Influence of endogenous and exogenous oestrogens on posterior pituitary secretion in women

Aquaporins in Diabetes Insipidus

Disruption of water and electrolyte balance is frequently encountered in clinical medicine. Regulating water metabolism is critically important. Diabetes insipidus (DI) presented with excessive water loss from the kidney is a major disorder of water metabolism. To understanding the molecular and cellular mechanisms and pathophysiology of DI and rationales of clinical management of DI is important for both research and clinical practice. This chapter will first review various forms of DI focusing on central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI). This is followed by a discussion of regulatory mechanisms underlying CDI and NDI, with a focus on the regulatory axis of vasopressin, vasopressin receptor 2 (V2R) and the water channel molecule, aquaporin 2 (AQP2). The clinical manifestation, diagnosis, and management of various forms of DI will also be discussed with highlights of some of the latest therapeutic strategies that are developed from in vitro experiments and animal studies.

Oxytocin therapy in hypopituitarism: Challenges and opportunities

Patients with hypopituitarism display impaired quality of life and excess morbidity and mortality, despite apparently optimal pituitary hormone replacement. Oxytocin is a neuropeptide synthesized in the anterior hypothalamus which plays an important role in controlling social and emotional behaviour, body weight and metabolism. Recent studies have suggested that a deficiency of oxytocin may be evident in patients with hypopituitarism and craniopharyngioma, and that this may be associated with deficits in cognitive empathy. Preliminary data hint at potential benefits of oxytocin therapy in improving these deficits and the accompanying metabolic disturbances that are common in these conditions. However, several challenges remain, including an incomplete understanding of the regulation and mechanisms of action of oxytocin, difficulties in accurately measuring oxytocin levels and in establishing a diagnosis of oxytocin deficiency, and a need to determine both the optimal mode of administration for oxytocin therapy and an acceptable safety profile with long-term use. This review considers the data linking oxytocin to the neuropsychological and metabolic disturbances evident in patients with craniopharyngioma and hypopituitarism, and describes the challenges that need to be overcome before replacement therapy can be considered as a therapeutic option in clinical practice.

Menstrual cycle-related fluctuations in oxytocin concentrations: A systematic review and meta-analysis

Oxytocin affects physiological and psychological functions that are often expressed sex-specifically, suggesting interactions between oxytocin and sex hormones. As female sex hormone concentrations change during the menstrual cycle, oxytocin might fluctuate, too. This systematic review and meta-analysis investigated endogenous oxytocin concentrations across menstrual cycle phases in healthy women. Data from 13 studies (120 women) showed a significant increase of oxytocin concentrations from the early follicular phase to ovulation (g = 0.39 [0.25; 0.53], p < .001) and a significant decrease from ovulation to the mid-luteal phase (g = -0.50 [-0.81; -0.18], p < .001). There were no significant differences between the early follicular and mid-luteal phase (g = -0.19 [-0.70; -0.32], p = .471). These findings contribute to a deeper understanding of differences in normal and abnormal psychobiological processes in women. They highlight the necessity to consider the menstrual cycle phase in studies on oxytocin in women.

Water intake keeps type 2 diabetes away? Focus on copeptin

INTRODUCTION

In both diabetic subjects and animal models high levels of vasopressin (AVP) have beendetected. The relationship between AVP and glucose metabolism is mediated through several direct andindirect effects and most of them are still unknown.

METHODS

We have reviewed 100 manuscripts retrieved from Cochrane Library, Embase and Pubmeddatabases in order to highlight a possible relationship between copeptin and type 2 diabetes and to provideinsights on the molecular mechanism that could explain this association.

