Melatonin inhibits oxytocin and vasopressin release from the neurointermediate lobe of the hamster pituitary

Hypothalamic gonadotropin-releasing hormone receptor activation stimulates oxytocin release from the rat hypothalamo-neurohypophysial system while melatonin inhibits this process

The present study was undertaken to investigate the influence of gonadotropin-releasing hormone (GnRH) and its agonist and antagonist on oxytocin (OT) release from the rat hypothalamo-neurohypophysial (H-N) system. An additional aim was to determine whether the possible response of oxytocinergic neurons to these peptides could be modified by melatonin through a cAMP-dependent mechanism. The results show that the highly selective GnRH agonist (i.e., [Des-Gly(10),d-His(Bzl)(6),Pro-NHEt(9)]-LHRH; Histrelin) stimulates the secretion of OT from an isolated rat H-N system. Melatonin significantly inhibited basal and histrelin-induced release of OT in vitro, and displayed no significant influence on OT release in the presence of GnRH or its antagonist. Addition of melatonin to a medium containing forskolin resulted in significant reduction of OT secretion from the H-N system. On the other hand, addition of forskolin to a medium containing both histrelin and melatonin did not further alter the inhibitory influence of melatonin on the histrelin-dependent secretion of OT in vitro. Intracerebroventricular (icv) infusion (experiment in vivo) of a GnRH antagonist resulted in substantial inhibition of OT release, thus revealing the stimulatory action of endogenous GnRH. In melatonin-treated animals, blood plasma OT levels were not changed in comparison to the vehicle. Our present data strongly suggests that activation of the GnRH receptor in the hypothalamus is involved in stimulation of OT secretion from the rat H-N system. It has also been shown, under experimental in vitro conditions, that melatonin fully suppresses the response of oxytocinergic neurons to the GnRH agonist - histrelin. The effect of melatonin on OT release is mediated by the cAMP-dependent mechanism, although other mechanisms of action are also possible.

A review of the renal system and diurnal variations of renal activity in livestock

Kidneys are the main organs regulating water-electrolyte homeostasis in the body. They are responsible for maintaining the total volume of water and its distribution in particular water spaces, for electrolyte composition of systemic fluids and also for maintaining acid-base balance. These functions are performed by the plasma filtration process in renal glomeruli and the processes of active absorption and secretion in renal tubules, all adjusted to an 'activity-rest' rhythm. These diurnal changes are influenced by a 24-hour cycle of activity of hormones engaged in the regulation of renal activity. Studies on spontaneous rhythms of renal activity have been carried out mainly on humans and laboratory animals, but few studies have been carried out on livestock animals. Moreover, those results cover only some aspects of renal physiology. This review gives an overview of current knowledge concerning renal function and diurnal variations of some renal activity parameters in livestock, providing greater understanding of general chronobiological processes in mammals. Detailed knowledge of these rhythms is useful for clinical, practical and pharmacological purposes, as well as studies on their physical performance.

Melatonin inhibits the substance P-induced secretion of vasopressin and oxytocin from the rat hypothalamo-neurohypophysial system: in vitro studies

The aim of the present investigations was to study the influence of substance P (a member of a family of peptides known as tachykinins) on basal and K(+)-evoked vasopressin (AVP) and oxytocin (OT) release from rat hypothalamo-neurohypophysial system in vitro as well as to determine whether this effect of substance P is sensitive to melatonin. The present results show that substance P stimulates basal AVP and OT release from isolated hypothalamo-neurohypophysial system, when used at the concentrations of 10(-6) and 10(-7)M/l. At the concentration of 10(-9)M/l, however, substance P was found to stimulate the in vitro secretion of AVP, but not that of OT. Melatonin diminished basal release of AVP; it also significantly inhibited the substance P-stimulated secretion of AVP and OT. K(+)-evoked release of the neurohypophysial hormones was not further modified by either substance P or melatonin. The present results show that the stimulatory effect of substance P on basal release of AVP and OT from rat hypothalamo-neurohypophysial system in vitro is sensitive to inhibitory influence of melatonin.

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.

