Prenatal exposure to dexamethasone alters hippocampal drive on hypothalamic-pituitary-adrenal axis activity in adult male rats

Glucocorticoids are essential for normal hypothalamic-pituitary-adrenal (HPA) axis activity; however, recent studies warn that exposure to excess endogenous or synthetic glucocorticoid during a specific period of prenatal development adversely affects HPA axis stability. We administered dexamethasone (DEX) to pregnant rats during the last week of gestation and investigated subsequent HPA axis regulation in adult male offspring in unrestrained and restraint-stressed conditions. With the use of real-time PCR and RIA, we examined the expression of regulatory genes in the hippocampus, hypothalamus, and pituitary, including corticotropin-releasing hormone (CRH), arginine vasopressin (AVP), glucocorticoid receptors (GR), mineralcorticoid receptors (MR), and 11-beta-hydroxysteroid dehydrogenase-1 (11beta-HSD-1), as well as the main HPA axis hormones, adrenal corticotropic hormone (ACTH) and corticosterone (CORT). Our results demonstrate that the DEX-exposed group exhibited an overall change in the pattern of gene expression and hormone levels in the unrestrained animals. These changes included an upregulation of CRH in the hypothalamus, a downregulation of MR with a concomitant upregulation of 11beta-HSD-1 in the hippocampus, and an increase in circulating levels of both ACTH and CORT relative to unrestrained control animals. Interestingly, both DEX-exposed and control rats exhibited an increase in pituitary GR mRNA levels following a 1-h recovery from restraint stress; however, the increased expression in DEX-exposed rats was significantly less and was associated with a slower return to baseline CORT compared with controls. In addition, circulating levels of ACTH and CORT as well as hypothalamic CRH and hippocampal 11beta-HSD-1 expression levels were significantly higher in the DEX-exposed group compared with controls following restraint stress. Taken together, these data demonstrate that late-gestation DEX exposure in rats is associated with persistent changes in both the modulation of HPA axis activity and the HPA axis-mediated response to stress.     

Vasopressinergic regulation of the hypothalamic-pituitary-adrenal axis: implications for stress adaptation

In addition to its role on water conservation, vasopressin (VP) regulates pituitary ACTH secretion by potentiating the stimulatory effects of corticotropin releasing hormone (CRH). The pituitary actions of VP are mediated by plasma membrane receptors of the V1b subtype, coupled to calcium-phospholipid signaling systems. VP is critical for adaptation of the hypothalamic-pituitary-adrenal (HPA) axis to stress as indicated by preferential expression of VP over CRH in parvocellular neurons of the hypothalamic paraventricular nucleus, and the upregulation of pituitary VP receptors during stress paradigms associated with corticotroph hyperresponsiveness. V1b receptor mRNA levels and coupling of the receptor to phospolipase C are stimulated by glucocorticoids, effects which may contribute to the refractoriness of VP-stimulated ACTH secretion to glucocorticoid feedback. The data suggest that vasopressinergic regulation of the HPA axis is critical for sustaining corticotroph responsiveness in the presence of high circulating glucocorticoid levels during chronic stress.     

Gastric inhibitory polypeptide stimulates glucocorticoid secretion in rats, acting through specific receptors coupled with the adenylate cyclase-dependent signaling pathway.

