Persistent phosphorylation of NKCC1 and WNK1 in the epicenter of the spinal cord following contusion injury

Bilobalide Activates Autophagy and Enhances the Efficacy of Bone Marrow Mesenchymal Stem Cells on Spinal Cord Injury Via Upregulating FMRP to Promote WNK1 mRNA Decay

Transplantation of bone marrow mesenchymal stem cells (BMSCs) represents an encouraging strategy for the repair of spinal cord injury (SCI), however, its effectiveness on treating SCI remains controversial. Bilobalide isolated from Ginkgo biloba leaves shows significant neuroprotective effects. We examined the role and underlying mechanism of bilobalide in the efficacy of BMSC transplantation on SCI. Primary BMSCs were isolated from neonatal rats, and cell viability was assessed by MTT assay. Neuronal markers (MAP-2, NeuN, NSE and Tuj1), autophagy markers (LC3 and Beclin1), and Fragile X mental retardation protein (FMRP)/With-no-lysine kinase-1 (WNK1) signaling were measured using RT-qPCR and western blotting. The relationship of FMRP and WNK1 was estimated by RNA immunoprecipitation, while WNK1 mRNA stability was assessed with actinomycin D assay. In a SCI rat model, tissue injury was examined using HE and Nissl staining. Bilobalide treatment facilitated neural differentiation of BMSCs, as well as enhanced autophagy and inhibited WNK1 signaling. The promotive effect of bilobalide on BMSC differentiation was antagonized when overexpressing WNK1 or inhibiting autophagy. Bilobalide upregulated FMRP to promote WNK1 mRNA decay, thus reducing WNK1 expression. FMRP knockdown reversed the promoted functions of bilobalide on autophagy and neuronal differentiation in BMSCs. Additionally, compared to either monotherapy, simultaneous treatments with bilobalide and BMSCs further facilitated autophagy and neuronal differentiation, thereby enhancing the repair of SCI in rats. Bilobalide enhances autophagy activity to promote BMSC neuronal differentiation via FMRP/WNK1 axis, thus improving functional recovery following SCI, which indicates a promising therapeutic approach for SCI.

NKCC1 inhibition reduces periaxonal swelling, increases white matter sparing, and improves neurological recovery after contusive SCI

Ultrastructural studies of contusive spinal cord injury (SCI) in mammals have shown that the most prominent acute changes in white matter are periaxonal swelling and separation of myelin away from their axon, axonal swelling, and axonal spheroid formation. However, the underlying cellular and molecular mechanisms that cause periaxonal swelling and the functional consequences are poorly understood. We hypothesized that periaxonal swelling and loss of connectivity between the axo-myelinic interface impedes neurological recovery by disrupting conduction velocity, and glial to axonal trophic support resulting in axonal swelling and spheroid formation. Utilizing in vivo longitudinal imaging of Thy1YFP+ axons and myelin labeled with Nile red, we reveal that periaxonal swelling significantly increases acutely following a contusive SCI (T13, 30 kdyn, IH Impactor) versus baseline recordings (laminectomy only) and often precedes axonal spheroid formation. In addition, using longitudinal imaging to determine the fate of myelinated fibers acutely after SCI, we show that ∼73% of myelinated fibers present with periaxonal swelling at 1 h post SCI and ∼ 51% of those fibers transition to axonal spheroids by 4 h post SCI. Next, we assessed whether cation-chloride cotransporters present within the internode contributed to periaxonal swelling and whether their modulation would increase white matter sparing and improve neurological recovery following a moderate contusive SCI (T9, 50 kdyn). Mechanistically, activation of the cation-chloride cotransporter KCC2 did not improve neurological recovery and acute axonal survival, but did improve chronic tissue sparing. In distinction, the NKKC1 antagonist bumetanide improved neurological recovery, tissue sparing, and axonal survival, in part through preventing periaxonal swelling and disruption of the axo-myelinic interface. Collectively, these data reveal a novel neuroprotective target to prevent periaxonal swelling and improve neurological recovery after SCI.

Optogenetic and Chemogenic Control of Pain Signaling: Molecular Markers

Pain is a complex experience that involves physical, emotional, and cognitive aspects. This review focuses specifically on the physiological processes underlying pain perception, with a particular emphasis on the various types of sensory neurons involved in transmitting pain signals to the central nervous system. Recent advances in techniques like optogenetics and chemogenetics have allowed researchers to selectively activate or inactivate specific neuronal circuits, offering a promising avenue for developing more effective pain management strategies. The article delves into the molecular targets of different types of sensory fibers such as channels, for example, TRPV1 in C-peptidergic fiber, TRPA1 in C-non-peptidergic receptors expressed differentially as MOR and DOR, and transcription factors, and their colocalization with the vesicular transporter of glutamate, which enable researchers to identify specific subtypes of neurons within the pain pathway and allows for selective transfection and expression of opsins to modulate their activity.

