Melatonin effects on brain. Interaction with microtubule protein, inhibition of fast axoplasmic flow and induction of crystaloid and tubular formations in the hypothalamus

From Chronodisruption to Sarcopenia: The Therapeutic Potential of Melatonin

Sarcopenia is an age-related condition that involves a progressive decline in muscle mass and function, leading to increased risk of falls, frailty, and mortality. Although the exact mechanisms are not fully understood, aging-related processes like inflammation, oxidative stress, reduced mitochondrial capacity, and cell apoptosis contribute to this decline. Disruption of the circadian system with age may initiate these pathways in skeletal muscle, preceding the onset of sarcopenia. At present, there is no pharmacological treatment for sarcopenia, only resistance exercise and proper nutrition may delay its onset. Melatonin, derived from tryptophan, emerges as an exceptional candidate for treating sarcopenia due to its chronobiotic, antioxidant, and anti-inflammatory properties. Its impact on mitochondria and organelle, where it is synthesized and crucial in aging skeletal muscle, further highlights its potential. In this review, we discuss the influence of clock genes in muscular aging, with special reference to peripheral clock genes in the skeletal muscle, as well as their relationship with melatonin, which is proposed as a potential therapy against sarcopenia.

The Zebrafish, an Outstanding Model for Biomedical Research in the Field of Melatonin and Human Diseases

The zebrafish has become an excellent model for the study of human diseases because it offers many advantages over other vertebrate animal models. The pineal gland, as well as the biological clock and circadian rhythms, are highly conserved in zebrafish, and melatonin is produced in the pineal gland and in most organs and tissues of the body. Zebrafish have several copies of the clock genes and of aanat and asmt genes, the latter involved in melatonin synthesis. As in mammals, melatonin can act through its membrane receptors, as with zebrafish, and through mechanisms that are independent of receptors. Pineal melatonin regulates peripheral clocks and the circadian rhythms of the body, such as the sleep/wake rhythm, among others. Extrapineal melatonin functions include antioxidant activity, inducing the endogenous antioxidants enzymes, scavenging activity, removing free radicals, anti-inflammatory activity through the regulation of the NF-κB/NLRP3 inflammasome pathway, and a homeostatic role in mitochondria. In this review, we introduce the utility of zebrafish to analyze the mechanisms of action of melatonin. The data here presented showed that the zebrafish is a useful model to study human diseases and that melatonin exerts beneficial effects on many pathophysiological processes involved in these diseases.

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.

Cellular Mechanisms of Melatonin: Insight from Neurodegenerative Diseases

Neurodegenerative diseases are the second most common cause of death and characterized by progressive impairments in movement or mental functioning in the central or peripheral nervous system. The prevention of neurodegenerative disorders has become an emerging public health challenge for our society. Melatonin, a pineal hormone, has various physiological functions in the brain, including regulating circadian rhythms, clearing free radicals, inhibiting biomolecular oxidation, and suppressing neuroinflammation. Cumulative evidence indicates that melatonin has a wide range of neuroprotective roles by regulating pathophysiological mechanisms and signaling pathways. Moreover, melatonin levels are decreased in patients with neurodegenerative diseases. In this review, we summarize current knowledge on the regulation, molecular mechanisms and biological functions of melatonin in neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, vascular dementia and multiple sclerosis. We also discuss the clinical application of melatonin in neurodegenerative disorders. This information will lead to a better understanding of the regulation of melatonin in the brain and provide therapeutic options for the treatment of various neurodegenerative diseases.

Melatonin Target Proteins: Too Many or Not Enough?

The neurohormone N-acetyl-5-methoxytryptamine, better known as melatonin, is a tryptophan derivative with a wide range of biological effects that is present in many organisms. These effects are believed to rely either on the chemical properties of melatonin itself as scavenger of free radicals or on the binding of melatonin to protein targets. More than 15 proteins, including receptors (MT1, MT2, Mel1c, CAND2, ROR, VDR), enzymes (QR2, MMP-9, pepsin, PP2A, PR-10 proteins), pores (mtPTP), transporters (PEPT1/2, Glut1), and other proteins (HBS, CaM, tubulin, calreticuline), have been suggested to interact with melatonin at sub-nanomolar to millimolar melatonin concentrations. In this review we assemble for the first time the available information on proposed melatonin targets and discuss them in a comprehensive manner to evaluate the robustness of these findings in terms of methodology, physiological relevance, and independent replication.

Melatonin deficiency at tissue level: a possible aetiological factor in nasal polyposis

OBJECTIVE

The aim of this study was to examine whether melatonin is involved in the pathogenesis of nasal polyposis.

METHOD

This study included 29 patients with nasal polyposis and undergoing functional endoscopic sinus surgery. As a control group, 26 patients who had been operated on for a deviated nasal septum and concha bullosa were enrolled. Samples were taken from the nasal polyp tissue and from the resected middle concha bullosa mucosa of the control group. Serum samples were taken from all patients.

RESULTS

It was found that the tissue and serum melatonin levels in the nasal polyp group were significantly lower compared with the tissue and serum melatonin levels in the control group.

