Cellular lining of the sheep pineal recess studied by light-, transmission-, and scanning electron microscopy: morphologic indications for a direct secretion of melatonin from the pineal gland to the cerebrospinal fluid

Crosstalk Between Aging, Circadian Rhythm, and Melatonin

Circadian rhythms (CRs) are 24-hour periodic oscillations governed by an endogenous circadian pacemaker located in the suprachiasmatic nucleus (SCN), which organizes the physiology and behavior of organisms. Circadian rhythm disruption (CRD) is also indicative of the aging process. In mammals, melatonin is primarily synthesized in the pineal gland and participates in a variety of multifaceted intracellular signaling networks and has been shown to synchronize CRs. Endogenous melatonin synthesis and its release tend to decrease progressively with advancing age. Older individuals experience frequent CR disruption, which hastens the process of aging. A profound understanding of the relationship between CRs and aging has the potential to improve existing treatments and facilitate development of novel chronotherapies that target age-related disorders. This review article aims to examine the circadian regulatory mechanisms in which melatonin plays a key role in signaling. We describe the basic architecture of the molecular circadian clock and its functional decline with age in detail. Furthermore, we discuss the role of melatonin in regulation of the circadian pacemaker and redox homeostasis during aging. Moreover, we also discuss the protective effect of exogenous melatonin supplementation in age-dependent CR disruption, which sheds light on this pleiotropic molecule and how it can be used as an effective chronotherapeutic medicine.

Non-Excitatory Amino Acids, Melatonin, and Free Radicals: Examining the Role in Stroke and Aging

The aim of this review is to explore the relationship between melatonin, free radicals, and non-excitatory amino acids, and their role in stroke and aging. Melatonin has garnered significant attention in recent years due to its diverse physiological functions and potential therapeutic benefits by reducing oxidative stress, inflammation, and apoptosis. Melatonin has been found to mitigate ischemic brain damage caused by stroke. By scavenging free radicals and reducing oxidative damage, melatonin may help slow down the aging process and protect against age-related cognitive decline. Additionally, non-excitatory amino acids have been shown to possess neuroprotective properties, including antioxidant and anti-inflammatory in stroke and aging-related conditions. They can attenuate oxidative stress, modulate calcium homeostasis, and inhibit apoptosis, thereby safeguarding neurons against damage induced by stroke and aging processes. The intracellular accumulation of certain non-excitatory amino acids could promote harmful effects during hypoxia-ischemia episodes and thus, the blockade of the amino acid transporters involved in the process could be an alternative therapeutic strategy to reduce ischemic damage. On the other hand, the accumulation of free radicals, specifically mitochondrial reactive oxygen and nitrogen species, accelerates cellular senescence and contributes to age-related decline. Recent research suggests a complex interplay between melatonin, free radicals, and non-excitatory amino acids in stroke and aging. The neuroprotective actions of melatonin and non-excitatory amino acids converge on multiple pathways, including the regulation of calcium homeostasis, modulation of apoptosis, and reduction of inflammation. These mechanisms collectively contribute to the preservation of neuronal integrity and functions, making them promising targets for therapeutic interventions in stroke and age-related disorders.

Brain washing and neural health: role of age, sleep, and the cerebrospinal fluid melatonin rhythm

The brain lacks a classic lymphatic drainage system. How it is cleansed of damaged proteins, cellular debris, and molecular by-products has remained a mystery for decades. Recent discoveries have identified a hybrid system that includes cerebrospinal fluid (CSF)-filled perivascular spaces and classic lymph vessels in the dural covering of the brain and spinal cord that functionally cooperate to remove toxic and non-functional trash from the brain. These two components functioning together are referred to as the glymphatic system. We propose that the high levels of melatonin secreted by the pineal gland directly into the CSF play a role in flushing pathological molecules such as amyloid-β peptide (Aβ) from the brain via this network. Melatonin is a sleep-promoting agent, with waste clearance from the CNS being highest especially during slow wave sleep. Melatonin is also a potent and versatile antioxidant that prevents neural accumulation of oxidatively-damaged molecules which contribute to neurological decline. Due to its feedback actions on the suprachiasmatic nucleus, CSF melatonin rhythm functions to maintain optimal circadian rhythmicity, which is also critical for preserving neurocognitive health. Melatonin levels drop dramatically in the frail aged, potentially contributing to neurological failure and dementia. Melatonin supplementation in animal models of Alzheimer's disease (AD) defers Aβ accumulation, enhances its clearance from the CNS, and prolongs animal survival. In AD patients, preliminary data show that melatonin use reduces neurobehavioral signs such as sundowning. Finally, melatonin controls the mitotic activity of neural stem cells in the subventricular zone, suggesting its involvement in neuronal renewal.

