Melatonin, a natural antioxidant therapy in spinal cord injury

Spinal cord injury (SCI) is a sudden onset of disruption to the spinal neural tissue, leading to loss of motor control and sensory function of the body. Oxidative stress is considered a hallmark in SCI followed by a series of events, including inflammation and cellular apoptosis. Melatonin was originally discovered as a hormone produced by the pineal gland. The subcellular localization of melatonin has been identified in mitochondria, exhibiting specific onsite protection to excess mitochondrial reactive oxygen species and working as an antioxidant in diseases. The recent discovery regarding the molecular basis of ligand selectivity for melatonin receptors and the constant efforts on finding synthetic melatonin alternatives have drawn researchers’ attention back to melatonin. This review outlines the application of melatonin in SCI, including 1) the relationship between the melatonin rhythm and SCI in clinic; 2) the neuroprotective role of melatonin in experimental traumatic and ischemia/reperfusion SCI, i.e., exhibiting anti-oxidative, anti-inflammatory, and anti-apoptosis effects, facilitating the integrity of the blood–spinal cord barrier, ameliorating edema, preventing neural death, reducing scar formation, and promoting axon regeneration and neuroplasticity; 3) protecting gut microbiota and peripheral organs; 4) synergizing with drugs, rehabilitation training, stem cell therapy, and biomedical material engineering; and 5) the potential side effects. This comprehensive review provides new insights on melatonin as a natural antioxidant therapy in facilitating rehabilitation in SCI.

Oxidative stress is a hallmark of secondary injury.It exists from the acute to chronic phases after traumatic and ischemia/reperfusion SCI (Erten et al., 2003;Song et al., 2013).Briefly, the balance between pro-oxidants and antioxidants is disrupted, excessive reactive oxygen species (ROS) are generated, and lipid peroxidation happens, which results in oxidative stress (McDonald and Sadowsky, 2002).The neurons and glial cells in the spinal cord cannot resist the oxidative stress, and cell death consequently happens followed by large amounts of debris (Bains and Hall, 2012).The deteriorated microenvironment initiates secondary injuries, e.g., neuroinflammation and cellular apoptosis (Wang H. et al., 2022).

Melatonin rhythm is affected in spinal cord injury
Primary sleep disorders, like circadian rhythm sleep-wake disorders and insomnia, are commonly encountered in SCI patients, especially in patients with quadriplegia (Zeitzer et al., 2000).Attributing to the role of melatonin (N-acetyl-5-methoxy tryptamine) in regulating the circadian rhythm sleep-wake cycle (Vasey et al., 2021), Zeitzer et al. (2000) first reported that plasma melatonin concentration failed to increase at night in three lower cervical SCI patients in the year 2000 (Zeitzer et al., 2000).Afterward, the temporal dynamics of melatonin has been measured in SCI patients with various injury degrees at different segments (Table 1).Notably, melatonin secretion decreases and rhythm loss occurs in the tetraplegic patients with complete cervical SCI, while they are not affected in complete thoracic SCI (Zeitzer et al., 2000;Scheer et al., 2006;Verheggen et al., 2012;Thøfner Hultén et al., 2018).A melatonin replacement therapy was proposed by Scheer et al. (2006) to improve sleep quality in tetraplegic patients (Scheer et al., 2006).Paralleled with the clinical reports, a similar pattern of changes in the melatonin rhythm has been revealed in the rat SCI model (Table 1) (Gezici et al., 2010).It has been reported that the rhythmic secretion of melatonin from the pineal gland is regulated by the suprachiasmatic nucleus (SCN) in the anterior hypothalamus (Cardinali and Pévet, 1998).The efferent nerve fibers from the SCN to the pineal gland cross the upper part of the cervical medulla and are connected to the preganglionic cells of the cervical sympathetic ganglion (Møller and Baeres, 2002), which hints the causality of patients with SCI at or above C8 with inadequate melatonin secretion, disrupted melatonin rhythm, and poor sleep quality (Verheggen et al., 2012).However, patients with injuries below the T4 spinal cord maintain their melatonin rhythm (Whelan et al., 2020).

