Assessment of effects of pinealectomy and exogenous melatonin administration on rat sciatic nerve suture repair: an electrophysiological, electron microscopic, and immunohistochemical study

Melatonin signalling in Schwann cells during neuroregeneration

It has widely been thought that in the process of nerve regeneration Schwann cells populate the injury site with myelinating, non–myelinating, phagocytic, repair, and mesenchyme–like phenotypes. It is now clear that the Schwann cells modify their shape and basal lamina as to accommodate re–growing axons, at the same time clear myelin debris generated upon injury, and regulate expression of extracellular matrix proteins at and around the lesion site. Such a remarkable plasticity may follow an intrinsic functional rhythm or a systemic circadian clock matching the demands of accurate timing and precision of signalling cascades in the regenerating nervous system. Schwann cells react to changes in the external circadian clock clues and to the Zeitgeber hormone melatonin by altering their plasticity. This raises the question of whether melatonin regulates Schwann cell activity during neurorepair and if circadian control and rhythmicity of Schwann cell functions are vital aspects of neuroregeneration. Here, we have focused on different schools of thought and emerging concepts of melatonin–mediated signalling in Schwann cells underlying peripheral nerve regeneration and discuss circadian rhythmicity as a possible component of neurorepair.

Melatonin Induced Schwann Cell Proliferation and Dedifferentiation Through NF-ĸB, FAK-Dependent but Src-Independent Pathways

BACKGROUND

Peripheral nerve injury (PNI) is a common condition that compromises motor and sensory functions. Peripheral nerves are known to have regenerative capability and the pineal hormone, melatonin, is known to aid nerve regeneration. However, the role of Schwann cells and the pathways involved remain unclear. Thus, the aim of this study is to identify the effects of melatonin on Schwann cell proliferation, dedifferentiation, and the involvement of nuclear factor kappa light chain enhancer of activated B cells (NF-ĸB), focal adhesion kinase (FAK) and proto-oncogene tyrosine-protein kinase, Src pathways in this process.

METHODS

Schwann cells was treated with melatonin and its proliferation and dedifferentiation were identified using MTT assay and immunofluorescence staining for SRY (sex determining region Y)-box 2 (SOX2). Next, the protein expressions of NF-ĸB, FAK and Src pathways were identified by Western blot.

RESULTS

MTT results confirmed increased proliferation of Schwann cells with melatonin treatment, and it was highest at 10 μM melatonin. Immunofluorescent staining revealed an increase in the green fluorescence staining for SOX2 in melatonin-treated cells, showing enhanced dedifferentiation. Western blot assay revealed melatonin increased phospho-NF-ĸB (PNF-ĸB), IKK-α, FAK (D2R2E), phospho-FAK (Tyr 576/577 and Tyr 397) protein expressions as compared with control. However, Src (32G6), Lyn (C13F9), Fyn, Csk (C74C1) protein expressions were not increased as compared with control.

CONCLUSION

Melatonin promotes Schwann cell proliferation and dedifferentiation via NF-ĸB, FAK-dependent but Src-independent pathways.

Melatonin promotes regeneration of injured motor axons via MT1 receptors

Melatonin is an ancient multi-tasking molecule produced by the pineal gland and by several extrapineal tissues. A variety of activities has been ascribed to this hormone in different physiological and pathological contexts, but little is known about its role in peripheral neuroregeneration. Here, we have exploited two different types of injury to test the capability of melatonin to stimulate regeneration of motor axons: (a) the acute and reversible presynaptic degeneration induced by the spider neurotoxin α-Latrotoxin and (b) the compression/transection of the sciatic nerve. We found that in both cases melatonin administration accelerates the process of nerve repair. This pro-regenerative action is MT1 -mediated, and at least in part due to a sustained activation of the ERK1/2 pathway. These findings reveal a receptor-mediated, pro-regenerative action of melatonin in vivo that holds important clinical implications, as it posits melatonin as a safe candidate molecule for the treatment of a number of peripheral neurodegenerative conditions.

