Recovery of the hypothalamo-pituitary-adrenocortical axis in the rat after long-term dexamethasone treatment

Male rats were treated for 14 days with dexamethasone (2.6 mumol/l in the drinking water) and killed at various times after withdrawal of the drug. Some animals were subjected to stress (ether or sham adrenalectomy) just before killing. The recovery of responsiveness of the components of the hypothalamo-pituitary-adrenocortical axis was assessed by measuring plasma and tissue concentrations of hormones, and the response of the tissue in vitro to appropriate stimuli. In vitro, bioactive corticotrophin-releasing factor (CRF) release in response to acetylcholine and adrenal corticosterone release in response to adrenocorticotrophin (ACTH) were significantly suppressed until 3 days after withdrawal. However, release of immunoreactive or bioactive ACTH in response to ovine CRF or hypothalamic extract did not return to normal until day 5. This was correlated with a reduction in pituitary immunoreactive ACTH content and bioactive plasma ACTH, which were suppressed until days 5 and 4, respectively. No change in hypothalamic immunoreactive CRF content could be detected after treatment, or after stress (ether or sham adrenalectomy) in either treated or control animals. Stress (ether) had no effect on the subsequent response of the anterior pituitary gland in vitro to ovine CRF. The large rises in plasma ACTH and adrenal corticosterone measured after stress (ether) in control animals were completely abolished after dexamethasone treatment and did not return to control values until 5 days after withdrawal. Therefore, it appears that after cessation of chronic dexamethasone treatment in the rat, the responsiveness of the hypothalamus and adrenal gland return to normal before that of the pituitary gland.     

In vivo corticotropin-releasing factor-induced secretion of adrenocorticotropin, beta-endorphin, and corticosterone

A 41-residue peptide purified as a corticotropin-releasing factor/beta-endorphin-releasing factor (CRF) in vitro was tested for its ability to stimulate the secretion of ACTH, beta-endorphin, and corticosterone in three animal groups: 1) unanesthesized rats bearing indwelling venous cannulae, 2) rats pretreated with chloropromazine plus morphine sulfate plus pentobarbital (CPZ-MS-Nb, and 3) rats with hypothalamic deafferentiations in the frontal and lateral retrochiasmatic areas. In all three bioassays iv administration of 0.1-10 micrograms CRF elicited a dose-related increase in plasma ACTH and beta-endorphin values over a 5- to 15-min period. Corticosterone secretion was also elevated but responded maximally with all doses of CRF tested. Pretreatment of CPZ-MS-Nb animals with 20 micrograms dexamethasone 4 h before assay abolished the CRF-induced hormone secretion. These data suggest that CRF may play a physiological role in the regulation of the hypothalamic-pituitary-adrenal axis.     

Effects of treatment with adrenocorticotrophin on the hypothalamo-pituitary-adrenal system

Long-term treatment with adrenocorticotrophin (ACTH) inhibited the stress-induced response of the hypophysial-adrenocortical system 24 h after the final ACTH injection. The mechanism of this phenomenon was studied in both normal and adrenalectomized rats, the latter receiving corticosterone at various doses. The effect of electrical stimulation of the medial basal hypothalamus on the concentration of corticosterone in plasma (an indicator of ACTH secretion), the corticotrophin releasing factor (CRF) content of the stalk median eminence (SME), the ACTH content of the pituitary gland and the in-vitro release of ACTH by the pituitary gland incubated with or without addition of SME extract were investigated. Electrical stimulation of the medial basal hypothalamus failed to induce a rise in concentrations of corticosterone in plasma of normal rats treated with ACTH; moreover the levels of hypothalamic CRF and hypophysial ACTH were significantly decreased. Hemipituitary glands of ACTH-treated rats released markedly less ACTH in vitro in response to SME extract than did the control glands. This indicated that long-term hormone administration caused a serious impairment of the responsiveness of the corticotrophic cells toward CRF. Pituitary ACTH content and in-vitro responsiveness of pituitary glands obtained from ACTH-treated, adrenalectomized rats receiving corticosterone replacement seemed to be dependent on the amount of exogenous corticosteroid, but not on that of exogenous ACTH. Our previous and present findings suggest that long-term treatment with ACTH elicits repeatedly increased secretion of endogenous corticosterone, impairing the stress-induced CRF-ACTH release at both the hypothalamic and hypophysial levels. Our data challenge the view that ACTH itself is able to inhibit its own secretion.     

