Protective Effects of Melatonin on Neurogenesis Impairment in Neurological Disorders and Its Relevant Molecular Mechanisms

Neurogenesis is the process by which functional new neurons are generated from the neural stem cells (NSCs) or neural progenitor cells (NPCs). Increasing lines of evidence show that neurogenesis impairment is involved in different neurological illnesses, including mood disorders, neurogenerative diseases, and central nervous system (CNS) injuries. Since reversing neurogenesis impairment was found to improve neurological outcomes in the pathological conditions, it is speculated that modulating neurogenesis is a potential therapeutic strategy for neurological diseases. Among different modulators of neurogenesis, melatonin is a particularly interesting one. In traditional understanding, melatonin controls the circadian rhythm and sleep–wake cycle, although it is not directly involved in the proliferation and survival of neurons. In the last decade, it was reported that melatonin plays an important role in the regulation of neurogenesis, and thus it may be a potential treatment for neurogenesis-related disorders. The present review aims to summarize and discuss the recent findings regarding the protective effects of melatonin on the neurogenesis impairment in different neurological conditions. We also address the molecular mechanisms involved in the actions of melatonin in neurogenesis modulation.


Introduction
Adult neurogenesis, namely, generation of new neurons, was discovered in the 1960s [1], while its involvement in behaviors and diseases were discovered in the last two decades since the discovery of the neurogenesis-promoting property of antidepressants [2]. Neurogenesis is implicated in different behaviors and brain functions [3]. For instance, neurogenesis in the subventricular zone (SVZ) is hypothesized to be responsible for the sexual and olfactory behavior, whereas neurogenesis taking place in the hippocampus is important for the learning and memory processes as well as pattern separation [4][5][6][7].
Melatonin is a hormone secreted by the pineal gland in the mammalian brain. Its first identified role is to regulate the circadian rhythm and the sleep-wake cycle [8]. Previously, it was hypothesized that melatonin is synthesized in the cytosol of cells; however, recent findings have shown that mitochondria is the original site of melatonin synthesis [9]. Melatonin synthesis has been found in mitochondria of different cells including oocytes, pinealocytes, endothelial cells, plant cells, and neurons [10]. Serotonin N-acetyltransferase (SNAT) is a melatonin synthetic enzyme, whereas N-acetyl-coenzyme A is an

Roles of Melatonin in Modulation of Neurogenesis
Melatonin has been shown to be involved in the modulation of neurogenesis in both in vivo and in vitro models [21] (Figure 1).

In Vitro Model
Melatonin promotes viability, proliferation, and neuronal differentiation of mouse embryonic cortical neural stem cells (NSCs) [22][23][24]. It enhances differentiation of embryonic NSCs into neurons through melatonin receptors (MT1/MT2) with the CBP/p300-mediated acetylation of histone H3 lysine 14 via extracellular signal-regulated kinase (ERK) signaling pathway [23]. Similar effects on the neural stem cell line were observed, where the expression of neuronal markers increased in the PC12 and C17.2 NSCs after treatment of melatonin via the activation of the PI3K/Akt pathway [25,26]. With the application of melatonin receptor antagonist luzindole, the effects of melatonin on the PC12 cells was abolished, leading to reduction in neurite outgrowth and decrease in the number of mature neurons [25]. In another cellular model, melatonin enhanced dopaminergic neuronal differentiation of embryonic day 14 (E14) rat midbrain NSCs, with the effect being potentially brought out by the increased production of brain-derived neurotropic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) in the NSCs [27]. Timing of melatonin treatment was found to be critical in the differentiation of NSCs. It was shown that melatonin promoted 1% fetal bovine serum (FBS)-induced neuronal differentiation during the proliferation period of E15.5 ganglionic eminence NSCs but it decreased neuronal differentiation of the NSCs during the differentiation period [28]. Melatonin also promoted the neuronal differentiation of amniotic fluid mesenchymal stem cells and mouse-induced pluripotent stem cells, in which ERK/CaMKII and PI3K/Akt signaling pathways were, respectively, involved [29,30]. In adult rat hippocampal organotypic culture experiments, melatonin was found to promote dendritogenesis by the activation of CaMKII, protein kinase C (PKC), and ERK1/2 with partial involvement of the melatonin receptors [31,32]. These results support the roles of melatonin on neurogenesis (Table 1).  Mouse NSCs (E15.5; from ganglionic eminence) 5 µM, 4-day treatment of melatonin Suppressed epidermal growth factor (EGF)-stimulated NSC proliferation Treatment during the proliferation period promoted 1% FBS-induced neural differentiation of NSCs Treatment during the differentiation period decreased neural differentiation of the NSCs Not examined in the study Moriya et al., 2007 [28] Human amniotic fluid mesenchymal stem cells (16 to 20 week of pregnancy)

