Potentially beneficial cardiovascular effects of melatonin administration in women

Melatonin as a therapeutic agent for alleviating endothelial dysfunction in cardiovascular diseases: Emphasis on oxidative stress

The vascular endothelium is vital in maintaining cardiovascular health by regulating vascular permeability and tone, preventing thrombosis, and controlling vascular inflammation. However, when oxidative stress triggers endothelial dysfunction, it can lead to chronic cardiovascular diseases (CVDs). This happens due to oxidative stress-induced mitochondrial dysfunction, inflammatory responses, and reduced levels of nitric oxide. These factors cause damage to endothelial cells, leading to the acceleration of CVD progression. Melatonin, a natural antioxidant, has been shown to inhibit oxidative stress and stabilize endothelial function, providing cardiovascular protection. The clinical application of melatonin in the prevention and treatment of CVDs has received widespread attention. In this review, based on bibliometric studies, we first discussed the relationship between oxidative stress-induced endothelial dysfunction and CVDs, then summarized the role of melatonin in the treatment of atherosclerosis, hypertension, myocardial ischemia-reperfusion injury, and other CVDs. Finally, the potential clinical use of melatonin in the treatment of these diseases is discussed.

The Role of the Circadian Rhythm in Dyslipidaemia and Vascular Inflammation Leading to Atherosclerosis

Cardiovascular diseases (CVD) are among the leading causes of death worldwide. Many lines of evidence suggest that the disturbances in circadian rhythm are responsible for the development of CVDs; however, circadian misalignment is not yet a treatable trait in clinical practice. The circadian rhythm is controlled by the central clock located in the suprachiasmatic nucleus and clock genes (molecular clock) located in all cells. Dyslipidaemia and vascular inflammation are two hallmarks of atherosclerosis and numerous experimental studies conclude that they are under direct influence by both central and molecular clocks. This review will summarise the results of experimental studies on lipid metabolism, vascular inflammation and circadian rhythm, and translate them into the pathophysiology of atherosclerosis and cardiovascular disease. We discuss the effect of time-respected administration of medications in cardiovascular medicine. We review the evidence on the effect of bright light and melatonin on cardiovascular health, lipid metabolism and vascular inflammation. Finally, we suggest an agenda for future research and recommend on clinical practice.

What do we really know about the safety and efficacy of melatonin for sleep disorders?

Melatonin is a hormonal product of the pineal gland, a fact that is often forgotten. Instead it is promoted as a dietary supplement that will overcome insomnia, as an antioxidant and as a prescription only drug in most countries outside the United States of America and Canada. The aim of this review is to step back and highlight what we know about melatonin following its discovery 60 years ago. What is the role of endogenous melatonin; what does melatonin do to sleep, body temperature, circadian rhythms, the cardiovascular system, reproductive system, endocrine system and metabolism when administered to healthy subjects? When used as a drug/dietary supplement, what safety studies have been conducted? Can we really say melatonin is safe when it has not been systematically studied and many studies show interactions with a wide range of physiological processes? Finally the results of studies investigating the efficacy of melatonin as a drug to alleviate insomnia are critically evaluated. In summary, melatonin is an endogenous pineal gland hormone with specific physiological functions in animals and humans, with its primary role in humans to maintain synchrony of sleep with the day/night cycle. When administered as a drug it affects a wide range of physiological systems and has clinically important drug interactions. With respect to efficacy for treating sleep disorders, melatonin can advance the time of sleep onset but the effect is modest and variable. In children with neurodevelopmental disabilities melatonin appears to have the greatest impact on sleep onset but little effect on sleep efficiency.

Acute and Chronic Pain Preclinical Models to Study the Analgesic Properties of Melatonergic Compounds

Melatonin (MLT) has been implicated in several pathophysiological states, including pain. MLT mostly activates two G protein-coupled receptors, MT₁ and MT₂. MLT displays analgesic properties in several animal paradigms of acute, inflammatory, and neuropathic pain. Although the analgesic mechanism of action of MLT is not yet completely elucidated, there is strong preclinical evidence suggesting the pharmacological potential of melatonergic compounds for treating pain. Importantly, MLT and melatonergic compounds seem to have a favorable toxicological profile than currently approved analgesic drugs. These compounds may thus deserve to be further developed as novel analgesic drugs, but this process relies on the use of appropriate and standardized experimental procedures. Therefore, in this chapter, we present the methodology to study the analgesic properties of MLT and melatonergic drugs in a preclinical model of chronic and acute pain. In addition to technical details of the surgical technique, details of anesthesia and perioperative care are also included.

