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doi:10.1111/jog.12177
J. Obstet. Gynaecol. Res. Vol. 40, No. 1: 1–11, January 2014
Melatonin and female reproduction
Hiroshi Tamura1, Akihisa Takasaki2, Toshiaki Taketani1, Manabu Tanabe1, Lifa Lee1,
Isao Tamura1, Ryo Maekawa1, Hiromi Aasada1, Yoshiaki Yamagata1 and
Norihiro Sugino1
1
Department of Obstetrics and Gynecology, Yamaguchi University Graduate School of Medicine, Ube, and 2Department of
Obstetrics and Gynecology, Saiseikai Shimonoseki General Hospital, Shimonoseki, Japan
Abstract
Melatonin (N-acetyl-5-methoxytryptamine) is secreted during the dark hours at night by the pineal gland.
After entering the circulation, melatonin acts as an endocrine factor and a chemical messenger of light and
darkness. It regulates a variety of important central and peripheral actions related to circadian rhythms and
reproduction. It also affects the brain, immune, gastrointestinal, cardiovascular, renal, bone and endocrine
functions and acts as an oncostatic and anti-aging molecule. Many of melatonin’s actions are mediated through
interactions with specific membrane-bound receptors expressed not only in the central nervous system, but
also in peripheral tissues. Melatonin also acts through non-receptor-mediated mechanisms, for example
serving as a scavenger for reactive oxygen species and reactive nitrogen species. At both physiological and
pharmacological concentrations, melatonin attenuates and counteracts oxidative stress and regulates cellular
metabolism. Growing scientific evidence of reproductive physiology supports the role of melatonin in human
reproduction. This review was conducted to investigate the effects of melatonin on female reproduction and to
summarize our findings in this field.
Key words: lipids metabolism, melatonin, ovary, pregnancy, reproduction.
Introduction
Melatonin is a neuroendocrine hormone secreted by
the pineal gland. Its secretion is regulated by light and
dark stimuli, and the hormone influences circadian
rhythms, such as the sleep cycle and body temperature.
Melatonin also regulates other general functions,
including lipid metabolism, sugar metabolism, carcinogenesis, immune regulation1–3 and reproduction.
In addition to its receptor-mediated actions as a neuroendocrine hormone, the discovery of melatonin’s
ability to act as a direct free radical scavenger has
greatly broadened understanding of the mechanisms of
melatonin that benefit reproductive physiology.4 Melatonin is thought to be a potent anti-aging agent due to
its cytoprotective action as an antioxidant.
Female reproduction, including puberty, ovarian
follicle growth, ovulation, luteinization, fertilization,
implantation, pregnancy, parturition, menopause and
climacteric, involves extremely dynamic changes that
are controlled by the delicate reproductive endocrinology system. Melatonin may play a role in pubertal
development and the reproductive function by regulating the hypothalamus–pituitary–gonadal axis, as the
blood melatonin levels decrease considerably during
childhood and puberty,5 and gonadotrophin release in
the hypothalamus–pituitary gland axis via specific
receptors.6 Melatonin production decreases with age
(Fig. 1) and removing the pineal gland (pinealectomy)
leads to the acceleration of many aspects of the aging
process,7,8 thus suggesting that melatonin might play
an anti-aging role.9 The decline in the serum estrogen
Reprint request to: Dr Hiroshi Tamura, Department of Obstetrics and Gynecology, Yamaguchi University Graduate School of
Medicine, Minamikogushi 1-1-1, Ube 755-8505, Japan. Email: hitamura@yamaguchi-u.ac.jp
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
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H. Tamura et al.
Figure 1 Night-time serum level of melatonin decreases
with age. The night-time serum melatonin levels were
measured in 63 women. Blood samples were drawn at
02:00 h. During blood sampling, the subjects were
recumbent on a bed and wore an eye mask to exclude
light from 21:00 h to the end of sampling. The
serum melatonin concentrations were measured with a
radioimmunoassay.
levels observed with ovarian aging results in menopause and is cause of considerable changes in lipid
metabolism.
