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

Short treatment with antalarmin alters adrenal gland receptors in the rat model of endometriosis

Endometriosis is a chronic inflammatory disorder in which endometrial tissue is found outside the uterine cavity. Previous reports suggest that there is a dysregulation of the hypothalamic pituitary adrenal axis during the progression of endometriosis. Our previous report showed that a short-term treatment with antalarmin, a corticotrophin releasing hormone receptor type 1 (CRHR1) antagonist decreases the number and size of endometriotic vesicles in the auto-transplantation rat model of endometriosis. Our current goal was to examine the mRNA expression of intra-adrenal receptors to better understand the mechanisms of the hypothalamic pituitary adrenal (HPA) axis involvement in endometriosis. We used two groups of female rats. The first received sham surgery or endometriosis surgery before collecting the adrenals after 7 days of the disease progression. The second group of animals received endometriosis surgery and a treatment of either vehicle or antalarmin (20 mg/kg, i.p.) during the first 7 days after endometriosis induction and then the disease was allowed to progress until day 60. Rats with sham surgery served as controls. Results showed that the mRNA expression of the mineralocorticoid (MRC2) receptor was lower in the rats after 7 days of endometriosis surgery and in rats with endometriosis that received antalarmin. In addition, the CRHR1 was significantly elevated in animals that received antalarmin and this was counteracted by a non-significant elevation in CRHR2 mRNA. The glucocorticoid receptor mRNA within the adrenals was not affected by endometriosis or antalarmin treatment. This report is one of the first to explore intra-adrenal mRNA for receptors involved in the HPA axis signaling as well as in the sympatho-adrenal signaling, calling for additional research towards understanding the role of the adrenal glands in chronic inflammatory diseases such as endometriosis.

Endogenous Ligands of PACAP/VIP Receptors in the Autocrine–Paracrine Regulation of the Adrenal Gland

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are the main endogenous ligands of a class of G protein-coupled receptors (Rs). Three subtypes of PACAP/VIP Rs have been identified and named PAC(1)-Rs, VPAC(1)-Rs, and VPAC(2)-Rs. The PAC(1)-R almost exclusively binds PACAP, while the other two subtypes bind with about equal efficiency VIP and PACAP. VIP, PACAP, and their receptors are widely distributed in the body tissues, including the adrenal gland. VIP and PACAP are synthesized in adrenomedullary chromaffin cells, and are released in the adrenal cortex and medulla by VIPergic and PACAPergic nerve fibers. PAC(1)-Rs are almost exclusively present in the adrenal medulla, while VPAC(1)-Rs and VPAC(2)-Rs are expressed in both the adrenal cortex and medulla. Evidence indicates that VIP and PACAP, acting via VPAC(1)-Rs and VPAC(2)-Rs coupled to adenylate cyclase (AC)- and phospholipase C (PLC)-dependent cascades, stimulate aldosterone secretion from zona glomerulosa (ZG) cells. There is also proof that they can also enhance aldosterone secretion indirectly, by eliciting the release from medullary chromaffin cells of catecholamines and adrenocorticotropic hormone (ACTH), which in turn may act on the cortical cells in a paracrine manner. The involvement of VIP and PACAP in the regulation of glucocorticoid secretion from inner adrenocortical cells is doubtful and surely of minor relevance. VIP and PACAP stimulate the synthesis and release of adrenomedullary catecholamines, and all three subtypes of PACAP/VIP Rs mediate this effect, PAC(1)-Rs being coupled to AC, VPAC(1)-Rs to both AC and PLC, and VPAC(2)-Rs only to PLC. A privotal role in the catecholamine secretagogue action of VIP and PACAP is played by Ca(2+). VIP and PACAP may also modulate the growth of the adrenal cortex and medulla. The concentrations attained by VIP and PACAP in the blood rule out the possibility that they act as true circulating hormones. Conversely, their adrenal content is consistent with a local autocrine-paracrine mechanism of action.