Melatonin Exerts Anti-Inflammatory, Antioxidant, and Neuromodulatory Effects That Could Potentially Be Useful in the Treatment of Vertigo

The acute phase of vertigo involves multiple neurotransmitters, inflammatory mediators, and products of oxidative stress. The vestibular pathway has multiple melatonin receptors distributed along its path, both centrally and peripherally. In addition, melatonin has been shown to be a powerful antioxidant and anti-inflammatory agent against factors related to vertigo, such as Bax/caspases, interleukins, and chemokines. Likewise, it exerts central GABAergic, antidopaminergic, and anti-migraine functions and regulates sympathetic activity in a similar way to the drugs classically used in acute vestibular crisis. In this review, the role of melatonin as a potential treatment of the acute phase of vertigo is discussed.

Aortic Arch Calcification on Routine Chest Radiography is Strongly and Independently Associated with Non-Dipper Blood Pressure Pattern

Background

Non-dipper blood pressure (NDBP) is one of the important causes of hypertension-related target organ damage and future cardiovascular events. Currently, there is no practical tool to predict NDBP pattern.

Objectives

The aim of this study was to investigate the relationship between aortic arch calcification (AAC) on chest radiography and NDBP pattern.

Methods

All patients referred for ambulatory BP monitoring test were approached for the study participation. NDBP was defined as the reduction of ≤10% in nighttime systolic BP as compared to the daytime values. AAC was evaluated with chest radiography and inter-observer agreement was analyzed by using kappa statistics. Univariate and multivariate logistic regression analysis was conducted to assess the association of AAC and NDBP pattern. A 2-tailed p-value < 0.05 was considered statistically significant.

Results

A total of 406 patients (median age: 51.3) were included. Of these, 261(64%) had NDBP pattern. Overall, the prevalence of AAC was 230 (57%). Non-dipper group had significantly higher prevalence of AAC (70% vs. 33%, p < 0.0001) as compared to the dipper group. Presence of AAC was a strong and independent predictor of NDBP pattern (OR 3.919, 95%CI 2.39 to 6.42) in multivariate analysis.

Conclusions

Presence of AAC on plain chest radiography is strongly and independently associated with the presence of NDBP pattern.

Melatonin and cardioprotection against ischaemia/reperfusion injury: What's new? A review

Melatonin is a pleiotropic hormone with several functions. It binds to specific receptors and to a number of cytosolic proteins, activating a vast array of signalling pathways. Its potential to protect the heart against ischaemia/reperfusion damage has attracted much attention, particularly in view of its possible clinical applications. This review will focus mainly on the possible signalling pathways involved in melatonin-induced cardioprotection. In particular, the role of the melatonin receptors and events downstream of receptor activation, for example, the reperfusion injury salvage kinase (RISK), survivor activating factor enhancement (SAFE) and Notch pathways, the sirtuins, nuclear factor E2-related factor 2 (Nrf2) and translocases in the outer membrane (TOM70) will be discussed. Particular attention is given to the role of the mitochondrion in melatonin-induced cardioprotection. In addition, a brief overview will be given regarding the status quo of the clinical application of melatonin in humans.