Gastric inhibitory polypeptide (GIP) is a 42-amino acid peptide, belonging to the VIP-secretin-glucagon superfamily, some members of this group are able to regulate adrenocortical function. GIP-receptor mRNA has been detected in the rat adrenal cortex, but investigations on the effect of GIP on steroid-hormone secretion in this species are lacking. Hence, we have investigated the distribution of GIP binding sites in the rat adrenal gland and the effect of their activation in vivo and in vitro. Autoradiography evidenced abundant [125I]GIP binding sites exclusively in the inner adrenocortical layers, and the computer-assisted densitometric analysis of autoradiograms demonstrated that binding was displaced by cold GIP, but not by either ACTH or the selective ACTH-receptor antagonist corticotropin-inhibiting peptide (CIP). The intraperitoneal (IP) injection of GIP dose-dependently raised corticosterone, but not aldosterone plasma concentration: the maximal effective dose (10 nmol/rat) elicited a twofold increase. GIP did not affect aldosterone and cyclic-AMP release by dispersed zona glomerulosa cells. In contrast, GIP enhanced basal corticosterone secretion and cyclic-AMP release by dispersed inner adrenocortical cells in a concentration-dependent manner, and the maximal effective concentration (10(-7) M) evoked 1.5- and 2.4-fold rises in corticosterone and cyclic-AMP production, respectively. GIP (10(-7) M) did not display any additive or potentiating effect on corticosterone and cyclic-AMP responses to submaximal or maximal effective concentrations of ACTH. The corticosterone secretagogue action of 10(-7) M GIP was abolished by the protein kinase A (PKA) inhibitor H-89 (10(-5)M), and unaffected by CIP (10(-6)M). Collectively, these findings indicate that GIP exerts a moderate but statistically significant stimulatory effect on basal glucocorticoid secretion in rats, acting through specific receptors coupled with the adenylate cyclase/PKA-dependent signaling pathway.     

Mathematical modeling of the hypothalamic–pituitary–adrenal gland (HPA) axis,including hippocampal mechanisms

This paper presents a mathematical model of the HPA axis. The HPA axis consists of the hypothalamus, the pituitary and the adrenal glands in which the three hormones CRH, ACTH and cortisol interact through receptor dynamics. Furthermore, it has been suggested that receptors in the hippocampus have an influence on the axis.     

Age-related changes in the dog hypothalamic-pituitary-adrenocortical system: neuroendocrine activity and corticosteroid receptors.

Aging is associated with a progressive dysfunctioning of the hypothalamic-pituitary-adrenocortical (HPA) axis. We have studied the response of the HPA axis to stress and a hormonal (ovine corticotropin releasing factor (o-CRF) challenge in young (1.5-2 years) and aged (greater than 11 years) dogs. Compared to the young dogs, the aged animals displayed an increased basal concentration of both ACTH and cortisol. In addition, in response to an o-CRF challenge (1 microgram/kg i.v.) or an electric footshock (1 mA, alternatively on/off for 2 s) or immobilization (45 min) stress, the aged dogs showed significantly larger increments in ACTH and cortisol. Following the challenge test, the young and aged dogs reached their respective basal hormone levels at the same time, except for the o-CRF test. In the latter case, in contrast to the young controls, the aged dogs still showed an increased plasma cortisol level compared to the pre-challenge basal hormone concentration. Concerning the effect of aging on the brain and hypophyseal corticosteroid receptors, a selective decline (minus 50-75%) in mineralocorticoid receptor (MR) was observed in all measured brain regions (dorsal and ventral hippocampus, septum, hypothalamus) and anterior pituitary, whereas no change was found in brain glucocorticoid receptor (GR) number. The GR level in the anterior pituitary was even increased by 70%. In light of the role that MR and GR seem to play in the regulation of the HPA axis, it is concluded that the diminished MR number in the aged dog brain may underly the increased basal hormone levels and the elevated responsiveness of the HPA axis in these animals. The observation that the stress-induced elevations of cortisol and ACTH were not prolonged at senescence suggests that the GR-mediated negative feedback action of glucocorticoids is not altered, which is in line with the unchanged brain GR numbers in the aged dogs.     

The role of endogenous cannabinoids in the hypothalamo-pituitary-adrenal axis regulation: in vivo and in vitro studies in CB1 receptor knockout mice

