New Metabolic Influencer on Oxytocin Release: The Ghrelin.

BACKGROUND: The hypothalamic⁻pituitary axis by secreting neuropeptides plays a key role in metabolic homeostasis. In light of the metabolic regulation, oxytocin is a potential neuropeptide for therapies against obesity and related disorders. The aim of our study is to measure ghrelin-induced oxytocin secretion in rats and to detect the changes after administration of ghrelin antagonist. METHODS: Ghrelin was administrated centrally (intracerebroventricular, i.c.v., 1.0, 10.0, and 100.0 pmol) or systemically (intravenous, i.v., 1.0, and 10.0 nmol). [d-Lys³]-GHRP-6 ghrelin antagonist was injected 15 min before ghrelin injection in a dose of 10.0 pmol i.c.v. and 10.0 nmol i.v. RESULTS: Either i.c.v. or i.v. administration of ghrelin dose-dependently increased the plasma oxytocin concentration. Following pretreatment with the ghrelin antagonist [d-Lys³]-GHRP-6, the high plasma oxytocin level induced by ghrelin was significantly reduced. CONCLUSION: The results indicate that the release of oxytocin is influenced directly by the ghrelin system. Examination of the mechanism of ghrelin-induced oxytocin secretion is a new horizon for potential therapeutic options.

The osmotically and histamine-induced enhancement of the plasma vasopressin level is diminished by intracerebroventricularly administered orexin in rats.

The effects of the centrally administered neuropeptides orexin-A on water intake and vasopressin (VP) secretion were studied in male Wistar rats (180-250 g). Different doses (10, 30, and 90 µg/10 µl) of the orexins and the specific orexin receptor-1 (OX(1)) antagonist SB 408124 (30 µg/10 µl) were administered intracerebroventricularly (i.c.v.) under anaesthesia, and the water consumption was measured during 6 h. A plasma VP level elevation was induced by histamine (10 mg/kg) or 2.5% NaCl (10 ml/kg) administered intraperitoneally (i.p.). The plasma VP levels were measured by radioimmunoassay. Increased water consumption was observed after the administration of 30 µg/10 µl orexin-A. There were no changes in basal VP secretion after the administration of different doses of the orexins. A significant increase in plasma VP concentration was detected following histamine administration. After 2.5% NaCl administration, there was a moderate VP level enhancement. Intracerebroventricularly administered orexin-A (30 µg/10 µl) blocked the VP level increase induced by either histamine or 2.5% NaCl administration. The inhibitory effects were prevented by the specific OX(1) receptor antagonist. In conclusion, the orexins increased water consumption. After 30 µg/10 µl orexin-A administration, the polydipsia was more pronounced. The OX(1) receptor antagonist significantly decreased the polydipsia. Histamine or hyperosmotic VP release enhancement was blocked by previously administered orexin. This inhibition was not observed following OX(1) receptor antagonist administration. Our results suggest that the effects of the orexins on water consumption or blockade of the histamine and osmosis-induced VP level increase are mediated by the OX(1) receptor.

Biological half-life and organ distribution of [3H]8-arginine vasopressin following administration of vasopressin receptor antagonist OPC-31260.

The effects of the antidiuretic (V(2)) non-peptide receptor antagonist OPC-31260 on the plasma vasopressin level and the biological half-life and organ distribution of radiochemically pure, biologically active [(3)H]8-arginine vasopressin [spec. act.: 15.9 mCi/mmol (588 GBq/mmol)] were studied in Wistar rats. The plasma vasopressin level increased significantly throughout the whole experimental period (24 h). There was no change in the fast phase of the curves of total radioactivity disappearance from the plasma after the administration of [(3)H]arginine vasopressin (control: 1.51+/-0.17 min, OPC-31260-treated: 1.42+/-0.12 min, n=10). The fast phase of the disappearance curves of intact [(3)H]arginine vasopressin did not change either following the administration of OPC-31260 in a dose of 30 mg/kg p.o. (control: 1.06+/-0.19 min, OPC-31260-treated: 1.00+/-0.15 min, n=6). The slow phase of the biological half-life, which is characteristic for the examined compound, proved to be significantly longer (total radioactivity control: 9.29+/-0.61 min, OPC-31260-treated: 12.33+/-0.42 min, P<0.05, n=10; [(3)H]arginine vasopressin radioactivity: control: 5.96+/-0.58 min, OPC-31260-treated: 8.90+/-0.37 min, P<0.05, n=6). In the control rats, the radioactivity was accumulated to the greatest extent in the neurohypophysis, adenohypophysis and kidney. Following OPC-31260 administration, significantly more radioactive compounds accumulated in the kidney (control: 0.30+/-0.052 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.50+/-0.133 total radioactivity %/100 mg organ weight, P<0.05, n=10) and neurohypophysis (control: 0.37+/-0.053 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.52+/-0.076 total radioactivity %/100 mg organ weight, P<0.05, n=10). Our results permit the conclusion that the antidiuretic antagonist OPC-31260 not only blocks the V(2) receptors, but also increases the biological half-life of vasopressin. The longer biological half-life of vasopressin following OPC-31260 administration may play a role in the elevation of the plasma vasopressin level.