Role of Microglia and Astrocytes in Spinal Cord Injury Induced Neuropathic Pain

Background:

Spinal cord injuries incite varying degrees of symptoms in patients, ranging from weakness and incoordination to paralysis. Common amongst spinal cord injury (SCI) patients, neuropathic pain (NP) is a debilitating medical condition. Unfortunately, there remain many clinical impediments in treating NP because there is a lack of understanding regarding the mechanisms behind SCI-induced NP (SCINP). Given that more than 450,000 people in the United States alone suffer from SCI, it is unsatisfactory that current treatments yield poor results in alleviating and treating NP.

Summary:

In this review, we briefly discussed the models of SCINP along with the mechanisms of NP progression. Further, current treatment modalities are herein explored for SCINP involving pharmacological interventions targeting glia cells and astrocytes.

Key message:

The studies presented in this review provide insight for new directions regarding SCINP alleviation. Given the severity and incapacitating effects of SCINP, it is imperative to study the pathways involved and find new therapeutic targets in coordination with stem cell research, and to develop a new gold-standard in SCINP treatment.

Phospho-regulation, nucleotide binding and ion access control in potassium-chloride cotransporters

Potassium-coupled chloride transporters (KCCs) play crucial roles in regulating cell volume and intracellular chloride concentration. They are characteristically inhibited under isotonic conditions via phospho-regulatory sites located within the cytoplasmic termini. Decreased inhibitory phosphorylation in response to hypotonic cell swelling stimulates transport activity, and dysfunction of this regulatory process has been associated with various human diseases. Here, we present cryo-EM structures of human KCC3b and KCC1, revealing structural determinants for phospho-regulation in both N- and C-termini. We show that phospho-mimetic KCC3b is arrested in an inward-facing state in which intracellular ion access is blocked by extensive contacts with the N-terminus. In another mutant with increased isotonic transport activity, KCC1Δ19, this interdomain interaction is absent, likely due to a unique phospho-regulatory site in the KCC1 N-terminus. Furthermore, we map additional phosphorylation sites as well as a previously unknown ATP/ADP-binding pocket in the large C-terminal domain and show enhanced thermal stabilization of other CCCs by adenine nucleotides. These findings provide fundamentally new insights into the complex regulation of KCCs and may unlock innovative strategies for drug development.

Effects of α7 Nicotinic Acetylcholine Receptor Positive Allosteric Modulator on BDNF, NKCC1 and KCC2 Expression in the Hippocampus following Lipopolysaccharide-Induced Allodynia and Hyperalgesia in a Mouse Model of Inflammatory Pain

BACKGROUND & OBJECTIVES

Hyperalgesia and allodynia are frequent symptoms of inflammatory pain. Neuronal excitability induced by the Brain-Derived Neurotrophic Factor (BDNF)-tyrosine receptor kinase B (TrkB) cascade has a role in the modulation of inflammatory pain. The effects of 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), an α7 nicotinic Acetylcholine Receptor Positive Allosteric Modulator (nAChR PAM), on hippocampal BDNF, cation-chloride cotransporters, NKCC1 and KCC2, expression in inflammatory pain are not known. The objective of the study was to determine the effects of TQS on BDNF, NKCC1, and KCC2 expression in the hippocampus following lipopolysaccharide (LPS)-induced allodynia and hyperalgesia in a mouse model of inflammatory pain.

METHODS

Mice were treated with TQS followed by LPS (1 mg/kg, ip) administration. The effects of TQS on mRNA and BDNF in the hippocampus were examined using qRT-PCR and Western blot, respectively. Immunoreactivity of BDNF, NKCC1, and KCC2 in the hippocampus was measured after LPS administration using immunofluorescence assay. Allodynia and hyperalgesia were determined using von Frey filaments and hot plate, respectively.

