Melatonin as a Potential Regulator of Oxidative Stress, and Neuroinflammation: Mechanisms and Implications for the Management of Brain Injury-Induced Neurodegeneration

Introduction

A review paper by Kaur and Sharma defined Traumatic Brain Injury (TBI) as “the injury resulting from mechanical energy to the brains from any sort of external physical stimulus”.¹ It occurs from the absorption by the brain tissues of the energy linked with a mechanical force, which does not act directly to the head of an individual, which causes a myriad of metabolic, molecular, and biochemical effects, affecting the overall cell’s homeostasis causing permanent or temporary cognitive and motor dysfunction, consciousness, or psychological capabilities.² Some of the well-known causes of TBI include fallings, assault, traffic accidents, and sport-associated concussions.³ Traumatic brain injury affects more than 10 million people yearly in the world, representing 30–40% of all injury-related disabilities and mortalities among individuals of all ages, causing tremendous economic and social impact.⁴ Epidemiological studies have suggested a 2–3-times higher incidence of TBI-associated consequences in the next ten years compared to Alzheimer’s disease or other neurodegenerative conditions.⁵ Interestingly, two of the most studied risk factors for the TBI are sex and age, as the males are three times more prone to suffer from a TBI compared to the females. While in case of age, the persons of 65 years or older, and kids under 14 are more likely to suffer from TBI. Annually, up to 2 million Americans suffer from a TBI, which accounts for around 1.4 million emergency visits, 2.75 million hospitalizations, and 5.2 thousand deaths annually in the US, contributing 30% of all deaths in the USA per year. A complete discussion and facts related to the epidemiology of TBI may be found somewhere else.⁶ Not different in East Asian countries. For example, according to the Lancet Neurology (2019), China has the most TBI patients than almost all countries of the world, making it a major public health concern. Population-based mortality of TBI is estimated to be 13 cases per 100 000 people, which is not less than reported in the other countries.⁷ Similarly, a retrospective longitudinal cohort study evaluated TBI trends from 2008 to 2017, acquired from Korean National Health Insurance Service–National Health Information Database (NHIS-NHID) (2020). Interestingly, mortality per 100,000 people was significantly higher among people aged ≥70 years than in the other age group.

It is also confirmed from a worldwide meta-analysis of the incidence of TBI that males have the most TBI among the adult population. Also, in the meta-analysis by continent showed in all age groups, there was no significant difference among continents including European and North American studies. The result obtained from this study is essential, as it is their first systematic review and meta-analysis that highlighted the heterogeneity of TBI incidence.

Although significant research has been made to develop therapeutic strategies against traumatic brain injury-induced neurodegeneration, it still represents the major global and public health challenge among all ages overall in the world regardless of the patient’s financial status and income level.⁸ Previously, a wide set of studies has been published on the role of melatonin in the management of brain injury-induced neurodegeneration.⁹ Therefore, the overall aim of this review is to summarize the role of melatonin against brain injury, including subarachnoid hemorrhage and cellular models of brain injury. In this review, broadly three broad thematic areas will be discussed, including the processes and etiology of TBI, evidence on therapeutic options from pre-clinical and clinical studies of TBI, and the use of melatonin as a potential regulator of TBI in multiple pathways will be discussed. The motivation behind the compilation of this review article was our previous study conducted on the role of melatonin against traumatic brain injury induced-neurodegeneration.¹⁰ For a collection of the articles, several independent databases, including but not limited to PubMed (https://pubmed.ncbi.nlm.nih.gov), Google Scholar (https://scholar.google.co.kr), and Web of Sciences (https://apps.webofknowledge.com). The papers were randomly selected, care was taken to avoid biases and repetition, and the main findings of the articles were collected from the abstracts, and some established protocols were followed.¹¹ The paper has two major parts, one is traumatic brain-injury and the protective effects of melatonin against brain injury, the contents of the study has been shown in Figure 1.

Figure 1 Diagram, showing the overall contents of the manuscript covering the introduction and the contents related to melatonin.

