Decreased intracellular GABA levels contribute to spinal cord stimulation-induced analgesia in rats suffering from painful peripheral neuropathy: the role of KCC2 and GABA(A) receptor-mediated inhibition

Characterization of preclinical models to investigate spinal cord stimulation for neuropathic pain: a systematic review and meta-analysis

Despite advancements in preclinical and clinical spinal cord stimulation (SCS) research, the mechanisms of SCS action remain unclear. This may result from challenges in translatability of findings between species. Our systematic review (PROSPERO: CRD42023457443) aimed to comprehensively characterize the important translational components of preclinical SCS models, including stimulating elements and stimulation specifications. Databases (Embase, PubMed, Web of Science, and WikiStim) were searched on October 5, 2023, identifying 78 studies meeting the search criteria. We conducted a post hoc meta-analysis, including subgroup analyses and meta-regression, to assess SCS efficacy on mechanical hypersensitivity in rats subjected to neuropathic pain. Although monopolar electrodes were predominantly used as stimulating elements until 2013, quadripolar paddle and cylindrical leads gained recent popularity. Most research was conducted using 50 Hz and 200 µs stimulation. Motor threshold (MT) estimation was the predominant strategy to determine SCS intensity, which was set to 71.9% of MT on average. Our analysis revealed a large effect size for SCS (Hedge g = 1.13, 95% CI: [0.93, 1.32]) with similar magnitudes of effect between conventional (≤100 Hz) and nonconventional SCS paradigms while sham SCS had nonsignificant effect size. In addition, different stimulation intensity, frequency, and electrode design did not affect effect size. The risk of bias was assessed using Systematic Review Centre for Laboratory animal Experimentation criteria and was unclear, and only the frequency subgroup analysis showed publication bias. In summary, our review characterizes the critical components of preclinical SCS models and provides recommendations to improve reproducibility and translatability, thereby advancing the scientific foundation for SCS research.

Insights into the pathophysiology and response of persistent spinal pain syndrome type 2 to spinal cord stimulation: a human genome-wide association study

BACKGROUND

Spinal cord stimulation (SCS) provides pain relief for some patients with persistent spinal pain syndrome type 2 (PSPS 2), but the precise mechanisms of action and prognostic factors for a favorable pain response remain obscure. This in vivo human genome-wide association study provides some pathophysiological clues.

METHODS

We performed a high-density oligonucleotide microarray analysis of serum obtained from both PSPS 2 cases and pain-free controls who had undergone lower back spinal surgery at the study site. Using multivariate discriminant analysis, we tried to identify different expressions between mRNA transcripts from PSPS 2 patients relative to controls, SCS responders to non-responders, or SCS responders to themselves before starting SCS. Gene ontology enrichment analysis was used to identify the biological processes that best discriminate between the groups of clinical interest.

RESULTS

Thirty PSPS 2 patients, of whom 23 responded to SCS, were evaluated together with 15 pain-free controls. We identified 11 significantly downregulated genes in serum of PSPS 2 patients compared with pain-free controls and two significantly downregulated genes once the SCS response became apparent. All were suggestive of enhanced inflammation, tissue repair mechanisms and proliferative responses among the former. We could not identify any gene differentiating patients who responded to SCS from those who did not respond.

CONCLUSIONS

This study points out various biological processes that may underlie PSPS 2 pain and SCS therapeutic effects, including the modulation of neuroimmune response, inflammation and restorative processes.

Spinal cord stimulation for the symptomatic treatment of rigidity and painful spasm in a case of stiff person syndrome

BACKGROUND

Stiff person syndrome (SPS) is a rare neuroimmunological disorder characterized by rigidity and painful spasm primarily affecting the truncal and paraspinal musculature due to autoimmune-mediated neuronal hyperexcitability. Spinal cord stimulation (SCS) is an approved therapy for managing painful neuropathic conditions, including diabetic peripheral neuropathy and refractory angina pectoris. We describe the novel use of SCS for the treatment of spasm and rigidity in a 49-year-old man with seropositive stiff person syndrome (SPS). The patient was treated with intravenous immunoglobulin (IVIG) and oral medications over a 13-month period with minimal improvement, prompting consideration of SCS. To our knowledge, this is the first report of the successful use of SCS in SPS with the demonstration of multifaceted clinical improvement.

METHODS

Following a successful temporary SCS trial, permanent implantation was performed. Spasm/stiffness (Distribution of Stiffness Index; Heightened Sensitivity Scale; Penn Spasm Frequency Scale, PSFS), disability (Oswestry Disability Index, ODI; Pain Disability Index, PDI), depression (Patient Health Questionnaire-9, PHQ-9), sleep (Pittsburgh Sleep Quality Index, PSQI), fatigue (Fatigue Severity Scale, FSS), pain (Numerical Pain Rating Scale, NPRS), quality of life (EuroQoL 5 Dimension 5 Level, EQ-5D-5L), and medication usage were assessed at baseline, 6-month, and 10-month postimplantation.

RESULTS

ODI, PHQ-9, FSS, NPRS, PSQI, and EQ-5D-5L scores showed a notable change from baseline and surpassed the defined minimal clinically important difference (MCID) at 6-month and 10-month follow-up. Oral medication dosages were reduced.

CONCLUSIONS

The novel use of SCS therapy in seropositive SPS resulted in functional improvement and attenuation of symptoms. We present possible mechanisms by which SCS may produce clinical response in patients with SPS and aim to demonstrate proof-of-concept for a future comprehensive pilot study evaluating SCS-mediated response in SPS.

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

Evoked compound action potential (ECAP)-controlled closed-loop spinal cord stimulation in an experimental model of neuropathic pain in rats

BACKGROUND

Preclinical models of spinal cord stimulation (SCS) are lacking objective measurements to inform translationally applicable SCS parameters. The evoked compound action potential (ECAP) represents a measure of dorsal column fiber activation. This measure approximates the onset of SCS-induced sensations in humans and provides effective analgesia when used with ECAP-controlled closed-loop (CL)-SCS systems. Therefore, ECAPs may provide an objective surrogate for SCS dose in preclinical models that may support better understanding of SCS mechanisms and further translations to the clinics. This study assessed, for the first time, the feasibility of recording ECAPs and applying ECAP-controlled CL-SCS in freely behaving rats subjected to an experimental model of neuropathic pain.

METHODS

Adult male Sprague-Dawley rats (200-300 g) were subjected to spared nerve injury (SNI). A custom-made six-contact lead was implanted epidurally covering T11-L3, as confirmed by computed tomography or X-ray. A specially designed multi-channel system was used to record ECAPs and to apply ECAP-controlled CL-SCS for 30 min at 50 Hz 200 µs. The responses of dorsal column fibers to SCS were characterized and sensitivity towards mechanical and cold stimuli were assessed to determine analgesic effects from ECAP-controlled CL-SCS. Comparisons between SNI rats and their controls as well as between stimulation parameters were made using omnibus analysis of variance (ANOVA) tests and t-tests.

RESULTS

The recorded ECAPs showed the characteristic triphasic morphology and the ECAP amplitude (mV) increased as higher currents (mA) were applied in both SNI animals and controls (SNI SCS-ON and sham SCS-ON). Importantly, the use of ECAP-based SCS dose, implemented in ECAP-controlled CL-SCS, significantly reduced mechanical and cold hypersensitivity in SNI SCS-ON animals through the constant and controlled activation of dorsal column fibers. An analysis of conduction velocities of the evoked signals confirmed the involvement of large, myelinated fibers.