Exogenous cannabinoids affect multiple hormonal systems including the hypothalamo-pituitary-adrenocortical (HPA) axis. These data suggest that endogenous cannabinoids are also involved in the HPA control; however, the mechanisms underlying this control are poorly understood. We assessed the role of endogenous cannabinoids in the regulation of the HPA-axis by studying CB1 receptor knockout (KO) and wild type (WT) mice. Basal and novelty stress-induced plasma levels of adrenocorticotropin (ACTH) and corticosterone were higher in CB1-KO than in WT mice. We investigated the involvement of the pituitary in the hormonal effects of CB1 gene disruption by studying the in vitro release of ACTH from anterior pituitary fragments using a perifusion system. Both the basal and corticotropin releasing hormone (CRH)-induced ACTH secretion were similar in CB1-KO and WT mice. The synthetic glucocorticoid, dexamethasone suppressed the CRH-induced ACTH secretion in both genotypes; thus, the negative feedback of ACTH secretion was not affected by CB1 gene disruption. The cannabinoid agonist, WIN 55,212-2 had no effects on basal and CRH-stimulated ACTH secretion by anterior pituitary slices. In our hands, the disruption of the CB1 gene lead to HPA axis hyperactivity, but the pituitary seems not to be involved in this effect. Our data are consistent with the assumption that endogenous cannabinoids inhibit the HPA-axis via centrally located CB1 receptors, however the understanding of the exact underlying mechanism needs further investigation.     

Effects of secretin and gastric inhibitory polypeptide on human pancreatic growth hormone-releasing factor(1-40)-stimulated growth hormone levels in the rat

Synthetic human pancreatic growth hormone-releasing factor containing 40 amino acids ([hpGRF (1-40)]-OH) significantly stimulated plasma growth hormone (GH) levels in both sodium pentobarbital and urethane anesthetized rats. Synthetic secretin, gastric inhibitory polypeptide (GIP), and glucagon significantly decreased plasma GH levels while synthetic vasoactive intestinal peptide (VIP) had no effect. Secretin and GIP also altered the in vivo plasma GH response to [hpGRF(1-40)]-OH. Whether this effect is the result of an interaction at the pituitary level or is due to an extra-pituitary effect of secretin and GIP awaits further study.     

Regulation and role of p21-activated kinase 3 by corticotropin-releasing factor in mouse pituitary

Corticotropin-releasing factor (CRF) is a major regulatory peptide in the hypothalamic-pituitary-adrenal (HPA) axis under stress conditions. In response to stress, CRF, produced in the hypothalamic paraventricular nucleus, releases adrenocorticotropic hormone (ACTH) from the anterior pituitary (AP). ACTH in turn stimulates the release of glucocorticoid from the adrenal glands. Glucocorticoid then inhibits hypothalamic production of CRF and pituitary production of ACTH. Mice lacking a functional gene for CRF (CRF KO) showed severe impairment of the HPA axis, indicating that CRF is required for its regulation. We applied oligonucleotide microarray analysis to the AP of CRF KO to identify gene expression induced by CRF. Twenty-four genes showed less than 60% expression in CRF KO compared with normal mice. Real-time PCR analysis revealed that p21-activated kinase 3 (Pak3), prohormone convertase type 1 (PC1), and CRF-binding protein (BP) mRNA expression levels were increased by CRF in AP cells. Both Pak3 and PC1 were also increased by dexamethasone in AP cells, while CRF-BP mRNA levels were reduced. Therefore, both Pak3 and PC1 mRNA levels would be regulated by both CRF and glucocorticoids. Pak3 knockdown inhibited CRF-induced cell viability in AtT-20 cells, suggesting the important role of Pak3 in the proliferation of corticotrophs.     

Gonadal Steroid Hormone Receptors and Sex Differences in the Hypothalamo-Pituitary-Adrenal Axis

The rapid activation of stress-responsive neuroendocrine systems is a basic reaction of animals to perturbations in their environment. One well-established response is that of the hypothalamo-pituitary-adrenal (HPA) axis. In rats, corticosterone is the major adrenal steroid secreted and is released in direct response to adrenocorticotropin (ACTH) secreted from the anterior pituitary gland. ACTH in turn is regulated by the hypothalamic factor, corticotropin-releasing hormone. A sex difference exists in the response of the HPA axis to stress, with females reacting more robustly than males. It has been demonstrated that in both sexes, products of the HPA axis inhibit reproductive function. Conversely, the sex differences in HPA function are in part due to differences in the circulating gonadal steroid hormone milieu. It appears that testosterone can act to inhibit HPA function, whereas estrogen can enhance HPA function. One mechanism by which androgens and estrogens modulate stress responses is through the binding to their cognate receptors in the central nervous system. The distribution and regulation of androgen and estrogen receptors within the CNS suggest possible sites and mechanisms by which gonadal steroid hormones can influence stress responses. In the case of androgens, data suggest that the control of the hypothalamic paraventricular nucleus is mediated trans-synaptically. For estrogen, modulation of the HPA axis may be due to changes in glucocorticoid receptor-mediated negative feedback mechanisms. The results of a variety of studies suggest that gonadal steroid hormones, particularly testosterone, modulate HPA activity in an attempt to prevent the deleterious effects of HPA activation on reproductive function.     

