ACTH and corticosterone secretion in rats following removal of the neurointermediate lobe of the pituitary gland

Recovery from Liver Failure and Fibrosis in a Rat Portacaval Anastomosis Model after Neurointermediate Pituitary Lobectomy

Liver diseases, including cirrhosis, viral hepatitis, and hepatocellular carcinoma, account for approximately two million annual deaths worldwide. They place a huge burden on the global healthcare systems, compelling researchers to find effective treatment for liver fibrosis-cirrhosis. Portacaval anastomosis (PCA) is a model of liver damage and fibrosis. Arginine vasopressin (AVP) has been implicated as a proinflammatory-profibrotic hormone. In rats, neurointermediate pituitary lobectomy (NIL) induces a permanent drop (80%) in AVP serum levels. We hypothesized that AVP deficiency (NIL-induced) may decrease liver damage and fibrosis in a rat PCA model. Male Wistar rats were divided into intact control (IC), NIL, PCA, and PCA+NIL groups. Liver function tests, liver gene relative expressions (IL-1, IL-10, TGF-β, COLL-I, MMP-9, and MMP-13), and histopathological assessments were performed. In comparison with those in the IC and PCA groups, bilirubin, protein serum, and liver glycogen levels were restored in the PCA+NIL group. NIL in the PCA animals also decreased the gene expression levels of IL-1 and COLL-I, while increasing those of IL-10, TGF-β, and MMP-13. Histopathology of this group also showed significantly decreased signs of liver damage with lower extent of collagen deposition and fibrosis. Low AVP serum levels were not enough to fully activate the AVP receptors resulting in the decreased activation of cell signaling pathways associated with proinflammatory-profibrotic responses, while activating cell molecular signaling pathways associated with an anti-inflammatory-fibrotic state. Thus, partial reversion of liver damage and fibrosis was observed. The study supports the crucial role of AVP in the inflammatory-fibrotic processes and maintenance of immune competence. The success of the AVP deficiency strategy suggests that blocking AVP receptors may be therapeutically useful to treat inflammatory-fibrotic liver diseases.

The relative importance of hypothalamic neurons containing corticotropin-releasing factor or vasopressin in the regulation of adrenocorticotropic hormone secretion

Corticotropin-releasing factor (CRF-41) and arginine vasopressin (AVP) are the two major factors that regulate adrenocorticotropic hormone (ACTH) secretion. The two neurohormones are co-localized in the parvocellular neurons of the paraventricular nuclei (PVN) of the hypothalamus and are capable of potentiating each others' action on freshly excised anterior pituitary fragments or cells in vitro. Transection of all axons entering the medial basal hypothalamus from anterior and lateral directions blocks ACTH release induced by either adrenalectomy or ether-surgery stress. Adrenalectomy-induced ACTH release is almost completely suppressed by a long-term lesion of the PVN. Stress-induced ACTH release is blocked for only a few days after PVN lesion and the pituitary-adrenal response to ether-surgery stress returns to a large extent by a few weeks after PVN lesioning. This remarkable plasticity can be observed also in the homozygous Brattleboro rat, therefore it is not dependent on mediation by AVP. When parvocellular CRF-41- and AVP-containing cells are present, and the anterior lobe ACTH cells are desensitized to the stimulating effects of AVP, the ACTH response to haemorrhage and immobilization is markedly decreased. This indicates that AVP may partially mediate ACTH release under normal conditions. The hypothalamic control of the pituitary-adrenocortical system has a remarkable degree of redundancy which may compensate, at least under stressful conditions, for disruption of the function of CRF-41-containing cells of the paraventricular nucleus, the major source of CRF-41 in the stalk-median eminence.

