Melatonin inhibits Arg-vasopressin release via MT(2) receptor in the suprachiasmatic nucleus-slice culture of rats

Melatonin is involved in the modulation of the hypothalamic and pituitary activity in the South American plains vizcacha, Lagostomus maximus

Melatonin, the key messenger of photoperiodic information, is synthesized in the pineal gland by arylalkylamine N-acetyltransferase enzyme (AANAT). It binds to specific receptors MT₁ and MT₂ located in the hypothalamus and pituitary gland. Melatonin can modulate the reproductive axis affecting the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). The South American plains vizcacha, Lagostomus maximus, shows natural poliovulation of up to 800 oocytes per estrous cycle, a 154-day long pregnancy, and reactivation of the reproductive axis at mid-gestation with pre-ovulatory follicular recruitment, presence of active corpora lutea, and variations of the endocrine status. Here we analyzed the involvement of melatonin in the modulation of the hypothalamic and pituitary gland physiology of vizcacha thorough several approaches, including histological localization of melatoninergic system components, assessment of melatoninergic components expression throughout the reproductive cycle, and evaluation of the effect of melatonin on hypothalamic and pituitary activities during the follicular and luteal phases of the estrous cycle. AANAT and melatonin receptors were localized in the pineal gland and preoptic area of the hypothalamus. Increase in pineal AANAT and serum melatonin expression was observed as pregnancy progressed, with the lowest hypothalamic MT₁ and MT₂ levels at mid-pregnancy. Pulsatility assays demonstrated that melatonin induces GnRH and LH secretion at luteal phase. The melatoninergic system effects on hypothalamic and pituitary gland hormones secretion during pregnancy pinpoint to melatonin as a potential key factor underlying the reactivation of the reproductive axis activity at mid-gestation.

The Effects of Melatonin on Brain Arginine Vasotocin: Relationship with Sex and Seasonal Differences in Melatonin Receptor Type 1 in Green Treefrogs (Hyla cinerea)

The neuroendocrine mechanisms by which animals synchronise their physiological state with environmental cues are vital to timing life-history events appropriately. One important endocrine transducer of environmental cues in vertebrates is the pineal hormone melatonin, the secretion of which is directly sensitive to photoperiod and temperature. Melatonin modulates arginine vasotocin (AVT)-immunoreactive (-IR) cell number in the brain of green treefrogs (Hyla cinerea) during the summer breeding season, and this modulation is sexually dimorphic. In the present study, we investigated whether the influence of melatonin on vasotocin varies seasonally. We show that treatment of nonreproductive male green treefrogs with melatonin-filled silastic implants for 4 weeks during the winter does not alter vasotocin-IR cell number in any brain region (i.e. nucleus accumbens, amygdala, preoptic area, suprachiasmatic nucleus or ventral hypothalamus). Taken together, these results suggest that the influence of melatonin on AVT is associated with sex and seasonal variation in melatonin receptor expression. We tested this hypothesis by using immunohistochemistry to characterise the distribution of melatonin receptor type 1 (MT1, also known as Mel1a) in the brain of reproductive and nonreproductive male and female frogs. We quantified MT1-IR cell number in regions known to contain AVT cell populations. Reproductive males had significantly more MT1-IR cells than nonreproductive males in all brain regions, including the combined nucleus accumbens, diagonal band of Broca and septum, striatum, amygdala, combined preoptic area and suprachiasmatic nucleus, as well as the ventral hypothalamus. In the accumbens region, where the effect of melatonin on AVT is known to be sexually dimorphic, males had significantly more MT1-IR cells than females during the summer breeding season. Based on these findings, we suggest that MT1 plays a role in mediating the interactions between melatonin and vasotocin that regulate seasonal and sexually dimorphic changes in sociosexual behaviour.

Alterations of melatonin receptors MT1 and MT2 in the hypothalamic suprachiasmatic nucleus during depression

BACKGROUND

The pineal hormone melatonin regulates circadian rhythms, largely by feedback on the central biological clock of the brain, the hypothalamic suprachiasmatic nucleus (SCN). This feedback is mediated by the melatonin receptors, melatonin receptor 1 (MT1) and melatonin receptor 2 (MT2). The circadian system may play a role in the pathophysiology of mood disorders, and indeed, melatonin-receptor agonists are considered a potential therapy for depression.