In Vivo Models
Melatonin promotes adult neurogenesis in different models. From gain-of-function experiments, when mice were administered 8 mg/kg body weight (BW) melatonin daily for 7 or 14 days exogenously, the number of doublecortin-positive (DCX+) neuronal precursor cells in the dentate gyrus (DG) increased [38]. In addition, melatonin increased the structural plasticity of mossy fiber projection in the 8-week-old mice [39]. The roles of melatonin on adult neurogenesis were further confirmed by the loss-of-function pinealectomy experiments. In the pinealectomy experiments, the pineal gland, which is the primary production site of melatonin, was surgically removed from the animals. After rats were pinealectomized, the levels of melatonin decreased and neurogenesis in the hippocampus declined; however, the neurogenesis impairment was reversed by the melatonin treatment [40]. Male offspring of the mouse mothers, which were pinealectomized before pregnancy, also exhibited neurogenesis disruption, but the impairment could be alleviated by the melatonin treatment [41]. Apart from promoting neurogenesis, melatonin enhanced survival and dendritic maturation of the immature neurons in the hippocampus [33] ( Table 2).
The neurogenic effects of melatonin were observed when the mice were treated at a dose as low as 2 mg/kg BW daily for 7 days [24]. This dosage of melatonin increased the number of BrdU+/NeuN+ cells in the DG and modulated the mitochondrial DNA copy number and oxidative phosphorylation proteins, including COX I, COX IV, ATP-5β, and NDUFB8 in the hippocampus [24]. In the aging animal studies, melatonin delayed the decline of the hippocampal neurogenesis in the 6-to 9-month-old aging mice [42]. Increased cell proliferation and higher number of calretinin+ neurons were observed in the DG in the aging mice which received the melatonin treatment [42,43]. Moreover, melatonin potentiated the running-wheel activity-induced cell survival and neurogenesis in the DG in 2-3-month-old mice [44] ( Table 2).

Melatonin and Depression
According to the neurogenesis hypothesis of depression, the disturbance of mood and emotion is linked to the impairment of neurogenesis, and restoration of neurogenesis would be a critical factor for the remission from the disease [2,[46][47][48]. If neurogenesis is suppressed, the beneficial effects of antidepressants would be abolished, which may indicate that reversing neurogenesis deficits is a potential therapeutic target of depression [49]. The protective effects of melatonin on depression has been well documented. Depression-like behavior in male BALB/c mice was reduced when they were treated with 5 and 10 mg/kg BW melatonin daily for 7 days [50]. Melatonin reduced the immobility behavior of the mice in the forced swim test [50], and coincidently, neurogenesis and dendrite maturation in those mice were promoted [50]. Corticosterone (CORT) contributes to the onset of depression-like behavior in rodents. The CORT depression model is a well-established model for studying depression. It was shown that 3-week daily treatment of 10 mg/kg BW melatonin prevented CORT-induced reduction in cell proliferation in the DG and reduced depression-and anxiety-like behavior of the mice in the forced swim test, open field test, and novelty suppressed feeding test [51]. The anti-depressive and neurogenetic effects of melatonin was potentially brought about by the inhibition of the acid sphingomyelinase/ceramide system as well as the decrease in vesicular monoamine transporter 2 (VMAT2) levels and increase in the monoamine oxidase A MAO-A levels in the hippocampus [52,53]. Using combination treatment of melatonin and citalopram (MLTCITAL), 14-day daily treatment of 2.57 mg/kg BW MLTCITAL promoted neurogenesis and reduced depression-like behavior in adult mice [54] (Table 3).
Agomelatine is a mixed MT1/MT2 melatonin receptor agonist and 5HT2C serotonin receptor antagonist. Agomelatine was able to reverse the decline of neurogenesis regulators including p-CREB, mGlu2/3, and mGlu5 receptor levels in the hippocampus, and prevented the neurogenesis impairment and depression-and anxiety-like behaviors in the adult male offspring of mouse mothers who were subjected to restraint stress during pregnancy [55]. In addition, agomelatine also protected against light stress-induced neurogenesis deficits by upregulating BDNF levels and inhibiting the apoptotic signaling pathway [56]. It was also reported that agomelatine increased neurogenesis via the 5HT2C receptor with the activation ERK1/2, protein kinase B, and GSK3β [57,58] (Table 3).

