Hypothalamic-pituitary-adrenal axis up-regulation in rats submitted to pituitary stalk compression

The present study investigated the hypothalamic-pituitary-adrenal (HPA) axis activity in response to stress in adult male rats submitted to pituitary stalk compression (PSC) or sham operation. Animals received water or oral salt loading (2% NaCl) for one or eight days before the day of the experiment. On the 14th day post-surgery rats were killed under basal conditions or after 15 min immobilization stress. In the PSC group urine output increased significantly and plasma vasopressin (AVP) levels failed to respond to osmotic stimuli. Short-term salt load induced a significant increase in AVP levels in the sham-operated group. The PSC group presented higher adrenocorticotrophin (ACTH) and corticosterone levels compared with sham-operated rats, both in water intake and salt load conditions. Immobilization stress induced a similar increase in plasma ACTH and corticosterone concentrations in sham-operated and PSC groups under water intake. However, long-term salt load blunted the ACTH and corticosterone responses to immobilization stress in sham-operated rats. PSC rats submitted to short- and long-term salt loading presented no changes in ACTH and corticosterone levels after immobilization. Immobilization stress caused neither AVP responses nor plasma osmolality changes in sham and PSC groups. There was no difference in median eminence AVP content among all groups. In conclusion, the high ACTH and corticosterone levels found in PSC rats under water intake and salt loading conditions suggest an up-regulation of the HPA axis, with a preserved adaptive mechanism to chronic stress.     

Vasopressin and CRF-ACTH in adrenalectomized and dexamethasone-treated rats

Median eminence corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) and pituitary and peripheral plasma adrenocorticotropin (ACTH) and AVP were measured in male Wistar rats 1 and 2 weeks after bilateral adrenalectomy (ADX), sham operation (SHAM) or dexamethasone-treatment (DEX). Median eminence AVP content was unchanged 1 week after ADX but was significantly elevated 2 weeks after ADX, whereas CRF activity was reduced at 1 week after ADX and returned to control range at 2 weeks. Anterior pituitary ACTH content was elevated but posterior pituitary AVP content was reduced at 1 and 2 weeks after ADX. Plasma ACTH was greatly elevated in ADX rats and reduced in DEX rats, whereas plasma AVP did not differ significantly between these two groups or the control group. When ADX and SHAM rats were laparotomized under ether, plasma ACTH increased greatly, but this elevation was prevented by DEX treatment. The plasma AVP level was elevated in all three groups 2.5 min after onset of stress but returned to the basal range at 20 min. Median eminence CRF and AVP and pituitary ACTH and AVP were not significantly changed after onset of stress. These results indicate that the vasopressin and CRF-ACTH responses were not consistent in the median eminence, pituitary and peripheral plasma and suggest that vasopression is not involved in the feedback and acute stress mechanism of CRF-ACTH secretion. However, we have to measure CRF activity and AVP concentration in the hypophysial portal blood to confirm this conclusion.     

Effect of stress, adrenocorticotropin or corticosteroid treatment, adrenalectomy, or hypophysectomy on hypothalamic immunoreactive adrenocorticotropin concentrations.

Despite variations in plasma and/or pituitary ACTH concentrations, no changes in median eminence or medial basal hypothalamic immunoreactive ACTH-like concentrations were seen in rats studied 6 weeks posthypophysectomy, 12 h or 28 days after bilateral adrenalectomy, 6 days after implantation of corticosterone pellets (75 mg), after dexamethasone administration (300 micrograms/100 g BW, ip, twice daily for 4 days), or after chronic immobilization stress. The lack of concordant variation of brain, pituitary, and plasma ACTH concentrations further supports the suggestion of a nonpituitary, central nervous system origin of some portion of brain immunoreactive ACTH-like activity.     

