Melatonin reduces the expression of alpha-synuclein in the dopamine containing neuronal regions of amphetamine-treated postnatal rats

Melatonin derivative 6a as a PARP-1 inhibitor for the treatment of Parkinson's disease

Both continuous oxidative stress and poly (ADP-ribose) polymerase 1 (PARP-1) activation occur in neurodegenerative diseases such as Parkinson's disease. PARP-1 inhibition can reverse mitochondrial damage and has a neuroprotective effect. In a previous study, we synthesized melatonin derivative 6a (MD6a) and reported that it has excellent antioxidant activity and significantly reduces α-synuclein aggregation in Caenorhabditis elegans; however, the underlying mechanism is largely unknown. In the present study, we revealed that MD6a is a potential PARP-1 inhibitor, leading to mammalian targe of rapamycin/heat shock factor 1 signaling downregulation and reducing heat shock protein 4 and 6 expression, thus helping to maintain protein homeostasis and improve mitochondrial function. Together, these findings suggest that MD6a might be a viable candidate for the prevention and treatment of Parkinson's disease.

Pharmacological interventions targeting α-synuclein aggregation triggered REM sleep behavior disorder and early development of Parkinson's disease

Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia characterized by elevated motor behaviors and dream enactments in REM sleep, often preceding the diagnosis of Parkinson's disease (PD). As RBD could serve as a biomarker for early PD developments, pharmacological interventions targeting α-synuclein aggregation triggered RBD could be applied toward early PD progression. However, robust therapeutic guidelines toward PD-induced RBD are lacking, owing in part to a historical paucity of effective treatments and trials. We reviewed the bidirectional links between α-synuclein neurodegeneration, progressive sleep disorders, and RBD. We highlighted the correlation between RBD development, α-synuclein aggregation, and neuronal apoptosis in key brainstem regions involved in REM sleep atonia maintenance. The current pharmacological intervention strategies targeting RBD and their effects on progressive PD are discussed, as well as current treatments for progressive neurodegeneration and their effects on RBD. We also evaluated emerging and potential pharmacological solutions to sleep disorders and developing synucleinopathies. This review provides insights into the mechanisms and therapeutic targets underlying RBD and PD, and explores bidirectional treatment effects for both diseases, underscoring the need for further research in this area.

Melatonin reduces the endoplasmic reticulum stress and polyubiquitinated protein accumulation induced by repeated anesthesia exposure in Caenorhabditis elegans

Endoplasmic reticulum (ER) stress has been linked to anesthesia-induced neurotoxicity, but melatonin seems to play a protective role against ER stress. Synchronized Caenorhabditis elegans were exposed to isoflurane during the developmental period; melatonin treatment was used to evaluate its role in preventing the defective unfolded protein response (UPR) and ER-associated protein degradation (ERAD). The induced expression of hsp-4::GFP by isoflurane was attenuated in the isoflurane-melatonin group. Isoflurane upregulated the expression of ire-1, whereas melatonin did not induce ire-1 expression in C. elegans even after isoflurane exposure. With luzindole treatment, the effect of melatonin on the level of ire-1 was significantly attenuated. The reduced expression of sel-1, sel-11, cdc-48.1, and cdc-48.2 due to isoflurane was restored by melatonin, although not up to the level of the control group. The amount of polyubiquitinated proteins was increased in the isoflurane group; however, melatonin suppressed its accumulation, which was significantly inhibited by a proteasome inhibitor, MG132. The chemotaxis index of the isoflurane-melatonin group was improved compared with the isoflurane group. Melatonin may be a potential preventive molecule against defective UPR and ERAD caused by repeated anesthesia exposure. The ire-1 branch of the UPR and ERAD pathways can be the target of melatonin to reduce anesthesia-induced ER stress.