Association of urinary levels of 6-sulfatoxymelatonin (aMT6s) with prevalent and incident hypertension

Laboratory studies indicate that melatonin has beneficial vascular effects. However, epidemiologic studies on the relationship between endogenous levels of melatonin and hypertension in humans are limited. We examined the association of quartile levels of 6-sulfatoxymelatonin (aMT6s) in first morning urines with prevalent and incident hypertension in 777 postmenopausal women who were originally part of a case-control study of breast cancer nested in the Women's Health Initiative Observational Study. A total of 321 prevalent and 172 incident cases of hypertension were studied. In cross-sectional analyses, higher quartile level of aMT6s was associated with lower odds of hypertension (Q4 versus Q1; odds ratio = 0.57; 95% confidence interval [CI]: 0.3-0.9), after adjustment for age, body mass index and other risk factors. We also examined the association between baseline aMT6s levels and risk of incident hypertension. Compared to women in the lowest quartile of urinary aMT6s, the multivariable-adjusted hazard ratios and 95% CIs of incident hypertension for women in the second, third and highest quartile were 1.16 (0.8-1.8), 0.96 (0.6-1.5) and 1.02 (0.6-1.6), respectively. The mean change in systolic and diastolic blood pressure over 3 years also did not vary by baseline quartile levels of aMT6s. Although we found no evidence of a prospective association between urinary levels of aMT6s and risk of incident hypertension in postmenopausal women, our cross-sectional results provide some possible evidence of a role for physiologic levels of melatonin in hypertension. Additional larger studies are warranted, preferably with a wider range of ages, both genders and multiple melatonin measurements.

Melatonin and Human Cardiovascular Disease

The possible therapeutic role of melatonin in the pathophysiology of coronary artery disorder (CAD) is increasingly being recognized. In humans, exogenous melatonin has been shown to decrease nocturnal hypertension, improve systolic and diastolic blood pressure, reduce the pulsatility index in the internal carotid artery, decrease platelet aggregation, and reduce serum catecholamine levels. Low circulating levels of melatonin are reported in individuals with CAD, arterial hypertension, and congestive heart failure. This review assesses current literature on the cardiovascular effects of melatonin in humans. It can be concluded that melatonin deserves to be considered in clinical trials evaluating novel therapeutic interventions for cardiovascular disorders.

Melatonin prevents kidney injury in a high salt diet-induced hypertension model by decreasing oxidative stress

Melatonin, a potent antioxidant molecule, plays a role in blood pressure regulation. We hypothesized that melatonin may generate a protective effect in a high salt diet (HSD) rodent model mediated by decreasing renal oxidative stress. Dahl salt-sensitive rats were divided into three groups according to diet: normal chow (control); HSD; HSD with melatonin [30/mg/kg/day]) placed in their water (HSD + Mel) over an 8-wk period. Blood pressure was measured by the tail cuff method. Kidney injury was evaluated by 24 H urine protein excretion. Glomerular injury index (GII) (fibrotic glomeruli/100 glomeruli) was evaluated from a Masson's trichrome-stained section. Kidney oxidative stress was determined by superoxide production via dihydroethidium staining. Expression of oxidative stress-related genes was measured by reverse transcriptase-qPCR. Melatonin had no effect on blood pressure increase induced by HSD and attenuated proteinuria induced by HSD (HSD--50.7 ± 12, HSD + Mel--22.3 ± 4.3, controls--6.5 ± 1.0 gram protein/gram creatinine, P < 0.001). HSD-induced glomerular damage was significantly diminished by melatonin (GII in HSD--24 ± 6, HSD + Mel--3.6 ± 0.8, controls--0.8 ± 0.5, P < 0.05). Superoxide production was significantly higher in kidneys of HSD fed rats than the controls (99 ± 9 versus 60 ± 7 relative fluorescent units (RFU)/μm(2), respectively, P < 0.05). Melatonin also decreased superoxide production (74 ± 5 RFU/μm(2), P < 0.05). The expression of kidney inducible nitric oxide synthase and p67(phox) mRNA was significantly higher in HSD than in the controls and HSD + Mel rats. Treatment with melatonin eliminated the deleterious effect of HSD in the kidneys of Dahl salt-sensitive rats. The beneficial effect of melatonin is not mediated by lowering blood pressure but by a direct antioxidative effect.

Therapeutic effects of melatonin receptor agonists on sleep and comorbid disorders

Several melatonin receptors agonists (ramelteon, prolonged-release melatonin, agomelatine and tasimelteon) have recently become available for the treatment of insomnia, depression and circadian rhythms sleep-wake disorders. The efficacy and safety profiles of these compounds in the treatment of the indicated disorders are reviewed. Accumulating evidence indicates that sleep-wake disorders and co-existing medical conditions are mutually exacerbating. This understanding has now been incorporated into the new Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5). Therefore, when evaluating the risk/benefit ratio of sleep drugs, it is pertinent to also evaluate their effects on wake and comorbid condition. Beneficial effects of melatonin receptor agonists on comorbid neurological, psychiatric, cardiovascular and metabolic symptomatology beyond sleep regulation are also described. The review underlines the beneficial value of enhancing physiological sleep in comorbid conditions.

