Vasopressin and oxytocin release as influenced by thyrotropin-releasing hormone in euhydrated and dehydrated rats.

Since the thyrotropin-releasing hormone (TRH) can modulate the processes of vasopressin (AVP) and oxytocin (OT) biosynthesis and release mainly at the hypothalamo-neurohypophysial level, the present experiments were undertaken to estimate whether TRH, administered intravenously in different doses, modifies these mechanisms under conditions of osmotic stimulation, brought about by dehydration. AVP and OT contents in the hypothalamus and neurohypophysis as well as plasma levels of AVP, OT, free thyroxine (FT4) and free triiodothyronine (FT3) were studied after intravenously TRH treatment in euhydrated and dehydrated for two days male rats. Under conditions of equilibrated water metabolism TRH diminished significantly the hypothalamic and neurohypophysial AVP and OT content but was without the effect on plasma oxytocin level; however, TRH in a dose of 100 ng/100 g b.w. raised plasma AVP level. TRH, injected i.v. to dehydrated animals, resulted in a diminution of AVP content in the hypothalamus but did not affect the hypothalamic OT stores. After osmotic stimulation, neurohypophysial AVP and OT release was significantly restricted in TRH-treated rats. Under the same conditions, injections of TRH were followed by a significant decrease of plasma OT level. I.v. injected TRH enhanced somewhat FT3 concentration in blood plasma of euhydrated animals but diminished FT4 plasma level during dehydration. Data from the present study suggest that TRH displays different character of action on vasopressin and oxytocin secretion in relation to the actual state of water metabolism.     

Galanin influences vasopressin and oxytocin release from the hypothalamo-neurohypophysial system of salt loaded rats.

Galanin is a peptide present in the nervous system and peripheral tissues which exerts a broad range of physiological functions. The influence of centrally administered galanin (Gal; 100 pM i.c.v.) on arginine vasopressin (AVP) and oxytocin (OT) content in the hypothalamus and neurohypophysis as well as on their blood plasma concentration was estimated in male Wistar rats drinking ad libitum 2% solution of natrium chloride per 48 hours. In euhydrated rats and subsequently applied i.c.v. with Gal a significant fall in the hypothalamic and neurohypophysial content of OT but not AVP was observed, however, without simultaneous changes in these neurohormones blood plasma concentration. On the contrary, i.c.v. injection of Gal to salt-loaded rats caused a marked raise in AVP and OT level in the hypothalamus and neurohypophysis with subsequent diminution of both neurohormones concentration in blood plasma. These results suggest that in euhydrated rats Gal has an inhibitory influence on the biosynthesis as well as axonal transport of OT, but not AVP. On the contrary, in salt-loaded rats galanin restricts secretion of both neurohormones into the systemic circulation.     

Galanin affects vasopressin and oxytocin release from the hypothalamo-neurohypophysial system in haemorrahaged rats.

The effect of centrally administered galanin (Gal; 100 pM i.c.v.) on the hypothalamo-neurohypophysial storage as well as blood plasma level of vasopressin and oxytocin was estimated in haemorrhaged (1 ml per 100 g b.w.) male Wistar rats. Gal i.c.v. treatment did not alter vasopressin and oxytocin content both in the hypothalamus and neurohypophysis as well as their concentration in blood plasma of not haemorrhaged rats. Haemorrhage decreased the hypothalamic and neurohypophysial vasopressin and oxytocin storage but increased the neurohormones plasma level in animals injected with vehicle solution. During the haemorrhage, the increase in plasma vasopressin and oxytocin was inhibited in rats previously treated i.c.v. with galanin. The hypothalamic and neurohypophysial vasopressin as well as oxytocin content significantly increased in animals treated with galanin and subsequently haemorrhaged. These results suggest that galanin may have a regulatory role in the hypothalamo-neurohypophysial function especially under condition of hypovolemia.     

The hypothalamic and neurohypophysial vasopressin and oxytocin content under various states of adrenergic transmission in dehydrated and subsequently rehydrated rats

