple regulatory factors acting in conjunction at various levels of the axis. The primary rate-limiting steps, however, are probably at the brain and adrenal levels. The ultimate "goal" appears to be to keep CORT levels within the narrow range of concentrations required for normal development.

Pituitary corticotrophs proliferate temporarily after adrenalectomy

Relationship of corticotroph proliferation answer and survival time after adrenalectomy was examined. Corticotroph proliferation rates were detected by short-term 3H-thymidine radiolabeling, then ACTH immunostaining and autoradiography. Effect of adrenalectomy on corticotroph proliferation rate was examined in vivo and an elevation was demonstrated first on the second postoperative day, increasing on the third-fourth day postoperatively and then decreasing. Effects of different secretagogues on corticotroph proliferation were examined in short-term pituitary monolayer cultures taken from ADX rats. CRF and Forskolin treatment potentiated corticotroph proliferation in cultures taken from adrenalectomized rats, but not in the controls. We suggest that croticotroph proliferation is stimulated via the cAMP-proteinkinase A pathway, while adrenalectomy plays a permissive role.

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.

Alcohol exposure in utero results in diminished T-cell function and alterations in brain corticotropin-releasing factor and ACTH content

The long-term teratogenic effects of prenatal ethanol exposure during the last week of gestation on immune responsiveness and levels of pituitary ACTH and hypothalamic corticotropin-releasing factor (CRF) were examined in Sprague-Dawley rats. Immune responsiveness was measured by T-lymphocyte proliferation in response to mitogenic stimulation with Con A (3 micrograms/ml) in spleen and thymus cells of 21-old-day male rats who were exposed to alcohol in utero. The proliferative response was 8-fold lower in spleen and twofold lower in thymus cells from alcohol-exposed animals compared to responses measured in control rats. Thymus weight was significantly smaller at birth in alcohol exposed males, but significantly larger at 21 days of age compared to controls. Alterations in the content of ACTH and CRF, hormones, known to be direct or indirect modulators of immune responsiveness, were also observed in alcohol exposed males. Hypothalamic content of CRF and pituitary content of ACTH were significantly lower in alcohol exposed males on postnatal Day 1, but hypothalamic ACTH content was significantly higher compared to controls. These results indicate that alcohol exposure during the last week of gestation can produce alterations of the hypothalamic-pituitary-adrenal function in addition to teratogenic effects on the immune system which have been previously observed only with a much longer alcohol exposure regimen.

ACTH, prolactin, corticosterone and pituitary cyclic AMP responses to repeated stress

The present experiment was conducted to determine whether the plasma hormonal and pituitary cyclic AMP responses observed following a single exposure to an acute stressor would diminish following reexposures to the same stressor. Fifteen-min stress exposures (forced running) were separated by 45-min recovery periods. Separate groups of control and stressed animals were sacrificed before and after each of four 15-min stress periods and after each recovery period. The first exposure to 15 min of forced running raised plasma ACTH, corticosterone and pituitary cyclic AMP levels approximately 6-fold and more than tripled levels of plasma prolactin. Plasma ACTH and pituitary cyclic AMP responses to the second, third and fourth stress exposures were very similar to the responses to the first stress exposure, and levels of these substances returned to prestress levels during each 45-min recovery period. Plasma prolactin responses to the four stress sessions were somewhat variable but no significant trend among the responses was seen. Plasma prolactin levels also returned to prestress levels between stress exposures. Corticosterone levels were similar following each of the four stress sessions but levels remained elevated compared to prestress levels between stress exposures. These data suggest that pituitary responses to acute stress are rapid, that return to prestress levels is also rapid, with the exception of corticosterone, and that repeated responses of the same magnitude may be evoked when stressors are separated by short recovery periods.

Isolated pituitary cells: glucocorticoids do not rapidly suppress ACTH secretion in response to CRF

The possibility of a direct rapid suppressive effect of glucocorticoids on stimulated adrenocorticotropic hormone (ACTH) release was investigated in perifused normal pituitary cells attached to microcarriers. Forty-eight hours after attachment to Cytodex beads, cells were transferred to two columns (one experimental, one control), perifused at a rate of 300-350 microliters/min, and equilibrated for 3 h. Either rat or ovine corticotropin-releasing factor (CRF; 2 nM) were used to stimulate ACTH release, and fractions collected every 5 min were assayed for immunoreactive ACTH. Concomitant treatment with CRF and glucocorticoids (dexamethasone 100 nM or corticosterone 1 microM), or glucocorticoid pretreatment for up to 2 h, did not affect the release of ACTH occasioned by repetitive 5-min exposures to CRF at 30-min intervals. In addition, when ovine CRF was given as two 30-min infusions 1 h apart, neither concomitant steroid administration nor steroid pretreatment for 90 min affected the release of ACTH compared with controls. The lack of rapid steroid inhibition was not an artifact of enzymatic dispersion or microcarrier attachment, since no rapid inhibitory response was seen with acutely perifused rat anterior pituitary quarters. We thus conclude that in vitro rapid inhibitory effects of glucocorticoids on ACTH release do not occur at the level of the pituitary. Accordingly such action in vivo presumably reflects acute steroid-induced effects on the hypothalamus or higher centers.