CONCLUSIONS

The use of ECAP-based SCS dose implemented in ECAP-controlled CL-SCS produced analgesia in animals subjected to an experimental model of neuropathic pain. This approach may offer a better method for translating SCS parameters between species that will improve understanding of the mechanisms of SCS action to further advance future clinical applications.

Measures of Dosage for Spinal-Cord Electrical Stimulation: Review and Proposal

This manuscript proposes an electrical definition of therapeutic dose for spinal-cord systems used for the treatment of chronic pain, analogous to the pharmacological definition. Dose-response relationships are fundamental to pharmacology, radio-therapy, and other treatments, but have never been properly established for neuromodulation. This manuscript offers a robust measure of dose, pre-requisite to establishing a reliable and repeatable dose-response relationship. The new definition, enabled by the system transresistance obtained from measurement of evoked action potentials, recognizes the mechanism of action of spinal cord stimulation (SCS), and should improve acceptance of the therapy as compared to pharmacological treatments which are currently used more frequently for the treatment of chronic pain. The new definition suggests methods for personalization and standardization of the dose in SCS, and is potentially generalizable to all neuromodulation therapies in which nervous tissue is excited including sacral nerve stimulation (SNS), vagal nerve stimulation (VNS) and deep-brain stimulation (DBS). Formulas are provided, and applied using patient data. Powerful conclusions are drawn from application of the new measure.

Combination therapy with simultaneous delivery of spinal cord stimulation modalities: COMBO randomized controlled trial

Aim: The Combining Mechanisms for Better Outcomes randomized controlled trial assessed the effectiveness of various spinal cord stimulation (SCS) modalities for chronic pain. Specifically, combination therapy (simultaneous use of customized sub-perception field and paresthesia-based SCS) versus monotherapy (paresthesia-based SCS) was evaluated. Methods: Participants were prospectively enrolled (key inclusion criterion: chronic pain for ≥6 months). Primary end point was the proportion with ≥50% pain reduction without increased opioids at the 3-month follow-up. Patients were followed for 2 years. Results: The primary end point was met (n = 89; p < 0.0001) in 88% of patients in the combination-therapy arm (n = 36/41) and 71% in the monotherapy arm (n = 34/48). Responder rates at 1 and 2 years (with available SCS modalities) were 84% and 85%, respectively. Sustained functional outcomes improvement was observed out to 2 years. Conclusion: SCS-based combination therapy can improve outcomes in patients with chronic pain. Clinical Trial Registration: NCT03689920 (ClinicalTrials.gov), Combining Mechanisms for Better Outcomes (COMBO).

Spinal cord stimulation: Beyond pain management

INTRODUCTION

The gate control theory of pain was the starting point of the development of spinal cord stimulation (SCS). We describe the indications for the treatment in pain management and other uses not related to pain.

DEVELOPMENT

There are currently several paradigms for SCS: tonic, burst, and high frequency. The main difference lies in the presence of paraesthesias. SCS is most beneficial for treating neuropathic pain. Patients with failed back surgery syndrome show the best response rates, although a considerable reduction in pain is also observed in patients with complex regional pain syndrome, diabetic neuropathy, radiculopathy, and low back pain without previous surgery. Phantom pain or pain related to cardiovascular or peripheral vascular disease may improve, although there is a lack of robust evidence supporting generalisation of its use. SCS also improves cancer-related pain, although research on this issue is scarce. Non-pain-related indications for SCS are movement disorders, spasticity, and sequelae of spinal cord injury. The main limiting factors for the use of SCS are mechanical complications and the cost of the treatment.

CONCLUSION

In its 50-year history, SCS has progressed enormously. The perfection of hardware and software may improve its effectiveness and reduce the rate of complications. Indications for SCS could include other diseases, and its use could be expanded, if the costs of the technology are reduced.

Spinal Cord Stimulation and Treatment of Peripheral or Central Neuropathic Pain: Mechanisms and Clinical Application

Spinal cord stimulation (SCS) as an evidence-based interventional treatment has been used and approved for clinical use in a variety of pathological states including peripheral neuropathic pain; however, until now, it has not been used for the treatment of spinal cord injury- (SCI-) induced central neuropathic pain. This paper reviews the underlying mechanisms of SCS-induced analgesia and its clinical application in the management of peripheral and central neuropathic pain. Evidence from recent research publications indicates that nociceptive processing at peripheral and central sensory systems is thought to be modulated by SCS through (i) inhibition of the ascending nociceptive transmission by the release of analgesic neurotransmitters such as GABA and endocannabinoids at the spinal dorsal horn; (ii) facilitation of the descending inhibition by release of noradrenalin, dopamine, and serotonin acting on their receptors in the spinal cord; and (iii) activation of a variety of supraspinal brain areas related to pain perception and emotion. These insights into the mechanisms have resulted in the clinically approved use of SCS in peripheral neuropathic pain states like Complex Regional Pain Syndrome (CRPS) and Failed Back Surgery Syndrome (FBSS). However, the mechanisms underlying SCS-induced pain relief in central neuropathic pain are only partly understood, and more research is needed before this therapy can be implemented in SCI patients with central neuropathic pain.

Spinal cord stimulation using differential target multiplexed programming modulates neural cell-specific transcriptomes in an animal model of neuropathic pain

Spinal cord stimulation is a proven effective therapy for treating chronic neuropathic pain. Previous work in our laboratory demonstrated that spinal cord stimulation based on a differential target multiplexed programming approach provided significant relief of pain-like behavior in rodents subjected to the spared nerve injury model of neuropathic pain. The relief was significantly better than obtained using high rate and low rate programming. Furthermore, transcriptomics-based results implied that differential target multiplexed programming modulates neuronal–glial interactions that have been perturbed by the pain process. Although differential target multiplexed programming was developed to differentially target neurons and glial cells, our previous work did not address this. This work presents transcriptomes, specific to each of the main neural cell populations (neurons, microglia, astrocytes, and oligodendrocytes), obtained from spinal cord subjected to continuous spinal cord stimulation treatment with differential target multiplexed programming, high rate programming, or low rate programming compared with no spinal cord stimulation treatment, using the spared nerve injury model. To assess the effect of each spinal cord stimulation treatment on these cell-specific transcriptomes, gene expression levels were compared with that of healthy animals, naïve to injury and interventional procedures. Pearson correlations and cell population analysis indicate that differential target multiplexed programming yielded strong and significant correlations to expression levels found in the healthy animals across every evaluated cell-specific transcriptome. In contrast, high rate programming only yielded a strong correlation for the microglia-specific transcriptome, while low rate programming did not yield strong correlations with any cell types. This work provides evidence that differential target multiplexed programming distinctively targeted and modulated the expression of cell-specific genes in the direction of the healthy state thus supporting its previously established action on regulating neuronal–glial interaction processes in a pain model.

Driving effect of BDNF in the spinal dorsal horn on neuropathic pain

Neuropathic pain (NP) is caused by direct or indirect damage to the nervous system and is a common symptom of many diseases. The mechanisms underlying the onset and persistence of NP are unclear. Therefore, research concerning these mechanisms has become an important focus in the medical field. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family of signaling molecules. BDNF is an important regulator of neuronal development, synaptic transmission, and cellular and synaptic plasticity, which are essential for nerve maintenance and repair. However, BDNF is upregulated in the spinal dorsal horn and can promote NP by activating glial cells, reducing inhibitory functions and enhancing excitement after nociceptive stimulation. This review considers the relationship between NP and BDNF signaling in the spinal dorsal horn and discusses potentially related pathological mechanisms.