Recovery of the components of the hypothalamo-pituitary-adrenocortical axis in the rat after chronic treatment with prednisolone

Male Wistar-derived rats (200-250 g) were treated for 14 days with prednisolone 21-sodium succinate at a concentration of 1035 mumol/l in their drinking water. The drug was then replaced with normal tap water and groups of animals were killed at various times during recovery, trunk blood being collected after decapitation. At the same time, hypothalamic slices, anterior pituitary gland fragments and adrenals were removed and their responsiveness assessed by exposure to appropriate stimuli in vitro. Tissues were also extracted to measure changes in content of hormones during recovery. Treatment with prednisolone produced marked reductions in body weight gain, adrenal weight and pituitary ACTH content, but no significant change in hypothalamic corticotrophin-releasing factor (CRF) bio- or immunoreactivity. The ACTH content was restored by 5 days after withdrawal but adrenal weight remained significantly reduced after 9 days of recovery. The responsiveness of the hypothalamus to acetylcholine in vitro was markedly inhibited and was still significantly reduced 7 days after withdrawal. The responsiveness of the anterior pituitary gland to synthetic CRF or arginine vasopressin and that of the adrenal gland to ACTH added in vitro were restored simultaneously after 7 days of withdrawal. In vivo, recovery was assessed by measurement of the response to laparotomy stress. Treatment with prednisolone prevented the increase in the plasma concentrations of ACTH and corticosterone produced by stress, and these responses recovered by 5 days (corticosterone) and 7 days (ACTH) after withdrawal. The abolition of the circadian rhythms of ACTH and corticosterone by treatment was also reversed by 5 days after withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)     

A superfusion system technique for the study of the sites of action of glucocorticoids in the rat hypothalamus-pituitary-adrenal system in vitro. II. Hypothalamus-pituitary cell-adrenal cell superfusion.

Basal and stimulated CRF release by hypothalamic blocks was studied by coupling the effluent of superfused hypothalamus tissue to a joint pituitary cell-adrenal cell superfusion system and measuring corticosterone production. Log dose-response curves of the adrenal cells for ACTH and of the pituitary cell-adrenal cell system for CRF were linear over the ranges used. Ca++-independent basal CRF release by the hypothalamus could be blocked in vitro by 0.2 mug/ml dexamethasone in the medium, or in vivo by treating the hypothalamus donor rats with corticosterone, 1 mg/rat ip 30 min before decapitation. These treatments did not impair CRF release caused by Veratridine (5 x 10(-6)M or by electrical stimulation. Adrenalectomy increased only basal but not stimulated CRF release. These results indicate that glucocorticoids have a hypothalamic site of action.     

Regulation of the messenger ribonucleic acid for corticotropin-releasing factor in the paraventricular nucleus and other brain sites of the rat