Melatonin and melatonin receptors 2.1 Melatonin
Melatonin is an amphiphilic molecule first isolated by Lerner et al. (1958).Melatonin is synthesized in the pineal gland in the central nervous system (CNS) and peripheral organs, including the retina, intestine, bone marrow, pancreas, and kidney (Reiter, 1991;Ren et al., 2017;Zhang Y. et al., 2018).The subcellular localization of melatonin has been identified in mitochondria due to that the melatonin synthetic enzyme serotonin N-acetyltransferase (SNAT) was found in the matrix and intermembrane space of mitochondria (Reiter and Tan, 2019).Mitochondria are the major source of ROS, the birth place and battle ground of melatonin, and also the site of melatonin metabolism (Reiter and Tan, 2019).Melatonin exhibits specific on-site protection against mitochondrial ROS, which is underlying melatonin's anti-oxidative mechanism.Reiter and Tan (2019) proposed that melatonin exhibits its major protective effects on neurons at the level of mitochondria (Reiter and Tan, 2019).Melatonin receptor (MT1) is located on the mitochondrial membrane, and it regulates mitochondrial physiology through the MT 1 -mediated sirtuin 1 (SIRT1)-peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α)-nuclear factor erythroid 2-related factor 2 (NRF2)-peroxisome proliferator-activated receptor-   gamma (PPAR-γ) signaling transduction through the MT 1mediated sirtuin 1 (SIRT1)-peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α)-nuclear factor erythroid 2-related factor 2 (NRF2)-peroxisome proliferatoractivated receptor-gamma (PPAR-γ) signaling transduction (Guo et al., 2014).It has been reported that melatonin maintains mitochondrial homeostasis by regulating mitochondrial fission, mitophagy, and mitochondrial permeability transition pore (Zheng et al., 2021).Specifically, mitochondrial ROS in cancer amplifies the tumorigenic phenotype, accelerates the accumulation of additional mutations, and leads to metastatic behaviors (Sabharwal and Schumacker, 2014); the cytotoxicity of melatonin to Caco-2 cells (an immortalized cell line of human colorectal adenocarcinoma cells) and Jurkat cells (an immortalized cell line of human T lymphocyte cells), in turn, evidences the effectiveness of melatonin in regulating the mitochondrial metabolism (Wölfler et al., 2001).Plasma melatonin concentration peaks at night in mammals (Zhang et al., 2022).It has been suggested by experts that the exposure to Sun during the day and avoidance of light at night benefit human wellbeing by improving melatonin production (Tan et al., 2023).In clinic, melatonin has been used to treat primary insomnia (Rondanelli et al., 2011).Moreover, the effectiveness of melatonin in combating oxidative stress, inflammation, and cellular apoptosis has been studied for decades in mood disorders and cognitive dysfunctions (Agahi et al., 2018), stroke and other neurodegenerative diseases (Hosseinzadeh et al., 2023), tumors, and cancers (Rohilla et al., 2023) Recently, the cryogenic electron microscopy (cryo-EM) structures of the active melatonin receptors (MTs) have been revealed (Wang Q. et al., 2022), which is going to take melatonin research forward.