RESULTS

The LPS (1 mg/kg) upregulates mRNA of BDNF and downregulates mRNA of KCC2 in the hippocampus and pretreatment of TQS (4 mg/kg) reversed the effects induced by LPS. In addition, the TQS decreased LPS-induced upregulation of BDNF and p-NKCC1 immunoreactivity in the dentate gyrus and CA1 region of the hippocampus. BDNF receptor (TrkB) antagonist, ANA12 (0.50 mg/kg), and NKCC1 inhibitor bumetanide (30 mg/kg) reduced LPS-induced allodynia and hyperalgesia. Blockade of TrkB with ANA12 (0.25 mg/kg) enhanced the effects of TQS (1 mg/kg) against LPS-induced allodynia and hyperalgesia. Similarly, bumetanide (10 mg/kg) enhanced the effects of TQS (1 mg/kg) against allodynia and hyperalgesia.

CONCLUSION

These results suggest that antinociceptive effects of α7 nAChR PAM are associated with downregulation of hippocampal BDNF and p-NKCC1 and upregulation of KCC2 in a mouse model of inflammatory pain.

Rehabilitation Decreases Spasticity by Restoring Chloride Homeostasis through the Brain-Derived Neurotrophic Factor-KCC2 Pathway after Spinal Cord Injury

Activity-based therapy is routinely integrated in rehabilitation programs to facilitate functional recovery after spinal cord injury (SCI). Among its beneficial effects is a reduction of hyperreflexia and spasticity, which affects ∼75% of the SCI population. Unlike current anti-spastic pharmacological treatments, rehabilitation attenuates spastic symptoms without causing an active depression in spinal excitability, thus avoiding further interference with motor recovery. Understanding how activity-based therapies contribute to decrease spasticity is critical to identifying new pharmacological targets and to optimize rehabilitation programs. It was recently demonstrated that a decrease in the expression of KCC2, a neuronal Cl- extruder, contributes to the development spasticity in SCI rats. Although exercise can decrease spinal hyperexcitability and increase KCC2 expression on lumbar motoneurons after SCI, a causal effect remains to be established. Activity-dependent processes include an increase in brain-derived neurotrophic factor (BDNF) expression. Interestingly, BDNF is a regulator of KCC2 but also a potent modulator of spinal excitability. Therefore, we hypothesized that after SCI, the activity-dependent increase in KCC2 expression: 1) functionally contributes to reduce hyperreflexia, and 2) is regulated by BDNF. SCI rats chronically received VU0240551 (KCC2 blocker) or TrkB-IgG (BDNF scavenger) during the daily rehabilitation sessions and the frequency-dependent depression of the H-reflex, a monitor of hyperreflexia, was recorded 4 weeks post-injury. Our results suggest that the activity-dependent increase in KCC2 functionally contributes to H-reflex recovery and critically depends on BDNF activity. This study provides a new perspective in understanding how exercise impacts hyperreflexia by identifying the biological basis of the recovery of function.

Repeated anodal trans-spinal direct current stimulation results in long-term reduction of spasticity in mice with spinal cord injury

KEY POINTS

Spasticity is a disorder of muscle tone that is associated with lesions of the motor system. This condition involves an overactive spinal reflex loop that resists the passive lengthening of muscles. Previously, we established that application of anodal trans-spinal direct current stimulation (a-tsDCS) for short periods of time to anaesthetized mice sustaining a spinal cord injury leads to an instantaneous reduction of spasticity. However, the long-term effects of repeated a-tsDCS and its mechanism of action remained unknown. In the present study, a-tsDCS was performed for 7 days and this was found to cause long-term reduction in spasticity, increased rate-dependent depression in spinal reflexes, and improved ground and skill locomotion. Pharmacological, molecular and cellular evidence further suggest that a novel mechanism involving Na-K-Cl cotransporter isoform 1 mediates the observed long-term effects of repeated a-tsDCS.

ABSTRACT

Spasticity can cause pain, fatigue and sleep disturbances; restrict daily activities such as walking, sitting and bathing; and complicate rehabilitation efforts. Thus, spasticity negatively influences an individual's quality of life and novel therapeutic interventions are needed. We previously demonstrated in anaesthetized mice that a short period of trans-spinal subthreshold direct current stimulation (tsDCS) reduces spasticity. In the present study, the long-term effects of repeated tsDCS to attenuate abnormal muscle tone in awake female mice with spinal cord injuries were investigated. A motorized system was used to test velocity-dependent ankle resistance and associated electromyographical activity. Analysis of ground and skill locomotion was also performed, with electrophysiological, molecular and cellular studies being conducted to reveal a potential underlying mechanism of action. A 4 week reduction in spasticity was associated with an increase in rate-dependent depression of spinal reflexes, and ground and skill locomotion were improved following 7 days of anodal-tsDCS (a-tsDCS). Secondary molecular, cellular and pharmacological experiments further demonstrated that the expression of K-Cl co-transporter isoform 2 (KCC2) was not changed in animals with spasticity. However, Na-K-Cl cotransporter isoform 1 (NKCC1) was significantly up-regulated in mice that exhibited spasticity. When mice were treated with a-tsDCS, down regulation of NKCC1 was detected, and this level did not significantly differ from that in the non-injured control mice. Thus, long lasting reduction of spasticity by a-tsDCS via downregulation of NKCC1 may constitute a novel therapy for spasticity following spinal cord injury.

