Effect of TNF-alpha on the melatonin synthetic pathway in the rat pineal gland: basis for a 'feedback' of the immune response on circadian timing

Partners in health and disease: pineal gland and purinergic signalling

In mammal's pineal glands, ATP interacts with the high-affinity P2Y1 and the low-affinity P2X7 receptors. ATP released from sympathetic nerve terminals potentiates noradrenaline-induced serotonin N-acetyltransferase (Snat) transcription, N-acetylserotonin (NAS), and melatonin (MLT) synthesis. Circulating melatonin impairs the expression of adhesion molecules in endothelial cells, blocking the migration of leukocytes. Acute defence response induced by pathogen- and danger/damage-associated molecular patterns (PAMPs and DAMPs) triggers the NF-κB pathway in pinealocytes and blocks the transcription of Snat. Therefore, the darkness hormone is not released, and neutrophils and monocytes migrate to the lesion sites. ATP released in high amounts from apoptotic and death cells was considered a DAMP, and the blockage of P2X7 receptors was tested as a new class of drugs for treating brain damage. However, this is not a simple equation. High ATP injected in a lateral ventricle blocked MLT, but not NAS, synthesis as it impairs the transcription of acetyl serotonin N-methyltransferase. NAS is released in the plasma and the cerebral spinal fluid. NAS also blocks the rolling and adhesion of leukocytes to endothelial cells. Otherwise, it is metabolised specifically in each brain area to provide the requested concentration of MLT as a neuroprotector. As observed in physiological conditions, high extracellular ATP, different from the other DAMPs, reports the environmental light/dark cycle rhythm because NAS substitutes MLT as the nocturnal chemical indicator, the darkness hormone. Thus, blocking the P2X7R should not be considered a universal therapy for improving acute strokes, as MLT and ATP are partners in health and disease.

Systemic administration of propranolol reduces bone resorption and inflammation in apical periodontitis of chronically stressed rats

AIM

To evaluate the effect of systemic administration of propranolol on the severity of apical periodontitis (AP) in chronically stressed rats.

METHODOLOGY

Twenty-four 70-day-old male Wistar rats (Rattus norvegicus, albinus) were distributed into three groups (n = 8): rats with AP without stressful conditions (AP-Control), rats with AP and submitted to a chronic unpredictable stress (CUS) protocol (AP + S) and rats with AP and submitted to a CUS protocol treated with propranolol (AP + S + PRO). Stress procedures were applied daily until the end of the experiment. After 3 weeks of CUS, AP was induced in all groups by exposing the pulpal tissue of mandibular and maxillary first molars to the oral environment. Propranolol treatment was administered orally once a day for the entire period of the experiment. Rats were sacrificed at 42 days, and the blood was collected for stress biomarkers serum dosage by multiplex assay. Mandibles were removed and submitted to microtomography and histopathological analyses. Periapical tissue surrounding the upper first molar was homogenized and subjected to RT-PCR analysis to evaluate the mRNA expression of RANKL, TRAP and OPG. Parametric data were assessed using one-way ANOVA followed by Tukey's test while the nonparametric data were analysed by the Kruskal-Wallis followed by Dunn's test. Significance level was set at 5% (p < .05) for all assessed parameters.

RESULTS

Micro-CT revealed statistically significant differences in bone resorption which was greater in the AP + S group (p < .05), but no differences were observed between the Control and AP + S + PRO groups (p > .05). The AP + S + PRO group had a lower intensity and extent of inflammatory infiltrate compared to the AP + S group with smaller areas of bone loss (p < 0.05). The gene expression of RANKL and TRAP was significantly higher in the stressed group AP + S compared to the control group (p < .05), and a significantly higher OPG expression was observed in AP + S + PRO compared to the AP + S group (p < .05).

CONCLUSIONS

Oral administration of propranolol had a significant effect on the AP severity in stressed rats, suggesting an anti-inflammatory effect and a protective role on bone resorption of AP in stressed animals. Further research is necessary to fully comprehend the underlying mechanisms.

Current Aspects of Selected Factors to Modulate Brain Health and Sports Performance in Athletes

This review offers a comprehensive evaluation of current aspects related to nutritional strategies, brain modulation, and muscle recovery, focusing on their applications and the underlying mechanisms of physiological adaptation for promoting a healthy brain, not only in athletes but also for recreationally active and inactive individuals. We propose that applying the rule, among others, of good sleep, regular exercise, and a properly balanced diet, defined as "SPARKS", will have a beneficial effect on the function and regeneration processes of the gut-brain-muscle axis. However, adopting the formula, among others, of poor sleep, stress, overtraining, and dysbiosis, defined as "SMOULDER", will have a detrimental impact on the function of this axis and consequently on human health as well as on athletes. Understanding these dynamics is crucial for optimizing brain health and cognitive function. This review highlights the significance of these factors for overall well-being, suggesting that adopting the "SPARKS" approach may benefit not only athletes but also older adults and individuals with health conditions.

Immune and endocrine alterations at the early stage of inflammatory assemblage in toads after stimulation with heat-killed bacteria (Aeromonas hydrophila)

The red-leg syndrome in amphibians is a condition commonly associated with the bacteria Aeromonas hydrophila and has led to population declines. However, there is little information concerning the inflammatory assemblage in infected anurans. We evaluated immune and endocrine alterations induced by stimulation with heat-killed A. hydrophila injected in Rhinella diptycha toads. Control animals were not manipulated, while the others were separated into groups that received intraperitoneal injection of 300 μl of saline or heat-killed bacteria: groups A1 (3 × 107 cells), A2 (3 × 108 cells), and A3 (3 × 109 cells). Animals were bled and euthanized six hours post-injection. We evaluated neutrophil: lymphocyte ratio (NLR), plasma bacterial killing ability (BKA), testosterone (T), melatonin (MEL), and corticosterone (CORT) plasma levels. Heat-killed A. hydrophila increased CORT and NLR, and decreased MEL, especially at higher concentrations. There was no effect of treatment on T and BKA. We then selected the saline and A3 groups to conduct mRNA expression of several genes including glucocorticoid receptor (GR), toll-like receptor-4 (TLR-4), interferon-γ (IFN-γ), interleukin (IL)-1β, IL-6, and IL-10. We found higher expression of IL-6, IL-1β, IL-10, and IFN-γ in group A3 compared to the saline group. These results indicate the beginning of an inflammatory assemblage, notably at the two highest concentrations of bacteria, and give a better understanding of how anurans respond to an infection within an integrated perspective, evaluating different physiological aspects. Future studies should investigate later phases of the immune response to elucidate more about the inflammation in amphibians challenged with A. hydrophila.

IL-10-induced STAT3/NF-κB crosstalk modulates pineal and extra-pineal melatonin synthesis

Immune-pineal axis activation is part of the assembly of immune responses. Proinflammatory cytokines inhibit the pineal synthesis of melatonin while inducing it in macrophages by mechanisms dependent on nuclear factor-κB (NF-κB) activation. Cytokines activating the Janus kinase/signal transducer and activator of transcription (STAT) pathways, such as interferon-gamma (IFN-γ) and interleukin-10 (IL-10), modulate melatonin synthesis in the pineal, bone marrow (BM), and spleen. The stimulatory effect of IFN-γ upon the pineal gland depends on STAT1/NF-κB interaction, but the mechanisms controlling IL-10 effects on melatonin synthesis remain unclear. Here, we evaluated the role of STAT3 and NF-κB activation by IL-10 upon the melatonin synthesis of rats' pineal gland, BM, spleen, and peritoneal cells. The results show that IL-10-induced interaction of (p)STAT3 with specific NF-κB dimmers leads to different cell effects. IL-10 increases the pineal's acetylserotonin O-methyltransferase (ASMT), N-acetylserotonin, and melatonin content via nuclear translocation of NF-κB/STAT3. In BM, the nuclear translocation of STAT3/p65-NF-κB complexes increases ASMT expression and melatonin content. Increased pSTAT3/p65-NF-κB nuclear translocation in the spleen enhances phosphorylated serotonin N-acetyltransferase ((p)SNAT) expression and melatonin content. Conversely, in peritoneal cells, IL-10 leads to NF-κB p50/p50 inhibitory dimmer nuclear translocation, decreasing (p)SNAT expression and melatonin content. In conclusion, IL-10's effects on melatonin production depend on the NF-κB subunits interacting with (p)STAT3. Thus, variations of IL-10 levels and downstream pathways during immune responses might be critical regulatory factors adjusting pineal and extra-pineal synthesis of melatonin.

