Melatonin biosynthesis in the thymus of humans and rats

Melatonin is an indoleamine widely distributed in the evolution that shows a great functional versatility, playing an important role as a transmitter of photoperiodic information and exhibiting antioxidant, oncostatic, anti-aging and immunomodulatory properties. In vertebrates, this molecule is produced by the pineal gland and other extrapineal sites. The present study was carried out to investigate the presence of melatonin in thymus and the possibility of an endogenous melatonin synthesis in this organ, in which T cells are matured. In this work, we demonstrate in humans and rats that thymus contains melatonin, expresses the mRNAs encoding N-acetyltransferase and hydroxyindol-O-methyltransferase, the two key enzymes of the melatonin synthesis, and has this biosynthetic machinery activated. In addition, rat thymocytes cultured for 24 h exhibited high levels of melatonin. The results presented here suggest that human and rat thymuses are able to synthesize melatonin, which could have intracrine, autocrine and paracrine functions. Received 30 September 2006; received after revision 30 December 2006; accepted 15 February 2007     

The contribution of extrapineal sites of melatonin synthesis to circulating melatonin levels in higher vertebrates

While the production of melatonin in higher vertebrates occurs in other organs and tissues besides the pineal, the contribution of extrapineal sites of melatonin synthesis such as the retina, the Harderian glands and the gut to circulating melatonin levels is still a matter of debate. The amount of melatonin found in the gastrointestinal tract is much higher than in any other organ including the pineal and the gut appears to make a significant contribution to circulating melatonin at least under certain conditions. The gut has been identified to be the major source of the elevated plasma concentrations of melatonin seen after tryptophan administration and of the changes of circulating melatonin level induced by the feeding regime. Whereas the circadian and circannual fluctuations of the concentration of melatonin in the blood seem to be triggered by changes of the photoenvironment and its effect of pineal melatonin formation, basal daytime melatonin levels and the extent of their elevation at nighttime appear to be additionally controlled by nutritional factors, such as the amount and the composition of ingested food and therefore availability of tryptophan as a rate-limiting precursor of melatonin formation by the enterochromaffin cells of the gastrointestinal tract.     

Melatonin,clock genes and mitochondria in sepsis

After the characterization of the central pacemaker in the suprachiasmatic nucleus, the expression of clock genes was identified in several peripheral tissues including the immune system. The hierarchical control from the central clock to peripheral clocks extends to other functions including endocrine, metabolic, immune, and mitochondrial responses. Increasing evidence links the disruption of the clock genes expression with multiple diseases and aging. Chronodisruption is associated with alterations of the immune system, immunosenescence, impairment of energy metabolism, and reduction of pineal and extrapineal melatonin production. Regarding sepsis, a condition coursing with an exaggerated response of innate immunity, experimental and clinical data showed an alteration of circadian rhythms that reflects the loss of the normal oscillation of the clock. Moreover, recent data point to that some mediators of the immune system affects the normal function of the clock. Under specific conditions, this control disappears reactivating the immune response. So, it seems that clock gene disruption favors the innate immune response, which in turn induces the expression of proinflammatory mediators, causing a further alteration of the clock. Here, the clock control of the mitochondrial function turns off, leading to a bioenergetic decay and formation of reactive oxygen species that, in turn, activate the inflammasome. This arm of the innate immunity is responsible for the huge increase of interleukin-1β and entrance into a vicious cycle that could lead to the death of the patient. The broken clock is recovered by melatonin administration, that is accompanied by the normalization of the innate immunity and mitochondrial homeostasis. Thus, this review emphasizes the connection between clock genes, innate immunity and mitochondria in health and sepsis, and the role of melatonin to maintain clock homeostasis.     

Melatonin,mitochondria, and the skin

The skin being a protective barrier between external and internal (body) environments has the sensory and adaptive capacity to maintain local and global body homeostasis in response to noxious factors. An important part of the skin response to stress is its ability for melatonin synthesis and subsequent metabolism through the indolic and kynuric pathways. Indeed, melatonin and its metabolites have emerged as indispensable for physiological skin functions and for effective protection of a cutaneous homeostasis from hostile environmental factors. Moreover, they attenuate the pathological processes including carcinogenesis and other hyperproliferative/inflammatory conditions. Interestingly, mitochondria appear to be a central hub of melatonin metabolism in the skin cells. Furthermore, substantial evidence has accumulated on the protective role of the melatonin against ultraviolet radiation and the attendant mitochondrial dysfunction. Melatonin and its metabolites appear to have a modulatory impact on mitochondrion redox and bioenergetic homeostasis, as well as the anti-apoptotic effects. Of note, some metabolites exhibit even greater impact than melatonin alone. Herein, we emphasize that melatonin–mitochondria axis would control integumental functions designed to protect local and perhaps global homeostasis. Given the phylogenetic origin and primordial actions of melatonin, we propose that the melatonin-related mitochondrial functions represent an evolutionary conserved mechanism involved in cellular adaptive response to skin injury and repair.     

