Arginine Vasopressin mRNA Expression Research Articles

The Hypothalamo-Pituitary-Adrenal Axis Response to Experimental Traumatic Brain Injury

Alterations in the hypothalamo-pituitary-adrenal (HPA) axis following traumatic brain injury have not been documented in detail. We used fluid percussion injury (FPI) to evaluate the early changes in components of the HPA axis following experimental traumatic brain injury. Wistar rats were sacrificed at 2 or 4 h following sham or FPI surgery. In situ hybridization histochemistry was used to determine the expression of mRNAs of corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) in the hypothalamus and pro-opiomelanocortin (POMC) in the pituitary. A group of animals undergoing no surgery were used as control. Repeated blood sampling from an indwelling catheter demonstrated that plasma corticosterone (CORT) levels peaked 30 min following surgery in sham and FPI animals but there was no significant difference in CORT concentration between these groups at any time. Pituitary POMC expression was increased following sham and FPI surgery (compared with control non-operated animals) but with no significant difference between the two groups undergoing surgery. Hypothalamic CRH mRNA expression was significantly higher in animals undergoing FPI compared with sham surgery. Hypothalamic AVP mRNA expression was not significantly increased when compared with control nonoperated animals. These data indicate that the anaesthesia and/or surgery associated with FPI or sham surgery induces a generalised activation of the HPA axis. The selective increase in CRH mRNA in animals undergoing FPI may be due to specific effects of traumatic brain injury rather than a general stress response and may suggest an additional neurotransmitter role for CRH following head injury. The absence of an AVP response suggests that the effects of FPI may be mediated through the CRH-alone-containing subpopulation of neurons.

Maternal undernutrition in early gestation alters molecular regulation of the hypothalamic-pituitary-adrenal axis in the ovine fetus.

We have demonstrated previously that plasma adrenocorticotropin hormone and cortisol responses to exogenous and endogenous stimuli are reduced in fetuses of mildly undernourished ewes. In the present study, we examined the molecular regulation of fetal hypothalamic-pituitary-adrenal (HPA) axis function at 127-130 days gestation (dGA) following 15% reduction in maternal nutrition between 0 and 70 dGA. Using in situ hybridization, we found that corticotropin releasing hormone (CRH) mRNA expression in the hypothalamic paraventricular nucleus (PVN) was lower in fetuses from nutrient restricted ewes than in controls. Restricted fetuses also had greater levels of mRNA encoding preproenkephalin (PENK) and magnocellular arginine vasopressin (AVP) in the PVN. Expression of oxytocin mRNA and parvocellular AVP mRNA in the PVN and pro-opiomelanocortin mRNA in the pituitary were unchanged. Glucocorticoid receptor mRNA expression was unaltered at the PVN, but was reduced (> 40%) in the anterior pituitary of restricted fetuses. Northern blot analysis demonstrated that levels of adrenal P450scc mRNA and P450(C17) mRNA were not different between the groups. We conclude that the reduction in HPA function reported previously is mediated, at least in part, by a decrease in expression of CRH mRNA and increase in PENK mRNA in the PVN.

Androgens Alter Corticotropin Releasing Hormone and Arginine Vasopressin mRNA Within Forebrain Sites Known to Regulate Activity in the Hypothalamic‐Pituitary‐Adrenal Axis

To reveal direct effects of androgens, independent of glucocorticoids, we studied the effects of gonadectomy (GDX) in adrenalectomized (ADX) rats with or without androgen replacement on corticotropin releasing hormone (CRH) and arginine vasopressin (AVP) mRNA expression within various forebrain sites known to regulate the hypothalamic-pituitary-adrenal axis. These included the medial parvocellular portion of the paraventricular nucleus of the hypothalamus (mp PVN), the central and medial nuclei of the amygdala and bed nuclei of the stria terminalis (BNST). In the mp PVN, ADX stimulated both CRH and AVP mRNA expression. Combined ADX + GDX inhibited only AVP, and testosterone and dihydrotestosterone (DHT) restored AVP mRNA. In the central nucleus of the amygdala, ADX decreased CRH mRNA expression, and this response was unaffected by GDX +/- testosterone or DHT replacement. In the medial amygdala, AVP mRNA expression was decreased by ADX, abolished by ADX + GDX, and restored by androgen replacement. ADX had no effect on CRH and AVP mRNA expression in the BNST. GDX + ADX, however, reduced CRH mRNA expression only within the fusiform nuclei of the BNST and reduced the number of AVP-expressing neurones in the posterior BNST. Androgen replacement reversed both responses. In summary, in ADX rats, AVP, but not CRH mRNA expression in the amygdala and mp PVN, is sensitive to GDX +/- androgen replacement. Both CRH- and AVP-expressing neurones in the BNST respond to GDX and androgen replacement, but not to ADX alone. Because androgen receptors are not expressed by hypophysiotropic PVN neurones, we conclude that glucocorticoid-independent, androgenic influences on medial parvocellular AVP mRNA expression are mediated upstream from the PVN, and may involve AVP-related pathways in the medial amygdala, relayed to and through CRH- and AVP-expressing neurones of the BNST.

