Melatonin and jet lag syndrome: experimental model and clinical implications

Inflammation-induced depression: Its pathophysiology and therapeutic implications

Inflammation is not the only cause of depression and cannot explain its entire pathophysiology, but it is an important pathogenic factor that explains one possible mechanism of depression, with the kynurenine (KYN) pathway of tryptophan at its center. In particular, greater impairment seems to exist in the KYN pathway in inflammation-induced depression related to immunotherapy, autoimmune disease, and infection. In patients with these conditions, immunopharmacology is likely to be an important therapy. To develop this therapy, clear evidence of the immune-KYN pathway must be established via multiple types of experiments. This paper reviews the body of evidence, not only for the action of tryptophan (TRY) and consequent serotonin depletion, but also for the detrimental effects of TRY catabolites and the key enzymes in the KYN pathway that play important roles in the pathophysiology of inflammation-induced depression. In addition, this paper explores a potential treatment strategy for inflammation-induced depression using KYN metabolism.

Stable isotope dilution ultra-high performance liquid chromatography-tandem mass spectrometry quantitative profiling of tryptophan-related neuroactive substances in human serum and cerebrospinal fluid

Many compounds related to L-tryptophan (L-TRP) have interesting biological or pharmacological activity, and their abnormal neurotransmission seems to be linked to a wide range of neurodegenerative and psychiatric diseases. A high-throughput method based on ultra-high performance liquid chromatography connected to electrospray tandem mass spectrometry (UHPLC-ESI-MS/MS) was developed for the quantitative analysis of L-TRP and 16 of its metabolites in human serum and cerebrospinal fluid (CSF), representing both major and minor routes of L-TRP catabolism. The combination of a fast LC gradient with selective tandem mass spectrometry enabled accurate analysis of almost 100 samples in 24h. The standard isotope dilution method was used for quantitative determination. The method's lower limits of quantification for serum and cerebrospinal fluid ranged from 0.05 to 15nmol/L and 0.3 to 45nmol/L, respectively. Analytical recoveries ranged from 10.4 to 218.1% for serum and 22.1 to 370.0% for CSF. The method's accuracy ranged from 82.4 to 128.5% for serum matrix and 90.7 to 127.7% for CSF matrix. All intra- and inter-day coefficients of variation were below 15%. These results demonstrate that the new method is capable of quantifying endogenous serum and CSF levels of a heterogeneous group of compounds spanning a wide range of concentrations. The method was used to determine the physiological levels of target analytes in serum and CSF samples from 18 individuals, demonstrating its reliability and potential usefulness in large-scale epidemiological studies.

Insulin resistance and dysregulation of tryptophan-kynurenine and kynurenine-nicotinamide adenine dinucleotide metabolic pathways

Insulin resistance (IR) underlines aging and aging-associated medical (diabetes, obesity, dyslipidemia, hypertension) and psychiatric (depression, cognitive decline) disorders. Molecular mechanisms of IR in genetically or metabolically predisposed individuals remain uncertain. Current review of the literature and our data presents the evidences that dysregulation of tryptophan (TRP)-kynurenine (KYN) and KYN-nicotinamide adenine dinucleotide (NAD) metabolic pathways is one of the mechanisms of IR. The first and rate-limiting step of TRP-KYN pathway is regulated by enzymes inducible by pro-inflammatory factors and/or stress hormones. The key enzymes of KYN-NAD pathway require pyridoxal-5-phosphate (P5P), an active form of vitamin B6, as a cofactor. Deficiency of P5P diverts KYN-NAD metabolism from production of NAD to the excessive formation of xanthurenic acid (XA). Human and experimental studies suggested that XA and some other KYN metabolites might impair production, release, and biological activity of insulin. We propose that one of the mechanisms of IR is inflammation- and/or stress-induced upregulation of TRP-KYN metabolism in combination with P5P deficiency-induced diversion of KYN-NAD metabolism towards formation of XA and other KYN derivatives affecting insulin activity. Monitoring of KYN/P5P status and formation of XA might help to identify subjects at risk for IR. Pharmacological regulation of the TRP-KYN and KYN-NAD pathways and maintaining of adequate vitamin B6 status might contribute to prevention and treatment of IR in conditions associated with inflammation/stress-induced excessive production of KYN and deficiency of vitamin B6, e.g., type 2 diabetes, obesity, cardiovascular diseases, aging, menopause, pregnancy, and hepatitis C virus infection.