Ghrelin-Induced Enhancement of Vasopressin and Oxytocin Secretion in Rat Neurohypophyseal Cell Cultures.

The effects of ghrelin on vasopressin and oxytocin secretion were studied in 13-14-day cell cultures of isolated rat neurohypophyseal tissue. The vasopressin and oxytocin contents of the supernatant were determined by radioimmunoassay after a 1- or 2-h incubation. Significantly increased levels of vasopressin and oxytocin production were detected in the cell culture media following ghrelin administration, depending on the ghrelin doses. The oxytocin level proved to be more elevated than that of vasopressin. The increase of vasopressin and oxytocin secretion could be totally blocked by previous administration of the ghrelin receptor antagonist ([D-Lys3]-growth hormone-releasing peptide-6). Application of the ghrelin receptor antagonist after ghrelin administration proved ineffective. The results indicate that vasopressin and oxytocin release is influenced directly by the ghrelin system, and the effects of ghrelin on vasopressin and oxytocin secretion from the neurohypophyseal tissue in rats can occur at the level of the posterior pituitary. Our observations lend support to the view that neurohypophysis contains ghrelin receptors.

Inhibitory effects of different galanin compounds and fragments on osmotically and histamine-induced enhanced vasopressin secretion in rats.

The effects of rat, porcine and human galanin, and the human 1-16 and human 16-30 terminal galanin fragments on vasopressin secretion were studied in rat. The plasma vasopressin level was determined by radioimmunoassay (RIA). There were no changes in the basal vasopressin secretion after galanin administration. A significant increase in vasopressin concentration was detected following 2.5% NaCl or histamine administration. I.c.v. injected rat, porcine or human galanin or the 1-16 N-terminal galanin fragment prevented the plasma vasopressin level enhancement. Following the i.v. administration of rat galanin or the i.c.v. injected 16-30 C-terminal galanin fragment, the vasopressin concentration did not return to the normal level. Administration of the galanin antagonist galantid (M15) i.c.v. before the rat galanin i.c.v. injection prevented the inhibitory effect on the increased plasma vasopressin level following 2.5% NaCl solution or histamine administration. The results indicate that there is no significant difference in the inhibitory effect of rat, porcine or human galanin or the 1-16 galanin fragment on the enhanced plasma vasopressin secretion induced by hyperosmosis or histamine administration. Our findings suggest that galanin, as a peptide modulator, is physiologically involved in the regulation of vasopressin release following different forms of stimulation: an osmotic response or histamine administration.

Synergistic interaction of endogenous platelet-activating factor and vasopressin in generating angina in rats.