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.

Pharmacological Effects of Melatonin as Neuroprotectant in Rodent Model: A Review on the Current Biological Evidence

The progressive loss of structure and functions of neurons, including neuronal death, is one of the main factors leading to poor quality of life. Promotion of functional recovery of neuron after injury is a great challenge in neuroregenerative studies. Melatonin, a hormone is secreted by pineal gland and has antioxidative, anti-inflammatory, and anti-apoptotic properties. Besides that, melatonin has high cell permeability and is able to cross the blood-brain barrier. Apart from that, there are no reported side effects associated with long-term usage of melatonin at both physiological and pharmacological doses. Thus, in this review article, we summarize the pharmacological effects of melatonin as neuroprotectant in central nervous system injury, ischemic-reperfusion injury, optic nerve injury, peripheral nerve injury, neurotmesis, axonotmesis, scar formation, cell degeneration, and apoptosis in rodent models.

Effects of Melatonin and Dexamethasone on Facial Nerve Neurorrhaphy

OBJECTIVES

To investigate the effects of topical and systemic administrations of melatonin and dexamethasone on facial nerve regeneration.

MATERIALS AND METHODS

In total, 50 male albino Wistar rats underwent facial nerve axotomy and neurorrhaphy. The animals were divided into 5 groups: control, topical melatonin, systemic melatonin, topical dexamethasone, and systemic dexamethasone. Nerve conduction studies were performed preoperatively and at 3, 6, 9, and 12 weeks after drug administrations. Amplitude and latency of the compound muscle action potentials were recorded. Coapted facial nerves were investigated under light and electron microscopy. Nerve diameter, axon diameter, and myelin thickness were recorded quantitatively.

RESULTS

Amplitudes decreased and latencies increased in both the melatonin and dexamethasone groups. At the final examination, the electrophysiological evidence of facial nerve degeneration was not significantly different between the groups. Histopathological examinations revealed the largest nerve diameter in the melatonin groups, followed by the dexamethasone and control groups (p<0.05). Axon diameter of the control group was smaller than those of the melatonin (topical and systemic) and topical dexamethasone groups (p<0.05). The melatonin groups had almost normal myelin ultrastructure.

CONCLUSION

Electrophysiological evaluation did not reveal any potential benefit of dexamethasone and melatonin in contrast to histopathological examination, which revealed beneficial effects of melatonin in particular. These agents may increase the regeneration of facial nerves, but electrophysiological evidence of regeneration may appear later.

The multiple functions of melatonin in regenerative medicine

Melatonin research has been experiencing hyper growth in the last two decades; this relates to its numerous physiological functions including anti-inflammation, oncostasis, circadian and endocrine rhythm regulation, and its potent antioxidant activity. Recently, a large number of studies have focused on the role of melatonin in the regeneration of cells or tissues after their partial loss. In this review, we discuss the recent findings on the molecular involvement of melatonin in the regeneration of various tissues including the nervous system, liver, bone, kidney, bladder, skin, and muscle, among others.

Useful Effects of Melatonin in Peripheral Nerve Injury and Development of the Nervous System

This review summarizes the role of melatonin (MLT) in defense against toxic-free radicals and its novel effects in the development of the nervous system, and the effect of endogenously produced and exogenously administered MLT in reducing the degree of tissue and nerve injuries. MLT was recently reported to be an effective free radical scavenger and antioxidant. Since endogenous MLT levels fall significantly in senility, these findings imply that the loss of this antioxidant could contribute to the incidence or severity of some age-related neurodegenerative diseases. Considering the high efficacy of MLT in overcoming much of the injury not only to the peripheral nerve but also to other organs, clinical trials for this purpose should be seriously considered.

