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.     

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 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.     

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 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.     

The release of neurohypophysial hormones as influenced by stimulation of alpha-adrenergic transmission in long-term dehydrated male white rats. Information 2: hypothalamic and neurohypophysial oxytocic activity

Rats dehydrated up to 12 days were given intraperitoneally methoxamine hydrochloride in a daily dose of 1.0 mg/100 g of initial body weight. The only dose of methoxamine injected into normally hydrated animals did not influence significantly the oxytocic activity neither in the hypothalamus nor in the neural lobe. Following four days of dehydration a distinctly more marked depletion of the hypothalamic (both in the NSO and NPV region) and neurohypophysial oxytocin content was found in animals treated with methoxamine. For the neurohypophysis, a similar effect has been noted under severe dehydration (8th and 12th day) as well.     

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.     

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 cholinergic influences on the oxytocin activity of the hypothalamus and neurohypophysis in long-term dehydrated male rats

The cholinergic influences on the oxytocin activity of the hypothalamus and neurohypophysis in long-term dehydrated male rats. Acta Physiol. Pol., 1978, 29 (1): 17-25. Rats dehydrated up to 12 days were injected intraperitoneally with carbachol or atropine sulfate in daily doses of 20 microgram/100 g and 1.0 mg/100 g, respectively. In not dehydrated rats atropine increased the oxytocin activity of the hypothalamus and neurohypophysis; carbachol did not influence the oxytocin potency of the hypothalamus but augmented it in the neurohypophysis. During long-term dehydration both carbachol and atropine intensified the depletion of oxytocin in the hypothalamus as well as in the neurohypophysis.     

Immunoreactive galanin-like material in magnocellular hypothalamoneurohypophysial neurones of the rat

Summary Immunoreactive galanin-like material was recently shown to co-exist with vasopressin in parvocellular and magnocellular perikarya of the paraventricular nucleus in the anterior hypothalamus of the rat (Melander et al. 1986). Since this distribution pattern differed from our observation of oxytocin-associated galanin-like immunoreactivity (LI) in the neurohypophysis, we compared in series of 0.5-m thick sections the localisation of galanin-LI with the localisation of oxytocin and vasopressin/dynorphin in the hypothalamus, the median eminence and the neurohypophysis. In the oxytocin system, galanin-LI was intense in oxytocin varicosities of the neurohypophysis. Oxytocin perikarya of the hypothalamic supraoptic and paraventricular nuclei exhibited galanin-LI only after intraventricular injection of colchicine and when sections were treated with trypsin prior to application of the antibody. In the vasopressin/dynorphin system galanin-LI was intense in hypothalamic perikarya after colchicine injection and in neurohypophysial varicosities after treatment of the sections with trypsin. In these neurones, galanin-LI was absent or weak in all elements when treatments with colchicine or trypsin were omitted. Galanin-LI in the neurohypophysis was not co-localised with the numerous fine endings showing GABA-LI. These observations indicate that galanin-like material coexists with vasopressin and oxytocin in the respective magnocellular neurones, although not always in an immunoreactive form.     

The pineal and oxytocin synthesis.

The aim of this study was to investigate the effect of pineal removal on oxytocin synthesis in the hypothalamus using the colchicine method. To this end, rats were injected intracerebroventricularly (i.c.v.) with colchicine solution (5 microg/5 microl) or normal saline and decapitated 20 h later. The animals were either pinealectomized or sham-operated two or eight weeks before i.c.v. injection. The oxytocin content in the hypothalamus was significantly higher in colchicine-treated rats whereas no significant differences were seen in the neurohypophysial hormone level between saline- or colchicine-injected animals. Thus, colchicine inhibited the hormonal transport but probably did not affect the function of the neurohypophysis. Two weeks after pinealectomy neither the oxytocin synthesis rate nor its neurohypophysial content were significantly different from control values. The oxytocin synthesis rate was increased markedly eight weeks after pineal removal. At that time, the neurohypophysial oxytocin content was reduced suggesting the increased secretion of the hormone. It is concluded that the pineal has an inhibitory impact on both oxytocin synthesis and release.     

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.     