Conventional Dorsal Root Ganglion Stimulation in an Experimental Model of Painful Diabetic Peripheral Neuropathy: A Quantitative Immunocytochemical Analysis of Intracellular γ-Aminobutyric Acid in Dorsal Root Ganglion Neurons

Background and Objective

The sensory cell somata in the DRG contain all equipment necessary for extensive GABAergic signaling and are able to release GABA upon depolarization. With this study, we hypothesize that pain relief induced by conventional dorsal root ganglion stimulation (Con‐DRGS) in animals with experimental painful diabetic peripheral neuropathy is related to the release of GABA from DRG neurons. With use of quantitative immunocytochemistry, we hypothesize DRGS to result in a decreased intensity of intracellular GABA‐immunostaining in DRG somata.

Materials and Methods

Female Sprague‐Dawley rats (n = 31) were injected with streptozotocin (STZ) in order to induce Diabetes Mellitus. Animals that developed neuropathic pain after four weeks (Von Frey) were implanted with a unilateral DRGS device at L4 (n = 14). Animals were then stimulated for 30 min with Con‐DRGS (20 Hz, pulse width = 0.2 msec, amplitude = 67% of motor threshold, n = 8) or Sham‐DRGS (n = 6), while pain behavior (von Frey) was measured. DRGs were then collected and immunostained for GABA, and a relation to size of sensory cell soma diameter (small: 12–26 μm, assumed to be C‐fiber related sensory neurons; medium: 26–40 μm, assumed to be Aδ related sensory neurons; and large: 40–54 μm, assumed to be Aβ related sensory neurons) was made.

Results

DRGS treated animals showed significant reductions in STZ‐induced mechanical hypersensitivity. No significant differences in GABA immunostaining intensity per sensory neuron cell soma type (small‐, medium‐, or large‐sized) were noted in DRGs of stimulated (Con‐DRGS) animals versus Sham animals. No differences in GABA immunostaining intensity per sensory cell soma type in ipsi‐ as compared to contralateral DRGs were observed.

Conclusion

Con‐DRGS does not affect the average intracellular GABA immunofluorescence staining intensity in DRG sensory neurons of those animals which showed significant pain reduction. Similarly, no soma size related changes in intracellular GABA immunofluorescence were observed following Con‐DRGS.

Overexpression of chloride importer NKCC1 contributes to the sensory-affective and sociability phenotype of rats following neonatal maternal separation

BACKGROUND

Early life stress is known to affect the development of the nervous system and its function at a later age. It increases the risk to develop psychiatric disorders as well as chronic pain and its associated affective comorbidities across the lifespan. GABAergic inhibition is important for the regulation of central function and related behaviors, including nociception, anxiety or social interactions, and requires low intracellular chloride levels. Of particular interest, the oxytocinergic (OTergic) system exerts potent anxiolytic, analgesic and pro-social properties and is known to be involved in the regulation of chloride homeostasis and to be impaired following early life stress.

METHODS

We used behavioral measures to evaluate anxiety, social interactions and pain responses in a rat model of neonatal maternal separation (NMS). Using quantitative PCR, we investigated whether NMS was associated with alterations in the expression of chloride transporters in the cerebrum and spinal cord. Finally, we evaluated the contribution of OTergic signaling and neuro-inflammatory processes in the observed phenotype.

RESULTS

NMS animals displayed a long-lasting upregulation of chloride importer Na-K-Cl cotransporter type 1 (NKCC1) expression in the cerebrum and spinal cord. Neonatal administration of the NKCC1 inhibitor bumetanide or oxytocin successfully normalized the anxiety-like symptoms and the lack of social preference observed in NMS animals. Phenotypic alterations were associated with a pro-inflammatory state which could contribute to NKCC1 upregulation.

CONCLUSIONS

This work suggests that an impaired chloride homeostasis, linked to oxytocin signaling dysfunction and to neuro-inflammatory processes, could contribute to the sensori-affective phenotype following NMS.

The Effect of Clinically Controllable Factors on Neural Activation During Dorsal Root Ganglion Stimulation

OBJECTIVE

Dorsal root ganglion stimulation (DRGS) is an effective therapy for chronic pain, though its mechanisms of action are unknown. Currently, we do not understand how clinically controllable parameters (e.g., electrode position, stimulus pulse width) affect the direct neural response to DRGS. Therefore, the goal of this study was to utilize a computational modeling approach to characterize how varying clinically controllable parameters changed neural activation profiles during DRGS.

MATERIALS AND METHODS

We coupled a finite element model of a human L5 DRG to multicompartment models of primary sensory neurons (i.e., Aα-, Aβ-, Aδ-, and C-neurons). We calculated the stimulation amplitudes necessary to elicit one or more action potentials in each neuron, and examined how neural activation profiles were affected by varying clinically controllable parameters.

RESULTS

In general, DRGS predominantly activated large myelinated Aα- and Aβ-neurons. Shifting the electrode more than 2 mm away from the ganglion abolished most DRGS-induced neural activation. Increasing the stimulus pulse width to 500 μs or greater increased the number of activated Aδ-neurons, while shorter pulse widths typically only activated Aα- and Aβ-neurons. Placing a cathode near a nerve root, or an anode near the ganglion body, maximized Aβ-mechanoreceptor activation. Guarded active contact configurations did not activate more Aβ-mechanoreceptors than conventional bipolar configurations.

CONCLUSIONS

Our results suggest that DRGS applied with stimulation parameters within typical clinical ranges predominantly activates Aβ-mechanoreceptors. In general, varying clinically controllable parameters affects the number of Aβ-mechanoreceptors activated, although longer pulse widths can increase Aδ-neuron activation. Our data support several Neuromodulation Appropriateness Consensus Committee guidelines on the clinical implementation of DRGS.

Prolonged Analgesia by Spinal Cord Stimulation Following a Spinal Injury Associated With Activation of Adult Neural Progenitors

OBJECTIVES

Responses of spinal progenitors to spinal cord stimulation (SCS) following spinal cord injury (SCI) in rats were assessed to reveal their potential contribution to SCS-induced analgesia.

METHODS

Spinal epidural electrodes were implanted in rats at T12 rostral to a quadrant dorsal horn injury at T13. Further groups additionally received either a microlesion to the dorsolateral funiculus (DLF) or gabapentin (10 mg/kg). SCS was performed at 25 Hz for 10 minutes on day 4 (early SCS) and at 10 Hz for 10 minutes on day 8 (late SCS) after injury. Paw withdrawal threshold (PWT) was measured before injury, 30 minutes before or after SCS, and before cull on day 14, followed by immunostaining assessment.

RESULTS

Paw withdrawal thresholds in uninjured animals (51.0 ± 4.0 g) were markedly reduced after SCI (17.3 ± 2.2 g). This was significantly increased by early SCS (38.5 ± 5.2 g, P < 0.01) and further enhanced by late SCS (50.9 ± 1.9 g, P < 0.01) over 6 days. Numbers of neural progenitors expressing nestin, Sox2, and doublecortin (DCX) in the spinal dorsal horn were increased 6 days after SCS by 6-fold, 2-fold, and 2.5-fold, respectively (P < 0.05 to 0.01). The elevated PWT evoked by SCS was abolished by DLF microlesions (48.9 ± 2.6 g vs. 19.0 ± 3.9 g, P < 0.01) and the number of nestin-positive cells was reduced to the level without SCS (P < 0.05). Gabapentin enhanced late SCS-induced analgesia from 37.0 ± 3.9 g to 54.0 ± 0.8 g (P < 0.01) and increased gamma-aminobutyric acid (GABA)-ergic neuronal marker vesicular GABA transporter-positive newborn cells 2-fold (P < 0.01).