Unveiling the role of melatonin MT2 receptors in sleep, anxiety and other neuropsychiatric diseases: a novel target in psychopharmacology

BACKGROUND

Melatonin (MLT) is a pleiotropic neurohormone controlling many physiological processes and whose dysfunction may contribute to several different diseases, such as neurodegenerative diseases, circadian and mood disorders, insomnia, type 2 diabetes and pain. Melatonin is synthesized by the pineal gland during the night and acts through 2 G-protein coupled receptors (GPCRs), MT1 (MEL1a) and MT2 (MEL1b). Although a bulk of research has examined the physiopathological effects of MLT, few studies have investigated the selective role played by MT1 and MT2 receptors. Here we have reviewed current knowledge about the implications of MT2 receptors in brain functions.

METHODS

We searched PubMed, Web of Science, Scopus, Google Scholar and articles' reference lists for studies on MT2 receptor ligands in sleep, anxiety, neuropsychiatric diseases and psychopharmacology, including genetic studies on the MTNR1B gene, which encodes the melatonin MT2 receptor.

RESULTS

These studies demonstrate that MT2 receptors are involved in the pathophysiology and pharmacology of sleep disorders, anxiety, depression, Alzheimer disease and pain and that selective MT2 receptor agonists show hypnotic and anxiolytic properties.

LIMITATIONS

Studies examining the role of MT2 receptors in psychopharmacology are still limited.

CONCLUSION

The development of novel selective MT2 receptor ligands, together with further preclinical in vivo studies, may clarify the role of this receptor in brain function and psychopharmacology. The superfamily of GPCRs has proven to be among the most successful drug targets and, consequently, MT2 receptors have great potential for pioneer drug discovery in the treatment of mental diseases for which limited therapeutic targets are currently available.

Effect of melatonin on nocturnal blood pressure: meta-analysis of randomized controlled trials

Background

Patients with nocturnal hypertension are at higher risk for cardiovascular complications such as myocardial infarction and cerebrovascular insult. Published studies inconsistently reported decreases in nocturnal blood pressure with melatonin.

Methods

A meta-analysis of the efficacy and safety of exogenous melatonin in ameliorating nocturnal blood pressure was performed using a random effects model of all studies fitting the inclusion criteria, with subgroup analysis of fast-release versus controlled-release preparations.

Results

Seven trials (three of controlled-release and four of fast-release melatonin) with 221 participants were included. Meta-analysis of all seven studies did not reveal significant effects of melatonin versus placebo on nocturnal blood pressure. However, subgroup analysis revealed that controlled-release melatonin significantly reduced nocturnal blood pressure whereas fast-release melatonin had no effect. Systolic blood pressure decreased significantly with controlled-release melatonin (−6.1 mmHg; 95% confidence interval [CI] −10.7 to −1.5; P = 0.009) but not fast-release melatonin (−0.3 mmHg; 95% CI −5.9 to 5.30; P = 0.92). Diastolic blood pressure also decreased significantly with controlled-release melatonin (−3.5 mmHg; 95% CI −6.1 to −0.9; P = 0.009) but not fast-release melatonin (−0.2 mmHg; 95% CI −3.8 to 3.3; P = 0.89). No safety concerns were raised.

Conclusion

Add-on controlled-release melatonin to antihypertensive therapy is effective and safe in ameliorating nocturnal hypertension, whereas fast-release melatonin is ineffective. It is necessary that larger trials of longer duration be conducted in order to determine the long-term beneficial effects of controlled-release melatonin in patients with nocturnal hypertension.

Relation between body mass index and endothelium-dependent vasodilatation in healthy postmenopausal women

OBJECTIVE

To evaluate whether endothelium-dependent vasodilatation is related to anthropometric parameters in 105 healthy postmenopausal women 47-68 years of age.