Melatonin, Aging, and Neurodegenerative Diseases
Melatonin can decelerate the progress of aging. Melatonin improved spatial memory deficits in the mouse model of d-galactose (d-gal)-induced aging and restored the reduction of Ki67+ proliferative cells and DCX+ neuronal precursor cells caused by the d-gal in the DG [59]. Senescence-accelerated (SAMP8) mice is another mouse model of aging, which presented age-related defects such as cognitive disability and motor dysfunction [60]. When the SAMP8 mice were given melatonin treatment from the ages of 1 month to 10 months old, the levels of acetylated p53, NF-κB, and amyloid β (Aβ) in the brain decreased and the levels of α-secretase and Bcl-2 XL increased, which suggested that melatonin exerted anti-aging effects by modulating the pro-survival and pro-death signals in the brain [61]. However, the study did not examine the relationship between the survival/death signals and neurogenesis (Table 4). Parkinson's disease (PD) is a neurodegenerative disease that is linked to motor and learning dysfunction [67]. Melatonin attenuates neurogenesis impairment in different PD animal models. For instance, 7-day treatment of 5 mg/kg BW melatonin reversed the decline of Nestin, DCX, and Beta-III tubulin expression in the hippocampus in the methamphetamine (METH)-induced PD mice by modulating the MAPK signaling activity, N-methyl-d-aspartate (NMDA) receptor subunits (NR2A and NR2B), as well as CaMKII levels [62]. Transplantation of 25 µM melatoninpretreated SVZ NSCs to the hippocampus and striatum also improved neuronal restoration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice [63]. Moreover, combination treatment of melatonin and NSC transplantation rescued tyrosine hydroxylase (TH) neurons and attenuated the behavioral deficits in the 6-hydroxydopamine (6 OHDA)-induced PD mouse model [64].
In the in vitro experiments, melatonin was shown to prevent the METH-induced inhibition of proliferation of NSCs by modulating the tumor suppressor p53 and cycle inhibitor p21CIP as well as regulating the CaMKII, NR2A, and NR2B [68] (Tables 4 and 5). Apart from Parkinson's disease, melatonin was found to rescue neurogenesis impairment in demyelinating disease. Melatonin prevented reduction of DCX+ neuronal precursor cells and Ki-67 proliferating cells in the DG through the phosphorylation of CREB and upregulation of glucose transporter 3 (GLUT3) and BDNF levels [65]. Melatonin is also neuroprotective against Alzheimer's disease (AD) [63,66]. Melatonin alleviated the AD-induced spatial learning and memory deficits by reducing the Aβ levels and modulating the mitochondrial biogenesis factors and DNA copy number in the brain [66]. However, the role of melatonin in the regulation of neurogenesis in AD has not yet been fully studied (Table 4).