It is very interesting to investigate how melatonin
participates in female reproduction. This review was
conducted to investigate the roles of melatonin in
female reproduction, especially the involvement of
melatonin in ovarian function, pregnancy and climacteric (lipid metabolism).
Synthesis of Melatonin
Melatonin (N-acetyl-5-methoxytryptamine), the hormone of the pineal gland, exhibits a circadian rhythm
that is generated by the circadian pacemaker situated
in the suprachiasmatic nucleus (SCN) of the hypothalamus, which is synchronized to 24 h, primarily by
the light–dark cycle acting via the SCN.10 The serum
melatonin concentrations are low during the day
and significantly increase at night. The initial precursor
of melatonin biosynthesis is an amino acid, tryptophan. Pinealocytes take up tryptophan from the
blood and convert it to serotonin via hydroxylation
and decarboxylation. Serotonin is then converted
to N-acetylserotonin by the enzyme arylalkylamine N-acetyltransferase (NAT).11 N-acetylserotonin is subsequently methylated to form melatonin,
a step that requires the enzyme hydroxyindole-O-
2
methyltransferase.12 Once synthesized, melatonin is
not stored in pineal cells but is quickly released into the
bloodstream and then into other body fluids, such as
bile,13 cerebrospinal fluid,14 saliva,15 semen,16 amniotic
fluid17 and ovarian follicular fluid.18 Two mammalian
subtypes of G protein-coupled melatonin receptors,
MT1 (Mel 1a) and MT2 (Mel 1b), have been cloned and
characterized.19 Melatonin receptors have been identified in hypothalamic neurons governing the release of
pituitary gonadotropins20 and in the gonadotrophs of
the anterior pituitary.21 The melatonin receptor expression in peripheral human tissues is also well documented.22 The mRNA and/or protein expression of
melatonin receptors (MT1, MT2) has been identified in
human reproductive tissues, including the breast
epithelium,23 uterine myometrium24 and ovarian
granulosa and luteal cells.25
Melatonin as an Antioxidant
Reactive oxygen species (ROS) are involved in a variety
of different cellular processes, ranging from physiological to pathological responses. It is well known that
ROS can promote cell survival, proliferation and differentiation at the physiological levels and cell death via
apoptosis or necrosis at higher levels.26 Oxidative stress
can be defined as an imbalance between cellular
oxidant species, such as ROS and antioxidants, and has
a direct toxic effect on cells, which leads to lipid peroxidation, protein oxidation and DNA damage.
It has been discovered that melatonin is a powerful
free radical scavenger and a broad-spectrum antioxidant.4,27 Due to its small size and highly lipophilic properties,27 melatonin crosses all cell membranes and
easily reaches subcellular compartments, including
mitochondria and nuclei, where it accumulates in high
concentrations.28 Melatonin prevents lipid peroxidation29 and protein30 and DNA damage.31 In particular,
melatonin has been found to preserve an optimal mitochondrial function and homeostasis by reducing and
preventing mitochondrial oxidative stress, thereby curtailing subsequent apoptotic events and cell death.32
The ability of melatonin to scavenge the ·OH is much
higher than that of other antioxidants, including mannitol, glutathione and vitamin E.33 Melatonin is a powerful and broad antioxidant and has been shown to
scavenge different types of free radicals, including
superoxide anion (O2·-), hydroxyl radical (·OH),
singlet oxygen (1O2), hydrogen peroxide (H2O2),
hypochlorous acid (HOCl), nitric oxide (NO·) and the
peroxynitrite anion (ONOO-).4 Not only is melatonin
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
Melatonin regulates female reproduction
itself a direct free radical scavenger, the metabolites that are formed during these interactions
(i.e. cyclic 3-hydroxmelatonin, N1-acetyl-N2-formyl-5methoxykynuramine and N1-acetyl-5-methoxykynuramine) are likewise excellent scavengers of toxic
reactants.4 Furthermore, melatonin plays an important
role in activating antioxidant defenses, such as superoxide dismutase (SOD), catalase (CAT), glutathione
peroxidase (GSHPx), glutathione reductase (GSH-Rd)
and glucose-6-phosphate dehydrogenase (G6PD).34
Ovulation and Oxidative Stress
The mechanism of ovulation has been compared to an
inflammatory reaction.35 ROS are important mediators
of inflammatory reactions and have been reported to