CONCLUSIONS

Spinal progenitor cells appear to be activated by SCS via descending pathways, which may be enhanced by gabapentin and potentially contributes to relief of SCI-induced neuropathic pain.

Mechanisms and mode of action of spinal cord stimulation in chronic neuropathic pain

Tonic spinal cord stimulation (SCS) has been used as a treatment for chronic neuropathic pain ever since its discovery in late 1960s. Despite its clinical successes in a subset of chronic neuropathic pain syndromes, several limitations such as insufficient pain relief and uncomfortable paresthesias have led to the development of new targets, the dorsal root ganglion, and new stimulation waveforms, such as burst and high frequency. The aim of this review is to provide a brief overview of the main mechanisms behind the mode of action of the different SCS paradigms. Tonic SCS mainly acts via a segmental spinal mechanism where it induces GABA-release from inhibitory interneurons in the spinal dorsal horn. Tonic SCS concurrently initiates neuropathic pain modulation through a supraspinal-spinal feedback loop and serotonergic descending fibers. Mechanisms of stimulation of the DRG as well as those related to new SCS paradigms are now under investigation, where it seems that burst SCS not only stimulates sensory, discriminative aspects of pain (like Tonic SCS) but also emotional, affective, and motivational aspects of pain. Initial long-term study results on closed-loop SCS systems hold promise for improvement of future SCS treatment.

Active Recharge Burst and Tonic Spinal Cord Stimulation Engage Different Supraspinal Mechanisms: A Functional Magnetic Resonance Imaging Study in Peripherally Injured Chronic Neuropathic Rats

OBJECTIVES

To assess the supraspinal working mechanisms of the burst spinal cord stimulation (SCS) mode, we used functional magnetic resonance imaging (fMRI) in chronic neuropathic rats. We hypothesized that active recharge burst SCS would induce a more profound blood oxygenation level-dependent (BOLD) signal increase in areas associated with cognitive-emotional aspects of pain, as compared to tonic SCS.

METHODS

Sprague Dawley rats (n = 17) underwent a unilateral partial sciatic nerve ligation, which resulted in chronic neuropathic pain. Quadripolar SCS electrodes were epidurally positioned on top of the dorsal columns at Th13. Isoflurane-anesthetized (1.5%) rats received either tonic SCS (n = 8) or burst SCS (n = 9) at 66% of motor threshold. BOLD fMRI was conducted before, during, and after SCS using a 9.4-T horizontal bore scanner.

RESULTS

Overall, both tonic and burst SCS induced a significant increase of BOLD signal levels in areas associated with the location and intensity of pain, and areas associated with cognitive-emotional aspects of pain. Additionally, burst SCS significantly increased BOLD signal levels in the raphe nuclei, nucleus accumbens, and caudate putamen. Tonic SCS did not induce a significant increase in BOLD signal levels in these areas.

CONCLUSIONS

In conclusion, active recharge burst and tonic SCS have different effects on the intensity and localization of SCS-induced activation responses in the brain. This work demonstrates that active recharge burst is another waveform that can engage brain areas associated with cognitive-emotional aspects of pain as well as areas associated with location and intensity of pain. Previous studies showing similar engagement used only passive recharge burst.

Enhancing neuronal chloride extrusion rescues α2/α3 GABAA-mediated analgesia in neuropathic pain

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis.

Nonlinear Relation Between Burst Dorsal Root Ganglion Stimulation Amplitude and Behavioral Outcome in an Experimental Model of Painful Diabetic Peripheral Neuropathy

Background and objective

Dorsal root ganglion stimulation (DRGS) has recently emerged as a neuromodulation modality in the treatment of chronic neuropathic pain. The objective of this study was to compare the efficacy of different Burst‐DRGS amplitudes in an experimental model of painful diabetic peripheral neuropathy (PDPN).

Methods

Diabetes mellitus was induced in female Sprague–Dawley rats by intraperitoneal injection of streptozotocin (STZ, n = 28). Animals were tested for mechanical hypersensitivity (von Frey paw withdrawal test) before, and four weeks after STZ injection. PDPN rats (n = 13) were implanted with a unilateral bipolar electrode at the L5 DRG. Animals received Burst‐DRGS at 0%, 10%, 33%, 50%, 66%, and 80% of motor threshold (MT) in a randomized crossover design on post‐implantation days 2–7 (n = 9). Mechanical hypersensitivity was assessed before stimulation onset, 15 and 30 min during stimulation, and 15 and 30 min after stimulation.

Results

Burst‐DRGS at amplitudes of 33%, 50%, 66%, and 80% MT resulted in significant attenuation of STZ‐induced mechanical hypersensitivity at 15 and 30 min during stimulation, as well as 15 min after cessation of stimulation. No effect on mechanical hypersensitivity was observed for Burst‐DRGS at 0% MT and 10% MT. Optimal pain relief and highest responder rates were achieved with Burst‐DRGS at 50–66% MT, with an estimated optimum at 52% MT.

Conclusion

Our findings indicate a nonlinear relationship between Burst‐DRGS amplitude and behavioral outcome, with an estimated optimal amplitude of 52% MT. Further optimization and analysis of DRGS driven by insights into the underlying mechanisms related to the various stimulation paradigms is warranted.

Dorsal Root Ganglion Stimulation in Experimental Painful Diabetic Polyneuropathy: Delayed Wash-Out of Pain Relief After Low-Frequency (1Hz) Stimulation

OBJECTIVE

Up until now there is little data about the pain relieving effect of different frequency settings in DRGS. The aim of this study was to compare the pain relieving effect of DRGS at low-, mid-, and high-frequencies and Sham-DRGS in an animal model of painful diabetic neuropathy (PDPN).

MATERIAL AND METHODS

Diabetes mellitus was induced by an intraperitoneal injection of streptozotocin in 8-week-old female Sprague-Dawley rats (n = 24; glucose ≥15 mmol/L: n = 20; mechanical hypersensitivity: n = 15). Five weeks later, a DRGS device was implanted at the L5 DRG. Ten animals were included for stimulation, alternating 30 minutes of low (1 Hz)-, mid (20 Hz)-, and high (1000 Hz)-frequencies and Sham-DRGS during four days, with a pulse width of 0.2 msec (average amplitude: 0.19 ± 0.01 mA), using a randomized cross-over design. The effect on mechanical hypersensitivity of the hind paw to von Frey filaments was evaluated.

RESULTS

All DRGS frequencies resulted in a complete reversal of mechanical hypersensitivity and "a clinically relevant reduction" was achieved in 70-80% of animals. No significant differences in maximal pain relieving effect were found between the different frequency treatments (p = 0.24). Animals stimulated at 1000 and 20 Hz returned to baseline mechanical hypersensitivity values 15 and 30 min after stimulation cessation, respectively, while animals stimulated at 1 Hz did not.

CONCLUSIONS

These results show that DRGS is equally effective when applied at low-, mid-, and high-frequency in an animal model of PDPN. However, low-frequency-(1 Hz)-DRGS resulted in a delayed wash-out effect, which suggests that this is the most optimal frequency for pain therapy in PDPN as compared to mid- and high-frequency.