Types of TBI

There are several systems to analyze the severity of TBI, the most common one is the Glasgow Coma Scale (GCS). The GCS classifies TBI into mild (GCS range 13–15), moderate (GCS range 9–12), and severe (GCS range 3–8), which are based on scores obtained from specific clinical evaluations, such as eye-opening, verbal communication and motor functions, which has been given elsewhere.¹² The TBI can be divided into three categories based on its distinctive physical insult: closed head, penetrating, and explosive blast TBI. The closed-head TBI is the highest occurring incident among civilians caused by blunt objects, such as falling, vehicle accidents, and sports injuries. A brain injury in which a foreign body penetrates the brain parenchyma, causing focal damages, intracranial hemorrhage, edema, and ischemic conditions is called penetrating TBI.¹³ Explosive TBI is a form of TBI caused by the explosion, introduced in the 20th century, which is war-related TBI.¹⁴ Here, in explosion blast TBI, the brain is badly affected by rapid pressure shock waves, transmitting a significant amount of energy from the skull into the brain parenchyma.¹⁵

Pathophysiology of TBI

The pathophysiological consequences of TBI can be divided into the following two categories: primary brain injury, and secondary brain injury.

Primary Brain Injury

The primary brain injury comprises the direct physical insult to the neuronal, vascular, and glial cells.¹⁶ It may cause deformation, compression, displacement, shearing, stretching, tearing, and crushing of brain cells and the blood circulatory system. The direct head injury can cause injury in adjacent (coup) brain cells or may damage the opposite (contrecoup) sides of the cells. The concussion involves the primary injury, which is not severe and is associated with rapid loss of consciousness (for more than several minutes) and occurs as a result of coup and contrecoup lesions. The contusion causes glial and vascular cell injury, while in severe cases it may cause neuronal injury. A primary brain insult is an irreversible event that causes neuro-electro-chemical events (secondary injury) that ends in neurodegeneration.²

Secondary Brain Injury

The molecular and biochemical changes caused by the primary neuronal injury cause secondary brain injury. Several factors may contribute to secondary brain injury, including mitochondrial dysfunction, excitotoxicity, oxidative stress and nitrosative stress, neuroinflammation, lipid peroxidation, and axonal degeneration.^17,18 A sketch (Figure 2) shows the pathophysiology of brain injury.

Figure 2 Pathophysiology of traumatic brain injury. A brief illustration of the primary brain injury, which is consisted of stretching, compression, and tearing followed by secondary brain injury which leads to further induce oxidative stress, neuroinflammation, excitotoxicity, apoptotic cell death, blood-brain barrier disruption, dendritic degeneration, synaptic loss, and ultimately progress towards neurodegeneration and cognitive dysfunctions.

Role of Oxidative and Nitrosative Stress in TBI

Studies related to TBI have extensively highlighted the role of Reactive Oxygen Species (ROS) and free radicals in brain injury, facilitating lipid peroxidation, protein oxidation, and DNA damage, causing excitotoxicity (glutamates), ionic dysregulation, and mitochondrial apoptosis.¹⁹ The excitotoxicity and release of glutamate cause an influx of Ca²⁺ into cells through the N-methyl D-aspartate (NMDA) receptors,²⁰ causing mitochondrial dysfunction with oxidative phosphorylation, and energy dyshomeostasis.²¹ Among ROS, superoxides are produced from the malfunctioning of the mitochondrial electron transport chain and activation of microglia and macrophages.²² The superoxide anion is converted into H₂O₂ + O₂ by Superoxide dismutase (SOD), through the glutathione peroxidase. The overproduction of superoxide coupled with acidosis favors the H₂O₂ to react with extravasated hemoglobin, producing hydroxyl radicals, which react with several biological molecules (capable of donating an electron).²³ The nuclear factor (erythroid-derived 2)-like 2 (Nrf-2), encoded by the NFE2L2 gene, is an important transcription factor acting as antioxidant defense system.²⁴ Nrf-2 is known as the molecular switch turning on/off the Nrf-2 signaling acting as an integral component of the oxidative-stress pathway, which is activated in response to the oxidative damage.²⁵ Under normal physiological conditions, Nrf2 is found in the cytoplasm bound to Kelch-like erythroid cell-derived protein with CNC homology- (ECH-) associated protein 1 (Keap-1), acting as a primary inhibitor of Nrf-2, where it undergoes proteasomal degradation and ubiquitination. Under physiological stress, the Nrf-2 is separated from the Keap-1, permeated into the nucleus, where it attaches with Antioxidant responsive element (ARE) and induces the transcription of ARE-associated antioxidant enzymes, such as catalases, superoxide dismutase (SOD), glutathione S-transferase (GSTP),²⁶ heme oxygenase-1 (HO-1).²⁷