Redox Biology of Melatonin: Discriminating Between Circadian and Noncircadian Functions

Melatonin has not only to be seen as a regulator of circadian clocks. In addition to its chronobiotic functions, it displays other actions, especially in cell protection. This includes antioxidant, anti-inflammatory, and mitochondria-protecting effects. Although protection is also modulated by the circadian system, the respective actions of melatonin can be distinguished and differ with regard to dose requirements in therapeutic settings. It is the aim of this article to outline these differences in terms of function, signaling, and dosage. Focus has been placed on both the nexus and the dissecting properties between circadian and noncircadian mechanisms. This has to consider details beyond the classic view of melatonin's role, such as widespread synthesis in extrapineal tissues, formation in mitochondria, effects on the mitochondrial permeability transition pore, and secondary signaling, for example, via upregulation of sirtuins and by regulating noncoding RNAs, especially microRNAs. The relevance of these findings, the differences and connections between circadian and noncircadian functions of melatonin shed light on the regulation of inflammation, including macrophage/microglia polarization, damage-associated molecular patterns, avoidance of cytokine storms, and mitochondrial functions, with numerous consequences to antioxidative protection, that is, aspects of high actuality with regard to deadly viral and bacterial diseases. Antioxid. Redox Signal. 37, 704-725.

An ultrastructural study of the deep pineal gland of the Sprague Dawley rat using transmission and serial block face scanning electron microscopy: cell types, barriers, and innervation

The morphology of the deep pineal gland of the Sprague Dawley rat was investigated by serial block face scanning electron microscopy. Cells were three-dimensionally (3-D) reconstructed using the software Fiji TrackEM. The deep pineal gland consisted of 2-5 layers of electron-lucent pinealocytes, with a euchromatic nucleus, endowed with one or two processes. Laterally, the deep pineal merged with the habenula and the stria medullaris thalami, via an intermediate area containing cells with more electron-dense cytoplasm and an indented nucleus with heterochromatin. Neither nerve terminals nor capillaries were observed in the deep pineal itself but present in the intermediate parts of the gland. The deep pineal was in contact with the third ventricle via the pineal and suprahabenular recesses. The ependymal lining in these recesses was an epithelium connected by tight junctions between their lateral cell membranes. Several intraventricular nerve terminals were in contact with the ependyma. 3-D reconstructions showed the ependymal cells endowed with long slender process penetrating the underlying pineal parenchyma. Few "tanocyte-like" ependymal cells, endowed with a process, reaching the subarachnoid space on the inferior surface of the deep pineal were observed. In addition, pinealocyte and astrocyte processes, often connected by gap junctions, bordered the inferior surface. In summary, the rat deep pineal gland is a neuroendocrine structure connected to the habenula. We here report specialized ependymal cells that might transmit signals from the cerebrospinal fluid to the deep pineal parenchyma and a "trans-pineal tanocyte-like cell" that connects the ventricular system with the subarachnoid space.

Divergent Importance of Chronobiological Considerations in High- and Low-dose Melatonin Therapies

Melatonin has been used preclinically and clinically for different purposes. Some applications are related to readjustment of circadian oscillators, others use doses that exceed the saturation of melatonin receptors MT1 and MT2 and are unsuitable for chronobiological purposes. Conditions are outlined for appropriately applying melatonin as a chronobiotic or for protective actions at elevated levels. Circadian readjustments require doses in the lower mg range, according to receptor affinities. However, this needs consideration of the phase response curve, which contains a silent zone, a delay part, a transition point and an advance part. Notably, the dim light melatonin onset (DLMO) is found in the silent zone. In this specific phase, melatonin can induce sleep onset, but does not shift the circadian master clock. Although sleep onset is also under circadian control, sleep and circadian susceptibility are dissociated at this point. Other limits of soporific effects concern dose, duration of action and poor individual responses. The use of high melatonin doses, up to several hundred mg, for purposes of antioxidative and anti-inflammatory protection, especially in sepsis and viral diseases, have to be seen in the context of melatonin's tissue levels, its formation in mitochondria, and detoxification of free radicals.