Rat resistance to rheumatoid arthritis induction as a function of the early-phase adrenal-pineal crosstalk

Chronic inflammatory diseases are triggered by causal stimuli that might occur long before the appearance of the symptoms. Increasing evidence suggests that these stimuli are necessary but not always sufficient to induce the diseases. The murine model of type II collagen emulsified in Freund's incomplete adjuvant (collagen-induced arthritis) to induce rheumatoid arthritis (RA) follows this pattern as some animals do not develop the chronically inflamed phenotype. Considering that in the immune-pineal axis (IPA) theory adrenal-pineal cross-talk adjusts early phases of inflammatory processes, we investigated whether differences in IPA activation could explain why some animals are resistant (RES) while others develop RA. We observed a similar increase in 6-sulfatoxymelatonin (aMT6s) excretion from day 3 to 13 in both RES and RA animals, followed by a significant decrease in RA animals. This pattern of aMT6s excretion positively correlated with plasma corticosterone (CORT) in RES animals. Additionally, RA animals presented a lower aMT6s/CORT ratio than saline-injected or RES animals. Plasmatic levels of tumour necrosis factor were similar in both groups, but interleukin (IL)-1β and monocyte chemotactic protein 1 (MCP-1) levels were lower in RES compared to RA animals. IL-2 and IL-4 were decreased in RES animals compared to saline-injected animals. The aMT6s/CORT ratio inversely correlated with the paw thickness and the inflammatory score (levels of IL-1β, MCP-1, IL-2 and IL-4 combined). Thus, adrenocortical-pineal positive interaction is an early defence mechanism for avoiding inflammatory chronification. KEY POINTS: Immune-pineal axis imbalance is observed in early-phase rheumatoid arthritis development. Only resistant animals present a positive association between adrenal and pineal hormones. The 6-sulfatoxymelatonin/corticosterone ratio is decreased in animals that develop rheumatoid arthritis. The inflammatory score combining the levels of nocturnal interleukin (IL)-1β, monocyte chemotactic protein 1, IL-2 and IL-4 presents a very strong positive correlation with the size of inflammatory lesion. The 6-sulfatoxymelatonin/corticosterone ratio presents a strong negative correlation with the inflammatory score and paw oedema size.

Pineal Gland from the Cell Culture to Animal Models: A Review

This review demonstrates current literature on pineal gland physiology, pathology, and animal model experiments to concisely explore future needs in research development with respect to pineal gland function and neuro-regenerative properties. The pineal gland plays an integral role in sleep and recovery by promoting physiologic circadian rhythms via production and release of melatonin. Yet, the current literature shows that the pineal gland has neuroprotective effects that modulate both peripheral and central nerve injuries through several direct and indirect mechanisms, such as angiogenesis and induction of growth factors and anti-inflammatory mediators. Animal models have also shown correlations between pineal gland function and metabolic homeostasis. Studies have shown that a functional pineal gland is essential in preventing and slowing the progression of certain diseases such as diabetes, osteoporosis, vertebral osteoarthritis, and neurodegenerative processes. Lastly, the array of cell culturing methods and animal models that can be used to further develop the study of pineal gland function and nervous system injury were reviewed.

Breast Milk and the Importance of Chrononutrition

During pregnancy the human fetus receives timed cues from the circadian rhythms of temperature, metabolites, and hormones from the mother. This influence is interrupted after parturition, the infant does not secrete melatonin and their circadian rhythms are still immature. However, evolution provided the solution to this problem. The newborn can continue receiving the mother's timed cues through breastmilk. Colostrum, transitional, and mature human milk are extraordinary complex biofluids that besides nutrients, contain an array of other non-nutritive components. Upon birth the first milk, colostrum, is rich in bioactive, immunological factors, and in complex oligosaccharides which help the proper establishment of the microbiome in the gut, which is crucial for the infants' health. Hormones, such as glucocorticoids and melatonin, transfer from the mother's plasma to milk, and then the infant is exposed to circadian cues from their mother. Also, milk components of fat, proteins, amino acids, and endogenous cannabinoids, among others, have a markedly different concentration between day and night. In the present review, we give an overview of nutritive and non-nutritive components and their daily rhythms in human milk and explore their physiological importance for the infant. Finally, we highlight some interventions with a circadian approach that emphasize the importance of circadian rhythms in the newborn for their survival, proper growth, and development. It is estimated that ~600,000 deaths/year are due to suboptimal breastfeeding. It is advisable to increase the rate of exclusive breastfeeding, during the day and night, as was established by the evolution of our species.

Neuroinflammation, Sleep, and Circadian Rhythms

Molecules involved in innate immunity affect sleep and circadian oscillators and vice versa. Sleep-inducing inflammatory molecules are activated by increased waking activity and pathogens. Pathologies that alter inflammatory molecules, such as traumatic brain injury, cancer, cardiovascular disease, and stroke often are associated with disturbed sleep and electroencephalogram power spectra. Moreover, sleep disorders, such as insomnia and sleep disordered breathing, are associated with increased dysregulation of inflammatory processes. Inflammatory molecules in both the central nervous system and periphery can alter sleep. Inflammation can also modulate cerebral vascular hemodynamics which is associated with alterations in electroencephalogram power spectra. However, further research is needed to determine the interactions of sleep regulatory inflammatory molecules and circadian clocks. The purpose of this review is to: 1) describe the role of the inflammatory cytokines interleukin-1 beta and tumor necrosis factor-alpha and nucleotide-binding domain and leucine-rich repeat protein-3 inflammasomes in sleep regulation, 2) to discuss the relationship between the vagus nerve in translating inflammatory signals between the periphery and central nervous system to alter sleep, and 3) to present information about the relationship between cerebral vascular hemodynamics and the electroencephalogram during sleep.

The Role of Delivery Route on Colostrum Melatonin and Serum Il-6 Levels: a Prospective Controlled Study

INTRODUCTION

the aim of this study was to determine whether maternal serum IL-6 and postnatal melatonin levels change with the mode of delivery.

MATERIALS AND METHODS

a prospective controlled study was performed on pregnant women (17-43 years) over 37 weeks of pregnancy. Patients were divided into three groups according to the route of delivery: Group 1) 30 women delivering by vaginal route; Group 2) 30 delivering by iterative cesarean section (CS); Group 3) delivering by emergency CS. Maternal serum IL-6 levels were measured before and after delivery, and maternal colostrum melatonin levels after delivery, and the results between the 3 groups compared.

RESULTS

pre-delivery and post-delivery maternal serum IL-6 levels were significantly higher in patients who delivered vaginally than in patients who delivered by the abdominal route (p<0.01). Maternal colostrum melatonin levels of patients after delivery were significantly higher in patients who delivered vaginally (32.88±7.16 ng/L) than in patients who delivered by elective and emergent cesarean deliveries (24.86±2.40 ng/L and 23.73±4.03 ng/L, respectively) (p<0.01).

