Luteinizing hormone-releasing hormone and oxytocin response to hyperosmotic stimulation: in vitro study.

It was shown previously that luteinizing hormone-releasing hormone (LHRH) affects the neurohypophysial oxytocin release in water-deprived rats. However, the detailed mechanisms by which LHRH modifies the oxytocin response to hyperosmotic stimulation have not been explained so far. Using the isolated hypothalamo-neurohypophysial explants obtained from euhydrated rats, the effect of LHRH on the oxytocin secretion was studied under conditions of direct osmotic (i.e., Na(+)- evoked) as well as nonosmotic (i.e., K(+)-evoked) stimulation. Additionally, the oxytocin response to LHRH was investigated using the explants obtained from animals drinking 2% saline for eight days (systemic, i. e., both direct and indirect, osmotic stimulation). LHRH significantly enhanced Na(+)- and K(+)-evoked oxytocin release from explants taken from rats drinking tap water, indicating that LHRH could affect the Na(+)/K(+)-dependent depolarization of perikarya of oxytocin neurones. In contrast, LHRH significantly diminished the K(+)-stimulated hormone release when the neurohypophysial complex was obtained from previously salt-loaded rats, suggesting that peripheral osmotic stimulation somehow modifies the sensitivity of oxytocinergic neurones to LHRH (possible mechanisms are discussed). It is concluded that LHRH may participate in the regulation of oxytocin secretion via both direct and indirect impact on magnocellular oxytocinergic neurones depending on the current functional status of the hypothalamo-neurohypophysial complex.

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 effect of melatonin on vasopressin release under stress conditions in pinealectomized male rats.

The findings here reported showed that the response of vasopressinergic neurons to immobilization stress is augmented by melatonin. The effectiveness of melatonin in functional modification of these neurons' activity under conditions of stress changes after pineal removal.

Luteinizing hormone-releasing hormone and function of the magnocellular vasopressinergic system.

Mechanisms by which luteinizing hormone-releasing hormone (LHRH) affects vasopressin secretion were investigated using the isolated rat hypothalamo-neurohypophysial explants. LHRH in a concentration of 4 x 10(-7)M inhibited both the basal and K(+)-stimulated vasopressin release from explants isolated from euhydrated rats. When, however, the tissue was obtained from animals previously salt-loaded, the inhibitory effect of LHRH was completely abolished, thus implying a decrease in the sensitivity to LHRH. LHRH did not affect vasopressin secretion under conditions of generalized blockade of synaptic inputs by 15 mM MgSO(4), suggesting the indirect action of this neurohormone on the hypothalamic magnocellular system. It is concluded that LHRH may play the role of a neuromodulator of vasopressinergic neurones in the rat.

Thyrotropin-releasing hormone affects the oxytocin, vasopressin and prolactin release in female rats during midlactation: relation to suckling.

The effect of thyrotropin-releasing hormone (TRH; 200 ng i.c.v.) on oxytocin (OT), vasopressin (AVP) and prolactin (PRL) release was estimated in female Wistar rats during midlactation. The hypothalamo-neurohypophysial radioimmunoassayed OT and AVP storage as well as blood plasma level of both neurohypophysial hormones and PRL in females suckled or not suckled have been studied. I.c.v. administration of TRH increased AVP content both in the hypothalamus and neurohypophysis of suckled females; however, plasma AVP level did not change. TRH increased the hypothalamic as well as neurohypophysial OT content during suckling. Simultaneously, TRH inhibited OT release into the blood plasma. On the contrary, in not suckled females TRH increased OT plasma concentration. I.c.v. TRH raised the PRL concentration in plasma of lactating but, at the moment, not suckled females. On the contrary, i.c.v. TRH injection into females just suckled was followed by a decrease in PRL plasma level. TRH probably acts in the central nervous system as an inhibitory neuromodulating factor for the vasopressin release. Also, it cannot be excluded that TRH--otherwise known to enhance the PRL release--suppresses the oxytocin-prolactin positive feedback mechanism when activated temporarily by suckling.

Effects of methylphenidate on oxytocin and vasopressin levels in pinealectomized rats during light-dark cycle.

Although previous reports have shown that methylphenidate (MPH), in addition to its known behavioral effect, can influence the hypothalamo-pituitary-adrenal axis by increasing the plasma ACTH, the pineal gland seems to be involved in neuroendocrinological processes too, e.g., in hypothalamic synthesis and release of oxytocin (OXY) and vasopressin (AVP). Therefore, a study was performed to measure the OXY and AVP content of the hypothalamus, neurohypophysis, and plasma after application of MPH in the morning and evening in pinealectomized (PE) as well as sham-operated control (SO) rats. Pinealectomy influenced both the daily pattern (reversed in the neurohypophysis) and the levels of OXY and AVP. Starting from this different situation, application of MPH produced diverse effects. Hypothalamus: PE, increase in both hormones in the morning and evening; SO, decrease in morning OXY level. Neurohypophysis: PE, increase in morning OXY level; SO, decrease in both hormones even though in the morning only. Plasma: PE, decrease in morning OXY concentration; SO, increase in both hormones in the morning and decrease in the evening. The present results indicate that MPH application influences the hypothalamo-neurohypophysial system. Furthermore, the hypothesis has been supported that this influence may be dependent on the circadian activity of the pineal gland as well.