Aging selectively suppresses vasoactive intestinal peptide messenger RNA expression in the suprachiasmatic nucleus of the Syrian hamster

Aging leads to many changes in the expression of circadian rhythms, including reduced amplitude, altered relationship to the environmental illumination cycle, and reduced sensitivity to phase resetting signals. Neuropeptide synthesizing neurons in the suprachiasmatic nucleus (SCN), the principal circadian pacemaker in mammals, play a role in regulating pacemaker function and in coupling the pacemaker to overt circadian rhythms. Aging may alter the activity of neuropeptide neurons in the SCN, which could be reflected in changes in mRNA expression. Therefore, this study investigated whether aging alters the level or rhythm of expression of neuropeptide mRNAs in the SCN of male Syrian hamsters, a well established model for the study of age-related changes in circadian rhythms. Three age groups of hamsters ( young [3–5 months old], middle-aged [12–15 months old] and old [19–22 months old] were sacrificed at five times of day. Their brains were dissected and sections through the suprachiasmatic nucleus were prepared and used for in situ hybridization for mRNAs for vasoactive intestinal peptide (VIP), arginine vasopressin (AVP) and somatostatin (SS). Aging selectively decreased the SCN expression of VIP mRNA without affecting AVP mRNA or SS mRNA. Also, only AVP mRNA expression exhibited a robust 24-h rhythm, in contrast to previous findings in other species that VIP mRNA and SS mRNA, as well as AVP mRNA, exhibit 24-h rhythms in the SCN. The present findings suggest that age-related reductions in VIP mRNA expression may contribute to the alterations in entrainment and attenuated sensitivity to phase resetting signals that are characteristic of aging. Furthermore, the results demonstrate that neuropeptide gene expression in the SCN is differentially regulated by aging and varies among species.

Corticosterone differentially modulates expression of corticotropin releasing factor and arginine vasopressin mRNA in the hypothalamic paraventricular nucleus following either acute or repeated restraint stress.

Exposing rats to repeated restraint stress induces well-characterized adaptations in the expression of either corticotropin-releasing factor (CRF) or arginine-vasopressin (AVP) mRNA in the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN). The effects of regulating corticosterone levels on this adaptation was studied in male rats. In intact rats, acute restraint stress increased the expression of CRF mRNA whilst AVP mRNA expression was no different to control. Repeated exposure resulted in habituation of CRF expression, whereas AVP mRNA increased above that seen in either non stressed or acutely stressed animals. In adrenalectomised rats with replacement pellets of corticosterone that replicated blood levels approximating to the daily trough (mean levels 37--65 ng/mL), basal CRF expression levels were raised, but the response to acute stress was still observed. However, the habituation seen in normal animals that had been repeatedly stressed was prevented, so that CRF mRNA levels continued to be raised after repeated stress. By contrast, the AVP response to both acute and repeated stress was unaltered in these low-dose corticosterone-treated rats compared with controls. Higher dose pellets, which resulted in blood levels around those of the daily maximum (mean 118--141 ng/mL) had the opposite effects. There was no change compared to intact rats in the expression of CRF mRNA following either acute or repeated stress, but the expected increase in AVP following repeated restraint was prevented. These experiments show that corticosterone has important modulating effects on the adaptive pattern of both CRF and AVP mRNA expression in the parvocellular PVN. The 'set-point' of corticosterone differs; for CRF, experiencing higher levels is necessary for subsequent adaptation to repeated restraint to occur, whereas for AVP a return to lower levels is necessary to allow this peptide to respond to repeated stress.

