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Melatonin and its possible role in regulation of water and electrolyte metabolism

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Abstract

Melatonin, the principal neurohormon of the pineal gland influences many physiological functions within the mammalians' body. The nocturnal peak of melatonin secretion may play a role in the regulation of the function of many organs and body systems (likewise the circadian rhythms and their functions), enabling them to maintain homeostasis and to adapt physiological functions to changing environmental conditions. New data suggests that melatonin may act directly on the water and electrolyte metabolism and renal function. It is possible that melatonin plays a role in the regulation of the functioning of the endocrine system through the nuclei in the hypothalamus, in order to inhibit synthesis and/or secretion of the pituitary vasopressin. Moreover, the presence of putative melatonin binding sites in the kidney suggests the possibility of the direct action of melatonin on the renal system. Melatonin may also indirectly affect the water and electrolyte metabolism through the influence on the behavioral effects.

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... (Carlson et al. 1989;Forcada et al., 1999;Chemineau et al., 2008 andJackson et al., 2014). The pineal gland produces and releases melatonin, which is a neurohormone (Skotnicka and Heynczak, 2001). The pineal gland's secretion is influenced by lighting conditions in a cyclic fashion (Kowiak et al., 1999). ...
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... Melatonin is classified as one of the neuro -hormones produced and release rhythmically from the pineal gland (3). Melatonin rises and declines in response to lighting conditions (4). ...
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... Melatonin is classified as one of the neuro -hormones produced and release rhythmically from the pineal gland (3). Melatonin rises and declines in response to lighting conditions (4). ...
Article
Full-text available
A total of twenty-four healthy ewes aged between 2-3 years old and weighed about 35.09 kg of body weight were divided randomly into four groups (6 animals for each group). The first group included the non-treated control group, while the other three groups were subjected to various oral doses of melatonin 9 mg, 12 mg and 15 mg/head, respectively. All treatments were applied at 6 o'clock in the morning, to investigate the effect of treatment with melatonin during breeding season on reproductive performance and levels of some biochemical parameters in Arabi ewes from 1 May to 1 October 2017. Blood samples 7ml were collected after one hour of treatment and then monthly during the pregnancy months to determine the concentrations of biochemical parameters. After Postpartum, lambed ewes/lambs number, birth weight, placenta empty weight and cotyledon number were recorded. The results showed significant (P
... Melatonin is one of the neuro hormones produced and released from pineal gland (Skotnicka and Heynczak, 2001). Thesecreting * Corresponding author: Waleed Y. Kassim from this gland rhythmically depends on the lighting conditions (Kowiak et al., 1999). ...
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... Melatonin is one of the neuro hormones produced and released from pineal gland (Skotnicka and Heynczak, 2001). Thesecreting * Corresponding author: Waleed Y. Kassim from this gland rhythmically depends on the lighting conditions (Kowiak et al., 1999). ...
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Arginine vasopressin (AVP) neurons were preferentially localized in the dorsomedial part of the suprachiasmatic nucleus (SCN). To know the role of AVP neurons in the SCN, male rats were kept under a normal light-dark cycle (L-D), or under constant darkness (D-D) for 20 days. In L-D condition, AVP levels in the SCN showed the circadian change. In D-D condition, the patterns in AVP levels showed a free-running rhythm, and an about 12-h shift per 20 days. This result suggests that the activity of AVP neurons may be closely associated with the endogenous circadian rhythm of the SCN.
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Melatonin transduces the effect of photoperiod on the neuroendocrine system. Synthesis of melatonin in the pineal gland is well described, but the location of its target(s) and the mechanism of its action are little known. In attempt to localize melatonin target(s), the presence of high affinity binding sites in rat brain was determined. Such sites were detected in discrete brain areas, including the hypothalamus and anterior pituitary. Subcellular analysis indicated these binding sites were on plasma membranes, which suggests that melatonin modulates cell functions through intracellular second messengers. The effects of melatonin on second messengers were studied using the neonatal anterior pituitary, in which melatonin is known to inhibit the LHRH-induced release of LH. Studies on the effects of melatonin on second messenger indicated [corrected] that melatonin inhibits accumulation of cAMP and cGMP as well as synthesis of diacylglycerol and release of arachidonic acid. Time-course analysis indicates that inhibition by melatonin of the LHRH-induced release of LH increases following long preincubation. Since the effect of melatonin on LHRH-induced release of LH is prevented by dibutyryl cAMP, we conclude that melatonin might act by inhibiting production of cAMP.