CRF, from the paraventricular nucleus of the hypothalamus (PVN), is the major hypothalamic releasing factor that controls pituitary ACTH. Recently, the mRNA for CRF and the CRF peptide have been detected in other brain sites. However, there is little information on the function and regulation of CRF in brain sites outside the paraventricular nucleus. We investigated the content of CRF mRNA in the PVN, the central nucleus of the amygdala (CN), the bed nucleus of the stria terminalis (BN), and the supraoptic nucleus (SON). Northern gel analysis showed that the mRNA for CRF is present in the BN, CN, and SON as well as the PVN, and that all are the same size. In response to adrenalectomy, the level of hybridizable mRNA increased 2.75-fold over 7 days in the PVN; there was no change in the CN, BN, or SON. High dose dexamethasone decreased, but did not eliminate, the PVN CRF mRNA; it was without effect in the other sites. Glucocorticoid replacement with constant low blood levels of corticosterone (5.6 +/- 0.3 micrograms/100 ml) suppressed plasma ACTH and decreased thymus weight while reducing, but not eliminating, PVN CRF mRNA. We conclude that the same sized mRNA for CRF is synthesized in the PVN, BN, CN, and SON, but only the PVN mRNA responds to alterations of peripheral glucocorticoid status. This may imply that only CRF from the PVN is involved in control of the hypothalamic-pituitary-adrenal axis.     

Physiological role of corticotropin-releasing factor in the control of adrenocorticotropin-mediated corticosterone release from the rat adrenal gland.

Marked fluctuations in adrenal sensitivity to ACTH have been reported under both physiological (e.g. diurnal) and experimental conditions. Recently, we reported that immunoneutralization of CRF reduces resting corticosterone (cort) levels in rats without inducing concomitant reductions in plasma ACTH. We postulated an endogenous CRF mechanism that controls the adrenal sensitivity to ACTH. In the present study, this hypothesis was tested by iv infusion of human ACTH (0, 1, 3, and 10 ng/kg.min for 60 min) into dexamethasone-treated anaesthetized male Wistar rats. Serial blood samples were taken for the determination of ACTH and corticosterone by RIA (ACTHi, corti). Infusion of ACTH resulted in dose-dependent steady state plasma ACTHi levels, ranging from 50-600 pg/ml, which were not affected by prior administration of a rat monoclonal antibody to rat CRF (PFU 83). As expected, infusion of ACTH resulted in a dose-dependent increase in plasma corti. In PFU 83-treated rats, preinfusion plasma corti levels were reduced compared to those of rat immunoglobulin G-treated controls (7.8 +/- 1.2 vs 25.3 +/- 3.2 ng/ml). In addition, the corti responses to infusion of 1 and 3 ng/kg.min ACTH were suppressed by PFU 83. However, at a (near) maximally effective dose of ACTH (10 ng/kg.min), no differences in plasma corti were found between PFU 83 and immunoglobulin G-treated rats. These findings suggest that immunoneutralization of endogenous CRF results in a 3-fold reduction of the adrenal sensitivity to ACTH. Subsequently, we studied the possible effects of exogenous CRF on the isolated perfused adrenal gland in situ. In this preparation, CRF alone (1-100 pmol) or ACTH alone (5 fmol) did not affect the corti secretion rate or the flow rate of the perfusion medium through the gland. However, when given together a marked (up to 3.2 times) CRF dose-dependent stimulation of corti secretion and an increase (up to 1.7 times) in adrenal flow rate were obtained. In experiments with freshly dispersed adrenal cells in vitro, PFU 83 (1 microM) or CRF (0.1-10 nM) did not influence corti secretion when given alone and did not affect ACTH-induced corti secretion. It is unlikely, therefore, that CRF acts directly on the steroid-producing cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticotrope response to removal of releasing factors and corticosteroids in vivo