Melatonin and Central Nervous System (CNS) Injury
Melatonin was shown to confer neuroprotective effects in stroke with yet elusive mechanisms. It was hypothesized that melatonin protected against stroke due to its anti-inflammatory and anti-oxidant properties. Recent studies reported that melatonin may also promote stroke recovery by regulating post-stroke neurogenesis. When mice were subjected to the middle cerebral ischemic/ reperfusional injury (CI/R), it was found that melatonin treatment could reduce the post-stroke free radical production, increasing the number of the DCX+ neuroblasts and Ki67+ proliferative cells in the peri-infarcted area (e.g., the cortex) [69]. For those Ki-67+ cells, they were not only DCX+, but the majority of them were also Nestin+ and NG2+ [69]. Nestin and NG2 are the markers of NSC and oligodendrocyte precursors, respectively. The findings were further supported by another study that showed that melatonin increased PCNA+ and NG2+ oligodendrocyte progenitor cells in the SVZ and white matter in the rat after the focal cerebral ischemia with the downregulation of inflammatory factors, including TLR4, NF-κB, and IL-1β [70] (Table 6).
On neurological functions, melatonin improved motor and coordination of post-stroke mice in the grip strength and rotarod tests [71]. Melatonin also attenuated the hyperactivity and anxiety behavior of the mice in the open field test after stroke [71]. It was found that the improvements were associated with the promotion of endogenous neurogenesis in the lateral ventricle, striatum, and cortex after melatonin treatment [71]. Nevertheless, in the global forebrain ischemia experiment, which used 3-to 4-month-old Mongolian gerbils as the animal model, it was found that chronic but not acute melatonin treatment could improve the post-stroke behavioral outcome [72], and instead of enhancing the post-stroke neurogenesis, both chronic and acute melatonin suppressed the increased neurogenesis after stroke [72] (Table 6).
Melatonin facilitated the efficiency of post-stroke mesenchymal stem cell (MSC) transplantation therapy. Melatonin enhanced the survival of MSCs through the ERK1/2 signaling pathway via the melatonin receptors [73]. When the MSCs were pretreated with 5 mM melatonin for 24 h and transplanted to the ipsilateral striatum in the rats after stroke, the rats exhibited improved behavioral outcomes with an increase in the levels of neurogenesis and angiogenesis. The effects may be brought by the upregulation of vascular endothelial growth factor (VEGF) after the transplantation therapy [73] (Table 6). Apart from stroke, melatonin protects against spinal cord injury (SCI). Melatonin treatment increased dendritic spine density in the peri-lesion site in the rats after SCI [74]. When the SCl rats received combined treatment of melatonin and treadmill exercise training, the hindlimb function in the injured animals were significantly improved in the Basso, Beattie, and Bresnahan (BBB) locomotor recovery scale. Meanwhile, the combination treatment also increased the number of BrdU+ proliferative cells and NSCs in the peri-lesion area [74] (Table 6).
In addition to SCI, melatonin was found to be protective on early-life CNS injuries such as perinatal hypoxia. Melatonin prevented microglial activation and downregulated the inflammatory mediators (e.g., TNFα, IL-1β, and nitric oxide) in the PND1 mice, which were subjected to the hypoxia at 5% oxygen and 95% nitrogen for 2 h [75]. Moreover, melatonin promoted neurogenesis and improved long-term deficits of the hypoxia-injured mice. It also attenuated the sensorimotor and locomotor function impairments, learning and memory deficits, and hyperactivity behavior of the mice 30 days after hypoxia [75]. In the in vitro study, when E12.5 mouse cortical NSCs were treated with 100 nM melatonin before being subjected to 12-h 95% N 2 and 5% CO 2 hypoxia incubation, the proliferation and neuronal differentiation of NSCs were restored by the phosphorylation of ERK1/2 via the MT1 receptor [77] (Tables 6 and 7). miR-363 was upregulated in the vitamin A deficiency (VAD)-induced congenital spinal deformities model. It was found that melatonin could promote proliferation, increased Nestin expression, and enhanced neuronal differentiation of the miR-363-transfected E13.5 rat NSCs [78]. In another CNS injury model, rats were exposed to kainic acid by intracerebroventricular administration on PND7 for the induction of neurodevelopmental injury. Melatonin protected against kainic acid-induced neurodevelopmental injury by preventing hippocampal neuronal loss without altering neurogenesis [76] (Tables 6 and 7).