Chronic melatonin treatment and the hypothalamo-pituitary-adrenal axis in the rat: Attenuation of the secretory response to stress and effects on hypothalamic neuropeptide content and release

The pituitary-adrenal secretory response to acute and chronic stress, suppressibility of adrenocortical secretions by exogenous glucocorticoids, and hypothalamic content and in vitro release of the two major peptidergic activators of the hypothalamo-pituitary-adrenal (HPA) axis, corticotropinreleasing hormone (CRH) and arginine-vasopressin (AVP), were examined in rats receiving daily melatonin (MEL) injections coincident with the circadian increment of endogenous pineal and adrenocortical secretory activity. After 7 days of MEL administration, the rats displayed a significant attenuation of the adrenocortical secretory response to acute and chronic stress. Chronic MEL treatment also prevented the decline in adrenocorticotropic hormone (ACTH) release resulting from chronic stress exposure. Hypothalamic CRH content was significantly lower in rats receiving MEL treatment, while AVP remained largely unaltered; however, MEL administration counteracted the chronic stress-induced decrease in hypothalamic AVP content and in vitro release. When exposed to dexamethasone in vitro, hypothalamic explants from MEL-treated rats responded with a stronger suppression of CRH and AVP release than those originating from vehicle-injected animals. These observations indicate that MEL attenuates the adrenocortical response to stress and influences the biosynthesis, release and glucocorticoid responsiveness of hypothalamic ACTH secretagogues.     

Effect of neuropeptide Y on the hypothalamic-pituitary-adrenal axis in the dog

There is increasing evidence that neuropeptide Y (NPY) affects the release of pituitary hormones, including adrenocorticotropic hormone (ACTH). The present study was designed to clarify the mechanism by which NPY activates the hypothalamic-pituitary-adrenal (HPA) axis in the dog. Mongrel dogs were equipped with a chronic cannula allowing intra-third (i.t.v.) or intra-lateral (i.l.v.) cerebroventricular administration. A 1.19 nmol, i.t.v. dose of NPY produced as great an ACTH and cortisol response as did equimolar ovine corticotropin releasing factor (CRF). This action of NPY was dose-dependent and shared by peptide YY (PYY) and pancreatic polypeptide (PP), other members of the PP family peptide. Intravenously (i.v.) administered NPY (1.19-11.9 nmol) was much less potent than i.v. CRF in stimulating ACTH and cortisol secretion. However, i.v. NPY significantly increased plasma ACTH and cortisol concentrations, raising the possibility that NPY may modulate the activity of corticotrophs. We have next investigated the possible relationship between NPY and CRF on the HPA axis. Pretreatment with a novel CRF antagonist, alpha-helical CRF9-41 (130.9 nmol i.t.v. or 261.8 nmol i.v.), partly but significantly attenuated the ACTH and cortisol responses to i.t.v. NPY (1.19 nmol). Furthermore, adding a subthreshold dose of i.t.v. NPY (0.119 nmol) to i.t.v. CRF (1.19 nmol) or i.v. NPY (2.38 nmol) to i.v. CRF (0.595 nmol) resulted in the potentiation of CRF-induced ACTH secretion. These results indicate that NPY may activate the HPA axis in concert with CRF probably at hypothalamic and/or pituitary levels. The present findings that NPY evokes ACTH secretion and potentiates the effectiveness of CRF as a secretagogue, together with high concentrations of NPY in the hypothalamus and pituitary portal blood, suggest that NPY is involved in the multihormonal control of ACTH release.     