Increased Colocalization of Corticotropin‐Releasing Factor and Arginine Vasopressin in Paraventricular Neurones of the Hypothalamus in Lactating Rats: Evidence from Immunotargeted Lesions and Immunohistochemistry

AbstractIn lactating female rats, tonically elevated glucocorticoid secretion is accompanied by blunted stress responsiveness, reduced expression of hypothalamic corticotropin‐releasing factor (CRF) mRNA and modest increases in arginine vasopressin (AVP) expression in the paraventricular nucleus (PVN). To determine the relative contribution of CRF and AVP to parvocellular function, we performed selective CRF (CRF‐Tx) or AVP (AVP‐Tx) lesions in the PVN neurones of ovariectomized virgin or lactating females (day 2 of lactation) by using ricin A associated with monoclonal antibodies directed towards CRF or AVP. We also performed double immunohistochemical labelling of CRF and AVP in the PVN of control rats injected with immunoglobulin (Ig)Gs associated with the ricin A (IgG‐Tx). Brains were collected 12 days after the lesion and processed for in situ hybridization of CRF and AVP mRNA or for double fluorescence CRF and AVP immunohistochemistry. We found that lactating females exhibit a high degree of CRF and AVP colocalization in parvocellular PVN neurones, hypothalamic processes and median eminence terminals compared to virgins. While CRF mRNA is significantly reduced in lactating rats, AVP mRNA and protein levels are greatly enhanced in parvocellular PVN neurones during lactation. Hypothalamic CRF or AVP ricin‐A lesions significantly reduced both CRF and AVP expression (15–35% decrease) as well as peptide immunoreactivity in PVN neurones in both groups of females. The specificity of the lesions varied between virgins and lactators since in virgin females, AVP‐Tx did not affect CRF mRNA expression whereas in lactating females, this same lesion significantly reduced CRF mRNA expression, suggesting that parvocellular PVN neurones are more sensitive to the effects of the lesions during lactation. In both virgins and lactators, lesion with CRF‐Tx tended to increase AVP mRNA expression; however, in virgins, parvocellular PVN neurones were possibly compensating for the loss of CRF synthesis by increasing AVP expression and immunoreactivity. We conclude that lactation is associated with a high degree of CRF and AVP colocalization in parvoPVN neurones and that the increased AVP production in these neurones increases their sensitivity to immunotargeted lesions. The opposite regulation of CRF and AVP gene expression during lactation might provide a useful model to study differential sensitivity to glucocorticoid feedback or hypothalamic activation of transcription factors.

Increased colocalization of corticotropin-releasing factor and arginine vasopressin in paraventricular neurones of the hypothalamus in lactating rats: evidence from immunotargeted lesions and immunohistochemistry.

In lactating female rats, tonically elevated glucocorticoid secretion is accompanied by blunted stress responsiveness, reduced expression of hypothalamic corticotropin-releasing factor (CRF) mRNA and modest increases in arginine vasopressin (AVP) expression in the paraventricular nucleus (PVN). To determine the relative contribution of CRF and AVP to parvocellular function, we performed selective CRF (CRF-Tx) or AVP (AVP-Tx) lesions in the PVN neurones of ovariectomized virgin or lactating females (day 2 of lactation) by using ricin A associated with monoclonal antibodies directed towards CRF or AVP. We also performed double immunohistochemical labelling of CRF and AVP in the PVN of control rats injected with immunoglobulin (Ig)Gs associated with the ricin A (IgG-Tx). Brains were collected 12 days after the lesion and processed for in situ hybridization of CRF and AVP mRNA or for double fluorescence CRF and AVP immunohistochemistry. We found that lactating females exhibit a high degree of CRF and AVP colocalization in parvocellular PVN neurones, hypothalamic processes and median eminence terminals compared to virgins. While CRF mRNA is significantly reduced in lactating rats, AVP mRNA and protein levels are greatly enhanced in parvocellular PVN neurones during lactation. Hypothalamic CRF or AVP ricin-A lesions significantly reduced both CRF and AVP expression (15-35% decrease) as well as peptide immunoreactivity in PVN neurones in both groups of females. The specificity of the lesions varied between virgins and lactators since in virgin females, AVP-Tx did not affect CRF mRNA expression whereas in lactating females, this same lesion significantly reduced CRF mRNA expression, suggesting that parvocellular PVN neurones are more sensitive to the effects of the lesions during lactation. In both virgins and lactators, lesion with CRF-Tx tended to increase AVP mRNA expression; however, in virgins, parvocellular PVN neurones were possibly compensating for the loss of CRF synthesis by increasing AVP expression and immunoreactivity. We conclude that lactation is associated with a high degree of CRF and AVP colocalization in parvoPVN neurones and that the increased AVP production in these neurones increases their sensitivity to immunotargeted lesions. The opposite regulation of CRF and AVP gene expression during lactation might provide a useful model to study differential sensitivity to glucocorticoid feedback or hypothalamic activation of transcription factors.