METHOD

In order to investigate melatonin receptors in the SCN during depression, and their relationship to the major neuropeptides in the SCN, vasopressin (AVP) and vasoactive intestinal peptide (VIP), we studied the SCN in 14 depressed patients (five major depression and nine bipolar disorder) and 14 matched controls by immunocytochemistry.

RESULTS

We show here that hypothalamic MT2 receptor immunoreactivity was limited to SCN, the supraoptic nucleus and paraventricular nucleus. We found that numbers of MT1-immunoreactive (MT1-ir) cells and AVP and/or VIP-ir cells were increased in the central SCN in depression, but numbers of MT2-ir cells were not altered. Moreover, the number of MT1-ir cells, but not MT2-ir cells was negatively correlated with age at onset of depression, while positively correlated with disease duration. CONCLUSION AND LIMITATIONS: Although every post-mortem study has limitations, MT1 receptors appeared specifically increased in the SCN of depressed patients, and may increase during the course of the disease. These changes may be involved in the circadian disorders and contribute to the efficacy of MT agonists or melatonin in depression. Moreover, we suggest that melatonin receptor agonists for depression should be targeted towards the MT1 receptor selectively.

Sexually dimorphic effects of melatonin on brain arginine vasotocin immunoreactivity in green treefrogs (Hyla cinerea)

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), regulate a variety of social and reproductive behaviors, often with complex species-, sex- and context-dependent effects. Despite extensive evidence documenting seasonal variation in brain AVT/AVP, relatively few studies have investigated the environmental and/or hormonal factors mediating these seasonal changes. In the present study, we investigated whether the pineal hormone melatonin alters brain AVT immunoreactivity in green treefrogs (Hyla cinerea). Reproductively active male and female frogs were collected during the summer breeding season and a melatonin-filled or blank silastic capsule was surgically implanted subcutaneously. The duration of hormone treatment was 4 weeks, at which time frogs were eutha-nized and the brains and blood collected and processed for AVT immunohistochemistry and steroid hormone assay. We quantified AVT-immunoreactive (AVT-ir) cell bodies in the nucleus accumbens (NAcc), caudal striatum and amygda- la (AMG), anterior preoptic area, suprachiasmatic nucleus (SCN) and infundibular region of the ventral hypothalamus. Sex differences in AVT-ir cell number were observed in all brain regions except in the anterior preoptic area and ventral hypothalamus, with males having more AVT-ir cells than females in the NAcc, amygdala and SCN. Brain AVT was sensitive to melatonin signaling during the breeding season, and the effects of melatonin varied significantly with both region and sex. Treatment with melatonin decreased AVT immunoreactivity in both the NAcc and SCN in male H. cinerea. In contrast, brain AVT was relatively insensitive to melatonin signaling in females, indicating that the regulation of the AVT/AVP neuropeptide system by melatonin may be sexually dimorphic. Finally, melatonin did not significantly influence testosterone or estradiol concentrations of male or female frogs, respectively, suggesting that the effects of melatonin on AVT immunoreactivity are independent of changes in gonadal sex steroid hormones. Collectively, our results indicate that the AVT/AVP neuronal system may be an important target for melatonin in facilitating seasonal changes in reproductive physiology and social behavior.

The effect of pinealectomy on plasma vasopressin response to isotonic, hypertonic and hypovolemic treatments in rats supplemented with L-thyroxine

