Melatonin, the neurohormone of the pineal gland, is also produced by various other tissues and cells. It acts via G protein-coupled receptors expressed in various areas of the central nervous system and in peripheral tissues. Parallel signaling mechanisms lead to cell-specific control and recruitment of downstream factors, including various kinases, transcription factors and ion channels. Additional actions via nuclear receptors and other binding sites are likely. By virtue of high receptor density in the circadian pacemaker, melatonin is involved in the phasing of circadian rhythms and sleep promotion. Additionally, it exerts effects on peripheral oscillators, including phase coupling of parallel cellular clocks based on alternate use of core oscillator proteins. Direct central and peripheral actions concern the up- or downregulation of various proteins, among which inducible and neuronal NO synthases seem to be of particular importance for antagonizing inflammation and excitotoxicity. The methoxyindole is also synthesized in several peripheral tissues, so that the total content of tissue melatonin exceeds by far the amounts in the circulation. Emerging fields in melatonin research concern receptor polymorphism in relation to various diseases, the control of sleep, the metabolic syndrome, weight control, diabetes type 2 and insulin resistance, and mitochondrial effects. Control of electron flux, prevention of bottlenecks in the respiratory chain and electron leakage contribute to the avoidance of damage by free radicals and seem to be important in neuroprotection, inflammatory diseases and, presumably, aging. Newly discovered influences on sirtuins and downstream factors indicate that melatonin has a role in mitochondrial biogenesis.

Melatonin is an immune modulator that displays both pro‐ and anti‐inflammatory properties. Proinflammatory actions, which are well documented by many studies in isolated cells or leukocyte‐derived cell lines, can be assumed to enhance the resistance against pathogens. However, they can be detrimental in autoimmune diseases. Anti‐inflammatory actions are of particular medicinal interest, because they are observed in high‐grade inflammation such as sepsis, ischemia/reperfusion, and brain injury, and also in low‐grade inflammation during aging and in neurodegenerative diseases. The mechanisms contributing to anti‐inflammatory effects are manifold and comprise various pathways of secondary signaling. These include numerous antioxidant effects, downregulation of inducible and inhibition of neuronal NO synthases, downregulation of cyclooxygenase‐2, inhibition of high‐mobility group box‐1 signaling and toll‐like receptor‐4 activation, prevention of inflammasome NLRP3 activation, inhibition of NF‐κB activation and upregulation of nuclear factor erythroid 2‐related factor 2 (Nrf2). These effects are also reflected by downregulation of proinflammatory and upregulation of anti‐inflammatory cytokines. Proinflammatory actions of amyloid‐β peptides are reduced by enhancing α‐secretase and inhibition of β‐ and γ‐secretases. A particular role in melatonin's actions seems to be associated with the upregulation of sirtuin‐1 (SIRT1), which shares various effects known from melatonin and additionally interferes with the signaling by the mechanistic target of rapamycin (mTOR) and Notch, and reduces the expression of the proinflammatory lncRNA‐CCL2. The conclusion on a partial mediation by SIRT1 is supported by repeatedly observed inhibitions of melatonin effects by sirtuin inhibitors or knockdown.

Abstract: Kynuramines represent their own class of biogenic amines. They are formed either by decarboxylation of kynurenines or pyrrole ring cleavage of indoleamines.N²‐formylated compounds formed in this last reaction can be deformylated either enzymatically by arylamine formamidases or hemoperoxidases, or photochemically. The earlier literature mainly focussed on cardiovascular effects of kynuramine, 5‐hydroxykynuramine and theirN¹,N¹‐dimethylated analogs, including indirect effects via release of catecholamines or acetylcholine and interference with serotonin receptors. After the discovery ofN¹‐acetyl‐N²‐formyl‐5‐methoxykynuramine (AFMK) andN¹‐acetyl‐5‐methoxykynuramine (AMK) as major brain metabolites of melatonin, these compounds became of particular interest. They were shown to be produced enzymatically, pseudoenzymatically, by various free radical‐mediated and via photochemical processes. In recent years, AFMK and AMK were shown to scavenge reactive oxygen and nitrogen species, thereby forming several newly discovered 3‐indolinone, cinnolinone and quinazoline compounds, and to protect tissues from damage by reactive intermediates in various models. AMK is of special interest due to its properties as a potent cyclooxygenase inhibitor, NO scavenger forming a stable nitrosation product, inhibitor and/or downregulator of neuronal and inducible NO synthases, and a mitochondrial metabolism modulator. AMK easily interacts with aromates, forms adducts with tyrosyl and tryptophanyl residues, and may modify proteins.

Abstract:  Evidence is accumulating regarding the importance of circadian core oscillators, several associated factors, and melatonin signaling in the maintenance of health. Dysfunction of endogenous clocks, melatonin receptor polymorphisms, age‐ and disease‐associated declines of melatonin likely contribute to numerous diseases including cancer, metabolic syndrome, diabetes type 2, hypertension, and several mood and cognitive disorders. Consequences of gene silencing, overexpression, gene polymorphisms, and deviant expression levels in diseases are summarized. The circadian system is a complex network of central and peripheral oscillators, some of them being relatively independent of the pacemaker, the suprachiasmatic nucleus. Actions of melatonin on peripheral oscillators are poorly understood. Various lines of evidence indicate that these clocks are also influenced or phase‐reset by melatonin. This includes phase differences of core oscillator gene expression under impaired melatonin signaling, effects of melatonin and melatonin receptor knockouts on oscillator mRNAs or proteins. Cross‐connections between melatonin signaling pathways and oscillator proteins, including associated factors, are discussed in this review. The high complexity of the multioscillator system comprises alternate or parallel oscillators based on orthologs and paralogs of the core components and a high number of associated factors with varying tissue‐specific importance, which offers numerous possibilities for interactions with melatonin. It is an aim of this review to stimulate research on melatonin signaling in peripheral tissues. This should not be restricted to primary signal molecules but rather include various secondarily connected pathways and discriminate between direct effects of the pineal indoleamine at the target organ and others mediated by modulation of oscillators.