Rats dehydrated for 8 days and subsequently rehydrated were given intracerebroventricularly (i.c.v.) methoxamine hydrochloride (MX) or dihydroergotamine methanosulphonate (DHE), each in a daily dose of 10 micrograms dissolved in 10 microliter of 0.9% sodium chloride. A single dose of MX injected to normally hydrated animals increased the release of hypothalamic and neurohypophysial vasopressin but did not affect significantly the oxytocic activity in the hypothalamus as well as in the neurohypophysis. Under conditions of dehydration MX did not influence the hypothalamic vasopressin content but it stimulated the neurohypophysial vasopressin depletion. On the contrary, MX distinctly inhibited the decrease of hypothalamic and neurohypophysial oxytocin content in dehydrated animals. In rehydrated animals MX restrained some what the renewal of hypothalamic vasopressin and oxytocin storage but intensified this process in the neurohypophysis. A single dose of DHE decreased the vasopressin content in the hypothalamus as well as the oxytocin content both in the hypothalamus and neurohypophysis. Under conditions of dehydration DHE stimulated the depletion of hypothalamic vasopressin and oxytocin. On the contrary, DHE strongly inhibited the depletion of oxytocin in the neurohypophysis of dehydrated rats. DHE restrained the renewal of hypothalamic vasopressin and oxytocin stores as well as intensified this process in the neurohypophysis of subsequently rehydrated rats.     

The vasopressin and oxytocin neurohypophysial content as influenced by bleeding or dehydration: effect of cholecystokinin octapeptide.

The effect of CCK-8 (50 ng, i.c.v.) on the neurohypophysial vasopressin and oxytocin storage was estimated in haemorrhaged (1 ml per 100 g b.w.) male Wistar rats. In another experimental series rats dehydrated for three days were given CCK-8 in a daily i.c.v. dose of 50 ng. The neurohypophysial vasopressin and oxytocin content was bioassayed by pressor effect following Dekański or milk-ejection activity in vitro following van Dongen and Hays, respectively. The decrease of neurohypophysial vasopressin and oxytocin content, brought about by dehydration, was significantly less marked in animals treated with CCK-8. The depletion of neurohypophysial vasopressin and oxytocin content in haemorrhaged animals could be completely inhibited by earlier i.c.v. administration of CCK-8. It is suggested that hypothalamic cholecystokinin may serve as a modulator of neurohypophysial function.     

The hypothalamic and neurohypophysial vasopressor and oxytocic content as influenced by alpha-adrenergic blockade in stressed rats

The hypothalamic and neurohypophysial vasopressor and oxytocic content as influenced by alpha-adrenergic blockade in stressed rats. Acta physiol. pol., 1985, 36 (3): 193-200. The effects of phenoxybenzamine (PBA; an alpha-adrenergic blocker) on hypothalamic and neurohypophysial vasopressin and oxytocin were investigated in stressed rats. Immobilization resulted in a decrease of both vasopressor and oxytocic activities in the hypothalamus and neurohypophysis, whereas in rats, exposed to cold the vasopressin and oxytocin content in the hypothalamo-neurohypophysial system was increased. Under treatment with PBA the vasopressin and oxytocin content in the neurohypophysis was diminished in stressed (both immobilized and cold-exposed) rats when compared to respective groups of untreated animals subjected to appropriate kind of stress. The response of the vasopressinergic and oxytocinergic neurones seems, therefore, to be dependent on the type of stress. The alpha-adrenergic transmission is probably in some way involved in the mechanisms of modified neurohypophysial function in stressed animals.     

Plasma galanin, vasopressin, and oxytocin in patients with Addison's disease.

Galanin is colocalized with adrenocorticotrophin (ACTH) in the human pituitary and with corticotrophin releasing hormone, arginine, vasopressin, and oxytocin in the hypothalamus. Galanin, vasopressin, and oxytocin influence the secretion of pituitary ACTH. The aim of this study was to investigate if the endogenous stimulation of ACTH release in Addison's disease was reflected in plasma galanin, vasopressin, and oxytocin. ACTH, galanin, vasopressin, and oxytocin were measured in plasma from 14 patients with Addison's disease, one patient with Nelson's syndrome, and 14 healthy controls. Eight patients had elevated plasma ACTH whereas six patients and all controls had ACTH levels within the reference-range. There was no difference in galanin or vasopressin between patients and controls or between samples with low or high ACTH concentrations. In contrast, oxytocin was higher in patients with elevated plasma ACTH compared to patients and controls with normal or low ACTH. No relation was found between galanin or oxytocin and age or sex. A tendency towards lower vasopressin with increasing age was found among the men (p=0.057). The highest ACTH and galanin levels were found in the patient with Nelson's syndrome. In conclusion, increased plasma ACTH was not reflected in elevated plasma galanin or vasopressin. In contrast, elevated ACTH levels were accompanied by higher oxytocin levels.     

The vasopressin and oxytocin content in the hypothalamus and neurohypophysis as influenced by reserpine treatment during long-term dehydration in the white rat.