Response of interleukin-1beta in the magnocellular system to salt-loading

Drinking 2% NaCl decreases interleukin (IL)-1beta in the neural lobe and enhances IL-1 Type 1 receptor expression in magnocellular neurones and pituicytes. To quantify cytokine depletion from the neural lobe during progressive salt loading and determine whether the changes are reversible and correspond with stores of vasopressin (VP) or oxytocin (OT), rats were given water on day 0 and then 2% NaCl to drink for 2, 5, 8 or 5 days followed by 5 days of water (rehydration). Control rats drinking only water were pair-fed amounts eaten by 5-day salt-loaded animals. Animals were decapitated on day 8, the neural lobe frozen and plasma hormones analysed by radioimmunoassay (OT, VP) or enzyme-linked immunosorbent assay (IL-1beta). IL-1beta, VP and OT in homogenates of the neural lobe were quantified by immunocapillary electrophoresis with laser-induced fluorescence detection. Differences were determined by ANOVA, Tukey's t-test, Dunnett's procedure, Fisher's least significant difference and linear regression analysis. In response to salt-loading, rats lost body weight similar to pair-fed controls, drank progressively more 2% NaCl and excreted greater urine volumes. Plasma VP increased at days 2 and 8 of salt-loading, whereas osmolality, OT and cytokine were enhanced after 8 days with IL-1beta remaining elevated after rehydration. In the neural lobe, all three peptides decreased progressively with increasing duration of salt-loading (IL-1beta, r2 = 0.98; OT, r2 = 0.94; VP, r2 = 0.93), beginning on day 2 (IL-1beta; VP) or 5 (OT), with only VP replenished by rehydration. IL-1beta declined more closely (P < 0.0001; ANOVA interaction analysis) with OT (r2 = 0.96) than VP (r2 = 0.86), indicative of corelease from the neural lobe during chronic dehydration. Local effects of IL-1beta on magnocellular terminals, pituicytes and microglia in the neural lobe with activation of forebrain osmoregulatory structures by circulating cytokine may sustain neurosecretion of OT and VP during prolonged salt-loading.

The effect of the alpha-2-adrenergic agonist, clonidine, on secretion patterns and rates of adrenocorticotropic hormone and its secretagogues in the horse

Alpha-2-adrenoceptor activation may lower adrenocorticotropic hormone (ACTH) by reducing secretagogue input and/or increasing the release of an inhibitory factor (CIF). To investigate this, we gave clonidine, an alpha-2-agonist, to seven horses, and collected pituitary venous blood every minute for 20 min before treatment and 40 min after treatment. Six horses were given saline vehicle. Mean secretion rates of corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP) and ACTH were calculated before and during four 5-min then two 10-min periods after clonidine or saline. Reduction in ACTH secretion without corresponding changes in CRH and/or AVP would imply the presence of CIF. Secretion rates of ACTH (P = 0.008) and AVP (P = 0.0005) fell after clonidine and remained lower than baseline values for 20 min and 10 min, respectively. The CRH secretion rate decreased slightly but not significantly after clonidine. In controls, hormone secretion rates did not alter during the experiment. Multiple linear regression showed that CRH and AVP secretion accounted for 69% (treated) or 45% (controls) of the variation in ACTH secretion (P < 0.0001 for each). CRH alone contributed 80% (treated) or 76% (controls) of the fit to this model, which is consistent with the concept that CRH 'sets the gain' of the response of corticotrophs to fluctuations in AVP. Accordingly, minute-to-minute changes in pituitary concentrations of AVP and ACTH were synchronous when all data were considered (% concordant changes: controls, 68%, P < 0.0001; treated, 76%, P < 0.0001) and the percentage of concordant movement was unaffected by clonidine (before 72%; after 73%; P = 0.80). In treated horses but not controls, the ratio between the secretion rates of ACTH and AVP fell (P = 0.009), while the ACTH : CRH ratio tended to fall after clonidine, implying reduced responsiveness to stimulation. Moreover, one horse showed a drop in ACTH and a rise in CRH and AVP secretion after clonidine. We conclude that in horses alpha-2-adrenoceptor activation lowers ACTH secretion primarily by reducing the secretion of AVP and possibly CRH. While there was some evidence that a CIF may participate in the clonidine-induced suppression of ACTH, the subtlety of the discordance between ACTH and its secretagogues in most horses and the rarity of complete dissociation indicate that it does not play a major role.