be involved in ovulation.36 Macrophages, neutrophils
and vascular endothelial cells reside in follicles, and
ROS are produced by these cells during ovulation.37
Although ROS play a role in follicle rupture during
ovulation, they can potentially damage oocytes and
granulosa cells undergoing luteinization. ROS have
been reported to inhibit progesterone production by
luteal cells by inhibiting steroidogenic enzymes37 and
intracellular carrier proteins involved in the transport
of cholesterol to mitochondria.38 Although ROS are also
essential for oocyte maturation, excess amounts of
ROS may be involved in oxidative stress and poor
oocyte quality. ROS, such as superoxide radical (O2·-),
hydroxyl radical (·OH) and hydrogen peroxide
(H2O2), are known to be detrimental to oocytes. They
deteriorate cell membrane lipids, destroy DNA and
induce two-cell block, apoptosis and inhibition of fertilization in mice and hamsters.39,40 In addition, higher
levels of the oxidant H2O2 have been reported in fragmented human embryos compared with that observed
in non-fragmented embryos and unfertilized oocytes.41
Melatonin and Reproduction (Ovulation
and Luteinization)
Although high concentrations of melatonin in ovarian
follicular fluids have been reported,18,42 the role of melatonin in follicles has not been investigated. We focused
on the effects of melatonin as an antioxidant within
ovarian follicles.
A previous report demonstrated the tissue distribution of H3-melatonin when given by intravenous injection to cats.43 It was found that the concentration of
H3-melatonin in the ovaries is ten times higher than
that in plasma, and a high uptake of H3-melatonin by
the ovaries compared to that observed in other peripheral tissues has been demonstrated. The fact that circulating melatonin is highly concentrated by the ovaries is
of interest in view of the relation between melatonin
and ovarian function. We previously reported that the
melatonin concentrations in the ovaries exhibit phasic
variation, as in the pineal gland and serum; they are
high at mid-dark and low at mid-light in cyclic hamsters.44 The ovarian melatonin concentration at middark is significantly higher during proestrus than on
other days of the estrous cycle. Ovaries on proestrus
contain preovulatory follicles; therefore, it is likely that
melatonin is taken up from the circulation by the
ovaries during follicular growth. We also demonstrated
the melatonin concentrations in human follicular fluids
in patients undergoing in vitro fertilization and embryo
transfer (IVF-ET).45 The melatonin concentrations are
higher in the fluid of large follicles than in that of small
follicles, thus suggesting that increased levels of melatonin in preovulatory follicles may play an important
role in the ovulation process.
Some peripheral tissues, such as the retina, gastrointestinal tract, skin, leukocytes and bone marrow,
synthesize melatonin.46–48 When we assessed the NAT
expression of granulosa cells using PCR in rats and
humans, we did not detect an active NAT expression.
We also measured the melatonin concentrations in
human follicular fluids obtained from patients who
were administered melatonin. The melatonin concentrations were increased depending on the dose of
melatonin (Fig. 2). These findings suggest that the
melatonin present in follicular fluid is derived from the
circulation and the uptake of melatonin by ovarian follicles is increased depending on follicular growth.
To investigate the relation between oxidative stress
and sex steroid production, we analyzed the concentrations of a DNA-related oxidative stress marker
(8-hydroxy-2′-deoxyguanosine [8-OHdG]), antioxidants (Cu,Zn-SOD, glutathione, melatonin) and sex
steroids (progesterone, testosterone, estradiol) in
mature follicles obtained from patients undergoing
IVF-ET.49 The melatonin concentrations in the follicular
fluid exhibited a negative correlation with 8-OHdG,
whereas Cu,Zn-SOD and glutathione did not exhibit
any significant correlations with 8-OHdG. The progesterone concentrations in the follicular fluid exhibited a
positive correlation with melatonin, whereas the levels
of estradiol and testosterone did not exhibit any
significant correlations with that of melatonin. The
progesterone concentrations in the follicular fluid were
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
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H. Tamura et al.