The roles of melatonin and light in the pathophysiology and treatment of circadian rhythm sleep disorders

Normal circadian rhythms are synchronized to a regular 24 h environmental light-dark cycle, and the suprachiasmatic nucleus and the hormone melatonin have important roles in this process. Desynchronization of circadian rhythms, as occurs in chronobiological disorders, can produce severe disturbances in sleep patterns. According to the International Classification of Sleep Disorders, circadian rhythm sleep disorders (CRSDs) include delayed sleep phase syndrome, advanced sleep phase syndrome, non-24 h sleep-wake disorder, jet lag and shift-work sleep disorder. Disturbances in the circadian phase position of plasma melatonin levels have been documented in all of these disorders. There is compelling evidence to implicate endogenous melatonin as an important mediator in CRSD pathophysiology, although further research involving large numbers of patients will be required to clarify whether the disruption of melatonin secretion is a causal factor in CRSDs. In this Review, we focus on the use of exogenous melatonin and light therapy to treat the disturbed sleep-wake rhythms seen in CRSDs.

Effect of methylene blue and related redox dyes on monoamine oxidase activity; rat pineal content of N-acetylserotonin, melatonin, and related indoles; and righting reflex in melatonin-primed frogs

The ability of methylene blue (MB) to inhibit the nitric oxide-induced stimulation of N-methyl-D-aspartate receptors has been suggested as a possible mechanism of MB's clinical antidepressant action. This study evaluated the alternative/additional mechanisms of the antidepressant effect of MB on biochemical and behavior levels. Selective inhibition of monoamine oxidase type A (MAO-A) is widely accepted as a major mechanism of the clinical antidepressant effect. MB and the related redox dyes toluidine blue O (TBO), thionine (TN), brilliant cresyl blue (BCB), and toluylene blue (TB) were reversible competitive inhibitors of both MAO-A and MAO-B and were highly selective toward MAO-A. TBO was the most potent inhibitor, followed by TN, BCB, MB, and TB. The dyes studied increased rat pineal N-acetylserotonin (NAS) and melatonin content, in accordance with our previous observations of the stimulating effect of selective inhibition of MAO-A on pineal melatonin biosynthesis. The redox dyes exerted antidepressant-like activity in frogs; that is, they suppressed the righting reflex in melatonin-primed frogs. This study's results indicate that selective inhibition of MAO-A might mediate the clinical antidepressant effect of MB through NAS stimulation and melatonin biosynthesis.

Role of the melatonin system in the control of sleep: therapeutic implications

The circadian rhythm of pineal melatonin secretion, which is controlled by the suprachiasmatic nucleus (SCN), is reflective of mechanisms that are involved in the control of the sleep/wake cycle. Melatonin can influence sleep-promoting and sleep/wake rhythm-regulating actions through the specific activation of MT(1) (melatonin 1a) and MT(2) (melatonin 1b) receptors, the two major melatonin receptor subtypes found in mammals. Both receptors are highly concentrated in the SCN. In diurnal animals, exogenous melatonin induces sleep over a wide range of doses. In healthy humans, melatonin also induces sleep, although its maximum hypnotic effectiveness, as shown by studies of the timing of dose administration, is influenced by the circadian phase. In both young and elderly individuals with primary insomnia, nocturnal plasma melatonin levels tend to be lower than those in healthy controls. There are data indicating that, in affected individuals, melatonin therapy may be beneficial for ameliorating insomnia symptoms. Melatonin has been successfully used to treat insomnia in children with attention-deficit hyperactivity disorder or autism, as well as in other neurodevelopmental disorders in which sleep disturbance is commonly reported. In circadian rhythm sleep disorders, such as delayed sleep-phase syndrome, melatonin can significantly advance the phase of the sleep/wake rhythm. Similarly, among shift workers or individuals experiencing jet lag, melatonin is beneficial for promoting adjustment to work schedules and improving sleep quality. The hypnotic and rhythm-regulating properties of melatonin and its agonists (ramelteon, agomelatine) make them an important addition to the armamentarium of drugs for treating primary and secondary insomnia and circadian rhythm sleep disorders.