Neuronal Bmi-1 is critical for melatonin induced ubiquitination and proteasomal degradation of α-synuclein in experimental Parkinson's disease models

Epigenetic polycomb repressor complex-1 subunit BMI-1 plays a pivotal role in the process of gene repression to maintain the self-renewal and differentiation state of neurogenic tissues. Accumulating reports links lower expression of BMI-1 fails to regulate the repression of anti-oxidant response genes disrupt mitochondrial homeostasis underlying neurodegeneration. Interestingly, this negative relation between BMI-1 function and neurodegeneration is distinct but has not been generalized as a potential biomarker particularly in Parkinson's disease (PD). Hyperphosphorylated BMI-1 undergoes canonical polycomb E3 ligase function loss, thereby leads to reduce monoubiquitylation of histone 2A at lysine 119 (H2AK119ub) corroborates cellular accumulation of α-synuclein protein phosphorylated at serine 129 (pα-SYN (S129). In general, neuroprotectant suppressing pα-SYN (S129) level turns ineffective upon depletion of neuronal BMI-1. However, it has been observed that our neuroprotectant exposure suppresses the cellular pα-SYN (S129) and restore the the BMI-1 expression level in neuronal tissues. The pharmacological inhibition and activation of proteasomal machinery promote the cellular accumulation and degradation of neuronal pα-SYN (S129), respectively. Furthermore, our investigation reveals that accumulated pα-SYN (S129) are priorly complexed with BMI-1 undergoes ubiquitin-dependent proteasomal degradation and established as key pathway for therpeutic effect in PD. These findings linked the unestablished non-canonical role of BMI-1 in the clearance of pathological α-SYN and suspected to be a novel therapeutic target in PD.

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.

A New Threat to Dopamine Neurons: The Downside of Artificial Light

Growing awareness of adverse impacts of artificial light on human health has led to recognize light pollution as a significant global environmental issue. Despite, a large number of studies in rodent and monkey models of Parkinson's disease have reported that near infrared light has neuroprotective effects on dopaminergic neurons, recent findings have shown that prolonged exposure of rodents and birds to fluorescent artificial light results in an increase of neuromelanin granules in substantia nigra and loss of dopaminergic neurons. The observed detrimental effect seems to be dependent on a direct effect of light on the substantia nigra rather than a secondary effect of the alterations of circadian rhythms. Moreover, inferences from animal models to human studies have shown a positive correlation between the prevalence of Parkinson's disease and light pollution. The present article discusses experimental evidence supporting a potentially deleterious impact of light on dopaminergic neurons and highlights the mechanisms whereby light might damage neuronal tissue. Moreover, it analyses epidemiological evidence that suggests light pollution to be an environmental risk factor for Parkinson's disease.

Multi-Faceted Role of Melatonin in Neuroprotection and Amelioration of Tau Aggregates in Alzheimer's Disease

Alzheimer's disease (AD) is one of the major age related neurodegenerative diseases whose pathology arises due to the presence of two distinct protein aggregates, viz., amyloid-β plaques in extracellular matrix and tau neurofibrillary tangles in neurons. Multiple factors play a role in AD pathology, which includes familial mutations, oxidative stress, and post-translational modifications. Melatonin is an endocrine hormone, secreted during darkness, derived from tryptophan, and produced mainly by the pineal gland. It is an amphipathic molecule, which makes it suitable to cross not only blood-brain barrier, but also to enter several other subcellular compartments like mitochondria and endoplasmic reticulum. In this context, the neuroprotective effect of melatonin may be attributed to its role as an antioxidant. Melatonin's pleiotropic function as an antioxidant and neuroprotective agent has been widely studied. However, its direct effect on the aggregation of tau and amyloid-β needs to be explored. Furthermore, an important aspect of its function is its ability to regulate the process of phosphorylation of tau by affecting the function of kinases and phosphatases. In this review, we are focusing on the pleiotropic function of melatonin on the aspect of its neuroprotective function in tau pathology, which includes antioxidant function, regulation of enzymes, including kinases and enzymes involved in free radical scavenging and mitochondrial protection.

Circadian disruption and human health: A bidirectional relationship

Circadian rhythm disorders have been classically associated with disorders of abnormal timing of the sleep-wake cycle, however circadian dysfunction can play a role in a wide range of pathology, ranging from the increased risk for cardiometabolic disease and malignancy in shift workers, prompting the need for a new field focused on the larger concept of circadian medicine. The relationship between circadian disruption and human health is bidirectional, with changes in circadian amplitude often preceding the classical symptoms of neurodegenerative disorders. As our understanding of the importance of circadian dysfunction in disease grows, we need to develop better clinical techniques for identifying circadian rhythms and also develop circadian based strategies for disease management. Overall this review highlights the need to bring the concept of time to all aspects of medicine, emphasizing circadian medicine as a prime example of both personalized and precision medicine.