CONCLUSION

In nasal polyposis, the melatonin level in the serum and tissue is lower than in individuals without polyposis. This deficiency may play a role in the pathogenesis of nasal polyposis.

Melatonin and age-related cardiovascular diseases

The pineal gland is a neuroendocrine gland closely related to human aging. Melatonin is a kind of indole neuroendocrine hormone secreted by the pineal gland, which is essential for maintaining physiological function. Many researches found that melatonin plays a key role in anti-aging-related cardiovascular diseases. In this paper, the latest advances in the study of melatonin and aging-related cardiovascular diseases are reviewed, and their related physiological functions and mechanisms are discussed.

Melatonin and its agonist ramelteon in Alzheimer's disease: possible therapeutic value

Alzheimer's disease (AD) is an age-associated neurodegenerative disease characterized by the progressive loss of cognitive function, loss of memory and insomnia, and abnormal behavioral signs and symptoms. Among the various theories that have been put forth to explain the pathophysiology of AD, the oxidative stress induced by amyloid β-protein (Aβ) deposition has received great attention. Studies undertaken on postmortem brain samples of AD patients have consistently shown extensive lipid, protein, and DNA oxidation. Presence of abnormal tau protein, mitochondrial dysfunction, and protein hyperphosphorylation all have been demonstrated in neural tissues of AD patients. Moreover, AD patients exhibit severe sleep/wake disturbances and insomnia and these are associated with more rapid cognitive decline and memory impairment. On this basis, the successful management of AD patients requires an ideal drug that besides antagonizing Aβ-induced neurotoxicity could also correct the disturbed sleep-wake rhythm and improve sleep quality. Melatonin is an effective chronobiotic agent and has significant neuroprotective properties preventing Aβ-induced neurotoxic effects in a number of animal experimental models. Since melatonin levels in AD patients are greatly reduced, melatonin replacement has the potential value to be used as a therapeutic agent for treating AD, particularly at the early phases of the disease and especially in those in whom the relevant melatonin receptors are intact. As sleep deprivation has been shown to produce oxidative damage, impaired mitochondrial function, neurodegenerative inflammation, and altered proteosomal processing with abnormal activation of enzymes, treatment of sleep disturbances may be a priority for arresting the progression of AD. In this context the newly introduced melatonin agonist ramelteon can be of much therapeutic value because of its highly selective action on melatonin MT₁/MT₂ receptors in promoting sleep.

Light deprivation improves melatonin related suppression of hippocampal plasticity

In early postnatal life, sensory inputs deeply influence development as well as function of the brain. Plasticity of synaptic transmission including its experimentally induced form, long-term potentiation (LTP), is affected by sensory deprivation in neocortex. This study is devoted to assess if dark rearing and a dark phase synthesized hormone melatonin influence LTP in the hippocampus, an area of brain involved in learning and memory. In vivo experiments were carried out on two groups of 45-days-old male Wistar rats kept in standard 12-h light/dark condition [light reared (LR) tested during the light phase] or in complete darkness [dark reared (DR)] since birth to testing. Each group, in turn, was divided to two, vehicle- and melatonin-treated, groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus extracellular postsynaptic potentials (EPSPs) were recorded in the CA1 area. Having the stable baseline responses to the test pulses, the hippocampus was perfused by either vehicle or 2 microg melatonin and EPSPs were recorded for 30 min. Then, for induction of LTP, the tetanus was applied to the Schaffer collaterals and the field potentials were pooled for 120-min post-tetanus. The light deprivation resulted in a significant augmentation in the amplitude of baseline responses. Also, we observed a melatonin-induced increase in amplitude of the baseline recordings in either LR or DR animals. Tetanic stimulation elicited LTP of EPSPs in both LR and DR groups, robustly in the former where it lasted for about 90 min. Generally, melatonin inhibited the production of LTP in the two groups especially in the LR animals leading to a noticeable depression. We concluded that higher level of neuronal activity in the DR rats gives rise to a lower level of LTP. Weaker effect of melatonin on blocking the potentiation of post-tetanus EPSPs in the DR rats may be the result of a desensitization of melatonin receptors due to chronically increased levels of this hormone in the visually deprived rats.

Malaria: therapeutic implications of melatonin

Malaria, which infects more than 300 million people annually, is a serious disease. Epidemiological surveys indicate that of those who are affected, malaria will claim the lives of more than one million individuals, mostly children. There is evidence that the synchronous maturation of Plasmodium falciparum, the parasite that causes a severe form of malaria in humans and Plasmodium chabaudi, responsible for rodent malaria, could be linked to circadian changes in melatonin concentration. In vitro melatonin stimulates the growth and development of P. falciparum through the activation of specific melatonin receptors coupled to phospholipase-C activation and the concomitant increase of intracellular Ca2+. The Ca2+ signaling pathway is important to stimulate parasite transition from the trophozoite to the schizont stage, the final stage of intraerythrocytic cycle, thus promoting the rise of parasitemia. Either pinealectomy or the administration of the melatonin receptor blocking agent luzindole desynchronizes the parasitic cell cycle. Therefore, the use of melatonin antagonists could be a novel therapeutic approach for controlling the disease. On the other hand, the complexity of melatonin's action in malaria is underscored by the demonstration that treatment with high doses of melatonin is actually beneficial for inhibiting apoptosis and liver damage resulting from the oxidative stress in malaria. The possibility that the coordinated administration of melatonin antagonists (to impair the melatonin signal that synchronizes P. falciparum) and of melatonin in doses high enough to decrease oxidative damage could be a novel approach in malaria treatment is discussed.