Figure 2 Concentrations of melatonin in follicular fluid
(following one tablet of melatonin administered
orally). To examine the melatonin concentrations in
human follicular fluids, follicular fluids were obtained
from patients who underwent in vitro fertilization and
embryo transfer. The patients were given a melatonin
tablet (1, 3, 6 mg) orally at 22:00 h from the fifth day of
the previous menstrual cycle until the day of oocyte
retrieval. The melatonin concentrations were measured
with a radioimmunoassay.
negatively correlated with the 8-OHdG concentrations.
Our data suggest that melatonin is an important antioxidant within follicles and contributes to progesterone production by luteinized granulosa cells.
It would be interesting to elucidate the physiological
role of melatonin in follicular fluid, especially with
respect to the high concentrations observed in preovulatory follicles, during the ovulation process. Oxidative
stress caused by ROS during the ovulation process is
detrimental to oocytes and granulosa cells, and it is
possible that impaired oocyte maturation, fertilization
and luteinization are important causes of infertility
(Fig. 3). The protective role of melatonin as an antioxidant within follicles is discussed below.
Oocyte quality
Oxidative stress in oocytes caused by ROS must be
limited in order for a good embryo to be produced.
ROS induce lipid peroxidation of membranes and
DNA damage in oocytes and are expected to cause
harmful effects in cell division, metabolite transport
and the mitochondrial function. We recently reported
the direct effects of ROS and melatonin on oocyte
maturation.50 To investigate the effects of H2O2 on
oocyte maturation, denuded oocytes obtained from
immature mice treated with PMSG were cultured in
incubation medium with various concentrations of
4
Figure 3 Balance between reactive oxygen species (ROS)
and antioxidants in follicles. ROS are produced within
follicles, especially during the ovulation process
induced by the luteinizing hormone surge. Antioxidant
enzymes (such as superoxide dismutase [SOD], glutathione peroxidase [GPx] and catalase) and nonenzymatic antioxidants (such as vitamin E, vitamin C,
glutathione, uric acid and albumin) are present in follicles. Melatonin, which is secreted by the pineal gland
and is taken up into the follicular fluid from the blood,
is an antioxidant present in follicles. Excess amounts of
ROS may be involved in oxidative stress in oocytes and
granulosa cells. The balance between ROS and antioxidants within follicles may be critical for oocyte maturation and luteinization of granulosa cells. CAT,
catalase; GSH, glutathione.
H2O2. After 12 h of incubation, oocytes with the first
polar body (MII stage oocytes) were counted. The percentage of mature oocytes (MII stage oocytes with a
first polar body) was significantly decreased in a dosedependent manner (>200 mM) following the addition
of H2O2. When oocytes were incubated with melatonin
in the presence of H2O2 (300 mM), melatonin dosedependently blocked the inhibitory effects of H2O2 on
oocyte maturation, with a significant effect at a melatonin concentration of 10 ng/mL. To further investigate the intracellular role of melatonin, oocytes were
incubated with dichlorofluorescein (DCF-DA). The
non-fluorescent DCF-DA was oxidized by intracellular
ROS to form highly fluorescent DCF. Then, intracellular ROS formation was visualized using fluorescence
images and the fluorescent intensity was analyzed.51
When oocytes were incubated without H2O2, there
was no observable fluorescent intensity. However, high
fluorescent intensities were observed in the presence
of H2O2 (300 mm). The increased fluorescent intensity
of oocytes incubated with H2O2 was significantly
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
Melatonin regulates female reproduction
decreased by melatonin treatment. These results
suggest that H2O2 inhibits oocyte maturation by producing ROS, while melatonin demonstrates protective
activity against oxidative stress caused by H2O2.
As summarized above, a growing amount of literature has demonstrated that melatonin and/or melatonin treatment may have beneficial effects on oocyte
maturation and embryo development. Poor oocyte
quality is one of the most intractable causes of infertility in women. Melatonin can be a useful infertility
treatment and therefore has recently been applied in
infertility patients for the first time.