Melatonin: Clinical Perspectives in Neurodegeneration

Prevention of neurodegenerative diseases is presently a major goal for our Society and melatonin, an unusual phylogenetically conserved molecule present in all aerobic organisms, merits consideration in this respect. Melatonin combines both chronobiotic and cytoprotective properties. As a chronobiotic, melatonin can modify phase and amplitude of biological rhythms. As a cytoprotective molecule, melatonin reverses the low degree inflammatory damage seen in neurodegenerative disorders and aging. Low levels of melatonin in blood characterizes advancing age. In experimental models of Alzheimer's disease (AD) and Parkinson's disease (PD) the neurodegeneration observed is prevented by melatonin. Melatonin also increased removal of toxic proteins by the brain glymphatic system. A limited number of clinical trials endorse melatonin's potentiality in AD and PD, particularly at an early stage of disease. Calculations derived from animal studies indicate cytoprotective melatonin doses in the 40-100 mg/day range. Hence, controlled studies employing melatonin doses in this range are urgently needed. The off-label use of melatonin is discussed.

Effects of Melatonin Levels on Neurotoxicity of the Medial Prefrontal Cortex in a Rat Model of Parkinson's Disease

Background:

Damage of the medial prefrontal cortex (mPFC) results in similar characteristics to the cognitive deficiency seen with the progress of Parkinson's disease (PD). Since the course of mPFC damage is still unclear, our study aimed to investigate the effects of melatonin (MT) on neurotoxicity in the mPFC of a rat model of PD.

Methods:

One hundred and fifty-four normal, male Wistar rats were randomly divided into the following five groups: normal + normal saline (NS), normal + 6-hydroxydopamine (6-OHDA), sham pinealectomy (PX) + 6-OHDA, PX + 6-OHDA, and MT + 6-OHDA. 6-OHDA was injected into the right substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) of each group, except normal + NS, 60 days after the PX. In the MT treatment group, MT was administered immediately after the intraperitoneal injection at 4 p.m. every day, for 14 days. Neuronal apoptosis in the mPFC was examined using the TUNEL method, while the expression of tyrosine hydroxylase (TH), Bax, and Bcl-2 in this region was measured using immunohistochemistry. The concentration of malondialdehyde (MDA) in the mPFC was examined using the thiobarbituric acid method.

Results:

Rats in the normal + 6-OHDA and sham PX + 6-OHDA groups were combined into one group (Group N + 6-OHDA) since there was no significant discrepancy between the groups for all the detected parameters. Apoptosis of cells in the NS, MT + 6-OHDA, N + 6-OHDA, and PX + 6-OHDA groups was successively significantly increased (Hc = 256.25, P < 0.001). The gray value of TH (+) fibers in the NS, MT + 6-OHDA, N + 6-OHDA, and PX + 6-OHDA groups was also successively significantly increased (F = 99.33, P < 0.001). The staining intensities of Bax and Bcl-2 were as follows: Group NS +/+, Group MT + 6-OHDA ++/+, Group N + 6-OHDA ++/+, and PX + 6-OHDA +++/+. The concentrations of MDA in the NS, MT + 6-OHDA, N + 6-OHDA, and PX + 6-OHDA groups were significantly increased in sequence (Hc = 296.309, P < 0.001).

Conclusions:

Neuronal damage of the VTA by 6-OHDA might induce VTA-mPFC nerve fibers to undergo anterograde nerve damage, in turn inducing transneuronal damage of the mPFC. PX significantly exacerbated the neurotoxicity in the mPFC, which was induced by the neuronal injury of the VTA. However, MT replacement therapy significantly alleviated the neurotoxicity in the mPFC.

