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.     

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 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 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.     

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 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.     

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.     

Changes in hypothalamic and extra-hypothalamic vasopressin content of water-deprived rats

A correlative radioimmunoassay (RIA) and immunocytochemical (ICC) study was carried out on vasopressin (VP) distribution and content in brains of normal and 3-day water-deprived rats. By RIA there were statistically significant differences in brain VP per pg/mg between normal and osmotically stressed specimens in hypothalamus (338.4 versus 134.4), thalamus (4.8 versus 0.9), septum (18.0 versus 3.4), striatum (1.6 versus 0.7) and amygdala (17.3 versus 1.3), but not in other brain regions measured. Pituitary VP decreased from 71.1 to 8.7 ng/mg, and plasma VP rose from 3.6 to 19.3 pg/ml during water deprivation. Application of the peroxidase-anti-peroxidase ICC method of Sternberger to vibratome sections showed that VP-immunoreactivity in dehydrated specimens decreased in perikarya of paraventricular nucleus and suprachiasmatic nucleus, while intrahypothalamic immunoreactive magnocellular fibers appeared more conspicuous due to proliferation of large Herring bodies. In extrahypothalamic sites VP-immunoreactivity in water-deprived rats was visibly reduced in periventricular thalamus and septum. Thus it is apparent that both intra- and extrahypothalamic VP are affected by osmotic stress, and these results are discussed within the context of current ideas relating to co-activation of neurosecretory cells that project to different sites.     

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 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.     

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.     

Transitory coexistence of oxytocin and vasopressin in the hypothalamo neurohypophysial system of parturient rats.

We used in situ hybridization and immunocytochemistry to investigate a possible coexistence of vasopressin and oxytocin in hypothalamic neurons of parturient rats. We found that a fraction of magnocellular neurons in the paraventricular and supraoptic nuclei contained immunostaining for both peptides as well as oxytocin and vasopressin mRNA hybridization. Colocalization of immunoreactive vasopressin and oxytocin could be observed in some of the Herring bodies in the median eminence and the posterior lobe. No coexistence of vasopressin and oxytocin was found in pregnant or in lactating animals, indicating that the observed coexistence is transitory, perhaps mediated through changing hormonal conditions peri partum.     

Changes of hypothalamic and plasma vasopressin in rats with deoxycorticosterone-acetate induced salt appetite.

Mineralocorticoids play a predominant role in development of salt appetite and hypertension. Since vasoactive peptides could mediate the central effects of mineralocorticoids, we evaluated changes of immunoreactive (IR) arginine vasopressin (AVP) in the paraventricular (PVN) and supraoptic (SON) hypothalamic nucleus during DOCA-induced salt appetite. In one model, rats having free access to water and 3% NaCl during 9 (prehypertensive stage) or 21 days (hypertensive stage) received DOCA (s.c., 10 mg/rat/in alternate days). A decrease in the IR cell area, number of IR cells and staining intensity was obtained in magnocellular PVN of rats treated during 9 days. After 21 days IR cell area and number of cells in the PVN also decreased, but staining intensity of remaining cells was normal. The same parameters were unchanged in the SON. In another model, animals treated with DOCA during 9 days had only access to 3% NaCl or water. The IR cell area in PVN and SON significantly increased in mineralocorticoid-treated and control animals, both drinking 3% NaCl. Staining intensity (PVN and SON) and number of IR cells (PVN) also augmented in DOCA-treated animals drinking salt respect of a group drinking water. Plasma AVP in rats treated with DOCA and offered salt and water, exhibited a 2-2.5 fold increase at the time of salt appetite induction. Plasma AVP was substantially higher in rats drinking salt only, while the highest levels were present in salt-drinking DOCA-treated rats. Thus, peptide depletion in the PVN may be due to increased release, because reduced levels of hypothalamic and posterior pituitary AVP were measured in this model. In rats drinking salt only the substantial increase of IR AVP in the PVN and SON, may be due to dehydration and hyperosmosis. Because DOCA-salt treated rats showed higher AVP levels in the PVN compared to untreated rats drinking salt only, it is possible that DOCA sensitized PVN cells to increase AVP production. The results suggest the vasopressinergic system could mediate some central functions of mineralocorticoids.     

Influence of hypergravity on hypothalamic vasopressin and oxytocin neurons in rats

This study was aimed to evaluate the reaction of the vasopressin (VP) and oxytocin (OT) neurons of the supraoptic nucleus (SON) in rats to single or repeated hypergravity (HG). Special attention was paid to the tyrosine hydroxylase (TH) expression in VP neurons as a marker of the neuron activation. Rats were revolved in a centrifuge with overloading 2G for 5 days or 34 days as well as for 34 days plus 5 days with an interval of 39 days between two rotations. Control rats were kept in a centrifuge room. Radioimmununoassay, quantitative and semi-quantitative immunocytochemistry and in situ hybridization were used to evaluate: a) VP concentration in the pituitary posterior lobe (PL) and in plasma; b) the number of VP-, OT- and TH-immunoreactive neurons in the SON; c) the optic density of VP-, OT- and TH-immunoreactive materials in cell bodies (SON) and distal axons (PL), d) the optic density of VP and OT mRNAs signals (S35) in the whole SON on microfilms. According to our data, VP neurons were strongly activated during HG (5 days or 34 days) that was manifested in the functional hypertrophy of the neurons, greatly increased concentrations of VP mRNA in the SON and VP in plasma, the onset of the TH expression. The neurons showed initially (5 days) the functional insufficiency (VP release > VP synthesis) followed by their adaptation (subsequent 29 days) to the increased need in VP (VP release < VP synthesis). No reaction of VP neurons was observed to repeated HG. In contrast to VP neurons, OT neurons did not react to short-term HG or showed functional depression after the long-term treatment.     

The hypothalamic syndrome in rats.

The original conception of the hypothalamus controlling feeding by the activity of two specific and reciprocally inhibitory centers has now been largely abandoned. Detailed neural research using a wide variety of methods has demonstrated the complex morphological and functional organization of this part of the brain and has modified the earlier simplistic approach. However, examination of the feeding responses to a variety of stimuli that represent components of control of feeding indicates that much or even most feeding control is extrahypothalamic. As demonstrated by the obesity or aphagia resulting from hypothalamic damage or from reversible hypothalamic interference, the hypothalamus influences or modulates feeding control, possibly by an enabling action, but it does not itself substantially control food intake either in the short or the long term. In the cachaxia of cancer, which can tentatively be regarded as a negative obesity, and which is closely reproducible in a rat model, the decline of food intake can be attributed to failure of control components that are all extrahypothalamic, and the deterioration of control of feeding appears to be quite independent of the hypothalamus. The very detailed reconstruction of intrahypothalamic circuitry that has been developed in recent years has not yet had any real impact on the problem of where or how the active control of food intake is generated or the way in which the hypothalamus influences this control.     

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.