Burst and Tonic Spinal Cord Stimulation Both Activate Spinal GABAergic Mechanisms to Attenuate Pain in a Rat Model of Chronic Neuropathic Pain

BACKGROUND

Experimental and clinical studies have shown that tonic spinal cord stimulation (SCS) releases gamma-aminobutyric acid (GABA) in the spinal dorsal horn. Recently, it was suggested that burst SCS does not act via spinal GABAergic mechanisms. Therefore, we studied spinal GABA release during burst and tonic SCS, both anatomically and pharmacologically, in a well-established chronic neuropathic pain model.

METHODS

Animals underwent partial sciatic nerve ligation (PSNL). Quantitative immunohistochemical (IHC) analysis of intracellular GABA levels in the lumbar L4 to L6 dorsal spinal cord was performed after 60 minutes of burst, tonic, or sham SCS in rats that had undergone PSNL (n = 16). In a second pharmacological experiment, the effects of intrathecal administration of the GABA_A antagonist bicuculline (5 μg) and the GABA_B antagonist phaclofen (5 μg) were assessed. Paw withdrawal thresholds to von Frey filaments of rats that had undergone PSNL (n = 20) were tested during 60 minutes of burst and tonic SCS 30 minutes after intrathecal administration of the drugs.

RESULTS

Quantitative IHC analysis of GABA immunoreactivity in spinal dorsal horn sections of animals that had received burst SCS (n = 5) showed significantly lower intracellular GABA levels when compared to sham SCS sections (n = 4; P = 0.0201) and tonic SCS sections (n = 7; P = 0.0077). Intrathecal application of the GABA_A antagonist bicuculline (5 μg; n = 10) or the GABA_B antagonist phaclofen (5 μg; n = 10) resulted in ablation of the analgesic effect for both burst SCS and tonic SCS.

CONCLUSIONS

In conclusion, our anatomical and pharmacological data demonstrate that, in this well-established chronic neuropathic animal model, the analgesic effects of both burst SCS and tonic SCS are mediated via spinal GABAergic mechanisms.

Mechanism of dorsal root ganglion stimulation for pain relief in painful diabetic polyneuropathy is not dependent on GABA release in the dorsal horn of the spinal cord

AIMS

It is hypothesized that dorsal root ganglion stimulation (DRGS), sharing some of the mechanisms of traditional spinal cord stimulation (SCS) of the dorsal columns, induces γ-aminobutyric acid (GABA) release from interneurons in the spinal dorsal horn.

METHODS

We used quantitative immunohistochemical analysis in order to investigate the effect of DRGS on intensity of intracellular GABA-staining levels in the L4-L6 spinal dorsal horn of painful diabetic polyneuropathy (PDPN) animals. To establish the maximal pain relieving effect, we tested for mechanical hypersensitivity to von Frey filaments and animals received 30 minutes of DRGS at day 3 after implantation of the electrode. One day later, 4 Sham-DRGS animals and four responders-to-DRGS received again 30 minutes of DRGS and were perfused at the peak of DRGS-induced pain relief.

RESULTS

No significant difference in GABA-immunoreactivity was observed between DRGS and Sham-DRGS in lamina 1-3 of the spinal levels L4-6 neither ipsilaterally nor contralaterally.

CONCLUSIONS

Dorsal root ganglion stimulation does not induce GABA release from the spinal dorsal horn cells, suggesting that the mechanisms underlying DRGS in pain relief are different from those of conventional SCS. The modulation of a GABA-mediated "Gate Control" in the DRG itself, functioning as a prime Gate of nociception, is suggested and discussed.

The Elevated Serum Level of IFN-γ in Patients with Failed Back Surgery Syndrome Remains Unchanged after Spinal Cord Stimulation

Objectives

We investigated the influence of spinal cord stimulation (SCS) on IFN-γ, IL-1β, IL-6, TNF-α, IL-10, and TGF-β serum levels in failed back surgery syndrome (FBSS) patients. The study will try to give new insights into the mechanism of SCS action and the role of IFN-γ and other cytokines in neuropathic pain (NP) development.

Materials and Methods

Clinical and biochemical assessment was conducted in four groups of patients: group 0 consisted of 24 FBSS patients qualified to SCS therapy, group 1 included 17 patients who were one month after implantation, group 2 featured 12 patients who were 3 months after the implantation, and group C (the control group) with no NP. Clinical status was assessed with the use of Numeric Rating Scale (NRS), the Pain Rating Index of McGill Pain Questionnaire (SF-MPQ), the Oswestry Disability Index (ODI), and Beck Depression Inventory (BDI). The plasma concentrations of IFN-γ were ascertained by an immunoenzymatic method.

Results

We found a significant difference between the patients before SCS and controls' serum level of IFN-γ. Similarly, a significantly higher level of TNF-α and significantly lower level of IL-10 in FBSS patients than controls were observed. The significant differences were not observed between SCS patients 3 months after the procedure and controls' serum level of IFN-γ and other cytokines. We noticed a positive correlation between IFN-γ concentration with NRS back value before SCS and positive correlation between IFN-γ concentration after SCS with NRS leg value before SCS. Higher IFN-γ concentrations accompanied higher NRS values. Levels of TGF-β and IL-10 may correlate with physical ability and depressive behavior.

Conclusions

SCS did not influence serum cytokine levels significantly. Serum concentration of IFN-γ may be recognized as an occasional pain factor because of its significantly higher level in FBSS patients versus controls and higher IFN-γ value accompanying higher pain intensity.

Dorsal Root Ganglion Stimulation in Experimental Painful Diabetic Peripheral Neuropathy: Burst vs. Conventional Stimulation Paradigm

Objectives

Painful diabetic peripheral neuropathy (PDPN) is a long‐term complication of diabetes mellitus (DM). Dorsal Root Ganglion Stimulation (DRGS) has recently emerged as a neuromodulation modality in the treatment of chronic neuropathic pain. The objective of this study was to compare the effect of burst DRGS (Burst‐DRGS) and conventional DRGS (Con‐DRGS) in an experimental model of PDPN.

Materials and Methods

DM was induced in female Sprague–Dawley rats by intraperitoneal injection of streptozotocin (STZ, n = 48). Animals were tested for mechanical hypersensitivity (50% hind paw withdrawal threshold on Von Frey test) before, and 4 weeks after STZ injection. PDPN rats were then implanted with a unilateral bipolar lead at the L5 DRG (n = 22) and were stimulated for 30 min at days 2 and 3 postimplantation. Animals received Con‐DRGS and Burst‐DRGS in a randomized crossover design (n = 10), or received Sham‐DRGS (n = 7) for 30 min, and were tested for mechanical hypersensitivity at baseline, 15 and 30 min during DRGS, and 15 and 30 min following DRGS. Five animals were withdrawn from the study due to electrode‐related technical problems.

Results

Con‐DRGS and Burst‐DRGS normalized STZ‐induced mechanical hypersensitivity at 15 and 30 min during stimulation. A significant difference in terms of mechanical hypersensitivity was observed between both of the stimulated groups and the Sham‐DRGS group at 15 and 30 min during stimulation. Interestingly, Burst‐DRGS showed signs of a residual effect at 15 min after cessation of stimulation, while this was not the case for Con‐DRGS.