Corticotropin-releasing hormone receptor expression on normal and tumorous human adrenocortical cells

Corticotropin-releasing hormone (CRH) is not only the principal regulator of the central hypothalamic-pituitary-adrenal (HPA) axis but also exerts direct actions on peripheral tissues. We analyzed the expression of CRH receptors in microdissected preparations of normal human adrenal glands and in adrenocortical and adrenomedullary tumors, employing immunohistochemistry, quantitative RT-PCR of microdissected adrenal tissues, and in situ hybridization. The effect of CRH on adrenal steroidogenesis was tested in adrenal cells. Immunoreactive CRH1R was found primarily within the zona reticularis. In addition, we found a higher expression of CRH type-1 and 2 receptors mRNAs in preparations of adrenal cortices as compared to pheochromocytomas, a 6-fold increase in preparations of clinically unapparent adrenocortical adenomas, and a 10- to 60-fold increase in cortisol-producing adrenal adenomas. Stimulation of the adrenal tumor cell line NCI-H295R with CRH elicited a 1.4-fold increase in DHEA secretion. This result could be reproduced in a culture of primary human adrenocortical cells. We conclude that adrenocortical cells exhibit a higher expression of functional CRH receptors than chromaffin cells and that CRH acts on adrenal DHEA production. The data support the assertion of a direct action of CRH on human adrenocortical cells in addition to an intra-adrenal CRH receptor/adrenocorticotropin system. Enhanced CRH1R expression may be involved in adrenocortical tumorigenesis.

Regulation of rat pituitary cocaine- and amphetamine-regulated transcript (CART) by CRH and glucocorticoids

Cocaine- and amphetamine-regulated transcript (CART) was originally isolated from rat brain, but CART is also synthesized and stored in the anterior pituitary. The localization of pituitary CART and factors regulating its synthesis are largely unknown. The regulation of pituitary CART synthesis and release in response to CRH and glucocorticoids was examined in vitro and in vivo. CART immunoreactivity (CART-IR) was released from anterior pituitary segments. This release was increased 15-fold in response to corticotropin-releasing hormone (CRH). Intraperitoneal administration of CRH to rats significantly increased plasma CART-IR. Furthermore, CART-IR content and plasma CART-IR were significantly increased in adrenalectomized rats, and anterior pituitary CART mRNA expression, CART-IR content, and plasma CART-IR were significantly decreased in corticosterone-treated rats. Plasma CART-IR showed a pattern of diurnal variation similar to that of ACTH and corticosterone, and plasma CART-IR was positively correlated with corticosterone. CART-IR was detectable in the medium of the corticotroph cell line AtT-20. Dual in situ hybridization for prepro-CART (ppCART) mRNA expression and immunocytochemistry for ACTH showed localization of ppCART mRNA to a subpopulation of ACTH-immunoreactive cells. These findings demonstrate that pituitary CART expression and release are regulated by CRH and the glucocorticoid environment and that pituitary CART is partly localized to corticotrophs.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

5-HT1A receptor knockout mice and mice overexpressing corticotropin-releasing hormone in models of anxiety

Pharmacological experiments have implicated a role for serotonin (5-HT)(1A) receptors in the modulation of anxiety. More recent is the interest in corticotropin-releasing hormone (CRH) system as a potential target for the treatment of anxiety disorders. However, selective pharmacological tools for the CRH system are limited, hampering research in this field. Gene targeting is a relatively new approach to study mechanisms underlying anxiety disorders. 5-HT(1A) receptor knockout (1AKO) mice have been created on three different background strains, and two different lines of mice, overexpressing CRH (CRH-OE), have been generated. In the present review, behavioural and physiological findings reported for 1AKO mice and CRH-OE mice will be reviewed. As behavioural phenotyping is often limited to one or two approach avoidance paradigms, we extended these observations and also tested 1AKO and CRH-OE mice in a conditioned fear paradigm. This paradigm reflects essentially different aspect of anxiety than approach avoidance paradigms. 1AKO mice on a 129/Sv background strain showed similar freezing as wild-type (WT) mice. In CRH-OE mice, less freezing was observed than in the corresponding wild-type mice. The fact that the anxious phenotype of these genetically altered mice seems less clear than initially reported will be discussed. Rather than studying the direct consequences of alterations in the targeted gene, 1AKO and CRH-OE mice seem very valuable to study compensatory processes that seem to have taken place in reaction to life-long changes in gene expression.