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.

Dim light at night: physiological effects and ecological consequences for infectious disease

Light pollution has emerged as a pervasive component of land development over the past century. Several detrimental impacts of this anthropogenic influence have been identified in night shift workers, laboratory rodents, and a plethora of wildlife species. Circadian, or daily, patterns are interrupted by the presence of light at night and have the capacity to alter rhythmic physiological or behavioral characteristics. Indeed, biorhythm disruption can lead to metabolic, reproductive, and immunological dysfunction depending on the intensity, timing, duration, and wavelength of light exposure. Light pollution, in many forms and by many pathways, is thus apt to affect the nature of host-pathogen interactions. However, no research has yet investigated this possibility. The goal of this manuscript is to outline how dim light at night, a relevant and common form of light pollution, may affect disease dynamics by interrupting circadian rhythms and regulation of immune responses as well as opportunities for host-parasite interactions and subsequent transmission risk including spillover into humans. We close by proposing some promising interventions including alternative lighting methods or vector control efforts.

Melatonin Uptake by Cells: An Answer to Its Relationship with Glucose?

Melatonin, N-acetyl-5-methoxytryptamine, is an indole mainly synthesized from tryptophan in the pineal gland and secreted exclusively during the night in all the animals reported to date. While the pineal gland is the major source responsible for this night rise, it is not at all the exclusive production site and many other tissues and organs produce melatonin as well. Likewise, melatonin is not restricted to vertebrates, as its presence has been reported in almost all the phyla from protozoa to mammals. Melatonin displays a large set of functions including adaptation to light: dark cycles, free radical scavenging ability, antioxidant enzyme modulation, immunomodulatory actions or differentiation–proliferation regulatory effects, among others. However, in addition to those important functions, this evolutionary ‘ancient’ molecule still hides further tools with important cellular implications. The major goal of the present review is to discuss the data and experiments that have addressed the relationship between the indole and glucose. Classically, the pineal gland and a pinealectomy were associated with glucose homeostasis even before melatonin was chemically isolated. Numerous reports have provided the molecular components underlying the regulatory actions of melatonin on insulin secretion in pancreatic beta-cells, mainly involving membrane receptors MTNR1A/B, which would be partially responsible for the circadian rhythmicity of insulin in the organism. More recently, a new line of evidence has shown that glucose transporters GLUT/SLC2A are linked to melatonin uptake and its cellular internalization. Beside its binding to membrane receptors, melatonin transportation into the cytoplasm, required for its free radical scavenging abilities, still generates a great deal of debate. Thus, GLUT transporters might constitute at least one of the keys to explain the relationship between glucose and melatonin. These and other potential mechanisms responsible for such interaction are also discussed here.

Emerging Role of the Cerebrospinal Fluid - Neuronal Interface in Neuropathology

The choroid plexus and cerebrospinal fluid have recently begun to emerge as essential regulators of neural function. Factors produced by the choroid plexus are released into the ventricular environment and thus provide a rich source of extracellular signaling molecules throughout the central nervous system. Identified factors in the cerebrospinal fluid include growth factors, hormones, proteins, peptides, lipids, glucose, microRNAs (miRNAs), messenger RNA (mRNA), and enzymes. In addition to mediating neural function, these factors have the potential to serve as biomarkers of disease states. In this review, we highlight recent advances demonstrating the importance of extracellular signaling mechanisms in mediating neural function and provide recent evidence for their role in neuropathology.

Is pineal melatonin released in the third ventricle in humans? A study in movement disorders

In order to determine sources and metabolism of melatonin in human cerebrospinal fluid (CSF), melatonin and 6-sulfatoxymelatonin (aMT6S) concentrations were measured in CSF sampled during neurosurgery in both lateral and third ventricles in patients displaying movement disorder (Parkinson's disease, essential tremor, dystonia or dyskinesia) and compared with their plasma levels. Previous determinations in nocturnal urine had showed that the patients displayed melatonin excretion in the normal range, compared with healthy controls matched according to age. A significant difference in melatonin concentration was observed between lateral and third ventricles, with the highest levels in the third ventricle (8.75±2.75pg/mL vs. 3.20±0.33pg/mL, P=0.01). CSF aMT6s levels were similar in both ventricles and of low magnitude, less than 5pg/mL. They were not correlated with melatonin levels or influenced by the area of sampling. Melatonin levels were significantly higher in third ventricle than in the plasma, whereas there was no difference between plasma and lateral ventricle levels. These findings show that melatonin may enter directly the CSF through the pineal recess in humans. The physiological meaning of these data remains to be elucidated.