To document the association between melatonin and
ovarian oxidative stress, human follicular fluids were
sampled during oocyte retrieval for the purpose of IVFET, and the concentrations of melatonin and 8-OHdG
were measured. This study revealed an inverse correlation between the intrafollicular concentrations of
melatonin and 8-OHdG, suggesting that the melatonin
present in follicles diffuses into the cumulus and
oocytes to protect them from free radical damage.
When patients were given a 3-mg tablet of melatonin
orally at 22:00 h from the fifth day of the previous menstrual cycle until the day of oocyte retrieval, the
intrafollicular concentrations of melatonin rose from
112 pg/mL in the control cycle (without melatonin
treatment) to 432 pg/mL after daily melatonin treatment.50,52 The intrafollicular concentrations of 8-OHdG
and hexanoyl-lysine adduct (HEL), a damaged lipid
product, were decreased following melatonin treatment compared to those observed in the prior cycle.
This result demonstrates that melatonin treatment
reduces intrafollicular oxidative damage. To investigate the clinical usefulness of melatonin administration, the effects of melatonin treatment on the clinical
outcomes of IVF-ET were examined in 115 patients
who failed to become pregnant during the previous
IVF-ET cycle with a low fertilization rate (<50%).
Among 56 patients who received melatonin treatment,
the fertilization rate (50.0 ⫾ 38.0%) was markedly
improved compared with that observed in the previous
IVF-ET cycle (20.2 ⫾ 19.0%), and 11 of 56 patients
(19.6%) achieved pregnancy. On the other hand, among
59 patients who were not given melatonin, the fertilization rate (22.8 ⫾ 19.0% vs 20.9 ⫾ 16.5%) did not significantly change, and only six of 59 patients (10.2%)
achieved pregnancy.50,52 These results show that melatonin administration increases the intrafollicular melatonin concentrations, reduces intrafollicular oxidative
damage and elevates fertilization and pregnancy
rates.
To our knowledge, our study represents the first
clinical evidence of the usefulness of melatonin treatment in infertility patients. Melatonin is likely to
become a treatment used to improve oocyte quality in
women who cannot become pregnant due to poor
oocyte quality.
Granulosa cells (luteinization)
In the ovaries, the corpus luteum is formed after ovulation, after which it begins to produce progesterone,
which is necessary for the establishment and maintenance of pregnancy. ROS have been reported to inhibit
progesterone production by luteal cells,37 mediated by
the inhibition of steroidogenic enzymes and intracellular carrier proteins involved in the transport of cholesterol to mitochondria.38 To examine the effects of
melatonin on progesterone production, luteinized
granulosa cells were obtained at the time of oocyte
retrieval in women undergoing IVF-ET. The luteinized
granulosa cells were incubated with or without H2O2
(30, 50 or 100 mM) in serum-free incubation medium
for 12 h in the presence or absence of melatonin
(1, 10, 100 mg/mL). After incubation, the progesterone
concentration in culture medium was measured. Progesterone production was significantly inhibited
by H2O2 (30 mM:54.9 ⫾ 18.8%; 50 mM:30.1 ⫾ 18.8%;
100 mM:17.4 ⫾ 6.0%, the values represent the mean ⫾
standard error of the mean), and the inhibitory effects
of H2O2 on progesterone production were reversed by the addition of melatonin (0 mg/mL:21.4 ⫾
2.8%; 1 mg/mL:38.0 ⫾ 7.8%; 10 mg/mL:65.5 ⫾ 22.1%;
100 mg/mL:99.7 ⫾ 31.4%, the values represent the
mean ⫾ standard error of the mean).49 Our results also
showed that ROS reduce progesterone production by
luteinized granulosa cells; however, melatonin abolished the inhibitory effects of H2O2 on progesterone
production. Therefore, this study suggests that melatonin protects granulosa cells from ROS in follicles
during ovulation and contributes to the luteinization of
granulosa cells.
Luteal phase defects have been implicated as a cause
of infertility and spontaneous miscarriage. However,
the cause of luteal phase defects is complicated by the
fact that the causes of the condition are highly varied.