Overexpression of corticotropin-releasing hormone in transgenic mice and chronic stress-like autonomic and physiological alterations

To gain a greater insight into the relationship between hyperactivity of the corticotropin-releasing hormone (CRH) system and autonomic and physiological changes associated with chronic stress, we developed a transgenic mouse model of central CRH overproduction. The extent of central and peripheral CRH overexpression, and the amount of bioactive CRH in the hypothalamus were determined in two lines of CRH-overexpressing (CRH-OE) mice. Furthermore, 24 h patterns of body temperature, heart rate, and activity were assessed using radiotelemetry, as well as cumulative water and food consumption and body weight gain over a 7-day period. CRH-OE mice showed increased amounts of CRH peptide and mRNA only in the central nervous system. Despite the presence of the same CRH transgene in their genome, only in one of the two established lines of CRH-OE mice (line 2122, but not 2123) was overexpression of CRH associated with increased levels of bioactive CRH in the hypothalamus, increased body temperature and heart rate (predominantly during the light (inactive) phase of the diurnal cycle), decreased heart rate variability during the dark (active) phase, and increased food and water consumption, when compared with littermate wildtype mice. Because line 2122 of the CRH transgenic mice showed chronic stress-like neuroendocrine and autonomic changes, these mice appear to represent a valid animal model for chronic stress and might be valuable in the research on the consequences of CRH excess in situations of chronic stress.

HPA axis dysregulation in mice overexpressing corticotropin releasing hormone

BACKGROUND

Hypersecretion of corticotropin-releasing hormone (CRH) in the brain has been implicated in stress-related human pathologies. We developed a transgenic mouse line overexpressing CRH (CRH-OE) exclusively in neural tissues to assess the effect of long-term CRH overproduction on regulation of the hypothalamic-pituitary-adrenal (HPA) axis.

METHODS

Male transgenic CRH-OE(2122) mice on a C57BL/6J background were used. Littermate wildtype mice served as control animals. Basal plasma corticotropin and corticosterone concentrations were measured, and adrenal gland weight was determined. A dexamethasone suppression test measured the effects of long-term CRH hypersecretion on negative feedback control. Additionally, we measured plasma corticosterone concentrations in reaction to stress.

RESULTS

CRH-OE(2122) mice showed elevated basal plasma corticosterone concentrations, hypertrophy of the adrenal gland, and dexamethasone nonsuppression. Basal plasma ACTH concentrations of wildtype and CRH-OE(2122) mice did not differ significantly. In reaction to stress, CRH-OE(2122) mice showed a normal corticosterone response.

CONCLUSIONS

The HPA axis abnormalities observed in CRH-OE(2122) mice suggest that long-term hypersecretion of CRH in the brain can be a main cause of HPA axis dysregulation. The alterations in HPA axis regulation are reminiscent of changes reported in major depressive disorder. As such, these CRH -OE(2122) mice may model the neuroendocrine changes observed in major depressive disorder.

Corticosterone-dependent driving influence of the suprachiasmatic nucleus on adrenal sensitivity to ACTH

We investigated the effects of ablation of the suprachiasmatic nucleus (SCN) on corticosterone (CORT) responses to synthetic ACTH given in either the morning or evening. After dexamethasone treatment, evening ACTH injections in intact rats produced a significantly larger increase in plasma CORT compared with morning ones. In rats with SCN lesions, the ACTH-induced CORT secretion was independent of time of day, providing direct evidence for a driving influence of the SCN on the diurnal rhythm of adrenal sensitivity to ACTH. In the absence of dexamethasone treatment, the SCN-lesioned rats were selected for morning-like (ML) or evening-like (EL) basal levels of CORT. Responses to ACTH were not different in ML rats compared with sham-lesioned morning controls. In contrast, EL rats compared with sham-lesioned evening controls showed an approximately 60% decrease in increment of CORT levels within the first 15 min postinjection. These results indicate that the SCN upregulates ACTH sensitivity of the adrenal cortex during the ascending phase of the daily CORT secretion and point to a critical role of glucocorticoids in determining SCN action.

Effects of a novel corticotropin-releasing-hormone receptor type I antagonist on human adrenal function