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This discussion of the roles of transmitters in the circadian system has focused mostly on the entrainment mechanism because it is not clear to what extent neurotransmission is important to the other major function of circadian systems, rhythm generation. Schwartz et al (1987) have presented evidence that the circadian pacemaker in the SCN continues to run, but cannot be entrained by light, when tetrodotoxin is used to block sodium-dependent action potentials. Although other forms of intercellular communication are not ruled out, these results suggest that classical synaptic neurotransmission is important for entrainment but not for rhythm generation. That the SCN contains a cholinergic marker like ChAT and is responsive to cholinergic agents but does not bind nicotine or ACh reflects a general problem in reconciling functional, physiological, and anatomical markers of neurotransmission. A mismatch between the anatomical distributions of transmitters and their receptor-binding sites is a common observation, the meaning of which remains enigmatic (Herkenham 1987). Also, the neurophysiological consequences of injections of drugs into parts of the brain involved in rhythm regulation remain largely unknown. Interpretations of the effects of these treatments on rhythms are predicated on assumptions that may not be valid; e.g. that a bolus injection of an excitatory substance has its primary effect by activating neurons. Still to be established is whether the effects of drugs when they are administered in behavioral pharmacology studies reflect their effects on cellular functions and on neuronal responses to photic cues when they are delivered at near-physiological levels to single neurons.
Article
A variety of reports indicate that some kind of interaction may exist between the pineal gland and cations. Of particular interest are the reports that indicate comparatively high levels of copper, manganese, and zinc in the pineal gland and that the pineal gland exhibits a circadian rhythm in calcium, magnesium, potassium, and sodium. There are, unfortunately, no reports suggesting a functional role for these findings. This study investigated circadian rhythms in circulating plasma cations in rats under 12/12 h light-dark cycle and in rats whose pineal function had been suppressed by exposure to constant light for 1 and 7 days. Neither of the treatments affected circulating potassium levels but had some significant effects on sodium concentration at a number of time points without affecting the total amount of sodium circulating in a 24 h period. Calcium, magnesium, and zinc plasma concentrations were little affected by 1 day of constant light, while 7 days of constant light caused a dramatic and highly significant increase in the circulating levels of the three cations. The plasma levels of copper, on the other hand, while also being unaffected by 1 day were significantly depressed by 7 days of constant light. It is apparent, therefore, that the pineal gland may be involved in regulation of circulating levels of the cations measured. The functional significance of these observations is not clear at this stage but warrants further investigation.
Article
Administration of methoxyindoles to rats in both the morning and the afternoon caused significant changes in circulating plasma cation levels. All the methoxyindoles except melatonin were without effect at doses below 100 micrograms/kg; the observed changes caused by doses of 100 micrograms/kg can, therefore, probably be best described as pharmacological rather than having physiological significance. Melatonin at all doses tested (25, 50, and 100 micrograms/kg) led to a decrease in plasma magnesium levels 3 h after administration in the morning, and this finding is discussed in the light of a possible feedback mechanism between melatonin and circulating magnesium. Melatonin administration in the afternoon was without effect on magnesium levels. Zinc levels were also decreased significantly 3 h after a morning dose of melatonin (50 and 100 micrograms/kg), and the possible functional significance of this observation is discussed. Most of the observed effects can be considered nonspecific and pharmacological, and further work is needed to fully elucidate the involvement, if any, of the pineal gland in regulation of cations.