Melatonin and Sleep Deprivation
Sleep deprivation (SD) suppresses neurogenesis in the adult hippocampus [79]. SD also causes adverse effects on neurological behavioral outcomes [80]. It is speculated that hippocampal neurogenesis impairment is responsible for SD-induced behavioral deficits (Table 8).
However, melatonin could rescue the neurogenesis deficits in SD. When mice were subjected to SD for 96 h, a 4-day daily treatment of 10 mg/kg BW melatonin restored the SD-induced reduction in the number of Sox2+/BrdU+ NSCs in the DG by increasing the levels of methyl-CpG-binding protein 2 (MECP2) and decreasing the levels of Sirtuin 1 (SIRT1) [81]. In addition, melatonin could also increase the number of BrdU/Nestin+ NSCs in the SGZ in SD mice by upregulating the Bcl-2 and Bcl-xL levels [82]. N-acetylserotonin (NAS) is an immediate precursor of melatonin. It was found that NAS could prevent the suppression of NSC proliferation induced by SD via the TrkB signaling pathway [83] (Table 8). Apart from SD, melatonin protected against the photoperiod alterations simulating "jet lag" [84]. Melatonin prevented the reduction of cell proliferation and attenuated the cognitive deficits caused by "jet lag" [84]. However, the underlying mechanism remains unclear ( Table 8).

Melatonin, Inflammation, and Oxidative Stress
Due to its antioxidant properties, melatonin helps reduce inflammation and oxidative stress. Melatonin can scavenge free radicals, reactive oxygen species (ROS), and reactive nitrogen species (RNS) directly [85]. It can also scavenge hydrogen peroxide (H 2 O 2 ) and neutralize the toxic hydroxyl radicals [86]. Inducible NO synthase (iNOS) generates nitric oxide (NO), which is a free radical [87]. Excessive amount of NO generated by iNOS can cause cytotoxic changes in cells [87]. Melatonin is able to inhibit iNOS and decrease NO levels [88,89]. It was also reported that, by regulating NOS expression, melatonin protected the brain in terms of pathological conditions such as ischemic brain injury [90]. In addition, pretreatment of melatonin was found to reduce nitric oxide (NO) levels and prevent E14 cortical NSCs from apoptosis after lipopolysaccharide (LPS) exposure [91] (Table 9).
Melatonin protects NSCs against inflammation and oxidative stress, not only through the regulation of NO levels. The study also showed that melatonin protected NSCs from LPS-induced cell death by activating the PI3K/Akt/Nrf2 signaling pathway [91] (Table 9). H 2 O 2 induces oxidative stress to iPSC-derived NSCs. H 2 O 2 was shown to decrease proliferation and viability of iPSC-derived NSCs and reduced their mitochondrial membrane potential [92]. Melatonin, however, could reverse all the adverse effects induced by H 2 O 2 by the activation of the PI3K/Akt signaling pathway via the melatonin receptors [92]. IL-18 is a cytokine that is detrimental to the NSCs. Melatonin, moreover, could promote the production of BDNF and GDNF in the IL-18-stimulated NSCs and suppressed the IL18-induced inhibition of proliferation, neurosphere formation, and neuronal differentiation of the NSCs [93] (Table 9).

Melatonin and Neurogenesis Impairment Caused by Environmental Factors
Neurogenesis impairment can be caused by environmental factors, which leads to different neurological deficits. Scopolamine is a prescription drug used for the prevention of nausea and vomiting. When mice were treated with scopolamine for 2 weeks, it was found that the number of DCX+ neuronal precursor cells and Ki67+ proliferative cells decreased in the DG, wherein the mice exhibited spatial learning and short-term memory impairments in the Morris water maze test and passive avoidance test [94]. However, when the mice were treated with melatonin, neurogenesis was restored and behavioral deficits were attenuated [94]. Apart from scopolamine, other drugs and chemicals such as 5-fluorouracil (5-FU), valproic acid (VPA), and methotrexate (MTX) also caused hippocampal neurogenesis impairment in rats, but the impairments could be reversed by melatonin treatment [95][96][97] (Table 10).  Dexamethasone (DEX) is a type of high potency corticosteroid medication. It causes depressive-like behavior and neurogenesis disruption in mice after exposure. It was found that pre-treatment of melatonin could prevent DEX-induced abnormality by modulating the glucocorticoid receptor (GR) and ERK1/2 expression [98]. The findings were further supported by an in vitro study, which showed that melatonin could reduce the DEX-induced decline in Ki67 and Nestin expression in the neurosphere of adult hippocampal NSCs by regulating the ERK1/2, cyclin E, and CDK2 via the melatonin and glucocorticoid receptors [107] (Tables 10 and 11). Table 11. Table showing the main findings regarding the effects of melatonin on neurogenesis impairment caused by environmental factors and the respective molecular mechanisms in the in vitro studies.