CONCLUSION

These data support, should there ever be a further need, the benefit of vaginal delivery over cesarean section, in which cytokine and melatonin levels are reduced compared to vaginal delivery.

Possible Role of Pineal and Extra-Pineal Melatonin in Surveillance, Immunity, and First-Line Defense

Melatonin is a highly conserved molecule found in prokaryotes and eukaryotes that acts as the darkness hormone, translating environmental lighting to the whole body, and as a moderator of innate and acquired defense, migration, and cell proliferation processes. This review evaluates the importance of pineal activity in monitoring PAMPs and DAMPs and in mounting an inflammatory response or innate immune response. Activation of the immune-pineal axis, which coordinates the pro-and anti-inflammatory phases of an innate immune response, is described. PAMPs and DAMPs promote the immediate suppression of melatonin production by the pineal gland, which allows leukocyte migration. Monocyte-derived macrophages, important phagocytes of microbes, and cellular debris produce melatonin locally and thereby initiate the anti-inflammatory phase of the acute inflammatory response. The role of locally produced melatonin in organs that directly contact the external environment, such as the skin and the gastrointestinal and respiratory tracts, is also discussed. In this context, as resident macrophages are self-renewing cells, we explore evidence indicating that, besides avoiding overreaction of the immune system, extra-pineal melatonin has a fundamental role in the homeostasis of organs and tissues.

The Circadian Rhythms of STAT3 in the Rat Pineal Gland and Its Involvement in Arylalkylamine-N-Acetyltransferase Regulation

In rodents, the melatonin production by the pineal gland is controlled through adrenergic signaling from the suprachiasmatic nuclei and regulation of the principal enzyme in its synthesis, arylalkylamine-N-acetyltransferase (AANAT). In the present study, we identified increased isoprenaline-induced aa-nat expression and nocturnal AANAT activity in the pineal glands in response to the silencing of the signal transducer and activator of transcription 3 (STAT3) with siRNA or STAT3 inhibitors WP1066 and AZD1480. This AANAT activity enhancement in vivo did not interfere with light-induced AANAT suppression. Systemic or in vitro lipopolysaccharide (LPS) administration markedly increased Stat3 expression and STAT3 phosphorylation, but it did not significantly affect AANAT expression or activity. Simultaneous LPS administration and Stat3 silencing enhanced the aa-nat transcription and AANAT activity to a similar extent as Stat3 inhibition without LPS co-administration. Furthermore, we describe the circadian rhythmicity in Stat3 expression and the phosphorylated form of STAT3 protein in the rat pineal gland. Our data suggest that the higher nocturnal endogenous level of STAT3 in the pineal gland decelerates or hampers the process of NA-induced AANAT activation or affects the AANAT enzyme stability.

Daily and LPS-induced variation of endocrine mediators in cururu toads (Rhinella icterica)

Increased plasma glucocorticoids (corticosterone - CORT, in amphibians) and melatonin (MEL) are associated with the daily activity phase and with environmental darkness, respectively. Besides, CORT and MEL also play pivotal immunomodulatory roles in several vertebrates. Herein we described the daily profile of plasma MEL and CORT for Rhinella icterica toads in captivity. Thereafter, we investigated the effects of lipopolysaccharide (LPS)-induced systemic inflammation on the production of CORT and MEL in the R. icterica. Captive toads showed CORT and MEL diurnal variation typical of nocturnal species, with increased values for CORT at ZT12 (18 h) and MEL peak at ZT18 (24 h). LPS-induced hormonal changes included increased plasma CORT and decreased ocular and plasma MEL when compared to those from toads treated with saline 2 h post-injection. Our results demonstrated the presence of a diurnal CORT and MEL variation in toads. We also showed the crosstalk between CORT and MEL during the toad's systemic inflammation in response to an immune challenge with LPS. Additionally, our results demonstrated that anuran eyes' MEL production might be regulated during the inflammatory processes.

The Coronavirus Disease 2019 (COVID-19): Key Emphasis on Melatonin Safety and Therapeutic Efficacy

Viral infections constitute a tectonic convulsion in the normophysiology of the hosts. The current coronavirus disease 2019 (COVID-19) pandemic is not an exception, and therefore the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, like any other invading microbe, enacts a generalized immune response once the virus contacts the body. Melatonin is a systemic dealer that does not overlook any homeostasis disturbance, which consequently brings into play its cooperative triad, antioxidant, anti-inflammatory, and immune-stimulant backbone, to stop the infective cycle of SARS-CoV-2 or any other endogenous or exogenous threat. In COVID-19, the corporal propagation of SARS-CoV-2 involves an exacerbated oxidative activity and therefore the overproduction of great amounts of reactive oxygen and nitrogen species (RONS). The endorsement of melatonin as a possible protective agent against the current pandemic is indirectly supported by its widely demonstrated beneficial role in preclinical and clinical studies of other respiratory diseases. In addition, focusing the therapeutic action on strengthening the host protection responses in critical phases of the infective cycle makes it likely that multi-tasking melatonin will provide multi-protection, maintaining its efficacy against the virus variants that are already emerging and will emerge as long as SARS-CoV-2 continues to circulate among us.

The paraventricular thalamus serves as a nexus in the regulation of stress and immunity

Many temperate zone animals exhibit seasonal rhythms in physiology and behavior, including seasonal cycles of reproduction, energetics, stress responsiveness, and immune function, among many others. These rhythms are driven by seasonal changes in the duration of pineal melatonin secretion. The neural melatonin target tissues that mediate several of these rhythms have been identified, though the target(s) mediating melatonin's regulation of glucocorticoid secretion, immune cell numbers, and bacterial killing capacity remain unspecified. The present results indicate that one melatonin target tissue, the paraventricular nucleus of the thalamus (PVT), is necessary for the expression of these seasonal rhythms. Thus, while radiofrequency ablations of the PVT failed to alter testicular and body mass response to short photoperiod exposure, they did block the effect of short day lengths on cortisol secretion and bacterial killing efficacy. These results are consistent with the independent regulation by separate neural circuits of several physiological traits that vary seasonally in mammals.

Circadian disruption, melatonin rhythm perturbations and their contributions to chaotic physiology

The aim of this report is to summarize the data documenting the vital nature of well-regulated cellular and organismal circadian rhythms, which are also reflected in a stable melatonin cycle, in supporting optimal health. Cellular fluctuations in physiology exist in most cells of multicellular organisms with their stability relying on the prevailing light:dark cycle, since it regulates, via specialized intrinsically-photoreceptive retinal ganglion cells (ipRGC) and the retinohypothalamic tract, the master circadian oscillator, i.e., the suprachiasmatic nuclei (SCN). The output message of the SCN, as determined by the light:dark cycle, is transferred to peripheral oscillators, so-called slave cellular oscillators, directly via the autonomic nervous system with its limited distribution. and indirectly via the pineal-derived circulating melatonin rhythm, which contacts every cell. Via its regulatory effects on the neuroendocrine system, particularly the hypothalamo-pituitary-adrenal axis, the SCN also has a major influence on the adrenal glucocorticoid rhythm which impacts neurological diseases and psychological behaviors. Moreover, the SCN regulates the circadian production and secretion of melatonin. When the central circadian oscillator is disturbed, such as by light at night, it passes misinformation to all organs in the body. When this occurs the physiology of cells becomes altered and normal cellular functions are compromised. This physiological upheaval is a precursor to pathologies. The deterioration of the SCN/pineal network is often a normal consequence of aging and its related diseases, but in today's societies where manufactured light is becoming progressively more common worldwide, the associated pathologies may also be occurring at an earlier age.