Photoimmunomodulation and melatonin

The seasons, and daily physical rhythms can have a profound effect on the physiology of the living organism, which includes immune status. The immune system can be influenced by a variety of signals and one of them is photic stimulus. Light may regulate the immunity through the neuroendocrine system leading to the most recent branch of research the "Photoimmunomodulation". Mammals perceive visible light (400-700 nm) through some specialized photoreceptors located in retina like retinal ganglion cells (RGC). This photic signal is then delivered to the visual cortex from there to the suprachiasmatic nucleus (SCN) of the hypothalamic region. Melatonin--one of the universally accepted chronobiotic molecule secreted by the pineal gland is now emerging as one of the most effective immunostimulatory compound in rodents and as oncostatic molecule at least in human. Its synthesis decreases with light activation along with norepinephrine and acetylcholine. The changes in level of melatonin may lead to alterations (stimulatory/inhibitory) in immune system. The evidences for the presence of melatonin receptor subtypes on lymphoid tissues heralded the research area about mechanism of action for melatonin. Further, melatonin receptor subtypes-MT1 and MT2 was noted on pars tuberalis, SCN and on lymphatic tissues suggesting a direct action of melatonin in modulation of immunity by photoperiod as well. The nuclear receptors (ROR, RZR etc.) of melatonin are known for its free radical scavenging actions and might be indirectly controlling the immune function.

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

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

Photoperiodism in birds

Birds show a circadian rhythm in melatonin secretion and, as expected, the pattern of output changes with photoperiod. Somewhat surprisingly then, in view of the mechanisms in mammals, birds do not seem to use this seasonal message in the photoperiodic control of reproduction. Some further experiments are needed, however, because in birds the pineal gland is not the only source of melatonin. Another difference from mammals is that birds detect the photoperiodic light not with the retina but by brain photoreceptors, which probably lie in the hypothalamus. An action spectrum for these receptors has now been obtained for the quail and this shows a peak absorption at 492 nm, suggesting that the photoreceptor is rhodopsin-based. The sensitivity of the brain receptors to 500 nm light was calculated at 2 X 10(4) photons mm-2s-1. For light to induce the photoperiodic response it must be interpreted by the bird's clock as a long day. This happens if the light falls 12-20 h after dawn and coincides with a rhythm of photosensitivity. The subsequent neuroendocrine response to the light signal is both precise and relatively long-term. A single 4 h light pulse initiates a wave of gonadotropin secretion lasting for 10 days. The light stimulus can be replaced by a brief (2 min) daily electrical stimulus given to the hypothalamus 10-12 h after dawn. Over the next few years it should be possible to disentangle further the neural processes involved.

Recovery of experimental Parkinson's disease with the melatonin analogues ML-23 and S-20928 in a chronic, bilateral 6-OHDA model: a new mechanism involving antagonism of the melatonin receptor

Over the past 10 years, there has been a resurgence of interest in examining the role of melatonin in health and disease. While the brunt of research in this area has portrayed melatonin in a favorable light, there is a growing body of evidence suggesting that melatonin may possess adverse effects contributing to the development of various neuropsychiatric disease states. In preclinical models of Parkinson's disease (PD), melatonin has been shown to enhance the severity of this condition while its antagonism, using constant light or pinealectomy, facilitates recovery. To test this hypothesis further, the present study employed the melatonin analogues ML-23 and S-20928 in a post-6-OHDA injection regime to determine whether they may have a favorable effect on the symptoms of this more chronic model of PD. When ML-23 was injected I.P. in a dose of 3 mg/kg twice daily for 3.5 days after 6-OHDA, significant improvement in motor function and regulatory deficits was observed. Similarly, the injection of S-20928 in a 1 mg/kg dose (I.P.), in the same regimen, facilitated modest improvement in motor function and regulatory deficits while the larger dose enhanced the severity of behavioural deficits and produced severe side effects causing deterioration in condition during the course of drug administration. ML-23 administration totally abolished the 6-OHDA-induced mortality, which accompanies dopamine (DA) degeneration, while S-20928 had no effect on this parameter. These results suggest that some melatonin analogues can aid in recovery from DA depleting lesions after DA degeneration has commenced and the recovery is not attributable to the antioxidative properties of this hormone. While the exact mechanism by which ML-23 and S-20928 are exerting their therapeutic effect is unclear, it is possible that antagonism of melatonin receptors may play some role and this should be considered when assessing the potential of melatonin analogues for treatment of human neuropsychiatric disorders.