RESULTS AND CONCLUSIONS

AVP potentiates CRH action at pituitary level resulting in an increased ACTH secretion and in turn in an increased cortisol secretion that escapes the negative feedback loop. Further, AVP regulates insulin and glucagon secretion through V1b receptor and promotes hepatic glycogenolysis and gluconeogenesis through V1a receptor. In addition to worsen glucose metabolism, AVP has been reported to have a role in the pathogenesis of diabetic complications such as cardiovascular diseases, kidney and ocular complications. Due to the very low concentration of AVP in the blood, the small size and poor stability, the assay of AVP is very difficult to perform. Thus, copeptin, the stable C-terminal portion of the prepro-vasopressin peptide has been identified as an easier assay to be measured and that mirrors AVP activity. Although there are promising evidence that copeptin could be involved in the pathogenesis of type 2 diabetes, further studies need to demonstrate the importance of copeptin as clinical marker to predict glucose metabolism derangements.

Diabetes Insipidus

Disruption of water and electrolyte balance is frequently encountered in clinical medicine. Regulating water metabolism is critically important. Diabetes insipidus (DI) presented with excessive water loss from the kidney is a major disorder of water metabolism. To understand the molecular and cellular mechanisms and pathophysiology of DI and rationales of clinical management of DI is important for both research and clinical practice. This chapter will first review various forms of DI focusing on central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI ) . This is followed by a discussion of regulatory mechanisms underlying CDI and NDI , with a focus on the regulatory axis of vasopressin, vasopressin receptor 2 (V2R ) and the water channel molecule, aquaporin 2 (AQP2 ). The clinical manifestation, diagnosis and management of various forms of DI will also be discussed with highlights of some of the latest therapeutic strategies that are developed from in vitro experiments and animal studies.

Copeptin levels remain unchanged during the menstrual cycle

BACKGROUND

Copeptin, a surrogate marker for arginin vasopressin production, is evaluated as an osmo-dependent stress and inflammatory biomarker in different diseases. We investigated copeptin during the menstrual cycle and its relationship to sex hormones, markers of subclinical inflammation and estimates of body fluid.

METHODS

In 15 healthy women with regular menstrual cycles, blood was drawn on fifteen defined days of their menstrual cycle and was assayed for copeptin, progesterone, estradiol, luteinizing hormone, high-sensitive C-reactive protein, tumor necrosis factor-alpha and procalcitonin. Symptoms of fluid retention were assessed on each visit, and bio impedance analysis was measured thrice to estimate body fluid changes. Mixed linear model analysis was performed to assess the changes of copeptin across the menstrual cycle and the relationship of sex hormones, markers of subclinical inflammation and estimates of body fluid with copeptin.

RESULTS

Copeptin levels did not significantly change during the menstrual cycle (p = 0.16). Throughout the menstrual cycle, changes in estradiol (p = 0.002) and in the physical premenstrual symptom score (p = 0.01) were positively related to copeptin, but changes in other sex hormones, in markers of subclinical inflammation or in bio impedance analysis-estimated body fluid were not (all p = ns).

CONCLUSION

Although changes in estradiol and the physical premenstrual symptom score appear to be related to copeptin changes, copeptin does not significantly change during the menstrual cycle.

Estrogen receptors: their roles in regulation of vasopressin release for maintenance of fluid and electrolyte homeostasis

Long standing interest in the impact of gonadal steroid hormones on fluid and electrolyte balance has led to a body of literature filled with conflicting reports about gender differences, the effects of gonadectomy, hormone replacement, and reproductive cycles on plasma vasopressin (VP), VP secretion, and VP gene expression. This reflects the complexity of gonadal steroid hormone actions in the body resulting from multiple sites of action that impact fluid and electrolyte balance (e.g. VP target organs, afferent pathways regulating the VP neurons, and the VP secreting neurons themselves). It also reflects involvement of multiple types of estrogen receptors (ER) in these diverse sites including ERs that act as transcription factors regulating gene expression (i.e. the classic ERalpha as well as the more recently discovered ERbeta) and potentially G-protein coupled, membrane localized ERs that mediate rapid non-genomic actions of estrogen. Furthermore, altered expression of these receptors in physiologically diverse conditions of fluid and electrolyte balance contributes to the difficulty of using simplistic approaches such as gender comparisons, gonadectomy, and hormone replacement to assess the role of gonadal steroids in regulation of VP secretion for maintenance of fluid and electrolyte homeostasis. This review catalogs these inconsistencies and provides a frame work for understanding them by describing: (1) the effect of gonadal steroids on target organ responsiveness to VP; (2) the expression of multiple types of estrogen receptors in the VP neurons and in brain regions monitoring feedback signals from the periphery; and (3) the impact of dehydration and hyponatremia on expression of these receptors.