Reciprocal interactions between sleep, circadian rhythms and Alzheimer's disease: focus on the role of hypocretin and melatonin

AD, sleep and circadian rhythm physiology display an intricate relationship. On the one hand, AD pathology leads to sleep and circadian disturbances, with a clear negative influence on quality of life. On the other hand, there is increasing evidence that both sleep and circadian regulating systems exert an influence on AD pathology. In this review we describe the impairments of both sleep regulating systems and circadian rhythms in AD and their link to clinical symptoms, as this may increase knowledge on appropriate diagnosis and adequate treatment of sleep problems in AD. Furthermore we discuss how sleep regulating systems, and especially neurotransmitters such as melatonin and hypocretin, may affect AD pathophysiology, as this may provide a role for lack of sleep and circadian rhythm deterioration in the onset of AD.

Developmental and diurnal expression of the synaptosomal-associated protein 25 (Snap25) in the rat pineal gland

Snap25 (synaptosomal-associated protein) is a 25 kDa protein, belonging to the SNARE-family (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) of proteins, essential for synaptic and secretory vesicle exocytosis. Snap25 has by immunohistochemistry been demonstrated in the rat pineal gland but the biological importance of this is unknown. In this study, we demonstrate a high expression of mRNA encoding Snap25 in all parts of the rat pineal complex, the superficial-, and deep-pineal gland, as well as in the pineal stalk. Snap25 showed a low pineal expression during embryonic stages with a strong increase in expression levels just after birth. The expression showed no day/night variations. Neither removal of the sympathetic input to the pineal gland by superior cervical ganglionectomy nor bilateral decentralization of the superior cervical ganglia significantly affected the expression of Snap25 in the gland. The pineal expression levels of Snap25 were not changed following intraperitoneal injection of isoproterenol. The strong expression of Snap25 in the pineal gland suggests the presence of secretory granules and microvesicles in the rat pinealocyte supporting the concept of a vesicular release. At the transcriptional level, this Snap25-based release mechanism does not exhibit any diurnal rhythmicity and is regulated independently of the sympathetic nervous input to the gland.

Melatonin metabolism in the central nervous system

The metabolism of melatonin in the central nervous system is of interest for several reasons. Melatonin enters the brain either via the pineal recess or by uptake from the blood. It has been assumed to be also formed in some brain areas. Neuroprotection by melatonin has been demonstrated in numerous model systems, and various attempts have been undertaken to counteract neurodegeneration by melatonin treatment. Several concurrent pathways lead to different products. Cytochrome P₄₅₀ subforms have been demonstrated in the brain. They either demethylate melatonin to N-acetylserotonin, or produce 6-hydroxymelatonin, which is mostly sulfated already in the CNS. Melatonin is deacetylated, at least in pineal gland and retina, to 5-methoxytryptamine. N¹-acetyl-N²-formyl-5-methoxykynuramine is formed by pyrrole-ring cleavage, by myeloperoxidase, indoleamine 2,3-dioxygenase and various non-enzymatic oxidants. Its product, N¹-acetyl-5-methoxykynuramine, is of interest as a scavenger of reactive oxygen and nitrogen species, mitochondrial modulator, downregulator of cyclooxygenase-2, inhibitor of cyclooxygenase, neuronal and inducible NO synthases. Contrary to other nitrosated aromates, the nitrosated kynuramine metabolite, 3-acetamidomethyl-6-methoxycinnolinone, does not re-donate NO. Various other products are formed from melatonin and its metabolites by interaction with reactive oxygen and nitrogen species. The relative contribution of the various pathways to melatonin catabolism seems to be influenced by microglia activation, oxidative stress and brain levels of melatonin, which may be strongly changed in experiments on neuroprotection. Many of the melatonin metabolites, which may appear in elevated concentrations after melatonin administration, possess biological or pharmacological properties, including N-acetylserotonin, 5-methoxytryptamine and some of its derivatives, and especially the 5-methoxylated kynuramines.

