Body Weight-Supported Treadmill Training Ameliorates Motoneuronal Hyperexcitability by Increasing GAD-65/67 and KCC2 Expression via TrkB Signaling in Rats with Incomplete Spinal Cord Injury

Body weight-supported treadmill training reduces glial scar overgrowth in SCI rats by decreasing the reactivity of astrocytes during the subacute phase

BACKGROUND

Spinal cord injury is followed by glial scar formation, which was long seen mainly as a physical barrier preventing axonal regeneration. Glial scar astrocytes lead to glial scar formation and produce inhibitory factors to prevent axons from growing through the scar, while inhibiting the conversion of reactive astrocytes into glial scar-forming astrocytes may represent an ideal treatment for CNS injury. Exercise is a non-invasive and effective therapeutic intervention for clinical rehabilitation of spinal cord injury. However, its precise therapeutic mechanisms still need to be continuously explored.

METHODS

30 rats were randomly assigned to three groups (Sham, SCI, SCI + BWSTT; n = 10 rats per group). In this study, we employed the BBB scales and gait analysis system to examine the behavioral functions of the rats in each group. Furthermore, we utilized immunoblotting of spinal cord tissue at the injury site, in addition to histological staining and immunofluorescence staining, to explore glial scar aggregation and axonal regeneration in each group of rats.

RESULTS

Our results revealed that hindlimb motor function was significantly improved in SCI rats after a sustained subacute period of BWSTT, accompanied by the promotion of histological repair and nerve regeneration. Subsequent immunofluorescence staining and immunoblotting showed diminished astrocyte reactivity in the region surrounding the spinal cord injury as well as reduced expression and distribution of collagen fibers near the lesion after BWSTT. Additionally, a significant decrease in the expression of MMP-2/9, which is closely related to astrocyte migration, was observed in the vicinity of spinal cord tissue lesions.

CONCLUSION

Our study demonstrates that a sustained BWSTT intervention during the subacute phase of spinal cord injury can effectively reduce astrocyte reactivity and glial scarring overgrowth, thereby facilitating functional recovery after SCI.

BDNF-TrkB Signaling Pathway in Spinal Cord Injury: Insights and Implications

Spinal cord injury (SCI) is a neurodegenerative disorder that has critical impact on patient's life expectance and life span, and this disorder also leads to negative socioeconomic features. SCI is defined as a firm collision to the spinal cord which leads to the fracture and the dislocation of vertebrae. The current available treatment is surgery. However, it cannot fully treat SCI, and many consequences remain after the surgery. Accordingly, finding new therapeutics is critical. BDNF-TrkB signaling is a vital signaling in neuronal differentiation, survival, overgrowth, synaptic plasticity, etc. Hence, many studies evaluate its impact on various neurodegenerative disorders. There are several studies evaluating this signaling in SCI, and they show promising outcomes. It was shown that various exercises, chemical interventions, etc. had significant positive impact on SCI by affecting BDNF-TrkB signaling pathway. This study aims to accumulate and evaluate these data and inspect whether this signaling is effective or not.

Continuous Use During Disuse: Mechanisms and Effects of Spontaneous Activity of Unloaded Postural Muscle

In most mammals, postural soleus muscles are involved in the maintenance of the stability of the body in the gravitational field of Earth. It is well established that immediately after a laboratory rat is exposed to conditions of weightlessness (parabolic flight) or simulated microgravity (hindlimb suspension/unloading), a sharp decrease in soleus muscle electrical activity occurs. However, starting from the 3rd day of mechanical unloading, soleus muscle electrical activity begins to increase and reaches baseline levels approximately by the 14th day of hindlimb suspension. This phenomenon, observed in the course of rat hindlimb suspension, was named the "spontaneous electrical activity of postural muscle". The present review discusses spinal mechanisms underlying the development of such spontaneous activity of rat soleus muscle and the effect of this activity on intracellular signaling in rat soleus muscle during mechanical unloading.