Neuroprotective effects of melatonin on amphetamine-induced dopaminergic fiber degeneration in the hippocampus of postnatal rats

Chronic amphetamine (AMPH) abuse leads to damage of the hippocampus, the brain area associated with learning and memory process. Previous results have shown that AMPH-induced dopamine neurotransmitter release, reactive oxygen species formation, and degenerative protein aggregation lead to neuronal death. Melatonin, a powerful antioxidant, plays a role as a neuroprotective agent. The objective of this study was to investigate whether the protective effect of melatonin on AMPH-induced hippocampal damage in the postnatal rat acts through the dopaminergic pathway. Four-day-old postnatal rats were subcutaneously injected with 5-10 mg/kg AMPH and pretreated with 10 mg/kg melatonin prior to AMPH exposure for seven days. The results showed that melatonin decreased the AMPH-induced hippocampal neuronal degeneration in the dentate gyrus, CA1, and CA3. Melatonin attenuated the reduction in the expression of hippocampal synaptophysin, PSD-95, α-synuclein, and N-methyl-D-aspartate (NMDA) receptor protein and mRNA caused by AMPH. Melatonin attenuated the AMPH-induced reduction in dopamine transporter (DAT) protein expression in the hippocampus and the reduction in mRNA expression in the ventral tegmental area (VTA). Immunofluorescence demonstrated that melatonin not only prevented the AMPH-induced loss of DAT and NMDA receptor but also prevented AMPH-induced α-synuclein overexpression in the dentate gyrus, CA1, and CA3. Melatonin decreased the AMPH-induced reduction in the protein and mRNA of the NMDA receptor downstream signaling molecule, calcium/calmodulin-dependent protein kinase II (CaMKII), and the melatonin receptors (MT1 and MT2). This study showed that melatonin prevented AMPH-induced toxicity in the hippocampus of postnatal rats possibly via its antioxidative effect and mitochondrial protection.

Melatonin inhibits rotenone-induced SH-SY5Y cell death via the downregulation of Dynamin-Related Protein 1 expression

Previous studies have shown that melatonin can protect cells against rotenone-induced cell death. Yet, the mechanism involved in this protection requires further research. In this study, we aimed to further investigate the effects of melatonin on inhibiting rotenone-induced SH-SY5Y cells and the underlying molecular mechanisms. Human neuroblastoma SH-SY5Y cells were treated with 0.3 or 1μM rotenone for 6 or 12h. Cell viability was measured with an MTS assay, the mitochondrial membrane potential was determined with a Rhodamine 123 staining assay, and the protein expression levels of the markers of autophagy, including cytochrome C release (Cyt C), light chain 3B (LC3 B) and Dynamin-Related Protein 1 (Drp1) were analyzed by western blotting. The co-localization of Drp1 and TOM20 proteins in the mitochondria of SH-SY5Y cells was measured by immunofluorescence coupled with confocal microscopy and the overexpression of the Drp1 gene was then conducted. The viability and expression levels of Cyt C and LC3 B in rotenone and melatonin + rotenone-treated Drp1-overexpressed SH-SY5Y cells were analyzed with MTS and western blotting, respectively. We found that rotenone effectively induced SH-SY5Y cell death by causing mitochondrial dysfunction and increasing Cyt C expression. Drp1 expression and its regulation of mitochondrial translocation mediated the rotenone-induced cell death and melatonin inhibited this process. Overexpression of Drp1 protein attenuated melatonin's inhibition of rotenone-induced SH-SY5Y cell death. In conclusion, melatonin effectively inhibits rotenone-induced neuronal cell death via the regulation of Drp1 expression.