Conclusions

Under the conditions tested, Con‐DRGS and Burst‐DRGS are equally effective in attenuating STZ‐induced mechanical hypersensitivity in an animal model of PDPN. Burst‐DRGS showed signs of a residual effect at 15 min after cessation of stimulation, which requires further investigation.

Dexmedetomidine attenuates persistent postsurgical pain by upregulating K+-Cl- cotransporter-2 in the spinal dorsal horn in rats

Background

Dexmedetomidine (DEX) could have an analgesic effect on pain transmission through the modulation of brain-derived neurotrophic factor (BDNF). In addition, KCC2-induced shift in neuronal Cl− homeostasis is crucial for postsynaptic inhibition mediated by GABAA receptors. Accumulating evidence shows that nerve injury, peripheral inflammation and stress activate the spinal BDNF/TrkB signal, which results in the downregulation of KCC2 transport and expression, eventually leads to GAGAergic disinhibition and hyperalgesia. The aim of this experiment was to explore the interaction between DEX and KCC2 at a molecular level in rats in the persistent postsurgical pain (PPSP).

Methods

PPSP in rats was evoked by the skin/muscle incision and retraction (SMIR). Mechanical hypersensitivity was assessed with the Dynamic Plantar Aesthesiometer. Western blot and immunofluorescence assay were used to assess the expressions of related proteins.

Results

In the first part of our experiment, the results revealed that the BDNF/TrkB-KCC2 signal plays a critical role in the development of SMIR-evoked PPSP; the second part showed that intraperitoneal administrations of 40 µg/kg DEX at 15 min presurgery and 1 to 3 days post-surgery significantly attenuated SMIR-evoked PPSP. Simultaneously, SMIR-induced KCC2 downregulation was partly reversed, which coincided with the inhibition of the BDNF/TrkB signal in the spinal dorsal horn. Moreover, intrathecal administrations of KCC2 inhibitor VU0240551 significantly reduced the analgesic effect of DEX on SMIR-evoked PPSP.

Conclusion

The results of our study indicated that DEX attenuated PPSP by restoring KCC2 function through reducing BDNF/TrkB signal in the spinal dorsal horn in rats, which provides a new insight into the treatment of chronic pain in clinical postsurgical pain management.

Long-Term Spinal Cord Stimulation Alleviates Mechanical Hypersensitivity and Increases Peripheral Cutaneous Blood Perfusion in Experimental Painful Diabetic Polyneuropathy

Objectives

This study utilizes a model of long‐term spinal cord stimulation (SCS) in experimental painful diabetic polyneuropathy (PDPN) to investigate the behavioral response during and after four weeks of SCS (12 hours/day). Second, we investigated the effect of long‐term SCS on peripheral cutaneous blood perfusion in experimental PDPN.

Methods

Mechanical sensitivity was assessed in streptozotocin induced diabetic rats (n = 50) with von Frey analysis. Hypersensitive rats (n = 24) were implanted with an internal SCS battery, coupled to an SCS electrode covering spinal levels L2–L5. The effects of four weeks of daily conventional SCS for 12 hours (n = 12) or Sham SCS (n = 12) were evaluated with von Frey assessment, and laser Doppler imaging (LDI).

Results

Average paw withdrawal thresholds (PWT) increased during long‐term SCS in the SCS group, in contrast to a decrease in the Sham group (Sham vs. SCS; p = 0.029). Twenty‐four hours after long‐term SCS average PWT remained higher in the SCS group. Furthermore, the SCS group showed a higher cutaneous blood perfusion during long‐term SCS compared to the Sham group (Sham vs. SCS; p = 0.048). Forty‐eight hours after long‐term SCS, no differences in skin perfusion were observed.

Discussion

We demonstrated that long‐term SCS results in decreased baseline mechanical hypersensitivity and results in increased peripheral blood perfusion during stimulation in a rat model of PDPN. Together, these findings indicate that long‐term SCS results in modulation of the physiological circuitry related to the nociceptive system in addition to symptomatic treatment of painful symptoms.

Activation of spinal alpha-7 nicotinic acetylcholine receptor shortens the duration of remifentanil-induced postoperative hyperalgesia by upregulating KCC2 in the spinal dorsal horn in rats

Background Accumulating evidence has shown that the signal from spinal brain-derived neurotrophic factor/tyrosine receptor kinase B-K+-Cl- cotransporter-2 plays a critical role in the process of pain hypersensitivity. The activation of alpha-7 nicotinic acetylcholine receptors could have an analgesic effect on remifentanil-induced postoperative hyperalgesia. Nevertheless, whether intrathecal administration of PNU-120596, an alpha-7 nicotinic acetylcholine receptors selective type II positive allosteric modulator, before surgery could affect the duration of remifentanil-induced postoperative hyperalgesia remains unknown, and the effects of alpha-7 nicotinic acetylcholine receptors activation on the brain-derived neurotrophic factor/tyrosine receptor kinase B-K+-Cl- cotransporter-2 signal in the spinal dorsal horn of rats with remifentanil-induced postoperative hyperalgesia is still enigmatic. Results We demonstrated that the brain-derived neurotrophic factor/tyrosine receptor kinase B-K+-Cl- cotransporter-2 signal played a critical role in the development of remifentanil-induced postoperative hyperalgesia. Intrathecal administration of PNU-120596 (8 µg/kg, 15 min before surgery) was associated with earlier signs of recovery from remifentanil-induced postoperative hyperalgesia. Simultaneously, remifentanil-induced postoperative hyperalgesia-induced K+-Cl- cotransporter-2 downregulation was partly reversed and coincided with a decreased expression of brain-derived neurotrophic factor/tyrosine receptor kinase B in the spinal dorsal horn, approximately correlating with the time course of the nociceptive behavior. Moreover, intrathecal administration of the K+-Cl- cotransporter-2 inhibitor VU0240551 significantly reduced the analgesic effect of PNU-120596 on remifentanil-induced postoperative hyperalgesia. Conclusions The activation of alpha-7 nicotinic acetylcholine receptors induced a shorter duration of remifentanil-induced postoperative hyperalgesia by restoring the brain-derived neurotrophic factor/tyrosine receptor kinase B-K+-Cl- cotransporter-2 signal in the spinal dorsal horn of rats, which provides new insight into treatment in clinical postoperative pain management.

6-Methoxyflavanone attenuates mechanical allodynia and vulvodynia in the streptozotocin-induced diabetic neuropathic pain

BACKGROUND

Diabetic neuropathy is the most prevalent, persistent and debilitating complication of diabetes mellitus often coupled with vulvodynia that may present as an isolated symptom or as a part of constellation of other neuropathic abnormalities.

OBJECTIVE

Flavonoids have selective affinity for GABA receptors and 6-methoxyflavanone (6-MeOF) is a positive allosteric modulator of GABA responses at human recombinant GABA_A receptors. GABAergic and opioidergic system inhibition have been shown to facilitate neuropathic pain.

METHODS

6-MeOF was evaluated for analgesic effect in the hot plate test and streptozotocin-induced diabetic neuropathic pain in female rats using von Frey hairs. The possible involvement of opioidergic and GABAergic mechanisms was investigated using naloxone and pentylenetetrazole (PTZ) antagonists, respectively. The biodistribution of 6-MeOF in plasma and CNS was examined using a validated HPLC/UV analytical method. The binding affinity of 6-MeOF with opioid and GABA receptors was studied using molecular docking simulation approach.