Chronic treatment with desmethylimipramine increases the oxytocin content in the hypothalamus and neurohypophysis of normal and pinealectomized male rats

The effect of chronic treatment with desmethylimipramine (i.p., 10 mg/kg; twice daily over 5 days) on the content of oxytocin in the hypothalamus and neurohypophysis of normal and pinealectomized male rats has been investigated. Pinealectomy resulted in a decrease of oxytocin content in the hypothalamus and neurohypophysis. Treatment with desmethylimipramine (desipramine; DMI) was followed by a distinct increase of the oxytocin potency in the hypothalamus and neurohypophysis in both normal and pinealectomized rats. It may be supposed that chronic treatment with DMI inhibits the oxytocin release from neurohypophysis.     

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.     

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.     

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.     

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.     

The effect of microwave irradiation on vasopressin in plasma and hypothalamo-neurohypophyseal system

Radioimmunoreactive vasopressin was measured in plasma, neurohypophysis and hypothalamus of the rats after different procedures of killing: a) microwave irradiation; b) decapitation; c) decapitation following a stress induced by immobilization in a restrainer. Vasopressin content in the neurohypophysis and hypothalamus was much lower in microwave irradiated than in both decapitated and stressed decapitated rats. In addition, the data from microwave technique were inconsistent with a large scatter. Plasma vasopressin concentration was elevated in both the microwave irradiated and stressed decapitated rats, demonstrating that restraining of the animals induced an excessive stress. Microwave irradiation technique including the necessary manipulation of the animal proved to be less suitable than decapitation technique for the measurement of vasopressin. It is likely that vasopressin in the hypothalamus and neurohypophysis is relatively resistant against post-mortem proteolysis.     

Multifaceted purinergic regulation of stimulus-secretion coupling in the neurohypophysis

The neurohypophysis is an original model of the CNS secretory system releasing vasopressin (AVP) and oxytocin (OXT), two neuropeptides hormones synthesized by the magnocellular neurons of the hypothalamus. Specific patterns of action potentials originating from cellular bodies of magnocellular neurons control the release of AVP and OT, but intra-neurohypophysis regulations do modulate the neuropeptides release. There is now good evidence for the effects of extracellular purines in the control of neurohypophysial secretion. This paper brings together evidence for the multiple, intricate actions of purines in the extracellular space of the neurohypophysis. It covers four main points. First, the activity-dependent release of endogenous ATP in the neurohypophysis. Second, the action of ATP on both neuronal and non-neuronal compartments of the neural lobe. Third, the termination of ATP positive feedback by ecto-nucleotidases. And finally the possible involvement of adenosine in the regulation of neurohypophysial secretion and glial plasticity. The data suggest that ATP and adenosine are physiological modulators of the release of neurohypophysial peptides by acting directly on nerve terminals and indirectly on neurohypophysial astrocytes. Since purinergic receptors are widespread in nervous and endocrine systems, the neurohypophysis appears as an useful model for studying the role of purines in the regulation of stimulus-secretion coupling and neuron-glia interactions. The feedback mechanisms found in the neurohypophysis could be ubiquitous, occurring throughout the central nervous system and in other secretory systems.     

Changes in oxytocin and vasopressin content in posterior pituitary and hypothalamus following pantethine treatment

Pantethine, a cysteamine precursor, depletes somatostatin in the cerebral cortex and hypothalamus and prolactin in the anterior pituitary and hypothalamus. This study investigated the effect of pantethine on oxytocin and arginine vasopressin content in the posterior pituitary and hypothalamus. Male Long-Evans rats were injected intraperitoneally with escalating doses of pantethine (i.e., 146.7 mg, 293.4 mg and 586.6 mg/100 gm body weight). Hormone content was determined by radioimmunoassay. Three hours after pantethine treatment, the oxytocin content in the posterior pituitary and the hypothalamus was markedly reduced with all doses of the drug. Vasopressin content in the posterior pituitary and hypothalamus was decreased but to a lesser extent than oxytocin and only with the highest dose of pantethine. Pantethine may act to reduce oxytocin and vasopressin content through intracellular conversion to cysteamine. The exact mechanism of action of pantethine on oxytocin and vasopressin remains to be elucidated.