Hydroxyindole-O-methyltransferase is another target for L-glutamate-evoked inhibition of melatonin synthesis in rat pinealocytes

Long-Term Effects of Perinatal Exposure to a Glyphosate-Based Herbicide on Melatonin Levels and Oxidative Brain Damage in Adult Male Rats

Concerns have been raised regarding the potential adverse health effects of the ubiquitous herbicide glyphosate. Here, we investigated long-term effects of developmental exposure to a glyphosate-based herbicide (GBH) by analyzing serum melatonin levels and cellular changes in the striatum of adult male rats (90 days old). Pregnant and lactating rats were exposed to 3% GBH (0.36% glyphosate) through drinking water from gestational day 5 to postnatal day 15. The offspring showed reduced serum melatonin levels (43%) at the adult age compared with the control group. The perinatal exposure to GBH also induced long-term oxidative stress-related changes in the striatum demonstrated by increased lipid peroxidation (45%) and DNA/RNA oxidation (39%) together with increased protein levels of the antioxidant enzymes, superoxide dismutase (SOD1, 24%), glutamate-cysteine ligase (GCLC, 58%), and glutathione peroxidase 1 (GPx1, 31%). Moreover, perinatal GBH exposure significantly increased the total number of neurons (20%) and tyrosine hydroxylase (TH)-positive neurons (38%) in the adult striatum. Mechanistic in vitro studies with primary rat pinealocytes exposed to 50 µM glyphosate demonstrated a decreased melatonin secretion partially through activation of metabotropic glutamate receptor 3 (mGluR3), while higher glyphosate levels (100 or 500 µM) also reduced the pinealocyte viability. Since decreased levels of the important antioxidant and neuroprotector melatonin have been associated with an increased risk of developing neurodegenerative disorders, this demonstrates the need to consider the melatonin hormone system as a central endocrine-related target of glyphosate and other environmental contaminants.

Mass-spectrometry analysis of the human pineal proteome during night and day and in autism

The human pineal gland regulates day-night dynamics of multiple physiological processes, especially through the secretion of melatonin. Using mass-spectrometry-based proteomics and dedicated analysis tools, we identify proteins in the human pineal gland and analyze systematically their variation throughout the day and compare these changes in the pineal proteome between control specimens and donors diagnosed with autism. Results reveal diverse regulated clusters of proteins with, among others, catabolic carbohydrate process and cytoplasmic membrane-bounded vesicle-related proteins differing between day and night and/or control versus autism pineal glands. These data show novel and unexpected processes happening in the human pineal gland during the day/night rhythm as well as specific differences between autism donor pineal glands and those from controls.

Trough Melatonin Levels Differ between Early and Late Phases of Alzheimer Disease

Objective

Melatonin has been considered to have an essential role in the pathophysiology of Alzheimer’s disease (AD) for its regulatory function on circadian rhythm and interaction with glutamate for the modulation of learning and memory. Previous studies revealed that melatonin levels decreased in patients with AD. However, melatonin supplement didn’t show promising efficacy for AD. This study compared trough melatonin levels among elderly people with different severities of cognitive deficits.

Methods

We enrolled 270 elder individuals (consisting four groups healthy elderly, amnestic mild cognitive impairment [MCI], mild AD, and moderate-severe AD) in the learning cohort. Trough melatonin levels in plasma were measured using ELISA. Cognitive function was evaluated by Clinical Dementia Rating Scale (CDR) and Mini-Mental State Examination (MMSE). An independent testing cohort, also consisting of four groups, was enrolled for ascertainment.

Results

In the learning cohort, trough melatonin levels decreased in the MCI group but elevated in the mild and moderate to severe AD groups. Trough melatonin levels were associated with CDR and MMSE in MCI or AD patients significantly. In the testing cohort, the results were similar to those in the learning cohort.

Conclusion

This study demonstrated that trough melatonin levels in the peripheral blood were decreased in MCI but increased with the severity of AD. The finding supports the trials indicating that melatonin showed efficacy only in MCI but not in AD. Whether trough melatonin level has potential to be a treatment response biomarker for AD, especially its early phase needs further studies.

