Effect of exogenous melatonin on vascular reactivity and nitric oxide in postmenopausal women: role of hormone replacement therapy

Essential Hypertension and Oxidative Stress: Novel Future Perspectives

Among cardiovascular diseases, hypertension is one of the main risk factors predisposing to fatal complications. Oxidative stress and chronic inflammation have been identified as potentially responsible for the development of endothelial damage and vascular stiffness, two of the primum movens of hypertension and cardiovascular diseases. Based on these data, we conducted an open-label randomized study, first, to evaluate the endothelial damage and vascular stiffness in hypertense patients; second, to test the effect of supplementation with a physiological antioxidant (melatonin 1 mg/day for 1 year) in patients with essential hypertension vs. hypertensive controls. Twenty-three patients of either gender were enrolled and randomized 1:1 in two groups (control and supplemented group). The plasmatic total antioxidant capacity (as a marker of oxidative stress), blood pressure, arterial stiffness, and peripheral endothelial function were evaluated at the beginning of the study and after 1 year in both groups. Our results showed that arterial stiffness improved significantly (p = 0.022) in supplemented patients. The endothelial function increased too, even if not significantly (p = 0.688), after 1 year of melatonin administration. Moreover, the supplemented group showed a significative reduction in TAC levels (p = 0.041) correlated with the improvement of arterial stiffness. These data suggest that melatonin may play an important role in reducing the serum levels of TAC and, consequently, in improving arterial stiffness.

Melatonin as an Anti-Aging Therapy for Age-Related Cardiovascular and Neurodegenerative Diseases

The concept of “aging” is defined as the set of gradual and progressive changes in an organism that leads to an increased risk of weakness, disease, and death. This process may occur at the cellular and organ level, as well as in the entire organism of any living being. During aging, there is a decrease in biological functions and in the ability to adapt to metabolic stress. General effects of aging include mitochondrial, cellular, and organic dysfunction, immune impairment or inflammaging, oxidative stress, cognitive and cardiovascular alterations, among others. Therefore, one of the main harmful consequences of aging is the development and progression of multiple diseases related to these processes, especially at the cardiovascular and central nervous system levels. Both cardiovascular and neurodegenerative pathologies are highly disabling and, in many cases, lethal. In this context, melatonin, an endogenous compound naturally synthesized not only by the pineal gland but also by many cell types, may have a key role in the modulation of multiple mechanisms associated with aging. Additionally, this indoleamine is also a therapeutic agent, which may be administered exogenously with a high degree of safety. For this reason, melatonin could become an attractive and low-cost alternative for slowing the processes of aging and its associated diseases, including cardiovascular and neurodegenerative disorders.

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.

Melatonin and the health of menopausal women: A systematic review

Melatonin is involved in multiple changes that characterize the aging and can potentially be a safe and effective treatment for menopausal women. The aim of this study was to carry out a systematic review of the medical literature on the health benefits of oral melatonin administration on menopausal women. The electronic databases PubMed, Scopus, and Web of Science were searched systematically on interventional studies that evaluated the association between oral melatonin administration and the health of menopausal women. Risk for bias was assessed for randomized, controlled studies by the RoB v.2 tool and for non-randomized trials by the ROBINS-I tool. Twenty-four studies on melatonin treatment in various aspects of women's health were included in the final systematic review. The studies included 1,173 participants. No evidence was found for an independent effect of melatonin on hemodynamic measures or markers of glucose metabolism. There is some evidence that very low-density lipoprotein and triglycerides levels increase during melatonin administration. There is a fair amount of evidence that melatonin treatment has a favorable effect on bone density and BMI. Melatonin treatment improves EEG patterns and subjective sleep quality in postmenopausal women with preexisting sleep impairment. In a dose of 3 mg and above, melatonin improves climacteric symptoms in one or more domains. The vast majority of the studies had a low risk for bias. In light of multiple health benefits and an excellent safety profile, melatonin administration should be considered in menopausal women.

Actigraphy-derived rest-activity rhythms are associated with nocturnal blood pressure in young women

INTRODUCTION

Misalignment between lifestyle behaviors and endogenous circadian rhythms is associated with elevated nocturnal blood pressure (BP) in experimental studies; however, less is known about free-living (i.e. nonlaboratory) circadian disruption and nocturnal BP. Additionally, sex-specific cardiovascular implications of circadian disruption are unclear.