Article
The influence of melatonin on circadian and photoperiodic functions in numerous species is well documented. It is known that the effect of melatonin on circadian rhythmicity is mediated via the suprachiasmatic nucleus (SCN), the biological clock of the brain. It is not known however where the photoperiodic effects of melatonin are mediated. Evidence from brain lesioning and melatonin implant studies point to a site in or near the medial hypothalamus. In contrast to these studies, melatonin receptors have been reported in widespread areas of the brain, the pituitary and in peripheral tissues. The characteristics of the reported melatonin receptors vary widely between studies and consequently no definitive description of a physiologically relevant melatonin receptor has received universal recognition. This review marshals recent evidence for the localization and characterization of the melatonin receptor and discusses these findings in the context of the known effects of the hormone in different species.
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Anatomical and electrophysiological studies have revealed a widespread innervation of the brain by arginine vasopressin (AVP)-containing fibers. There is evidence that these central AVP pathways may be activated simultaneously with endocrine pathways. Stimulation of hypothalamic nuclei that contain AVP cell bodies causes changes in electrical activity of neurons in areas receiving AVP projections; in these same regions, release of immunoreactive AVP can be detected in response to appropriate stimuli or hypothalamic stimulation. These parts of the brain have also been shown to contain AVP receptors, and application of AVP to cells in these areas alters spontaneous activity or modifies the responses to other transmitters. AVP appears to act as a neurotransmitter involved in the central control of the cardiovascular, renal, and thermoregulatory systems. AVP may act centrally to coordinate autonomic and endocrine responses to homeostatic perturbations.
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Pinealectomized (PX), sham-operated and non operated control rats were injected intraperitoneally (i.p.), once daily at 8.00 over five days, with: (a) 0.9% sodium chloride, (b) propranolol hydrochloride in a dose of 10 mg/kg (= 0.1 ml solution per 100 g b.w.). Three hours following the last injection the animals were decapitated and the content of vasopressin and oxytocin was bioassayed in the hypothalamus and neurointermediate lobe. PX was followed by known decrease of both vasopressin and oxytocin in the hypothalamus and neurohypophysis. In rats not-PX propranolol did not change the vasopressin and oxytocin content in the hypothalamus and neurointermediate lobe. In PX-rats, treatment with propranolol resulted in a distinct increase of the vasopressin in the neurohypophysis. It may be therefore supposed that the beta-adrenergic transmission is in some way involved in the regulatory mechanisms of pineal-neurohypophysial functional relationship.
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The effects of melatonin on blood pressure and heart rate were studied in 23-week-old male spontaneously hypertensive rats. Melatonin infused i.p. at a dose of 6 mg/rat per day for 5 days using an osmotic minipump produced a significant reduction of blood pressure and a slight but significant decrease of heart rate in the conscious and unrestrained state. These cardiovascular effects of melatonin developed gradually. Plasma renin concentration tended to decrease after melatonin treatment. These results demonstrate that melatonin has an antihypertensive action. The mechanism of the antihypertensive action of melatonin requires further study.
Article
Melatonin injected in a single intraperitoneal dose of 100 μg/100 g b. w. to euhydrated rats resulted in a decrease of neurohypophysial oxytocin content but the hypothalamic oxytocin storage as well as the hypothalamo-neurohypophysial storage of vasopressin were not changed. Following 8 d of once-daily melatonin treatment the hypothalamic and neurohypophysial oxytocin and vasopressin content was decreased. It might be therefore suggested that melatonin increases the release of neurohypophysial hormones and/or decreases their synthesis. Melatonin did not significantly modify the neurohypophysial vasopressin depletion rate in animals deprived of water up to 8 days. No consistent effects of melatonin on the decrease of hypothalamo-neurohypophysial content of oxytocin were noted under conditions of dehydration and simultaneous administration of melatonin up to 8 d.
Article
The spontaneous locomotor activity of unoperated and pinealectomized (PE) rats was recorded electromagnetically and the blood pressure was measured without anesthesia by the tail cuff method. During several different lighting schedules PE rats had an increased tendency to a 6 hour ultradian activity rhythm. During a 15 hour light 9 hour dark schedule PE rats showed greatly increased locomotor activity in darkness. The psychosedative drug oxypertine (50 mg/l in drinking water), which has a methoxyindole structure resembling the pineal hormone melatonin, reduced motor activity in PE rats less than in unoperated rats. Oxypertine treatment also increased the tendency to ultradian activities, in both unoperated and PE rats, as well as wholly preventing the development of pinealectomy induced hypertension. In view of the known effects of pineal indoles on the brain monoaminergic mechanisms, it is suggested that melatonin and/or related pineal indoles may act as natural antihypertensive agents, possibly by stimulating central adrenergic pathways. Further, the pineal indoles may also inhibit the level of spontaneous locomotor activity via central 5 hydroxytryptaminergic mechanisms. Possibly also the pineal gland influences the centres which generate the rhythm of locomotor activity.