The role of steroid hormones in the regulation of vasopressin and oxytocin release and mRNA expression in hypothalamo-neurohypophysial explants from the rat

Vasopressin and oxytocin release from the neural lobe, and the vasopressin and oxytocin mRNA contents of the supraoptic and paraventricular nuclei are increased by hypertonicity of the extracellular fluid. The factors regulating these parameters can be conveniently studied in perifused explants of the hypothalamo-neurohypophysial system that include the supraoptic nucleus (but not the paraventricular nucleus) with its axonal projections to the neural lobe. Vasopressin and oxytocin release and the mRNA content of these explants respond appropriately to increases in the osmolality of the perifusate. This requires synaptic input from the region of the organum vasculosum of the lamina terminalis. Glutamate is a likely candidate for transmitting osmotic information from the organum vasculosum of the lamina terminalis to the magnocellular neurones, because agonists for excitatory amino acid receptors stimulate vasopressin and oxytocin release, and because increased vasopressin release and mRNA content induced in hypothalamo-neurohypophysial explants by a ramp increase in osmolality are blocked by antagonists of both NMDA (N-methyl-D-aspartate) and non-NMDA glutamate receptors. Osmotically stimulated vasopressin release is also blocked by testosterone, dihydrotestosterone, oestradiol and corticosterone. Both oestrogen and dihydrotestosterone block NMDA stimulation of vasopressin release, and in preliminary studies oestradiol blocked AMPA stimulation of vasopressin release. Thus, steroid inhibition of osmotically stimulated vasopressin secretion may reflect inhibition of mechanisms mediated by excitatory amino acids. Recent studies have demonstrated numerous mechanisms by which steroid hormones may impact upon neuronal function. Therefore, additional work is warranted to understand these effects of the steroid hormones on vasopressin and oxytocin secretion and to elucidate the potential contribution of these mechanisms to regulation of hormone release in vivo.

Neurohypophysial hormone and melatonin secretion over the natural and suppressed menstrual cycle in premenopausal women

OBJECTIVE

Vasopressin, oxytocin and melatonin have been reported to be influenced by ovarian steroids. The neurohypophysial hormones have also been shown to display a diurnal pattern of secretion in men. We therefore studied the diurnal pattern of neurohypophysial hormone and melatonin secretion in premenopausal women and in women on oral contraceptives.

DESIGN

Healthy normally cycling premenopausal women were studied over 24 hours during the midfollicular and midluteal phases of the menstrual cycle. Healthy premenopausal women on oral contraceptives were studied over 24 hours at similar times.

SUBJECTS

Eight healthy normally cycling women and 7 healthy premenopausal women on oral contraceptives.

MEASUREMENTS

Plasma vasopressin, oxytocin and melatonin were measured by radioimmunoassay.

RESULTS

Vasopressin concentrations and its nocturnal peak were highest in the follicular phase of the natural menstrual cycle and attenuated in the women on oral contraceptives. Oxytocin concentrations did not vary between the two phases of the menstrual cycle, but increased on oestrogen administration. Overall melatonin secretion was augmented in the women on oral contraceptives.

CONCLUSIONS

Vasopressin release and its nocturnal peak were greatest in the follicular phase of the menstrual cycle, while melatonin secretion was augmented in the women on oral contraception.