METHODS

Flow-dependent, endothelium-dependent vasodilatation was considered as the maximal dilatation following deflation of a cuff placed on the forearm and inflated to supra-systolic blood pressure values for 4 min. Endothelium-independent vasodilatation was considered as the maximal dilatation induced by sublingual nitroglycerine (400 microg).

RESULTS

Among parameters such as height, weight, body mass index (BMI), waist, hip, waist/hip ratio, lipids, glucose or insulin, only BMI, an indirect index of adiposity, was independently and directly related to baseline brachial artery diameter (b = 0.042, r = 0.269, p = 0.0055) and flow-mediated endothelium-dependent vasodilatation either expressed as net (b = 0.034, r = 0.315, p = 0.001) or percentage (b = 0.376, r = 0.202, p = 0.039) change. Stratification for BMI categories showed that women with BMI < 22 kg/m(2) had an endothelium-dependent vasodilatation, significantly lower than that of women with BMI >or= 30 kg/m(2) (0.711 +/- 0.076 mm vs. 1.107 +/- 0.141 mm; p = 0.0114). BMI was not related to endothelium-independent vasodilatation.

CONCLUSIONS

Present results show that, in healthy postmenopausal women, endothelium-dependent vasodilatation is related to BMI, arteries of slender women dilating less than those of their heavier counterparts. A low BMI does not appear to be beneficial for artery vasodilatation in healthy postmenopausal women.

17Beta-estradiol modulates hMT1 melatonin receptor function

Estrogen modulates expression and function of G-protein-coupled receptors. The goal of this study was to assess the effect of 17beta-estradiol (10 nM) exposure for 1 (E1) or 6 (E6) days on density and function of hMT1 and hMT2 melatonin receptors expressed in Chinese hamster ovary (CHO) cells (CHO-MT1/CHO-MT2 cells). This strain of CHO cells expressed both estrogen receptor alpha and beta mRNAs, as determined by RT-PCR amplification. 17beta-Estradiol treatment did not modify the affinity of either receptor; however, it significantly increased the density of 2-[125I]iodomelatonin-binding sites in CHO-MT2 cells. 17beta-Estradiol treatment (1-6 days) did not affect the potency of melatonin to inhibit forskolin stimulation of cAMP formation through activation of either MT1 or MT2 receptors; however, it significantly attenuated the maximal inhibition of forskolin-stimulated cAMP formation induced by melatonin (0.01-1 microM) in CHO-MT1 cells. Melatonin stimulation of [35S]GTPgammaS binding to CHO-MT1 cell membranes was also attenuated following estradiol treatment. The inverse agonist luzindole reduced basal [35S]GTPgammaS binding in estradiol-treated cells but not in control CHO-MT1 cells, suggesting that estradiol promotes constitutive activity of MT1 melatonin receptors. We suggest that 17beta-estradiol differentially affects MT1 and MT2 melatonin receptor functions, attenuates melatonin responses through activation of MT1 receptors, and increases the MT2 receptors density.

The relevance of melatonin to sports medicine and science

The pineal hormone, melatonin, has widespread effects on the body. The aim of this review is to consider the specific interactions between melatonin and human physiological functions associated with sport and exercise medicine. Separate researchers have reported that melatonin concentrations increase, decrease and remain unaffected by bouts of exercise. Such conflicting findings may be explained by inter-study differences in lighting conditions and the time of day the study participants have exercised. Age and fitness status have also been identified as intervening factors in exercise-mediated changes in melatonin concentration. The administration of exogenous melatonin leads to hypnotic and hypothermic responses in humans, which can be linked to immediate reductions in short-term mental and physical performance. Depending on the dose of melatonin, these effects may still be apparent 3-5 hours after administration for some types of cognitive performance, but effects on physical performance seem more short-lived. The hypothesis that the hypothermic effects of melatonin lead to improved endurance performance in hot environments is not supported by evidence from studies involving military recruits who exercised at relatively low intensities. Nevertheless, no research group has examined such a hypothesis with athletes as study participants and with the associated more intense levels of exercise. The fact that melatonin has also been found to preserve muscle and liver glycogen in exercised rats adds weight to the notion that melatonin might affect endurance exercise in humans. Melatonin has been successfully used to alleviate jet lag symptoms of travellers and there is also a smaller amount of evidence that the hormone helps shiftworkers adjust to nocturnal regimens. Nevertheless, the symptoms of jet lag and shiftwork problems have primarily included sleep characteristics rather than performance variables. The few studies that have involved athletes and performance-related symptoms have produced equivocal results. Melatonin has also been found to be useful for treating some sleeping disorders, but interactions between sleep, melatonin and exercise have not been studied extensively with trained study participants. It is unknown whether melatonin plays a role in some exercise training-related problems such as amenorrhoea and over-training syndrome.