Multinucleated Arginine-Vasopressin Neurons in the Human Supraoptic Nucleus: A Hallmark of Pulmonary Pathology

The human supraoptic nucleus (SON) is the main production site of plasma arginine-vasopressin (AVP). The present study aimed to define the subpopulation of patients in which multinucleated SON neurons (MNN) are present. We determined the proportion of MNN in immunocytochemically defined SON AVP population and compared AVP mRNA levels in the SON of 29 patients with and without MNN. Interestingly, MNN appeared to be present in a high proportion in patients with pneumonia and other pulmonary pathologies (Pul P). The percentage of MNN in the SON of Pul P patients turned out to be age- and sex-dependent. In young women with Pul P their proportion was 10 times higher than in women without such a pathology and in young men with Pul P their frequency was 22 times higher than in other men. In those patients with the highest proportion of MNN, i.e. young females and males with Pul P, AVP mRNA expression in the SON was the lowest. In addition, young women (≤50 years old) had lower AVP mRNA levels than young men and than elderly women (>50 years old). In conclusion, our study suggests that multinucleated neurons are a hallmark of Pul P in the human SON and that this phenomenon may be accompanied by lower AVP production in young subjects.

Centrally Administered Murine Leptin Stimulates Plasma Arginine-Vasopressin Secretion and Increases the Level of mRNA Expression in the Supraoptic Nucleus of Conscious Rats

The product of the ob gene protein, leptin, has been suggested to function as an endogenous mediator of the cardiovascular system via sympathetic nerve activity. Moreover, extensive distribution of leptin receptor-like immunoreactivity has been demonstrated in the choroid plexus, cerebral cortex, hippocampus, thalamus and hypothalamus, especially in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In this study, we have investigated the in vivo effects of leptin on plasma arginine-vasopressin (AVP) secretion and the level of AVP messenger ribonucleotic acid (AVP mRNA) in the SON of conscious rats. Intracerebroventricularly administered leptin increased plasma AVP concentration in a dose-dependent manner (0–400 pmol/rat). The maximal effect was obtained at 15 min after the administration of leptin. Furthermore, in Northern blot analyses, the levels of AVP mRNa in the SON increased approximately 2-fold from the basal level after the administration of leptin. AVP mRNA expression in the PVN was also increased by leptin. However, leptin had no effects on plasma oxytocin (OXT) secretion and OXT gene expression in the SON. In conclusion, leptin is involved in AVP secretion via the central nervous system, however, its physiological role is unknown.

Independent and Overlapping Effects of Corticosterone and Testosterone on Corticotropin-Releasing Hormone and Arginine Vasopressin mRNA Expression in the Paraventricular Nucleus of the Hypothalamus and Stress-Induced Adrenocorticotropic Hormone Release

Adrenocorticotropin (ACTH) release is regulated by both glucocorticoids and androgens; however, the precise interactions are unclear. We have controlled circulating corticosterone (B) and testosterone (T) by adrenalectomy (ADX) +/- B replacement and gonadectomy (GDX) +/- T replacement, comparing these to sham-operated groups. We hoped to reveal how and where these neuroendocrine systems interact to affect resting and stress-induced ACTH secretion. ADX responses. In gonadal-intact rats, ADX increased corticotropin-releasing factor (CRH) and vasopressin (AVP) mRNA in hypothalamic parvocellular paraventricular nuclei (PVN) and ACTH in pituitary and plasma. B restored these toward normal. GDX blocked the increase in AVP but not CRH mRNA and reduced plasma, but not pituitary ACTH in ADX rats. GDX+T restored increased AVP mRNA in ADX rats, although plasma ACTH remained decreased. Stress responses. Restraint-induced ACTH responses were elevated in ADX gonadally intact rats, and B reduced these toward normal. GDX in adrenal-intact and ADX+B rats increased ACTH responses. Without B, T did not affect ACTH; together with B, T restored ACTH responses to normal. The magnitude of ACTH responses to stress was paralleled by similar effects on the number of c-fos staining neurons in the hypophysiotropic PVN. We conclude that gonadal regulation of ACTH responses to ADX is determined by T dependent effects on AVP biosynthesis, whereas CRH biosynthesis is B-dependent. Stress-induced ACTH release is not explained by B and T interactions at the PVN, but is determined by B- and T-dependent changes in drive to PVN motorneurons.

Sex differences in the daily rhythm of vasoactive intestinal polypeptide but not arginine vasopressin messenger ribonucleic acid in the suprachiasmatic nuclei.