Corticotropin-releasing hormone (CRH) is the principal regulator of the hypothalamic-pituitary-adrenal (HPA) axis and an activator of the sympathoadrenal (SA) and systemic sympathetic (SS) systems. Mental disorders, including major depression and, more recently, Alzheimer's disease have been associated with dysregulation of the HPA axis and the SA/SS systems. Treatment of rats or monkeys with the novel CRH receptor type 1 (CRH-R1) antagonist antalarmin inhibits the HPA and/or the SA/SS axes. This is the first study to examine the potential direct effect of antalarmin on human adrenal function. Adrenocortical and adrenomedullary cells were characterized by double-immunohistochemistry with anti-17 alpha hydroxylase (cortical cells) and anti-chromogranin A (chromaffin cells). Expression of CRH, ACTH, CRH type I and type II receptor mRNA were analyzed by reverse-transcription (RT) PCR. Human adrenal cortical and/or chromaffin cells in co-culture were incubated with CRH, antalarmin, and both CRH and antalarmin in vitro. Exposure of these cells to corticotropin or vehicle medium served as positive and negative controls, respectively. Cortical and chromaffin tissues were interwoven in the human adrenals, and both in situ and in the co-culture system the endocrine cell types were in close cellular contact. ACTH, CRH, and CRH-R1 and CRH-R2 mRNAs were expressed in the human adrenal as determined by RT-PCR. CRH (10-8 M) led to a moderate increase of cortisol release (145.7 +/- 20.0%) from cortical and chromaffin adrenal cells in co-culture. This effect corresponded to 41.8% of the maximal increase induced by ACTH (10-8 M). The action of CRH was completely inhibited by antalarmin. CRH, ACTH, and both CRH-R1 and CRH-R2 mRNAs are expressed in the adult human adrenal gland. CRH stimulates cortisol production in cortical and chromaffin cell co-cultures. This effect is blocked by antalarmin, a selective CRH-R1 receptor antagonist, suggesting that CRH-R1 receptors are involved in an intraadrenal CRH/ACTH control system in humans.

Induction of corticotropin-releasing hormone gene expression by glucocorticoids: implication for understanding the states of fear and anxiety and allostatic load

Evidence supports the idea of two distinct corticotropin-releasing hormone (CRH) systems in the brain: one which is constrained by glucocorticoids and the other which is not. It is this latter system that includes two primary sites (central nucleus of the amygdala and the lateral bed nucleus of the stria terminalis) in which the regulation of CRH gene expression can be disassociated from that of the paraventricular nucleus of the hypothalamus. It is this other system that we think is linked to fear and anxiety and to clinical syndromes (excessively shy fearful children, melancholic depression, post-traumatic stress disorder and self-administration of psychotropic drugs). The excess glucocorticoids and CRH, and the state of anticipatory anxiety, contribute to allostatic load, a new term that refers to the wear and tear on the body and brain arising from attempts to adapt to adversity.

KAT45, a noradrenergic human pheochromocytoma cell line producing corticotropin-releasing hormone

KAT45 cells were derived from a human pheochromocytoma, which also caused ectopic Cushing's syndrome, and developed into a cell line spontaneously after the continuous primary culture of the tumor cells. These human pheochromocytoma cells were compared with the extensively characterized PC12 rat pheochromocytoma cell line. KAT45 cells resembled PC12 cells in morphology, proliferation rate, response to cholinergic stimuli, and the development of dendrite-like projections after exposure to nerve growth factor. They produced norepinephrine and epinephrine in a ratio of 50:1, as opposed to production of dopamine by PC12 cells, in amounts 1 order of magnitude higher compared with PC12. Because of the ectopic Cushing's syndrome in our patient, her normal ACTH level, and the knowledge that PC12 cells and even normal rat chromaffin cells appear to produce CRH, we examined whether KAT45 cells also produced this neuropeptide. Indeed, KAT45 cells released authentic CRH and contained an apparently intact CRH transcript. Nicotine and KCl depolarization stimulated the secretion of CRH, whereas interleukin-1beta, glucocorticoids, and nerve growth factor stimulated its synthesis. In addition to the potential systemic effects of CRH, which in our patient produced ectopic Cushing's syndrome, CRH can exert paracrine effects within normal or tumoral adrenals. We used KAT45 cells as a model for the study of the local role of CRH. CRH affected several parameters of KAT45 cell metabolism, including their proliferation rate, synthesis of catecholamines, and production of POMC-derived peptides. KAT45 cells, in addition to the data they provided regarding the in vitro profile of a human CRH-producing pheochromocytoma, may prove to be a valuable auxiliary to the PC12 cell line.