RESULTS

6-MeOF (10 and 30mg/kg) attenuated the acute phasic thermal nociception in the hot plate test while in the case of streptozotocin-induced diabetic neuropathy model, 6-MeOF (10 and 30mg/kg) produced static/dynamic anti-allodynic (increased paw withdrawal threshold and latency) as well as static/dynamic anti-vulvodynic effects (increased flinching response threshold and latency), when compared to the vehicle and standard gabapentin (75mg/kg). In silico studies depicted the preference of 6-MeOF for the delta- and kappa-opioid and GABA_A receptors. Moreover, the pharmacokinetic profile revealed a quick appearance of 6-MeOF in the systemic circulation and brain areas with maximum concentration observed after 30min in the amygdala, brain stem and cerebral cortex.

CONCLUSION

6-MeOF readily crosses the blood brain barrier and may be effective in attenuating the diabetes-induced allodynia as well as vulvodynia, probably through interactions with the GABAergic and opioidergic systems.

Neurochemistry study of spinal cord in non-human primate (Sapajus spp.)

The spinal cord is involved in local, ascending and descending neural pathways. Few studies analyzed the distribution of neuromediators in the laminae of non-human primates along all segments. The present study described the classic neuromediators in the spinal cord of the non-human primate Sapajus spp. through histochemical and immunohistochemical methods. Nicotinamide adenine dinucleotide hydrogen phosphate-diaphorase (NADPH-d) method showed neuronal somata in the intermediolateral column (IML), central cervical nucleus (CCN), laminae I, II, III, IV, V, VI, VII, VIII and X, besides dense presence of nerve fibers in laminae II and IX. Acetylcholinesterase (AChE) activity was evident in the neuronal somata in laminae V, VI, VII, VIII, IX, CCN, IML and in the Clarke’s column (CC). Immunohistochemistry data revealed neuronal nitric oxide synthase (nNOS) immunoreactivity in neuronal somata and in fibers of laminae I, II, III, VII, VIII, X and IML; choline acetyltransferase (ChAT) in neuronal somata and in fibers of laminae VII, VIII and IX; calcitonin gene-related peptide (CGRP) was noticed in neuronal somata of lamina IX and in nerve fibers of laminae I, II, III, IV, V, VI and VII; substance P (SP) in nerve fibers of laminae I, II, III, IV, V, VI, VII, VIII, IX, X, CCN, CC and IML; serotonin (5-HT) and vesicular glutamate transporter-1 (VGLUT1) was noticed in nerve fibers of all laminae; somatostatin (SOM) in neuronal somata of laminae III, IV, V, VI, VII, VIII and IX and nerve fibers in laminae I, II, V, VI, VII, X and IML; calbindin (Cb) in neuronal somata of laminae I, II, VI, VII, IX and X; parvalbumin (PV) was found in neuronal somata and in nerve fibers of laminae III, IV, V, VI, VII, VIII, IX and CC; finally, gamma-amino butyric acid (GABA) was present in neuronal somata of laminae V, VI, VII, VIII, IX and X. This study revealed interesting results concerning the chemoarchitecture of the Sapajus spp. spinal cord with a distribution pattern mostly similar to other mammals. The data corroborate the result described in literature, except for some differences in CGRP, SP, Cb, PV and GABA immunoreactivities present in neuronal somata and in nerve fibers. This could suggest certain specificity for the neurochemistry distribution in this non-human primate species, besides adding relevant data to support further studies related to processes involving spinal cord components.

Genomics of the Effect of Spinal Cord Stimulation on an Animal Model of Neuropathic Pain

BACKGROUND

Few studies have evaluated single-gene changes modulated by spinal cord stimulation (SCS), providing a narrow understanding of molecular changes. Genomics allows for a robust analysis of holistic gene changes in response to stimulation.

METHODS

Rats were randomized into six groups to determine the effect of continuous SCS in uninjured and spared-nerve injury (SNI) animals. After behavioral assessment, tissues from the dorsal quadrant of the spinal cord (SC) and dorsal root ganglion (DRG) underwent full-genome microarray analyses. Weighted Gene Correlation Network Analysis (WGCNA), and Gene Ontology (GO) analysis identified similar expression patterns, molecular functions and biological processes for significant genes.

RESULTS

Microarray analyses reported 20,985 gene probes in SC and 19,104 in DRG. WGCNA sorted 7449 SC and 4275 DRG gene probes into 29 and 9 modules, respectively. WGCNA provided significant modules from paired comparisons of experimental groups. GO analyses reported significant biological processes influenced by injury, as well as the presence of an electric field. The genes Tlr2, Cxcl16, and Cd68 were used to further validate the microarray based on significant response to SCS in SNI animals. They were up-regulated in the SC while both Tlr2 and Cd68 were up-regulated in the DRG.

CONCLUSIONS

The process described provides highly significant interconnected genes and pathways responsive to injury and/or electric field in the SC and DRG. Genes in the SC respond significantly to the SCS in both injured and uninjured animals, while those in the DRG significantly responded to injury, and SCS in injured animals.

Burst and Tonic Spinal Cord Stimulation Differentially Activate GABAergic Mechanisms to Attenuate Pain in a Rat Model of Cervical Radiculopathy

OBJECTIVE

Spinal cord stimulation (SCS) is widely used to treat neuropathic pain. Burst SCS, an alternative mode of stimulation, reduces neuropathic pain without paresthesia. However, the effects and mechanisms of burst SCS have not been compared to conventional tonic SCS in controlled investigations. This study compares the attenuation of spinal neuronal activity and tactile allodynia, and the role of γ-aminobutyric acid (GABA) signaling during burst or tonic SCS in a rat model of cervical radiculopathy.

METHODS

The effects of burst and tonic SCS were compared by recording neuronal firing before and after each mode of stimulation at day 7 following a painful cervical nerve root compression. Neuronal firing was also recorded before and after burst and tonic SCS in the presence of the GABAB receptor antagonist, CGP35348.

RESULTS

Burst and tonic SCS both reduce neuronal firing. The effect of tonic SCS, but not burst SCS, is blocked by CGP35348. In a separate study, spinal cord stimulators were implanted to deliver burst or tonic SCS beginning on day 4 after painful nerve root compression; allodynia and serum GABA concentration were measured through day 14. Burst and tonic SCS both reduce allodynia. Tonic SCS attenuates injury-induced decreases in serum GABA, but GABA remains decreased from baseline during burst SCS.

CONCLUSION AND SIGNIFICANCE

Together, these studies suggest that burst SCS does not act via spinal GABAergic mechanisms, despite its attenuation of spinal hyperexcitability and allodynia similar to that of tonic SCS; understanding other potential spinal inhibitory mechanisms may lead to enhanced analgesia during burst stimulation.

Cerebrospinal fluid levels of glial cell-derived neurotrophic factor correlate with spinal cord stimulation frequency in patients with neuropathic pain: a preliminary report

STUDY DESIGN

Case series.

OBJECTIVES

To evaluate relationships between spinal cord stimulation (SCS) parameters and levels of glial cell-derived neurotrophic factor (GDNF).

SETTING

Ambulatory pain clinic of St James's Hospital, Dublin, Ireland.

METHODS

Nine patients with an implanted SCS and Failed Back Surgery Syndrome (FBSS) were administered the Brief Pain Inventory and Short Form (36) Health Survey. Following a lumbar puncture, levels of GDNF in cerebrospinal fluid (CSF) were assayed and correlated with stimulation parameters. Controls were patients with arthritic back pain who were matched for age, gender and SF-36 score.