Corticotrophin-releasing peptides in rat hypophysial portal blood after paraventricular lesions: a marked reduction in the concentration of corticotrophin-releasing factor-41, but no change in vasopressin

Previous data show that corticotrophin-releasing factor-41 (CRF-41), arginine vasopressin (AVP) and oxytocin are released into hypophysial portal blood. It has been presumed that the CRF-41 originates mainly from parvicellular neurones of the paraventricular nuclei (PVN); however, AVP and oxytocin could also be derived as a consequence of preterminal release from magnocellular projections to the neurohypophysis. The latter has been suggested to be the case for AVP as assessed by studies of the median eminence in vitro. Here we have investigated the source of CRF-41, AVP and oxytocin in hypophysial portal blood of adult male Wistar rats 8-10 days after surgical lesioning of the PVN. In PVN-lesioned animals the output of CRF-41 into hypophysial portal blood was reduced by about 90%, and that of oxytocin by about 40%: however, the output of AVP into portal blood was reduced only by about 10%. The release of AVP into portal blood increased after adrenalectomy; this increased release could be returned to normal by treatment with dexamethasone. No change of AVP release occurred after adrenalectomy in animals in which the PVN had been lesioned. These results show (i) that most of the CRF-41 released into hypophysial portal blood is derived from the PVN, (ii) that in PVN-lesioned animals AVP and oxytocin release remains at near normal or 60% of normal respectively, suggesting that a substantial amount of both neuropeptides in portal blood is derived as a consequence of preterminal release from supraoptic nuclei projections in the median eminence, and (iii) that glucocorticoid feedback inhibition of AVP release is exerted at the level of the PVN.     

Atriopeptin inhibits stimulated secretion of adrenocorticotropin in rats: evidence for a pituitary site of action

The aim of this study was to resolve previous controversies regarding the effect of atriopeptin on the secretion of ACTH in vivo. Male Wistar rats were used throughout. The animals were subjected to lesioning of the hypothalamic paraventricular nucleus (PVN) or sham operation and implanted with indwelling jugular cannulae 5 days later for blood sampling and drug infusion. Two days after the insertion of the cannulae the animals were treated with saline or 103-126 amino acid residue atriopeptin iv: a bolus injection was given (200 or 40 pmol/rat) followed by an infusion (40 or 8 pmol/min) which was maintained for the entire duration of the experiment (70 min). Ten minutes after the bolus of atriopeptin the animals received iv a combination of 1 pmol 41-residue CRF and 10 pmol arginine vasopressin (CRF/AVP) to stimulate ACTH secretion. Serial blood samples (0.1 ml) were obtained at -10 min and immediately before the injection of CRF/AVP and at 5, 10, 20, 30, and 60 min afterwards. Plasma ACTH concentration was measured by RIA. In sham-operated rats CRF/AVP caused a 4-fold increase in plasma ACTH which peaked at 5 min and returned to baseline by 60 min. In sham-operated rats the higher dose of atriopeptin (200 pmol bolus, 40 pmol/min infusion) did not alter the effect of the stimulus between 5 and 30 min, and augmented plasma ACTH at 60 min. The smaller dose of atriopeptin reduced plasma ACTH at 10 and 20 min by 54% and 48%, respectively, and also decreased by 48% the net amount of ACTH released over 30 min in response to CRF/AVP. When given alone, the higher dose of atriopeptin caused a persistent (60 min) 10-13% reduction of mean arterial blood pressure, while the lower dose decreased blood pressure by about 9% for less than 10 min. In parallel, the higher dose of atriopeptin increased plasma ACTH concentration while the lower dose produced no change. In PVN-lesioned rats the CRF/AVP induced ACTH response was similar to that seen in sham-operated controls. Only the higher dose of atriopeptin was tested, and this markedly reduced CRF/AVP stimulated ACTH secretion at 5-60 min after CRF/AVP. Given alone, atriopeptin had no marked effect on plasma ACTH in PVN-lesioned rats, while its hypotensive action was similar to that in sham-operated animals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticotrophin-releasing factor-41, vasopressin and oxytocin release into hypophysial portal blood in the rat: effects of electrical stimulation of the hypothalamus, amygdala and hippocampus

The role of the paraventricular nuclei (PVN), amygdala and hippocampus in the control of the hypothalamic-pituitary-adrenal axis has been studied by determining the effect of electrical stimulation of the PVN, amygdala and hippocampus on the release of corticotrophin-releasing hormone (CRF-41) and arginine vasopressin (AVP) into hypophysial portal blood and ACTH and corticosterone into peripheral blood. Adult female Wistar rats were anaesthetized with sodium pentobarbitone and stimulation was carried out through previously implanted bipolar, glass-insulated platinum electrodes. Hypophysial portal blood was collected 30 min before and 30 min during the application of the stimulus which consisted of trains (30 s on and 30 s off) of biphasic rectangular pulses with a frequency of 50 Hz, pulse width 1 ms and amplitude 1 mA. Bilateral stimulation of the PVN increased while unilateral stimulation of the amygdala decreased the release of CRF-41 into hypophysial portal blood. The threefold increase in release of CRF-41 induced by PVN stimulation correlated with a marked increase in peripheral plasma concentrations of ACTH and corticosterone. Stimulation of the hippocampus had no significant effect on CRF-41 release, and stimulation of each of the three brain regions had no effect on AVP release into portal blood. These findings were extended in a second study to compare the effects of unilateral bipolar electrical stimulation of the PVN and of the supraoptic nucleus (SON) on the release of CRF-41, AVP and oxytocin. This study was carried out on adult male rats, anaesthetized with sodium pentobarbitone, in which the stimulus was applied through previously implanted concentric stainless-steel electrodes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increase in adrenocorticotropin release during wallerian degeneration of peripheral sympathetic neurons after superior cervical ganglionectomy of rats