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.     

Melatonin,mitochondria and hypertension

Melatonin, due to its multiple means and mechanisms of action, plays a fundamental role in the regulation of the organismal physiology by fine tunning several functions. The cardiovascular system is an important site of action as melatonin regulates blood pressure both by central and peripheral interventions, in addition to its relation with the renin–angiotensin system. Besides, the systemic management of several processes, melatonin acts on mitochondria regulation to maintain a healthy cardiovascular system. Hypertension affects target organs in different ways and cellular energy metabolism is frequently involved due to mitochondrial alterations that include a rise in reactive oxygen species production and an ATP synthesis decrease. The discussion that follows shows the role played by melatonin in the regulation of mitochondrial physiology in several levels of the cardiovascular system, including brain, heart, kidney, blood vessels and, particularly, regulating the renin–angiotensin system. This discussion shows the putative importance of using melatonin as a therapeutic tool involving its antioxidant potential and its action on mitochondrial physiology in the cardiovascular system.     

Increased melatonin levels after hemorrhagic shock in male and female C3H/HeN mice

Although hemorrhagic shock leads to significant alterations of several hormones, e.g. ACTH, corticosterone and -endorphin, it is not known whether plasma melatonin levels are affected under this condition and if so, whether the effects are comparable in males and females. Using a radioimmunoassay, it was found that plasma melatonin levels were significantly increased in male and proestrus female C3H/HeN mice immediately after hemorrhagic shock. However, in male mice, by two hours after hemorrhage and resuscitation, plasma melatonin returned to levels comparable to those seen in control and sham-operated animals. Proestrus female mice, on the other hand, showed significantly increased plasma melatonin levels at two hours after surgery when compared to unoperated control animals. Although the significance and biological role of the transient increased plasma melatonin levels after hemorrhagic shock remain to be determined, it appears that the pineal gland and/or an extrapineal source of melatonin, of both male and proestrus female mice responds to severe hypotension by increased release of melatonin.     

Development of rhythmic melatonin synthesis in cultured pineal glands and pineal cells isolated from chick embryo

The chick pineal gland exhibits circadian rhythms in melatonin synthesis under in vivo and in vitro conditions. A daily rhythm of melatonin production was first detectable in pineal glands isolated from chick embryos at embryonic day 16 and incubated under a LD cycle. All pineal glands isolated from 17-day-old and older embryos were rhythmic while no gland isolated at embryonic day 14 and 15 exhibited a daily rhythm in melatonin synthesis. Melatonin production in static cultures of embryonic pineal cells was rhythmic over 48 h if the cells were kept under a LD cycle. When embryonic pineal cells were incubated in constant darkness the rhythm in melatonin production was damped within 48 h. These results suggest that chick pineal cells from embryonic day 16 onwards are photosensitive but that the endogenous component of the melatonin rhythm is not completely developed at that age. A soluble analogue of cAMP stimulated and norepinephrine inhibited melatonin synthesis in cultured embryonic pineal cells. These findings indicate that the stimulatory and inhibitory pathways controlling melatonin synthesis in the mature pineal gland are effective in pineal cells isolated from chick embryos at least 2 days before hatching.     

Melatonin and cell death: differential actions on apoptosis in normal and cancer cells

Melatonin is a natural compound synthesized by a variety of organs. It has been shown to function as a cell-protective agent. Since 1994, when the first paper was published documenting the role of melatonin in apoptosis, the number of reports in this area has increased rapidly. Much of the research conducted falls into three major categories: first, the role of melatonin in inhibiting apoptosis in immune cells; second, the role of melatonin in preventing neuronal apoptosis and finally, the role of melatonin in increasing apoptotic cell death in cancer cells. The mechanisms whereby melatonin influences apoptosis have not clarified, although a number of mechanistic options have been suggested. Apoptotic cell death is a physiological phenomenon related to homeostasis and proper functioning of tissues and organs; however, a failure in the apoptotic program is related to a number of diseases. The participation of melatonin in apoptosis in numerous cell types and its potential importance in a variety of diseases such as immunodeficiency, neurodegeneration and cancer is summarized in this review.Received 14 November 2002; received after revision 16 January 2003; accepted 10 February 2003     