Changes in a proposed new neuroendocrine marker of oestrogen receptor function in postpartum women

We describe a novel neuroendocrine test which reflects a central response to activation of oestrogen receptors. This is achieved by measurement of plasma levels of oestrogen-stimulated neurophysin (ESN) following an oestrogen challenge. In normal women the ESN response to ethinyl oestradiol is dose-dependent. This response is attenuated in normal women during the first postpartum month, although it is unchanged in patients with anorexia nervosa, in spite of their similar concurrent hypo-oestrogenic state. The altered puerperal response may result from the acute oestrogen withdrawal which occurs at delivery. The time course of the altered ESN response coincides with the period of maximum risk for puerperal psychosis. The ESN response to oestrogen provides a novel neuroendocrine measure to test the relevance of changes in central oestrogen receptor responsiveness in the pathogenesis of puerperal psychosis.

The effect of an osmotic stimulus on the release of neurohypophysial hormones in the cat: preferential release of vasopressin with a possible involvement of the area postrema

Abstract In cats anaesthetized with intravenous chloralose, the injection of 0.05 to 0.4 ml 1.54 M NaCl solution (hypertonic saline, HS) into a lateral cerebral ventricle caused a large release of vasopressin. The concentration of vasopressin greatly exceeded that of oxytocin in the same samples of plasma. Vasopressin was also released when HS was injected into the fourth ventricle and into the cisterna magna from which there is no access in the cat to the ventricles, but it was less effective by these routes than when injected into a lateral ventricle in the same cat. This suggests a possible action of HS on circumventricular organs related to the third ventricle but also indicates an additional site of action reached from the subarachnoid space which would give access to the ventral and dorsal surfaces of the brainstem. Vasopressin was not released on topical application of HS to the 'nicotine sensitive area' on the ventral surface of the brainstem where nicotine acts to release vasopressin without oxytocin. Vasopressin, however, was released without detectable oxytocin on topical appliction of HS to the dorsal surface of the brainstem either outside the fourth ventricle or to the floor of the ventricle at its distal extremity, in the region of the obex. A possible site where HS acts to cause a preferential release of vasopressin on injection into a lateral ventricle is the area postrema, a circumventricular organ which impinges on the walls of the fourth ventricle at the obex. Preferential release of vasopressin might then be mediated by a selective neural input, possibly through the nucleus of the tractus solitarius, from osmoreceptors in the area postrema to the vasopressin-secreting cells in the supraoptic and paraventricular nuclei.

Central inhibition by gamma-aminobutyric acid and muscimol of the release of vasopressin and oxytocin by an osmotic stimulus in the rat

1. In water-loaded rats under ethanol anaesthesia, the injection of 2-4 microliters 1.54M NaCl solution (hypertonic saline:HS) into a lateral cerebral ventricle (i.c.v.) produced an antidiuretic and a pressor response, together with increased urinary excretion of vasopressin and 'oxytocin-like radioimmunoreactivity' (OLRI). In lactating rats HS also produced a milk-ejection response which was shown to be due to the release of oxytocin. 2. The injection of 20-40 micrograms gamma-aminobutyric acid (GABA) or 40-80 ng muscimol i.c.v. 2 min before HS inhibited the antidiuretic, pressor and milk-ejection responses and reduced the urinary excretion of vasopressin and OLRI. 3. The pressor response to HS was abolished by a ganglion blocking agent but it was not reduced by a vasopressin antagonist. After the antagonist, the antidiuretic response to HS was abolished and the pressor response was accompanied by a diuresis both of which were blocked by muscimol. 4. The threshold dose of HS for an antidiuretic response was 4-8 times higher on injection into the cisterna magna (i.cist.) than when injected i.c.v. GABA, i.v. or i.cist, did not inhibit the response to HS i.c.v. 5. The results confirm other evidence that, in the rat, in contrast some other species, an osmotic stimulus causes release of both vasopressin and oxytocin. This release is blocked by GABA and muscimol. These drugs and HS act at a site reached not from the subarachnoid space but from the cerebral ventricles, probably the hypothalamus. The pressor response to HS under the experimental conditions used is due entirely to central sympathetic stimulation and this effect, as well as the release of vasopressin and oxytocin, is blocked by muscimol.