The changes in adrenocorticotropin (ACTH) release before, during and after sympathetic nerve degeneration following superior cervical ganglionectomy (SCGx) were examined in male rats. A 12-fold increase of circulating ACTH was found in both SCGx and sham-operated rats 6 h after surgery. In sham-operated rats, plasma ACTH decreased by about half 16-22 h after surgery, whereas in SCGx rats it remained at a high concentration from 16 to 54 h after surgery, attaining basal values by 120 h post-SCGx. In SCGx rats, MBH corticotropin-releasing hormone (CRH) content decreased significantly from 16 to 54 h after surgery, while in controls it remained unmodified. Significantly smaller arginine vasopressin (AVP) contents were found in MBH of SCGx rats as compared to sham-operated controls, 16-54 h after surgery. In rats exposed to ether or immobilization stress 22 h after SCGx, plasma ACTH levels were significantly higher than in controls; however, since unstressed ACTH levels were about twice as high in SCGx rats, the percent increase of ACTH was smaller in the SCGx group. A decreased response of plasma ACTH to ether or immobilization stress was found in rats 7 days after SCGx. In rats subjected to a simultaneous adrenalectomy (Adx) and SCGx or sham-SCGx, plasma ACTH levels increased to a similar extent in both groups. ACTH increase after Adx was accompanied by decreases in MBH CRH, and absence of significant changes in MBH AVP contents. Rats subjected to pinealectomy (Px) or sham-Px 1 week earlier and killed 22 h earlier exhibited similar responses in plasma ACTH and MBH CRH to SCGx regardless of pineal intactness.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic psychosocial stress enhances vasopressin, but not corticotropin-releasing factor, in the external zone of the median eminence of male rats: relationship to subordinate status.

Male Wistar rats living in hierarchically structure male/female colonies were used to investigate the effects of chronic psychosocial stress on the hypothalamus-pituitary-adrenal system. Colony-housed subordinates were compared to control rats housed in male-female pairs. Classical parameters of chronic stress (thymus involution, impaired somatic growth, and elevated resting plasma corticosterone level) were found in all subordinate rats. Changes in vasopressin (AVP) and CRF stored in the external zone of the median eminence (ZEME) were measured by quantitative immunocytochemistry. Chronic psychosocial stress for 19-28 days increased AVP immunostaining in the ZEME to 160-190% of that in pair-housed controls, whereas CRF immunostaining in the ZEME remained unchanged. Within colonies, subordinates differed in avoidance behavior and aggression received (subordinate status). This intracolony subordination rank was correlated with AVP in the ZEME (P less than 0.01). Although resting corticosterone was elevated in subordinate rats (P less than 0.01), the increase in AVP was not associated with detectable secretion of AVP and/or CRF from the ZEME, as measured after blockade of axonal transport. In control rats, interaction with a dominant male increased plasma ACTH and corticosterone levels and caused depletion of AVP, but not CRF, from the ZEME. Subordinates showed suppressed hypothalamic (AVP depletion), pituitary (plasma ACTH) and adrenal (plasma corticosterone) responses to interaction with the dominant male, which may reflect suppressive actions of elevated corticosterone on CRF neurons or suprahypothalamic centers.     