The neurochemistry of peripheral nerve regeneration

Peripheral nerve injuries (PNIs) can be most disabling, resulting in the loss of sensitivity, motor function and autonomic control in the involved anatomical segment. Although injured peripheral nerves are capable of regeneration, sub-optimal recovery of function is seen even with the best reconstruction. Distal axonal degeneration is an unavoidable consequence of PNI. There are currently few strategies aimed to maintain the distal pathway and/or target fidelity during regeneration across the zone of injury. The current state of the art approaches have been focussed on the site of nerve injury and not on their distal muscular targets or representative proximal cell bodies or central cortical regions. This is a comprehensive literature review of the neurochemistry of peripheral nerve regeneration and a state of the art analysis of experimental compounds (inorganic and organic agents) with demonstrated neurotherapeutic efficacy in improving cell body and neuron survival, reducing scar formation and maximising overall nerve regeneration.

The role of oxidative stress and inflammatory response in high-fat diet induced peripheral neuropathy

OBJECTIVE

Earlier studies suggest that high-calorie diet is an important risk factor for neuronal damage resulting from oxidative stress of lipid metabolism. In our experimental study of rats under high-fat diet, oxidative stress markers and axonal degeneration parameters were used to observe the sciatic nerve neuropathy. The aim of this study is to evaluate the pathophysiology of neuropathy induced by high-fat diet.

METHODS

A total of 14 male rats (Wistar albino) were randomly divided into two experimental groups as follows; control group (n=7) and the model group (n=7); while control group was fed with standard diet; where the model group was fed with a high-fat diet for 12 weeks. At the end of 12 weeks, the lipid profile and blood glucose levels, interleukin-1β (IL-1), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and transforming growth factor-β (TGF-β) levels were studied. Tissue malondialdehyde (MDA), nitric oxide (NO) levels and super-oxide dismutase (SOD), paraoxonase-1 (PON-1) and glutathione peroxidase (GPx) activities were studied. The distal blocks of the left sciatic nerves were evaluated for histomorphological analysis (including mean axon area, axon numbers, nerve fiber diameters, axon diameters, and thickness of myelin sheets).

RESULTS

Body weights, serum glucose and high-density lipoprotein (HDL) levels of rats were found not statistically significantly different compared between the model and the control groups (p>0.05). Serum cholesterol, triglyceride, TGF-β and TNF-α levels were significantly higher in the model group when compared with the control group (p<0.05). IL-1 and IL-6 levels were not statistically significantly different compared between the model group and the control group (p>0.05). The MDA and NO levels and the SOD and GPx activities of the sciatic nerves in model group were statistically significantly higher than the control group (p<0.05). In addition, the activities of PON-1 were statistically significantly lower in the model group when compared with the control group (p<0.05). The difference in the total number of myelinated axons between the control group and the model group was not statistically significant (p>0.05). The nerve fiber diameter and the thickness of the myelin sheet were statistically significantly lower in the model group when compared with the control group (p<0.05). The axon diameter and area were significantly decreased in the model group when compared with the control group (p<0.05).

CONCLUSION

Our results support that dyslipidemia is an independent risk factor for the development of neuropathy. In addition, we postulated that oxidative stress and inflammatory response may play an important role in the pathogenesis of high-fat diet induced neuropathy.

The effect of melatonin and platelet gel on sciatic nerve repair: an electrophysiological and stereological study

Nerve regeneration after surgical reconstruction is far from optimal, and thus effective strategies for improving the outcome of nerve repair are being sought. In this experiment, we verified if postoperative intraperitoneal melatonin (MLT) administration after intraoperative platelet gel application improves peripheral nerve regeneration. In adult male rats, 1-cm long sciatic nerve defects were repaired using four different strategies: autologous nerve graft repair followed by MLT (NM, n = 5), collagen conduit repair followed by MLT (CM, n = 5), platelet gel-enriched collagen conduit repair followed by MLT (CGM, n = 6), and platelet gel-enriched collagen conduit (CG, n = 5) repair followed by no substance administration. Sham operated animals were used as controls (Cont, n = 5). Ninety days after surgery, the nerve regeneration outcome was comparatively assessed by means of electrophysiological and stereological analysis. Electrophysiology revealed no significant differences between the experimental and the sham control groups. Stereological analysis showed no significant differences among the experimental groups regarding axon size and myelin thickness, but the axon number was significantly lower in the CM compared to Cont and NM group. Moreover, there was no significant difference between number of axons in CG and Cont groups, between CGM and CM, and between CM and NM. Although it was observed that platelet gel have a positive effect on nerve regeneration, but a combination of local platelet gel with MLT does not have the same effect on nerve repair.