The therapeutic effects of dopamine replacement therapy and its psychiatric side effects are mediated by pineal function

There are reports that melatonin secretion from the pineal gland gradually diminishes with advancing age. It has been suggested that various forms of neuropsychiatric disease, in particular, Parkinson's disease (PD), is consequentially related to this decrease by virtue of increased oxidative stress which enhances the process of dopamine (DA) degeneration. There is, however, considerable disagreement on this theme as very little is generally known about the role of the pineal gland in the aetiology and treatment of PD. To assess the role of the pineal gland in PD and in dopamine replacement therapy (DART), the effect of three anti-Parkinsonian drugs on motor and psychiatric function was assessed in normal, pinealectomized (PX) and DA deficient, PX rats. In the first study, rats underwent PX or sham operation and were then injected (IP) with Amantadine (30 or 50 mg/kg), Bromocriptine (5 or 10 mg/kg) or L-Dopa (30 or 60 mg/kg plus 50 mg/kg of R-044602) 3-8 weeks after surgery. Open field performance and motor reflex tests were assessed during the light and dark phases of the L/D cycle. In a second study, clinically effective doses of Bromocriptine (10 mg/kg) and L-Dopa (30 and 100 mg/kg with 50 mg/kg R-044602) were injected into depleted, PX or sham operated rats. In study I, sham operated and PX rats responded differently to Bromocriptine and L-Dopa, while Amantadine did not differentially effect motor performance in the two groups. In study II, 6-OHDA induced degeneration of the nigro-striatal system abolished the effects of Bromocriptine and dramatically altered the effects of L-Dopa seen in study I, in sham operated versus PX rats. DART significantly altered emotionality, as measured by escape attempts, agitation and rage in sham operated animals, compared to PX rats. DA deficiency abolished the tendency to escape in all groups except those treated with 100mg/kg of L-Dopa. Conversely, agitation and rage scores were greater after 100 mg/kg of L-Dopa, in rats with intact pineal function, than in PX rats. These results provide compelling evidence that altered pineal function plays a major role in the aetiology of PD, the therapeutic effect of anti-Parkinsonian drugs and in the psychiatric side effects of DART.

Melatonin induction of filamentous structures in non-neuronal cells that is dependent on expression of the human mt1 melatonin receptor

Melatonin has gained recent popularity as a treatment for insomnia and other sleep disorders; however, its cellular effects are unknown. We report the effects of melatonin on the cellular morphology of Chinese hamster ovary (CHO) cells transformed to express the human melatonin receptors, mt1 and MT2. Our results show that melatonin exerts a strong influence on cellular shape and cytoskeletal organization in a receptor-dependent and possibly subtype-selective manner. The cell shape change that we see after a 5-h treatment of these non-neuronal cells with a pharmacological concentration of melatonin consists of the formation of long filamentous outgrowths that are reminiscent of the neurite processes produced by differentiating nerve cells. This morphological change occurs exclusively in cells expressing the mt1 receptor. We find that the microtubule and microfilament organization within these outgrowths is similar to that of neurites. Microtubules are required for the shape change to occur as Colcemid added in combination with melatonin completely blocks outgrowth formation. We demonstrate that the number of cells showing the altered cell shape is dependent on melatonin concentration, constant exposure to melatonin and that outgrowth frequencies increase when protein kinase A (PKA) is inhibited. Concomitant melatonin-dependent increases in MEK 1/2 and ERK 1/2 phosphorylation are noted in mt1-CHO cells only. The production of filamentous outgrowths is dependent on the translation of new protein but not the transcription of new mRNA. Outgrowth number is not controlled by centrosomes but is instead controlled by the polymerization state of the actin cytoskeleton. The results of this work show that the organization of the cytoskeleton is affected by processes specifically mediated or regulated by the mt1 receptor and may represent a novel alternative mechanism for the stimulation of process formation.

Melatonin alters the metabolism of the beta-amyloid precursor protein in the neuroendocrine cell line PC12

The deposition of amyloid plaques in brain parenchyma is one of the major pathological hallmarks of Alzheimer's disease (AD). The amyloid in senile plaques is composed of the amyloid beta-peptide (A beta) of 39-43 amino acid residues derived from a larger beta-amyloid precursor protein (beta APP). Soluble derivatives of beta APP (sAPP) lacking the cytoplasmic tail, transmembrane domain, and a small portion of the extracellular domain are generated proteolytically by "secretases." Using cell cultures, the authors analyzed the level of sAPP in neuroblastoma and pheochromocytoma (PC12) cells by immunoblotting samples from conditioned media and cell lysates. Normal levels of secretion of sAPP into conditioned media were severely inhibited by treating cells with melatonin (3-4 mM). The inhibitory effect of melatonin on the secretion of sAPP can be reversed. When the cells that were pretreated with melatonin for 10 h were washed, the normal level of secretion of sAPP was restored. Northern blot analyses indicated that the treatment of PC12 cells with melatonin resulted in a significant decrease in the level of mRNA encoding beta APP, beta-actin, and glyceraldehyde-3-phosphate dehydrogenase, and that the treatment of a human neuroblastoma cell line with melatonin resulted in no change in levels of these messages. The secretion of sAPP into the conditioned medium was substantially reduced in the differentiated cells similar to reductions observed in melatonin-treated undifferentiated PC12 cells. Melatonin was found to potentiate the nerve growth factor-mediated differentiation in PC12 cells at 24 h. Taken together, these data suggest that melatonin regulates the metabolism of beta APP and other housekeeping genes in a cell-type specific manner, and that melatonin accelerates the early process of neuronal differentiation.