Regulation of basal corticotropin-releasing hormone and arginine vasopressin messenger ribonucleic acid expression in the paraventricular nucleus: effects of selective hypothalamic deafferentations

There exists considerable evidence to suggest that CRH and arginine vasopressin (AVP)-secreting parvocellular neurosecretory neurons of the hypothalamic paraventricular nucleus (PVN) are central integrators of negative feedback effects evoked by circulating glucocorticoid hormones. Most evidence suggests that these neurons may be receptive to circulating glucocorticoid levels, either via glucocorticoid receptors indigenous to these cells and/or via extrahypothalamic glucocorticoid-receptive neurons interacting with the PVN secretory cell. In an effort to address this issue, we performed anterior (ANT), posterior (POST) and total (TOT) deafferentations of the PVN region in male Sprague-Dawley rats using microknives fashioned from narrow-gauge spinal needles. Effective knife cuts were verified immunohistochemically, and deemed acceptable only if they avoided damage to the PVN proper and fibers of CRH and AVP-containing neurons coursing through the hypothalamus en route to the median eminence, while effectively eliminating neuronal input into the PVN region. Subsequent to surgery, levels of mRNA encoding for CRH and AVP in the parvocellular and magnocellular PVN were assayed via semiquantitative in situ hybridization histochemistry. Results indicate that TOT deafferentations resulted in significant increases in CRH mRNA expression in the PVN, and a slight but noticeable induction of AVP mRNA in the medial parvocellular but not posterior magnocellular divisions of the PVN. ANT lesions also produced an up-regulation of CRH and AVP mRNA relative to operated control rats. POST lesions did not produce a clear induction in either CRH or AVP mRNA. The data indicate that in the absence of neuronal input coming from anterior structures, CRH mRNA expression is up-regulated, suggesting that local effects of glucocorticoids on the PVN neuron are ineffective in maintaining normal CRH mRNA expression. These results support a role for neuronal feedback in regulation of the CRH neuron. The limited up-regulation (compared with adrenalectomized rats) of AVP mRNA in the TOT group suggests that while neuronal input may have some control of AVP mRNA expression, local glucocorticoid feedback is clearly able to restrict AVP message to levels considerably less than those seen in steroid-deficient animals. Analysis of knife-cut effects on plasma corticosterone and ACTH levels reveals that POST and TOT, but not ANT, deafferentations prohibit the secretory activity of the hypothalamo-pituitary-adrenocortical (HPA) axis seen pursuant to the anesthesia/thoracotomy in lesion and operated control groups.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hypophysial-portal plasma levels, median eminence content, and immunohistochemical staining of corticotropin-releasing factor, arginine vasopressin, and oxytocin after pharmacological adrenalectomy

Changes in immunostaining, median eminence content, and secretion into the hypophysial-portal circulation of immunoreactive CRF (irCRF), arginine vasopressin (irAVP), and oxytocin (irOT) were directly evaluated after pharmacological adrenalectomy (PHADX). Mean circulating levels of ACTH rose from 270 +/- 57 (+/- SE) to 1560 +/- 283 pg/ml after 72 h of treatment with metyrapone and aminoglutethimide. Initially, hypophysial-portal plasma irCRF levels decreased to 52.6% (12 h) and 21.7% (24 h) of control levels (230 +/- 41 pg/ml). Accompanying changes in the patterns of CRF immunostaining in the paraventricular nuclei (PVN) or in median eminence irCRF content at 24 h did not parallel alterations in portal plasma irCRF levels at this time. By 72 h posttreatment, portal irCRF levels were elevated 2.2-fold, while the number of detectable CRF-positive perikarya in the PVN increased 3.0-fold. The mean hypophysial-portal plasma irAVP concentration was unchanged from the control value (1312 +/- 287 pg/ml) at 12 h, but was only 34.9% of the control value at 24 h. Inverse changes in median eminence irAVP content were noted at these times, whereas the number of AVP-immunostained cells exhibited a tendency toward an increase at 24 h, in parallel with significantly increased content. By 72 h post-PHADX, portal irAVP, median eminence irAVP content, irAVP immunostaining intensity, and AVP-immunopositive cell number were elevated. Approximately 64% of CRF-positive perikarya in the parvocellular PVN costained for AVP at this time, whereas no colocalization was evident in untreated rats. These changes were prevented by corticosterone replacement. irOT staining intensity, irOT-positive cell number, median eminence irOT content, and portal plasma irOT concentration remained stable at all times examined. We conclude that: removal of adrenal steroids by PHADX results in a sequence of changes in CRF and AVP within the PVN (as determined by immunocytochemistry) and the median eminence (as determined by peptide content) similar to those observed after surgical adrenalectomy; after steroid removal the secretion of both irCRF and irAVP changes in a biphasic manner characterized by reduced secretion at 24 h and greatly enhanced secretion at 72 h; neither immunostaining nor median eminence content alone proved to be a reliable index or secretory activity during the initial phases of steroid blockade; and the hypophysiotropic OT system of normal male rats appears to be insensitive to adrenal steroid influences.     