Apart from oxidative stress, nitrosative stress also plays a pivotal role in the execution of neuronal cell loss in TBI²⁸ and other neurodegenerative conditions.²⁹ It affects the level of nitric oxide (NO), affecting the extracellular and intracellular signaling, which is crucial for cell survival and functioning, and is produced by the activity of nitric oxide synthase (NOS).³⁰ The elevated level of peroxynitrite and RNS actively suppresses the endogenous antioxidant mechanisms, causing lipid peroxidation of mitochondrial membranes,³¹ affecting the Ca2+ homeostasis,³² formation of the membrane permeability transition pores,³³ and neurodegeneration.²⁹ Figure 3 has been given, showing the role of oxidative and nitrosative stress and its consequences.

Figure 3 Brain injury-induced oxidative and nitrosative stress and its consequences. Diagrammatic representation, showing that brain injury induces oxidative and nitrosative stress, which causes further deterioration by disrupting the blood-brain barrier, activating the astrocytes and microglial cells, releasing the inflammatory cytokines, infiltration of the lymphocytes, and may cause the accumulation of misfolded proteins and neurodegeneration.

Traumatic Brain Injury and Neuroinflammation

Within 24 hours of TBI, the permeability of the blood-brain barrier (BBB) increases the infiltration of circulating monocytes, neutrophils, and lymphocytes into the brain parenchyma,³⁴ which activates the complement system and release inflammatory cytokines such as tissue necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6).^35,36 The release of these inflammatory cytokines is associated with BBB dysfunction and brain edema. It was established that upon injury the activated microglia and macrophages reach the site of insult to mitigate the deleterious impacts of the brain injury and provide a safe environment.³⁷ The activated microglia may cause the release of inflammatory cytokines, reactive oxygen species, and release of glutamate, which exacerbates the injury and neuronal cell death.³⁸ The maturation of IL-1β and its release into the systemic circulation is mainly regulated by nucleotide oligomerization-like receptor protein 3 (NLRP3) inflammasome, which is localized in immune cells.³⁹ The NLRP3 inflammasome is made of NLRP3, oligomers apoptosis-associated speck-like (ASC) adapter protein, and pro-caspase-1, cleaving caspase-1 and maturation of IL-1β, and neuroinflammation.⁴⁰ Figure 4 shows the overall brain injury associated with neuroinflammation and its consequences.

Figure 4 Traumatic brain injury and neuroinflammation. The figure illustrates the activation of microglia (Iba-1 and microglial receptor TLR-4), astrocytes (GFAP), MAP kinases (such as p-JNK and p-38), the release of the inflammatory cytokines (TNF-α and IL-1β), and release of chemokines, which may induce neuroinflammation and neurodegeneration.

Use of Melatonin Against Traumatic Brain Injury-Induced Neurodegeneration

The therapeutic approaches deeply vary among the individuals because of the complex nature, limited knowledge of the pathophysiological mechanisms of TBI, and the response of the patient to the drugs. The most frequently used drugs in the management of TBI include osmotic therapy such as mannitol, which regulates osmotic pressure, decreases cerebral edema, and cerebral blood flow. Other drugs used to reduce secondary brain injury include opioids (morphine, to reduce the pain), and sedatives (benzodiazepines, barbiturates) which reduce neuronal activity.⁴¹ Table 1 depicts a summarized chart on the therapeutic drugs or approaches to treating TBI in the pre-clinical and clinical trial stage. As mentioned, there is no therapeutic drug available against TBI-induced neurodegeneration, although several pre-clinical drugs, such as melatonin and plant-based polyphenols have drawn much attention.⁴² Failure to develop effective drugs against brain injury-induced neurodegeneration may be partly because most drugs target a single aspect of the disease. So, current efforts are being made to develop drugs having multi-targeted beneficial effects against injury. In this regard, melatonin, a hormone from the pineal gland has drawn much attention, due to its multi-targeted effects against injuries. Here, we give a list of previously used compounds and drugs against TBI. Table 1.

