Centrally administered galanin modifies vasopressin and oxytocin release from the hypothalamo-neurohypophysial system of euhydrated and dehydrated rats.

Galanin (Gal) as a neuropeptide with widespread distribution in the central nervous system may be involved in the mechanisms of vasopressin (AVP) and oxytocin (OT) release from the hypothalamo-neurohypophysial system. Vasopressin and oxytocin content in the hypothalamus and neurohypophysis as well as plasma level of both neurohormones were studied after galanin treatment in euhydrated and dehydrated rats. In not dehydrated rats intracerebroventricular (i.c.v.) injections of Gal did not affect the hypothalamic and neurohypophysial OT content, however, distinctly increased plasma OT concentration. In the same animals Gal diminished the hypothalamic AVP content but was without the effect on neurohypophysial AVP storage; plasma AVP level then raised. Galanin, administered i.c.v. to rats deprived of water, distinctly inhibited AVP and OT release from the hypothalamo-neurohypophysial system. Simultaneously, plasma AVP and OT level was significantly diminished after Gal treatment in dehydrated rats. These results suggest that modulatory effect of galanin on vasopressin and oxytocin release depends on the actual state of water metabolism. Gal acts as an inhibitory neuromodulator of AVP and OT secretion under conditions of the dehydration but stimulates this process in the state of equilibrated water metabolism.     

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.     

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.     

Monoaminergic influences on the incorporation of 35S in the hypothalamic and neurohypophysial proteins following intracerebroventricular injection of 35S-cysteine in dehydrated rats

In the rats dehydrated for 48 h the mean specific activity of hypothalamic and neurohypophysial TCA-precipitable proteins following intracerebroventricular injection of L-cysteine-35S-hydrochloride was significantly higher than that found in the dehydrated and reserpinized. Under influence of amphetamine sulfate however, no change of 35S-uptake by TCA-precipitable hypothalamic and neurohypophysial proteins could be detected in rats similarly dehydrated.     

The neurohypophysial vasopressin content as influenced by modified cholinergic or adrenergic transmission during long-term dehydration in the white rat.

In rats dehydrated up to 12 days the neurohypophysial vasopressin content was determined by Dekański's method. Carbamylcholine inhibited somewhat the vasopressin depletion in the neurohypophysis, but not earlier than under severe dehydration (8th and 12th day). A single dose of atropine given 24 h prior to sacrifice to not dehydrated animals resulted in a diminution of the vasopressin content in the neurohypophysis; in animals dehydrated for four days and parallely atropinized the decrease of the neurohypophyseal vasopressin content was, on the contrary, considerably inhibited. Under severe dehydration, the treatment with atropine did not change the vasopressin stores in the neural lobe. Phenoxybenzamine inhibited the vasopressin depletion in the neural lobe following four days of dehydration. Under severe dehydration, amphetamine potentiated the effect of osmoreceptor stimulation. It is supposed that impulses of osmoreceptor origin are of some importance in determining the vasopressin release following changes of cholinergic or adrenergic transmission.     

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.     

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.     

Centrally administered atriopeptin III reduces water intake and vasopressin secretion in dehydrated sheep.

Two experiments were carried out to investigate the responses of sheep (N = 6) to intracerebroventricular (ICV) injections of atriopeptin III [atrial natriuretic factor (5-28), AP III]. In Experiment, 1, 24-h dehydrated animals were given 0 (saline vehicle control), 10 or 30 micrograms AP III directly before the presentation of water. The highest dose of AP III significantly (p less than 0.02) reduced the amount of water drunk in the subsequent 20 min. In Experiment 2, blood samples were taken at various intervals before and after ICV injection of 0 or 30 micrograms AP III when the sheep were water replete or 24-h dehydrated. Plasma concentrations of vasopressin (AVP) and cortisol were measured and estimates were made of plasma osmolality. In the dehydration condition, AVP levels were somewhat reduced (p less than 0.059) after AP III administration but no decrease was observed when the animals were euhydrated. No significant changes in plasma osmolality or cortisol concentrations were observed in response to AP III or the saline vehicle. Because a large dose of AP III (30 micrograms ICV) was required to produce the comparatively small behavioral and endocrine effects observed in this study, the results are suggestive of a pharmacological, rather than a physiological, action of the peptide in this species.     

Atrial natriuretic factor inhibits vasopressin secretion in conscious sheep

To test the hypothesis that atrial natriuretic factor (ANF) has a centrally mediated action on body fluid homeostasis, the effects of intracerebroventricularly (ICV) infused ANF on plasma vasopressin (AVP) concentration and urinary water and electrolyte excretion were investigated in euhydrated and water-deprived conscious sheep. ICV ANF decreased plasma AVP concentration and increased urinary free water excretion in euhydrated sheep, with excretion of Na and K unaltered. However, ICV ANF did not affect urinary volume, free water clearance, or excretion of Na and K in dehydrated animals, although plasma AVP concentration was significantly decreased. The relationship between urine volume and plasma AVP concentration was fitted by a power curve: urine volume = 0.79 X [AVP]-0.71; urine volume changes very little as a function of AVP concentration at the higher ranges. Intravenous infusion of the same amount of ANF was without effect on plasma AVP concentration or urinary excretion in both euhydrated and dehydrated animals. Mean arterial pressure was unchanged throughout all experiments. These results are consistent with the hypothesis that central ANF inhibits AVP secretion.     

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.     