The timing of the preovulatory surge of LH in female rodents is tightly coupled to the environmental light/dark cycle. This coupling is mediated by the circadian pacemaker located in the suprachiasmatic nuclei (SCN). Studies indicate that vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), which are synthesized in the SCN, transmit circadian information from the SCN to GnRH neurons, thereby regulating the timing of the LH surge. However, to date, the rhythmic expression of these two peptides in the SCN has only been examined in males. The pattern of VIP expression in males is difficult to reconcile with its role in the LH surge. The purpose of the present study was to assess the rhythm of VIP messenger RNA (mRNA) levels in the SCN of female rats under several endocrine conditions. We compared this rhythm to that in males and to AVP mRNA rhythms in all experimental groups. In all groups of females, VIP mRNA levels were rhythmic, with peak expression occurring during the light phase and a nadir occurring during the dark phase. The rhythm was approximately 12 h out of phase compared with that in males. The rhythmic expression of AVP mRNA in the SCN was virtually identical in all groups of animals. Based on these results, we conclude that 1) the rhythm of VIP seen in the SCN of females during the day may serve as a facilitory signal from the SCN to GnRH neurons; 2) the sex-specific pattern of VIP mRNA does not depend on estradiol; and 3) AVP gene expression within the SCN is not sexually differentiated or altered by estradiol.

Pathophysiology of renal fluid retention

Central to a unifying hypothesis of body fluid regulation is maintenance of arterial circulatory integrity. This may be disturbed by arterial underfilling, either from reduction in cardiac output or by peripheral arterial vasodilation. In cardiac failure (CF), cardiac output falls and the nonosmotic release of arginine vasopressin (AVP) and expression of AVP mRNA in the hypothalamus are stimulated. V2 AVP receptor antagonists correct the impaired water excretion in rats with low-output CF, increase solute free water clearance, correct the hyponatremia in congestive CF patients, and normalize urinary concentrations of the aquaporin-2 (AQP-2) water channels. In conditions associated with peripheral vasodilation, such as cirrhosis, nonosmotic release of AVP also occurs, and AQP-2 gene expression in the rat kidney is up-regulated. In cirrhosis, nitric oxide-mediated vasodilation occurs early prior to water retention. V2 antagonists reverse the latter. In normal pregnancy, plasma AVP is relatively high for the degree of hypoosmolality. Pregnant rats up-regulate AQP-2 in the renal papilla, an effect reversed by V2 receptor antagonists. This supports the hypothesis that AVP is an important mediator of renal water retention in pregnancy. In summary, AVP-mediated water retention through collecting duct AQP-2 water channels is important in both low-output CF and high-output states such as cirrhosis and pregnancy. V2 receptor antagonists reverse the water retention and down-regulate AQP-2 water channels.

Neuronal histamine and expression of corticotropin-releasing hormone, vasopressin and oxytocin in the hypothalamus: relative importance of H1 and H2 receptors.

Centrally administered histamine (HA) stimulates the secretion of the pro-opiomelanocortin-derived peptides ACTH and beta-endorphin as well as prolactin. The effect of HA on secretion of these adenohypophysial hormones is indirect and may involve activation of hypothalamic neurons containing corticotropin-releasing hormone (CRH), arginine-vasopressin (AVP) or oxytocin (OT). We studied the effect of activating central HA receptors by central infusion of HA, HA agonists or antagonists on expression of CRH, AVP and OT mRNA in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Intracerebroventricular infusion of HA (270 nmol), the H1-receptor agonist 2-thiazolylethylamine or the H2-receptor agonist 4-methylhistamine increased the level of CRH mRNA in the PVN, and OT mRNA in the SON. In contrast, none of these compounds had any effect on expression of AVP mRNA in the PVN or SON. Administration of the H1-receptor antagonist mepyramine or the H2-receptor antagonist cimetidine had no effect on basal expression of CRH, AVP or OT mRNA in the PVN and/or SON except for a slight inhibitory effect of cimetidine on CRH mRNA expression in the PVN. Pretreatment with mepyramine or cimetidine before HA administration inhibited the HA-induced increase in OT mRNA levels but had no effect on the HA-induced increase in CRH mRNA levels in the PVN. We conclude that HA stimulates hypothalamic CRH and OT neurons by increasing mRNA levels, and this effect seems to be mediated via activation of both HA H1 and H2 receptors.