Melatonin receptors and melatonergic ligands
Melatonin regulates sleep and the circadian rhythm by activating two high-affinity G protein-coupled receptors, i.e., MT 1 and MT 2 (Cecon et al., 2018).In the brain, MT 1 is expressed in the locus coeruleus (LC) and perifornical lateral hypothalamus (PFH), regulating the rapid eye movement (REM), while MT 2 is found in the thalamic reticular nucleus (TRN) in charge of the non-REM (NREM) activity (Lacoste et al., 2015;Waly and Hallworth, 2015).Initially, melatonin is extracted from the pineal gland and urine; currently, all melatonin used in clinic is obtained from chemical methods, such as the prolonged melatonin preparation (Circadin ® ) and the synthesized ligand agonist to both MT 1 and MT 2 (agomelatine, ramelteon, and tasimelteon) (Wang Q. et al., 2022).The technological breakthrough, obtaining biological melatonin from microbes, algae, and plants, has been achieved a few years ago (Arnao et al., 2023).Notably, the microorganisms, including the genetically modified Saccharomyces and Escherichia coli and microbial fermentation Lactobacillus sp., Bifidobacterium sp., Enterococcus sp., and Streptococcus thermophilus, haven been  employed to produce melatonin industrially in bioreactors (Arnao et al., 2023).Despite the development of the chemical and biological melatonergic ligands, the in vivo stability and subtype selectivity, especially the MT 1 selective drug, are yet to be solved (Boutin et al., 2020).From the pharmacological standpoint, a full characterization of the melatonin receptor subtypes is advocated.Wang H. et al. (2022) revealed that the cryo-EM structures of active MTs with heterotrimeric G i protein engaged in the receptor core (Wang Q. et al., 2022).Though MT 1 and MT 2 possess a high similarity in the orthosteric ligand-binding pockets, they display distinctive features in the cryo-EM, i.e., the distinctive structure of the N4.60-Y5.38-H5.46motif and the "longitudinal channel" in MT 1 and the larger sub-pocket in MT 2 (Wang Q. et al., 2022).
Attributing to the relationship between the melatonin rhythm and SCI in clinic (Table 1), efforts have been paid to bond the causality between rehabilitation and melatonin in SCI.To provide a thorough summary of melatonin and its effect associated with SCI, literature searches were conducted on the databases of PubMed, Web of Science, and Google Scholar from inception to March 2023 with the keywords combining "spinal cord injury" or "SCI" with "melatonin" or "N-acetyl-5-methoxy tryptamine."The eligibility criteria of the selected studies are 1) relevant studies with full text published in English and assessable online; 2) outcomes obtained from animal models (mouse, rat, and rabbit); 3) reporting information including model type, treatment, dosage/ duration of melatonin, and outcome assessment; and 4) modeling with contusion, compression, radiation, photo thrombosis, and spinal cord ischemia-reperfusion injury (SCIR).

Melatonin confers neuroprotection in spinal cord injury
The neuroprotective effects and underlying mechanisms of melatonin were explored in experimental SCI (Table 2).

Melatonin maintains the integrity of the blood-spinal cord barrier and decreases edema
The blood-spinal cord barrier (BSCB) consists of vascular endothelial cells, astrocytes, pericytes, and basement membrane.It regulates molecular exchange between blood and the spinal cord and plays an important role in maintaining the fluid microenvironment of the spinal cord (Haddadi et al., 2013).The compromised integrity of the BSCB increases its permeability to plasma components, e.g., neutrophil infiltration happens at the site of injury following traumatic SCI, leading to vasogenic spinal cord edema (Wu et al., 2014).In traumatic SCI, the BSCB permeability is increased quickly within 48 h post SCI; the melatonin treatment maintains the integrity of the BSCB via regulating the tight junction proteins and protecting endothelial cells and pericytes (Wu et al., 2014).Jing et al. (2016) showed that melatonin attenuates edema by decreasing the expression of aquaporin 4 (AQP4) and intercellular cell adhesion molecule-1 (ICAM1), which restrains microvessel loss and sustains microcirculation (Wu et al., 2014).In the irradiated spinal cord tissue, the mRNA expression of the vascular endothelial growth factor (VEGF), which is critical in BSCB destruction (Haddadi et al., 2013), is increased at 20th and 22nd week after irradiation (Haddadi et al., 2013).Melatonin preconditioning decreases the expression of VEGF and significantly reduces vasodilation, congestion, and cavitation in the white matter, which promotes functional recovery in the irradiated animals (Haddadi et al., 2013).