In the intact rat, adrenalectomy (ADX) is known to result in increased ACTH synthesis, content, and secretion from the anterior pituitary compared with those in the sham-adrenalectomized control. Treatment of adrenalectomized, rats with corticosterone prevents or reverses these changes in ACTH. Because corticosterone is known to act both at the corticotrope and at the level of CRF secretion, it is not clear to what extent the ACTH response to ADX is a result of removal of glucocorticoids from the pituitary per se. To test the role of brain input as well as the role of glucocorticoids on the corticotrope response to ADX, we performed the following experiment. Rats were prepared with anterolateral hypothalamic deafferentations (lesion) which severed CRF and arginine vasopressin cell bodies in the hypothalamus from their axonal endings in the median eminence and posterior pituitary. Control rats were subjected to sham lesions. Two days later, half of the rats in each group were subjected to either ADX or sham ADX; a subgroup of the lesioned rats was provided at the time of adrenal surgery with a constant infusion of rat CRF. Five days later, all rats were killed, and anterior pituitary levels of proopiomelanocortin (POMC) mRNA, ACTH, and protein; plasma ACTH and corticosterone, and adrenal and thymus weights were measured. In sham-lesioned rats, ADX resulted in increases in POMC mRNA, and plasma ACTH of 2.5- and 12-fold, respectively, compared to sham-adrenalectomized controls. In the absence of hypothalamic drive (lesion only), there were no responses of any of these variables to ADX. In lesioned rats driven with CRF, ADX resulted in increases in POMC mRNA and plasma ACTH of 2.2- and 2.6-fold, respectively, compared to sham ADX. After consideration of the three variables indicating ACTH synthesis, storage, and secretion and comparison of the results of ADX vs. sham ADX within and across the sets of animals, we conclude that 1) there is no autonomous response of the corticotrope to ADX; 2) the removal of corticosterone from the anterior pituitary may account for the majority of the effects of ADX on ACTH synthesis; and 3) the normal response to ADX requires secretion of CRF and increased secretion of another ACTH-releasing factor (possibly arginine vasopressin) that causes increased secretion but little synthesis of ACTH.     

Studies on the site of action of vasopressin in inducing adrenocorticotropin secretion.

Hypophysectomized rats bearing three transplanted pituitaries under the kidney capsule responded to synthetic lysine vasopressin or pitressin with a significant elevation of plasma corticosterone, whereas hypophysectomized rats with no grafts did not. This response was completely abolished by pretreatment of animals with dexamethasone but was unaltered by central hypothalamic destruction. Corticotropin-releasing factor content of the hypothalamic median eminence, hypophyseal stal-, or pars nervosa of the posterior pituitary of intact rats was unchanged 5 or 10 min after ip injection of vasopressin compared to the basal level. We conclude that vasopressin and dexamethasone act directly on the adenohypophysis in vivo to exert their stimulatory or inhibitory effect on ACTH secretion.     

Plasma catecholamines do not participate in pituitary-adrenal activation by immobilization stress in rats with transection of nerve fibers to the median eminence

The effect of immobilization stress was studied in rats in which the CRF and arginine vasopressin-containing innervation of the median eminence was destroyed by an anterolateral cut (ALC) around the medial basal hypothalamus. One week after surgery, the rats with ALC were subjected to immobilization and they showed a normal rise in plasma corticosterone, a smaller than normal rise of plasma ACTH, and an increased response of plasma epinephrine and norepinephrine. When in rats with an ALC the response of plasma catecholamines was prevented by guanethidine pretreatment and adrenal enucleation the small rise in plasma ACTH was unchanged during immobilization. In addition, the plasma corticosterone and ACTH rises during immobilization in the rats with ALC were not influenced by prior treatment with phentolamine (2.5 mg/kg ip) or propranolol (2.5 mg/kg ip). These findings suggest that the large rises in plasma epinephrine and norepinephrine levels during immobilization do not contribute to changes in plasma ACTH or corticosterone levels when hypothalamic regulation via CRF and/or arginine vasopressin is interrupted by ALC.     