Table 1 Summary of the Drugs Used Against Traumatic Brain Injury-Induced Neurodegeneration, Therapeutic Targets, and Implications

Melatonin, Its Target Sites, and Receptors

Melatonin (N-acetyl-5-hydroxytryptamine), a hormone of darkness synthesized within the pinealocytes from tryptophan, once synthesized it is released into the peripheral system and cerebrospinal fluid (CSF). Melatonin acts both centrally and peripherally and the binding sites for melatonin are found in several areas of the brain including the hypothalamus, pars tuberalis, immune cells, gonads, kidneys, and heart.⁵⁴ Melatonin mediates its effects through receptor-dependent and independent mechanisms. The non-receptor-mediated effects are due to amphipathic properties, meaning that it can easily cross the cell and nuclear membranes, as it can be easily found in the nuclei of several cells in the brain and other body tissues.⁵⁵ A prominent example of the non-receptor-mediated effects of melatonin includes antioxidant action against different models of oxidative stress. Melatonin acts as a free radical scavenger and potent anti-oxidant in vivo and in vitro models, independent of the presence of the receptor.⁵⁶ Previously, several studies have suggested strong antioxidant, anti-inflammatory, and neuroprotective effects against different models of neurodegeneration.¹⁰ In this review, we have collected data from different sources concerning the use of melatonin in the post-injury period to counteract the effects of TBI-associated neurodegeneration.

Melatonin as a Regulator of Oxidative Stress in Traumatic Brain Injury

Oxidative stress plays a pivotal role in the pathophysiology of TBI, which suggests that antioxidant therapeutic approaches may provide a rescuing effect against TBI-induced oxidative damage.^57,58 Several natural compounds have shown beneficial effects against brain injury-induced oxidative stress-mediated neurodegeneration, such as melatonin, quercetin, and other plyphenols^10,59 that. A study conducted on melatonin against the controlled weight‐drop TBI model has shown rescuing effects of melatonin against the elevated oxidative stress (as shown by reduced expression of total glutathione, an enhanced ratio of oxidized glutathione to the total glutathione). Moreover, there was a decrease in STAT1 DNA‐binding activity, and neuronal cells. There was a marked upregulation in the mRNA level of the inhibitor of cytokine signaling (SOCS3), interleukin‐6 (IL‐6), inducible nitric oxide synthase (iNOS), and a reduced expression of protein inhibitor of activated STAT (PIAS1), after 24 hours of the brain contusion. As SOCS3 and PIAS1 are the endogenous inhibitors of STAT1. Interestingly, they have suggested that activation of SOCS‐3 and elevated oxidative stress in the TBI may aid in the activation of STAT1 inactivation, and melatonin may reduce the TBI-induced oxidative stress, STAT1 inhibition, and increase in the expression of SOCS‐3 and inflammatory mediators.⁶⁰

To evaluate the effect of melatonin against subarachnoid hemorrhage (SAH)-induced neurodegeneration and oxidative stress, the SAH model was developed by injection of arterial fresh autologous blood (non-heparinized) into the chiasmatic cistern. To the SAH + melatonin co-treated group, melatonin was injected at a dose of 150 mg/kg 2 and 24 hours after the SAH, and brain samples were collected after 48 hours of SAH. Their findings suggested that melatonin significantly enhanced the levels of Nrf-2‐ARE signaling and associated genes, such as Nrf-2, NAD (P) H: quinone oxidoreductase-1, heme oxygenase‐1 (HO-1), and glutathione S‐transferase α‐1. Moreover, melatonin regulated the blood-brain barrier (BBB) dysfunction, cortical apoptosis, brain-edema, and neurological impairment. The findings suggest that melatonin may rescue the SAH-induced neurodegeneration and oxidative stress by regulating the endogenous antioxidant regulators.⁶¹