Mechanisms of Melatonin in Alleviating Alzheimer's Disease

Alzheimer's disease (AD) is a chronic, progressive and prevalent neurodegenerative disease characterized by the loss of higher cognitive functions and an associated loss of memory. The thus far "incurable" stigma for AD prevails because of variations in the success rates of different treatment protocols in animal and human studies. Among the classical hypotheses explaining AD pathogenesis, the amyloid hypothesis is currently being targeted for drug development. The underlying concept is to prevent the formation of these neurotoxic peptides which play a central role in AD pathology and trigger a multispectral cascade of neurodegenerative processes post-aggregation. This could possibly be achieved by pharmacological inhibition of β- or γ-secretase or stimulating the nonamyloidogenic α-secretase. Melatonin the pineal hormone is a multifunctioning indoleamine. Production of this amphiphilic molecule diminishes with advancing age and this decrease runs parallel with the progression of AD which itself explains the potential benefits of melatonin in line of development and devastating consequences of the disease progression. Our recent studies have revealed a novel mechanism by which melatonin stimulates the nonamyloidogenic processing and inhibits the amyloidogenic processing of β-amyloid precursor protein (βAPP) by stimulating α -secretases and consequently down regulating both β- and γ-secretases at the transcriptional level. In this review, we discuss and evaluate the neuroprotective functions of melatonin in AD pathogenesis, including its role in the classical hypotheses in cellular and animal models and clinical interventions in AD patients, and suggest that with early detection, melatonin treatment is qualified to be an anti-AD therapy.

Melatoninergic System in Parkinson's Disease: From Neuroprotection to the Management of Motor and Nonmotor Symptoms

Melatonin is synthesized by several tissues besides the pineal gland, and beyond its regulatory effects in light-dark cycle, melatonin is a hormone with neuroprotective, anti-inflammatory, and antioxidant properties. Melatonin acts as a free-radical scavenger, reducing reactive species and improving mitochondrial homeostasis. Melatonin also regulates the expression of neurotrophins that are involved in the survival of dopaminergic neurons and reduces α-synuclein aggregation, thus protecting the dopaminergic system against damage. The unbalance of pineal melatonin synthesis can predispose the organism to inflammatory and neurodegenerative diseases such as Parkinson's disease (PD). The aim of this review is to summarize the knowledge about the potential role of the melatoninergic system in the pathogenesis and treatment of PD. The literature reviewed here indicates that PD is associated with impaired brain expression of melatonin and its receptors MT₁ and MT₂. Exogenous melatonin treatment presented an outstanding neuroprotective effect in animal models of PD induced by different toxins, such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, paraquat, and maneb. Despite the neuroprotective effects and the improvement of motor impairments, melatonin also presents the potential to improve nonmotor symptoms commonly experienced by PD patients such as sleep and anxiety disorders, depression, and memory dysfunction.

Melatonin: A Potential Anti-Oxidant Therapeutic Agent for Mitochondrial Dysfunctions and Related Disorders

Mitochondria play a central role in cellular physiology. Besides their classic function of energy metabolism, mitochondria are involved in multiple cell functions, including energy distribution through the cell, energy/heat modulation, regulation of reactive oxygen species (ROS), calcium homeostasis, and control of apoptosis. Simultaneously, mitochondria are the main producer and target of ROS with the result that multiple mitochondrial diseases are related to ROS-induced mitochondrial injuries. Increased free radical generation, enhanced mitochondrial inducible nitric oxide synthase (iNOS) activity, enhanced nitric oxide (NO) production, decreased respiratory complex activity, impaired electron transport system, and opening of mitochondrial permeability transition pores have all been suggested as factors responsible for impaired mitochondrial function. Because of these, neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and aging, are caused by ROS-induced mitochondrial dysfunctions. Melatonin, the major hormone of the pineal gland, also acts as an anti-oxidant and as a regulator of mitochondrial bioenergetic function. Melatonin is selectively taken up by mitochondrial membranes, a function not shared by other anti-oxidants, and thus has emerged as a major potential therapeutic tool for treating neurodegenerative disorders. Multiple in vitro and in vivo experiments have shown the protective role of melatonin for preventing oxidative stress-induced mitochondrial dysfunction seen in experimental models of PD, AD, and HD. With these functions in mind, this article reviews the protective role of melatonin with mechanistic insights against mitochondrial diseases and suggests new avenues for safe and effective treatment modalities against these devastating neurodegenerative diseases. Future insights are also discussed.

Comparison of the Effects of 13-cis Retinoic Acid and Melatonin on the Viabilities of SH-SY5Y Neuroblastoma Cell Line

Objective

Neuroblastoma is one of common childhood tumors. Although its mortality is very high, there is no effective treatment yet. The aim of this project is to evaluate cytotoxic effects of melatonin (MLT) an endogen hormone and 13-cis retinoic acid (13-cis-RA) also named as isotretinoin an analogue of vitamin A on neuroblastoma SH-SY5Y cell line.