OBJECTIVE

To examine the associations between rest--activity rhythms (RAR), a field-based estimate of circadian disruption, and nocturnal BP characteristics in male and female young adults.

METHODS

Fifty participants (20 ± 1 years; 20 men/30 women) underwent 24-h ambulatory BP monitoring following 14 days of wrist actigraphy. RAR variables of interdaily stability (day-to-day consistency in RAR), intradaily variability (within-day fragmentation of RAR), and relative amplitude (difference between peak vs. trough activity) were derived from actigraphy. Multivariable regression models of mean nocturnal SBP, DBP, and SBP dipping were generated to test main associations with RAR variables, and sex × RAR interactions. Daytime BP, race, BMI, physical activity, sleep duration, alcohol, caffeine, and sodium intake were considered as covariates.

RESULTS

In the full sample, no main associations between RAR and nocturnal BP characteristics were found. Sex interacted with RAR such that in women, higher interdaily stability (β = -5.39, 95% CI = -10.04 to -0.73, P = 0.024) and relative amplitude (β = -4.78, 95% CI = -9.22 to -0.34, P = 0.036) were both associated with lower nocturnal SBP. Sex-stratified multivariable models of nocturnal BP also revealed associations between interdaily stability and relative amplitude with SBP dipping in women (all P ≤ 0.01). No associations were apparent in men.

CONCLUSION

Consistent and high-amplitude RAR are favorably associated with nocturnal BP characteristics in young female adults.

Melatonin and CIDR improved the follicular and luteal haemodynamics, uterine and ovarian arteries vascular perfusion, ovarian hormones and nitric oxide in cyclic cows

This study hypothesizes that melatonin with exogenous progesterone (CIDR) can improve follicular, luteal, ovarian and uterine haemodynamic of heat-stressed cows. Holstein cows (N = 12) studied for two spontaneous oestrous cycles during winter then divided equally during summer into the CIDR group received CIDR for 7 days and the melatonin group (Mel) received three injections of melatonin (75 mg/head) at the CIDR insertion, removal and ovulation days. Blood samples were collected to assay oestradiol (E2), progesterone (P4) and nitric oxide (NO). On day 0 (Ovulation), Mel had more small follicles (p < .05), higher ipsilateral and contralateral ovarian arteries (Ov.A.) peak systolic velocity (PSV), higher ipsilateral uterine artery (Ut.A.) PSV (p = .031) and blood flow volume (BFV), also Mel elevated contralateral Ut.A. PSV and BFV (p < .0001) but lowered contra Ut.A. pulsatility index (PI, p < .0001), E2 (p < .01) and NO (p < .0001). Mel increased the corpus luteum diameter (CL, p < .001), coloured area (p < .007) and P4 (p < .0001) on day 5 and reduced them (p < .05; p < .01) on Day 14. On day 10, Mel obtained CL diameter (p < .03) and coloured area (p < .002) of spontaneous that was higher than CIDR and decreased P4 (p < .003). Mel increased CL diameter, area and coloured area and decreased them thereafter. Mel increased the ipsilateral ovarian and uterine arteries PSV and BFV before ovulation and until day 8. Mel increased P4 and decreased NO until days 6 and 14. In conclusion, the improvement in follicular, luteal, ovarian and uterine haemodynamic and the decrease of NO production proved our hypothesis Melatonin doses higher than 75 mg/head is recommended to improve the heat-stressed cow's fertility.

Co-administering Melatonin With an Estradiol-Progesterone Menopausal Hormone Therapy Represses Mammary Cancer Development in a Mouse Model of HER2-Positive Breast Cancer