Assessment of processed human amniotic membrane as a protective barrier in rat model of sciatic nerve injury

Following nerve injury, scar formation is thought to be a considerable impediment to axonal regeneration at the nerve injury site. Nerve wrapping can protect the regenerating axons, and human amniotic membrane (HAM) derived from human placenta is an effective material for that purpose. The impact of nerve wrapping with HAM on functional recovery after nerve injuries, especially after autograft repair of long gap lesions, has not been comprehensively investigated. In the current study, we investigated whether the application of HAM as a nerve wrap to a 10mm segment of transected and repaired nerve would reduce scar formation and permit better axonal regeneration and/or functional recovery in rats. The outcome was assessed with morphological and functional measures. We found that nerves wrapped with HAM had significantly fewer adhesions and less scar formation than controls. Although the final outcome, both functionally and morphologically, was not significantly improved by wrapping the nerve with HAM, the observed decrease in adhesions and scar formation might help the nerve retain its mobility and thus prevent traction injury and ischemia, which are caused by nerve tethering to the adjacent tissue during the healing process.

Stereological analysis of sciatic nerve in chickens following neonatal pinealectomy: an experimental study

Background

Although the injury to the peripheral nervous system is a common clinical problem, understanding of the role of melatonin in nerve degeneration and regeneration is incomplete.

Methods

The current study investigated the effects of neonatal pinealectomy on the sciatic nerve microarchitecture in the chicken. The chickens were divided into two equal groups: unpinealectomized controls and pinealectomized chickens. At the end of the study, biochemical examination of 10 sciatic nerve samples from both groups was performed and a quantitative stereological evaluation of 10 animals in each group was performed. The results were compared using Mann-Whitney test.

Results

In this study, the results of axon number and thickness of the myelin sheath of a nerve fiber in newly hatched pinealectomy group were higher than those in control group. Similarly, surgical pinealectomy group had significantly larger axonal cross-sectional area than the control group (p < 0.05). In addition, the average hydroxyproline content of the nerve tissue in neonatal pinealectomy group was higher than those found in control group. Our results suggest that melatonin may play a role on the morphologic features of the peripheral nerve tissue and that melatonin deficiency might be a pathophysiological mechanism in some degenerative diseases of peripheral nerves. The changes demonstrated by quantitative morphometric methods and biochemical analysis has been interpreted as a reflection of the effects of melatonin upon nerve tissue.

Conclusion

In the light of these results from present animal study, changes in sciatic nerve morphometry may be indicative of neuroprotective feature of melatonin, but this suggestion need to be validated in the human setting.

Inhibitory effect of pinealectomy on the development of cerebellar granule cells in the chick: A stereological study

Melatonin has some effects upon morphological features of various structures in small animals and human being. However, there has been no investigation concerning its physiological role on development of cerebellar granule cells. In this study, the changes induced by pinealectomy procedure on cerebellar development and their granule cell numbers in the chick were investigated using quantitative stereological methods. A total of 15 Hybro Broiler newly hatched chicks were randomly divided into three equal groups: pinealectomy group (n=5) and non-pinealectomized control group (n=5) and sham-operated group (n=5). Pinealectomy procedure and sham operation were done in 3-day-old chicks and all animals were sacrificed for histopathological evaluation and subsequent stereological analysis in the 8th week. Here, it was observed that pinealectomy significantly reduces the granular cell number in cerebellar cortex of the chicks (P<0.001). The present study is the first stereological study to evaluate the histomorphological effects of pinealectomy on the cerebellar granule cells of the chick. We suggest that the granule cell loss in the cerebellar cortex is due to developmental retardation in early postnatal period, although its exact mechanism is not clear. Based on our findings, we intimate that pineal gland/melatonin might play an important role in the development of cerebellar granule cells in the chick.