The regulation of brain states by neuroactive substances distributed via the cerebrospinal fluid; a review

The cerebrospinal fluid (CSF) system provides nutrients to and removes waste products from the brain. Recent findings suggest, however, that in addition, the CSF contains message molecules in the form of actively released neuroactive substances. The concentrations of these vary between locations, suggesting they are important for the changes in brain activity that underlie different brain states, and induce different sensory input and behavioral output relationships.The cranial CSF displays a rapid caudally-directed ventricular flow followed by a slower rostrally-directed subarachnoid flow (mainly towards the cribriform plate and from there into the nasal lymphatics). Thus, many brain areas are exposed to and can be influenced by substances contained in the CSF. In this review we discuss the production and flow of the CSF, including the mechanisms involved in the regulation of its composition. In addition, the available evidence for the release of neuropeptides and other neuroactive substances into the CSF is reviewed, with particular attention to the selective effects of these on distant downstream receptive brain areas. As a conclusion we suggest that (1) the flowing CSF is involved in more than just nutrient and waste control, but is also used as a broadcasting system consisting of coordinated messages to a variety of nearby and distant brain areas; (2) this special form of volume transmission underlies changes in behavioral states.

Melatonin is released in the third ventricle in humans. A study in movement disorders

In order to determine sources and metabolism of melatonin in human cerebrospinal fluid (CSF), melatonin and 6-sulfatoxymelatonin (aMT6S) concentrations were measured in CSF sampled during neurosurgery in both lateral and third ventricles in patients displaying movement disorder (Parkinson's disease, essential tremor, dystonia or dyskinesia) and compared with their plasma levels. Previous determinations in nocturnal urine had showed that the patients displayed melatonin excretion in the normal range, compared with healthy controls matched according to age. A significant difference in melatonin concentration was observed between lateral and third ventricles, with the highest levels in the third ventricle (8.75+/-2.75 pg/ml vs. 3.20+/-0.33 pg/ml, p=0.01). CSF aMT6s levels were similar in both ventricles and of low magnitude, less than 5 pg/ml. They were not correlated with melatonin levels or influenced by the area of sampling. Melatonin levels were significantly higher in third ventricle than in the plasma, whereas there was no difference between plasma and lateral ventricle levels. These findings show that melatonin may enter directly the CSF through the pineal recess in humans. The physiological meaning of these data remains to be elucidated.

Melatonin: signaling mechanisms of a pleiotropic agent

Melatonin acts both as a hormone of the pineal gland and as a local regulator molecule in various tissues. Quantities of total tissue melatonin exceed those released from the pineal. With regard to this dual role, to the orchestrating, systemic action on various target tissues, melatonin is highly pleiotropic. Numerous secondary effects result from the control of the circadian pacemaker and, in seasonal breeders, of the hypothalamic/pituitary hormonal axes. In mammals, various binding sites for melatonin have been identified, the membrane receptors MT(1) and MT(2), which are of utmost chronobiological importance, ROR and RZR isoforms as nuclear receptors from the retinoic acid receptor superfamily, quinone reductase 2, calmodulin, calreticulin, and mitochondrial binding sites. The G protein-coupled receptors (GPCRs) MT(1) and MT(2) are capable of parallel or alternate signaling via different Galpha subforms, in particular, Galpha(i) (2/) (3) and Galpha(q), and via Gbetagamma, as well. Multiple signaling can lead to the activation of different cascades and/or ion channels. Melatonin frequently decreases cAMP, but also activates phospholipase C and protein kinase C, acts via the MAP kinase and PI3 kinase/Akt pathways, modulates large conductance Ca(2+)-activated K(+) and voltage-gated Ca(2+) channels. MT(1) and MT(2) can form homo and heterodimers, and MT(1) interacts with other proteins in the plasma membrane, such as an orphan GPCR, GPR50, and the PDZ domain scaffolding protein MUPP1, effects which negatively or positively influence signaling capacity. Cross-talks between different signaling pathways, including influences of the membrane receptors on nuclear binding sites, are discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Ventricular cerebrospinal fluid melatonin concentrations investigated with an endoscopic technique