Yoga: Can it be integrated with treatment of neuropathic pain?

Background

Neuropathic pain (NP) is a debilitating condition that may result from spinal cord injury (SCI). Nearly 75% of all SCI results in NP affecting 17,000 new individuals in the United States every year, and an estimated 7–10% of people worldwide. It is caused by damaged or dysfunctional nerve fibers sending aberrant signals to pain centers in the central nervous system causing severe pain that affects daily life and routine. The mechanisms underlying NP are not fully understood, making treatment difficult. Identification of specific molecular pathways that are involved in pain syndromes and finding effective treatments has become a major priority in current SCI research. Yoga has therapeutic applications may prove beneficial in treating subjects suffering chronically with SCI induced NP, chronic back and associated pains if necessary experimental data is generated

Summary

This review aims to discuss the implications of various mechanistic approaches of yoga which can be tested by new study designs around various nociceptive molecules including matrix metalloproteinases (MMPs), cation-dependent chloride transporter (NKCC1) etc in SCI induced NP patients.

Key messages

Thus, yogic practices could be used in managing SCI induced NP pain by regulating the action of various mechanisms and its associated molecules. Modern prescriptive treatment strategies combined with alternative approaches like yoga should be used in rehabilitation centers and clinics in order to ameliorate chronic NP. We recommend practical considerations of careful yoga practice as part of an integrative medicine approach for NP associated with SCI.

Folic Acid Modulates Matrix Metalloproteinase-9 Expression Following Spinal Cord Injury

Background

Treatment of spinal cord injury (SCI) induced neuropathic pain (NP) proves to be extremely clinically challenging as the mechanism behind SCINP is poorly understood. Matrix metalloproteinase (MMP) is largely responsible for the early disruption of the blood spinal cord barrier. This system initiates macrophage infiltration and degradation of myelin, which plays a pivotal role in how NP occurs. In a recent study, we demonstrated that folic acid (FA) treatment to cSCI rats reduced NP and improved functional recovery by repressing MMP-2 expression. We hypothesize that MMP-2 expression is suppressed because FA actively methylates the DNA sequence that encodes for the MMP-2 protein. However, modulation of MMP-2 expression for alleviation of NP is only pertinent to the mid- to late-phase of injury. Therefore, we need to explore alternate therapeutic methods to target the early- to mid-phase of injury to wholly alleviate NP.

Purpose

Furthering our previous findings on inhibiting MMP-2 expression by FA in mid- and late- phase following cSCI in rats, we hypothesized that FA will methylate and suppress MMP-9 expression during the early- phase, day 1, 3, 7 post cSCI and mid- phase (day 18 post cSCI), in comparison with MMP-2 expression during mid- and the late-phase of cSCI.

Methods

Adult male Sprague Dawley rats (250–270g) underwent cSCI, using a NYU impactor, with 12.5 gm/cm injury. The spinal cord-injured animals were treated intraperitoneally (i.p.) with a standardized dose of FA (80 μg/kg body weight) on day 1, 2, 3, prior to cSCI, followed by daily injection up to 14 or 17 days post-cSCI in different experiments. Animals were euthanized on day 1, 3, 7 post cSCI (early- phase), day 18 post cSCI (mid- phase), and day 42 post cSCI (late-phase) and the epicenter region of injured spinal cord were harvested for MMP-9 and MMP-2 expression analysis by Western blots technique.

Results

i) During early-phase on day 1, 3, and 7, the quantitation displayed no statistical significance in MMP-9 expression, between water- and FA- injected rats. ii) On day 18 post-cSCI, FA significantly modulates the expression of MMP-9 (p = 0.043) iii) Comparing results with MMP-2 expression and inhibition, FA significantly modulates the expression of MMP-2 on day 18 post cSCI (FA- and water-injected rats (p = 0.003). iv) In addition, FA significantly modulates the expression of MMP-2 on day 42 post-cSCI comparing FA- and water- injected rat groups (p = 0.034).