In dehydrated rats both neurohypophysial hormones diminished in hypothalamus as well as in the neurohypophysis. Oxytocin disappearef from the hypothalamus and neurohypophysis at a more rapid rate than vasopressin did. The minimal content of vasopressin and oxytocin in the hypothalamus was observed during 3rd--4th day, but even in extreme dehydration it was found to be relatively high: 65 per cent of vasopressin and 27 per cent of oxytocin as compared with intact animals. At that time the neurohypophysial vasopressin and oxytocin content were almost fully exhausted. In dehydrated and additionally reserpinized animals (10 mg/kg intraperitoneally, then each 48 hr 5 mg/kg of initial body weight) the vasopressin and and oxytocin hypothalamus and neurohypophysis changed in a similar manner. In some experimental groups the decrease of neurohormones in both sites was more marked under reserpine treatment. The drug seems therefore rather to potentiate the effects of physiological stimulation of osmodetectors. So the existence of monoaminergic stimulatory synapses, directly involved in the neural pathway between the osmodetector and the neurosecretory cell, appears to be hardly probable.     

Arginine-vasopressin and oxytocin response to cholecystokinin-tetrapeptide.

This study examined the effects of i.v. administration of cholecystokinin-tetrapeptide (CCK-4) on plasma release of arginine vasopressin (AVP) and oxytocin (OT) in women with premenstrual dysphoric disorder (PMDD) and control women, during both the follicular phase and the luteal phase of their menstrual cycle. Plasma AVP and OT concentrations increased following CCK-4 administration. AVP and OT response to CCK-4 was similar for PMDD and control women and unaffected by menstrual cycle phase. AVP and OT may play a role in the hypothalamo-pituitary adrenal (HPA) axis activity associated with the panic response induced by CCK-4.     

Effect of dehydration on the endogenous opiate content of the art neuro-intermediate lobe

The relationship of endogenous opiate peptides of rat neuro-intermediate lobe to the release of neurohypophysial peptides has been investigated. Both dehydrated rats, with increased oxytocin and vasopressin release, as well as rats homozygous for hypothalamic diabetes insipidus (DI) of the Brattleboro strain, with increased oxytocin release, showed significantly decreased levels of pituitary opiate peptides. We suggest that neuro-intermediate lobe opiate peptides may modulate the release of neurohypophysial antidiuretic peptides.     

Dissociation of oxytocin, vasopressin and corticotropin secretion during different types of stress

Oxytocin (OT), vasopressin (AVP), and corticotropin (ACTH) levels were measured in peripheral plasma of male rats subjected to one of three models of stress: restraint, cold, or ether. ACTH secretion was increased in all three groups compared to unstressed controls. OT secretion was increased in rats subjected to restraint or ether but not cold. AVP secretion was increased only by ether stress. The data suggest that the hypothalamic and neurohypophysial contribution to the control of ACTH secretion may vary in response to different types of stress.     

Intracerebroventricular insulin affects the neurohypophysial vasopressin content in euhydrated and dehydrated rats

Rats euhydrated or dehydrated for four days were given intracerebroventricular insulin once daily in a dose of 100 ng (not affecting blood sugar level). In euhydrated rats, insulin decreased significantly the neurohypophysial vasopressin content. In dehydrated animals the neurohypophysial content depleted by deprivation of water could be further reduced by intracerebroventricular treatment with insulin. These results may suggest a possible regulatory role of brain insulin in the mechanisms of vasopressin release.     

Hypothalamic action of atrial natriuretic factor to inhibit vasopressin secretion

The ability of synthetic atrial natriuretic factor (ANF) to inhibit vasopressin (AVP) release, as well as its action to inhibit water intake and salt preference in the rat, suggest a role for the peptide in the hypothalamic control of fluid volume in addition to its established actions in the kidney. We report here evidence for a direct, hypothalamic site of action of ANF to inhibit, specifically, AVP secretion. Third cerebroventricular infusion of 1.0 (p less than 0.05) and 2.0 (p less than 0.025) nmoles ANF significantly inhibited AVP release in euvolemic, normally hydrated rats while IV doses of ANF failed to significantly alter AVP release except when 5 nmoles (p less than 0.05) were infused. No significant effects on oxytocin (OT) release were observed. Vasopressin release from median eminence or pituitary, neural lobe explants during static, in vitro incubations was not significantly altered by doses of ANF ranging from 10(-12) to 10(-7) molar. Release of AVP during perifusion of neural lobe explants in the presence of ANF was similarly unaffected. However, AVP and not OT release from hypothalamo-neurohypophysial system explants was significantly inhibited in the presence of 10(-8) and 10(-7) M ANF, suggesting an action of the peptide at the levels of the AVP-producing cell bodies in the included supraoptic nucleus either directly or via an action on an interneuron, and not at the AVP-containing terminal fields in the median eminence or neural lobe.     