Chronic antioxidant treatment improves sympathetic functions and beta-adrenergic pathway in the spontaneously hypertensive rats

BACKGROUND

Correlations have been found between oxidative stress and hypertension.

OBJECTIVE

To determine whether antioxidants can normalize sympathetic dysfunction at pre- or postsynaptic levels in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

Untreated SHRs and Wistar-Kyoto (WKY) rats were compared with rats treated with melatonin (30 mg/kg per day) or N-acetylcysteine (NAC) (4 g/kg per day) given in drinking water for 4 weeks. At the presynaptic level, SHRs had greater plasma noradrenaline concentrations (P <0.01) and an enhanced release of [3H]noradrenaline from isolated atria (P <0.001). At the postsynaptic level, they exhibited an increased proportion of beta2-adrenoceptors in the heart (P <0.001)and a decrease in the chronotropic and mean arterial pressure (MAP) responses to isoproterenol (P <0.001). Melatonin and NAC decreased MAP (P <0.001) and heart rate (P <0.05), and restored the plasma noradrenaline concentrations (P<0.01 and P <0.001, respectively), the chronotropic response to isoproterenol (P <0.05) and the proportions of beta(1)/beta(2)-adrenoceptors in the heart (P <0.05) in SHRs to the levels found in WKY rats. The same treatments decreased the release of [3H]noradrenaline from isolated atria (P <0.05), and melatonin slightly improved the relaxation in the aorta in SHRs only (P <0.05). Plasma concentrations of adrenaline, the isoproterenol-induced relaxation in mesenteric arteries, the total density and affinity of beta-adrenoceptors in the heart, and the adenylate cyclase reactivity of cardiac membranes to isoproterenol, forskolin, sodium fluoride and guanylylimidophosphate were not altered by the treatments.

CONCLUSION

These results suggest that NAC and melatonin decreased the MAP and heart rate and improved the chronotropic response to isoproterenol in SHRs, in association with an inhibition of sympathetic activity and the restoration of cardiac beta-adrenoceptor function.

Influence of sleep deprivation coupled with administration of melatonin on the ultrastructure of rat pineal gland

The effects of sleep deprivation with or without melatonin treatment on the pineal morphology in rats were studied. Five days after sleep deprivation and using electron microscopy, many of the pinealocytes exhibited structural alterations including dilation of the cisternae of the rough/smooth endoplasmic reticulum, Golgi saccules and mitochondria, and an increase in the numbers of lipid droplets, vacuoles and dense-core vesicles. These features were considered as morphological evidence of increased synthesis or secretion by the pineal gland. In addition, numerous membranous profiles, considered to be degraded cellular organelles, were observed in some pinealocytes and sympathetic nerve terminals. It is suggested that the occurrence of degenerating organelles had resulted from the deleterious effect of sleep deprivation. This may be attributed to an overload of secretory activity of the pineal gland during stress elicited by the long-term sleep deprivation, leading to functional exhaustion and irreversible damage of the oxidation-related organelles. In sleep-deprived rats receiving a single injection of melatonin (10 mg/kg) for 5 consecutive days, the above features indicative of pinealocytic activation were attenuated. In fact, all signs of degeneration of cellular organelles were rarely found. These results suggest that the pineal gland is itself a target for exogenously administered melatonin. Thus, melatonin when administered systemically may be used as a potential neuroprotective drug against neuronal damage induced by sleep deprivation.