Methods

In this study, SH-SY5Y cell line was used. After cell culture, the cells were exposed to different doses of MLT and 13-cis-RA. 24 and 48 hours later. While the viabilities was estimated with MTT cell viability assay test, apoptotic indexes were calculated after staining with TUNEL based apoptosis kit.

Results

It was observed that MLT has very effective cytotoxic potential than 13-cis-RA on neuroblastoma cell line. At the same time, when MLT and 13-cis-RA were combined, this effect was potentiated. On the other hand, it was found that the effect of 13-cis-RA individually on neuroblastoma cells was very slight.

Conclusion

We suggest that in the treatment of patient with neuroblastoma, MLT is very effective and also this effect can be augmented by combination with 13-cis-RA.

Melatonin inhibits manganese-induced motor dysfunction and neuronal loss in mice: involvement of oxidative stress and dopaminergic neurodegeneration

Excessive manganese (Mn) induces oxidative stress and dopaminergic neurodegeneration. However, the relationship between them during Mn neurotoxicity has not been clarified. The purpose of this study was to investigate the probable role of melatonin (MLT) against Mn-induced motor dysfunction and neuronal loss as a result of antagonizing oxidative stress and dopaminergic neurodegeneration. Mice were randomly divided into five groups as follows: control, MnCl2, low MLT + MnCl2, median MLT + MnCl2, and high MLT + MnCl2. Administration of MnCl2 (50 mg/kg) for 2 weeks significantly induced hypokinesis, dopaminergic neurons degeneration and loss, neuronal ultrastructural damage, and apoptosis in the substantia nigra and the striatum. These conditions were caused in part by the overproduction of reactive oxygen species, malondialdehyde accumulation, and dysfunction of the nonenzymatic (GSH) and enzymatic (GSH-Px, superoxide dismutase, quinone oxidoreductase 1, glutathione S-transferase, and glutathione reductase) antioxidative defense systems. Mn-induced neuron degeneration, astrocytes, and microglia activation contribute to the changes of oxidative stress markers. Dopamine (DA) depletion and downregulation of DA transporter and receptors were also found after Mn administration, this might also trigger motor dysfunction and neurons loss. Pretreatment with MLT prevented Mn-induced oxidative stress and dopaminergic neurodegeneration and inhibited the interaction between them. As a result, pretreatment with MLT significantly alleviated Mn-induced motor dysfunction and neuronal loss. In conclusion, Mn treatment resulted in motor dysfunction and neuronal loss, possibly involving an interaction between oxidative stress and dopaminergic neurodegeneration in the substantia nigra and the striatum. Pretreatment with MLT attenuated Mn-induced neurotoxicity by means of its antioxidant properties and promotion of the DA system.

Melatonin and mitochondrial dysfunction in the central nervous system

Cell death and survival are critical events for neurodegeneration, mitochondria being increasingly seen as important determinants of both. Mitochondrial dysfunction is considered a major causative factor in Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Increased free radical generation, enhanced mitochondrial inducible nitric oxide (NO) synthase activity and NO production, and disrupted electron transport system and mitochondrial permeability transition, have all been involved in impaired mitochondrial function. Melatonin, the major secretory product of the pineal gland, is an antioxidant and an effective protector of mitochondrial bioenergetic function. Both in vitro and in vivo, melatonin was effective to prevent oxidative stress/nitrosative stress-induced mitochondrial dysfunction seen in experimental models of AD, PD and HD. These effects are seen at doses 2-3 orders of magnitude higher than those required to affect sleep and circadian rhythms, both conspicuous targets of melatonin action. Melatonin is selectively taken up by mitochondria, a function not shared by other antioxidants. A limited number of clinical studies indicate that melatonin can improve sleep and circadian rhythm disruption in PD and AD patients. More recently, attention has been focused on the development of potent melatonin analogs with prolonged effects which were employed in clinical trials in sleep-disturbed or depressed patients in doses considerably higher than those employed for melatonin. In view that the relative potencies of the analogs are higher than that of the natural compound, clinical trials employing melatonin in the range of 50-100mg/day are needed to assess its therapeutic validity in neurodegenerative disorders.