Denervation of the rat posterior pituitary gland: validation of a stereotaxic method

A stereotaxic surgical method was developed for interrupting the nerve fibres running through the rat pituitary stalk to the posterior pituitary gland without obliterating the hypothalamo-pituitary portal circulation. The pituitary stalk was compressed by the blunt tip of an L-shaped rotating knife. Successful operations produced mild diabetes insipidus, disappearance of arginine vasopressin from the neural lobe, accumulation of arginine vasopressin and neurosecretory material in the pituitary stalk and no infarction in the anterior lobe of the pituitary gland. In female rats, the oestrous cycle was only temporarily disturbed. Plasma prolactin and corticosterone levels were high during the first 24 h after the stalk compression but returned to normal baseline levels from the second day after the operation. One week after the operation plasma adrenocorticotropin and prolactin levels were in the control range while plasma alpha-melanocyte-stimulating hormone was elevated. Denervation of the posterior pituitary gland may help in studying the neural control of intermediate lobe function and the role of the neural lobe in various endocrine conditions, and may serve as a model for lesions of the pituitary stalk and formation of ectopic neurohypophysis in the human.

Interactions between neurohypophysial hormones and the ACTH-adrenocortical axis

Figure 5 summarizes the aspects of the interaction of the HPA and magnocellular systems discussed above. Hormones classically considered confined to the magnocellular-neurohypophysial system are found in the parvocellular-long portal system and are known to be paramount in the hypophysiotropic control of ACTH release. It is now clear that hormones from the posterior pituitary can influence the secretion of ACTH via the short portal circulation and, possibly, by recirculation. There is some evidence that circulating AVP may affect adrenal sensitivity to ACTH. Corticosteroids, in addition to inhibiting parvocellular CRH and ACTH release, may inhibit the release of AVP from the neurohypophysis. The converse is also true in that patients with adrenal insufficiency may have an SIAD-like scenario. CRH may be synthesized in, and secreted from, magnocellular OT neurons and may be involved in the control of neurohypophysial function.

Vasopressin response to haemorrhage in rats: effect of hypoxia and water restriction

1. The aim of the present study was to determine the effect of water restriction and/or hypoxia on the vasopressin response to haemorrhage in conscious rats. 2. Male, Long-Evans rats (n = 39) were prepared with chronically indwelling femoral artery and vein catheters and exposed to 24 h of one of the following: normoxia with ad lib drinking water (N + W); normoxia with water restriction (N - W); hypoxia with ad lib drinking water (H + W); and hypoxia with water restriction (H - W). At the end of 24 h, a 15 mL/kg arterial haemorrhage was performed. 3. Water restricted rats had elevated pre-haemorrhage vasopressin levels. Haemorrhage induced an increase in vasopressin in all groups. Water restriction (N - W) or hypoxia (H + W) each augmented the vasopressin response to haemorrhage. However, the combination of hypoxia and water restriction (H - W) failed to augment the vasopressin response to haemorrhage as compared to normoxic, water replete (N + W) rats. 4. Hypoxia or water restriction per se augment the vasopressin response to haemorrhage. This augmented vasopressin response to haemorrhage is not maintained when hypoxia and water restriction are combined.