Not only low blood flow of the corpus luteum,53,54 but
also oxidative stress are associated with luteal phase
defects.
To analyze the clinical effectiveness of melatonin
administration in patients with luteal phase defects, 25
infertility patients (26–42 years of age) with luteal
phase defects who did not exhibit a decreased luteal
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
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H. Tamura et al.
blood flow were enrolled.49 These patients were diagnosed as having luteal phase defects (the serum
progesterone concentrations during the midluteal
phase were <10 ng/mL) and were not diagnosed as
having a decreased luteal blood flow. The patients were
divided into two groups during the subsequent treatment cycle: 14 patients were given 3 mg/day of melatonin orally at 22:00 h after human chorionic
gonadotrophin (hCG) injection throughout the luteal
phase, and 11 patients were given no medications after
hCG injection as a control. In the melatonin treatment
group, nine of 14 (64.3%) patients exhibited improved
serum progesterone concentrations of more than
10 ng/mL, and the mean progesterone concentration
was 11.0 ⫾ 2.6 ng/mL. In the control group, only two
of 11 (18.2%) patients exhibited normal serum progesterone concentrations, and the mean progesterone concentration was 8.9 ⫾ 2.2 ng/mL. The improvement
rates in the two groups were significantly different
(P < 0.05). The results strongly suggest that oxidative
stress is a cause of luteal phase defects and that melatonin protects luteinized granulosa cells and increases
progesterone production by the corpus luteum by
reducing oxidative stress. Melatonin supplementation
can therefore be a useful treatment for luteal phase
defects related to oxidative stress.
Melatonin, Pregnancy and Delivery
Human fetuses exhibit circadian rhythms in hormones,
behavior, heart rate and sleep.55 The photoperiodic
information perceived by the mother is thought to play
a role in synchronizing fetal physiology.56 The information about day length and circadian phase, presumably
mediated by the maternal melatonin rhythm, is transferred to the fetus. On the other hand, circadian variations of melatonin in the maternal circulation during
pregnancy have been reported in sheep.57 In humans,
the serum melatonin levels during pregnancy and
labor are reportedly significantly higher than those
observed during the postpartum period.58 We previously measured daytime (14:00 h) and night-time
(02:00 h) serum melatonin concentrations in normal
women during pregnancy59 (Fig. 4a). The daytime
serum melatonin levels showed incremental changes
toward the end of pregnancy; however, the rise was not
Figure 4 Changes of maternal serum melatonin in pregnancy. (a) Levels of maternal serum melatonin during the night (solid
line) and day (dotted line) in normal singleton pregnancy. The data were modified from Nakamura et al.59 Values are
means ⫾ standard error of the mean. (b) Changes in maternal serum melatonin levels at night-time during pregnancy in
rats. Maternal circulating night-time melatonin concentrations were measured on days 7, 12, 15, 17, 19 and 21 of pregnancy,
and on day 2 of postpartum. Data are shown as the mean ⫾ standard error of the mean for 7–10 rats. The data were
modified from Tamura et al.60
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© 2013 The Authors
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Melatonin regulates female reproduction
significant in normal singleton pregnancies. In contrast, the night-time serum melatonin levels were significantly higher than the daytime values throughout
pregnancy, gradually increasing after 24 weeks of gestation and exhibiting significantly higher levels after 32
weeks of gestation. Thereafter, they declined to the
non-pregnant levels on the 2nd day of puerperium in
normal singleton pregnancies. We also recently measured the maternal serum melatonin levels throughout
pregnancy in rats60 (Fig. 4b). The maternal night-time
(00:00–01:00 h) melatonin levels in rats with a normal
pregnancy increased toward day 21 of pregnancy and
rapidly dropped to the non-pregnancy levels after parturition. The increased levels of melatonin in serum
may be involved in delivery, especially with respect to
the parturition time. To investigate the effects of melatonin on the parturition time, we evaluated a pinealectomy (PINX) model in rats61 (Fig. 5). The pregnant rats
showed a daytime preference for parturition and an
afternoon preference on days 22 and 23 of pregnancy;
however, the PINX rats exhibited constantly low levels
of melatonin and a loss of the daytime preference for
parturition. The PINX rats implanted with melatonin
capsules exhibited constantly high serum melatonin
Figure 5 Effects of pinealectomy (PINX) or melatonin
(Mel) supplementation to pinealectomized rats on parturition times. Black dots indicate parturition time of
rats who gave birth during mean ⫾ 2 standard deviation (SD) period of control group, and white dots indicate those of rats who gave birth at the other times.