Melatonin prevents neural death and facilitates neuroplasticity
Neurons are the structural and functional units of the nervous system, while glial cells support neurons.Neurons communicate to one another based on a synaptic structure.Oligodendrocytes form the myelin sheath around axons, which promises an efficient longdistance impulse transmission from neuronal soma to the presynaptic membrane.
In SCI, the deteriorated microenvironment after SCI jeopardizes neuronal vitality and causes neuronal death.Wang Q. et al. (2022) found that melatonin inhibits neuronal death at the acute phases of SCI and promotes rehabilitation afterward by attenuating oxidative stress and preventing mitochondrial dysfunction (Wang H. et al., 2022).Moreover, intrathecal melatonin administration significantly preserves the myelinated areas and increases the number of sensory neurons in the white matter after SCI (Fakhri et al., 2021).In a word, melatonin administration prevents axonal degeneration and demyelination (Majidpoor et al., 2020a;Majidpoor et al., 2020b).Parallelly, the endogenous melatonin content in the rostrocaudal region of the spinal cord is increased by a 12-h light/12-h dark cycle, promoting oligodendrogenesis and excitatory synaptic formation (Hong et al., 2019).Neuroplasticity drives functional restoration in SCI.Melatonin facilitates neuronal remodeling by increasing the Frontiers in Cell and Developmental Biology frontiersin.orgexpression of the brain-derived neurotrophic factor (BDNF) and growth-associated protein-43 (GAP-43) (Jing et al., 2017).Bi et al. (2021) found that melatonin administration after SCI significantly increases the expression of GAP-43, synaptic protein 1 (SYN1), neurofilament 200 (NF200), and postsynaptic density protein 95 (PSD-95) (Bi et al., 2021).

Melatonin decreases scar formation and promotes axon regeneration
Reactive astrogliosis forms the core of fibrotic scar at the SCI lesion site.The glial scar, a dense limiting border outside the injury site, is the primary cause of the failure in axonal regeneration, and reducing glial scar formation after SCI contributes to the regrowth of transected axons (Krityakiarana et al., 2016).It has been reported that melatonin suppresses the proliferation and accumulation of astrocytes (Yang et al., 2020).Krityakiarana et al. (2016) reported that melatonin reduces inflammatory response and scar formation, which promotes axonal outgrowth (Krityakiarana et al., 2016).Moreover, Hong et al. (2019) reported that the melatonin precursor N-acetylserotonin (NAS) activates the tropomyosin receptor kinase B (TRKB)-induced protein kinase B (AKT)-extracellular-regulated kinase (ERK) signaling cascade, which facilitates axonal growth after SCI (Hong et al., 2019).

Melatonin attenuates oxidative stress
Melatonin is an effective free-radical scavenger, sustaining the balance between reduced glutathione (GSH) and oxidized glutathione disulfide (GSSG) and preventing excess lipid peroxidation in the aged mouse brain (Carretero et al., 2009).In traumatic and ischemia/reperfusion SCI, melatonin corrects the level of malondialdehyde (MDA), GSH, superoxide dismutase (SOD), nitrite oxide (NO), xanthine oxidase (XO), thiobarbituric acid reactive substance (TBARS), inducible nitric oxide synthase (iNOS), and myeloperoxidase (MPO), which benefits rehabilitation (Fujimoto et al., 2000;Kaptanoglu et al., 2000;Taskiran et al., 2000;Erten et al., 2003;Liu et al., 2004;Song et al., 2013).Moreover, Gül et al. (2005) revealed a dosedependent effect of exogenous melatonin on protecting the axons and myelin in white matter at 24 h after SCI, but not on preventing early lipid peroxidation (Gül et al., 2005).Interestingly, Park et al. (2012) reported a loss of efficacy of melatonin treatment in decreasing the mRNA expression of iNOS and accelerating motor function recovery when exposing SCI rats to 24-h constant light, which suggests the significance of endogenous melatonin and melatonin rhythm in functional recovery in SCI (Park et al., 2012).