A similar study was conducted on melatonin against the mouse model of traumatic brain injury. Here, the Melatonin (10mg/kg/intraperitoneal injection) was administered at 0, 1, 2, 3, and 4 hours post-TBI. For confirmatory purposes, a second experiment was conducted where the Nrf2 wild-type (Nrf2 (+/+) group) and Nrf2-knockout mice were used, and the TBI was induced in these groups and injected with melatonin (10mg/kg, i.p) at the corresponding time points. The findings have suggested that melatonin after TBI markedly regulated the effects of brain injury-induced oxidative stress, cortical neuronal degeneration, and brain edema. Furthermore, melatonin boosted the nuclear translocation of Nrf2 from the cytoplasm; thereby enhancing the expression of Nrf2-ARE signaling and related proteins, such as heme oxygenase-1 and NAD(p)h: Quinone oxidoreductase-1; and reduced the antioxidant enzymes, such as glutathione peroxidase and superoxide dismutase. Interestingly, in Nrf2 knockout mice, the neuroprotective effects of melatonin were partly reduced,⁶² showing that melatonin through Nrf2 rescued the mice brains against TBI-induced neurodegeneration.

A study conducted on closed head injury (CHI) has suggested that melatonin (5 mg/kg) had pronounced effects on neurobehavioral recovery, suppression of the redox‐sensitive transcription factors nuclear factor‐kappa-B (NF‐κB), and AP‐1 in mice subjected to closed head injury. After one hour of CHI, melatonin facilitated recovery for at least 1 week and decreased lesion size by two-fold. Neuroprotection against closed head injury-induced neurodegeneration was achieved with melatonin. The findings have indicated that melatonin is neuroprotective, via potentiation of brain antioxidants and regulation of NF‐κB and AP‐1 activation.⁶³ Overall findings have suggested that melatonin may reduce brain injury-induced oxidative stress and its consequences. Figure 5 shows a summary of the protective effect of melatonin against brain-injury associated neuroinflammation and neurodegeneration, by counteracting the oxidative and nitrosative stress, boosting the endogenous antioxidant mechanism (Nrf-2, HO-1 and GSH), and lipid peroxidation.

Figure 5 Melatonin is a potent antioxidant in brain injury-induced neurodegeneration.

Melatonin as an Energy Regulator in Traumatic Brain Injury

Apart from the elevated oxidative stress, energy imbalance and dyshomeostasis also play a pivotal role in the progression of brain injury-induced neurodegeneration.⁶⁴ Both preclinical and clinical studies have suggested that brain injury causes a prolonged suppression in glucose metabolism that affects brain physiology.⁶⁵ Most of the metabolic dysfunctions occur in the cortical and hippocampal regions of the brain,⁶⁶ highlighting the roles of metabolic dysfunction in brain injury-induced neurodegeneration and its pathological consequences.⁶⁷ Melatonin boosts the energy sources (such as glucose and lactate) or provides alternative energy sources (glyceryl triacetate, acetyl-L-carnitine),^68,69 which may confer neuroprotection against brain injury-induced neurodegeneration. Several studies have indicated that boosting the energy sensors through its kinases may confer neuroprotection after TBI-induced neurodegeneration. A study conducted on the role of phospho-5′AMP-activated protein kinase (p-AMPK) has shown that pharmacological enhancement of p-AMPK expression is neuroprotective.⁷⁰ To unveil the role of melatonin against the TBI-induced downregulated expression of p-AMPK, we conducted a study on TBI, which suggested that TBI-evoked oxidative stress, induced neuroinflammation, dysregulated the brain energy homeostasis, and contributed to neurodegeneration and memory dysfunction in mice. As TBI reduced the expression of phospho-5′AMP-activated protein kinase (p-AMPK) and phospho- cAMP-response element-binding (p-CREB), and activated the phosphorylation of NF-κB in TBI mice brains, interestingly melatonin enhanced the expression of p-AMPK, p-CREB, and reduced the p-NF-κB in the TBI-induced mice. Moreover, melatonin regulated mitochondrial dysfunction, amyloidogenic markers, and behavioral dysfunctions in TBI-induced mice.¹⁰