Increased Expression of Magnocellular Vasopressin mRNA in Rats with Deoxycorticosterone-Acetate Induced Salt Appetite

The neuropeptides arginine vasopressin (AVP) and oxytocin (OT) have been implicated in the genesis of hypertension due to deoxycorticosterone acetate (DOCA)-salt treatment of uninephrectomized rats. In this work, we studied if DOCA treatment of intact rats in doses arousing a salt appetite (a prehypertensive state), modulated mRNA for AVP and OT in the hypothalamus. Male Sprague-Dawley rats were offered both tap water and 3% NaCl in separate bottles and received vehicle or subcutaneous injections of 10 mg DOCA on alternate days for 7 days (4 injections) or 17 days (9 injections). They developed a preference for 3% NaCl solutions 24–48 h after treatment. Brain slices from rats killed on the 8th or 18th day were exposed to ³⁵S-labeled probes encoding prepro-AVP mRNA or OT mRNA, respectively. Expression of these mRNAs was measured in the magnocellular and parvocellular divisions of the paraventricular nucleus (PVN) and magnocellular cells of the supraoptic nucleus (SON). No changes were obtained in neuropeptide mRNA levels in the parvocellular division of the PVN between control and the two groups of DOCA-treated rats. However, DOCA-treated animals presented an increased number of grains per cell for AVP mRNA in the magnocellular division of the PVN and in magnocellular cells of the SON, as shown by group mean comparisons and frequency histograms. No changes were detected for OT mRNA. In a second series of studies, control or DOCA-treated rats were offered 3% NaCl or water as the only choice. Animals drinking 3% NaCl showed increased AVP and OT mRNA levels, whether they received DOCA or not. However, AVP mRNA levels in both nuclei were higher in DOCA-treated rats drinking 3% NaCl than in controls drinking salt solution. In comparison, control and DOCA-treated rats drinking water showed lower levels of AVP mRNA. OT mRNA levels in the SON remained unchanged in the same groups. The results suggest that in the magnocellular cells of the PVN and SON, increments in AVP mRNA are obtained following increments in salt intake produced by either mineralocorticoid treatment or exclusive salt drinking. In rats offered salt solution and water to drink, DOCA effects on AVP mRNA developed before changes occurred in serum sodium levels. Because combined DOCA + salt treatment induced a higher response in terms of AVP mRNA expression, we suggest that AVP could be a target of the central effects of the mineralocorticoid.

Centrally administered parathyroid hormone (PTH)-related protein(1-34) but not PTH(1-34) stimulates arginine-vasopressin secretion and its messenger ribonucleic acid expression in supraoptic nucleus of the conscious rats.

It has been suggested that PTH-related protein (PTHrP) is an endogenous modulator of cardiovascular systems. We have reported that PTHrP(1-34), but not PTH(1-34), causes the release of arginine-vasopressin (AVP) from the supraoptic nucleus (SON) of the hypothalamus in vitro through a novel receptor distinct from the PTH/PTHrP receptors (type I or type II) described previously. In this study, we have investigated the in vivo effects of PTHrP(1-34) on AVP secretion and its, messenger RNA (mRNA) expression in the SON in conscious rats. Intracerebroventricular (i.c.v.) administration of PTHrP(1-34) resulted in an increase in plasma AVP concentration in a dose-dependent manner (0-400 pmol/rat). The maximal effect was obtained at 15 min after i.c.v. administration of PTHrP(1-34). Neither PTHrP(7-34) nor PTH(1-34) had any effect on plasma AVP levels. PTHrP(1-34)-induced AVP secretion was antagonized by pretreatment with PTHrP(7-34) but not by that with PTH(1-34). In addition, in situ hybridization study revealed that AVP mRNA expression in the SON and paraventricular nucleus was significantly increased 30 min after i.c.v. administration of PTHrP(1-34) and reached a maximum at 180 min. Furthermore, in Northern blot analyses, AVP mRNA expression in the SON was increased to approximately a 2-fold of basal level by PTHrP(1-34). On the other hand, neither PTHrP(7-34) or PTH(1-34) had any effect on the mRNA expression. The PTHrP(1-34)-stimulated AVP mRNA expression was eliminated by pretreatment with PTHrP(7-34) but not with PTH(1-34). These results suggest that, in the central nervous system, PTHrP(1-34) is involved in AVP secretion through a novel receptor distinct from the PTH/PTHrP receptors reported previously, playing a role in the body water and electrolyte homeostasis.

Sex difference in glucocorticoid regulation of vasopressin mRNA in the paraventricular hypothalamic nucleus.