Analysis of the dual mechanism of ACTH release by arginine vasopressin and its analogs in conscious rats

Plasma ACTH and/or corticosterone levels were measured in conscious rats 30 min after subcutaneous administration of arginine vasopressin (AVP), oxytocin (OT) and various analogs with a large range of activity on the vasopressor (V1), antidiuretic (V2) or oxytocic receptors. The comparison of their dose-response curves indicated that two different mechanisms are involved in the release of ACTH by neurohypophysial peptides and their analogs. AVP itself and a specific vasopressor agonist (Phe2, Orn8, OT) displayed a similar, high slope dose-response curve. Non-vasopressor analogs, such as dDAVP were characterized by a low slope dose-response curve. Furthermore, dDAVP potentiated CRF and neither its own ACTH-releasing action nor its potentiation of CRF were sensitive to previous VI- or V2-receptor blockade. These results, together with other available data, are interpreted as indicative of the existence of two mechanisms of action for ACTH release by AVP and its analogs in vivo: an indirect action via endogenous CRF release, mediated by a VI receptor mechanism, and a direct action on the pituitary, shared by dDAVP and other non-vasopressor analogs, with receptor characteristics different to both the V1 and the V2 classical types.     

The vasopressor and oxytocic activities of the hypothalamus and neurohypophysis influenced by stimulated alpha-adrenergic transmission during dehydration and subsequent rehydration in the white rat

Under conditions of equilibrated water metabolism a single dose of methoxamine increased the content of vasopressin in the hypothalamus as well as that of oxytocin both in the hypothalamus and neurohypophysis. During dehydration the depletion of hypothalamic and neurohypophysial vasopressin was more marked in methoxamine-treated animals; this effect, however, was absent in the neurohypophysis on the 2nd day and in the hypothalamus on the 8th day of water deprivation. After two days of dehydration methoxamine inhibited the decrease of oxytocin content in the hypothalamus; simultaneously (2nd and 4th day of dehydration) it intensified this process in the neurohypophysis. During rehydration methoxamine impaired the renewal of vasopressin both in the hypothalamus and neurohypophysis; this effect was most marked on the 8th day of rehydration. On the contrary, it favoured somewhat the renewal of hypothalamic oxytocin in rehydrated rats (such an event was not found on the 8th day of rehydration). Moreover, methoxamine restrained initially (on the 2nd and 4th day of rehydration) the restoration of neurohypophysial oxytocin stores; following eight days of rehydration an opposite effect was here found. It is concluded that the response of the vasopressinergic and oxytocinergic neurons to alpha-adrenergic stimulation, brought about by using methoxamine as pharmacological tool, seems to be depended on the actual state of water metabolism. Impulses from the osmoreceptors may be therefore of some importance in modifying the change in vasopressin and oxytocin synthesis, transport and release resulting from stimulation of alpha-adrenergic transmission through neural chains including units susceptible to methoxamine.     

Interactions of galanin with endomorphin-2, vasopressin and oxytocin in nociceptive modulation of the trigemino-hypoglossal reflex in rats

Galanin (GAL) is suggested to be a neuropeptide involved in pain transmission. In this study we tried to determine, whether the increase of GAL concentration in brain cells affects impulse transmission between the motor centers localized in the vicinity of the third and fourth cerebral ventricles. The experiments were carried out on rats under chloralose anesthesia. The study objectives were realized using the method allowing to record the amplitude of evoked tongue jerks (ETJ) in response to noxious tooth pulp stimulation during the perfusion of the cerebral ventricles with solutions containing tested compounds. Perfusion of the cerebral ventricles with GAL concentration-dependently inhibited the ETJ amplitude. The antinociceptive effect of GAL was blocked by a galanin receptor antagonist, galantide (GLT) and by opioid antagonists: non-selective naloxone (Nal) and micro-selective beta-funaltrexamine (beta-FNA). In contrast, a delta-opioid receptor antagonist, naltrindole (NTI) or the kappa-opioid receptor antagonist, nor-binaltrophimine (nor-BNI) did not inhibit the effect of GAL. The antinociceptive effect of GAL was more pronounced when GAL was perfused in combination with other neuropeptides/neurohormones, such as endomorphin-2 (EM-2), vasopressin (AVP) and oxytocin (OT). The present results demonstrate that in the orofacial area analgesic activity is modulated by GAL, OT and AVP and that EM-2-induced antinociception involves GAL.     