Differential effect of neurointermediate lobectomy on central oxytocin and vasopressin

Removal of the neurointermediate pituitary (NIL) affects the secretion of anterior pituitary (AP) hormones. It is not known if these effects are due to changes in central neuropeptide neurons. Two neuropeptides implicated in the neuroendocrine regulation of AP hormone secretion, and which are located in the NIL, are oxytocin and vasopressin. The present study evaluated whether removal of the NIL affected oxytocin and vasopressin concentrations in discrete brain areas containing cell bodies (paraventricular nucleus), fibers (arcuate nucleus), and/or terminals (median eminence) of these central neurons. Adult male rats underwent removal (NIL-X) or visualization (SHAM controls) of the NIL using a parapharyngeal approach. Oxytocin levels increased in the paraventricular nucleus and median eminence following NIL-X, whereas vasopressin concentrations were relatively unaffected by NIL-X. The data suggest that at least part of the influence of the NIL on AP hormone secretion may result from the ability of feedback from the NIL to differentially affect central neuropeptide neurons.

Osmotic stimulation affects neurohypophysial corticotropin releasing factor-41 content: effect of dexamethasone

In this study we examined the effects of 2% saline loading (S), partial restriction of water consumption (R) or a combination of S or R with dexamethasone (DEX) treatment for 14 days on corticotropin releasing factor (CRF)-41 content of the neurointermediate lobe (NIL) and supraoptic nuclei (SON) of male Wistar rats. Arginine vasopressin (AVP) and oxytocin (OXY) contents of the NIL and SON were also assayed as well as plasma corticosterone, ACTH, [Na+] and [Cl-] concentrations. S or R for 14 days resulted in an increase in CRF-41 content and a concomitant drop in AVP and OXY contents of the NIL. Dexamethasone treatment enhanced the effect of S but not of R on NIL CRF-41 content. Dexamethasone treatment abolished the decrease in the AVP content and partially reversed the decrease in the OXY content of the NIL in response to S but not in response to R. No changes were observed in CRF-41, AVP and OXY content of the SON. Unstressed plasma corticosterone concentrations were not changed in S rats but were elevated in R rats; DEX did not prevent this elevation. Plasma ACTH concentrations were low in all groups examined. Plasma [Na+] and [Cl-] increased in response to both S and R. Increases in [Na+] and [Cl-] evoked by S but not R were prevented by DEX treatment. The results show that in the NIL, osmotic stimulation decreases AVP and OXY content, while it increases the CRF-41 content.

Neuroendocrine responses to emotional stress: possible interactions between circulating factors and anterior pituitary hormone release

We have shown that a psychological stressor can elicit increases in plasma AVP levels in normal human subjects. Since AVP can enhance the release of ACTH, and the pituitary gland is outside the blood-brain barrier, AVP present in the general circulation might extend the time course of stress-induced, CRF-mediated release of ACTH from the anterior lobe. Since PRA is involved in the synthesis of angiotensin I, the precursor of AII, and AII is known to enhance CRF-mediated release of ACTH from pituitary cells and to stimulate release of AVP, it is possible that the increase in PRA also contributed to the release of AVP and ACTH in this study. Reports differ as to whether circulating catecholamines can release ACTH in vivo by direct action on the pituitary. Finally, it has been reported that beta-EP enhances the release of PRL, and inhibits release of AVP. Since the increase in beta-EP in the present study was quite robust, it might have extended the PRL release, and truncated the AVP response.

Long-term salt loading impairs pituitary responsiveness to ACTH secretagogues and stress in rats

Male Wistar rats were allowed to drink tap water ad lib (W), 2% saline (S) or 2% saline containing dexamethasone (S + D, 1 mg/l) for 7 days. On the 8th day rats were subjected to a 3-min ether stress. Plasma ACTH, corticosterone and prolactin concentrations were determined before and after ether exposure. Prestress concentrations of plasma ACTH were low and did not vary among the three groups. In response to ether stress W rats exhibited twice as high plasma ACTH concentrations as did S rats. Rats of the S + D group exhibited a small but statistically significant ACTH response. Plasma corticosterone concentration in S rats was increased while in S + D rats was significantly decreased under resting conditions compared to that in W rats. Ether stress caused large increases in plasma corticosterone concentrations in W and S rats while a small but statistically significant increase was observed in S + D rats. Prolactin responses to ether were smaller in groups S and S + D than in group W. To test whether the decreased ACTH response to ether exposure was a result of a decreased sensitivity of corticotrope cells to corticotropin releasing factor (CRF)-41 or arginine vasopressin (AVP), adenohypophysial fragments from W, S and S + D rats were incubated in the presence of different doses of CRF-41 or AVP. Pituitary fragments obtained from W rats secreted larger amounts of ACTH than did pituitaries from S rats in response to either CRF-41 or AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Renin response to graded haemorrhage in conscious rats