A study conducted on 7-day-old rat pups, which were subjected to contusion injury and were examined after 24 hours of TBI, showed elevated expression of thiobarbituric acid reactive substances (TBARS), and there was no increase in the antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPx). Contrarily, Melatonin at a single dose (5 mg/kg) reduced the level of TBARS levels in brain hemispheres, showing that melatonin reduced brain injury-induced oxidative stress in rat pups.⁷¹

Surgical brain injury (SBI) is another form of brain injury, which occurs as an inevitable consequence of neurosurgical procedures. SBI may cause postoperative complications such as brain edema and inflammation following blood-brain barrier (BBB) leakage and neurological disorders. They have suggested that administration of melatonin (5 mg/kg, 15 mg/kg) to the rodent models (before the surgical procedures) markedly reduced brain edema, reduced lipid peroxidation, and may protect the brains against SBI-induced neurodegeneration. They have shown that a higher dose of melatonin 150mg/kg is deleterious, as it may induce lipid peroxidation and brain edema.⁷²

To evaluate the effects of melatonin on the regeneration of motor axons in the TBI-induced model, two types of injury models were developed; reversible and acute presynaptic degeneration induced by α‐Latrotoxin and compression/transection of the sciatic nerves. The findings have suggested that melatonin accelerated the process of nerve repair via melatonin receptor-1 (MT1) and is partly due to sustained activation of the ERK1/2 signalings. These findings have suggested that the regenerative effects of melatonin are receptor-mediated and may offer a potential application for the management of several peripheral neurodegenerative conditions.⁷³

The neuronal‐specific K+–Cl− cotransporter‐2 (KCC2) is the main Cl− extruder in neuronal cells, playing a role in the Cl- homeostasis and physiological functions of the neurons. A study conducted on the role of melatonin on the expression of KCC2 in the TBI-induced rat model has suggested reduced expression of KCC2 (protein and mRNA) in the ipsilateral peri‐core parietal cortex. On the other hand, with the administration of melatonin (10 mg/kg intraperitoneally at 1, 2, 3, and 4 hours after TBI), the expression of KCC2 was significantly enhanced. Moreover, with the administration of melatonin, there was a significant increase in the expression of brain‐derived neurotrophic factor (BDNF) and phospho-extracellular signal‐regulated kinase (p‐ERK), reduced neuronal apoptosis, brain edema, and neurological dysfunction in the brain injury-induced rats. The overall findings suggest that melatonin may enhance the expression of KCC2, BDNF, and p-ERK, thereby reducing neurodegeneration and neurological deficits in TBI-induced animals.⁷⁴ Similarly, a study conducted by Nilgün Senol has also investigated the anti-oxidant effects of melatonin in the cerebral cortex and blood of traumatic brain injury-induced rats. The findings have suggested that TBI reduced the β-carotene, vitamin C, vitamin E, glutathione, and erythrocyte, glutathione levels, and plasma vitamin C level in the cerebral cortex of TBI-induced mice, which were enhanced with the administration of melatonin, suggesting that melatonin may regulate the level of anti-oxidant enzymes thereby regulate the oxidative stress associated with TBI. The overall findings support the anti-oxidant potentials of melatonin against brain injury-induced neurodegeneration.⁷⁵

Melatonin as a Regulator of Autophagic Dysfunction and Activated NLRP-3 Inflammasome in TBI-Induced Mice