Melatonin: Roles in influenza, Covid-19, and other viral infections

There is a growing appreciation that the regulation of the melatonergic pathways, both pineal and systemic, may be an important aspect in how viruses drive the cellular changes that underpin their control of cellular function. We review the melatonergic pathway role in viral infections, emphasizing influenza and covid-19 infections. Viral, or preexistent, suppression of pineal melatonin disinhibits neutrophil attraction, thereby contributing to an initial "cytokine storm", as well as the regulation of other immune cells. Melatonin induces the circadian gene, Bmal1, which disinhibits the pyruvate dehydrogenase complex (PDC), countering viral inhibition of Bmal1/PDC. PDC drives mitochondrial conversion of pyruvate to acetyl-coenzyme A (acetyl-CoA), thereby increasing the tricarboxylic acid cycle, oxidative phosphorylation, and ATP production. Pineal melatonin suppression attenuates this, preventing the circadian "resetting" of mitochondrial metabolism. This is especially relevant in immune cells, where shifting metabolism from glycolytic to oxidative phosphorylation, switches cells from reactive to quiescent phenotypes. Acetyl-CoA is a necessary cosubstrate for arylalkylamine N-acetyltransferase, providing an acetyl group to serotonin, and thereby initiating the melatonergic pathway. Consequently, pineal melatonin regulates mitochondrial melatonin and immune cell phenotype. Virus- and cytokine-storm-driven control of the pineal and mitochondrial melatonergic pathway therefore regulates immune responses. Virus-and cytokine storm-driven changes also increase gut permeability and dysbiosis, thereby suppressing levels of the short-chain fatty acid, butyrate, and increasing circulating lipopolysaccharide (LPS). The alterations in butyrate and LPS can promote viral replication and host symptom severity via impacts on the melatonergic pathway. Focussing on immune regulators has treatment implications for covid-19 and other viral infections.

Homeobox genes in melatonin-producing pinealocytes: Otx2 and Crx act to promote hormone synthesis in the mature rat pineal gland

Homeobox genes encode transcription factors that regulate developmental processes; however, in the pineal gland, a neuroendocrine organ responsible for nocturnal melatonin synthesis, expression of the homeobox genes Otx2 (orthodenticle homeobox 2) and Crx (cone-rod homeobox) persists postnatally. We here show that OTX2 and CRX are exclusively present in melatonin-producing pinealocytes of the rat pineal gland. To understand the roles of Otx2 and Crx in the mature pineal gland, we used siRNA technology in cultured rat pinealocytes with the nocturnal situation mimicked by adding norepinephrine to the culture media. siRNA-induced knockdown of Otx2 was found to reduce expression levels of the enzymes involved in melatonin synthesis at both transcript and protein levels. Similar results were obtained when knocking down Crx. Knocking down Otx2 and Crx simultaneously produced an even larger reduction in both transcript and protein levels of the melatonin-producing enzymes and also reduced the levels of melatonin released to the culture media. These results suggest that Otx2 and Crx, both alone and in combination, act to control pineal melatonin synthesis.

The environmental neurotoxin β-N-methylamino-L-alanine inhibits melatonin synthesis in primary pinealocytes and a rat model

The environmental neurotoxin β-N-methylamino-L-alanine (BMAA) is a glutamate receptor agonist that can induce oxidative stress and has been implicated as a possible risk factor for neurodegenerative disease. Detection of BMAA in mussels, crustaceans, and fish illustrates that the sources of human exposure to this toxin are more abundant than previously anticipated. The aim of this study was to determine uptake of BMAA in the pineal gland and subsequent effects on melatonin production in primary pinealocyte cultures and a rat model. Autoradiographic imaging of 10-day-old male rats revealed a high and selective uptake in the pineal gland at 30 minutes to 24 hours after ¹⁴ C-L-BMAA administration (0.68 mg/kg). Primary pinealocyte cultures exposed to 0.05-3 mmol/L BMAA showed a 57%-93% decrease in melatonin synthesis in vitro. Both the metabotropic glutamate receptor 3 (mGluR3) antagonist Ly341495 and the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate prevented the decrease in melatonin secretion, suggesting that BMAA inhibits melatonin synthesis by mGluR3 activation and PKC inhibition. Serum analysis revealed a 45% decrease in melatonin concentration in neonatal rats assessed 2 weeks after BMAA administration (460 mg/kg) and confirmed an inhibition of melatonin synthesis in vivo. Given that melatonin is a most important neuroprotective molecule in the brain, the etiology of BMAA-induced neurodegeneration may include mechanisms beyond direct excitotoxicity and oxidative stress.

Characterization of human pineal gland proteome

The pineal gland is a neuroendocrine gland located at the center of the brain. It is known to regulate various physiological functions in the body through secretion of the neurohormone melatonin. Comprehensive characterization of the human pineal gland proteome has not been undertaken to date. We employed a high-resolution mass spectrometry-based approach to characterize the proteome of the human pineal gland. A total of 5874 proteins were identified from the human pineal gland in this study. Of these, 5820 proteins were identified from the human pineal gland for the first time. Interestingly, 1136 proteins from the human pineal gland were found to contain a signal peptide domain, which indicates the secretory nature of these proteins. An unbiased global proteomic profile of this biomedically important organ should benefit molecular research to unravel the role of the pineal gland in neuropsychiatric and neurodegenerative diseases.

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.