Horizontal lines above control group show
mean ⫾ 2SD period of parturition time in control
group. Bottom white bar indicates light period, and
black bar indicates dark period. (a) P < 0.01 vs control
group. (b) P < 0.05 vs control group. The figure was
modified from Takayama et al.61
levels and gave birth across the 24-h light–dark cycle.
Melatonin administration in the PINX rat was effective
in restoring the daytime birth pattern when administered in the evening (20:00 h), although it was ineffectual when given in the morning (08:00 h). This result
demonstrates that the melatonin rhythm synchronized
with the photoperiodic rhythm is likely to be an important determinant of the parturition time in pregnant
rats. Increased levels of melatonin before delivery may
therefore serve as a key circadian signal for parturition.
We also analyzed the mechanisms underlying the
increased serum levels of melatonin observed in late
pregnancy. The maternal night-time (00:00–01:00 h)
melatonin levels increased toward day 21 of pregnancy
and rapidly dropped to the non-pregnancy levels after
parturition in rats with a normal pregnancy (more than
10 conceptions). However, the night-time serum melatonin levels in the 1-conceptus rats (the number of
conceptions was experimentally reduced to one on day
7 of pregnancy) were significantly lower on day 21 of
pregnancy. When the fetuses were removed by fetectomy (all fetuses but not the placentae) on day 12 of
pregnancy, the night-time serum melatonin concentrations on day 21 of pregnancy were not lower than
normal. To examine the effects of placental hormones
on maternal melatonin production, a conditioned
medium produced by incubating the placenta obtained
on day 20 of pregnancy with the medium was injected
into the 1-conceptus dams from day 17 to day 20 of
pregnancy. The conditioned medium significantly
increased the serum melatonin concentrations. To identify the source of the circulating maternal melatonin,
the NAT mRNA expression was examined in the placenta and fetal pineal gland. The amount of NAT
mRNA was negligible in the placenta and the pineal
gland of the fetus compared with the values observed
in the maternal pineal gland.60 The implication of this
finding is that maternal circulating melatonin is likely
of maternal pineal gland origin and is increased via the
actions of placental hormones.
The involvement of melatonin in human pregnancy,
including the fetal circadian rhythm, fetal development, pre-eclampsia and fetal brain damage, was summarized in our review article.62
Melatonin and Climacteric
(Lipid Metabolism)
Chronic estrogen deficiency in the postmenopausal
period can cause osteoporosis and cardiovascular
© 2013 The Authors
Journal of Obstetrics and Gynaecology Research © 2013 Japan Society of Obstetrics and Gynecology
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H. Tamura et al.
disease.63 After menopause, significant changes occur,
including increases in the total cholesterol and lowdensity lipoprotein (LDL) cholesterol levels and
decreases in the high-density lipoprotein (HDL)
cholesterol levels, that can induce cardiovascular diseases.63,64 Therefore, management and prevention of
menopausal hypercholesterolemia are important
issues.