Neuromodulation of the Pineal Gland via Electrical Stimulation of Its Sympathetic Innervation Pathway

Stimulation of the pineal gland via its sympathetic innervation pathway results in the production of N-acetylserotonin and melatonin. Melatonin has many therapeutic roles and is heavily implicated in the regulation of the sleep-wake cycle. In addition, N-acetylserotonin has recently been reported to promote neurogenesis in the brain. Upregulation of these indoleamines is possible via neuromodulation of the pineal gland. This is achieved by electrical stimulation of structures or fibres in the pineal gland sympathetic innervation pathway. Many studies have performed such pineal neuromodulation using both invasive and non-invasive methods. However, the effects of various experimental variables and stimulation paradigms has not yet been reviewed and evaluated. This review summarises these studies and presents the optimal experimental protocols and stimulation parameters necessary for maximal upregulation of melatonin metabolic output.

Rhythmic expression of the melatonergic biosynthetic pathway and its differential modulation in vitro by LPS and IL10 in bone marrow and spleen

Daily oscillation of the immune system follows the central biological clock outputs control such as melatonin produced by the pineal gland. Despite the literature showing that melatonin is also synthesized by macrophages and T lymphocytes, no information is available regarding the temporal profile of the melatonergic system of immune cells and organs in steady-state. Here, the expression of the enzymes arylalkylamine-N-acetyltransferase (AA-NAT), its phosphorylated form (P-AA-NAT) and acetylserotonin-O-methyltransferase (ASMT) were evaluated in phagocytes and T cells of the bone marrow (BM) and spleen. We also determined how the melatonergic system of these cells is modulated by LPS and the cytokine IL-10. The expression of the melatonergic enzymes showed daily rhythms in BM and spleen cells. Melatonin rhythm in the BM, but not in the spleen, follows P-AA-NAT daily variation. In BM cells, LPS and IL10 induced an increase in melatonin levels associated with the increased expressions of P-AA-NAT and ASMT. In spleen cells, LPS induced an increase in the expression of P-AA-NAT but not of melatonin. Conversely, IL10 induced a significant increase in melatonin production associated with increased AA-NAT/P-AA-NAT expressions. In conclusion, BM and spleen cells present different profiles of circadian production of local melatonin and responses to immune signals.

Melatonin and cannabinoids: mitochondrial-targeted molecules that may reduce inflammaging in neurodegenerative diseases

Generally, the development and progression of neurodegenerative diseases are associated with advancing age, so they are usually diagnosed in late adulthood. A primary mechanism underlying the onset of neurodegenerative diseases is neuroinflammation. Based on this background, the concept of "neuroinflammaging" has emerged. In this deregulated neuroinflammatory process, a variety of immune cells participate, especially glial cells, proinflammatory cytokines, receptors, and subcellular organelles including mitochondria, which are mainly responsible for maintaining redox balance at the cellular level. Senescence and autophagic processes also play a crucial role in the neuroinflammatory disease associated with aging. Of particular interest, melatonin, cannabinoids, and the receptors of both molecules which are closely related, exert beneficial effects on the neuroinflammatory processes that precede the onset of neurodegenerative pathologies such as Parkinson's and Alzheimer's diseases. Some of these neuroprotective effects are fundamentally related to its anti-inflammatory and antioxidative actions at the mitochondrial level due to the strategic functions of this organelle. The aim of this review is to summarize the most recent advances in the study of neuroinflammation and neurodegeneration associated with age and to consider the use of new mitochondrial therapeutic targets related to the endocannabinoid system and the pineal gland.

STAT1-NFκB crosstalk triggered by interferon gamma regulates noradrenaline-induced pineal hormonal production

Melatonin production by pineal glands is modulated by several immune signals. The nuclear translocation of nuclear factor kappa-B (NFκB) homodimers, lacking transactivation domains, once induced by lipopolysaccharide (LPS) or tumor necrosis factor (TNF), inhibits the expression of Aanat gene and the synthesis of noradrenaline (NA)-induced melatonin. Interferon gamma (IFN-γ), on the other hand, increases melatonin synthesis. Furthermore, this cytokine activates the signal transducer as well as the activator of transcription 1 (STAT1) pathway, which was never evaluated as a melatonin synthesis modulator before. Reports demonstrated that IFN-γ might also activate NFκB. The present study evaluated the role of STAT1-NFκB crosstalk triggered by IFN-γ regarding the regulation of NA-induced pineal glands' hormonal production. Moreover, IFN-γ treatment increased NA-induced Aanat transcription, in addition to the synthesis of N-acetylserotonin (NAS) and melatonin. These effects were associated with STAT1 nuclear translocation, confirmed by the co-immunoprecipitation of STAT1 and Aanat promoter. Pharmacological STAT1 enhancement augmented NA-induced Aanat transcription as well as NAS and melatonin production. Additionally, IFN-γ induced the nuclear translocation of RelA-NFκB subunits. The blockade of this pathway prevented IFN-γ effects on the pineal function. The present data show that STAT1 and NFκB crosstalk controls melatonin production through a synergistic mechanism, disclosing a new integrative mechanism regarding pineal hormonal activity control.

Pineal gland dysfunction in Alzheimer's disease: relationship with the immune-pineal axis, sleep disturbance, and neurogenesis

Alzheimer’s disease (AD) is a globally common neurodegenerative disease, which is accompanied by alterations to various lifestyle patterns, such as sleep disturbance. The pineal gland is the primary endocrine organ that secretes hormones, such as melatonin, and controls the circadian rhythms. The decrease in pineal gland volume and pineal calcification leads to the reduction of melatonin production. Melatonin has been reported to have multiple roles in the central nervous system (CNS), including improving neurogenesis and synaptic plasticity, suppressing neuroinflammation, enhancing memory function, and protecting against oxidative stress. Recently, reduced pineal gland volume and pineal calcification, accompanied by cognitive decline and sleep disturbances have been observed in AD patients. Here, I review current significant evidence of the contribution of pineal dysfunction in AD to the progress of AD neuropathology. I suggest new insights to understanding the relationship between AD pathogenesis and pineal gland function.

Melatonin in Synaptic Impairments of Alzheimer's Disease

Alzheimer's disease (AD) underlies dementia for millions of people worldwide with no effective treatment. The dementia of AD is thought stem from the impairments of the synapses because of their critical roles in cognition. Melatonin is a neurohormone mainly released by the pineal gland in a circadian manner and it regulates brain functions in various manners. It is reported that both the melatonin deficit and synaptic impairments are present in the very early stage of AD and strongly contribute to the progress of AD. In the mammalian brains, the effects of melatonin are mainly relayed by two of its receptors, melatonin receptor type 1a (MT1) and 1b (MT2). To have a clear idea on the roles of melatonin in synaptic impairments of AD, this review discussed the actions of melatonin and its receptors in the stabilization of synapses, modulation of long-term potentiation, as well as their contributions in the transmissions of glutamatergic, GABAergic and dopaminergic synapses, which are the three main types of synapses relevant to the synaptic strength. The synaptic protective roles of melatonin in AD treatment were also summarized. Regarding its protective roles against amyloid-β neurotoxicity, tau hyperphosphorylation, oxygenation, inflammation as well as synaptic dysfunctions, melatonin may be an ideal therapeutic agent against AD at early stage.