Sildenafil accelerates reentrainment of circadian rhythms after advancing light schedules

Mammalian circadian rhythms are generated by a master clock located in the suprachiasmatic nuclei and entrained by light-activated signaling pathways. In hamsters, the mechanism responsible for light-induced phase advances involves the activation of guanylyl cyclase, cGMP and its related kinase (PKG). It is not completely known whether interference with this pathway affects entrainment of the clock, including adaptation to changing light schedules. Here we report that cGMP-specific phosphodiesterase 5 is present in the hamster suprachiasmatic nuclei, and administration of the inhibitor sildenafil (3.5 mg/kg, i.p.) enhances circadian responses to light and decreases the amount of time necessary for reentrainment after phase advances of the light-dark cycle. These results suggest that sildenafil may be useful for treatment of circadian adaptation to environmental changes, including transmeridian eastbound flight schedules.

Melatonin, a potent agent in antioxidative defense: actions as a natural food constituent, gastrointestinal factor, drug and prodrug

Melatonin, originally discovered as a hormone of the pineal gland, is also produced in other organs and represents, additionally, a normal food constituent found in yeast and plant material, which can influence the level in the circulation. Compared to the pineal, the gastrointestinal tract contains several hundred times more melatonin, which can be released into the blood in response to food intake and stimuli by nutrients, especially tryptophan. Apart from its use as a commercial food additive, supraphysiological doses have been applied in medical trials and pure preparations are well tolerated by patients. Owing to its amphiphilicity, melatonin can enter any body fluid, cell or cell compartment. Its properties as an antioxidant agent are based on several, highly diverse effects. Apart from direct radical scavenging, it plays a role in upregulation of antioxidant and downregulation of prooxidant enzymes, and damage by free radicals can be reduced by its antiexcitatory actions, and presumably by contributions to appropriate internal circadian phasing, and by its improvement of mitochondrial metabolism, in terms of avoiding electron leakage and enhancing complex I and complex IV activities. Melatonin was shown to potentiate effects of other antioxidants, such as ascorbate and Trolox. Under physiological conditions, direct radical scavenging may only contribute to a minor extent to overall radical detoxification, although melatonin can eliminate several of them in scavenger cascades and potentiates the efficacy of antioxidant vitamins. Melatonin oxidation seems rather important for the production of other biologically active metabolites such as N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK) and N¹-acetyl-5-methoxykynuramine (AMK), which have been shown to also dispose of protective properties. Thus, melatonin may be regarded as a prodrug, too. AMK interacts with reactive oxygen and nitrogen species, conveys protection to mitochondria, inhibits and downregulates cyclooxygenase 2.

Debunking a myth: neurohormonal and vagal modulation of sleep centers, not redistribution of blood flow, may account for postprandial somnolence

It is widely believed that postprandial somnolence is caused by redistribution of blood flow from cerebral to mesenteric vessels after a meal. This belief persists despite its apparent contradiction with a well-known neurophysiologic principle that cerebral perfusion is preferentially maintained under a wide range of physiologic states. For instance, during exercise when a large amount of perfusion is diverted to muscles, blood flow to the brain is maintained. Furthermore, recent evidence suggests that there is no measurable change of blood flow in the common carotid artery during postprandial states. We propose an alternative hypothesis that postprandial release of gut-brain hormones and activation of vagal afferents may play a role in postprandial somnolence through modulation of sleep centers such as the hypothalamus. Feeding alters the milieu of hormones such as melatonin and orexins and also promotes central vagal activation. Emerging evidence suggest that these pathways are also modulators of neural sleep centers. Potential adaptive explanations of postprandial somnolence are explored from a Darwinian perspective.