Effects of cortisol treatment on brain and adrenal corticotropin-releasing hormone (CRH) content and other parameters regulated by CRH

Corticotropin-releasing hormone (CRH) has been found in both hypothalamic and extrahypothalamic sites of the brain and also in the adrenal medulla. To study the timing and location of delayed glucocorticoid action in rats, we measured the effects of 2-day and 7-day cortisol treatment on immunoreactive CRH concentrations in hypothalamus, cerebral cortex, hippocampus, cerebellum, and adrenal gland. The activity of the hypothalamo-pituitary-adrenal (HPA) axis and the sympathoadrenal system were also measured. Studies were carried out both in the afternoon and/or in the morning, to get information about possible circadian changes. CRH contents were not changed in any brain areas studied, except there was a trend of decrease in the hypothalamus compared to vehicle in the afternoon due to the lack of circadian increase after 7-day cortisol treatment. Pituitary ACTH content decreased significantly after 7-day treatment, while beta-endorphin did not. Plasma levels of ACTH, corticosterone, norepinephrine and epinephrine and adrenal ACTH and beta-endorphin contents decreased after 2-day, adrenal CRH content after 7-day treatment with cortisol. Our findings suggest, that chronic cortisol treatment inhibits the circadian activation of the HPA axis at all levels but has variable effects on baseline measures because it causes different changes in release and synthesis at different sites.     

Neuroendocrine aspects of hypercortisolism in major depression

A consistent finding in biological psychiatry is that hypothalamic-pituitary-adrenal (HPA) axis physiology is altered in humans with major depression. These findings include hypersecretion of cortisol at baseline and on the dexamethasone suppression test. In this review, we present a process-oriented model for HPA axis regulation in major depression. Specifically, we suggest that acute depressions are characterized by hypersecretion of hypothalamic corticotropin-releasing factor, pituitary adrenocorticotropic hormone (ACTH), and adrenal cortisol. In chronic depressions, however, enhanced adrenal responsiveness to ACTH and glucocorticoid negative feedback work in complementary fashion so that cortisol levels remain elevated while ACTH levels are reduced. In considering the evidence for hypercortisolism in humans, studies of nonhuman primates are presented and their utility and limitations as comparative models of human depression are discussed.     

Involvement of GABAB receptors in the regulation of the hypothalamo-pituitary-adrenocortical (HPA) axis in rats

Previous experiments have shown that the GABA_B receptor agonist -baclofen given subcutaneously to male rats significantly enhanced plasma concentrations of adrenocorticotropic hormone (ACTH) and the adrenocortical hormones corticosterone and aldosterone. The goal of the present study was to investigate whether the stimulatory effects on adrenocortical steroids elicited by -baclofen in vivo could be reversed by the selective GABA_B antagonist CGP 35 348. One hour before subcutaneous administration of 3 mg/kg -baclofen, a dose of 600 mg/kg CGP 35 348 or saline was administered intraperitoneally. The stimulatory effect of -baclofen on ACTH, corticosterone and aldosterone was significantly reduced by 60% after pretreatment with CGP 35 348. The GABA_B antagonist CGP 35 348 by itself had no effect on ACTH or the adrenocortical hormones. These results indicate that GABA_B receptors are involved in the -baclofen-induced activation of the HPA axis in rats. In vitro, however, neither -baclofen nor CGP 35 348 had any effects on corticosterone and aldosterone release from perifused adrenal cells. These results suggest that the participation of GABA_B receptors in the activation of the HPA axis induced by -baclofen in vivo does not occur at the level of the adrenal gland, and therefore must occur at the level of the pituitary or the brain.     

ACTH, alpha-MSH, and control of cortisol release: cloning, sequencing, and functional expression of the melanocortin-2 and melanocortin-5 receptor in Cyprinus carpio