Conclusion

We report that FA administration results in alleviating cSCI-induced NP by inhibiting MMP-9 in the proposed mid- phase of cSCI. However, FA administration resulted in MMP-2 decline during both mid- through late- phase following cSCI. Our study elucidates a new phase of cSCI, the mid-phase. We conclude that further investigation on discovering and quantifying the nature of the mid- phase of SCI injury is needed.

NKCC1 up-regulation contributes to early post-traumatic seizures and increased post-traumatic seizure susceptibility

Traumatic brain injury (TBI) is not only a leading cause for morbidity and mortality in young adults (Bruns and Hauser, Epilepsia 44(Suppl 10):210, 2003), but also a leading cause of seizures. Understanding the seizure-inducing mechanisms of TBI is of the utmost importance, because these seizures are often resistant to traditional first- and second-line anti-seizure treatments. The early post-traumatic seizures, in turn, are a contributing factor to ongoing neuropathology, and it is critically important to control these seizures. Many of the available anti-seizure drugs target gamma-aminobutyric acid (GABA_A) receptors. The inhibitory activity of GABA_A receptor activation depends on low intracellular Cl⁻, which is achieved by the opposing regulation of Na⁺–K⁺–Cl⁻ cotransporter 1 (NKCC1) and K⁺–Cl⁻–cotransporter 2 (KCC2). Up-regulation of NKCC1 in neurons has been shown to be involved in neonatal seizures and in ammonia toxicity-induced seizures. Here, we report that TBI-induced up-regulation of NKCC1 and increased intracellular Cl⁻ concentration. Genetic deletion of NKCC1 or pharmacological inhibition of NKCC1 with bumetanide suppresses TBI-induced seizures. TGFβ expression was also increased after TBI and competitive antagonism of TGFβ reduced NKKC1 expression, ameliorated reactive astrocytosis, and inhibited seizures. Thus, TGFβ might be an important pathway involved in NKCC1 up-regulation after TBI. Our findings identify neuronal up-regulation of NKCC1 and its mediation by TGFβ, as a potential and important mechanism in the early post-traumatic seizures, and demonstrate the therapeutic potential of blocking this pathway.

(WNK)ing at death: With-no-lysine (Wnk) kinases in neuropathies and neuronal survival

Members of With-no-lysine (WNK) family of serine-threonine kinase are key regulators of chloride ion transport in diverse cell types, controlling the activity and the surface expression of cation-chloride (Na(+)/K(+)-Cl(-)) co-transporters. Mutations in WNK1 and WNK4 are linked to a hereditary form of hypertension, and WNKs have been extensively investigated pertaining to their roles in renal epithelial ion homeostasis. However, some members of the WNK family and their splice isoforms are also expressed in the mammalian brain, and have been implicated in aspects of hereditary neuropathy as well as neuronal and glial survival. WNK2, which is exclusively enriched in neurons, is well known as an anti-proliferative tumor suppressor. WNK3, on the other hand, appears to promote cell survival as its inhibition enhances neuronal apoptosis. However, loss of WNK3 has been recently shown to reduce ischemia-associated brain damage. In this review, I surveyed the potentially context-dependent roles of WNKs in neurological disorders and neuronal survival.

Spinal cord injury induced neuropathic pain: Molecular targets and therapeutic approaches

Neuropathic pain, especially that resulting from spinal cord injury, is a tremendous clinical challenge. A myriad of biological changes have been implicated in producing these pain states including cellular interactions, extracellular proteins, ion channel expression, and epigenetic influences. Physiological consequences of these changes are varied and include functional deficits and pain responses. Developing therapies that effectively address the cause of these symptoms require a deeper knowledge of alterations in the molecular pathways. Matrix metalloproteinases and tissue inhibitors of metalloproteinases are two promising therapeutic targets. Matrix metalloproteinases interact with and influence many of the studied pain pathways. Gene expression of ion channels and inflammatory mediators clearly contributes to neuropathic pain. Localized and time dependent targeting of these proteins could alleviate and even prevent neuropathic pain from developing. Current therapeutic options for neuropathic pain are limited primarily to analgesics targeting the opioid pathway. Therapies directed at molecular targets are highly desirable and in early stages of development. These include transplantation of exogenously engineered cell populations and targeted gene manipulation. This review describes specific molecular targets amenable to therapeutic intervention using currently available delivery systems.