The regulation of circadian clock by tumor necrosis factor alpha

All organisms display circadian rhythms which are under the control of the circadian clock located in the hypothalamus at the suprachiasmatic nucleus, (SCN). The circadian rhythms allow individuals to adjust their physiological activities and daily behavior for the diurnal changes in the living environment. To achieve these, all metabolic processes are aligned with the sleep/wake and fasting/feeding cycles. Subtle changes of daily behavior or food intake can result in misalignment of circadian rhythms. This can cause development of variety of metabolic diseases and even cancer. Although light plays a pivotal role for the activation of the master clock in SCN, the peripheral secondary clocks (or non-SCN), such as melatonin, growth hormone (GH), insulin, adiponectin and Ghrelin also are important in maintaining the circadian rhythms in the brain and peripheral organs. In recent years, growing body of evidence strongly suggest that CA²⁺ signaling, tumor necrosis factor alpha (TNFα) and transforming growth factor beta (TGFβ) also play very important roles in the regulation of circadian rhythms by regulating the transcription of the clock genes.

Daily Onset of Light and Darkness Differentially Controls Hematopoietic Stem Cell Differentiation and Maintenance

Hematopoietic stem and progenitor cells (HSPCs) tightly couple maintenance of the bone marrow (BM) reservoir, including undifferentiated long-term repopulating hematopoietic stem cells (LT-HSCs), with intensive daily production of mature leukocytes and blood replenishment. We found two daily peaks of BM HSPC activity that are initiated by onset of light and darkness providing this coupling. Both peaks follow transient elevation of BM norepinephrine and TNF secretion, which temporarily increase HSPC reactive oxygen species (ROS) levels. Light-induced norepinephrine and TNF secretion augments HSPC differentiation and increases vascular permeability to replenish the blood. In contrast, darkness-induced TNF increases melatonin secretion to drive renewal of HSPCs and LT-HSC potential through modulating surface CD150 and c-Kit expression, increasing COX-2/αSMA+ macrophages, diminishing vascular permeability, and reducing HSPC ROS levels. These findings reveal that light- and darkness-induced daily bursts of norepinephrine, TNF, and melatonin within the BM are essential for synchronized mature blood cell production and HSPC pool repopulation.

Differential response of pineal microglia to surgical versus pharmacological stimuli

Microglial cells are one of the interstitial elements of the pineal gland (PG). We recently reported the pattern of microglia colonization and activation, and microglia-Pax6⁺ cell interactions during normal pineal ontogeny. Here, we describe the dynamics of microglia-Pax6⁺ cell associations and interactions after surgical or pharmacological manipulation. In adult rats, the superior cervical ganglia (SCG) were exposed, and either bilaterally excised (SCGx) or decentralized (SCGd). In the SCGx PGs, the density of Iba1⁺ microglia increased after surgery and returned to sham baseline levels 13 days later. Pineal microglia also responded to SCGd, a more subtle denervation. The number of clustered Iba1⁺ /PCNA⁺ /ED1⁺ microglia was higher 4 days after both surgeries compared to the sham-operated group. However, the number of Pax6⁺ /PCNA^- cells and the percentage of Pax6⁺ cells contacted by and/or phagocytosed by microglia increased significantly only after SCGx. Separate groups of rats were treated with either bacterial lipopolysaccharides (LPS) or doxycycline (DOX) to activate or inhibit pineal microglia, respectively. Peripheral LPS administration caused an increase in the number of clustered Iba1⁺ /PCNA⁺ /ED1⁺ microglial cells, and in the percentage of Pax6⁺ cells associated with and/or engulfed by microglia. In the LPS-treated PGs, we also noted an increase in the number of PCNA⁺ cells that were Iba1^- within the microglial cell clusters. The density of Pax6⁺ cells did not change after LPS treatment. DOX administration did not influence the parameters analyzed. These data suggest that pineal microglia are highly receptive cells capable of rapidly responding in a differential manner to surgical and pharmacological stimuli.

Night work effects on salivary cytokines TNF, IL-1β and IL-6

Shift work is unavoidable in modern societies, but at the same time disrupts biological rhythms and contributes to social distress and disturbance of sleep, health and well-being of shift workers. Shift work has been associated with some chronic diseases in which a chronic inflammatory condition may play a role. However, few studies investigating the association of cytokine and other inflammation markers with shift workers have been published in recent years. In this study we evaluated the effects of permanent night work on the production of tumor necrosis factor (TNF), interleukin-1β (IL-1β), IL-6 and melatonin in saliva. Another aim was to demonstrate the benefit of the use of salivary cytokines for studies in chronobiology, since it is an easy and non-invasive method that allows for sampling at several times. Thirty-eight healthy male workers, being 21 day workers and 17 night workers, agreed to participate in this study. Sleep was evaluated by actigraphy and activity protocols. Saliva was collected during three workdays approximately at the middle of the work shift and at bed and wake times of the main sleep episode. Saliva samples were then analyzed by enzyme-linked immunosorbent assay to measure TNF, IL-1β, IL-6 and melatonin levels, and the results were submitted to non-parametric statistical analysis. The use of saliva instead of blood allowed for a greater number of samples from the same subjects, allowing identifying alterations in the daily production patterns of salivary cytokines TNF, IL-1β and IL-6 that probably are linked to night work. Salivary TNF and IL-1β levels were similar for day and night workers, with higher daily production after awakening, in the morning hours for day workers and in the afternoon for night workers. Both groups presented a significant daily variation pattern of these two cytokines. Day and night workers produced similar amounts of salivary IL-6. Nevertheless, the daily variation pattern observed among day workers, with a peak after awakening, was absent among night workers. Thus, in our study, night workers showed partially adjusted daily variation patterns for salivary TNF and IL-1β, not seen for salivary IL-6. Results for salivary IL-6 could be better explained as a consequence of circadian disruption due to permanent night work. Our results suggest that the whole circadian system, including clocks and pineal gland, is involved in regulating cytokine profile in shift workers and that a coordinated production of these cytokines, important for an adequate inflammatory response, could be disturbed by shift work. The distinct effects that shift work may have on different cytokines could give some cues about the mechanisms involved in this association.

β-Adrenoceptors Trigger Melatonin Synthesis in Phagocytes

Melatonin (5-methoxy-N-acetylserotonin), the pineal hormone, is also synthesized by immune-competent cells. The pineal hormone signals darkness, while melatonin synthesized on demand by activated macrophages at any hour of the day acts locally, favoring regulatory/tolerant phenotypes. Activation of β-adrenoceptors in pinealocytes is the main route for triggering melatonin synthesis. However, despite the well-known role of β-adrenoceptors in the resolution macrophage phenotype (M2), and the relevance of macrophage synthesized melatonin in facilitating phagocytic activity, there is no information regarding whether activation of β-adrenoceptors would induce melatonin synthesis by monocytes. Here we show that catecholamines stimulate melatonin synthesis in bone marrow-derived dendritic cells and RAW 264.7 macrophages. Activation of β-adrenoceptors promotes the synthesis of melatonin by stimulating cyclic AMP/protein kinase A (PKA) pathway and by activating the nuclear translocation of NF-κB. Considering the great number of macrophages around sympathetic nerve terminals, and the relevance of this system for maintaining macrophages in stages compatible to low-grade inflammation, our data open the possibility that extra-pineal melatonin acts as an autocrine/paracrine signal in macrophages under resolution or tolerant phenotypes.