Corticosterone acts on the brain to inhibit adrenalectomy-induced adrenocorticotropin secretion

To determine the site (brain and/or pituitary) at which corticosterone (Cort) acts to inhibit adrenalectomy-induced ACTH secretion, the following experiments were performed in male rats. Rats in which brain and pituitary feedback sites were to be left intact were subjected to sham hypothalamic lesions. Rats in which only pituitary sites were to be left were subjected to either medial basal hypothalamic (MBH) or para-ventricular nuclei (PVN) lesions. Two days later all rats were adrenalectomized and replaced with varying amounts of Cort (by sc pellet). Lesioned rats also received sc pumps that delivered 0-5 micrograms/day rat CRF. All rats were killed 5 days after adrenalectomy. Sham-lesioned groups exhibited the expected dose-related inhibition of plasma and pituitary ACTH concentrations by Cort, with normal values obtained at plasma Cort levels between 4.4 and 7.7 micrograms/dl. By contrast, rats with MBH and PVN lesions exhibited no ACTH responses to adrenalectomy when CRF infusions were between 0 and 1 micrograms/day. In rats with MBH and PVN lesions receiving 5 micrograms/day CRF, plasma ACTH concentrations were elevated and were not inhibited by plasma Cort values up to 6 micrograms/dl. Plasma ACTH was inhibited in rats with MBH lesions infused with 5 micrograms/day CRF when plasma Cort levels were 30.6 micrograms/dl. These rats also exhibited marked thymic atrophy. We conclude from these results that Cort normally acts only on a brain site to inhibit adrenalectomy-induced increases in ACTH secretion. Only when plasma Cort concentrations are markedly elevated can evidence for pituitary feedback be demonstrated in vivo.     

Diurnal variations in responsiveness of the hypothalamo-pituitary-adrenocortical axis of the rat

Hypothalami, anterior pituitary gland segments and adrenal glands were removed from female Wistar-derived rats decapitated at various times of the day. Blood and tissue hormone concentrations were measured and the tissues challenged with appropriate stimuli in vitro. Both bioactive and immunoreactive corticotrophin-releasing factor (CRF) content of the hypothalami were significantly higher in the evening than in the morning, as was the basal release of bioactive CRF in vitro. The response of the hypothalami to serotonin or acetylcholine added in vitro did not change with time of day. Basal bioactive and immunoreactive adrenocorticotrophin (ACTH) release from the anterior pituitary gland was significantly increased in the evening, as was the response to synthetic ovine CRF in vitro. Plasma ACTH concentrations in intact rats given crude CRF (hypothalamic extract) in vivo were higher in the evening at all times after injection tested, but this difference was markedly reduced in animals with mediobasal hypothalamic lesions. Corticosterone released basally from adrenal glands in vitro was significantly increased in the evening and the response to added ACTH 1-24 was slightly enhanced. For adrenal glands removed from lesioned rats, the pattern was reversed, corticosterone release in vitro being lower in the evening for all doses of ACTH added. Similarly in vivo, in intact rats given ACTH 1-24, plasma corticosterone concentrations and corticosterone release in vitro from adrenal glands removed after the injection were higher in the evening. After the placement of basal hypothalamic lesions, the situation was reversed, the response to ACTH administration in vivo being greater in the morning.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that the hypothalamus mediates endotoxin stimulation of adrenocorticotropic hormone secretion.

The site of action of Escherichia coli endotoxin in inducing ACTH secretion was studied in vivo and in vitro. Hypophysectomized rats, bearing two to three transplanted pituitaries under the kidney capsule and "primed" with exogenous ACTH, responded to 2.0-7.5 microgram/100 g BW ip or iv endotoxin with a several-fold increase of plasma corticosterone. This response was markedly reduced by hypothalamic lesions and completely abolished by removing the entire forebrain. Endotoxin added directly to cultured rat adenohypophyseal cells in a concentration up to 10 microgram/ml did not induce significant ACTH secretion. We conclude that endotoxin-induced ACTH secretion from heterotopically transplanted pituitaries is mediated primarily by the hypothalamus, presumably through hypothalamic CRF that reaches the transplanted pituitaries via the systemic circulation.     