Effect of pinealectomy on the morphology of the chick cervical spinal cord: a stereological and histopathological study

Melatonin has some effects upon morphological features of various structures in small animals and human being. In this study, the changes induced by pinealectomy procedure on morphological features of developing cervical spinal cord and their neurons in the gray matter (GM) and white matter (WM) of cervical spinal cord in the chicken were investigated. A total of 15 Hybro Broiler newly hatched chicks were randomly divided into three equal groups: unoperated control group (n=5), sham-operated group (n=5) and pinealectomy group (n=5). Pinealectomy procedure and sham operation were done in 3-day-old chicks and all animals were sacrificed at the 8th week and the 6th cervical (C6) spinal cord segment was dissected out for histopathological evaluation and subsequent stereological analysis. The volume of spinal cord segment did not show a significant difference between unoperated and sham-operated controls, but the pinealectomy group has a declined volume value compared with those of the control and sham operated groups (P<0.01). By contrast, there was no statistically significant difference between unoperated and sham-operated controls and the pinealectomy group with regard to volume fraction of the GM and WM of the cervical spinal cord. Finally, it was observed that pinealectomy procedure significantly reduces neuron number in the GM and the volume of WM of the C6 segment of cervical spinal cord in the chicken (P<0.001). The present study is the first study at all to evaluate the effects of pinealectomy on quantitative feature of the spinal cord in the chicken. Based on our findings, we suggest that pineal gland/melatonin might play an important role in morphological features of the developing spinal cord in the chicken.

Immunohistochemical profile of transforming growth factor-β1 and basic fibroblast growth factor in sciatic nerve anastomosis following pinealectomy and exogenous melatonin administration in rats

Collagen scar formation at the cut end of a peripheral nerve, an important problem in clinical practice for neurosurgeons, obstructs sprouting of axons into appropriate distal fascicles, and thereby limits the regeneration process. Researchers have attempted to control collagen accumulation and neuroma formation with various physical and chemical methods, but with limited functional success. Recently, it has been demonstrated that transforming growth factor (TGF)-beta and basic fibroblast growth factor (bFGF) play an important role in collagen production by fibroblasts and in Schwann cell activity. In our study, rats were divided into a control group, a melatonin-treated group, a surgical pinealectomy group, and a group treated with melatonin following pinealectomy. They then underwent a surgical sciatic nerve transection and primary suture anastomosis. At 2 months after anastomosis, the animals were sacrificed and unilateral sciatic nerve specimens, including the anastomotic region, were removed and processed for immunohistochemical study from two animals in each group. For each antibody, immunoreactivity was assessed using a semiquantitative scoring system. Strong TGF-beta1 and/or bFGF expression was observed in the epineurium of animals that underwent pinealectomy, but no or weak staining was observed in animals in the control and melatonin treatment groups. Based on these data, we suggest that both TGF-beta1 and bFGF have important roles in control of collagen accumulation and neuroma formation at the anastomotic site, and that the pineal neurohormone melatonin has a beneficial effect on nerve regeneration.

Neonatal pinealectomy induces Purkinje cell loss in the cerebellum of the chick: A stereological study