Demyelination in Multiple Sclerosis: Reprogramming Energy Metabolism and Potential PPARγ Agonist Treatment Approaches

Demyelination in multiple sclerosis (MS) cells is the site of several energy metabolic abnormalities driven by dysregulation between the opposed interplay of peroxisome proliferator-activated receptor γ (PPARγ) and WNT/β-catenin pathways. We focus our review on the opposing interactions observed in demyelinating processes in MS between the canonical WNT/β-catenin pathway and PPARγ and their reprogramming energy metabolism implications. Demyelination in MS is associated with chronic inflammation, which is itself associated with the release of cytokines by CD4⁺ Th17 cells, and downregulation of PPARγ expression leading to the upregulation of the WNT/β-catenin pathway. Upregulation of WNT/β-catenin signaling induces activation of glycolytic enzymes that modify their energy metabolic behavior. Then, in MS cells, a large portion of cytosolic pyruvate is converted into lactate. This phenomenon is called the Warburg effect, despite the availability of oxygen. The Warburg effect is the shift of an energy transfer production from mitochondrial oxidative phosphorylation to aerobic glycolysis. Lactate production is correlated with increased WNT/β-catenin signaling and demyelinating processes by inducing dysfunction of CD4⁺ T cells leading to axonal and neuronal damage. In MS, downregulation of PPARγ decreases insulin sensitivity and increases neuroinflammation. PPARγ agonists inhibit Th17 differentiation in CD4⁺ T cells and then diminish release of cytokines. In MS, abnormalities in the regulation of circadian rhythms stimulate the WNT pathway to initiate the demyelination process. Moreover, PPARγ contributes to the regulation of some key circadian genes. Thus, PPARγ agonists interfere with reprogramming energy metabolism by directly inhibiting the WNT/β-catenin pathway and circadian rhythms and could appear as promising treatments in MS due to these interactions.

Immune-pineal axis - acute inflammatory responses coordinate melatonin synthesis by pinealocytes and phagocytes

Melatonin is well known for its circadian production by the pineal gland, and there is a growing body of data showing that it is also produced by many other cells and organs, including immune cells. The chronobiotic role of pineal melatonin, as well as its protective effects in vitro and in vivo, have been extensively explored. However, the interaction between the chronobiotic and defence functions of endogenous melatonin has been little investigated. This review details the current knowledge regarding the coordinated shift in melatonin synthesis from the pineal gland (circadian and monitoring roles) to the regulation of acute immune responses via immune cell production and autocrine effects, producing systemic interactions termed the immune-pineal axis. An acute inflammatory response drives the transcription factor, NFκB, to switch melatonin synthesis from pinealocytes to macrophages/microglia and, upon acute inflammatory resolution, back to pinealocytes. The potential pathophysiological relevance of immune-pineal axis dysregulation is highlighted, with both research and clinical implications, across several medical conditions, including host/parasite interaction, neurodegenerative diseases and cancer. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Endotoxin-induced inflammation disturbs melatonin secretion in ewe

OBJECTIVE

The study examined the effect of intravenous administration of bacterial endotoxin-lipopolysaccharide (LPS) -on the nocturnal secretion of melatonin and on the expression of enzymes of the melatonin biosynthetic pathway in the pineal gland of ewes, taking into account two different photoperiodic conditions: short-night (SN; n = 12) and long-night (LN; n = 12).

METHODS

In both experiments, animals (n = 12) were randomly divided into two groups: control (n = 6) and LPS-treated (n = 6) one. Two hours after sunset, animals received an injection of LPS or saline. Blood samples were collected starting one hour after sunset and continuing for 3 hours after the treatment. The ewes were euthanized 3 hours after LPS/saline treatment. The concentration of hormones in plasma was assayed by radioimmunoassay. In the pineal gland, the content of serotonin and its metabolite was determined by HPLC; whereas the expression of examined genes and protein was assayed using real-time polymerase chain reaction and Western Blot, respectively.

RESULTS

Endotoxin administration lowered (p<0.05) levels of circulating melatonin in animals from LN photoperiod only during the first hour after treatment, while in ewes from SN photoperiod only in the third hour after the injection. Inflammation more substantially suppressed biosynthesis of melatonin in ewes from SN photoperiod, which were also characterised by lower (p<0.05) cortisol concentrations after LPS treatment compared with animals from LN photoperiod. In the pineal gland of ewes subjected to SN photoperiod, LPS reduced (p<0.05) serotonin content and the expression of melatonin biosynthetic pathway enzymes, such as tryptophan hydroxylase and arylalkylamine-N-acetyltransferase. Pineal activity may be disturbed by circulating LPS and proinflammatory cytokines because the expression of mRNAs encoding their corresponding receptors was determined in this gland.

CONCLUSION

The present study showed that peripheral inflammation reduces the secretion of melatonin, but this effect may be influenced by the photoperiod.

Dietary Sources and Bioactivities of Melatonin

Insomnia is a serious worldwide health threat, affecting nearly one third of the general population. Melatonin has been reported to improve sleep efficiency and it was found that eating melatonin-rich foods could assist sleep. During the last decades, melatonin has been widely identified and qualified in various foods from fungi to animals and plants. Eggs and fish are higher melatonin-containing food groups in animal foods, whereas in plant foods, nuts are with the highest content of melatonin. Some kinds of mushrooms, cereals and germinated legumes or seeds are also good dietary sources of melatonin. It has been proved that the melatonin concentration in human serum could significantly increase after the consumption of melatonin containing food. Furthermore, studies show that melatonin exhibits many bioactivities, such as antioxidant activity, anti-inflammatory characteristics, boosting immunity, anticancer activity, cardiovascular protection, anti-diabetic, anti-obese, neuroprotective and anti-aging activity. This review summaries the dietary sources and bioactivities of melatonin, with special attention paid to the mechanisms of action.

Short day length enhances physiological resilience of the immune system against 2-deoxy-d-glucose-induced metabolic stress in a tropical seasonal breeder Funambulus pennanti

Studies demonstrate the importance of metabolic resources in the regulation of reproduction and immune functions in seasonal breeders. In this regard, the restricted energy availability can be considered as an environmental variable that may act as a seasonal stressor and can lead to compromised immune functions. The present study explored the effect of photoperiodic variation in the regulation of immune function under metabolic stress condition. The T-cell-dependent immune response in a tropical seasonal breeder Funambulus pennanti was studied following the inhibition of cellular glucose utilization with 2-deoxy-d-glucose (2-DG). 2-DG treatment resulted in the suppression of general (e.g., proliferative response of lymphocytes) and antigen-specific [anti-keyhole limpet hemocyanin IgG titer and delayed-type hypersensitivity response] T-cell responses with an activation of the hypothalamic-pituitary-adrenal axis, which was evident from the increased levels of plasma corticosterone. 2-DG administration increased the production of inflammatory cytokines [interleukin (IL)-1β and tumor necrosis factor (TNF)-α] and decreased the autocrine T-cell growth factor IL-2. The immunocompromising effect of 2-DG administration was retarded in animals exposed to short photoperiods compared with the control and long photoperiod-exposed groups. This finding suggested that short photoperiodic conditions enhanced the resilience of the immune system, possibly by diverting metabolic resources from the reproductive organs toward the immune system. In addition, melatonin may have facilitated the energy "trade-off" between reproductive and immune mechanisms, thereby providing an advantage to the seasonal breeders for their survival during stressful environmental conditions.

Dual Effect of Catecholamines and Corticosterone Crosstalk on Pineal Gland Melatonin Synthesis

BACKGROUND/AIM

The nocturnal production of melatonin by the pineal gland is triggered by sympathetic activation of adrenoceptors and may be modulated by immunological signals. The effect of glucocorticoids on nocturnal melatonin synthesis is controversial; both stimulatory and inhibitory effects have been reported. During pathophysiological processes, an increased sympathetic tonus could result in different patterns of adrenoceptor activation in the pineal gland. Therefore, in this investigation, we evaluated whether the pattern of adrenergic stimulation of the pineal gland drives the direction of the glucocorticoid effect on melatonin production.

METHODS

The corticosterone effect on the pineal hormonal production induced by β-adrenoceptor or β+α1-adrenoceptor activation was evaluated in cultured glands. We also investigated whether the in vivo lipopolysaccharide (LPS)-induced inhibition of melatonin is dependent on the interaction of glucocorticoids and the α1-adrenoceptor in adrenalectomized animals and on the in vivo blockade of glucocorticoid receptors (GRs) or the α1-adrenoceptor.