Melatonin reduces inflammation
ROS functions as a critical second messenger that modulates inflammation through redox-sensitive mechanisms.Melatonin works as an antioxidant, attenuating neuroinflammation by altering the oxidative stress-induced gene expression profiles in SCI, such as peroxisome proliferator-activated receptors α (PPAR-α), toll-like receptor-4 (TLR4), nuclear factor kappa B (NF-κB), and NRF2.Specifically, melatonin exhibits antiinflammatory effects against the secondary injury in wild-type mice by binding to PPAR-α, indicated by a loss of response to melatonin in the PPAR-α knockout (KO) mice (Paterniti et al., 2017).Moreover, melatonin administration during the acute phase after SCI suppresses inflammation through inhibiting the TLR4/NF-κB signaling pathway (Majidpoor et al., 2020a).Recently, Wang Q. et al. (2022) reported that melatonin reduces the generation of nucleotide-binding domain-like receptor protein 3 (NLRP3) inflammasomes via activating NRF2 and downstream antioxidant responsive element (ARE) signaling transduction, which reduces mitochondrial dysfunction and accelerates neurological functional recovery (Wang H. et al., 2022).
4 Melatonin maintains the function of peripheral organs and aids in spinal cord injury SCI causes functional disorders in the peripheral organs, including the gastrointestinal system (Zhang C. et al., 2018), kidney (Akakin et al., 2013), bladder (Erşahin et al., 2012), testes (Yuan et al., 2016), and corpus cavernosum (Tavukçu et al., 2014), which is attributed to the damaged autonomic innervation by the sympathetic and parasympathetic fibers derived from the spinal cord (Table 3).For example, the damaged autonomic nervous system loses gastrointestinal sympathetic control, leading to insufficient intestinal motility and deficient mucosal secretions (Zhang C. et al., 2018).The gut microbiota works as a virtual endocrine organ, regulating mood and behaviors via the microbiota-gut-brain axis and immunity via the intestinal neuro-immune axis (Jacobson et al., 2021;Huang et al., 2022).Hence, SCI affects the behavior of the gastrointestinal system, alters the constitution of gut microbial community, and causes systemic disorders.The alteration in gut microbiota is associated with the metabolite profiles in SCI (Kang et al., 2023).Gut dysbiosis impairs rehabilitation in SCI (Agahi et al., 2018).The gut microbiota-brain axis mediates the psychological stress after SCI (Musleh-Vega et al., 2022).Fecal microbiota transplantation exerts neuroprotective effects on SCI mice via modulating the microenvironment at the lesion site (Jing et al., 2022).Furthermore, melatonin mitigates the microbiota dysbiosis in the segment of the jejunum in the short-term sleep-deprived mice (Gao T. et al., 2020).Jing et al. (2019) showed that melatonin treatment reduces the relative abundance of Clostridium perfringens and increases that of Lactobacillus, which attenuates SCI-induced dysbiosis (Jing et al., 2019).Moreover, melatonin treatment improves the function of bladder (Erşahin et al., 2012), decreases the SCIincreased permeability of capillaries, and increases the SCIreduced testicular blood flow (Yuan et al., 2016) by coping with oxidative stress.

Synergistic effects of melatonin
Based on the bulk reports regarding melatonin's protective roles in SCI, the synergistic effects of melatonin are explored (Table 4), including interacting other medications, functioning as a supplement in rehabilitation training, and working as a preconditioning stimulus in stem cell therapy.Moreover, melatonin has been employed by biomedical material engineering as an antioxidant agent in treating SCI and a differentiation signal to activate the endogenous neural cells.

Melatonin administrated with other drugs
Methylprednisolone (MP) is a corticosteroid medication applied during the acute phase in SCI (Cayli et al., 2004).Cayli et al. (2004) reported that combining melatonin with MP results in a better neurological recovery during the subacute phase after SCI by inhibiting the accumulation of lipid peroxidation (Cayli et al., 2004).Moreover, melatonin administrated with the corticosteroid dexamethasone (DEX) significantly reduces tissue damage and promotes motor function recovery via reducing apoptosis and polymorphonuclear leukocyte infiltration indicated by the downregulated protein expressions of TNF-α and iNOS in the spinal cord (Genovese et al., 2007).The combined administration elevates the curative effect of DEX, i.e., reducing to a 10th of the original effective concentration, and avoids its side effects, such as infection, osteonecrosis/ osteoporosis, and depression (Genovese et al., 2007).Moreover, the combination therapy of melatonin and tadalafil (a medicine for erection problems) is more effective in restoring erectile function via attenuating the oxidative damage to cavernosum in SCI compared to the individual drugs (Tavukçu et al., 2014).