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.     

Stimulatory effect of caffeine on the hypothalamo-pituitary-adrenocortical axis in the rat

Intraperitoneal injection of caffeine (12.5-100 mg/kg) into rats caused a significant, dose-related increase in plasma corticosterone 2 h later, when the greatest response was measured. The corticosterone response to laparotomy stress or i.v. injection of ACTH(1-24) was unaffected by prior injection of caffeine. The response to stress or caffeine was unaffected by adrenal enucleation 28 days previously. In vitro, 10 mmol caffeine/l stimulated basal release of corticosterone from adrenal quarters and potentiated the response to a sub-maximal stimulatory concentration of cyclic AMP (cAMP). The drug had no effect on release stimulated by a sub-maximal concentration of ACTH(1-24). Release of ACTH from pituitary fragments incubated in vitro was stimulated in a dose-related manner by caffeine (0.01-10 mmol/l), and the responses to hypothalamic extract and sub-maximal concentrations of corticotrophin-releasing factor (CRF-41) or arginine vasopressin (AVP), but not cAMP, were significantly enhanced by 10 mmol caffeine/l. Release of immunoreactive CRF-41 (but not AVP) was significantly increased by caffeine (0.01-10 mmol/l) added to hypothalami incubated in vitro. The response to injection of caffeine in vivo was completely prevented by pharmacological blockade of endogenous CRF release. Taken together, these results show that caffeine at high concentrations can stimulate directly the release of the hormones of the hypothalamo-pituitary-adrenocortical axis in vitro, but the fact that these concentrations are unlikely to be reached after administration in vivo suggests that the effect of caffeine may be mediated centrally.     

Centrally administered murine-leptin stimulates the hypothalamus-pituitary- adrenal axis through arginine-vasopressin

Starvation induces a decrease in circulating leptin levels and activation of the hypothalamus-pituitary-adrenal (HPA) axis. Leptin inhibits the HPA axis in unfed rodents or genetically leptin-deficient ob/ob mice, whereas it stimulates corticotropin-releasing hormone (CRH) gene expression in the paraventricular nucleus (PVN). However, the interactions between leptin, CRH and the HPA axis are poorly understood and are likely to be complex. We recently demonstrated that central leptin administration caused increases in plasma arginine-vasopressin (AVP) and AVP gene expression of the PVN in nonstressful rats. AVP stimulates the release of adrenocorticotropic hormone (ACTH), but it also potentiates the action of CRH on ACTH release. In this study, we investigated the effects of leptin on plasma ACTH and corticosterone levels, CRH mRNA of the PVN and proopiomelanocortin (POMC) mRNA of the pituitary in nonstrained rats. Intracerebroventricularly administered leptin caused increases in plasma ACTH and corticosterone levels in dose-dependent manners. In Northern blot analyses, the leptin injection induced significant increases in the expression of CRH mRNA in the PVN and POMC mRNA in the pituitary. The increased plasma ACTH and corticosterone levels by leptin were attenuated with intracerebroventricular pretreatment of a V(1a) receptor antagonist (OPC-21268) or a V(1a)/V(1b) receptor antagonist (dP[Tyr(Me)(2)]AVP), but not with that of a V(2) receptor antagonist (OPC-31260). The leptin-induced CRH mRNA expression in the PVN and POMC mRNA expression in the pituitary were also reduced by the pretreatment with OPC-21268 and dP[Tyr(Me)(2)]AVP. These results suggest that intracerebroventricular leptin administration activates the HPA axis by AVP receptor activation through V(1a) receptors in the PVN which in turn activates CRH neurons to drive ACTH and corticosterone secretion in concert with AVP in nonstrained rats.     

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.     

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)     

Influence of the paraventricular nucleus of the hypothalamus in the alteration of neuroendocrine functions induced by intermittent footshock or interleukin.