Maturation of the pituitary-adrenal function in rat fetuses

Plasma adrenocorticotropic hormone (ACTH) and corticosterone were detectable in fetal plasma on day 16 of pregnancy. Thereafter, the levels of both hormones increased steadily in a parallel manner and reached a peak on day 19 of pregnancy. Administration of an antiserum anti-rat corticotropin-releasing factor (CRF) to pregnant rats was followed by a significant decrease in fetal plasma corticosterone as early as day 17. Plasma ACTH measured under the same experimental conditions on day 19 of gestation was also significantly decreased. Similar results have been obtained with fetal plasma collected from adrenalectomized pregnant rats, indicating that the plasma corticosterone decrease in fetuses after immunoneutralization of CRF reflects changes in fetal adrenal secretion and not a diminution of corticosterone transfer from the maternal to the fetal circulation. These results show that endogenous CRF begins to play a physiological role in the regulation of ACTH and corticosterone secretion as early as in 17-day-old fetuses. This effect may occur before the connections between the neurosecretory CRF axons and the hypophysial portal capillaries have been established. Therefore, endogenous CRF may enter the hypophysial portal circulation after intercellular diffusion in hypothalamic tissue.     

Ontogeny of the stress response in the rat: role of the pituitary and the hypothalamus

The neonatal rat shows a period of decreased responsiveness to noxious stimuli during the first 3 weeks of life, but the nature of this impairment is still controversial. To test the functionality of the hypothalamus-pituitary-adrenal axis during this period, we studied pituitary and adrenal responsiveness to exogenous ovine CRF and the ability of various stressors (ether vapors, electroshocks, and hypoxia) to elicit ACTH and corticosterone secretion. We also measured hypothalamic CRF content and pituitary ACTH content as well as CRF-binding sites in the anterior pituitary. From days 3-10, small elevations in plasma ACTH and corticosterone levels were observed after a 3-min exposure to ether vapors or electroshocks. In contrast, during this period, a 20-min exposure to hypoxia (5% O2 in N2) was unable to trigger measurable ACTH secretion, while corticosterone was significantly elevated. From days 14-21, plasma ACTH and corticosterone levels increased significantly after exposure to ether stress, hypoxia, and, to a lesser extent, electroshocks. By contrast, administration of urethane (1.2 g/kg BW) caused a significant increase in ACTH secretion on days 3, 5, and 10, an effect that was partially suppressed by pretreatment with an anti-CRF serum. This suggests that endogenous CRF can be released by at least some stimuli as early as day 3. Direct stimulation of the pituitary with synthetic oCRF (10 micrograms/kg BW) caused significant elevations in plasma ACTH levels at all ages tested (days 3 through 21), though these increases were significantly (P less than or equal to 0.01) smaller on day 3 (2.7-fold) than on day 21 (4.3-fold). Hypothalamic CRF content as well as ACTH content increased gradually with age, but the values reached by the third week of life were still low compared to the values on day 45. Finally, anterior pituitary CRF-binding sites averaged 317 +/- 48 fmol/mg protein on day 5 and 158 +/- 22 fmol/mg protein on day 17. The affinity (Kd) of the receptor for CRF was not significantly different on day 5, 17, or 45. These results show that although pituitary corticotrophs appear to be functional at birth, exposure to stress does not elicit marked increases in plasma ACTH until day 14 of age.     

In vivo and in vitro comparisons of biological activities of bovine, ovine and rat CRF (corticotrophin-releasing factor)

The biological activity of partially purified bovine hypothalamic CRF (corticotrophin- releasing factor) was compared to those of synthetic CRFs (ovine, rat) sauvagine and vasopressin in vivo and in vitro. ACTH-primed hypophysectomized rats with heterotopically transplanted pituitaries and medial basal hypothalamic ablation (H-T + MBHA ), and intact rats pre-treated with chlorpromazine, morphine and Nembutal (C-M-N) were used for in vivo CRF assays. Perifused rat adenohypophyseal fragments were employed for in vitro studies. CRF-A (void volume fractions, 'big' CRF) and CRF-B (Kav = 0.583) purified from bovine hypophyseal stalk, synthetic ovine and rat CRF, and sauvagine all induced significant stimulation of ACTH and/or corticosterone secretion in these systems. Synthetic ovine and rat CRF and sauvagine showed comparable CRF potency. The CRF dose-response slopes for bovine CRF were somewhat steeper than those for ovine CRF or sauvagine in the in vitro system. Vasopressin had the least steep dose-response slope. Intravenous bolus administration of ovine CRF caused a more prolonged (greater than 20 min) elevation of plasma ACTH compared to a relatively short duration after bovine CRF-A. These data suggest that bovine hypothalamus contains substance(s) which exhibits different CRF characteristics from those of ovine CRF.     