Melatonin plays an important role in certain physiological functions and morphological features of various structures. In the current study, the effects of pinealectomy on Purkinje cell number and morphological features of developing cerebellum in the chick were investigated using stereological methods. Fifteen Hybro Broiler newly hatched chicks were divided into three groups: a pinealectomized group (n = 5), sham-operated group (n = 5) and a non-pinealectomized control group (n = 5). Surgical pinealectomy was performed in 3-day-old chicks. In the 8th week, all animals were sacrificed for histopathological evaluation and subsequent stereological analysis. Each layer volume of molecular (+Purkinje cell), granular and white matter in the cerebellum was estimated in all animals. It was found that there was no significant difference for the volume of whole cerebellum and also molecular (+Purkinje cell) layer in these groups (P > 0.05). Nevertheless, the values of granular layer and white matter of sham-operated group were significantly different from those of control and pinealectomized animals (P < 0.01). It was also observed that pinealectomy significantly reduces the Purkinje cell number in cerebellar cortex (P < 0.01). The present study is the first stereological study to demonstrate the histomorphological effects of pinealectomy on the cerebellum in the chick. Our results suggest that pineal gland/melatonin might play an important role in morphological features of the developing cerebellum in the chick.

Beneficial effects of melatonin on morphological changes in postnatal cerebellar tissue owing to epileptiform activity during pregnancy in rats: light and immunohistochemical study

Although it has been demonstrated that maternal epilepsy has some harmful effects on newborn individuals, current data concerning the effects of epileptic phenomena in pregnant mothers on newborn pups are still limited. This study was undertaken to investigate the changes in the cerebellum of newborns of pinealectomized rats subjected to experimental epilepsy during pregnancy. In our study, the rats were randomly divided into six groups: intact control group, anesthesia control group, epilepsy group, melatonin-treated epileptic group, surgical pinealectomy group, and group of melatonin treatment following pinealectomy procedure. At 1 month after pinealectomy, an acute grand mal epileptic seizure was induced by 400 IU penicillin-G administration into their intrahippocampal CA3 region during the 13th day of their pregnancy in all animals except intact control group. On the neonatal first day, pups were perfused transcardially and the cerebellums removed were processed for light microscopic and immunohistochemical studies. Normal migration and maturation were determined in the postnatal rat cerebellum in both intact control and anesthesia (ketamine-xylazine) control groups, but the morphological structure of cerebellum in the epilepsy control group corresponded to the early embryonal period. It was found that experimental epilepsy or pinealectomy procedure enhanced nestin immunoreactivity, but exogenous melatonin treatment (30 microg/100 g body weight, i.p.) following pinealectomy inhibited increased nestin expression induced by melatonin deprival in vermis region of newborn rat cerebellum (P < 0.001). Our results confirm that epileptic seizures during pregnancy may impair neurogenesis and neuronal maturation in newborns, which are more dramatic in the presence of melatonin deficiency during pregnancy, explaining more harmful effects of epileptic seizures to embryos of aged mothers. To the best of our knowledge, this is the first study reporting the effects of maternal epilepsy during pregnancy in pinealectomized rats on nestin immunoexpression in the newborn rat cerebellum.

Role of melatonin in neurodegenerative diseases

The pineal product melatonin has remarkable antioxidant properties. It scavenges hydroxyl, carbonate and various organic radicals, peroxynitrite and other reactive nitrogen species. Melatonyl radicals formed by scavenging combine with and, thereby, detoxify superoxide anions in processes terminating the radical reaction chains. Melatonin also enhances the antioxidant potential of the cell by stimulating the synthesis of antioxidant enzymes like superoxide dismutase, glutathione peroxidase and glutathione reductase, and by augmenting glutathione levels. The decline in melatonin production in aged individuals has been suggested as one of the primary contributing factors for the development of age-associated neurodegenerative diseases, e.g., Alzheimer's disease. Melatonin has been shown to be effective in arresting neurodegenerative phenomena seen in experimental models of Alzheimer's disease, Parkinsonism and ischemic stroke. Melatonin preserves mitochondrial homeostasis, reduces free radical generation, e.g., by enhancing mitochondrial glutathione levels, and safeguards proton potential and ATP synthesis by stimulating complex I and IV activities. Therapeutic trials with melatonin have been effective in slowing the progression of Alzheimer's disease but not of Parkinson's disease. Melatonin's efficacy in combating free radical damage in the brain suggests that it may be a valuable therapeutic agent in the treatment of cerebral edema after traumatic brain injury.