Modulation of pineal melatonin synthesis by glutamate involves paracrine interactions between pinealocytes and astrocytes through NF-κB activation

The glutamatergic modulation of melatonin synthesis is well known, along with the importance of astrocytes in mediating glutamatergic signaling in the central nervous system. Pinealocytes and astrocytes are the main cell types in the pineal gland. The objective of this work was to investigate the interactions between astrocytes and pinealocytes as a part of the glutamate inhibitory effect on melatonin synthesis. Rat pinealocytes isolated or in coculture with astrocytes were incubated with glutamate in the presence of norepinephrine, and the melatonin content, was quantified. The expression of glutamate receptors, the intracellular calcium content and the NF-κB activation were analyzed in astrocytes and pinealocytes. TNF-α's possible mediation of the effect of glutamate was also investigated. The results showed that glutamate's inhibitory effect on melatonin synthesis involves interactions between astrocytes and pinealocytes, possibly through the release of TNF-α. Moreover, the activation of the astrocytic NF-κB seems to be a necessary step. In astrocytes and pinealocytes, AMPA, NMDA, and group I metabotropic glutamate receptors were observed, as well as the intracellular calcium elevation. In conclusion, there is evidence that the modulation of melatonin synthesis by glutamate involves paracrine interactions between pinealocytes and astrocytes through the activation of the astrocytic NF-κB transcription factor and possibly by subsequent TNF-α release.

Exocytosis of D-Aspartate from INS-1E Clonal .BETA. Cells

D-Aspartate is present in the central nervous system and various endocrine organs, and modulates their neuroendocrine function. In islets of Langerhans, alpha and beta cells contain D-aspartate. Here we show that INS-1E clonal beta cells contain the highest amount of D-aspartate. Immunohistochemical analysis with specific antibodies against D-aspartate indicated that D-aspartate is co-localized with insulin. Upon the addition of K(+), both D-aspartate and insulin are secreted from the cells in a Ca(2+)-dependent manner. A Ca(2+) ionophore, A23187, also triggers the release of D-aspartate and insulin in the presence of Ca(2+). Bafilomycin A(1), a specific inhibitor of V-ATPase and V-ATPase-linked secondary transport, inhibits the secretion of D-aspartate. These results support the idea that D-aspartate is present in insulin-containing secretory granules and co-secreted with insulin through exocytosis.

Immunohistochemical Localization of D-Aspartate in Islets of Langerhans

D-aspartate is a putative modulator of neuroendocrine functions, and is present in various neuroendocrine cells as well as the central nervous system. Here we show that the islet of Langerhans is a D-aspartate-containing endocrine organ. Immunohistochemical analysis with specific antibodies against D-aspartate indicated that D-aspartate is present in all islet cells, and predominantly present in alpha cells and a subpopulation of F-cells. Since these cells are glutamatergic in nature, it is possible that D-aspartate is involved in the glutamate signaling pathways in the islets.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

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

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

Glutamate receptor subunit delta2 is highly expressed in a novel population of glial-like cells in rat pineal glands in culture

The mammalian pineal gland uses L-glutamate as an intercellular chemical transmitter to regulate negatively melatonin synthesis. To receive glutamate signals, pinealocytes express at least three kinds of glutamate receptors: metabotropic receptor types 3 and 5 and an ionotropic receptor, GluR1. In this study, we examined whether or not the fourth class of ionotropic receptor, delta, which is known for its nondefinitive molecular function and its unique expression pattern in brain, is expressed in pineal gland. RT-PCR analyses with specific probes indicated the expression of mRNA of delta2 but not that of delta1 in pineal gland and cultured pineal cells. Western blotting analysis with polyclonal antibodies specific to the carboxyl-terminal region of the delta2 receptor recognized a single 110-kDa polypeptide of cerebellar membranes and specifically immunostained Purkinje cells. The delta2 antibodies recognized a 110-kDa polypeptide of pineal membranes and specifically immunostained huge glial-like cells with the occasional presence of several long, branching processes in a pineal cell culture. delta2 is not uniformly distributed throughout the cells and is relatively abundant at the periphery of the cell bodies and long processes, where the terminals of synaptophysin-positive processes of pinealocytes, a site for glutamate secretion, are frequently present. The delta2-positive cells constitute a very minor population among total pineal cells (approximately 0.03%). Double immunolabeling with delta2 antibodies and antibodies against marker proteins for pineal interstitial cells clearly distinguishes delta2-positive pineal cells and other known interstitial cells, including glial fibrillary acidic protein- or vimentin-positive glial-like cells. These results indicated that the delta2 glutamate receptor is expressed in a novel subpopulation of pineal glial-like cells in culture and suggest the presence of a glutamate-mediated intercellular signal transduction mechanism between pinealocytes and delta2-expressing cells. The pineal cells may provide a good experimental system for studies on the function of glutamate receptor delta2.