The documented ability of physical stress and cytokines to increase the secretion of corticotropin-releasing factor CRF by the paraventricular nucleus of the hypothalamus (PVN), coupled with our earlier demonstration that CRF acts within the brain to interfere with reproductive functions, led us to investigate the effect of lesions of the PVN on LH, testosterone, ACTH, and corticosterone (CORT) secretion. Bilateral lesions of the PVN were done electrolytically, and 2 weeks later a series of acute and chronic experiments were performed in intact or castrated male rats bearing indwelling jugular and/or intracerebroventricular cannulas. The first study involved a single 2-h exposure of intact male rats to footshocks (2 mA, 2-sec duration, 4 per min). Although PVN lesions did not measurably alter the ability of intermittent footshock to lower plasma testosterone levels, this treatment attenuated the rise in plasma ACTH and CORT. In a second study, which was done in castrated rats, shocks were delivered 2 h daily for 7 days, and circulating hormone levels were measured at the end of the last shock period. Plasma LH levels of stressed rats showed statistically comparable decreases in both sham- and PVN-lesioned animals. Chronic exposure to footshocks induced smaller increases of ACTH and CORT secretion in PVN-lesioned compared to sham-lesioned rats, but the lesions did not completely abolish stress-induced activation of the hypothalamic-pituitary adrenal (HPA) axis. The third experiment involved the central injection of the vehicle or 40 ng interleukin-1 beta (IL-1 beta) to castrated rats. As expected, IL-1 beta dramatically decreased plasma LH values and increased circulating ACTH and CORT levels measured 2 h later. Bilateral PVN lesion did not influence LH secretion after injection of the vehicle or IL-1 beta. In contrast, destruction of the PVN completely blocked the increase of HPA axis activity observed in sham-operated rats 2 h after the intracerebroventricular infusion of IL-1 beta. These results confirm our previous observation that exposure to either footshocks or interleukins alters the activity of the hypothalamic-pituitary gonadal and HPA axis. Additionally, the present studies suggest that the PVN, which represents the predominant hypothalamic nucleus controlling the response of the HPA axis to a number of stimuli, does not appear to be necessary for the antireproductive effects of intermittent footshock or immune activation.     

Abnormalities in the hypothalamo-pituitary-adrenal axis in spontaneously hypertensive rats during development of hypertension

Abnormalities in the hypothalamo-pituitary-adrenal axis in spontaneously hypertensive rats (SHR) during development of hypertension were investigated using in vivo and in vitro methods. Plasma ACTH responses to hemorrhage and ether stress were significantly smaller in 7-week-old SHR than in age-matched Wistar-Kyoto rats (WKY), while plasma corticosterone baseline levels and its response to stress were greater in SHR than in WKY. There was no significant difference in the plasma ACTH response to ether stress between bilaterally adrenalectomized SHR and WKY replaced with a 25% corticosterone pellet for 6 days. Adrenalectomy prevented the development of hypertension in SHR; however, corticosterone replacement restored hypertension. Plasma ACTH showed a smaller response to iv CRH injection in SHR than in WKY, while the ACTH response to arginine vasopressin was not different between SHR and WKY. CRH concentrations in the median eminence, posterior pituitary, and cerebral cortex were lower in SHR than in WKY, while the CRH concentration in the median eminence was not different in SHR and WKY when they were adrenalectomized with or without corticosterone replacement. Basal in vitro CRH release from hypothalamic tissue was reduced in SHR, while CRH release in response to 56 mM KCl was not different in SHR and WKY. These results suggest that adrenocortical function is enhanced in young SHR, that reduced ACTH response to stress and exogenous CRH in SHR may be ascribed to higher plasma corticosterone levels, and that corticosterone is essential for the development of hypertension in SHR.     

Atrial natriuretic peptide is a physiological inhibitor of ACTH release: evidence from immunoneutralization in vivo.

The brain is thought to exert a predominantly stimulatory action on ACTH secretion mediated mainly by corticotrophin-releasing factor-41 (CRF-41) and arginine vasopressin (AVP). Several data, however, also point to the existence of an ACTH-inhibiting factor. Atrial natriuretic peptide (ANP), at concentrations found in hypophysial portal blood, inhibits ACTH release in vitro. The aim of the present studies was to use ANP immunoneutralization to determine whether ANP does in fact inhibit ACTH release in vivo. Intracerebroventricular infusion (1 microliters/min for 30 min) of sheep anti-ANP serum into male rats anaesthetized with sodium pentobarbitone had no significant effect on jugular venous plasma concentrations of ACTH or LH but did decrease significantly the plasma concentrations of prolactin. Intravenous infusion of 0.8 ml sheep anti-ANP serum but not control (non-immune) sheep serum, through an indwelling intra-atrial cannula in conscious male rats resulted in a marked and significant increase in plasma ACTH and corticosterone concentrations. The ACTH and corticosterone response to a 30-s ether stress was not significantly potentiated in the same conscious rats infused with anti-ANP serum. Intra-atrial infusion of anti-ANP did not significantly affect plasma prolactin, LH, glucose or sodium concentrations or plasma osmolality. These results show for the first time that ANP is a potent inhibitor of ACTH secretion in the conscious male rat and that, therefore, ANP is a hypothalamic neurohormone which is likely to play an important inhibitory role in the neural control of ACTH release.     