Article
Neurosecretory activity of the hypothalamic magnocellular supraoptic and paraventricular nuclei (SON and PVN) was determined in pinealectomized and control young adult female rats subjected to a 12/12 h light rhythm, to continuous light, or to ovariectomy. The distribution of thiamine diphosphate-phosphohydrolase (TPP-ase), an enzyme specific for the Golgi apparatus, was used as a parameter for neurosecretory activity. In both the SON and the PVN, a decrease in neurosecretory activity was observed after pinealectomy, irrespective of additional experimental conditions. The significance of this finding is discussed, especially in regard to a possible general influence of the pineal gland on the neurosecretory nuclei.Copyright © 1972 S. Karger AG, Basel
Article
The possible role of aldosterone in pinealectomy induced hypertension was studied by using daily spironolactone administration (10 mg/kg orally) from the second postoperative day. induced blood pressure elevation was Pinealectomy partly inhibited and the reduction of serum potassium concentration antagonized by spironolactone. The urine volume was increased but the excretion of sodium and potassium decreased in pinealectomized animals. These changes, too, were prevented by spironolactone administration. In spironolactone treated pinealectomized rats the sodium concentration was slightly increased in both the heart and the arterial wall. The ability of spironolactone to antagonize the pinealectomy induced hypertension and the changes in the electrolyte concentrations in plasma and urine supports our earlier suggestion of an increased aldosterone secretion in this type of hypertension in rats.
Article
The effects of pineal gland on kidney-adrenal axis have been studied in male rats. Rats were pinealectomized and exposed to a photoperiod of 12 h light: 12 h dark. Plasma renin activity (PRA), corticosterone and corticotropin (ACTH) levels were measured at 10, 20 and 35 days postpinealectomy. Pinealectomy increased corticosterone and ACTH levels and decreased PRA in all age groups. A significant negative correlation was found between corticosterone and PRA, which suggest that changes in PRA were due to changes in circulating corticosterone, via feedback mechanism on renin secretion. On the other hand, melatonin administration prevents these effects of pinealectomy. It is suggested that the lack of this pineal indol is responsible for the pinealectomy-induced alterations in male rats.
Article
Diurnal changes in the intranuclear release of vasopressin (VP) and oxytocin (OT) in the suprachiasmatic (SCN), paraventricular (PVN) and supraoptic nuclei (SON) of the rat were studied by means of brain microdialysis. A significant diurnal variation in VP release in the SCN was detected, with the highest levels occurring during midday and a trough around midnight. OT release from the SCN was below detection limit. The release of neither of these neurohypophysial peptides showed diurnal variations within the PVN or SON.
Article
Putative melatonin receptors in normal kidney cortical tissues of patients with transitional cell carcinoma or renal cell carcinoma were characterized using a melatonin agonist, 2-[125I]iodomelatonin, as the radioligand. 2-[125I]Iodomelatonin-binding sites in the human kidney were stable, saturable, reversible, and of high affinity. The binding affinity was 15.2 +/- 2.5 pmol/L, and the binding density was 1.79 +/- 0.19 fmol/mg protein. The unity of the Hill coefficients and linearity of the Scatchard plots suggested that 2-[125I]iodomelatonin was bound to a single class of binding sites. Pharmacological characterization showed that these binding sites were highly specific to 2-iodomelatonin, melatonin, 6-hydroxymelatonin, and 6-chloromelatonin. Guanosine 5'-O-(3-thiotriphosphate) decreased the binding affinity and density of 2-[125I]iodomelatonin-binding sites in the kidney, suggesting that these binding sites are coupled to a G-protein. The characterization of 2-[125I]iodomelatonin-binding sites in normal kidney tissues taken from patients with transitional cell carcinoma or renal cell carcinoma suggests the existence of 2-[125I]iodomelatonin-binding sites in the human kidney cortex, which is in line with the findings of 2-[125I]iodomelatonin-binding sites in kidneys of other mammals and birds. The implication of a direct melatonin action on renal function in the human is proposed.