Melatonin site and mechanism of action: single or multiple?

By affecting the entrainment pathways of the biologic clock, melatonin has a major influence on the circadian and seasonal organization of vertebrates. In addition, a number of versatile functions that far transcend melatonin actions on photoperiodic time measurement and circadian entrainment have emerged. Melatonin is a free radical scavenger and antioxidant and it has a significant immunomodulatory activity, being presumably a major factor in an organism's defense toxic agents and invading organisms. Besides affecting specific receptors in cell membranes to exert its effects, the interaction of melatonin with nuclear receptor sites and with intracellular proteins, like calmodulin or tubulin-associated proteins, as well as the direct antioxidant effects of melatonin, may explain many general functions of the pineal hormone.

Effect of melatonin on beta-tubulin and MAP2 expression in NIE-115 cells

Physiological concentrations of the pineal hormone melatonin induce an increase of microtubules in neuroblastoma NIE-115 cells. This effect is due to an increase in the polymerization state of tubulin. Concomitantly, higher levels of soluble beta-tubulin are present in the treated cells. Unexpectedly, no significant changes in the levels of beta-tubulin or its mRNA occur in the presence of melatonin reflecting perhaps a strict control of its steady state in a physiological context. In contrast, higher amounts of microtubule-associated-protein 2 are found when the cells are exposed to melatonin. These findings support the idea that tubulin polymerization process is one of the targets of melatonin action. Furthermore, our results might explain the increase in the length and number of neurites present in these cells when they are treated with this hormone.

Effects of pinealectomy and melatonin treatments on serotonin uptake and release from synaptosomes of rat hypothalamic regions

This study examined the effects induced by long-term pinealectomy, daily melatonin treatment to pinealectomized and intact rats, and a single melatonin injection on [14C]-serotonin (5-HT) uptake and release from synaptosomes obtained of hypothalamic regions. Pinealectomy inhibited the accumulation of labeled 5-HT by synaptosomes of the preoptic area-anterior hypothalamus (POA-AH), but it failed to alter the [K+]-evoked 5-HT release. Melatonin treatment for 10 consecutive days to pinealectomized rats restored 5-HT uptake in POA-AH, and also increased 5-HT release in medial and posterior hypothalamus. These results suggest that pineal melatonin plays a stimulatory role on the serotoninergic terminals of the hypothalamus. Moreover, when daily melatonin treatment was administered to intact rats a significant increase in 5-HT uptake activity by synaptosomes of all the hypothalamic regions was observed, but 5-HT release was unaffected. In contrast, a single melatonin injection induced a significant decrease in 5-HT release from synaptosomes of the POA-AH was observed. The results suggest the existence of a differential sensitivity in the mechanisms mediating melatonin actions on 5-HT uptake/release, which depends on the presence of the pineal gland in the animals and on the frequency of the treatments with the pineal hormone.

Effects of melatonin on microtubule assembly depend on hormone concentration: role of melatonin as a calmodulin antagonist

Melatonin may play a key role in cytoskeletal rearrangements through its calmodulin antagonism. In the present work, we tested this hypothesis by studying melatonin effects on both microtubule polymerization in vitro and cytoskeletons in situ. Microtubule assembly is a dynamic process inhibited by Ca2+/calmodulin. Calmodulin antagonists prevent the inhibition by binding to Ca(2+)-activated calmodulin, thus causing microtubule enlargement. In the presence of calmodulin (5 microM) and CaCl2 (1 mM), polymerization at equilibrium was inhibited by 40%. Complete reversal of the Ca2+/calmodulin effect on microtubules was observed with 10(-9) M melatonin or with 10(-5) M trifluoperazine or 1 microgram/ml of compound 48/80. In the absence of Ca2+/calmodulin, melatonin at 10(-5) M inhibited tubulin polymerization like 10(-4) M trifluoperazine does. Melatonin effects on microtubule assembly at both nanomolar and micromolar ranges were corroborated in cytoskeletons in situ. Therefore, it is suggested that at a low concentration (10(-9) M), cytoskeletal melatonin effects are mediated by its antagonism to Ca2+/calmodulin. At a higher concentration (10(-5) M), non-specific binding of melatonin to tubulin occurs, thus overcoming the melatonin antagonism to Ca2+/calmodulin. The results support the hypothesis that under physiological conditions, melatonin synchronizes different body rhythms through cytoskeletal rearrangements mediated by its calmodulin antagonism.