Vasorelaxant effects of the chronic treatment with melatonin on mesenteric artery and aorta of spontaneously hypertensive rats

OBJECTIVE

To investigate the effect of a chronic treatment with melatonin on arterial pressure and a possible improvement of the vascular muscarinic and NO synthase (NOS) pathways in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats.

DESIGN AND METHODS

Mean arterial pressure (MAP), systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) were evaluated in conscious rats treated with 30 mg/kg per day of melatonin during 4 weeks. Changes in MAP were evaluated following an intravenous injection of the NOS inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME). Relaxant effects of acetylcholine (Ach), sodium nitroprusside (SNP), and the calcium ionophore A23187 were examined on mesenteric beds and aortic rings with or without treatment with melatonin.

RESULTS

Melatonin produced a significant reduction of MAP, SBP, DBP and HR in SHR (P < 0.05). L-NAME increased the MAP of melatonin-treated SHR by the same magnitude as that of WKY rats which was significantly higher than that of non-treated SHR (P< 0.05). Melatonin treatment improved the maximal relaxation of mesenteric arteries to A23187 in SHR (P < 0.001) to the WKY level and caused a slight increment in Ach- and A23187-induced vasodilations in aorta from SHR and WKY rats (P < 0.05).

CONCLUSION

The present study showed that melatonin exerted a bradycardic and an antihypertensive action in SHR. The enhancement by melatonin of the endothelium-dependent vasodilation (Ach and/or A23187) in mesenteric artery and aorta from SHR and WKY rats and the higher increase in MAP following L-NAME treatment in melatonin-treated SHR suggest the contribution of an improved vascular NOS pathway activity in the hypotensive effect of melatonin.

Weak vasoconstrictor activity of melatonin in human umbilical artery: relation to nitric oxide-scavenging action

We evaluated the nitric oxide (NO)-scavenging property of melatonin, demonstrated in a recent in vitro study, on vascular reactivity in the human umbilical artery. Helical sections of human umbilical artery were prepared following elective Cesarean deliveries near term. Changes in maximal tension induced by prostaglandin F(2 alpha)(5 x 10(-5) M) were measured in artery sections with an intact endothelium. Melatonin at concentrations higher than 10(-6) M increased prostaglandin F(2 alpha)-induced vascular tension. The vasospastic effect of melatonin was much less than that of L-N(G)-monomethylarginine (L-NMA, 2 x 10(-4) M), an inhibitor of NO synthesis (2.8+/-1.4%, 9.1+/-1.7%, 16.5+/-2.5%, and 29.6+/-5.9% of the L-NMA effect at melatonin concentrations of 10(-8), 10(-7), 10(-6), and 10(-5) M, respectively). Removal of the endothelium significantly reduced the vasoconstrictive effect of melatonin. Treatment with L-NMA (2 x 10(-4) M) prior to addition of prostaglandin F(2 alpha) also significantly reduced the vasoconstrictive effect of melatonin (10(-5) M). Treatments with melatonin (10(-5) M) did not affect calcium ionophore A 23187-induced relaxation or 5-hydroxytryptamine-induced constriction. The findings indicate that melatonin may potentiate vascular tension in human umbilical artery by scavenging endogenous endothelial NO, but not by inhibiting NO synthesis. However, the NO-scavenging vasoconstrictive effect of melatonin may be negligible at physiologic concentrations and very weak at pharmacologic concentrations below 10(-7) M.

Influence of exogenous melatonin on catecholamine levels in postmenopausal women prior and during oestradiol replacement

OBJECTIVE

In young individuals melatonin administration reduces circulating norepinephrine. Some effects of melatonin are reduced in elderly women and are modulated by gonadal steroids. Accordingly, the influence of melatonin on catecholamine levels was investigated in postmenopausal women without and with oestradiol replacement.

DESIGN

Prior to and after 2 months of transdermal oestradiol (50 microg/day), women were studied on two consecutive days, on which they received placebo or 1 mg of melatonin orally in a randomised and double-blind fashion.

PATIENTS

Fourteen healthy postmenopausal women.

MEASUREMENTS

Resting levels of epinephrine and norepinephrine and their responses to both a cold stimulus, performed by placing a hand in a basin of water and ice for 2 minutes, and to 10 minutes of upright position (upright test).