Melatonin supplements rehabilitation training
SCI disrupts the signaling transmission between the brain and body, and SCI rehabilitation exercise helps restore brain-body communication via facilitating neuronal regeneration in the injured spinal cord.Melatonin administration combined with motor-driven treadmill exercise suppresses neuroinflammatory response, reduces neuronal deformation, increases endogenous neural stem/progenitor cells, enhances neuronal differentiation, and improves the recovery of motor function after SCI (Park et al., 2010).In line with the previous report, Lee et al. (2014) found that the synergistic effect of melatonin combined with treadmill exercise.It creates a healing niche which facilitates proliferation and differentiation of endogenous neural stem/progenitor cells, promotes neuronal regeneration, and improves neurological function in SCI rats (Lee et al., 2014).

Melatonin serves in stem cell therapy
The melatonin precondition provides a promising extracellular vesicle-based or stem cell-based approach for treating SCI.It has been reported in mesenchymal stem cells (MSCs) that melatonin reduces methyltransferase-like 3 (METTL3) expression and global N6-methyladenosine (m 6 A) modification, which stabilizes the mRNA of ubiquitin specific peptidase 29 (USP29) (Liu et al., 2021).Liu et al. (2021) harvested USP29-contained extracellular vesicles from the melatoninpretreated MSCs (MEVs) (Liu et al., 2021).The USP29contained MEVs transfer USP29 to microglia and macrophages in the injured site, which stabilizes NRF2, facilitates M2-like polarization, and consequently, promotes functional behavioral recovery in the SCI mice (Liu et al., 2021).Moreover, melatonin, working as an antioxidant, facilitates the vitality of the adipose-derived stem cells (ADSCs/adMSCs) in vitro before stem cell transplantation (Naeimi et al., 2022).The melatonin-pretreated ADSC/adMSC positively affects the engraftment and neuronal differentiation in SCI (Naeimi et al., 2022).Similarly, melatonin administrated with wnt family member 4 (WNT4) promotes neural cell differentiation of the bovine amniotic epithelial cells, and the neural cell transplantation has been proven to accelerate the motor function recovery in SCI (Gao et al., 2016).

Melatonin is employed by biomedical material engineering
Melatonin is employed in biomedical material engineering to act directly on the endogenous stem cells.Two new sustainedrelease melatonin delivery systems to prolong melatonin release and enhance melatonin's role in facilitating M2 polarization are developed in consideration of the complicated conditions and significant inter-individual differences in SCI patients (Zhang et al., 2021).For in situ injection, an injectable Lap/MS@ Melatonin micro-gel compound, i.e., poly (lactic-co-glycolic acid) (PLGA) sustained-release microspheres (MSs) loaded with melatonin mixed with Laponite hydrogels, has been designed (Zhang et al., 2021).To sustain and for precisiontargeted delivery of melatonin intravenously, a nano-PM compound, namely, PM/MS@Melatonin, i.e., nanospheres coated with a platelet membrane (PM) loaded with melatonin, is designed (Zhang et al., 2021).Moreover, a 3Dprinted scaffold for a controlled release of melatonin has been developed for curing long-range nerve defects (Qian et al., 2018).It has been found that this melatonin/polycaprolactone nerve guide conduit reduces oxidative stress, neuroinflammation, and nerve cell apoptosis; provides energy for nerves; facilitates nerve debris clearance; and stimulates neural proliferation (Qian et al., 2018).