Calmodulin mediates melatonin cytoskeletal effects

In this article, we review the data concerning melatonin interactions with calmodulin. The kinetics of melatonin-calmodulin binding suggest that the hormone modulates cell activity through intracellular binding to the protein at physiological concentration ranges. Melatonin interaction with calmodulin may allow the hormone to modulate rhythmically many cellular functions. Melatonin's effect on tubulin polymerization, and cytoskeletal changes in MDCK and N1E-115 cells cultured with melatonin, suggest that at low concentrations (10(-9) M) cytoskeletal effects are mediated by its antagonism to Ca2+-calmodulin. At higher concentrations (10(-5)M) non-specific binding of melatonin to tubulin occurs thus overcoming the specific melatonin antagonism to Ca2+-calmodulin. Since the structures of melatonin and calmodulin are phylogenetically well preserved, calmodulin-melatonin interaction probably represents a major mechanism for regulation and synchronization of cell physiology.

The contribution of extrapineal sites of melatonin synthesis to circulating melatonin levels in higher vertebrates

While the production of melatonin in higher vertebrates occurs in other organs and tissues besides the pineal, the contribution of extrapineal sites of melatonin synthesis such as the retina, the Harderian glands and the gut to circulating melatonin levels is still a matter of debate. The amount of melatonin found in the gastrointestinal tract is much higher than in any other organ including the pineal and the gut appears to make a significant contribution to circulating melatonin at least under certain conditions. The gut has been identified to be the major source of the elevated plasma concentrations of melatonin seen after tryptophan administration and of the changes of circulating melatonin level induced by the feeding regimen. Whereas the circadian and circannual fluctuations of the concentration of melatonin in the blood seem to be triggered by changes of the photoenvironment and its effect of pineal melatonin formation, basal daytime melatonin levels and the extent of their elevation at nighttime appear to be additionally controlled by nutritional factors, such as the amount and the composition of ingested food and therefore availability of tryptophan as a rate-limiting precursor of melatonin formation by the enterochromaffin cells of the gastrointestinal tract.

Neuronal markers in the rodent pineal gland--an immunohistochemical investigation

Although some embryological and morphological features speak in favour of a neuronal character of rodent pinealocytes, histochemistry and ultrastructure let this issue appear controversial. Using antibodies to different neurofilaments, the neural adhesion molecule L1, synaptophysin and tubulin as neuronal markers, the pineal glands of rat and guinea-pig were studied by means of immunofluorescence. Neurofilament-immunoreactivity was present in some rat pineal nerve fibers and in the majority of guinea-pig pinealocytes, L1 decorated rat intrapineal nerve fibers, synaptophysin was almost ubiquitously distributed in the pineal of both species, while tubulin-immunofluorescence was seen in nerve fibers of rat and guinea-pig pineal and in some pinealocytes of the latter. These findings speak in favour of the neuronal character of guinea-pig pinealocytes. The lack of neurofilament- and tubulin-immunoreactivity in rat pinealocytes might be attributable to very low concentrations of these proteins or species differences as to their expression. Further studies including in situ-hybridisation of relevant mRNAs will be necessary to answer these questions definitely.

Time-dependent effect of melatonin on actin mRNA levels and incorporation of 35S-methionine into actin and proteins by the rat hypothalamus

The synthesis of the cytoskeletal protein actin exhibits, in the rat hypothalamus, a diurnal variation with maxima during morning hours. The objective of the present study was to assess whether melatonin injection could affect the in vitro incorporation of 35S-methionine into actin, as well as the levels of actin mRNA, in the hypothalamus of adult male rats treated either acutely or chronically with the hormone at 10:00 or 18:00. Injection of 100 micrograms/kg of melatonin for ten days at either time induced a significant depression in the incorporation of 35S-methionine into a 43 kDa protein with the electrophoretic mobility of actin. The specific activity of total soluble proteins after labeled methionine incubations decreased only after evening melatonin administration (100 micrograms/kg, ten days). Hypothalamic actin mRNA levels, quantitated by dot-blot analysis, decreased only after the injection of 100 micrograms/kg melatonin for ten days at 10:00. Neither a 10-micrograms/kg dose of melatonin, nor a single injection of 100 micrograms/kg melatonin, caused any significant change in the parameters examined. Melatonin (100 micrograms/kg for ten days) did not modify hypothalamic somatostatin or H-Ras mRNA concentration. These results suggest the existence of an inhibitory effect of melatonin on hypothalamic actin synthesis.

Melatonin effects on the cytoskeletal organization of MDCK and neuroblastoma N1E-115 cells