Melatonin, hormone of darkness and more – occurrence, control mechanisms, actions and bioactive metabolites

In its role as a pineal hormone, melatonin is a pleiotropic, nocturnally peaking and systemically acting chronobiotic. These effects are largely explained by actions via G protein-coupled membrane receptors found in the suprachiasmatic nucleus, but also in numerous other sites. Nuclear (ROR/RZR), cytoplasmic (quinone reductase-2, calmodulin, calreticulin) and mitochondrial binding sites and radical-scavenging properties contribute to the actions of melatonin. Regulation of pineal melatonin biosynthesis is largely explained by control mechanisms acting on arylalkylamine N-acetyltransferase, at the levels of gene expression and/or enzyme stability influenced by phosphorylation and interaction with 14-3-3 proteins. Melatonin is not only a hormone but is also synthesized in numerous extrapineal sites, in which it sometimes attains much higher quantities than in the pineal and the circulation. It is also present in many taxonomically distant groups of organisms, including bacteria, fungi, and plants. Moreover, melatonin is a source of bioactive metabolites, such as 5-methoxytryptamine, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine and N(1)-acetyl-5-methoxykynuramine.     

The melatonin rhythm: both a clock and a calendar

The paper briefly reviews the data which shows that the circadian production and secretion of melatonin by the pineal gland can impart both daily, i.e., clock, and seasonal, i.e., calendar, information to the organism. The paper summarizes the 3 patterns of nocturnal melatonin production that have been described. Clearly, regardless of the pattern of nocturnal melatonin production a particular species normally displays, the duration of nightime elevated melatonin is proportional to the duration of the night length. Since daylength under natural conditions changes daily the melatonin rhythm, which adjusts to the photoperiod sends time of year information to the organism. The melatonin receptors which subserve the clock message sent by the pineal gland in the form of a melatonin cycle may reside in the biological clock itself, namely, the suprachiasmatic nuclei (SCN). The melatonin receptors that mediate seasonal changes in reproductive physiology are presumably those that are located on the pars tuberalis cells of the anterior pituitary gland. Besides these receptors which likely mediate clock and calendar information, melatonin receptors have been described in other organs. Interestingly, the distribution of melatonin receptors is highly species-specific. Whereas the clock and calendar information that the melatonin cycle imparts to the organism relies on cell membrane receptors, a fact that is of some interest considering the high lipophilicity of melatonin, recent studies indicate that other functions of melatonin may require no receptor whatsoever.     

Melatonin biosynthesis in the mammalian pineal gland

Rhythmic production of melatonin by the mammalian pineal occurs in response to noradrenergic stimulation which produces a cascade of biochemical events within the pinealocyte. In the rat, massive changes in NAT activity result from an increase in intracellular c-AMP levels produced by a synergistic interaction whereby an alpha 1 activation amplifies beta-adrenergic stimulation. The intracellular events mediating this effect are described. A major aspect of the temporal control of melatonin production is the programmed down-regulation of responses to noradrenergic stimulation once the initial surge of c-AMP is produced. Noradrenergic activation of the gland also influences other enzymic functions, including tryptophan hydroxylase and HIOMT activities, and produces a dramatic increase in intracellular c-GMP levels. Other neurotransmitters and neuropeptides, e.g. VIP, may also influence pineal function and comparisons are made between the rat, the subject of the bulk of experimental studies, and other species.     

Melatonin receptors limit dopamine reuptake by regulating dopamine transporter cell-surface exposure

Melatonin, a neuro-hormone released by the pineal gland, has multiple effects in the central nervous system including the regulation of dopamine (DA) levels, but how melatonin accomplishes this task is not clear. Here, we show that melatonin MT₁ and MT₂ receptors co-immunoprecipitate with the DA transporter (DAT) in mouse striatal synaptosomes. Increased DA re-uptake and decreased amphetamine-induced locomotor activity were observed in the striatum of mice with targeted deletion of MT₁ or MT₂ receptors. In vitro experiments confirmed the interactions and recapitulated the inhibitory effect of melatonin receptors on DA re-uptake. Melatonin receptors retained DAT in the endoplasmic reticulum in its immature non-glycosylated form. In conclusion, we reveal one of the first molecular complexes between G protein-coupled receptors (MT₁ and MT₂) and transporters (DAT) in which melatonin receptors regulate the availability of DAT at the plasma membrane, thus limiting the striatal DA re-uptake capacity in mice.     