The neurohypophyseal vasopressin content under conditions of inhibited synthesis of prostaglandins in euhydrated and dehydrated rats

Rats euhydrated or dehydrated up to eight days were given intraperitoneally indomethacin (IM) twice daily in a dose of 0.2 mg/100 g of initial body weight. A single dose of IM injected to euhydrated animals did not affect the neurohypophyseal vasopressor activity; however, when administered repeatedly during four or eight days, IM increased significantly the neurohypophyseal vasopressor activity. Under conditions of severe dehydration (eight days of water deprivation) the known depletion of neurohypophyseal vasopressin store was not significantly affected by indomethacin. It is, therefore, suggested that impulses of osmoreceptor origin are of distinct importance in determining the function of vasopressinergic neurons under conditions of inhibited prostaglandin synthesis.     

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.     

Effects of oxytocin and vasopressin on the preadipocyte 3T3-F442A cell line

The 3T3-F442A mouse fibroblast cell line, triggered by factors present in fetal calf serum (FCS), converts either spontaneously or, in the simultaneous presence of FCS and insulin, at an accelerated rate into cells exhibiting the adipocyte phenotype. The effects of the neurohypophysial hormones in differentiated cells on glucose metabolism (glucose oxidation and lipogenesis) were compared with the stimulatory actions of insulin, which had its most pronounced effects in cells differentiated spontaneously with FCS in the absence of insulin. The differentiated 3T3-F442A cells were sensitive to physiological levels of insulin and exhibited manyfold increases in glucose metabolism in response to it. This result demonstrated that these cultured cells respond to insulin, in a manner analogous to freshly isolated adipocytes. In contrast to its insulin-like effects in isolated epididymal adipocytes, oxytocin was not reproducibly able to stimulate glucose metabolism in differentiated 3T3-F442A cells. Vasopressin was similarly inactive. In contrast, both oxytocin and vasopressin blocked adipocyte conversion triggered by FCS, either in the presence or absence of insulin; vasopressin was more potent than oxytocin, indicating that a vasopressin receptor was responsible for the observed inhibition of differentiation. Our work suggests that vasopressin could potentially play a role in the regulation of the adipocyte differentiation process.     

Melatonin and the synthesis of vasopressin in pinealectomized male rats

The pineal hormone, melatonin, is known to modify, under different experimental conditions, neurohypophysial hormone secretion in the rat. The aim of this study was to investigate the effect of melatonin on the vasopressin biosynthesis rate in the hypothalamus of either pinealectomized or sham-operated rats, using the colchicine method. To estimate whether colchicine affects the function of the neurohypophysis in these animals, the neurohypophysial and plasma vasopressin levels were also measured. The vasopressin synthesis rate was increased after pineal removal, when compared with sham-operated animals, and melatonin strongly inhibited the rise in the hormone synthesis due to pinealectomy. After pineal removal plasma vasopressin concentration was significantly elevated, and melatonin attenuated this effect. On the contrary, the neurohypophysial vasopressin content was significantly decreased after pinealectomy, but it was not further modified by melatonin.Thus, melatonin suppresses the synthesis and secretion of vasopressin in pinealectomized rats. The present results confirm our previous reports as to the inhibitory impact of the pineal on both vasopressin synthesis and release and suggest that melatonin may mediate the effect of the pineal gland on vasopressinergic neuron activity.     

Active dehydration impairs upper and lower body anaerobic muscular power

We examined the effects of active dehydration by exercise in a hot, humid environment on anaerobic muscular power using a test-retest (euhydrated and dehydrated) design. Seven subjects (age, 27.1 +/- 4.6 years; mass, 86.4 +/- 9.5 kg) performed upper and lower body Wingate anaerobic tests prior to and after a 1.5-hour recovery from a heat stress trial of treadmill exercise in a hot, humid environment (33.1 +/- 3.1C = 55.1 +/- 8.9% relative humidity) until a 3.1 +/- 0.3% body mass loss was achieved. Dehydration was confirmed by a significant body mass loss (P < 0.001), urine color increase (P = 0.004), and urine specific gravity increase (P = 0.041). Motivation ratings were not significantly different (P = 0.059), and fatigue severity was significantly (P = 0.009) increased 70% in the dehydrated compared to the euhydrated condition. Compared to the euhydrated condition, the dehydrated condition mean power was significantly (P = 0.014) decreased 7.17% in the upper body and 19.20% in the lower body. Compared to the euhydrated condition, the dehydrated condition peak power was significantly (P = 0.013) decreased 14.48% in the upper body and 18.36% in the lower body. No significant differences between the euhydrated and dehydrated conditions were found for decrease in power output (P = 0.219, power = 0.213). Our findings suggest that dehydration of 2.9% body mass decreases the ability to generate upper and lower body anaerobic power. Coaches and athletes must understand that sports performance requiring anaerobic strength and power can be impaired by inadequate hydration and may contribute to increased susceptibility to musculoskeletal injury.     

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.     

Mechanism of the post-haemorrhagic vasopressin release from the posterior pituitary lobe

The mechanism of post-haemorrhagic vasopressin release from the neurohypophysis was studied in rats anaesthetized with urethane. Neurohypophysial vasopressin content was determined according to Dekański and plasma renin activity by radioimmunoassay. In animals bled (1.5% body weight) 60 min after induction of anaesthesia and 30 min after bilateral nephrectomy vasopressin content of the posterior pituitary was significantly higher than in sham-nephrectomized rats. However, when haemorrhage was produced 240 min after induction of anaesthesia and 210 min after nephrectomy, the neurohypophysial vasopressin content was low and similar as that in non-nephrectomized animals. It is concluded that in the phase directly following haemorrhage vasopressin release depends on acute activation of the renin-angiotensin system. Other mechanisms, possibly circulatory reflexes, are involved in the late phase, during prolonged anaesthesia.     

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.