The present study was conducted to determine the effects of basal, isotonic as well as hypertonic and hypovolemic treatments on fluid-electrolyte balance and plasma AVP levels in rats supplemented with L-thyroxine and pinealectomized L-thyroxine. The animals were initially separated into 4 groups: control (n = 24), L-thyroxine treated (n = 24); L-thyroxine + sham-pinealectomy (n = 24) treated and 4-L-thyroxine + pinealectomy (n = 24) treated. L-thyroxine was given for 4 weeks. At the end of the 4-week experimental period, the sub-groups were formed before decapitation, which were classified as unstimulated (n = 6), isotonic (n = 6), hypertonic (n = 6) and hypovolemic (n = 6) stimulation. Plasma AVP, total triiodothyronine (TT3) and total thyroxine (TT4) levels were examined in plasma by RIA. Hematocrit and osmolality levels were also determined. It was found that the TT3 and TT4 levels showed significant increases in L-thyroxine treated groups (P <0.001). Also, plasma AVP levels increased in the group subjected to L-thyroxine treatment. However, this increase was depicted to be significantly more prominent in L-thyroxine + pinealectomy treated group (P <0.001). The results of the present study indicate that L-thyroxine treatment increases the basal and stimulated AVP release, which became more significant in the pinealectomy plus L-thyroxine treatment group. Moreover, the results indicate that AVP response to hypertonic and hypovolemic stimulations does not undergo any change due to supplementation by L-thyroxine treatment and/or pinealectomy plus L-thyroxine.

Influences of sex and saline intake on diurnal changes in plasma melatonin and osmoregulatory hormones of Pekin ducks (Anas platyrhynchos)

Melatonin (MT) inhibits salt gland secretion of Pekin ducks [Ching, A.C.T., Hughes, M.R., Poon, A.M.S., Pang, S.F., 1999. Melatonin receptors and melatonin inhibition of duck salt gland secretion. Gen. Comp. Endocrinol. 116, 229-240]. The present study examined simultaneous diurnal changes in plasma concentrations of MT and four osmoregulatory hormones of Pekin ducks to assess the possible interactions among these hormones that might contribute to inhibition of extrarenal salt secretion. The study compared diurnal changes in freshwater (FW) and saline-acclimated (SA) male and female ducks. We hypothesized plasma concentrations of osmoregulatory hormones: (1) differ between sexes (partially accepted); (2) vary diurnally (accepted, influenced by sex and treatment); (3) are correlated with MT (partially accepted); and their diurnal variation is affected by (4) sex (partially accepted); and (5) saline acclimation (partially accepted). We compared body mass, plasma osmolality (Osm(pl)), plasma volume, concentrations of plasma ions, and arginine vasotocin (AVT), angiotensin II (AII), prolactin, (PRL), and aldosterone (ALDO), in relation to plasma melatonin (MT). At night body mass increased in FW ducks and decreased in SA ducks. This likely reflects the higher plasma PRL concentration of female ducks and its middark increase only in SA ducks. Plasma volume increased at night in all but FW males. Plasma sodium and osmolality were higher in SA ducks and did not vary diurnally in either FW or SA ducks. Midday MT did not differ between males and females or between FW and SA ducks; at middark, all ducks, except FW females, increased MT. Midday AVT was higher in SA ducks. Only FW ducks increased AVT at middark. Changes in AVT and MT were positively correlated in males and negatively correlated in females. Males had higher AII and, at night, AII increased in SA ducks and decreased in FW ducks. AVT and AII were negatively correlated. ALDO was highly correlated with AII and unaffected by sex, saline, or time. At night, only AVT was elevated in FW ducks, while MT, AII, and PRL were elevated in SA ducks. FW and SA ducks appear to follow different, but equally effective, hormonal strategies to maintain osmotic homeostasis at night. We conclude that the effect of MT on salt gland secretion is probably a direct effect. These may be influenced by gender.

Signal transmission from the suprachiasmatic nucleus to the pineal gland via the paraventricular nucleus: analysed from arg-vasopressin peptide, rPer2 mRNA and AVP mRNA changes and pineal AA-NAT mRNA after the melatonin injection during light and dark periods

Arg-vasopressin (AVP) containing neurons are one of the output paths from the suprachiasmatic nucleus (SCN), the center of the biological clock. AVP mRNA transcription is controlled by a negative feedback loop of clock genes. Circadian rhythm of melatonin release from the pineal gland is regulated by the SCN via the paraventricular nucleus (PVN). To clarify the transduction system of circadian signals from the SCN to the pineal gland, we determined the effects of melatonin injection (1 mg/kg, i.p.) during light and dark periods on Per2 and AVP mRNAs in the SCN and PVN, in addition to arylalkylamine N-acetyltransferase (AA-NAT) and inducible cAMP early repressor (ICER) mRNAs in the pineal gland of rats using RT-PCR. AVP peptide contents were also measured in the SCN and PVN. AVP content in the SCN decreased during the light period, while no changes were observed in the PVN. In the SCN, Per2 mRNA increased during both light and dark periods. In the PVN, Per2 decreased during the light period and increased during the dark period at 180 min after melatonin injection. In the pineal gland, Per2 mRNA increased between 60 and 180 min after the melatonin injection during the light period, while it did not significantly change during the dark period. The AA-NAT mRNA varied similar to the Per2 mRNA changes. These results might suggest that the different responses to melatonin in the pineal gland during the light and dark periods was originated in the changes of Per2 in the PVN via SCN.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Methylphenidate-induced motor activity in rats: modulation by melatonin and vasopressin