Role of VIP, PACAP, and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are members of a family of regulatory peptides that are widely distributed in the body and share numerous biologic actions. The two peptides display a remarkable amino acid-sequence homology, and bind to a class of G protein-coupled receptors, named PACAP/VIP receptors (PVRs), whose signaling mechanism mainly involves the activation of adenylate-cyclase and phospholipase-C cascades. A large body of evidence suggests that VIP and PACAP play a role in the control of the hypothalamo--pituitary-adrenal (HPA) axis, almost exclusively acting in a paracrine manner, since their blood concentration is very low. VIP and PACAP are contained in both nerve fibers and neurons of the hypothalamus, and VIP, but not PACAP, is also synthesized in the pituitary gland. Both peptides are expressed in the adrenal gland, and especially in medullary chromaffin cells. All the components of the HPA axis are provided with PVRs. VIP and PACAP enhance pituitary ACTH secretion, VIP by eliciting the hypothalamic release of CRH and potentiating its secretagogue action, and PACAP by directly stimulating pituitary corticotropes. Through this central mechanism, VIP and PACAP may increase mineralo- and glucocorticoid secretion of the adrenal cortex. VIP but not PACAP also exerts a weak direct secretagogue action on adrenocortical cells by activating both PVRs and probably a subtype of ACTH receptors. VIP and PACAP raise aldosterone production via a paracrine indirect mechanism involving the stimulation of medullary chromaffin cells to release catecholamines, which in turn enhance the secretion of zona glomerulosa cells via a beta-adrenoceptor-mediated mechanism. PACAP appears to be able to evoke a glucocorticoid response through the activation, at least in the rat, of the intramedullary CRH/ACTH system. The relevance of these effects of VIP and PACAP under basal conditions is questionable, although there are indications that endogenous VIP is involved in the maintenance of the normal growth and steroidogenic capacity of rat adrenal cortex. However, indirect evidence suggests that these peptides might play a relevant role under paraphysiological conditions (e.g., in the mediation of HPA axis responses to cold and inflammatory stresses) or may be somehow involved in the pathogenesis of Cushing disease or some case of hyperaldosteronism associated with secreting pheochromocytomas.

Neurotensin stimulates CRH and ACTH release by rat adrenal medulla in vitro

Neurotensin (NT) is a 13-amino acid peptide, widely distributed in the central and peripheral nervous system, which is able to stimulate the activity of the hypothalamo-pituitary CRH-ACTH system. We investigated by RIA the effect of NT on the release of CRH and ACTH immunoreactivities (ir) by rat adrenal medulla in vitro. NT enhanced the release of both CRH-ir and ACTH-ir, the maximal effective concentration being 10(-8) M. [D-Trp11]-NT, a specific NT receptor antagonist, abrogated the effects of NT. The stimulatory effect of 10(-8) M NT on ACTH-ir release was blocked by alpha-helical-CRH (an antagonist of CRH receptors), thereby suggesting that the enhancement in ACTH secretion is consequent to the stimulation of CRH release. These findings suggest that NT is a stimulator not only of the central (hypothalamo-pituitary), but also of the peripheral (intramedullary), branch of the CRH-ACTH system.

Comparative study between normal rat chromaffin and PC12 rat pheochromocytoma cells: production and effects of corticotropin-releasing hormone

The adrenal medulla of several species and some human pheochromocytomas contain CRH. The first aim of the present work was to find out whether normal rat adrenal chromaffin cells and the PC12 rat pheochromocytoma cell line produce CRH in vitro and what regulates its production. CRH was measured and characterized in the media of both types of chromaffin cells under basal conditions and after exposure to K+, nicotine, interleukin-1 beta, and nerve growth factor (NGF). The second aim was to examine the biological effect of exogenous CRH (and of its antagonist) on the production of catecholamines from these two types of cells. Our results are as follows: 1) Both types of chromaffin cells contained and secreted comparable amounts of immunoreactive-CRH under basal conditions and after K(+)-induced depolarization, nicotine, and interleukin-1 beta; 2) the physicochemical characteristics of the immunoreactive-CRH in the cells and the media were identical to the putative CRH peptide on both sieve chromatography and RP-HPLC; 3) synthetic CRH induced the production of catecholamines from both cell types in a dose- and time-dependent manner; this effect was abolished by the antagonist, alpha helical CRH; 4) exposure of PC12 cells to NGF (for 1 week) resulted in their neuronal differentiation and the stimulation of their production of CRH by 30 times and of dopamine by 10 times, compared with parallel controls; this effect of NGF was abolished by alpha helical CRH. In conclusion, our data suggest that the production of CRH by PC12 cells represents the preservation of a normal chromaffin cell characteristic rather than a tumor-induced ectopic phenomenon.