Steroid inhibition of canine ACTH: in vivo evidence for feedback at the corticotrope

We infused submaximal feedback doses of either dexamethasone (DEX; 0.1 microgram.kg-1.min-1) or corticosterone and cortisol (B+F; 1.5 micrograms.kg-1.min-1) intravenously for 40 min into conscious dogs and measured the adrenocorticotropic hormone (ACTH) responses to hypoglycemia induced by insulin (0.1 U/kg) or to ovine corticotropin-releasing factor (oCRF; 1 microgram/kg); both agents were injected at 120 min. The dose of DEX was chosen to produce suppression of the ACTH response to oCRF equivalent to that produced by B+F. The purpose of the study was to determine 1) whether CRF- and hypoglycemia-induced ACTH secretion are equally inhibited by glucocorticoid treatment and 2) whether DEX and B+F have differential effects in the inhibition of stress-induced ACTH secretion. We found that peak ACTH responses to hypoglycemia and CRF were equally inhibited by DEX (36 +/- 6 and 52 +/- 9%, respectively). The peak ACTH responses to hypoglycemia and CRF were also equally inhibited after B+F infusion (45 +/- 13 and 65 +/- 5%, respectively). There was no significant interaction between the steroid administered and the stimulus given in controlling the ACTH response (by 2-way analysis of variance). The results suggest that pituitary feedback is of primary importance in suppression of canine ACTH secretion by delayed feedback and that the natural and synthetic steroids both act at this site.

Vasopressin and adrenalectomy-induced sensitivity to morphine

Arginine vasopressin, vasopressin antiserum and a specific vasopressin pressor antagonist were injected intracerebroventricularly into adrenalectomized rats before morphine-induced antinociception was tested. In these experiments we have exploited previous findings which showed that the antinociceptive effect of opioids was potentiated after adrenalectomy; rats that were adrenalectomized in the morning under basal resting conditions of the pituitary-adrenal system displayed significantly higher response latencies after morphine administration than rats adrenalectomized in the evening. These effects were measured 7 days after adrenalectomy. The same conditions were used in this study. Both, the vasopressin antiserum and the vasopressin antagonist abolished the morning adrenalectomy-induced hypersensitivity to centrally injected morphine and were not effective when administered to rats that had been adrenalectomized in the evening. The reverse was observed after intraventricular administration of vasopressin. The peptide significantly raised the sensitivity to morphine-induced antinociception of rats that had been adrenalectomized in the evening whereas it did not affect antinociception in animals that had been adrenalectomized in the morning. Vasopressin levels determined by radioimmunoassay in the cerebrospinal fluid were significantly higher in adrenalectomized animals. We propose that vasopressin is a critical neuropeptide factor involved in the adrenalectomy-induced hypersensitivity to morphine antinociception.

Effects of adrenalectomy and glucocorticoids on the peptides CRF-41, AVP and oxytocin in rat hypophysial portal blood

1. The effects of adrenalectomy (3 weeks) and dexamethasone (3 h) treatment on the release of corticotrophin-releasing factor-41 (CRF-41), arginine vasopressin (AVP), oxytocin (OT), adrenocorticotrophin (ACTH) and corticosterone were studied in adult female Wistar rats. 2. The animals were anaesthetized with sodium pentobarbitone which, as assessed by the effects on the circadian rhythm of plasma ACTH and corticosterone, appeared to be a better anaesthetic than either urethane or alphaxalone for studies on the hypothalamic-pituitary-adrenal system. 3. Adrenalectomy increased the concentrations of ACTH in peripheral plasma and the output of CRF-41 and AVP into hypophysial portal plasma. 4. Dexamethasone administered to adrenalectomized rats significantly reduced the concentration of ACTH in peripheral plasma and the amount of AVP released into portal plasma. However, dexamethasone did not affect the output of CRF-41 into portal blood. 5. The output of OT into portal plasma was unaffected by either adrenalectomy or dexamethasone treatment. 6. Dexamethasone administered to adrenalectomized rats reduced significantly the ACTH response to CRF-41. 7. These results show that the feed-back action of glucocorticoids is mediated by two mechanisms. The increased release of ACTH which follows adrenolectomy [corrected] is produced predominantly by an increased release of both CRF-41 and AVP into hypophysial portal blood. The intermediate negative feed-back of glucocorticoids is produced by a reduction in the output of AVP but not CRF-41 into portal blood and, as well, by a significant reduction in the responsiveness of the anterior pituitary gland to CRF-41.

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.