Effect of melatonin on arginine vasopressin secretion stimulated by physical exercise or angiotensin II in normal men

The present study was undertaken in order to establish the possible involvement of melatonin in the mechanisms underlying the arginine-vasopressin (AVP) responses to physical exercise and angiotensin II (ANG II). On two mornings at least 1 week apart, normal male subjects were tested with exercise on a bicycle ergometer (the workload was gradually increased at 3-min intervals until exhaustion and lasted about 15 min in all subjects) or ANG II (60-min infusion of ANG II (Asp 1, IIe 5 angiotensin II) dissolved in 5% glucose in successively increasing doses of 4, 8, 16 ng/kg/min; each dose for 20 min). Tests were carried out with the administration of either 6 mg melatonin or placebo. Melatonin treatment neither modified the basal concentrations of AVP nor changed the AVP response to ANG II. In contrast, plasma AVP levels rose 3.6 times during exercise in the absence of melatonin, but only 2.3 times in the presence of melatonin. These data indicate an involvement of melatonin in the mechanism underlying the AVP response to physical exercise, but not ANG II, in normal men.

The effect of melatonin on suckling-induced oxytocin and prolactin release in the rat

There is growing evidence that melatonin (MEL) inhibits oxytocin (OT) release when used in a low dose, while higher doses stimulate the release of the hormone in the rat. In the present study we investigated the effect of exogenous MEL, administered intracerebroventricularly (ICV), on suckling-induced OT and prolactin (PRL) release in the urethane-anesthetized rat. Lactating rats suckled by 8-12 pups were studied on days 8-12 of postpartum, and lactating pups-deprived rats on the same days of postpartum served as a control. Plasma OT and PRL levels as well as hypothalamic and neurohypophyseal OT contents were measured by RIA. Suckling stimulated the secretion of both OT and PRL. The ICV injection of 1 ng/ml MEL produced a significant inhibition of suckling-induced OT as well as PRL secretion. Melatonin in doses of 100 ng/ml or 10 micrograms/ml did not modify the OT release but significantly inhibited PRL release brought about by suckling; 10 pg/ml of MEL was not effective in this regard. Thus, exogenous MEL seems to inhibit suckling-induced OT as well as PRL secretion when applied at doses regarded to be in the range of the physiological level; when applied in higher doses, it was shown not to influence the release of OT following physiological stimulation such as suckling.

Effect of melatonin on hypoglycemia and metoclopramide-stimulated arginine vasopressin secretion in normal men

The present study was performed in order to establish whether melatonin (MEL) plays a role in the regulation of arginine vasopressin secretion (AVP) in normal human subjects. For this purpose, the effects of an oral administration of 6 or 12 mg MEL on basal and metoclopramide (MCP)- or hypoglycemia-stimulated AVP secretion was tested in 18 normal men. MCP was given at a dose of 20 mg as an intravenous (i.v.) bolus; hypoglycemia was induced with an i.v. bolus injection of 0.15 IU/kg body weight of insulin. In addition, in view of the well-known inhibitory effect of MEL on the growth hormone (GH) response to hypoglycemia, GH levels were measured during the insulin tolerance test (ITT), as an independent index of MEL activity. MEL did not produce any change in AVP secretory patterns in basal conditions or during the MCP test. In contrast, the mean peak AVP response to hypoglycemia was 2.33 times higher than baseline in the control ITT, whereas it was only 1.77 times higher than baseline in the ITT plus MEL tests. Also, the GH response to hypoglycemia was significantly lower in the presence than in the absence of MEL. For both AVP and GH, the inhibitory effect of MEL during ITT was similar, when either 6 or 12 mg MEL was given. These data indicate an involvement of MEL in the control of the AVP response to hypoglycemia, but not of basal and MCP-induced AVP secretion. In addition, the similar effects of MEL on GH and AVP secretions during ITT suggest that similar neuroendocrine mechanisms underlie these hormonal responses to hypoglycemia.