RESULTS

Corticosterone potentiated β-adrenoceptor-induced pineal melatonin synthesis, whilst corticosterone-dependent inhibition was observed when melatonin production was induced by β+α1-adrenoceptors agonists. The inhibitory effect of corticosterone is mediated by GR, as it was abolished in the presence of a GR antagonist. Moreover, LPS-induced reduction in melatonin nocturnal plasma content was reversed by adrenalectomy and by antagonizing GR or α1-adrenoceptors.

CONCLUSIONS

The dual effect of corticosterone on pineal melatonin synthesis is determined by the activation pattern of adrenoceptors (β or β+α1) in the gland during GR activation, suggesting that increased activation of the sympathetic system and the hypothalamic-pituitary-adrenal axis are necessary for the control of melatonin production during defense responses.

Avian biological clock - Immune system relationship

Biological rhythms in birds are driven by the master clock, which includes the suprachiasmatic nucleus, the pineal gland and the retina. Light/dark cycles are the cues that synchronize the rhythmic changes in physiological processes, including immunity. This review summarizes our investigations on the bidirectional relationships between the chicken pineal gland and the immune system. We demonstrated that, in the chicken, the main pineal hormone, melatonin, regulates innate immunity, maintains the rhythmicity of immune reactions and is involved in the seasonal changes in immunity. Using thioglycollate-induced peritonitis as a model, we showed that the activated immune system regulates the pineal gland by inhibition of melatonin production at the level of the key enzyme in its biosynthetic pathway, arylalkylamine-N-acetyltransferase (AANAT). Interleukin 6 and interleukin 18 seem to be the immune mediators influencing the pineal gland, directly inhibiting Aanat gene transcription and modulating expression of the clock genes Bmal1 and Per3, which in turn regulate Aanat.

ACTH modulation on corticosterone, melatonin, testosterone and innate immune response in the tree frog Hypsiboas faber

The modulation exerted by glucocorticoids in physiological responses to stressors is essential for maintaining short-term homeostasis. However, highly frequent and/or prolonged activation of the hypothalamic-pituitary-adrenal/interrenal axis may inhibit processes that are important to long-term fitness and health, including reproduction and immunocompetence. The present study evaluates the response to adrenocorticotropic hormone (ACTH) injection in the adult male tree frog, Hypsiboas faber, as indicated by levels of plasma corticosterone (CORT), plasma testosterone (T), ocular melatonin (MEL), hematocrit and immune functioning (total leukocyte count and bacterial killing ability against Escherichia coli). All levels were measured 1, 3 and 6h after treatment. ACTH increased CORT levels whilst decreasing T and MEL levels at 1h post-treatment. 6h after ACTH injection, hematocrit and MEL levels increased. ACTH treatment did not significantly modulate the immune measures over the time-range sampled. The hormonal changes observed in response to ACTH treatment suggest that stressors could act as inhibitors of reproductive activity, as well as differentially modulating melatonin levels at different time-points.

The RelA/cRel nuclear factor-κB (NF-κB) dimer, crucial for inflammation resolution, mediates the transcription of the key enzyme in melatonin synthesis in RAW 264.7 macrophages

Lipopolysaccharide (LPS) modulates the transcription of the gene that codifies the enzyme arylalkylamine-N-acetyltransferase (AA-NAT) through nuclear translocation of the transcription factor nuclear factor-κ-light-chain-enhancer of activated B cells (NF-κB). AA-NAT converts serotonin to N-acetylserotonin, the ultimate precursor of melatonin. Activation of kappa B elements (aa-nat-κB), localized in the promoter (nat-κB1 and nat-κB2), leads to Aa-nat transcription in RAW 264.7 macrophages. Competitive electrophoretic mobility shift assay (EMSA) with oligonucleotide probes corresponding to each of the two elements, as well as a NF-κB consensus corresponding probe, revealed different specificities for each κB element. In addition, activator protein-1 (AP-1) as well as signal transducers and activator of transcription-1 and 3 (STAT-1; STAT-3) competed with NF-κB for binding to nat-κB1, while only STAT-3 competed with NF-κB for binding to nat-κB2. According to co-immunoprecipitation (ChiP) assays, these two sites are able to distinguish NF-κB subunits. The sequence nat-κB1 bound dimers containing p52, RelA, and cRel, while nat-κB2 bound preferentially p50, p52, and RelA, and did not bind cRel. The expression of RelA and cRel is essential for the induction of Aa-nat expression and melatonin synthesis. Considering that the expression of cRel is induced by the earlier expressed p50/RelA, the differential effects of NF-κB dimers may be intimately associated with the temporal regulation of inflammatory responses, with the resolution phase being associated with paracrine and autocrine melatonin effects. Such data suggest that the proven effects of exogenous melatonin in the resolution phase of inflammation are paralleled by the effects of locally synthesized melatonin in immune cells.

Central Interleukin-1β Suppresses the Nocturnal Secretion of Melatonin

In vertebrates, numerous processes occur in a rhythmic manner. The hormonal signal reliably reflecting the environmental light conditions is melatonin. Nocturnal melatonin secretion patterns could be disturbed in pathophysiological states, including inflammation, Alzheimer's disease, and depression. All of these states share common elements in their aetiology, including the overexpression of interleukin- (IL-) 1β in the central nervous system. Therefore, the present study was designed to determine the effect of the central injection of exogenous IL-1β on melatonin release and on the expression of the enzymes of the melatonin biosynthetic pathway in the pineal gland of ewe. It was found that intracerebroventricular injections of IL-1β (50 µg/animal) suppressed (P < 0.05) nocturnal melatonin secretion in sheep regardless of the photoperiod. This may have resulted from decreased (P < 0.05) synthesis of the melatonin intermediate serotonin, which may have resulted, at least partially, from a reduced expression of tryptophan hydroxylase. IL-1β also inhibited (P < 0.05) the expression of the melatonin rhythm enzyme arylalkylamine-N-acetyltransferase and hydroxyindole-O-methyltransferase. However, the ability of IL-1β to affect the expression of these enzymes was dependent upon the photoperiod. Our study may shed new light on the role of central IL-1β in the aetiology of disruptions in melatonin secretion.

Adenosine triphosphate inhibits melatonin synthesis in the rat pineal gland

Adenosine triphosphate (ATP) is released onto the pinealocyte, along with noradrenaline, from sympathetic neurons and triggers P2Y1 receptors that enhance β-adrenergic-induced N-acetylserotonin (NAS) synthesis. Nevertheless, the biotransformation of NAS into melatonin, which occurs due to the subsequent methylation by acetylserotonin O-methyltransferase (ASMT; EC 2.1.1.4), has not yet been evaluated in the presence of purinergic stimulation. We therefore evaluated the effects of purinergic signaling on melatonin synthesis induced by β-adrenergic stimulation. ATP increased NAS levels, but, surprisingly, inhibited melatonin synthesis in an inverse, concentration-dependent manner. Our results demonstrate that enhanced NAS levels, which depend on phospholipase C (PLC) activity (but not the induction of gene transcription), are a post-translational effect. By contrast, melatonin reduction is related to an ASMT inhibition of expression at both the gene transcription and protein levels. These results were independent of nuclear factor-kappa B (NF-kB) translocation. Neither the P2Y1 receptor activation nor the PLC-mediated pathway was involved in the decrease in melatonin, indicating that ATP regulates pineal metabolism through different mechanisms. Taken together, our data demonstrate that purinergic signaling differentially modulates NAS and melatonin synthesis and point to a regulatory role for ATP as a cotransmitter in the control of ASMT, the rate-limiting enzyme in melatonin synthesis. The endogenous production of melatonin regulates defense responses; therefore, understanding the mechanisms involving ASMT regulation might provide novel insights into the development and progression of neurological disorders since melatonin presents anti-inflammatory, neuroprotective, and neurogenic effects.