Angiotensin II and adrenocorticotropin release: mediation by endogenous corticotropin-releasing factor

Recent reports indicate that the main effect of systemically administered angiotensin II (AII) on ACTH release is probably due to some central nervous system mechanism. The present studies were designed to investigate whether the central action of AII on ACTH release is directly mediated through CRF. In order to test the participation of endogenous CRF in the AII-induced ACTH release in vivo, intact and pharmacologically blocked (pretreated with chlorpromazine-morphine-nembutal) female rats were injected iv with AII (8 nmol/100 g BW). Plasma levels of ACTH as well as CRF content in the median eminence (ME) and medial basal hypothalamus (MBH) were evaluated before and 1, 2.5, and 5 min after treatment. These responses were compared with the effect of 1 min ether exposure on hypothalamic CRF content and plasma ACTH levels in unanesthetized animals. AII injection and ether exposure increased plasma ACTH levels several-fold at both 2.5 and 5 min post treatment in intact rats. Conversely, AII failed to induce any significant increase in plasma ACTH levels in centrally blocked rats at any interval studied. On the other hand, AII injection and ether inhalation acted in similar fashion on CRF content in ME, inducing fast depletion at 1 min post treatment, recovering to control values at 2.5 min after injection, and finally, accumulating peptide at 5 min post treatment. In addition, CRF content in the MBH decreased significantly at 5 min, under both experimental conditions; AII had no effect on hypothalamic CRF content in centrally blocked rats. In vitro experiments using whole MBH (containing ME) fragments incubated with either neural peptides or high K+ solutions indicate that AII possesses a CRF releasing effect at concentrations of 10(-6) M or more. Conversely, other hypothalamic peptides, such as LHRH, TRH, and somatostatin did not induce significant release of CRF at any of the concentrations assayed (10(-7) to 10(-5) M). On the other hand, high K+ solutions released CRF in a concentration-related manner (15-60 mM). These studies suggest that the central effect of AII stimulation on ACTH release in vivo could be, at least in part, through the release of hypothalamic CRF into the portal circulation.     

Effects of dexamethasone on central and peripheral ACTH systems in the rat

To investigate the simultaneous effects of dexamethasone on peripheral and central adrenocorticotropic hormone (ACTH) systems, rats were treated with dexamethasone or saline for 4 days. Pituitary, plasma, hypothalamus and cerebrospinal fluid (CSF) were then collected and analyzed for ACTH immunoreactivity. Additionally, hypothalamic tissue extracts were analyzed for corticotropin-releasing hormone (CRH) immunoreactivity. Dexamethasone significantly lowered peripheral levels of ACTH as measured in pituitary and plasma. Hypothalamic ACTH content significantly increased while CSF ACTH significantly decreased with dexamethasone treatment. Hypothalamic CRH concentrations showed a small but statistically insignificant decrease. These results suggest that prolonged exposure to dexamethasone affects central as well as peripheral ACTH activity, corroborate our previous findings in rhesus monkeys of decreased CSF ACTH in response to prolonged dexamethasone treatment, suggest that dexamethasone may inhibit the release of ACTH from hypothalamic neurons into the CSF, and provide evidence that the effect of dexamethasone on pituitary ACTH content is of greater magnitude than its effect on hypothalamic CRH.     