Rate-Dependent Depression of the Hoffmann Reflex: Practical Applications in Painful Diabetic Neuropathy

Measurement of the rate-dependent depression (RDD) of the Hoffmann (H) reflex, a technique developed over half a century ago, is founded on repeated stimulation of the H-reflex with tracking of sequentially evoked H-wave amplitudes in the resulting electromyogram. RDD offers insight into the integrity of spinal reflex pathways and spinal inhibitory regulation. Initially, RDD was predominantly utilized in the mechanistic exploration and evaluation of movement disorders characterized by spasticity symptoms, as may occur following spinal cord injury. However, there is increasing recognition that sensory input from the periphery is modified at the spinal level before ascending to the higher central nervous system and that some pain states can arise from, or be exaggerated by, disruption of spinal processing via a mechanism termed spinal disinhibition. This, along with the urgent clinical need to identify biological markers of pain generator and/or amplifier sites to facilitate targeted pain therapies, has prompted interest in RDD as a biomarker for the contribution of spinal disinhibition to neuropathic pain states. Current research in animals and humans with diabetes has revealed specific disorders of spinal GABAergic function associated with impaired RDD. Future investigations on RDD aim to further elucidate its underlying pathways and enhance its clinical applications.

Treadmill running on neuropathic pain: via modulation of neuroinflammation

Neuropathic pain is a type of chronic pain caused by an injury or somatosensory nervous system disease. Drugs and exercise could effectively relieve neuropathic pain, but no treatment can completely stop neuropathic pain. The integration of exercise into neuropathic pain management has attracted considerable interest in recent years, and treadmill training is the most used among exercise therapies. Neuropathic pain can be effectively treated if its mechanism is clarified. In recent years, the association between neuroinflammation and neuropathic pain has been explored. Neuroinflammation can trigger proinflammatory cytokines, activate microglia, inhibit descending pain modulatory systems, and promote the overexpression of brain-derived neurotrophic factor, which lead to the generation of neuropathic pain and hypersensitivity. Treadmill exercise can alleviate neuropathic pain mainly by regulating neuroinflammation, including inhibiting the activity of pro-inflammatory factors and over activation of microglia in the dorsal horn, regulating the expression of mu opioid receptor expression in the rostral ventromedial medulla and levels of γ-aminobutyric acid to activate the descending pain modulatory system and the overexpression of brain-derived neurotrophic factor. This article reviews and summarizes research on the effect of treadmill exercise on neuropathic pain and its role in the regulation of neuroinflammation to explore its benefits for neuropathic pain treatment.

Body Weight Support Treadmill Training Combined With Sciatic Nerve Electrical Stimulation Ameliorating Motor Function by Enhancing PI3K/Akt Proteins Expression via BDNF/TrkB Signaling Pathway in Rats with Spinal Cord Injury

OBJECTIVE

To investigate the effects of body weight support treadmill training (BWSTT) and sciatic nerve electrical stimulation (SNES) on motor function recovery in spinal cord injury (SCI) rats and its possible mechanism.

METHODS

Modified Allen's method was utilized for T10 incomplete SCI. The Basso-Beattie-Bresnahan (BBB) score and modified Tarlov score were applied to assess motor function. Pathologic alterations of the spinal cord and muscles were observed by hematoxylin and eosin (HE) staining. The positive staining region of collagen fibers was assessed with Masson staining. Immunofluorescence was applied to count the positive cells of brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB). BDNF, TrkB, phosphatidylinositol-3-kinase (PI3K), and protein kinase B (Akt) relative mRNA and protein expressions were evaluated by reverse transcription polymerase chain reaction (RT-PCR) and Western blotting.

RESULTS

On the 21st day of the intervention, the motor scores in SNES and BWSTT + SNES groups were higher than that in SCI group (P < 0.05). Compared with SCI group, mRNA and protein expressions of BDNF/TrkB and PI3K/Akt were more significant on the 21st day of the intervention in SNES and BWSTT + SNES groups (P < 0.05), but there was no difference in BWSTT group (P > 0.05).

CONCLUSIONS

This experiment demonstrated that BWSTT combined with SNES contributed to alleviating spinal cord tissue injury, delaying muscle atrophy and improving locomotion. One of the possible mechanisms may be related to the regulation of the BDNF/TrkB signaling pathway, which changes the expression of PI3K/Akt protein. Furthermore, it was discovered that the ultra-early BWSTT may not be conducive to recovery.

Impact of Endurance Training on Regeneration of Axons, Glial Cells, and Inhibitory Neurons after Spinal Cord Injury: A Link between Functional Outcome and Regeneration Potential within the Lesion Site and in Adjacent Spinal Cord Tissue