Article
Neurones of the rat suprachiasmatic nucleus (SCN) were tested with [Arg8]vasopressin (AVP) and the AVP receptor antagonist, [d(CH2)5,d-Tyr(OEt)2,Val4,Cit8]-vasopressin in vitro. 52% of AVP-responsive neurones showed an antagonist-induced decrease in activity, indicative of the presence of an endogenous excitatory tone. The magnitude of this effect declined significantly between subjective light and dark phases, consistent with the possibility that circadian fluctuations in endogenous AVP excitation contribute to the cycle of electrical activity within the SCN. However, similar fluctuations in basal activity between the light and dark phases was observed for both antagonist-sensitive and -insensitive neurones, indicating that endogenous AVP was not the only factor determining the circadian cycle.
Article
The excitatory effect of [Arg⁸]-vasopressin and its potential contribution to the circadian cycle of electrical activity in the suprachiasmatic nucleus of the rat was investigated using extracellular recordings from hypothalamic slices from virgin female rats. The majority of neurons tested for their responses to vasopressin and [Arg⁸]-vasotocin displayed coincident, dose-dependent excitation by both peptides, although the relative efficacy varied between neurons, with some showing a highly preferential excitation by vasotocin. Perifusion with the vasopressin receptor antagonistd(CH2)5[Tyr(OEt)²,Val⁴, Cit⁸]vasopressin was able to block the majority of responses to vasopressin or vasotocin (2025), and similar excitation could be induced by the selective agonist [Phe²,Orn⁸]-vasotocin, indicating a mainly V1 receptor-mediated effect. Few neurons (327; 11%) responded to the oxytocin-specific agonist, [Thr⁴,Gly⁷]-oxytocin, suggesting a low occurrence of oxytocin receptors. In addition to blocking the action of exogenous vasopressin, the V1 antagonist caused a reversible suppression of spontaneous basal activity in725 cases, consistent with the presence of an endogenous excitatory vasopressin tone. In agreement with previous reports, the activity of suprachiasmatic nucleus neurons showed a significant correlation between spontaneous activity and the light-dark cycle, with activity decreasing during the subjective dark phase. When neurons were divided on the basis of their response to vasopressin and/or vasotocin, the peptide-sensitive neurons continued to show a strong correlation (r = 0.513, P < 0.01) while the insensitive neurons showed no correlation (r = 0.136, P > 0.05).
Article
Arginine-vasopressin (AVP) gene expression in the rat suprachiasmatic nucleus (SCN) is subject to daily rhythmic changes. To determine whether this variation is endogenously generated, temporal changes in the SCN AVP mRNA level in constant dark (DD) condition was compared with changes occurring under the light-dark (LD) condition. In both lighting conditions, the presence of a rhythm in AVP mRNA level was observed in the SCN. In LD condition, peak level of AVP mRNA was found during the latter part of the day (zeitgeber time or ZT 8) and trough value during the night at ZT 20. Correspondingly, peak level of AVP mRNA under DD condition was observed during the latter part of the subjective day (circadian time or CT 8) and a trough during the subjective night (CT 20). Under both lighting conditions, a rapid increase and decrease of mRNA around the peak time was also observed. On the other hand, no significant daily variation in AVP mRNA was found in the supraoptic nucleus in both LD and DD conditions. These results provide evidence that a rhythmic change in AVP mRNA level is regulated by a circadian clock intrinsic to the SCN. The phase relationship of AVP mRNA rhythm to peptide rhythm in the SCN is discussed.