We examined the involvement of endogenous vasopressin and platelet-activating factor (PAF) in the pathogenesis of two types of experimental angina in urethane-anaesthetised male Wistar rats. In the first model, epinephrine (10 microg kg(-1)) was injected into the tail vein, followed at the development of the maximum blood pressure response, i.e., 30 s later, by phentolamine (15 mg kg(-1)). In the second model, the vasopressin V1 receptor agonist ornithine-vasopressin (ornipressin; 0.5 IU kg(-1), i.v.) was administered. The heart rate, mean arterial blood pressure and surface electrocardiogram (ECG, standard lead II) were registered simultaneously. As a measure of myocardial ischaemia, at 1 min after phentolamine or ornipressin administration, we found significant ST-segment depression, lasting for more than 10 or 5 min, respectively. Pretreatment (15 min, s.c.) with the vasopressin V1 receptor antagonist Mca1,Tyr(Me)2AVP (the Manning peptide; 0.02-0.2 microg kg(-1)) or the platelet-activating factor receptor antagonist ginkgolide B (BN 52021; 0.25-2.5 mg kg(-1)) alone caused a dose-dependent reduction of the ST-segment depression. Concurrent administration of the two antagonists in their threshold doses (0.02 microg kg(-1) and 0.25 mg kg(-1)) also attenuated the ST-segment depression in both models. Neither antagonist affected the blood pressure or heart rate changes throughout the studies. Our results suggest that endogenous vasopressin and platelet-activating factor interact synergistically in provoking myocardial ischaemia in vivo in experimental angina in the rat.

Raloxifene lowers ischaemia susceptibility by increasing nitric oxide generation in the heart of ovariectomized rats in vivo.

We studied the effects of a 2-week period of oral raloxifene therapy on the cardiac level of nitric oxide (NO) and on the susceptibility to angina in ovariectomized rats. Ovariectomy decreased the activity of Ca2+-dependent nitric oxide synthase (NOS) in the left ventricle, an effect restored by raloxifene (0.2-5 mg kg(-1) day(-1)) or 17beta-oestradiol (0.3 mg kg(-1) day(-1)). Ovariectomy led to a significant ST segment depression after the injection of (1) ornithine-vasopressin (0.5 IU kg(-1), i.v.) or (2) epinephrine (10 microg kg(-1), i.v.), followed 30 s later by phentolamine (15 mg kg(-1), i.v.); both effects were reversed by raloxifene or 17beta-oestradiol treatment. Inhibition of nitric oxide synthase (with NG-nitro-L-arginine methyl ester [L-NAME]; 5 mg kg(-1), s.c.) augmented the ST segment depression in the ovariectomized rat and abolished the anti-ischaemic effect of 17beta-oestradiol or raloxifene. Thus, an oestrogen deficiency down-regulates the cardiac constitutive nitric oxide synthase, which increases the susceptibility of the heart to ishaemia because both actions can be blocked by exogenous administration of the natural oestrogen 17beta-oestradiol or the selective oestrogen-receptor modulator (SERM) raloxifene. In the present in vivo system, raloxifene exerts oestrogen-agonist properties.

Effects of orexin-monoaminergic interactions on oxytocin secretion in rat neurohypophyseal cell cultures.

The effects of orexin-monoaminergic compound interactions on oxytocin release were studied in 14-day rat neurohypophyseal cell cultures prepared by an enzymatic dissociation technique. The oxytocin contents of the supernatants were determined by radioimmunoassay. Following the administration of orexin-A or orexin-B in increasing doses, significant changes were not observed in the oxytocin content of the supernatant media. The oxytocin level increased substantially in response to adrenaline, noradrenaline, serotonin, histamine, dopamine or K(+) treatment. Preincubation with orexin-A or orexin-B reduced the adrenaline-, histamine- or serotonin-induced oxytocin level increases, but the oxytocin concentrations of the supernatant media remained above the control level. There was no significant difference in decreasing effect between orexin-A and orexin-B. Neither orexin-A nor orexin-B induced changes in oxytocin release following monoaminergic compound treatment. The results indicate that the changes in oxytocin secretion induced by the monoaminergic system can be directly influenced by the orexin system. The effects of orexin on oxytocin release can be antagonized by an orexin-1 receptor-specific antagonist. It may be presumed that the orexins can play a role in the pathogenetic process of metabolic diseases (e.g. obesity) by reducing the effects of increased oxytocin release caused by monoaminergic compounds. The interactions between the monoaminergic and orexin systems regarding oxytocin secretion occur at both the hypothalamic and the neurohypophyseal levels.

The effects of orexins on monoaminerg-induced changes in vasopressin level in rat neurohypophyseal cell cultures.