Methylphenidate (MPH), a dopamine (DA) reuptake inhibitor, is well known to enhance motor activity, in part depending on the time of its application during the light-dark cycle. Moreover, after MPH administration, the hypothalamo-neurohypophysial axis including the neuropeptide vasopressin (AVP) was found influenced. Both the latter and behavioural effects of central AVP can also be modulated by the pineal gland with its light-dark-dependent activity. The present study was performed to investigate whether the pineal gland, its hormone melatonin (Mel), and AVP are involved in the MPH-evoked stimulation of activity. After application of 10 mg/kg MPH, the motor activity in pinealectomised (PE) rats was significantly higher than in sham-operated (SO) animals. After application of 250 microg Mel before MPH treatment, the stimulation of motor activity was diminished in PE rats and augmented in SO animals; however, when SO and PE rats were compared after Mel pretreatment, the reaction to MPH was nearly identical. Blocking the endogenous AVP by 25 or 1 microg of the V1a receptor antagonist d(CH(2))(5)[Tyr(Me)(2)]AVP (AAVP) before MPH treatment significantly augmented the motor activity in SO rats only and abolished the differences seen between SO and PE animals after MPH application. The present results indicate that the behavioural stimulation of MPH was modulated by both the pineal gland with its hormone Mel as well as the neuropeptide AVP.

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

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

Effects of melatonin injection on running-wheel activity and body temperature differ by the time of day

Melatonin, which is released from the pineal gland, influences many physiological events concerned with circadian rhythm. Body temperature levels and rhythmicity are tightly coupled with locomotor activity. To understand the functions of melatonin, we determined the effects of melatonin injection on locomotor activity measured by running-wheel activity and body temperature in rats. The rats were kept under a 12-h light and 12-h dark lighting condition, with the light on at zeitgeber time 0 (ZT 0, correspond to 7:00 JST). Melatonin injection, between ZT 3 and ZT 5 (light period) and between ZT 15 and ZT 17 (dark period), attenuated the wheel-running activity in a dose-dependent manner (10 microg to 1 mg/100 g body weight [bw]). A significant attenuation of activity by melatonin was recognized when injected at ZT 8, ZT 14 and ZT 20. After the injection of melatonin, the animal's body temperature was elevated at ZT 2 and ZT 8 (during light), while it fell at ZT 14 and ZT 20 (during darkness). We propose a plausible explanation underlying the observed changes in body temperature during the light and dark periods accompanying the suppression of activity induced by melatonin.

Circadian rhythm of melatonin release in pineal gland culture: arg-vasopressin inhibits melatonin release

The mammalian pineal gland is known to receive a noradrenergic sympathetic efferent signal from the suprachiasmatic nucleus (SCN) via the superior cervical ganglion. Arg-vasopressin (AVP) containing neurons in the SCN is one of the output paths of circadian information to the other brain areas. AVP release from the SCN is suppressed by melatonin. In turn, we determined the direct effect of AVP on melatonin release using pineal gland explant culture. AVP (1 microM) suppressed melatonin release. Noradrenaline stimulated melatonin release was attenuated by AVP. In turn, the expression of the melatonin synthesis enzyme arylalkylamine N-acetyltransferase mRNA in the rat SCN was reported. We measured melatonin content in the SCN in rats kept under the light-dark cycle and constant dim light. Melatonin in the SCN was higher during the dark period than that in the light. A similar tendency was also observed in the SCN of animals kept under a constant dim light. It was suggested that the reciprocal regulation of melatonin release and AVP release occurs in the SCN and pineal gland.