The role of adrenocorticotropin in the cortisol and aldosterone responses to angiotensin II in conscious dogs

The role of ACTH in the cortisol and aldosterone responses to iv angiotensin II (AII) infusion, (5, 10, and 20 ng kg-1 min-1) in dogs was evaluated by examining the effect of AII infusion in conscious dogs pretreated with dexamethasone to suppress endogenous ACTH secretion. AII infusion in untreated dogs produced dose-related increases in plasma cortisol and aldosterone concentrations. The plasma ACTH concentration also increased. Dexamethasone treatment lowered the basal cortisol concentration from 1.7 +/- 0.1 to 0.7 +/- 0.1 micrograms/dl (P less than 0.05) and the ACTH concentration from 52 +/- 3 to 41 +/- 4 pg/ml (P less than 0.05), and abolished the cortisol response to all doses of AII, indicating that ACTH was necessary for the response. On the other hand, the basal aldosterone concentration was not significantly affected by dexamethasone, although the aldosterone response to the highest dose of AII was reduced. Additional experiments were performed to determine if the cortisol and aldosterone responses to AII (20 ng kg-1 min-1) in dexamethasone-treated dogs are restored if the ACTH concentration is maintained near control levels by iv infusion of synthetic alpha ACTH-(1-24) (0.3 ng kg-1 min-1). AII still failed to increase the plasma cortisol concentration in this group of dogs; however, the aldosterone response was fully restored. To evaluate the effect of elevated ACTH levels on the steroidogenic effects of AII, dogs were treated with dexamethasone and a higher dose of ACTH (0.4 ng kg-1 min-1). This dose of ACTH increased the plasma cortisol concentration from 1.7 +/- 0.1 to 3.5 +/- 0.8 micrograms/dl (P less than 0.05), but did not significantly affect the plasma aldosterone concentration. In the presence of constant elevated levels of ACTH, AII (10 and 20 ng kg-1 min-1) increased the plasma cortisol concentration in dexamethasone-treated dogs, although the response to the 10 ng kg-1 min-1 dose was smaller than the response in untreated dogs. Infusion of AII at 5 ng kg-1 min-1 did not increase the plasma cortisol concentration. In contrast, the increased plasma aldosterone produced by AII infusion in dexamethasone-treated dogs was not altered in the presence of elevated ACTH levels. Finally, AII infusion did not alter the clearance of cortisol. Collectively, these results demonstrate that an increase in plasma ACTH is necessary for the cortisol response to AII infusion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interleukin-1 stimulates corticotropin-releasing factor gene expression in rat hypothalamus

To examine the effect of interleukin-1 (IL-1) on CRF and POMC gene expression, recombinant human IL-1 alpha and -beta were ip injected in rats. The plasma ACTH level showed a dose-related increase at 2 h after the injection of 0.5 and 2 micrograms IL-1 alpha and -beta, and also showed a sustained increase from 1 h until 5 h after the injection of 2 micrograms of IL-1 beta. CRF contents in the medial basal hypothalamus and ACTH contents in the anterior pituitary (AP) decreased at 2 h after the injection of 2 micrograms of IL-1 alpha and -beta, and such decreased levels were maintained until 5 h after the injection of 2 micrograms of IL-1 beta. The levels of CRF mRNA in the hypothalamus and POMC mRNA in AP significantly increased 3 h after the injection of 2 micrograms IL-1 alpha and -beta, and these levels were still higher at 5 h after the injection of 2 micrograms of IL-1 beta compared with those of the control. There was no significant change in the ACTH content and POMC mRNA levels in the intermediate-posterior pituitary or the hypothalamus or in the CRF contents and CRF mRNA levels in the cerebral cortex. These results indicate that acute administration of IL-1 alpha and -beta stimulates gene expression of hypothalamic CRF and CRF release, which causes the stimulation of ACTH release and POMC gene expression in AP.