Autophagy is a self-catabolic physiological mechanism by which cells recycle and conserve their organelles in a nutrient-deprived or stressed state, which is a lysosomal degradation mechanism responsible for survival, development, differentiation, and homeostasis. It serves as a safeguard protecting the organisms against diverse pathogenic events including cancer, infections, aging, neurodegeneration, and heart diseases.⁷⁶ Also, autophagic dysfunction is involved in several pathological conditions, including TBI and other brain injuries,⁷⁷ and boosting autophagy may counteract the pathological consequences of TBI. To explore the effects of melatonin on the autophagy-related markers in animal models of brain injury, several studies have been conducted on melatonin. A study has suggested that melatonin activates mitophagy via the mTOR signaling, thereby reducing the TBI‐induced inflammation. Moreover, melatonin markedly regulated neuronal cell death and behavioral dysfunctions in TBI-induced mice. The collective findings have suggested that melatonin may protect the mice’s brain against TBI‐induced neuroinflammation by activating the autophagy and inhibiting the IL-1β secretion via regulating the autophagy of damaged mitochondria.⁷⁸ Another study suggested that melatonin markedly regulated the secondary brain injury induced by TBI upregulated autophagy, accompanied by a reduction in translocation of Bax to mitochondria and the release of cytochrome C into the cytoplasm. Collectively, they have suggested that melatonin regulates autophagy and mitochondrial apoptosis, thereby protecting the mice’s brains against TBI-induced secondary brain injury.⁷⁹ In addition to oxidative stress, the innate immune response is also playing a critical role in the pathophysiology of brain-injury-induced neurodegeneration. One key mechanism of the innate immune system is NLRP-3 inflammasome activation, which is an important component of the innate immune response to inflammasome activities and tissue damage, and the abnormal activation of this system has been extensively reported in several neurological conditions, including AD, PD and TBI.⁴⁰ Several studies have suggested that inhibition of the NLRP-3 inflammasome activation in the TBI may reduce the activation of the inflammatory cytokines and neuroinflammation.⁸⁰ To analyze the effects of melatonin against the subarachnoid hemorrhage (SAH)-induced activated inflammasome, a comprehensive study has been conducted. The findings have indicated that melatonin through upregulation of autophagy (LC3-II/LC3-I and Atg 5), and mitophagy markers (Parkin and PINK-1) suppressed the NLRP-3 inflammasome activation. Moreover, melatonin regulated the pathological changes in mitochondria, reduced oxidative stress, and reduced the pro-inflammatory cytokine after SAH-induction. Interestingly, the inhibition of autophagy (through 3-MA) reversed the protective effects of melatonin against SAH-induced NLRP-3 inflammasome activation and release of the inflammatory mediators, showing the protective effects of melatonin are autophagy-dependent.⁸¹ Figure 6 shows the effects of melatonin against the activated NLRP-3 inflammasomes and subsequent neurodegeneration.

Figure 6 Effects of melatonin against Brain injury-induced activated NLRP-3 inflammasome and autophagic dysfunction. Graphical representation of effects of melatonin against brain injury-induced the activation of NLRP-3 inflammasome and inhibits the phosphorylation of mTOR. As the activated NLRP-3 inflammasome are responsible for the release of inflammatory cytokines, mitochondrial apoptosis, and elevated oxidative stress. Moreover, it has been suggested that melatonin has regulatory effects on the mTOR-mediated autophagy and NLRP-3 inflammasome activation.

Future Considerations, Research Gap, and Conclusion

The overall findings strongly support the notion that melatonin may counteract the brain-injury associated neurodegenerations by regulating oxidative stress, neuroinflammation, and autophagic dysfunction. Melatonin has shown neuroprotective effects in receptor dependent and independent manners.¹⁰ The receptor-independent effects are achieved through activating the endogenous antioxidant mechanisms, reducing lipid peroxidation, and reducing the reactive oxygen species. For example, melatonin attenuates oxidative insult by activating specific activator of transcription factor Nrf2 [REF]. The neuroprotective effects of melatonin on TBI were verified using Nrf2 knockout mice, which caused a loss in the neuroprotective effects of melatonin, indicating that the neuroprotective effects of melatonin are Nrf2 dependent.⁶² In addition, our study conducted on brain injury has suggested that melatonin may regulate the expression of p-AMPK and p-CREB, thereby reducing neuroinflammation and neurodegeneration [REF]. Also, our previous study spanned on the role of melatonin against ethanol-induced neurodegenerations in 7-day-old rat pups. Our finding is in agreement with the above studies that revealed related mechanisms of Nrf2 and melatonin significantly reduced neuroinflammation and neurodegeneration via the Nrf2 and its associated genes, as Nrf2 knocked down in HT-22 cells resulted in significant loss in the neuroprotective effects of melatonin [REF].