1. A haemorrhage volume/plasma renin activity (PRA) response relationship was established for five levels of acute haemorrhage ranging from 1.5 to 15 ml/kg in conscious rats. In addition, the effects of chronic indomethacin and/or acute propranolol administration on the PRA response to 5 and 10 ml/kg haemorrhage was assessed. 2. Mean arterial pressure decreased in a haemorrhage volume dependent manner which was not significantly altered by indomethacin and/or propranolol. 3. Haemorrhage volumes of 1.5 and 3.0 ml/kg did not significantly alter PRA. At haemorrhage volumes of 5.0 ml/kg and higher, PRA increased in a volume-dependent manner. Propranolol decreased basal PRA levels but had little effect on the response to haemorrhage. Indomethacin had no effect on basal PRA, but attenuated the response to haemorrhage somewhat. When propranolol and indomethacin were combined, the PRA response to haemorrhage was significantly attenuated. 4. The conscious cannulated rat model exhibits predictable and reproducible renin responses to haemorrhage and is an excellent model for studying the control of renin secretion.

Does cortisol inhibit vasopressin secretion in sheep?

Glucocorticoids inhibit the plasma vasopressin responses to hemorrhage and hypoxia in dogs. Attempts to demonstrate glucocorticoid inhibition of vasopressin secretion in fetal sheep have been unsuccessful, suggesting the possibility that there is an influence of development on the expression of this interaction, or that the interaction cannot be demonstrated in all mammalian species. This study was designed to investigate these two possibilities. Adult ewes chronically prepared with carotid arterial loops, were subjected to 5 hr infusions of cortisol at a rate of 6 ug/kg min or vehicle (5% ethanol in saline). The infusion of cortisol increased plasma cortisol concentration from 26 +/- 3 to 46 +/- 8 ng/ml, while vehicle infusion was associated with a decrease in plasma cortisol concentration from 23 +/- 4 to 15 +/- 3 ng/ml. One hr after the end of the cortisol or vehicle infusions, vasopressin secretion was stimulated by arterial hypotension produced by 10 min infusions of sodium nitroprusside (20 ug/kg min). Nitroprusside decreased arterial blood pressure equally in both groups. Plasma vasopressin concentrations were increased to peak concentrations of 92 +/- 33 and 116 +/- 20 pg/ml in the vehicle- and cortisol-infused groups, responses which were not significantly different as tested by ANOVA. We conclude that increases in plasma cortisol concentration, equal to those observed during responses to stressors, do not inhibit vasopressin secretion in this species.

Vasopressin and oxytocin: hypothalamic modulators of the stress response: a review

ACTH secretion is primarily controlled by hypothalamic secretion of corticotropin releasing factor (CRF) into pituitary portal blood. However arginine vasopressin (AVP) and oxytocin (OT) can modulate the actions of CRF and at times may be important mediators of stress-induced ACTH secretion. The relative contributions of CRF, AVP, and OT to the control of ACTH secretion vary with different types of stress. In general, AVP stimulates ACTH secretion in all species studied. OT also stimulates ACTH release in rats but is inhibitory in primates. The involvement of AVP and OT in the control of ACTH secretion may have important implications for physiological and pathological conditions associated with activation of the hypothalamo--hypophysial--adrenal cortical axis.