Endurance training prior to spinal cord injury (SCI) has a beneficial effect on the activation of signaling pathways responsible for survival, neuroplasticity, and neuroregeneration. It is, however, unclear which training-induced cell populations are essential for the functional outcome after SCI. Adult Wistar rats were divided into four groups: control, six weeks of endurance training, Th9 compression (40 g/15 min), and pretraining + Th9 compression. The animals survived six weeks. Training alone increased the gene expression and protein level of immature CNP-ase oligodendrocytes (~16%) at Th10, and caused rearrangements in neurotrophic regulation of inhibitory GABA/glycinergic neurons at the Th10 and L2 levels, known to contain the interneurons with rhythmogenic potential. Training + SCI upregulated markers for immature and mature (CNP-ase, PLP1) oligodendrocytes by ~13% at the lesion site and caudally, and increased the number of GABA/glycinergic neurons in specific spinal cord regions. In the pretrained SCI group, the functional outcome of hindlimbs positively correlated with the protein levels of CNP-ase, PLP1, and neurofilaments (NF-l), but not with the outgrowing axons (Gap-43) at the lesion site and caudally. These results indicate that endurance training applied before SCI potentiates the repair in damaged spinal cord, and creates a suitable environment for neurological outcome.

The role of KCC2 and NKCC1 in spinal cord injury: From physiology to pathology

The balance of ion concentrations inside and outside the cell is an essential homeostatic mechanism in neurons and serves as the basis for a variety of physiological activities. In the central nervous system, NKCC1 and KCC2, members of the SLC12 cation-chloride co-transporter (CCC) family, participate in physiological and pathophysiological processes by regulating intracellular and extracellular chloride ion concentrations, which can further regulate the GABAergic system. Over recent years, studies have shown that NKCC1 and KCC2 are essential for the maintenance of Cl^- homeostasis in neural cells. NKCC1 transports Cl^- into cells while KCC2 transports Cl^- out of cells, thereby regulating chloride balance and neuronal excitability. An imbalance of NKCC1 and KCC2 after spinal cord injury will disrupt CI^- homeostasis, resulting in the transformation of GABA neurons from an inhibitory state into an excitatory state, which subsequently alters the spinal cord neural network and leads to conditions such as spasticity and neuropathic pain, among others. Meanwhile, studies have shown that KCC2 is also an essential target for motor function reconstruction after spinal cord injury. This review mainly introduces the physiological structure and function of NKCC1 and KCC2 and discusses their pathophysiological roles after spinal cord injury.

Ionic Plasticity: Common Mechanistic Underpinnings of Pathology in Spinal Cord Injury and the Brain

The neurotransmitter GABA is normally characterized as having an inhibitory effect on neural activity in the adult central nervous system (CNS), which quells over-excitation and limits neural plasticity. Spinal cord injury (SCI) can bring about a modification that weakens the inhibitory effect of GABA in the central gray caudal to injury. This change is linked to the downregulation of the potassium/chloride cotransporter (KCC2) and the consequent rise in intracellular Cl- in the postsynaptic neuron. As the intracellular concentration increases, the inward flow of Cl- through an ionotropic GABA-A receptor is reduced, which decreases its hyperpolarizing (inhibitory) effect, a modulatory effect known as ionic plasticity. The loss of GABA-dependent inhibition enables a state of over-excitation within the spinal cord that fosters aberrant motor activity (spasticity) and chronic pain. A downregulation of KCC2 also contributes to the development of a number of brain-dependent pathologies linked to states of neural over-excitation, including epilepsy, addiction, and developmental disorders, along with other diseases such as hypertension, asthma, and irritable bowel syndrome. Pharmacological treatments that target ionic plasticity have been shown to bring therapeutic benefits.

Phenotypes of Motor Deficit and Pain after Experimental Spinal Cord Injury

Motor disability is a common outcome of spinal cord injury (SCI). The recovery of motor function after injury depends on the severity of neurotrauma; motor deficit can be reversible, at least partially, due to the innate tissue capability to recover, which, however, deteriorates with age. Pain is often a comorbidity of injury, although its prediction remains poor. It is largely unknown whether pain can attend motor dysfunction. Here, we implemented SCI for modelling severe and moderate neurotrauma and monitored SCI rats for up to 5 months post-injury to determine the profiles of both motor deficit and nociceptive sensitivity. Our data showed that motor dysfunction remained persistent after a moderate SCI in older animals (5-month-old); however, there were two populations among young SCI rats (1 month-old) whose motor deficit either declined or exacerbated even more over 4–5 weeks after identical injury. All young SCI rats displayed changed nociceptive sensitivity in thermal and mechanical modalities. The regression analysis of the changes revealed a population trend with respect to hyper- or hyposensitivity/motor deficit. Together, our data describe the phenotypes of motor deficit and pain, the two severe complications of neurotrauma. Our findings also suggest the predictability of motor dysfunction and pain syndromes following SCI that can be a hallmark for long-term rehabilitation and recovery after injury.