Pathogenesis of spinal cord injury induced edema and neuropathic pain: expression of multiple isoforms of wnk1

Background

Neuropathic pain (NP) is a common occurrence following spinal cord injury (SCI). Identification of specific molecular pathways that are involved in pain syndromes has become a major priority in current SCI research. We have investigated the role of a cation-dependent chloride transporter, Cl-regulatory protein Na⁺-K⁺-Cl^- 1 (NKCC1), phosphorylation profile of NKCC1 and its specific involvement in neuropathic pain following contusion SCI (cSCI) using a rat model. Administration of the NKCC1 inhibitor bumetanide (BU) increases the mean hindpaw withdrawal latency time (WLT), thermal hyperalgesia (TH) following cSCI. These results demonstrate implication of NKCC1 co-transporter and BUin SCI-induced neuropathic pain. The with-no-lysine (K)–1 (WNK1) kinase has been shown to be an important regulator of NKCC1 phosphorylation in many systems, including nocioception. Mutations in a neuronal-specific exon of WNK1 (HSN2) was identified in patients that have hereditary sensory neuropathy type II (HSANII) also implicates WNK1 in nocioception, such that these patients have loss of perception to pain, touch and heat. In our ongoing research we proposed two studies utilizing our contusion SCI (cSCI) NP model of rat.

Purpose

Study 1 aimed at NKCC1 expression and activity is up-regulated following cSCI in the early edema and chronic neuropathic pain phases. Study 2 aimed at identifying the expression profile of alternatively spliced WNK1 isoforms in animals exhibiting thermal hyperalgesia (TH) following cSCI.

Methods

Adult male Sprague Dawley rats (275–300 g) following laminectomy received cSCI at T9 with the NYU impactor-device II by dropping 10 g weight from the height of 12.5 mm. Control rats obtained laminectomy but no impaction. Following injury, functional recovery was assessed by BBB locomotor scores on day 1, 7, 14, 21, 35, and 42 and development of thermal hyperalgesia on day 21, 28, 35, and 42 day of injury by monitoring hind paw withdraw latency time (WLT) in seconds compared with the baseline data before injury.

Results

Increased NKCC1 may explain observed increase in magnetic resonance imaging (MRI) T2, exhibiting NKCC1 localization in neurons. This data supports NKCC1’s role in the pathogenesis of acute and chronic phases of injury, namely spinal cord edema and chronic phase neuropathic pain. NKCC1 dependent chloride influx requires the phosphorylation at specific residues. Probing for the HSN2 exon of WNK1 reveals two key findings: i) the HSN2 exon is found in alternatively spliced neuronal isoforms found at 250 kDa and 230 kDa; ii) the 250 kDa isoform is found only in tissue that is injured.

Conclusions

This data implicates the NKCC1/WNK1/WNK1HSN2 involvement in post-injury response that contributes to the development of neuropathic pain. Targeting this system may have therapeutic benefit.

Exercise modulates chloride homeostasis after spinal cord injury

Activity-based therapies are routinely integrated in spinal cord injury (SCI) rehabilitation programs because they result in a reduction of hyperreflexia and spasticity. However, the mechanisms by which exercise regulates activity in spinal pathways to reduce spasticity and improve functional recovery are poorly understood. Persisting alterations in the action of GABA on postsynaptic targets is a signature of CNS injuries, including SCI. The action of GABA depends on the intracellular chloride concentration, which is determined largely by the expression of two cation-chloride cotransporters (CCCs), KCC2 and NKCC1, which serve as chloride exporters and importers, respectively. We hypothesized that the reduction in hyperreflexia with exercise after SCI relies on a return to chloride homeostasis. Sprague Dawley rats received a spinal cord transection at T12 and were assigned to SCI-7d, SCI-14d, SCI-14d+exercise, SCI-28d, SCI-28d+exercise, or SCI-56d groups. During a terminal experiment, H-reflexes were recorded from interosseus muscles after stimulation of the tibial nerve and the low-frequency-dependent depression (FDD) was assessed. We provide evidence that exercise returns spinal excitability and levels of KCC2 and NKCC1 toward normal levels in the lumbar spinal cord. Acutely altering chloride extrusion using the KCC2 blocker DIOA masked the effect of exercise on FDD, whereas blocking NKCC1 with bumetanide returned FDD toward intact levels after SCI. Our results indicate that exercise contributes to reflex recovery and restoration of endogenous inhibition through a return to chloride homeostasis after SCI. This lends support for CCCs as part of a pathway that could be manipulated to improve functional recovery when combined with rehabilitation programs.