Melatonin has numerous anti-cancer properties reported to influence cancer initiation, promotion, and metastasis. With the need for effective hormone therapies (HT) to treat menopausal symptoms without increasing breast cancer risk, co-administration of nocturnal melatonin with a natural, low-dose HT was evaluated in mice that develop primary and metastatic mammary cancer. Individually, melatonin (MEL) and estradiol-progesterone therapy (EPT) did not significantly affect mammary cancer development through age 14 months, but, when combined, the melatonin-estradiol-progesterone therapy (MEPT) significantly repressed tumor formation. This repression was due to effects on tumor incidence, but not latency. These results demonstrate that melatonin and the HT cooperate to decrease the mammary cancer risk. Melatonin and EPT also cooperate to alter the balance of the progesterone receptor (PR) isoforms by significantly increasing PRA protein expression only in MEPT mammary glands. Melatonin significantly suppressed amphiregulin transcripts in MEL and MEPT mammary glands, suggesting that amphiregulin together with the higher PRA:PRB balance and other factors may contribute to reducing cancer development in MEPT mice. Melatonin supplementation influenced mammary morphology by increasing tertiary branching in the mouse mammary glands and differentiation in human mammary epithelial cell cultures. Uterine weight in the luteal phase was elevated after long-term exposure to EPT, but not to MEPT, indicating that melatonin supplementation may reduce estrogen-induced uterine stimulation. Melatonin supplementation significantly decreased the incidence of grossly-detected lung metastases in MEL mice, suggesting that melatonin delays the formation of metastatic lesions and/or decreases aggressiveness in this model of HER2⁺ breast cancer. Mammary tumor development was similar in EPT and MEPT mice until age 8.6 months, but after 8.6 months, only MEPT continued to suppress cancer development. These data suggest that melatonin supplementation has a negligible effect in young MEPT mice, but is required in older mice to inhibit tumor formation. Since melatonin binding was significantly decreased in older mammary glands, irrespective of treatment, melatonin supplementation may overcome reduced melatonin responsiveness in the aged MEPT mice. Since melatonin levels are known to decline near menopause, nocturnal melatonin supplementation may also be needed in aging women to cooperate with HT to decrease breast cancer risk.

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 supplementation to treat the metabolic syndrome: a randomized controlled trial

BACKGROUND

Supplemental melatonin may ameliorate metabolic syndrome (MetS) components, but data from placebo-controlled trials are lacking.

METHODS

We conducted a double-blind, placebo-controlled, crossover, Phase II randomized pilot clinical trial to estimate the effects of melatonin supplementation on MetS components and the overall prevalence of MetS. We randomized 39 subjects with MetS to receive 8.0 mg oral melatonin or matching placebo nightly for 10 weeks. After a 6-week washout, subjects received the other treatment for 10 more weeks. We measured waist circumference, triglycerides, HDL cholesterol, fasting glucose, and blood pressure (BP) in each subject at the beginning and end of both 10-week treatment periods. The primary outcome was the mean 10-week change in each MetS component, and a secondary outcome was the proportion of subjects free from MetS, after melatonin versus placebo.

RESULTS

The mean 10-week change for most MetS components favored melatonin over placebo (except fasting glucose): waist circumference -0.9 vs. +1.0 cm (p = 0.15); triglycerides -66.3 vs. -4.2 mg/dL (p = 0.17); HDL cholesterol -0.2 vs. -1.1 mg/dL (p = 0.59); fasting glucose +0.3 vs. -3.1 mg/dL (p = 0.29); systolic BP -2.7 vs. +4.7 mmHg (p = 0.013); and diastolic BP -1.1 vs. +1.1 mmHg (p = 0.24). Freedom from MetS tended to be more common following melatonin versus placebo treatment (after the first 10 weeks, 35.3% vs. 15.0%, p = 0.25; after the second 10 weeks, 45.0% vs. 23.5%, p = 0.30). Melatonin was well-tolerated.

CONCLUSIONS

Melatonin supplementation modestly improved most individual MetS components compared with placebo, and tended to increase the proportion of subjects free from MetS after treatment.

TRIAL REGISTRATION

NCT01038921, clinicaltrials.gov.