Effect of pinealectomy and melatonin on vasopressin-potentiated passive avoidance in rats.

The pineal indoleamine, melatonin, and the hypothalamic neuropeptide, vasopressin, facilitate passive avoidance behaviour in rats. The similarity of the effects suggest that interaction might occur between the two substances. Therefore, the effect of intraperitoneally applied vasopressin and/or melatonin on one-trial learning passive avoidance behaviour was studied in pinealectomized rats. Intraperitoneal treatment with 500 ng vasopressin 1 hr before the retention test increased passive avoidance latency of sham-operated rats. In pinealectomized rats, an identical amount of vasopressin was ineffective. In sham-operated rats, melatonin blocked the effect of vasopressin. It is concluded that vasopressin needs a regulated pineal function for developing effects in passive avoidance behaviour.

Cholecystokinin octapeptide and the daily rhythm of vasopressin and oxytocin release.

The daily rhythm of neurohypophysial hormone release was monitored in rats given intracerebroventricularly (i.c.v.) CCK-8 (50 ng/10 microliters--once daily over five days). In animals injected i.c.v. with vehicle solution (0.9% NaCl) plasma oxytocin and vasopressin concentrations were seen to rise significantly over the hours of daylight, decreasing during the night. The changes seen in the neurohypophysial vasopressin and oxytocin content were inversely related to the plasma concentrations. Under i.c.v. treatment with CCK-8, the daily rhythm of the vasopressin and oxytocin release was similar to daily rhythm in the control group; respective figures were, however, reduced for the hypothalamus and neurohypophysis as well as increased for the blood plasma. It is suggested that CCK-8 may be involved in some circadian regulatory processes related to vasopressin and oxytocin release from the rat hypothalamo-neurohypophysial system.

Neurohypophysial response to peptides of the cholecystokinin family: in vivo and in vitro studies.

I.c.v. injection of CCK-8 (50 ng/day) to euhydrated rats significantly decreased vasopressin and oxytocin content in the hypothalamo-neurohypophysial system. In rats drinking hypertonic saline and simultaneously treated with CCK-8, the decrease of vasopressin content was more marked in hypothalamus but somewhat restrained in the neurohypophysis. The decrease of hypothalamic and neurohypophysial oxytocin content (as brought about by the salt load) was significantly less marked in animals treated simultaneously with CCK-8. Incubation of neurointermediate lobes in medium enriched with CCK-33/CCK-39 or CCK-8 did not change significantly the vasopressin and oxytocin release from the neurointermediate lobes both under basal conditions and during potassium stimulation. It is suggested that afferent impulses of osmoreceptor origin may modify the response of vasopressinergic and oxytocinergic neurons to CCK-8. The events, related to the influence of CCK peptides on vasopressin or oxytocin release, do not seem to be localized at the neurohypophysial level.

Thyrotropin-releasing hormone (TRH) modulates vasopressin and oxytocin release from the hypothalamo-neurohypophysial system in dehydrated rats.

Rats euhydrated and dehydrated during two or four days were given intracerebroventricularly (i.c.v.) thyrotropin-releasing hormone (TRH) in a daily dose of 200 ng dissolved in 10 microliters of 0.9% sodium chloride. A single dose of TRH injected to euhydrated animals increased the hypothalamic vasopressin content but did not affect significantly the content of vasopressin in the neurohypophysis as well as that of oxytocin both in the hypothalamus and neurohypophysis. In rats deprived of water for two days TRH completely prevented the decrease of neurohypophysial oxytocin due to stimulation of osmoreceptor origin. Similarly, TRH restrained both the hypothalamic and the neurohypophysial vasopressin and oxytocin depletion in rats dehydrated for four days.

Insulin-like immunoreactivity (IRI) in the rat hypothalamo-neurohypophysial system: effect of dehydration and haemorrhage.

Hypothalamic IRI was not affected in haemorrhaged rats, but diminished considerably in the dehydrated ones. In the neurohypohysis, IRI was distinctly higher both in dehydrated and haemorrhaged rats, i.e., under disorders which stimulated vasopressin and/or oxytocin release. It is suggested that insulin-like substance(s) may be someway involved in regulation of vasopressin or oxytocin secretion.

Hypothalamic and neurohypophysial vasopressin and oxytocin content as influenced by haemorrhage in melatonin-treated male rats.