The effects of melatonin on vasopressin secretion in vivo: interactions with acetylcholine and prostaglandins

The pineal hormone melatonin influences the neurohypophysial hormone release from the isolated hypothalamus in vitro through the effect on the cholinergic pathways as well as the biosynthesis of prostaglandins. The aim of the present study was, therefore, to investigate the effects of melatonin (0.5, 1, or 5 ng) administered in vivo on the vasopressin and oxytocin release as well as to examine whether similar interactions between melatonin and acetylcholine or prostaglandins occur in vivo. In the initial study on the effect of melatonin male Sprague-Dawley rats were implanted under anaesthesia with an arterial and venous cannula. Melatonin in a dose of 0.5 ng injected intravenously had no effect on plasma vasopressin concentration. The higher dose of 1 ng caused a significant decrease in vasopressin release 10 min after injection, whereas 5 ng melatonin caused an increase in plasma hormone concentrations, the difference being significant 20 min after injection. No significant effects of melatonin on the oxytocin release was found. In the second study in which an I.C.V. cannula was additionally implanted, the cholinergic muscarinic receptor antagonist atropine (10 microg) injected I.C.V. abolished the melatonin-induced effects on plasma vasopressin level. On the other hand, a cyclo-oxygenase inhibitor ibuprofen (75 microg) injected I.C.V. blocked the vasopressin release induced by 5 ng melatonin and reversed the inhibitory effect of 1 ng melatonin. These results demonstrate that melatonin affects the neurosecretory function of the hypothalamo-neurohypophysial complex in vivo possibly via mechanisms involving cholinergic transmission and/or prostaglandin biosynthesis.

Neurohypophyseal vasopressin in the Syrian hamster: response to short photoperiod, pinealectomy, melatonin treatment, or osmotic stimulation

In the present study, the effect of photoperiod on vasopressin content in the pituitary neurointermediate lobe (NIL), as well as the ability of pinealectomy to prevent and melatonin to mimic the short photoperiod-induced changes in NIL vasopressin were studied in male Syrian hamsters. The ability of melatonin to modify the hyperosmotically stimulated vasopressin release was also determined. Exposure to short photoperiod (SD) for 4 or 10 weeks increased vasopressin content in the hamster NIL. In long photoperiod (LD)-exposed hamsters, pinealectomy induced a decrease in NIL vasopressin content, whereas no effect of melatonin injections on vasopressin storage in the NIL was detected. In SD-exposed animals, pineal removal failed to alter vasopressin content in the NIL. Hypertonic saline administration led to the expected decrease in vasopressin content in the NIL both in vehicle- and melatonin-treated animals. The hyperosmotically stimulated release of vasopressin was not modified by previous treatment with melatonin. The data from the present study show that, in male Syrian hamsters, exposure of animals to SD increases the vasopressin content in the posterior pituitary, but these changes appear not to be mediated by SD-induced changes in melatonin secretion. Furthermore, the exposure of animals to SD prevents the pinealectomy-induced changes in NIL vasopressin content. Melatonin does not modify the hyperosmotically stimulated vasopressin release in the male Syrian hamster.

The effects of short photoperiod, pinealectomy, and melatonin treatment on oxytocin synthesis and release in the male syrian hamster

The pineal gland has been shown to affect plasma oxytocin (OT) levels, but the mechanism of this action is not apparent. In the present study, the ability of the photoperiod to affect plasma OT levels, neurointermediate lobe (NIL) OT content, and hypothalamic OT mRNA levels was studied in male Syrian hamsters. In addition, the ability of pinealectomy to prevent and melatonin (MEL) to mimic the short photoperiod-induced changes were also determined. Exposure to short days (SD) led to the expected decrease in testes weight and plasma PRL levels, but plasma OT levels were unchanged. However, NIL OT content was increased in the SD-exposed animals. Hypothalamic OT mRNA levels were not significantly altered by SD exposure. Pinealectomy blocked the effects of SD on testes weight, whereas afternoon MEL injections mimicked the effects of SD. In long day (LD)-exposed hamsters, pinealectomy induced a decrease in NIL OT content without altering hypothalamic OT mRNA levels. In SD-exposed animals, NIL OT content was not affected by pinealectomy. Melatonin injections had no significant effect on NIL OT content or hypothalamic OT mRNA levels. The data from the present study suggest that exposure of male Syrian hamsters to short photoperiods influences some aspects of OT synthesis and/or transport to produce its increased accumulation in the NIL, but does not affect OT release. These changes are apparently not the result of SD-induced changes in MEL secretion, but conceivably could be related to the previously documented effects of SD on hypothalamic catecholamine turnover.