Synergism between melatonin and atorvastatin against endothelial cell damage induced by lipopolysaccharide

The beneficial effects of atorvastatin are based on both cholesterol-dependent and independent mechanisms. The latter probably include the ability of the estatin to enhance the expression of endothelial nitric oxide synthase (eNOS) and to cause a vasodilatation. In turn, the antioxidant and anti-inflammatory actions of melatonin are related to its vascular protection. In the present study, we investigated the efficacy of the combination of melatonin plus atorvastatin against endothelial cell damage induced by inflammation and oxidative stress injury. Human umbilical vein endothelial cells (HUVEC) were cultured with bacterial lipopolysaccharide (LPS) in the presence or absence of melatonin and/or atorvastatin. LPS inhibited eNOS mRNA and protein expression, which was reversed by atorvastatin and, to a lesser extent, by melatonin. Together, melatonin + atorvastatin induced higher eNOS protein expression than either compound alone. Melatonin, but not atorvastatin, reduced free radical generation, lipid peroxidation, and interleukin-6 levels induced by LPS. In the presence of atorvastatin, the effects of melatonin were maintained or even improved. These data suggest that melatonin improves the beneficial effects of atorvastatin and reduces its side effects in endothelial cells during inflammation and under conditions of oxidative stress.

Intracoronary melatonin increases coronary blood flow and cardiac function through β-adrenoreceptors, MT1/MT2 receptors, and nitric oxide in anesthetized pigs

Melatonin is involved in the regulation of the cardiovascular system through the modulation of sympathetic function and the nitric oxide (NO)-related pathway and interaction with MT1/MT2 receptors. However, information regarding its direct actions on coronary blood flow and cardiac function is scarce. This study therefore determined the primary in vivo effect of melatonin on cardiac function and perfusion and the involvement of the autonomic nervous system, MT1/MT2 receptors, and NO. In 35 pigs, melatonin infused into the coronary artery at 70 pg for each mL/min of coronary blood flow while preventing changes in heart rate and arterial pressure increased coronary blood flow, dP/dt(max), segmental shortening, and cardiac output by about 12%, 14%, 8%, and 23% of control values (P < 0.05), respectively. These effects were accompanied by an increase in coronary NO release of about 46% (P < 0.05) of control values. The aforementioned responses were graded in a further five pigs. Moreover, the blockade of muscarinic cholinoreceptors (n = 5) and α-adrenoreceptors (n = 5) did not abolish the observed responses to melatonin. After β(1)-adrenoreceptors blocking (n = 5), melatonin failed to affect cardiac function, whereas β(2)-adrenoreceptors (n = 5) and NO synthase inhibition (n = 5) prevented the coronary response and the effect of melatonin on NO release. Finally, all effects were prevented by MT1/MT2 receptor inhibitors (n = 10). In conclusion, melatonin primarily increased coronary blood flow and cardiac function through the involvement of MT1/MT2 receptors, β-adrenoreceptors, and NO release. These findings add new information about the mechanisms through which melatonin physiologically modulates cardiovascular function and exerts cardioprotective effects.

Beneficial effects of melatonin on cardiological alterations in a murine model of accelerated aging

This study investigated the effect of aging-related parameters such as inflammation, oxidative stress and cell death in the heart in an animal model of accelerated senescence and analyzed the effects of chronic administration of melatonin on these markers. Thirty male mice of senescence-accelerated prone (SAMP8) and 30 senescence-accelerated-resistant mice (SAMR1) at 2 and 10 months of age were used. Animals were divided into eight experimental groups, four from each strain: two young control groups, two old untreated control groups, and four melatonin-treated groups. Melatonin was provided at two different dosages (1 and 10 mg/kg/day) in the drinking water. After 30 days of treatment, the expression of inflammatory mediators (tumor necrosis factor-alpha, interleukin 1 and 10, NFkBp50 and NFkBp52), apoptosis markers (BAD, BAX and Bcl2) and parameters related to oxidative stress (heme oxygenases 1 and 2, endothelial and inducible nitric oxide synthases) were determined in the heart by real-time reverse transcription polymerase chain reaction (RT-PCR). Inflammation, as well as, oxidative stress and apoptosis markers was increased in old SAMP8 males, when compared to its young controls. SAMR1 mice showed significantly lower basal levels of the measured parameters and smaller increases with age or no increases at all. After treatment with melatonin, these age-altered parameters were partially reversed, especially in SAMP8 mice. The results suggest that oxidative stress and inflammation increase with aging and that chronic treatment with melatonin, a potent antioxidant, reduces these parameters. The effects were more marked in the SAMP8 animals.