Regulation of vasopressin secretion

With the development of sensitive and specific radio-immunoassays to measure the low circulating concentrations of vasopressin there has been a quantum leap in our understanding of the physiological processes involved in the regulation of its secretion. The results of Verney's pioneering studies in dogs led to the concept of 'osmoreceptors'. It is now appreciated that osmoregulation of vasopressin release is of principal importance in the maintenance of water balance. Functional characteristics of the osmoregulatory system have been defined clearly by independent laboratories, and more recently the physiological influences that can subtly alter this very finely controlled system have been described. Non-osmotic factors that release vasopressin have been recognized for many years. Secretion of vasopressin in response to haemodynamic influences has been characterized, and significant hypotension and/or hypovolaemia are potent stimuli to hormone release. Other non-osmotic factors--nausea/emesis, hypoglycaemia--may play important roles in disturbances of water balance. Vasopressin should not, however, be regarded as a stress hormone, since recent careful studies in a variety of species indicate that secretion is not enhanced following a series of different noxious stimuli.

Vasopressin: sexual dimorphism in secretion, cardiovascular actions and hypertension

We have investigated the issue of sexual dimorphism in the secretion of vasopressin, its pressor action, and the development of deoxycorticosterone (DOC)-salt hypertension. In normal human subjects on controlled salt intake, the basal secretion of vasopressin, indicated by plasma vasopressin levels and urinary excretion of vasopressin, was higher in men than in women and in blacks than in whites. Basal vasopressin secretion also was higher in male than in female rats. This effect was not associated with a difference in the metabolic clearance of the hormone. The sex-related difference in vasopressin release in rats was abolished by gonadectomy and restored by treatment of males with testosterone and females with ovarian hormones. The pressor responsiveness to vasopressin was higher in male than in randomly cycling female rats. Finally, DOC-salt hypertension, which is dependent on vasopressin, developed more rapidly in male than in female rats. Although there was no sex-related difference in the extent to which plasma vasopressin levels were elevated, pressor responsiveness to vasopressin was greater and baroreflex sensitivity was attenuated to a lesser extent in hypertensive males than in hypertensive females. Thus, it seems likely that gonadal hormones play a significant role in cardiovascular regulation.

The plasma arginine vasopressin response to insulin-induced hypoglycaemia in children with short stature is related to age and the onset of puberty

The plasma arginine vasopressin (AVP) response to insulin-induced hypoglycaemia was investigated in 27 children with short stature. None had diabetes insipidus. Six patients were excluded from further analysis because of hypothalamo-pituitary dysfunction. Of the remainder, 14 were prepubertal (Tanner 1) and seven were pubertal (Tanner 2-4). Both groups had similar height velocity retardation. There was a significant rise of AVP of 3.4 +/- 1.3 pmol/l at 30 min in the pubertal group (P less than 0.05) but no significant change in prepubertal patients. There was a significant relationship between chronological age and AVP response 30 min after insulin (r = 0.45, P less than 0.05) and a closer correlation between bone age and AVP response (r = 0.62, P less than 0.01). The data suggest that insulin-induced hypoglycaemia does not reliably stimulate AVP secretion in children and that this response is related to age and pubertal stage.

No effect of naloxone on plasma oxytocin in normal men

The role of endogenous opiates in the control of the secretion of oxytocin in the basal state in healthy male volunteers was investigated with the opiate antagonist naloxone. There was no change in plasma oxytocin levels, assessed for a 120 min period following the intravenous administration of naloxone (10 mg). Although there was no effect of opiate receptor blockade with naloxone in this basal situation, further studies are needed to evaluate the possible role of opioid regulation of oxytocin during lactation and parturition.