Article
The direct action of pineal melatonin on the renal system is supported by our demonstration of 2-[125I]iodomelatonin binding sites in the male guinea pig kidney. Scatchard analyses and Hill coefficients revealed a single type of binding site with an equilibrium dissociation constant (Kd) of 22.3 +/- 1.6 pmol/l and a maximum binding density (Bmax) of 0.99 +/- 0.03 fmol/mg protein (n = 7) at mid-light. There was no significant difference in the Kd and Bmax values between kidney tissues collected at the middle of light and dark periods. The pharmacological profile of these 2-[125I]iodomelatonin binding sites indicated high specificity for melatonin, 2-iodomelatonin and 6-chloromelatonin while kinetic studies generated a Kd value of 28.4 +/- 7.3 pmol/l (n = 5) which was comparable to that determined from Scatchard transformations. Our results suggest that these binding sites are stable, reversible, saturable, specific, and of high affinity. Regional distribution study showed that specific binding of 2-[125I]iodomelatonin was 8-fold higher in the cortical region than that in the medullary region. Studies of subcellular distribution showed that 59.3% of binding sites were localized in crude nuclear fractions followed by crude mitochondrial fractions (22.3%) and crude microsomal fractions (18.3%) with no detectable binding in cytosolic fractions. Our present findings suggest the presence of putative melatonin receptors in the guinea pig kidney, which support the hypotheses of melatonin-regulated renin secretion together with renal excretory functions via melatonin receptors.
Article
High-affinity melatonin receptors are expressed in the tail artery of the rat and arteries forming the circle of Willis of the rat and certain primates. The characteristics of the vascular melatonin receptors seem to be similar to the ones described in the central nervous system. The expression of vascular melatonin receptors in the rat is differentially regulated by factors such as strain and age, and in the female by reproductive hormones. Functional studies using the caudal artery of the rat suggest that melatonin regulates vascular tone. The highly restricted distribution of melatonin receptors in arteries involved in regulating blood flow to areas involved in heat dissipation suggests that vascular melatonin receptors may have a specific function in thermoregulatory homeostasis.
Article
The connection between the suprachiasmatic nucleus (SCN) and the paraventricular nucleus of the hypothalamus (PVN) forms an important component of the melatonin rhythm-generating system. However, the chemical identity of this projection is not known. To test the possible implication of the SCN peptides vasopressin (VP) and vasoactive intestinal peptide (VIP) in this projection, we performed microinfusions in the PVN during the first half of the dark period and subsequently monitored resulting plasma melatonin levels. Infusions for 7 hr of either VP or VIP, but not oxytocin, caused increased plasma melatonin levels in the middle of the dark period. These observations confirm the role of the PVN in the melatonin rhythm-generating pathway and indicate that both VP and VIP released at the level of the PVN, and probably derived from the SCN, are able to influence peripheral plasma melatonin levels.
Article
Plasma concentrations of neurohypophysial hormones show clear rhythms over 24 hr which can be suppressed by exposure to constant light, an observation consistent with pineal involvement. A study has therefore been performed on the changes in the hormone levels in the hypothalamus, posterior pituitary, and plasma over 24 hr in control, pinealectomised, and sham pinealectomised animals to determine if the pineal could play a role. Water intake, urine excretion, packed cell volume, plasma osmolality, and electrolytes were also monitored. Pinealectomy had little effect on fluid balance, but after 8 weeks for oxytocin and 2 weeks for vasopressin the morning values (0700-0800) for the circulating concentrations of the hormones were significantly higher in the pinealectomized group compared with the combined sham operated and unoperated groups (pineal intact). By contrast, the pituitary vasopressin was significantly lower in the pinealectomised group. The increase in plasma oxytocin and vasopressin seen over the hours of daylight and accompanying fall in plasma osmolality seen in the pineal intact group were absent in the pinealectomised group. Similarly, the evening fall in pituitary hormone concentrations and increase in hypothalamic hormone content were absent in the pinealectomised animals. After 10 days of exposure to constant light, the fall in plasma osmolality in the pineal-intact animals over the day was no longer significant; instead a significant increase in plasma osmolality and sodium was seen in the pinealectomised group. Exposure to constant light, while altering the patterns of neurohypophysial activity in the pineal intact group, had little effect on the pinealectomised animals.
