Spinal cord injury (SCI) is a sudden onset of disruption to the spinal neural tissue, e.g., spinal cord compression caused by accidents or tumors (Baptiste and Fehlings, 2006), leading to paraplegia, quadriplegia, pain hypersensitivity, and abnormal temperature sensation (Khorasanizadeh et al., 2019; Anjum et al., 2020; Eli et al., 2021). Moreover, a series of complications caused by SCI, such as neurogenic bladder, gastrointestinal dysfunction, and sexual dysfunction, decrease the quality of life of SCI patients, which brings a huge burden to society (Popa et al., 2010; Tollefsen and Fondenes, 2012; Sezer et al., 2015; Tate et al., 2020). Though the initial trauma in SCI is irretrievable, preventing adverse events brought by the secondary injury cascade over the subsequent weeks is achievable.

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).

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 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 MT₁-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 MT₁-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 (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 regulates sleep and the circadian rhythm by activating two high-affinity G protein–coupled receptors, i.e., MT₁ and MT₂ (Cecon et al., 2018). In the brain, MT₁ is expressed in the locus coeruleus (LC) and perifornical lateral hypothalamus (PFH), regulating the rapid eye movement (REM), while MT₂ 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 MT₁ and MT₂ (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 MT₁ 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 MT₁ and MT₂ 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-H5.46 motif and the “longitudinal channel” in MT₁ and the larger sub-pocket in MT₂ (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).

Oxidative stress is a potent apoptotic inducer, which disrupts Ca²⁺ homeostasis, causes mitochondrial dysfunction, e.g., mtDNA damage and cytochrome c release, and induces typical apoptotic proteins, e.g., the increased expression of cleaved caspase-3 (CASP3) and B-cell lymphoma 2 (BCL2) and the decrease in the bcl-2-like protein 4 (BAX) content (Aydemir et al., 2016). Aydemir et al. (2016) found that melatonin deficiency impedes sensory and motor function recovery in the ischemic spinal cord injured rats, while pretreatment with melatonin alleviates neuronal apoptosis and accelerates functional recovery by activating the phosphoinositide 3-kinase (PI₃K)–akt serine/threonine kinase 1 (AKT1) signaling pathway (Aydemir et al., 2016). Parallelly, the administration of melatonin during the first 3 days after traumatic SCI ameliorates neuronal apoptosis via provoking the wingless–int (Wnt)/β-catenin signaling transduction (Shen et al., 2017).

Oxidative stress damages cellular constituents, e.g., proteins, DNA, and lipids, which turns on the autophagic process (Li et al., 2019). The mechanistic target of rapamycin (mTOR) is a pharmacologic target for autophagy regulation. It has been found that melatonin enhances autophagy and inhibits neuronal apoptosis after SCI through activating the PI₃K/AKT/mTOR signaling pathway (Li et al., 2019). In addition, mTOR, 5′AMP-activated protein kinase (AMPK), and NAD-dependent deacetylase sirtuin-1 (SIRT1) interplay in regulating metabolic stress. Gao T. et al. (2020) indicated that melatonin exerts neuroprotection via activating autophagy through the SIRT1/AMPK signaling pathway (Gao K. et al., 2020).

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).

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).

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 (m⁶A) 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 melatonin-pretreated MSCs (MEVs) (Liu et al., 2021). The USP29-contained 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 in biomedical material engineering to act directly on the endogenous stem cells. Two new sustained-release 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 precision-targeted 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 3D-printed 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).