The role of melatonin in humans still remains unclear. Uncertainties persist about its effects on neurophysiology regarding its levels in human cerebrospinal fluid (CSF), as the bulk of knowledge on this subject mainly derives from studies conducted on animals. In this study, CSF was micro-sampled with a simple, new method from each cerebral ventricle of patients undergoing neuroendoscopy for hydrocephalus. Our purpose was to measure CSF melatonin levels and determine possible differences in its concentration among various significant areas in the cerebral ventricles (e.g. pineal recess, pituitary recess, lateral ventricle, fourth ventricle) and lumbar cistern. From 2002 to 2004, 10 hydrocephalic patients were operated on using a neuroendoscopic technique. The CSF specimens were investigated for melatonin concentrations (free plus protein-bound) after deproteinization; the measurement technique was high-performance liquid chromatography. The preliminary data obtained with this endoscopic micro-sampling technique (applied to humans for the first time) suggest that melatonin is more concentrated within the ventricles and its highest concentration is found in the third ventricle (IIIv), although the difference detected between the CSF of the IIIv and that of the pineal recess was not significant.

Dendritic peptide release and peptide-dependent behaviours

Neuropeptides that are released from dendrites, such as oxytocin and vasopressin, function as autocrine or paracrine signals at their site of origin, but can also act at distant brain targets to evoke long-lasting changes in behaviour. Oxytocin, for instance, has profound effects on social bonding that are exerted at sites that richly express oxytocin receptors, but which are innervated by few, if any, oxytocin-containing projections. How can a prolonged, diffuse signal have coherent behavioural consequences? The recently demonstrated ability of neuropeptides to prime vesicle stores for activity-dependent release could lead to a temporary functional reorganization of neuronal networks harbouring specific peptide receptors, providing a substrate for long-lasting effects.

Essential hypertension seems to result from melatonin-induced epigenetic modifications in area postrema

Essential hypertension is a complex multifactorial disorder with epigenetic and environmental factors contributing to its prevalence. Epigenetic system is a genetic regulatory mechanism that allows humans to maintain extraordinarily stable patterns of gene expression over many generations. Sympathetic nervous system plays a major role in the maintenance of hypertension and the rostral ventrolateral medulla is the main source of this sympathetic activation. A possible mechanism to explain the sympathetic hyperactivity in the rostral ventrolateral medulla is an action of the area postrema. Area postrema seems to be the region where a shift of the set-point to a higher operating pressure occurs resulting in hypertension. But, how can a shift occur in the area postrema. We propose that melatonin-induced epigenetic modifications in the neurons of area postrema plays a role in this shift. Area postrema is reported to contain high levels of melatonin receptors that play a role in the epigenetic modifications in certain cells. Environmental stressors cause epigenetic modifications in the neurons of area postrema via the pineal hormone melatonin and these changes lead to a shift in the set-point to a higher operating pressure. This signal is then sent via efferent projections to key medullary sympathetic nuclei in rostral ventrolateral medulla resulting in increases in sympathetic nerve activity. This model may explain the long-term alterations in sympathetic activity in essential hypertension.

Melatonin seems to be a mediator that transfers the environmental stimuli to oocytes for inheritance of adaptive changes through epigenetic inheritance system

Possibility of inheritance of epigenetic modifications have led us to consider that adaptive geographic variations in humans may result from interactions between environmental factors and epigenetic inheritance system. In this system melatonin seems to be a mediator that transfers the environmental stimuli to germ cells (oocytes). While environmental factors produce modifications in the body, they simultaneously induce epigenetic modifications in the oocytes with the help of melatonin, and these changes are inherited to offspring. In this way, adaptive changes could be passed on to the next generation. This kind of heritable long-term changes is generally labeled biological adaptation. But, how can melatonin cause epigenetic changes in oocytes? We suggest that melatonin induces epigenetic modifications by affecting the nuclear melatonin receptors that can in turn change the superstructure of DNA. It was previously suggested that biological adaptation is limited to neural crest derivatives such as, craniofacial tissues, melanocytes, and structures related to stature, hair form and body proportions. Thus, inheritance of adaptive changes is possible only where environmental factors affect the neural crest derivatives, including the cells that produce the next generation.