Side effects of melatonin
Adverse events have been reported in melatonin treating sleep disorders, including excessive sedation (Panda et al., 2022), cognitive disorders (Besag et al., 2019), nocturnal hypotension (Grossman et al., 2011), and impaired glucose tolerance (Rubio-Sastre et al., 2014).The suggested dose of melatonin in clinic is 0.3-1.0mg/day internationally.However, a clinical trial indicated no significant difference between melatonin and placebo arms in the rate of adverse events, even at a dose as high as 1,600 mg/day (Foley and Steel, 2019).Moreover, systemic reviews indicate no adverse events of life-threatening or of major clinical significance (Grossman et al., 2011;Besag et al., 2019;Panda et al., 2022).The bioavailability of exogenous melatonin is highly variable due to the first-pass metabolism and poor absorption, ranging from 1% to 37% (Andersen et al., 2016).Especially, in cirrhotic patients, the clearance of melatonin is slowed down (Chojnacki et al., 2013).Hence, caution should be exercised when administering melatonin with other drugs (Foster et al., 2015).For example, taking melatonin with sedatives might cause sleepiness and slow breathing (Panda et al., 2022).Furthermore, a case report indicated that a good initial response to melatonin was found at the beginning in treating intellectual disability and sleep disorders; the patients lost response to melatonin afterward potentially due to the delayed melatonin clearance (Braam et al., 2010).
To avoid the adverse effects, lowering effective concentration, increasing the effectiveness of melatonin treatment, and seeking melatonin analogs are considered.For example, the effect of βmethyl-6-chloromelatonin, which exerts a direct soporific effect in treating insomnia without changing in body temperature, heart rate, or blood pressure compared to melatonin (Zemlan et al., 2005), has been studied in SCI though no statistical significant outcome was found (Fee et al., 2010).Other novel synthetic analogs, mainly the melatonin derivatives, are under study, such as the 2-phenylindole derivatives (Suzen et al., 2006), 5bromoindole derivatives (Gürkök et al., 2009), 2-indole aldehyde derivatives (Suzen et al., 2013), and 5-chloroindole derivatives (Yılmaz et al., 2012).

Conclusion and perspectives
In this review, the relationship between the melatonin rhythm and SCI has been unveiled in clinic, and the feasibility of melatonin treatment in facilitating rehabilitation under the circumstances of various SCI degrees and segments has been verified in experimental SCI models.Oxidative stress is a primary cause in cytotoxicity and disease.Melatonin administrated independently or synergized with other medications, rehabilitation training, stem cell transplantation, and biomedical material engineering, as a natural antioxidant therapy, aids in SCI.Generally, melatonin copes with oxidative stress and mitigates subsequent neuroinflammation and cellular apoptosis in SCI, resulting in reduced neural death and debris, restored BSCB, attenuated edema, decreased scar formation, a better neuronal regeneration, and neuroplasticity and microcirculation in the injured spinal cord (Figure 1).Moreover, melatonin attenuates oxidative damage, prevents functional disorders in the peripheral organs, and has a beneficial influence on systemic recovery in SCI (Figure 1).Though the melatonin's protective roles in preventing secondary injuries in SCI have been studied for decades and many signaling pathways are reported involved in melatonin facilitating functional recovery and attenuating tissue damage, the receptordependent mechanism is rarely mentioned, e.g., MT 1 or MT 2 .Due to the potential rise of long-term melatonin exposure and the high dose of melatonin used in SCI [orally: 5 mg/kg, intraperitoneal injection: 5-100 mg/kg (reported in experimental SCI research, Table 2); compared to clinical dosage: physiological dose (0.3-1.0 mg/qd) and supraphysiological dose for occasional use (≥3 mg/qd), seeking melatonin analogs to prevent drug resistance and designing selective compound for an effective melatonergic therapy in SCI are advocated.

FIGURE 1
FIGURE 1Melatonin facilitates systemic recovery by preventing spinal cord dysfunctions and peripheral organ disorders.

TABLE 1
Relationship between the temporal dynamics of melatonin and spinal cord injury in patients and animal models.

TABLE 2
Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.
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TABLE 2 (
Continued) Non-comprehensive review of the neuroprotective effects of melatonin in experimental spinal cord injury.

TABLE 3
Non-comprehensive review of melatonin protecting peripheral organs in experimental spinal cord injury.
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TABLE 3 (
Continued) Non-comprehensive review of melatonin protecting peripheral organs in experimental spinal cord injury.

TABLE 4
Non-comprehensive review of the synergistic effects of melatonin in spinal cord injury.

TABLE 4 (
Continued) Non-comprehensive review of the synergistic effects of melatonin in spinal cord injury.
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