The effects of orexin-monoaminergic compound interactions on vasopressin release were studied in 14-day neurohypophyseal cell cultures from adult rats, prepared by an enzymatic dissociation technique. The vasopressin contents of the supernatants were determined by radioimmunoassay. Following administration of either orexin-A or orexin-B in increasing doses, significant changes were not observed in the vasopressin levels of the supernatant media. The vasopressin level substantially increased after epinephrine, norepinephrine, serotonin, histamine, dopamine or K(+) treatment. Preincubation with either orexin-A or orexin-B reduced the epinephrine-, histamine- or serotonin-induced increases in vasopressin level, but the vasopressin concentrations of the supernatant media remained above the control level. There was no significant difference in decreasing effect between orexin-A and orexin-B. Neither orexin-A nor orexin-B induced changes in vasopressin release following monoaminergic compound treatment. The results indicate that the changes in vasopressin secretion induced by the monoaminergic system can be directly influenced by orexin system. It may be presumed that the orexins can play a physiological role in the regulation of the water metabolism by reducing the effect of increased vasopressin release caused by monoaminergic compounds. The interactions between the monoaminergic and orexin systems regarding vasopressin secretion occur at both the hypothalamic and the neurohypophyseal level.

Inhibitory effect of galanin on adrenaline- and noradrenaline-induced increased oxytocin secretion in rat neurohypophyseal cell cultures.

The effects of the interactions between the 29 amino acid-containing peptide galanin and adrenaline or noradrenaline on the secretion of oxytocin were studied in 13- to 14-day cultures of isolated rat neurohypophyseal tissue. The alpha-receptor antagonist corynanthine blocked the adrenaline-induced increase of oxytocin secretion. When the beta-receptor antagonist propranolol was added before the noradrenaline treatment, the antagonist prevented the noradrenaline-induced enhancement of oxytocin release. Following the addition of galanin, the extent of oxytocin secretion into the supernatant medium decreased. Adrenaline and noradrenaline treatments increased the oxytocin level. Preincubation with galanin reduced the adrenaline- and noradrenaline-induced oxytocin level elevations. The blocking effect of galanin was prevented by previous treatment with the galanin receptor antagonist galantid (M15). When adrenaline or noradrenaline treatment was applied before galanin addition, the oxytocin secretion remained enhanced. The present results indicate that the changes in oxytocin secretion induced by the adrenergic system can be directly influenced by the galaninergic system. The interactions between the adrenergic and galaninergic systems from the aspect of oxytocin secretion can occur at the level of the posterior pituitary, independently of the hypothalamus.

Prevention of hypoxic brain oedema by the administration of vasopressin receptor antagonist OPC-31260.

The numerous situations which can result in cerebral hypoxic damage occur in newborn infants and in the elderly. In research aimed at more effective therapeutic intervention in ischaemic disorders of the brain, the animal model used and the principles of the causal therapy should be better outlined. The effects of the non-peptide AVPR (V2) antagonist 5-dimethylamino-1-[4-(2-methylbenzoylamino) benzoyl]-2,3,4,5-tetrahydro-1H-benzazepine hydrochloride (OPC-31260) on the cerebral oedema induced by general cerebral hypoxia were studied in rats. The general cerebral hypoxia was produced by bilateral common carotid ligation in Sprague-Dawley rats of the CFY strain. By 6h after the ligation, half of the rats had died, but the survival rate was significantly higher following OPC-31260 administration. Electron microscopic examinations revealed typical ischaemic changes after the carotid ligation, and OPC-31260 treatment did not significantly reduce the hypoxic signs in the brain cortex; only a certain decrease in the pericapillary oedema was observed. The carotid ligation increased the brain contents of water and Na(+) and enhanced the plasma AVP level. The increased brain water and Na(+) accumulation was prevented by OPC-31260 administration, but the plasma AVP level was further enhanced by OPC-31260. These results demonstrate the important role of AVP in the development of the disturbances in brain water and electrolyte balance in response to general cerebral hypoxia. The carotid ligation-induced cerebral oedema was significantly reduced following oral OPC-31260 administration. The protective mechanism exerted by OPC-31260 stems from its influence on the renal AVPR (V2). These observations might suggest an effective approach to the treatment of global hypoxia-induced cerebral oedema in humans.