Neuropeptide K and neurokinin A stimulate CRH and ACTH release by rat adrenal medulla in vitro

Tachykinins are a family of peptides that are able to modulate the activity of the hypothalamo-pituitary CRH-ACTH system. Mammalian tachykinins include neurokinin A (NKA), neurokinin B (NKB), neuropeptide K (NPK), and substance P (SP). We investigated by RIA the effects of tachykinins on the release of CRH and ACTH by rat adrenal medulla in vitro. NKA and NPK concentration-dependently enhanced the release of both CRH and ACTH, NPK being more active than NKA. NKB exerted only a minor stimulatory action exclusively on CRH release, and SP was ineffective. The stimulatory effect of both NKA and NPK on ACTH release was blocked by the CRH receptor antagonist alpha-helical-CRH, thereby suggesting that the increase in ACTH secretion is consequent to the stimulation of CRH release. These findings indicate that NKA and NPK are stimulators not only of the central (hypothalamo-pituitary), but also of the peripheral (intramedullary) branch of the CRH-ACTH system.

Arginine-vasopressin stimulates CRH and ACTH release by rat adrenal medulla, acting via the V1 receptor subtype and a protein kinase C-dependent pathway

Arginine-vasopressin (AVP) is a hypothalamic hormone that, like CRH, stimulates the pituitary release of ACTH, thereby activating adrenal glucocorticoid secretion. Evidence indicates that rat adrenal medulla contains a CRH-ACTH system duplicating that existing at the hypothalamo-pituitary level and involved in the paracrine stimulation of the cortex secretion. Therefore, we investigated by RIA the effect of AVP on the release of CRH and ACTH immunoreactivities (IR) by rat adrenal medulla in vitro. AVP concentration-dependently enhanced the release of both CRH-IR and ACTH-IR, and the effect was blocked by a selective antagonist of the V1 subtype of AVP receptors. The CRH receptor antagonist alpha-helical-CRH partially reversed AVP-evoked rise in ACTH-IR release, without altering either CRH response or basal secretions of CRH and ACTH. The specific inhibitors of protein kinase C Ro31-8220 and calphostin C abolished both CRH and ACTH responses to AVP. In conclusion, our present findings suggest that AVP stimulates intramedullary the CRH-ACTH system, acting via V1 receptors and activating protein kinase C.

Hypothalamic and suprahypothalamic effects of prolonged treatment with dexamethasone in the rat

Corticosteroid type I and II receptors mediate the negative feedback effects of these hormones at various central nervous system sites involved in the regulation of the hypothalamic-pituitary-adrenal (HPA) axis. To examine the effects of chronic treatment with dexamethasone (DEX), a type 2 receptor agonist, on the regulation of this axis, male Sprague-Dawley rats weighing 200-250 g were given daily injections of DEX for 1,2,3, and 4 weeks or were treated with a subcutaneously implanted DEX-releasing minipump for one week. At the end of treatment, the animals were weighed and brains and truncal blood were collected. Daily intermittent DEX treatment reduced the body weight of the rats in a time-dependent fashion, but had little or no effect on their wet brain weight. Plasma ACTH and corticosterone, measured by RIA, were fully suppressed after one week of intermittent treatment and did not show any further reduction in rats treated for longer periods. In these animals, the content of immunoreactive corticotropin-releasing hormone (iCRH), arginine vasopressin (AVP), ACTH and beta-endorphin (beta-EP) in the hypothalamus, hippocampus, cerebral cortex and cerebellum and pituitary ACTH content did not show any difference compared to vehicle-treated rats. In contrast, continuous DEX treatment increased iCRH content in the cortex, reduced AVP content in the cerebellum, increased ACTH content in the hippocampus, decreased ACTH and beta-EP content in the hypothalamus, and reduced pituitary ACTH content. Hypothalami explanted from rats treated with DEX for one week released lower basal amounts of iCRH in vitro and did not respond to a maximally stimulatory concentration of serotonin (5-HT), a known CRH secretagogue. Continuous DEX administration suppressed also potassium chloride-induced iCRH release. Interestingly, hypothalami explanted from rats receiving daily injection of vehicle, but not from unhandled, untreated controls, did not respond to 5-HT with an increase of iCRH release in vitro. In conclusion, prolonged and continuous, but not intermittent, administration of DEX had a strong effect on brain neuropeptide content. Both regimens of DEX reduced the hypothalamic iCRH responsiveness to stimuli in vitro. Chronic handling also decreased the responsiveness of the hypothalamus to a stimulatory neurotransmitter and may confound the interpretation of data pertinent to inhibitory mechanisms.