1. Arginine vasopressin (AVP) is synthesized in specific brain regions including the magnocellular and parvocellular divisions of the paraventricular nucleus (PVN). Whereas magnocellular AVP responds to osmotic stimuli and functions mainly--although not exclusively--as an antidiuretic hormone, that produced in the parvocellular region controls the hypothalamus-pituitary-adrenal (HPA) axis, in conjunction with CRF. 2. In view of the reported sex differences in control of the HPA axis, we studied if these also pertain to AVP mRNA in the PVN of ovariectomized-estrogenized female rats and male rats determined by in situ hybridization. AVP mRNA was measured in intact rats, adrenalectomized (ADX) rats and ADX receiving dexamethasone (DEX) of both sexes. 3. Computerized autoradiography showed that in both sexes, AVP mRNA levels in the parvocellular division of the PVN increased after adrenalectomy and decreased following DEX. However, the reduction by DEX was more pronounced in female rats. No changes were found for the magnocellular region. Grain counting analysis of the medial-medial (MMP) and medial-lateral (MLP) subdivisions of the parvocellular region showed that the average number of grains per cell area in the MMP region of adrenally intact female rats was higher than that in males. However, in females there was no clear-cut effect of adrenalectomy on AVP mRNA levels, although the reduction after DEX treatment was again greater than that in male rats. Frequency histograms constructed by plotting the number of cells vs the number of grains per area substantiated the enhanced glucocorticoid negative control of AVP mRNA in the MMP and MLP of female rats. 4. The results indicated a sexual dimorphism in the glucocorticoid-dependent plasticity of AVP mRNA levels in the PVN. Because AVP mRNA expression differs between sexes under basal levels, after adrenalectomy, and after DEX treatment, these plastic changes may differentially condition the response to stress. Taking into consideration that stress and AVP may play a role in neurogenic hypertension, the possibility of sexual dimorphisms in AVP control may be important to assess the role of sex hormones in stress and steroid-derived hypertension.

Photoperiodic exposure and time of day modulate the expression of arginine vasopressin mRNA and vasoactive intestinal peptide mRNA in the suprachiasmatic nuclei of Siberian hamsters.

In hamsters, changes in ambient photoperiod lead to alterations in the circadian rhythm of pineal melatonin secretion and subsequent changes in reproductive function. The present study examined whether photoperiod also alters 24-h rhythms in neuropeptide mRNA levels in the SCN of Siberian hamsters. In situ hybridization and quantitative autoradiography were used to assess messenger RNA levels for vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the SCN of hamsters sacrificed at six times of day following exposure to long (16 h light/day) or short (10 h light/day) photoperiod for 2 weeks. Both AVP mRNA and VIP mRNA in the SCN were significantly affected by time of day and photoperiodic exposure. The 24-h profiles of AVP mRNA and VIP mRNA showed different relationships to the light: dark cycle, suggesting that these profiles are differentially regulated. In general, short photoperiod tended to suppress AVP mRNA and VIP mRNA in the SCN; this effect on AVP mRNA was significant at two times of day. These results complement and extend previous findings of 24-h h profiles in neuropeptide mRNA expression in the rat SCN by showing that these 24-h profiles are also characteristic of the Siberian hamster SCN and that they can be modulated by photoperiod.

Evidence for Short‐Loop Feedback Effects of ACTH on CRF and Vasopressin Expression in Parvocellular Neurosecretory Neurons

Immuno- and hybridization histochemical methods were used to examine a possible role for adrenocorticotropic hormone (ACTH) in regulating the expression of corticotropin-releasing peptides in rat hypothalamus. Densitometric assessments of relative levels of mRNAs encoding corticotropin releasing factor (CRF), arginine vasopressin (AVP) and oxytocin (OT) in the parvocellular division of the paraventricular nucleus (PVH) were carried out in intact, adrenalectomized (ADX) and hypophysectomized (HYPOX) animals. Both surgeries resulted in comparable increases in relative levels of CRF and AVP transcripts in the parvocellular PVH; no effects on OT mRNA in this compartment were evident. In a second experiment, ACTH or saline vehicle were administered systemically via osmotic minipump for seven days to rats submitted to both HYPOX and ADX surgeries. Lower replacement doses of ACTH reduced the number of detectable AVP-immunoreactive (AVP-ir) cells in the parvocellular PVH to 53% of that seen in vehicle-treated HYPOX/ADX controls; the number of CRF-IR cells was not significantly affected. Higher doses of ACTH resulted in counts of AVP- and CRF-IR neurons that were reduced to 32% and 70%, respectively, of control values. Staining patterns for the two peptides in the external lamina of the median eminence generally followed the cell count data. Neither densitometric nor combined immunohistochemical (for CRF-ir) and hybridization histochemical (for AVP mRNA) assays revealed any marked effect of ACTH on AVP mRNA expression in the parvocellular PVH of HYPOX/ADX rats. The results indicate that ACTH is capable of inhibiting corticotropin-releasing peptide, but not mRNA, expression in hypophysiotropic neurons. The mechanisms underlying these effects remain to be fully clarified.