A role for melatonin in maintaining the pro- and anti-inflammatory balance by influencing leukocyte migration and apoptosis in carp

Melatonin is responsible for the synchronization of many physiological processes, including the immune response. Here we focus on the expression of melatonin MT1 receptors in/on leukocytes, and on the effects of melatonin administration on the inflammatory processes of carp. For the first time, we showed that fish leukocytes express MT1 receptors, implicating direct responsiveness to melatonin stimulation. Moreover, both in vitro and in vivo, melatonin modulated the immune response. The most potent effects of melatonin concerned the regulation of leukocyte migration. Melatonin reduced chemotaxis of leukocytes towards CXC chemokines in vitro. In vivo, during zymosan induced peritonitis, i.p. administration of melatonin reduced the number of neutrophils. This correlated with a melatonin-induced decrease of gene expression of the CXCa chemokine. Moreover, melatonin induced a decrease of the respiratory burst in inflammatory leukocytes. Although these data do suggest a potent anti-inflammatory function for this hormone, melatonin-induced inhibition of leukocyte apoptosis clearly indicates towards a dual function. These results show that also in carp, melatonin performs a pleiotropic and extra-pineal function that is important in maintaining the delicate pro- and anti-inflammatory balance during infection. They furthermore demonstrate that neuroendocrine-immune interaction via melatonin is evolutionary conserved.

Melatonin attenuates Leishmania (L.) amazonensis infection by modulating arginine metabolism

Acute inflammatory responses induced by bacteria or fungi block nocturnal melatonin synthesis by rodent pineal glands. Here, we show Leishmania infection does not impair daily melatonin rhythm in hamsters. Remarkably, the attenuated parasite burden and lesion progression in hamsters infected at nighttime was impaired by blockage of melatonin receptors with luzindole, whereas melatonin treatment during the light phase attenuated Leishmania infection. In vitro studies corroborated in vivo observations. Melatonin treatment reduced macrophage expression of Cat-2b, Cat1, and ArgI, genes involved in arginine uptake and polyamine synthesis. Indeed, melatonin reduced macrophage arginine uptake by 40%. Putrescine supplementation reverted the attenuation of infectivity by melatonin indicating that its effect was due to the arrest of parasite replication. This study shows that the Leishmania/host interaction varies in a circadian manner according to nocturnal melatonin pineal synthesis. Our results provide new data regarding Leishmania infectiveness and show new approaches for applying agonists of melatonin receptors in Leishmaniasis therapy.

Neuroendocrine control of photoperiodic changes in immune function

Seasonal variation in immune function putatively maximizes survival and reproductive success. Day length (photoperiod) is the most potent signal for time of year. Animals typically organize breeding, growth, and behavior to adapt to spatial and temporal niches. Outside the tropics individuals monitor photoperiod to support adaptations favoring survival and reproductive success. Changes in day length allow anticipation of seasonal changes in temperature and food availability that are critical for reproductive success. Immune function is typically bolstered during winter, whereas reproduction and growth are favored during summer. We provide an overview of how photoperiod influences neuronal function and melatonin secretion, how melatonin acts directly and indirectly to govern seasonal changes in immune function, and the manner by which other neuroendocrine effectors such as glucocorticoids, prolactin, thyroid, and sex steroid hormones modulate seasonal variations in immune function. Potential future research avenues include commensal gut microbiota and light pollution influences on photoperiodic responses.

Inflammation-sleep interface in brain disease: TNF, insulin, orexin

The depth, pattern, timing and duration of unconsciousness, including sleep, vary greatly in inflammatory disease, and are regarded as reliable indicators of disease severity. Similarly, these indicators are applicable to the encephalopathies of sepsis, malaria, and trypanosomiasis, and to viral diseases such as influenza and AIDS. They are also applicable to sterile neuroinflammatory states, including Alzheimer’s disease, Parkinson’s disease, traumatic brain injury, stroke and type-2 diabetes, as well as in iatrogenic brain states following brain irradiation and chemotherapy. Here we make the case that the cycles of unconsciousness that constitute normal sleep, as well as its aberrations, which range from sickness behavior through daytime sleepiness to the coma of inflammatory disease states, have common origins that involve increased inflammatory cytokines and consequent insulin resistance and loss of appetite due to reduction in orexigenic activity. Orexin reduction has broad implications, which are as yet little appreciated in the chronic inflammatory conditions listed, whether they be infectious or sterile in origin. Not only is reduction in orexin levels characterized by loss of appetite, it is associated with inappropriate and excessive sleep and, when dramatic and chronic, leads to coma. Moreover, such reduction is associated with impaired cognition and a reduction in motor control. We propose that advanced understanding and appreciation of the importance of orexin as a key regulator of pathways involved in the maintenance of normal appetite, sleep patterns, cognition, and motor control may afford novel treatment opportunities.

Selective protection of the cerebellum against intracerebroventricular LPS is mediated by local melatonin synthesis

Although melatonin is mainly produced by the pineal gland, an increasing number of extra-pineal sites of melatonin synthesis have been described. We previously demonstrated the existence of bidirectional communication between the pineal gland and the immune system that drives a switch in melatonin production from the pineal gland to peripheral organs during the mounting of an innate immune response. In the present study, we show that acute neuroinflammation induced by lipopolysaccharide (LPS) injected directly into the lateral ventricles of adult rats reduces the nocturnal peak of melatonin in the plasma and induces its synthesis in the cerebellum, though not in the cortex or hippocampus. This increase in cerebellar melatonin content requires the activation of nuclear factor kappa B (NF-κB), which positively regulates the expression of the key enzyme for melatonin synthesis, arylalkylamine N-acetyltransferase (AA-NAT). Interestingly, LPS treatment led to neuronal death in the hippocampus and cortex, but not in the cerebellum. This privileged protection of cerebellar cells was abrogated when G-protein-coupled melatonin receptors were blocked by the melatonin antagonist luzindole, suggesting that the local production of melatonin protects cerebellar neurons from LPS toxicity. This is the first demonstration of a switch between pineal and extra-pineal melatonin production in the central nervous system following a neuroinflammatory response. These results have direct implications concerning the differential susceptibility of specific brain areas to neuronal death.

Neurodegeneration in Parkinson's disease: interactions of oxidative stress, tryptophan catabolites and depression with mitochondria and sirtuins

The biological underpinnings to the etiology and course of neurodegeneration in Parkinson's disease are an area of extensive research that has yet to produce an early biological marker or disease-slowing or preventative treatment. Recent conceptualizations of Parkinson's disease have integrated immuno-inflammation and oxidative and nitrosative stress occurring in depression, somatization and peripheral inflammation into the course of Parkinson's disease. We review the data showing the importance of immuno-inflammatory processes and oxidative and nitrosative stress in such classically conceived 'comorbidities', suggesting that lifetime, prodromal and concurrent depression and somatization may be intricately involved in the etiology and course of Parkinson's disease, rather than psychiatric comorbidities. This produces a longer term developmental perspective of Parkinson's disease, which incorporates tryptophan catabolites (TRYCATs), lipid peroxidation, sirtuins, cyclic adenosine monophosphate, aryl hydrocarbon receptor, and circadian genes. This integrates wider bodies of data pertaining to neuronal loss in Parkinson's disease, emphasizing how these interact with susceptibility genes to drive changes in mitochondria, blood-brain barrier permeability and intercellular signalling. We review this data here in the context of neurodegeneration in Parkinson's disease and to the future directions indicated for slowing disease progression.