Antioxidative effects of melatonin administration in elderly primary essential hypertension patients

The imbalance of the redox state of the aging organism may be involved in the development of primary essential hypertension. Melatonin, a potent antioxidant agent, was found to exert a hypotensive effect and improve the function of the cardiovascular system. The aim of this study was to determine the influence of melatonin supplementation on oxidative stress parameters in elderly primary essential hypertensive (EH) patients, controlled by a diuretic (indapamide) monotherapy. The levels of malondialdehyde (MDA) and reduced glutathione (GSH), activities of Cu-Zn superoxide dismutase (SOD-1), catalase (CAT) and glutathione peroxidase (GSH-Px) in erythrocytes, the plasma level of nitrate/nitrite, the content of carbonyl groups of plasma proteins and morning melatonin levels in the serum of 17 elderly EH patients were determined at the baseline and after the 15th and 30th days of melatonin supplementation (5 mg daily). Melatonin administration resulted in a significant increase in the morning melatonin concentration, SOD-1 and CAT activities, and a reduction in the MDA level. Statistically significant alterations in the levels of GSH, nitrate/nitrite and carbonyl groups and the activity of GSH-Px were not observed. These results indicate an improvement in the antioxidative defense of the organism by melatonin supplementation in the examined group and may suggest melatonin supplementation as an additional treatment supporting hypotensive therapy in elderly EH patients.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Melatonin receptors in humans: biological role and clinical relevance

In addition to its antioxidative effects melatonin acts through specific nuclear and plasma membrane receptors. To date, two G-protein coupled melatonin membrane receptors, MT(1) and MT(2), have been cloned in mammals, while the newly purified MT(3) protein belongs to the family of quinone reductases. Screening studies have shown that various tissues of rodents express MT(1) and/or MT(2) melatonin receptors. In humans, melatonin receptors were also detected in several organs, including brain and retina, cardiovascular system, liver and gallbladder, intestine, kidney, immune cells, adipocytes, prostate and breast epithelial cells, ovary/granulosa cells, myometrium, and skin. This review summarizes the data published so far about MT(1) and MT(2) receptors in human tissues and human cells. Established and putative functions of melatonin after receptor activation as well as the clinical relevance of these findings will be discussed.

Melatonin for cognitive impairment

BACKGROUND

There are a number of studies that suggest a relationship between decline of melatonin function and the symptoms of dementia.

OBJECTIVES

The review assessed the evidence of clinical efficacy and safety of melatonin in the treatment of manifestations of dementia or cognitive impairment (CI).

SEARCH STRATEGY

The Cochrane Dementia and Cognitive Improvement Group's Specialized Register was searched for trials involving melatonin on 5 October 2005. The search terms used were MELATONIN, and N-ACETYL-5-METHOXYTRYPTAMINE. This Register contains records from all major health care databases as well as many ongoing trials databases and is updated regularly.

SELECTION CRITERIA

All relevant, randomized controlled trials in which orally administered melatonin in any dosage was compared with a control group for the effect on managing cognitive, behavioural (excluding sleep), and/or affective disturbances of people with dementia of any degree of severity.

DATA COLLECTION AND ANALYSIS

Two to three reviewers independently assessed the retrieved articles for relevance and methodological quality, and extracted data from the selected studies. Statistically significant differences in changes in outcomes from baseline to end of treatment between the melatonin and control groups were examined. Each study was summarized using a measure of effect (e.g. mean difference) and meta-analyses were conducted when appropriate.

MAIN RESULTS

Three studies met the inclusion criteria. This review revealed non-significant effects from the pooled estimates of MMSE cognitive, and ADAS-cognitive change scores. Individual study estimates for treatment effect demonstrated a significant improvement for melatonin compared with placebo in behavioural and affective symptoms as measured by the ADAS non-cognitive scale in a study of 20 patients, and the Neuropsychiatric Inventory (NPI) following treatment with 2.5 mg/day (SR) melatonin, but not with 10mg/day (IR) melatonin in a larger study of 157 patients. The remainder of the treatment effects for affect, behaviour and activities of daily living were non-significant.

AUTHORS' CONCLUSIONS

There is insufficient evidence to support the effectiveness of melatonin in managing the cognitive and non-cognitive sequelae of dementia.