Dehydration stimulates hypothalamic gene expression of histamine synthesis enzyme: importance for neuroendocrine regulation of vasopressin and oxytocin secretion

Dehydration associated with hyperosmolality and decreased extracellular volume stimulates arginine vasopressin (AVP) and oxytocin (OT) secretion from magnocellular neurons of the hypothalamus. The effects of hyperosmolality and decreased extracellular volume on the magnocellular neurons are mainly indirect and seem to be mediated centrally via several neurotransmitters and neuropeptides. Because histamine (HA), which serves as a central neurotransmitter, releases AVP and OT from the neurohypophysis when administered centrally, we investigated the possible role of HA in dehydration-induced AVP and OT secretion. To do this, we studied 1) the effect of dehydration on messenger RNA (mRNA) expression of the HA synthesis enzyme, histidine decarboxylase (HDC), in the tuberomammillary nucleus of the hypothalamus; and 2) the effect of HA synthesis inhibition during dehydration on AVP and OT mRNA expression in the supraoptic nucleus of the hypothalamus as well as on plasma AVP and OT levels. Forty-eight hours of dehydration increased the mRNA level of HDC in the tuberomammillary nuclei, whereas 24 h of dehydration had no effect. Pretreatment with the HA synthesis inhibitor alpha-fluoromethylhistidine (alpha FMH) increased the expression of HDC mRNA in 24-h dehydrated rats, but had no effect in euhydrated rats. In rats dehydrated for 48 h, the already increased level of HDC mRNA was not increased further by alpha FMH. Twenty-four and 48 h of dehydration increased AVP and OT mRNA levels in the supraoptic nucleus. This effect was inhibited by alpha FMH pretreatment. Dehydration increased the plasma levels of AVP and OT to an extent which depended on the duration of dehydration. Pretreatment with alpha FMH inhibited the hormone responses to 24 h of dehydration, but did not affect the responses to 48 h of dehydration. Twenty-four and 48 h of dehydration had no significant effect on the contents of AVP and OT in the neurohypophysis, whereas pretreatment with alpha FMH combined with 48 h of dehydration led to depletion of AVP stores in the neurohypophysis. Based on the present findings, we conclude that hypothalamic histaminergic neurons are involved in the regulation of dehydration-induced stimulation of magnocellular AVP and OT neurons.

Dehydration stimulates hypothalamic gene expression of histamine synthesis enzyme: importance for neuroendocrine regulation of vasopressin and oxytocin secretion.

Dehydration associated with hyperosmolality and decreased extracellular volume stimulates arginine vasopressin (AVP) and oxytocin (OT) secretion from magnocellular neurons of the hypothalamus. The effects of hyperosmolality and decreased extracellular volume on the magnocellular neurons are mainly indirect and seem to be mediated centrally via several neurotransmitters and neuropeptides. Because histamine (HA), which serves as a central neurotransmitter, releases AVP and OT from the neurohypophysis when administered centrally, we investigated the possible role of HA in dehydration-induced AVP and OT secretion. To do this, we studied 1) the effect of dehydration on messenger RNA (mRNA) expression of the HA synthesis enzyme, histidine decarboxylase (HDC), in the tuberomammillary nucleus of the hypothalamus; and 2) the effect of HA synthesis inhibition during dehydration on AVP and OT mRNA expression in the supraoptic nucleus of the hypothalamus as well as on plasma AVP and OT levels. Forty-eight hours of dehydration increased the mRNA level of HDC in the tuberomammillary nuclei, whereas 24 h of dehydration had no effect. Pretreatment with the HA synthesis inhibitor alpha-fluoromethylhistidine (alpha FMH) increased the expression of HDC mRNA in 24-h dehydrated rats, but had no effect in euhydrated rats. In rats dehydrated for 48 h, the already increased level of HDC mRNA was not increased further by alpha FMH. Twenty-four and 48 h of dehydration increased AVP and OT mRNA levels in the supraoptic nucleus. This effect was inhibited by alpha FMH pretreatment. Dehydration increased the plasma levels of AVP and OT to an extent which depended on the duration of dehydration. Pretreatment with alpha FMH inhibited the hormone responses to 24 h of dehydration, but did not affect the responses to 48 h of dehydration. Twenty-four and 48 h of dehydration had no significant effect on the contents of AVP and OT in the neurohypophysis, whereas pretreatment with alpha FMH combined with 48 h of dehydration led to depletion of AVP stores in the neurohypophysis. Based on the present findings, we conclude that hypothalamic histaminergic neurons are involved in the regulation of dehydration-induced stimulation of magnocellular AVP and OT neurons.