Lowest Dosage
Causing an Observable Effect

Main Findings Molecular Mechanisms References
Adult hippocampal NSCs (from 8-week-old rats) exposed to dexamethasone (DEX) In addition, melatonin also protected against the behavioral impairments caused by morphine sulfate and ketamine in rats and mice [99,100]. It was found that the protective effects were brought by the upregulation of BDNF and modulation of the MEK/ERK and PI3K/AKT signaling pathways [100]. However, the roles of melatonin on the neurogenesis regulation in the morphine sulfate-and ketamine-exposed animals have not yet been sufficiently studied (Table 10).
Environmental toxicants such as insecticide and industrial chemicals affect neurodevelopment. Exposure of fenvalerate (FEN) 5 h after fertilization caused abnormal swimming behavior in zebrafish [101]. However, melatonin could prevent the FEN-induced abnormal behavior and neurogenesis disruption by suppressing the pro-apoptotic genes such as Bax, Fas, caspase 8, caspase 9, and caspase 3 and upregulating the anti-apoptotic genes such as Bcl-2 [101]. Tri-ortho-cresyl phosphate (TOCP) is a material that is widely used in industries. Melatonin protected the mouse E12.5 NSCs from TOCP-induced cell death by suppressing the ROS levels and activating the ERK1/2 signaling pathway [108] (Tables 10 and 11).
Collectively, the findings suggest that melatonin exerts protective effects on the environmental factor-induced neurological deficits by modulating neurogenesis.

Melatonin and Other Diseases That Are Related to Neurogenesis Impairment
Melatonin has been found to also attenuate neurogenesis impairment in a wide range of other diseases that were not discussed above.
As a metabolic disease, diabetes mellitus (DM) can lead to neurological problems chronically. A high fat diet (HFD) and streptozotocin (STZ)-induced DM led to neurogenesis and synaptogenesis deficits and impaired spatial memory in rats [109]. Melatonin, however, could reverse the adverse effects caused DM by activating the p-ERK signaling pathway and upregulating the melatonin receptor as well as insulin receptor [109] (Table 12). DM also causes fetal neurodevelopmental impairment. When female mice were subjected to STZ before pregnancy for 3 consecutive days and then fertilized with healthy male mice, the E11.5 and E17.5 mouse embryos showed decreased proliferation and increased premature differentiation of NSCs in the brain [110]. Nevertheless, melatonin could attenuate the STZ-induced neurodevelopmental deficits by inhibiting autophagy and preventing apoptosis in the embryonic brain [110,111]. In the in vitro experiments, it was also reported that melatonin could protect the embryonic NSCs from cell death and impairments in proliferation and differentiation in the hyperglycemia condition [110,111] (Tables 12 and 13). Ts65Dn (TS) mice are a commonly used model of DS, which were found to exhibit different DS phenotypes including cognitive deficits and impairments in hippocampal functions [112]. It was reported that melatonin rescued the neurogenesis suppression in the Ts65Dn (TS) mice by increasing the density and activity of glutamatergic synapses [112]. Nevertheless, in another study, it was shown that melatonin had no effect on the improvement of neurogenesis and behavioral outcomes in TS mice, but only regulated the DS-induced oxidative stress [113] (Table 12).
Seizures impair neurogenesis. Induction of seizures to a rat mother led to early embryonic neurogenesis and delayed maturation of newborn neurons in the cerebellum and the Cornus ammonis 1 (CA1) region of the hippocampus in PND0 offspring [114,115]. Olfactory NSCs isolated from schizophrenia (SCZ) patients also exhibited an abnormal increase in the potassium-evoked secretion when compared with the NSCs of the healthy control [116]. Interestingly, melatonin treatment could reverse all the abnormalities caused by seizures [114][115][116] (Tables 12 and 13).