Cortisol release from fish head kidney during the acute phase of the stress response is controlled by the adrenocorticotropic hormone (ACTH) from the pituitary pars distalis (PD). Alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin, from the pars intermedia (PI), have been implicated in cortisol release during the chronic phase. The present study addresses the regulation of cortisol release by ACTH and alpha-MSH in common carp (Cyprinus carpio) and includes characterization of their receptors, namely, the melanocortin-2 and melanocortin-5 receptors (MC2R and MC5R). We could not demonstrate corticotropic activity of alpha-MSH, beta-endorphin, and combinations of these. We do show a corticotrope in the PI, but its identity is as yet uncertain. Carp restrained for 1 and 7 days showed elevated plasma cortisol and alpha-MSH levels; cortisol is still elevated but lower at day 7 than day 1 of restraint. Interrenal response capacity is unaffected, as estimated by stimulation with a maximum dose ACTH in a superfusion setup. MC2R and MC5R appear phylogenetically well conserved. MC2R is predominantly expressed in head kidney; a low abundance was found in spleen and kidney. MC5R is expressed in brain, pituitary PD, kidney, and skin. Quantitative PCR analysis of MC2R and MC5R expression in the head kidney of restrained fish reveals MC2R mRNA downregulation after 7 days restraint, in line with lower plasma cortisol levels seen. We discuss regulation of corticosteroid production from a phylogenetic perspective. We propose that increased levels of alpha-MSH exert a positive feedback on hypothalamic corticotropin-releasing hormone release to sustain a mild stress axis activity.     

Ghrelin is released from rat hypothalamic explants and stimulates corticotrophin-releasing hormone and arginine-vasopressin.

Ghrelin and synthetic growth hormone secretagogues have diverse effects on the hypothalamus including effects on appetite and the growth hormone axis as well as on the hypothalamus-pituitary-adrenal (HPA) axis. We previously studied the effect of synthetic growth hormone secretagogues on CRH and AVP release from rat hypothalami in vitro, and now report on the effects of ghrelin on CRH and AVP release. The ghrelin protein content and ghrelin output from rat hypothalamic explants was measured using a specific novel ghrelin enzyme immunoassay. The effect of 10(-8) M to 10(-6) M ghrelin on CRH and AVP release was studied in the rat hypothalamic explants, where stimulation with des-octanoyl ghrelin was used as control. The presence of both ghrelin mRNA and protein could be shown in the rat hypothalamus. Ghrelin output was detected in the incubation fluid of rat hypothalamic explants and could be stimulated with high potassium concentrations. Our data also demonstrated a dose-dependent effect of ghrelin on both CRH and AVP release, while des-octanoylated ghrelin showed no effect on either peptide. In summary, the current data suggest that ghrelin is expressed in the hypothalamus both at RNA and the protein levels. Ghrelin stimulates the HPA axis in the rat via stimulation of both CRH, and particularly, AVP release from the hypothalamus. The local autocrine/paracrine and endocrine effects of ghrelin in the hypothalamus could influence all the hormonal systems involved in ghrelin effects, including growth hormone release, the HPA axis and appetite.     

Leukemia inhibitory factor regulates glucocorticoid receptor expression in the hypothalamic-pituitary-adrenal axis

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine belonging to the gp130 family. LIF is induced peripherally and within the brain during inflammatory or chronic autoimmune diseases and is a potent stimulator of the hypothalamic-pituitary-adrenal (HPA) axis. Here we investigated the role of LIF in mediating glucocorticoid receptor (GR) expression in the HPA axis. LIF treatment (3 microg/mouse, i.p.) markedly decreased GR mRNA levels in murine hypothalamus (5-fold, P < 0.01) and pituitary (1.7-fold, P < 0.01) and downregulated GR protein levels. LIF decreased GR expression in murine corticotroph cell line AtT20 within 2 h, and this effect was sustained for 8 h after treatment. LIF-induced GR mRNA reduction was abrogated in AtT20 cells overexpressing dominant-negative mutants of STAT3, indicating that intact JAK-STAT signaling is required to mediate LIF effects on GR expression. Conversely, mice with LIF deficiency exhibited increased GR mRNA levels in the hypothalamus and pituitary (3.5- and 3.5-fold, respectively; P < 0.01 for both) and increased GR protein expression when compared with wild-type littermates. The suppressive effects of dexamethasone on GR were more pronounced in LIF-null animals. These data suggest that LIF maintains the HPA axis activation by decreasing GR expression and raise the possibility that LIF might contribute to the development of central glucocorticoid resistance during inflammation.