Another main mechanism is the regulation of autophagic dysfunction in brain injury, as the regulation of autophagy is responsible for counteracting neuroinflammation and neurodegeneration.⁷⁸ A study conducted on the role of melatonin against the TBI-induced neuroinflammation has suggested that melatonin reduced the expression of IL-1β level in the melatonin-treated groups compared to the TBI-induced mice. Similarly, melatonin enhanced the expression of IL-6, TNF-α IL-10 compared to the TBI model group (p < 0.001). These findings suggested that melatonin reversed the inflammatory cytokines involved in the brain injury-induced edema and neurodegeneration after TBI.⁸² Besides the receptor-independent mechanisms, several studies show that melatonin regulates ERK phosphorylation, thereby modulating the axonal regeneration via its melatonin receptor (MT1).⁷³ The available literatures strongly highlighted that the protective effects of melatonin against brain injury-induced neurodegeneration, but several aspects are left unexplored. For example, still there are limited studies available on whether regulation of sleep via melatonin in the TBI-induced model. In mammals and humans, the melatonin is secreted at night and the peak plasma concentration reaches at night (3–4 am) with a robust circadian rhythm. The rise of melatonin secretion is strongly associated with sleep propensity about 2 hours before the person’s regular bedtime.⁸³ Several studies have suggested that in TBI, there is a reduction in sleep quality, as melatonin has the sleep-promoting quality and is involved in the circadian regulation of the sleep-wake cycle. Abrogated melatonin concentration may contribute to sleep-related issues after TBI.⁸⁴ Studies have shown that TBI patients showed reduced (42% less) melatonin production overnight compared to healthy participants. Similarly, the timing of dim light melatonin onset (DLMO) was delayed by 1.5 hours in patients with TBI compared to the controls, proving that circadian regulation of melatonin synthesis is a feature of TBI, contributing to the sleep deprivation.⁸⁵ Another study has demonstrated a clear disruption of the diurnal rhythm in the TBI in the intensive care unit (ICU) patients. These patients with TBI peaked maximal endogenous serum melatonin level compared to control or patients with trauma without TBI, which is due to the stress caused by ICU. These findings widen the therapeutic window of melatonin, which might improve TBI patient recovery by balancing the circadian rhythm, strengthening the notion that administration of melatonin may counteract the TBI-associated sleep complications.⁸⁶

Few other shortcomings are in the timing of administration of melatonin, as melatonin is specifically activated in response to the darkness, and more detailed studies tackling whether administration or endogenous melatonin has therapeutic implication needs to be further investigated. Also, more mechanistic studies may be focused to unveil the specific dose, duration of treatment, pre- and post-treatment of melatonin in different disease states, and effects of sex differences on the efficacy of melatonin. One study has suggested that compared to the female rats, melatonin exhibited more significant neuroprotective effects (including improved locomotor and exploratory activities, elevated neuronal amount, and reduced neuronal apoptosis) in male rats exposed to TBI.⁸⁷

Conclusively, the overall findings suggest that melatonin may reduce brain injury-induced neurodegeneration by regulating oxidative stress, neuroinflammation, autophagy, and apoptotic cell death. The exact mechanisms behind the neuroprotective effects of melatonin are still not clear as mentioned previously, more efforts should be focused on unveiling the neuroprotective effects of melatonin. Current therapeutic approaches based on the regulation of oxidative stress, neuroinflammation, and neurodegeneration are quite generalized. Due to the heterogeneity of TBI mechanisms, current review focuses on oxidative and nitrosative stress after TBI. On the other hand, still very little attention has been given to underlying pathology on the association of melatonin and immune activation such as microglia and its receptors. Since immune system is very plastic, meaning that it can not only remain over a long period after injury, but immune activation also drive both injury and repair. This calls for future research, particularly to the long-term outcomes on immune responses after brain injury-induced neurodegeneration. Based on available studies, we suggest that melatonin may be further evaluated for its efficacy against brain injury-associated degenerative conditions.