Simultaneous and independent release of vasopressin and oxytocin in the rat

The relative dependence or independence of the secretion of the neurohypophysial hormones, arginine vasopressin and oxytocin, was investigated using a wide variety of stimuli reported to cause the secretion of one or the other hormone. Differences in species, animal preparations, sampling techniques, assays, and other factors make comparison of many previous studies difficult. The aim of this study was to overcome these problems by using the same methodology, animal species, and assays to compare vasopressin and oxytocin release. To further strengthen the analysis, determinations of vasopressin and oxytocin were done in the same blood samples. The results demonstrated that during simultaneous release of both hormones, vasopressin is released in greater proportion following restraint stress, hemorrhage, isotonic hypovolemia, and nicotine, whereas oxytocin is released in greater proportion following endotoxin or hypertonic saline. Vasopressin was released without oxytocin following diethylstilbestrol. Oxytocin was released without concomitant vasopressin release following exercise, hypothermia, hyperthermia, labour, and lactation. Neither oxytocin nor vasopressin release was observed following thyroid-releasing hormone or insulin-induced hypoglycemia. These data illustrate the marked flexibility of the hypothalamo-neurohypophysial system that regulates secretion of vasopressin and oxytocin.     

Reserpine inhibits release of vasopressin from the neural lobe of the pituitary in dehydrated rats

Summary The effects of reserpine on the osmotically induced release of pituitary vasopressin were studied (i) by measuring the urinary excretion and the vasopressin content of the neural lobe of the pituitary, and (ii) by examining the ultrastructural morphology of axons in the neural lobe of dehydrated rats. After water deprivation for two days, control rats displayed characteristic antidiuretic response including a 75% reduction of urinary excretion and a sixfold decrease in vasopressin content of the neural lobe associated with a dramatic depletion of neurosecretory granules in corresponding axons. In contrast, when they received two daily injections of reserpine, animals dehydrated for two days showed both urinary excretion and vasopressin contents in the neural lobe that remained at levels comparable to those measured in the normally hydrated rats. Additionally, neural-lobe axons of such dehydrated, reserpine-treated rats displayed a normal amount of neurosecretory granules. These data indicate that reserpine inhibits release of vasopressin from the neural lobe and favour the concept of a facilitatory role of the catecholaminergic innervation in the control of hypothalamo-neurohypophysial vasopressin-secreting neurons.     

Physiology of spontaneous [Ca2+]i oscillations in the isolated vasopressin and oxytocin neurones of the rat supraoptic nucleus

The magnocellular vasopressin (AVP) and oxytocin (OT) neurones exhibit specific electrophysiological behaviour, synthesise AVP and OT peptides and secrete them into the neurohypophysial system in response to various physiological stimulations. The activity of these neurones is regulated by the very same peptides released either somato-dendritically or when applied to supraoptic nucleus (SON) preparations in vitro. The AVP and OT, secreted somato-dendritically (i.e. in the SON proper) act through specific autoreceptors, induce distinct Ca²⁺ signals and regulate cellular events. Here, we demonstrate that about 70% of freshly isolated individual SON neurones from the adult non-transgenic or transgenic rats bearing AVP (AVP-eGFP) or OT (OT-mRFP1) markers, produce distinct spontaneous [Ca²⁺]_i oscillations. In the neurones identified (through specific fluorescence), about 80% of AVP neurones and about 60% of OT neurones exhibited these oscillations. Exposure to AVP triggered [Ca²⁺]_i oscillations in silent AVP neurones, or modified the oscillatory pattern in spontaneously active cells. Hyper- and hypo-osmotic stimuli (325 or 275 mOsmol/l) respectively intensified or inhibited spontaneous [Ca²⁺]_i dynamics. In rats dehydrated for 3 or 5 days almost 90% of neurones displayed spontaneous [Ca²⁺]_i oscillations. More than 80% of OT-mRFP1 neurones from 3 to 6-day-lactating rats were oscillatory vs. about 44% (OT-mRFP1 neurones) in virgins. Together, these results unveil for the first time that both AVP and OT neurones maintain, via Ca²⁺ signals, their remarkable intrinsic in vivo physiological properties in an isolated condition.     

The hypothalamic and neurohypophysial vasopressin content as influenced by diphenylhydantoin in dehydrated rats.

Rats dehydrated up to 8 days were treated with diphenylhydantoin given intraperitoneally in daily doses of 10 mg/100 g of the initial body weight. The single dose of diphenylhydantoin diminished the vasopressin content in the hypothalamus and neurohypophysis of normally hydrated rats. Under conditions of severe dehydration (8 days), DPH treatment resulted in a more marked decrease of vasopressin in the hypothalamo-neurohypophysial system.