The effect of haemorrhage (1 ml per 100 g b. w.) on the vasopressin and oxytocin storage in the hypothalamus and neurohypophysis of melatonin-treated male rats was determined. Melatonin treatment (100 micrograms/100 g b. w., once daily over 8 days) resulted in a known decrease of vasopressin as well as oxytocin content both in the hypothalamus and neurohypophysis. Haemorrhage decreased the neurohypophysial vasopressin and oxytocin storage in animals injected with vehicle solution or otherwise not treated. In melatonin-treated rats, however, bleeding did not affect the actual (i.e., decreased by melatonin) vasopressin and oxytocin content in the hypothalamo-neurohypophysial system. The results demonstrate that melatonin may be involved in mechanisms determining the rate of the response of vasopressinergic and oxytocinergic neurones to bleeding.

Neurohypophysial vasopressin and oxytocin as influenced by (6R)-5,6,7,8-tetrahydro-alpha-biopterin in euhydrated and dehydrated rats.

Experiments were performed to test a possible role of (6R)-5,6,7,8-tetrahydro-alpha-biopterin dihydrochloride (BH4) in the neurohypophysial vasopressin and oxytocin content in euhydrated and dehydrated rats. Chronic treatment with BH4, administered to not dehydrated animals, was followed by a significant decrease of neurohypophysial vasopressin, but not of neurohypophysial oxytocin. In dehydrated animals the neurohypophysial vasopressin content decreased progressively; this process was not affected significantly by BH4. In rats dehydrated and treated simultaneously with BH4 the decrease of neurohypophysial oxytocin content was distinctly less marked.

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 Dekanski 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.

Daily rhythms in the hormone content of the neurohypophysial system and release of oxytocin and vasopressin in the male rat: effect of constant light.

Patterns of neurohypophysial hormone secretion and changes in the hormone content of the hypothalamus and posterior pituitary lobe were monitored in the male rat for cycles of 24 h in association with changes in food and water intake and fluid excretion. Plasma oxytocin and vasopressin concentrations were seen to rise significantly over the hours of daylight, decreasing during the night. Parallel changes were seen in the immunoreactive material in the hypothalamus, whilst the content of the neurohypophysis was inversely related to plasma concentrations. The ratio of plasma oxytocin:vasopressin reached a significant peak at about 02.00 h which might be related to the feeding activity of the rats, food and water intake being largely confined to the night, as was fluid excretion. On exposure to constant light, despite initial disruption hormonal rhythms were still seen but showed a phase shift. The relationships between plasma and tissue levels were maintained. Patterns of food and water intake and urinary excretion were little affected by exposure to constant light, remaining largely confined to the former night phase. The hormonal rhythms appeared to be more closely related to the activity of the rats, which also showed a phase shift during constant light.

Atrial natriuretic peptide inhibits neurohypophysial hormones' release in the rat (in vitro and in vivo studies).

Intracerebroventricular hANP (50 nmol) inhibits release of vasopressin and oxytocin following dehydration as well as after haemorrhage. 10 nmol/L hANP markedly inhibits vasopressin and oxytocin release in vitro from the neurointermediate lobes both under basal condition as well as during stimulation with excess (56 mM) potassium. It is suggested that ANP may serve as a modulator of vasopressin and oxytocin release. The respective processes are localized, at least in part, at the neurohypophysial level.

Thyrotropin-releasing hormone (TRH) and vasopressin and oxytocin release: in vitro as well as in vivo studies.

Rats euhydrated and dehydrated for two or four days were given intracerebroventricularly (i.c.v.) thyrotropin-releasing hormone (TRH) in a daily dose of 200 ng dissolved in 10 microliters of 0.9% sodium chloride.) A single dose of TRH administered to euhydrated animals was followed by a significant increase of the vasopressin content in the neurohypophysis and hypothalamus as well as of the hypothalamic oxytocin content. On the contrary, a single dose of TRH decreased the oxytocin content in the neurohypophysis. Under conditions of dehydration TRH distinctly restrained the decrease of vasopressin and oxytocin in the hypothalamus. In animals dehydrated for two or four days the decrease of oxytocin in the neurohypophysis, brought about by stimulation of osmoreceptors, was distinctly more marked under treatment with TRH. On the contrary, the depletion of neurohypophysial vasopressin was significantly less apparent under such conditions. 28 nmol/L TRH markedly increased vasopressin release but inhibited that of oxytocin from the neurointermediate lobes incubated in vitro both under basal conditions as well as during stimulation with excess (56 mmol) potassium.

Melatonin, pinealectomy, and release of neurohypophysial hormones: in vitro studies.

The effect of melatonin as well as pinealectomy on the basal and K(+)-evoked release of vasopressin and oxytocin from the neurointermediate lobes in vitro was determined. Pineal removal resulted in a diminution of vasopressin and oxytocin release from the neurointermediate lobes in vitro. Melatonin (10(-3) or 10(-6) M/l) increased vasopressin and oxytocin release from neurointermediate lobes of sham-operated rats. Nevertheless, when pinealectomized rats served as donors of the neurointermediate lobes, melatonin (10(-3) or 10(-6) M/l) increased vasopressin release under basal conditions. For the same tissue, melatonin did not affect the oxytocin release either under basal conditions or during depolarization due to excess potassium. When 10(-7) M/l melatonin was used, no changes in either vasopressin or oxytocin release were observed in vitro.