Despite the fact that many physiological and pharmacological actions of melatonin (MEL) have been described, its mechanism of action at the subcellular level remains unclear. It has been suggested that MEL has effects on cellular processes that involve microfilaments and microtubules. In the present study MEL effects on the cytoskeleton were evaluated in MDCK and N1E-115 cells in which the microfilaments have been shown to participate in cell morphology and dome formation (MDCK) and the microtubules in neurite outgrowths. After one day of culture with 10(-11)-10(-7) M MEL MDCK cells showed an increase in the number of elongated cells. After four days with the hormone, an increase in the incidence of MDCK cells contacting neighboring cells through long cytoplasmic elongations was observed. Actin antibody stain showed the appearance of thicker fluorescent fibres beneath the cell membrane and over the nucleus in the MEL treated cells. An increase in dome formation in confluent cells was also observed. In N1E-115 cells MEL (10(-13)-10(-5) M) induced an increase in cell with neurite processes. Neurite outgrowth is clearly seen at 24 h after plating. MEL-treated cells grow in clusters with neurites forming intricate networks. Antitubulin antibody stain showed long fluorescent neurites in the N1E-115 MEL-treated cells. A decrease in N1E-115 neurite formation was observed with either serotonin or 6-hydroxymelatonin (6OH-MEL). However, the number of MDCK cells with cytoplasmic elongations was decreased only after 6OH-MEL. We conclude that MEL action at the cellular level involves a modification of the cytoskeletal organization.

The effects of melatonin and pinealectomy upon local cerebral glucose utilization in awake unrestrained rats are restricted to a few specific regions

The effects of the infusion of melatonin (MT) and/or of pinealectomy upon glucose utilization in anatomically discrete regions of the brain of freely moving rats have been studied by the quantitative autoradiographic 2-deoxy-D-[1-14C]glucose technique. The experiments were made from 14.00 to 16.00 h, when MT is normally not secreted by the pineal gland, in order to compare the local cerebral glucose utilization (LCGU) response to MT from animals with long-term (pinealectomized) or short-term (pineal intact) absence of MT secretion. The majority of the 98 brain areas examined showed no change in LCGU after MT administration and/or pinealectomy. Pinealectomy increased the LCGU in the median mammillary nucleus only, whereas following MT administration, an increase in LCGU was observed in 3 cerebral regions of intact rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexus of the third ventricle) and in 5 cerebral regions of pinealectomized rats (paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, choroid plexuses of the lateral ventricles, third and fourth ventricles). Except for the choroid plexuses of the fourth ventricle, there was no difference in LCGU response to MT between pinealectomized and intact animals. This does not favor the hypothesis of receptor supersensitivity under the conditions of this experiment. Our results suggest that the hypothalamus, nucleus of the solitary tract and choroid plexuses represent a neural substrate through which MT could influence the activity of the central nervous system when administered at a low dose under near-physiological conditions.

The pineal gland: anatomy, physiology, and clinical significance

Since the discovery of melatonin approximately 25 years ago, there has been intense study regarding the details of the structure and function of the pineal gland. This work is reviewed, with particular emphasis on those aspects of importance to human physiology and disease.

Daily rhythms of serotonin metabolism in the medial hypothalamus of the chicken: effects of pinealectomy and exogenous melatonin

Indoleamine levels in punches of the medial hypothalamus containing the suprachiasmatic nuclei (SCN) of 4-week-old cockerels were determined by HPLC-EC. Melatonin levels in punches were determined by radioimmunoassay (RIA). Daily rhythms of serotonin (5-HT) and of its metabolite 5-hydroxy-3-indoleacetic acid (5-HIAA) were observed; levels were higher at midnight than at midday. A daily rhythm with the same phase in punch melatonin content was also observed. Pinealectomy at 1 week after hatching abolished the 5-HIAA and melatonin rhythm in 4-week-old birds but did not abolish the 5-HT rhythm. Injections of melatonin (0.5 mg/kg) increased 5-HT, 5-HIAA and melatonin levels in the hypothalamic punches. These results indicate that circulating melatonin of pineal origin may act to increase 5-HT turnover and/or release in the SCN. They suggest a link between the circadian secretion of pineal melatonin and the regulation of 5-HT projections to the hypothalamus from the raphe nuclei in the brainstem of the chicken. We have previously shown that the rhythmic secretion of melatonin by the pineal is influenced by oscillators in the brain via the superior cervical ganglia. The results reported here indicate that melatonin in turn may regulate brain oscillators, suggesting a neuroendocrine loop within the avian circadian system.

Methoxyindoles and photoreceptor metabolism: activation of rod shedding

Using an in vitro eye-cup preparation, we have evaluated a potential relationship between methoxyindole metabolism and photoreceptor disk shedding. Melatonin, 6-chloromelatonin, and 5-methoxytryptophol all activate rod disk shedding in culture. The effect is compound specific since serotonin and N-acetylserotonin are without effect, and it is similar to shedding in vivo because it is evoked by light and is quantitatively comparable to a normal intact animal response. The results suggest the involvement of 5-methoxyindoles in the control of rhythmic photoreceptor metabolism.

Melatonin increases cGMP and decreases cAMP levels in rat medial basal hypothalamus in vitro

Measurement of cyclic nucleotide accumulation in rat medial basal hypothalamus (MBH) incubated in vitro with methoxyindoles indicated that melatonin (10(-8) M or greater) increased significantly cGMP and depressed cAMP levels. Only the effect on MBH cAMP was shared by 5-methoxytryptophol which exhibited a greater activity than melatonin. 6-Fluoromelatonin (10(-7) M) increased cGMP, whereas the effect of 6-hydroxymelatonin was not significant. Both 6-fluoro- and 6-hydroxymelatonin (10(-5) M) depressed MBH cAMP accumulation while only 6-fluoromelatonin affected it at 10(-7) M concentrations. These data suggest that melatonin and 5-methoxytryptophol affect differently cyclic nucleotide content of MBH at physiological concentrations.