Endocrinology of menopausal transition and its brain implications

The central nervous system is one of the main target tissues for sex steroid hormones, which act on both through genomic mechanisms, modulating synthesis, release, and metabolism of many neuropeptides and neurotransmitters, and through non-genomic mechanisms, influencing electrical excitability, synaptic function, morphological features, and neuron-glia interactions. During the climacteric period, sex steroid deficiency causes many neuroendocrine changes. At the hypothalamic level, estrogen withdrawal gives rise to vasomotor symptoms, to eating behavior disorders, and altered blood pressure control. On the other hand, at the limbic level, the changes in serotoninergic, noradrenergic, and opioidergic tones contribute to the modifications in mood, behavior, and nociception. Hormone replacement therapy (HRT) positively affects climateric depression throughout a direct effect on neural activity and on the modulation of adrenergic and serotoninergic tones and may modulate the decrease in cognitive efficiency observed in climaterium. The identification of the brain as a de novo source of neurosteroids, suggests that the modifications in mood and cognitive performances occurring in postmenopausal women may also be related to a change in the levels of neurosteroids. These findings open new perspectives in the study of the effects of sex steroids on the central nervous system and on the possible use of alternative and/or auxiliary HRT.

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.

Functional melatonin receptors in rat ovaries at various stages of the estrous cycle

This study investigated the receptor mechanism(s) by which the hormone melatonin directly affects ovarian function. Expression of MT1 and MT2 melatonin receptor mRNA was detected in the rat ovaries both by reverse transcriptase-polymerase chain reaction and in situ hybridization with digoxigenin-labeled oligoprobes. Specific 2-[125I]iodomelatonin binding was significantly higher in ovarian tissue from animals sacrificed during proestrus than in metestrus, suggesting regulation of melatonin receptors by estrogens. Additionally, basal and melatonin-mediated stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding to ovarian sections was higher in proestrus compared with metestrus. During proestrus, both luzindole (0.1 microM) and 4-phenyl-2-propionamidotetraline (4P-PDOT) (0.1 microM), acting as inverse agonists, inhibited basal [35S]GTPgammaS binding to ovarian sections, suggesting the presence of MT1 constitutively active melatonin receptors. In primary cultures of ovarian granulosa cells, melatonin inhibited forskolin-stimulated cAMP accumulation through activation of Gi-coupled melatonin receptors. This inhibition was blocked by both, luzindole, and 4P-PDOT, acting as competitive receptor antagonists. Exposure of granulosa cells in culture to 17beta-estradiol seems to alter the state of melatonin receptor coupling. Indeed, the efficacy of 4P-PDOT on forskolin-stimulated cAMP formation was reversed from an MT2 partial agonist in vehicle-treated cells to that of an MT1 inverse agonist in 17beta-estradiol (0.1 microM)-treated granulosa cells. We conclude that MT1 and MT2 melatonin receptors expressed in antral follicles and corpus luteum may affect steroidogenesis through cAMP-mediated signaling. These results underscore the implications of the levels of ovarian estrogen when melatonin receptor ligands are used as therapeutic agents.

Acute modifications in the levels of daytime melatonin do not influence leptin in postmenopausal women

Melatonin shows a clear circadian rhythm with peak values at night, and may act directly with fat cells. Leptin, the anorexic hormone synthesized mainly by adipocytes, is produced in a circadian fashion, similar to that of melatonin. Accordingly, in the present study, we investigated whether melatonin may contribute to the rise in circulating leptin. The study was performed in postmenopausal women with 2 months of treatment with placebo or estradiol (50 microg/day). Melatonin was administered in doses of 1 mg by mouth versus placebo. In experiment 1, melatonin was administered at 08:30 hr. In experiment 2, at 08:30 hr and 10:30 hr, and in experiment 3 at 15:30 hr. Three blood samples, one every 15 min, were collected prior to the administration of melatonin and 2 hr after the administration of the single melatonin dose or the second melatonin administration (experiment 2). Following its administration, circulating melatonin reached pharmacological levels. In the three experiments, levels of leptin were not modified by the daytime administration of melatonin. These data indicate that, at least in daytime hours, acute modifications in daytime melatonin levels do not influence levels of leptin of postmenopausal women either without or with estradiol replacement. Accordingly, the metabolic, endocrine, reproductive and biological modifications induced by acute daytime melatonin in women do not seem to be mediated by modifications in circulating leptin.