Melatonin and mitochondrial function during ischemia/reperfusion injury

Ischemia/reperfusion (IR) injury occurs in many organs and tissues, and contributes to morbidity and mortality worldwide. Melatonin, an endogenously produced indolamine, provides a strong defense against IR injury. Mitochondrion, an organelle for ATP production and a decider for cell fate, has been validated to be a crucial target for melatonin to exert its protection against IR injury. In this review, we first clarify the mechanisms underlying mitochondrial dysfunction during IR and melatonin’s protection of mitochondria under this condition. Thereafter, special focus is placed on the protective actions of melatonin against IR injury in brain, heart, liver, and others. Finally, we explore several potential future directions of research in this area. Collectively, the information compiled here will serve as a comprehensive reference for the actions of melatonin in IR injury identified to date and will hopefully aid in the design of future research and increase the potential of melatonin as a therapeutic agent.     

Relaxin: a pregnancy hormone as central player of body fluid and circulation homeostasis

The peptide relaxin has long been regarded as an important hormone of pregnancy, contributing to changes in connective tissue composition as well as to regulation of implantation, myometrial activity and labor. On the other hand, the astonishing pleiotropy of this hormone escaped scientific awareness. This review focuses on new facets of relaxin, including its antifibrotic effects, its role in the control of pituitary hormone release, its vasodilator and pro-angiogenic properties and its versatile myocardial actions. Recent progress in understanding relaxin's receptor and signaling mechanisms is also highlighted. The peptide will be characterized as potential regulator of body fluid and circulation homeostasis.     

A novel interplay between membrane and nuclear melatonin receptors in human lymphocytes: significance in IL-2 production

Human lymphocyte melatonin, through membrane and nuclear receptors binding, acts as an activator in IL-2 production. Antagonism of membrane melatonin receptors using luzindole exacerbates the drop of the IL-2 production induced by PGE₂ in peripheral blood mononuclear and Jurkat cells. This paper studies the melatonin membrane and nuclear receptors interplay in PGE₂-diminished IL-2 production. The decrease in IL-2 production after PGE₂ and/or luzindole administration correlated with downregulation in the nuclear receptor RORα. We also highlighted a role of cAMP in the pathway, because forskolin mimicked the effects of luzindole and/or PGE₂ in the RORα expression. Finally, a significant RORα downregulation was observed in T cells permanently transfected with inducible MT₁ antisense. In conclusion, we show a novel connection between melatonin membrane receptor signalling and RORα expression, opening a new way to understand melatonin regulation in lymphocyte physiology. Received 23 September 2008; received after revision 19 November 2008; accepted 21 November 2008     

Melatonin biosynthesis in the mammalian pineal gland

Summary Rhythmic production of melatonin by the mammalian pineal occurs in response to noradrenergic stimulation which produces a cascade of biochemical events within the pinealocyte. In the rat, massive changes in NAT activity result from an increase in intracellular c-AMP levels produced by a synergistic interaction whereby an ₁ activation amplifies -adrenergic stimulation. The intracellular events mediating this effect are described. A major aspect of the temporal control of melatonin production is the programmed down-regulation of responses to noradrenergic stimulation once the initial surge of c-AMP is produced. Noradrenergic activation of the gland also influences other enzymic functions, including tryptophan hydroxylase and HIOMT activities, and produces a dramatic increase in intracellular c-GMP levels. Other neurotransmitters and neuropeptides, e.g. VIP, may also influence pineal function and comparisons are, made between the rat, the subject of the bulk of experimental studies, and other species.     

Tryptophanhydroxylase im corpus pineale beim menschen

Summary Tryptophanhydroxylase has been demonstrated in the human pineal gland. The enzyme activity was 3–6 times greater than that of homogenates from cortical areas of human brain. The presence of tryptophan hydroxylase in the pineal gland implies that this organ is not dependent on the transport of 5-hydroxytryptophan but is able to synthesize this precursor of melatonin.This work was supported by grants from the Medical Faculty, University of Lund and the Swedish Cancer Society.