Is tubulin involved in the electrically-induced mechanical activity of the isolated rat sciatic nerve?

The electrically-induced mechanical activity of isolated segments of rat sciatic nerves remains unaffected following incubation with 10(-4) M colchicine, vinblastine or melatonin. Vinblastine depressed tubulin levels in incubated nerves. These results suggest that microtubules are not involved in nerve mechanical activity in vitro.

Melatonin blocks in vitro generation of prostaglandin by the uterus and hypothalamus

The effects of melatonin on the motility (isometric tension developed and contractile frequency) of uterine horns isolated from ovariectomized rats as well as on the mechanical responsiveness to added oxytocin or prostaglandin F2 alpha (PGF2 alpha) were explored. The pineal indole (10(-6) M or higher) depressed significantly the spontaneous motility of the uterus and reduced the responses evoked by oxytocin but not those evoked by PGF2 alpha. Melatonin was also tested on the prostaglandin (PG) release into the suspending media from either uterine horns from spayed rats or bovine medial basal hypothalamic (MBH) explants. Melatonin (10(-3) M) diminished the output from the uterus or the MBH of both PGE and PGF-"like material". Similarities between the effects of melatonin and indomethacin as well as the possible physiological relevance of the present findings are discussed.

Enhancement of serum FSH levels after pinealectomy

The effect of pinealectomy on the circadian periodicity in serum gonadotropins was investigated in adult male rats. Pinealectomy resulted in an elevation of the serum FSH concentration 7 days after the operation. A 4-fold increase in serum FSH over 24 h with no concommitant increase in serum LH following pinealectomy suggests that the control of FSH secretion could be mediated via the pineal gland.

Fast axonal transport in rat sciatic nerve. Inhibition by pineal indoles

The effect of pineal indoles on fast axoplasmic transport of proteins in the sciatic nerve was examined in rats injected with [3H]leucine in the sixth lumbar dorsal root ganglion. Melatonin (350-1100 nmol) applied locally in the sciatic nerve impaired significantly axonal transport. At a 350-nmol dose all other indoles tested (i.e., 5-hydroxyindoleacetic acid, 5-methoxyindoleacetic acid, serotonin, N-acetylserotonin, tryptamine, 5-methoxytryptamine) were less potent than melatonin to impair fast axonal flow. Vinblastine injected either into the ganglion or in the sciatic nerve markedly inhibited axonal transport.

The pineal gland as an APUD organ: supporting evidence and implications

It is only in recent years that the pineal gland has emerged from being thought of as non-functional and unimportant. The rise from obscurity has been the result of the interest of investigators of multidisplinary origins; such approaches, whilst clearly advancing understanding, also tend to leave knowledge fragmentary. In the last decade, a new neuroendocrine axis known as the APUD system has been delineated and scientifically accepted. The essence of the concept has been to link a physically widely dispersed and apparently unconnected series of glandular structures by a common embryological derivation and their secretion of closely similar functional products. The present proposal collates evidence for the inclusion of the pineal gland within the APUD system. In addition, because of the proven connections and response of the pineal gland to environmental changes, it is hypothesised that pineal products may provide the link between the environment and other members of the widely dispersed APUD system and thereby temper body homeostasis.

Effects of castration, estradiol and testosterone on tubulin levels of the medial basal hypothalamus and the adenohypophysis of the rat

Tubulin levels of the medial basal hypothalamus (MBH) were greater in male than in female rats. Orchidectomy brought about a decrease of MBH tubulin concentration, whereas testosterone injection augmented it in the MBH and adenohypophysis. Estradiol administration augmented MBH tubulin and protein concentration.

The pineal and psychiatry: a review

The pineal body in man is an active endocrine gland throughout life. Historically the gland has been associated with speculation on the nature of mind and its disorders. Modern research is now demonstrating it to be an endocrine gland capable of affecting the brain and behaviour. The implications for future research into this aspect of pineal function are discussed in the context of a critical review of the current literature.

Effects of melatonin treatment and environmental lighting on the ultrastructural appearence, melatonin synthesis, norepinephrine turnover and microtubule protein content of the rat pineal gland

Melatonin administration or exposure of rats to darkness for two weeks induced comparable changes in pineal ultrastructure, compatible with a generalized organ's activation. These include an increased number of ribosomes, procentrioles and microtubules, prominent nucleoli and Golgi apparatus, and annulate lamellae. Melatonin treatment resulted in a dose-dependent increase of hydroxyindole-O-methyl transferase and serotonin-N-acetyltransferase activities. In addition it increased by 85% the colchicine binding capacity of pineal homogenates, an estimation of the microtubule protein content of the gland. Pineal norepinephrine turnover was not affected by melatonin treatment. These data indicate that the pineal itself is a target organ for exogenously administered melatonin. Key words: Pineal gland, melatonin, norepinephrine, tubulin.