There is increasing evidence showing the involvement of melatonin in lipid metabolism. Several reports
have suggested that the administration of melatonin
to genetically hypercholesterolemic rats results in
reductions in the plasma cholesterol levels and
improvements in fatty changes in the liver.65–67
While melatonin has been shown to potentially exert
anti-hypercholesterolemic effects under experimental
conditions, there are no data concerning melatonin’s
effects in hypercholesterolemic patients. Therefore, we
investigated the effects of melatonin on lipid metabolism in peri- and postmenopausal women.2
First, the relation between the night-time (02:00 h)
serum melatonin levels and the serum levels of total
cholesterol, triglycerides, HDL-cholesterol and LDLcholesterol were investigated in peri- and postmenopausal women (Fig. 6). The night-time serum
melatonin levels exhibited a negative correlation with
Figure 6 Correlation of the serum night-time levels of melatonin and serum levels of (a) total cholesterol, (b) triglyceride, (c)
low-density lipoprotein (LDL) cholesterol and (d) high-density lipoprotein (HDL)-cholesterol. Blood samples for melatonin were drawn at 02:00 h in 36 women. The subjects wore an eye mask to exclude light from 21:00 h to the end of
sampling. Blood samples for cholesterol and triglycerides estimations were drawn at 09:00 h after an overnight fast.
Night-time serum melatonin levels had a negative correlation with serum levels of total cholesterol (r = 0.370, P < 0.05) and
LDL-cholesterol (r = 0.500, P < 0.01), and had a loose positive correlation with serum levels of HDL-cholesterol (r = 0.278,
P < 0.1). The figure was modified from Tamura et al.2
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Melatonin regulates female reproduction
the serum levels of total cholesterol and LDLcholesterol and a loose positive correlation with the
serum levels of HDL-cholesterol. No correlations were
found between the night-time serum melatonin values
and the serum triglyceride levels. These results
strongly suggest the presence of a correlation between
melatonin and lipid metabolism. In the next study, to
investigate the effects of melatonin on lipid metabolism, normolipidemic peri- and postmenopausal
women received melatonin treatment (1.0 mg of oral
melatonin daily for 1 month). Although there were no
significant differences in the concentrations of serum
total cholesterol, triglycerides or LDL-cholesterol
before and after melatonin treatment, the serum concentrations of HDL-cholesterol were significantly
increased by melatonin treatment. Taken together,
these results suggest that melatonin is likely to increase
the serum HDL-cholesterol levels, and melatonin
may influence cholesterol metabolism by augmenting
endogenous cholesterol clearance mechanisms.
Following our report, many researchers have
attempted to apply melatonin therapy to improve lipid
profiles. A recent report demonstrated that melatonin
treatment improves lipid profiles (decreases in LDL
cholesterol) and antioxidative defense (increases in
CAT activity).68 The beneficial effects of melatonin on
the plasma concentrations of lipids and liver enzymes
in patients with nonalcoholic steatohepatitis (NASH)
have also been reported.69,70 Melatonin administration
may become a new medical treatment used to improve
lipid metabolism and prevent cardiovascular disease in
peri- and postmenopausal women.
Conclusions
The involvement of melatonin in the female reproduction system has been summarized. We herein demonstrated that the night-time melatonin concentrations in
pregnant women increase toward parturition and are
regulated by placental hormones. Increased night-time
serum levels of melatonin may regulate the parturition
time. We also observed a relation between the serum
levels of melatonin and the lipid profiles in perimenopausal women and found that melatonin treatment
may improve lipid metabolism. Furthermore, we
focused on the intrafollicular role of melatonin in the
ovaries. Melatonin, which is secreted by the pineal
gland, is taken up into the follicular fluid from the
blood. ROS produced within the follicles, especially
during the ovulation process, are scavenged by melatonin, while reduced oxidative stress may be involved
in oocyte maturation, embryo development and luteinization of granulosa cells. Our clinical study demonstrated that melatonin treatment in infertile women
increases fertilization and pregnancy rates. It should be
noted that melatonin therapy may become a new treatment for improving oocyte quality in infertile women.
Acknowledgments
The authors would like to thank Dr Russel J. Reiter
(Department of Cellular & Structural Biology, The University of Texas Health Science Center, San Antonio,
TX, USA) and Dr Yasuhiko Nakamura (Department of
Obstetrics and Gynecology, Yamaguchi Grand Medical
Center, Hofu, Japan) for their advice. This work was
supported in part by Grants-in-Aid 20591918, 21592099
and 21791559 for Scientific Research from the Ministry
of Education, Science and Culture of Japan.
Disclosure
The authors declare that there is no conflict of interest.
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