Article
The pineal gland releases the "time-keeping' hormone melatonin following a rhythmic sympathetic input which translates light information. The aim of this work was to study the role and mechanism of action of the central vasopressinergic input on pineal cAMP-dependent melatonin synthesis in the rat. The pineal was found to display vasopressin receptors of the V1a subtype, as the V1a antagonist [125I]HO-LVA bound in a saturable manner to pineal membranes with a high affinity (kd = 10 pM) and a maximal binding capacity (B(max)) of 13 fmol/mg protein. Vasopressin was able to displace [125I]HO-LVA binding in a dose-dependent manner (k(i) = 1.9 nM). Vasopressin had no effect on the basal cAMP level and melatonin secretion in cultured rat pinealocytes. However, it clearly and dose-dependently (EC50 = 7 nM) potentiated by 2-3 times cAMP accumulation and by 1.5-2.5 times melatonin secretion induced by moderate noradrenergic stimulation. On strongly stimulated pinealocytes, however, vasopressin could potentiate cAMP accumulation, but not melatonin secretion. The potentiatory effect of vasopressin was inhibited in the presence of the V1a antagonist. These results indicate that vasopressin is a potent modulator of rat pineal synthetic activity.
Article
The aim of this investigation was to study whether melatonin affects the release of oxytocin and vasopressin by the pituitary neurointermediate lobe of the Syrian hamster in vitro. The effect of melatonin was studied on the unstimulated (pre- and post-K+ -stimulated) release of oxytocin and vasopressin and on the response to K+ stimulation. Melatonin significantly inhibited unstimulated release of these hormones in all concentrations (10(-11) M, 10(-9) M and 10(-7) M) studied. K+ -stimulated release of oxytocin and vasopressin was significantly decreased by the 10(-9) M dose of melatonin. It is concluded that melatonin is active in modifying the release of these peptides in the Syrian hamster neurointermediate lobe, as it has been previously demonstrated in the rat hypothalamus.
Article
The relationship between the pineal gland and pituitary function remains controversial, while the role of melatonin in the adaptation of the organism to the light-dark cycle of the environment is becoming increasingly recognized. The aim of this study was to investigate the effect of a manipulation of the melatonin rhythm on pituitary hormone secretion in man. Double-blind controlled clinical study. Ten adult healthy male volunteers, aged 21-33 years, were studied on two occasions: once after the administration of melatonin 5 mg orally for 4 days at 1700 hours and once after the administration of placebo, at similar times. On the day of each study the subjects undertook their normal duties but refrained from taking heavy exercise, from smoking and drinking alcohol. Serum cortisol, growth hormone, prolactin and plasma vasopressin, oxytocin, melatonin, sodium, potassium, osmolality and packed cell volume were measured over the following 24 hours. The cortisol peak was advanced and prolactin release increased after melatonin administration, while growth hormone was not affected. Vasopressin and oxytocin levels were found to increase during the night in the control study, but the period of the nocturnal increase in vasopressin concentrations was reduced after the administration of melatonin and the nocturnal increase of oxytocin was absent. Altering the melatonin rhythm may affect neuroendocrine function, influencing the nocturnal pattern of neurohypophysial hormone secretion, augmenting prolactin release and advancing the peak of cortisol release.
Article
Plasma concentration of arginine vasopressin (AVP) and melatonin and serum osmolality were measured at noon and at midnight in individuals living in the northern hemisphere on March 22-23, June 13-14, September 26-27, and December 12-13 in 35 healthy volunteers (15 men and 20 women) aged 60-74 years. The nocturnal increase in melatonin was highest in the autumn and lowest in the winter in both sexes. The midnight serum osmolality level was lower in the autumn than in any other time of the year. In both the men and the women the AVP level was higher in winter than in any other season (P < 0.01 and P < 0.0001, respectively). In men, the AVP level was higher at noon than at midnight in 49% of the investigated 24 hr periods, at the same level in 15% and lower in 36% (NS). The corresponding figures for women were 55%, 25%, and 20%, respectively (P < 0.05). This study suggests a possible relationship between melatonin and serum osmolality.