RESULTS

Prior to oestradiol, melatonin did not modify baseline or stimulated catecholamine levels. In contrast, during oestradiol, melatonin tended to reduce, although not significantly, baseline norepinephrine levels (P = 0.053), and significantly reduced peak values (P = 0.0061) and integrated norepinephrine response (P = 0.0076) to the cold stimulus. Responses of norepinephrine to the upright test were not modified, while those of epinephrine were increased (P = 0.042). During, but not prior to oestradiol replacement, modifications induced by melatonin (melatonin day-placebo day) in the norepinephrine response to the cold (r2 = 0. 457; P = 0.0079) and the upright (r2 = 0.747; P = 0.0001) tests were linearly and inversely related to the responses of the placebo day.

CONCLUSIONS

Melatonin does not modulate adrenergic activity in postmenopausal women without hormone replacement therapy. Oestradiol replacement restores the capability of melatonin to modulate adrenergic activity, particularly the norepinephrine response to stimuli.

Effects of melatonin on vascular reactivity, catecholamine levels, and blood pressure in healthy men

In this study it was investigated whether the oral administration of melatonin (1 mg) in comparison to placebo was able to reduce blood pressure, vascular reactivity, and catecholamines in men, as previously reported in young women. The administration of melatonin significantly reduced blood pressure, the pulsatility index in the internal carotid artery, and catecholamines levels within 90 minutes. The effect of melatonin on the artery pulsatility index was related to baseline values, being greater in men with higher baseline values. The present data indicate that melatonin may blunt the activity of the cardiovascular system and may have both physiopathologic and clinical implications.

Estradiol modulates vascular response to melatonin in rat caudal artery

The purpose of this study was to determine whether estrogen modulates the function of vascular melatonin receptors. We used the rat caudal artery and found that the contractile effects of melatonin were influenced by the estrous cycle, ovariectomy, and estrogen replacement. In arterial ring segments isolated from female rats, melatonin potentiated, in a concentration-dependent manner, contractions produced either by adrenergic nerve stimulation or by phenylephrine. Constrictor responses to melatonin were smaller in arteries from female rats in proestrus compared with other stages of the estrous cycle and after ovariectomy. Administration of 17beta-estradiol to ovariectomized female rats also resulted in decreased constriction of isolated arteries to melatonin; however, in vitro addition of 17beta-estradiol (10(-7) M) had no effect. In the caudal artery, melatonin appears to act on two receptor subtypes that mediate contraction and relaxation, respectively. The selective melatonin MT2-receptor antagonist 4-phenyl-2-propionamidotetraline (4P-PDOT) enhanced constrictor responses to melatonin in arterial segments from intact female rats, consistent with the inhibition of MT2 receptor-mediated relaxation. In contrast, 4P-PDOT had no significant effect in arteries from ovariectomized female rats. However, when estradiol was replaced in vivo, the effect of 4P-PDOT on melatonin responses was restored. Thus circulating estradiol appears to enhance MT2 melatonin-receptor function in the thermoregulatory caudal artery of the female rat resulting in increased vasodilatation in response to melatonin.

Melatonin modulates vascular smooth muscle tone

The molecular cloning of a family of melatonin receptors has created a renewed interest in the diverse actions of the hormone melatonin. The radioligand 2-[125I]iodomelatonin has identified potential sites of action for melatonin throughout the central nervous system and periphery of numerous species. Interestingly, in addition to the suprachiasmatic nucleus (the "biological clock"), 2-[125I]iodomelatonin binding sites have been localized to the rat caudal and cerebral arteries. Furthermore, in vitro, melatonin has been shown to induce a concentration-dependent vasoconstriction of rat caudal and cerebral arteries, and pig and human coronary arteries. The lack of melatonin receptor subtype-selective agonists and antagonists prevents the full pharmacological characterization of these responses. The physiological significance of the in vitro vasoconstrictive properties is far from clear, however; in rats, melatonin has been shown to reduce cerebral blood flow. The widespread use of melatonin warrants appropriately designed studies to probe the role of melatonin and its receptors in the modulation of in vitro vascular tone.