Stress-induced adrenocorticotropin secretion: diurnal responses and decreases during stress in the evening are not dependent on corticosterone.

To test whether the diurnal rhythm in stress responsiveness is dependent on corticosterone (B)-mediated negative feedback, the responses of intact (SHAM) and adrenalectomized (ADX) rats to restraint for 3-90 minutes or ip injection with saline in the morning (AM) and the evening (PM) were compared. In both SHAM and ADX rats, ACTH responses to restraint stress were larger in the AM. In intact rats, this could have resulted from both fast negative feedback, due to the rate of rise of B during the stress in the PM, and delayed negative feedback, due to the high basal concentrations of B before the stress in the PM. However, this diurnal pattern of stress responsiveness was not dependent on B, as the same relative responses to restraint and ip injection were found in ADX rats. To determine whether the lack of response of ADX rats in the PM to stress was due to a loss of sensitivity to endogenous secretagogues, ADX rats were given CRF + arginine vasopressin (AVP) while anesthetized with ether after 30 min of restraint. In both the AM and the PM, the pituitaries were able to respond to exogenous secretagogues. A second novel finding was that in the PM, but not the AM, plasma ACTH concentrations in the ADX rats decreased substantially during the period of restraint, despite the lack of B-mediated negative feedback. In the AM and the PM, ADX rats were restrained for 30 min and then stressed with ether for 6 min. The ACTH concentrations were not different before and after ether, suggesting that, although the pituitaries of ADX rats are able to respond to exogenous CRF + AVP after stress, an additional stress of ether exposure no longer stimulates endogenous CRF and AVP release after 30 min of restraint at either time of day. After 90 min of restraint in the AM and the PM, the relationship between ACTH and B was positive, not negative, providing no evidence of ongoing B-mediated negative feedback in the SHAM rats. Therefore, the same mechanism responsible for the decrease in ACTH secretion in ADX rats may occur in SHAM rats as well. From these results, we conclude that the diurnal rhythm in stress responsiveness and, in the PM in the ADX rats, the decrease in plasma ACTH during stress, are largely independent of B.     

Control of the hypothalamo-pituitary-adrenal axis in the neonatal period: adrenocorticotropin and corticosterone stress responses dissociate in vasopressin-deficient brattleboro rats

In adulthood the hypothalamo-pituitary-adrenal axis is controlled by both CRH and arginine vasopressin (AVP). However, in neonates CRH secretion is very low, whereas AVP secretion is fully functional. This suggests that the role of AVP is more pronounced in young than in adult rats. We investigated the role of AVP by studying stress responses in 5, 10, and 20-d-old AVP-deficient Brattleboro rats. Two different stressors were applied: 24-h maternal separation and Hypnorm Grove Oxford UK injections. In heterozygous controls (that do express AVP), both stressors increased plasma ACTH and corticosterone. The ACTH stress response disappeared in AVP-deficient rats, demonstrating that during the perinatal period, the secretion of this hormone is controlled by AVP. Surprisingly, corticosterone responses remained intact in AVP-deficient rats. Similar findings were obtained after 1-, 4-, 12-, and 24-h long maternal separations. Thus, preserved corticosterone stress responses were not explained by changes in the timing of ACTH secretion. In vitro experiments suggested that the dissociation of ACTH and corticosterone stress responses can only be partly explained by higher ACTH responsiveness of the adrenal cortex in AVP-deficient rats. Together, our results show that in neonatal periods, AVP is crucial for the expression of ACTH stress responses, but neither AVP nor ACTH is necessary for the induction of corticosterone stress responses. Discrepant ACTH and corticosterone stress responses may reflect compensatory mechanisms activated by AVP deficiency, but disparate findings suggest that they rather depict a neonate-specific mechanism of hypothalamo-pituitary-adrenal-axis control.