Melatonin and Peripheral Nerve Impairment
Apart from recusing neurogenesis impairment in the CNS, melatonin also promotes nerve regeneration after peripheral nerve injury (PNI).
Studies showed that 30-day 1 and 10 mg/kg BW melatonin treatment could promote nerve regeneration in the rats after PNI [117,118]. It was also found that melatonin improved the upper limb functional recovery and restored the number of re-innervated motor end plates on the target muscle in the rats after PNI [117]. The protection of melatonin against PNI was suggested to be brought by the suppression of CaMKII [117]. It was also suggested that the protective effects may be related to the increase of proliferation of the Schwann cells after the melatonin treatment, given that Schwann cells are crucial for axonal guidance and nerve regeneration after PNI [118]. Melatonin was shown to promote proliferation of Schwann cells by activating ERK 1/2 via the MT1 receptor [118] (Table 14). The modulation of the nitroxidergic system plays an important role in the protective effects of melatonin on nerve injury. After rats were subjected to hypoglossal nerve transection, melatonin suppressed NADPH-d/NOS expression and preserved superoxide dismutase (SOD) activity, which increased the number of motoneurons in the hypoglossal nucleus and further improved the functional recovery [119,120]. Chronic constriction injury (CCI) is an injury model that causes peripheral neuropathic pain on the sciatic nerve in animals. One study showed that melatonin modulated the iNOS and nNOS levels in the dorsal root ganglia, which alleviated thermal nociceptive hypersensitivity in rats after CCI [121]. Oxaliplatin is a compound that is used in the treatment of colorectal cancer but it has the side effect of causing peripheral neuropathy [122]. Melatonin, moreover, was found to modulate the nitroxidergic system in oxaliplatin-treated rats. It helped reduce the oxaliplatin-induced pain behavior and neuropathic deficits by alleviating nitrosative stress [122]. Apart from regulating the nitroxidergic system, melatonin also inhibited the apoptotic pathway and activated the autophagy pathway in the sciatic nerve and dorsal root ganglia in oxaliplatin-treated rats [122] (Table 14).
Due to its antioxidant properties, melatonin promoted nerve recovery after ischemia-reperfusion (I/R) injury. Melatonin salvaged the nerve fibers from ischemic degeneration and it decreased edema and damage in the myelin sheaths and axons after the I/R injury by reversing the I/R-induced increase in the MDA levels [123]. In the cut or crush sciatic nerve injury model, melatonin also preserved myelin sheath in the injured rats by decreasing lipid peroxidation, which helped promote the recovery [124]. In both injury models, melatonin treatment increased the SOD levels [123,124] (Table 14).
Last but not least, the roles of melatonin of nerve regeneration were further confirmed by the pinealectomy experiment, in which pinealectomy was performed in the rats 3 weeks before they were subjected to a surgical intervention consisting of bilateral sciatic nerve section and primary suture repair [125]. The results showed that the collagen content of the sciatic nerve and macroscopic neuroma formation increased in pinealectomy rats, but melatonin treatment reversed the changes [125]. Moreover, melatonin also helped reduce the stimulus intensities required to excite a nerve action potential response (NAP) in the pinealectomy rats [125] (Table 14).

Conclusions
From the traditional biological perspective, melatonin is well recognized as a key player of regulating circadian rhythm. Recent studies, however, have highlighted the pleiotropic nature of melatonin, which is widely related to the normal functioning of different systems, including the CNS. Due to its diverse effects, the melatonin signaling system could be a possible treatment target of disorders of the CNS. As increasing evidence has shown that melatonin is pro-neurogenic, it is a possible drug candidate that may be used for treating neurogenesis-related disorders, such as mood disorders, neurotrauma, and neurodegenerative disorders. However, as the neurogenic and treatment effect of melatonin has been just discovered in the past two decades, its potential as a drug remains to be further confirmed and explored.
Further exploration of the signaling mechanisms, treatment effect of the synthetic agonist/antagonist on different disorders, and potential side effects are possible future study directions in terms of applying melatonin in treating various CNS disorders. The lack of severe side effects and the affordable nature of melatonin would imply its applicability in clinical situations for chronic treatment.

Conflicts of Interest:
The authors declare no conflict of interest.