RESULTS

Concentrations of GDNF in CSF are higher in patients with FBSS than controls (P=0.002) and correlate with SCS frequency (P=0.029).

CONCLUSION

Concentrations of GDNF in CSF are higher in neuropathic pain and appear to be related to stimulation frequency. Further work is needed to evaluate this potential relationship, both in neuropathic pain and in other contexts such as locomotor dysfunction.

Analgesic mechanism of electroacupuncture in a rat L5 spinal nerve ligation model

The aim of this study was to investigate the analgesic mechanism of electroacupuncture (EA) in the treatment of neuropathological pain. A total of 60 Sprague-Dawley rats were randomly divided into three groups, namely the spinal nerve ligation (SNL), electroacupuncture (SNL + EA) and normal control groups, with 20 rats in each group. The up-down method was used to determine the bipedal 50% mechanical paw withdrawal threshold (PWT). The ultrastructure of the injured-side L5 nerve root (n=6) was observed by electron microscopy. The mRNA levels of brain-derived neurotrophic factor (BDNF) and purinergic receptor P2X, ligand-gated ion channel 4 (P2X4) in the spinal cord (n=14) were determined by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The postoperative PWT of the injured-side hindpaw in the SNL group at each time point was lower than that in the control group (P<0.01); there were differences of statistical significance between the PWT values of the SNL + EA and SNL groups on postoperative days 14 and 21 (P<0.05). Postoperatively, the PWT of the hindpaw on the uninjured-side was significantly lower in the SNL group when compared with that of the control group on days 10, 14 and 21 (P<0.05). Following the EA treatment, axonal demyelination was reduced and vascular proliferation was observed within the visual field. In addition, following the EA treatment, BDNF expression levels in the spinal dorsal horn increased (P<0.05), while the expression of P2X4 was not different from that in the SNL group. EA exerted an analgesic effect on the SNL model in a time-dependent manner, and improved the blood supply to the nerve root. Following the EA treatment, the expression of P2X4 did not change significantly compared with that in the SNL group, whereas the spinal secretion of BDNF increased. However, the exact mechanism requires further study.

The role of the dorsolateral funiculi in the pain relieving effect of spinal cord stimulation: a study in a rat model of neuropathic pain

Activation of the dorsal columns is relayed to supraspinal centers, involved in pain modulation, probably via the descending fibers in the dorsolateral funiculi (DLF). The present study examines the role of the DLF in the attenuation of pain-related signs by spinal cord stimulation (SCS). Several groups of rats were subjected to nerve injury and to chronic bilateral DLF lesions at C5-7 level. In each animal, two sets of miniature electrodes were implanted, a caudal system placed in the dorsal epidural space at low thoracic level and another implanted over the dorsal column nuclei, rostral to the lesions. Stimulation (50 Hz, 0.2 ms; 70 % of motor threshold) was applied for 5 min via either of the electrodes. Behavioral tests were used to assess the effects of SCS on the nerve injury-induced mechanical and cold hypersensitivity and heat hyperalgesia. Prior to application of SCS, antagonists to either of GABAA or B, 5-HT1 or 1-2 or α/β-adrenergic receptors were injected i.p. Both stimulations produced comparable decreases (80-90 % of the control) of neuropathic manifestations in rats with intact spinal cords. DLF lesions attenuated the effects of both types of stimulation by about 50 %. Pretreatment with receptor antagonists differentially counteracted the effects of rostral and caudal stimulation; the inhibition with rostral stimulation generally being more prominently influenced. These results provide further support to the notion of important involvement of brainstem pain modulating centers in the effects of SCS. A major component of the inhibitory spinal-supraspinal-spinal loop is mediated by fibers running in the DLF.

Spinal autofluorescent flavoprotein imaging in a rat model of nerve injury-induced pain and the effect of spinal cord stimulation

Nerve injury may cause neuropathic pain, which involves hyperexcitability of spinal dorsal horn neurons. The mechanisms of action of spinal cord stimulation (SCS), an established treatment for intractable neuropathic pain, are only partially understood. We used Autofluorescent Flavoprotein Imaging (AFI) to study changes in spinal dorsal horn metabolic activity. In the Seltzer model of nerve-injury induced pain, hypersensitivity was confirmed using the von Frey and hotplate test. 14 Days after nerve-injury, rats were anesthetized, a bipolar electrode was placed around the affected sciatic nerve and the spinal cord was exposed by a laminectomy at T13. AFI recordings were obtained in neuropathic rats and a control group of naïve rats following 10 seconds of electrical stimulation of the sciatic nerve at C-fiber strength, or following non-noxious palpation. Neuropathic rats were then treated with 30 minutes of SCS or sham stimulation and AFI recordings were obtained for up to 60 minutes after cessation of SCS/sham. Although AFI responses to noxious electrical stimulation were similar in neuropathic and naïve rats, only neuropathic rats demonstrated an AFI-response to palpation. Secondly, an immediate, short-lasting, but strong reduction in AFI intensity and area of excitation occurred following SCS, but not following sham stimulation. Our data confirm that AFI can be used to directly visualize changes in spinal metabolic activity following nerve injury and they imply that SCS acts through rapid modulation of nociceptive processing at the spinal level.

Modeling effects of spinal cord stimulation on wide-dynamic range dorsal horn neurons: influence of stimulation frequency and GABAergic inhibition

Spinal cord stimulation (SCS) is a clinical therapy for chronic, neuropathic pain, but an incomplete understanding of the mechanisms underlying SCS contributes to the lack of improvement in SCS efficacy over time. To study the mechanisms underlying SCS, we constructed a biophysically based network model of the dorsal horn circuit consisting of interconnected dorsal horn interneurons and a wide-dynamic range (WDR) projection neuron and representations of both local and surround receptive field inhibition. We validated the network model by reproducing cellular and network responses relevant to pain processing including wind-up, A fiber-mediated inhibition, and surround receptive field inhibition. We then simulated the effects of SCS on the activity of the WDR projection neuron and found that the response of the model WDR neuron to SCS depends on the SCS frequency; SCS frequencies of 30–100 Hz maximally inhibited the model WDR neuron, while frequencies under 30 Hz and over 100 Hz excited the model WDR neuron. We also studied the impacts on the effects of SCS of loss of inhibition due to the loss of either GABA or KCC2 function. Reducing the influence of local and surround GABAergic interneurons by weakening their inputs or their connections to the WDR neuron and shifting the anionic reversal potential of the WDR neurons upward each reduced the range of optimal SCS frequencies and changed the frequency at which SCS had a maximal effect. The results of this study provide insights into the mechanisms of SCS and pave the way for improved SCS parameter selection.

Mechanisms and models of spinal cord stimulation for the treatment of neuropathic pain

Spinal cord stimulation (SCS) is an established and cost-effective therapy for treating severe chronic pain. However, despite over 40 years of clinical practice and the development of novel electrode designs and treatment protocols, increases in clinical success, defined as the proportion of patients that experience 50% or greater self-reported pain relief, have stalled. An incomplete knowledge of the neural circuits and systems underlying chronic pain and the interaction of SCS with these circuits may underlie this plateau in clinical efficacy. This review summarizes prior work and identifies gaps in our knowledge regarding the neural circuits related to pain and SCS in the dorsal horn, supraspinal structures, and the Pain Matrix. In addition, this review discusses and critiques current experimental and computational models used to investigate and optimize SCS. Further research into the interactions between SCS and pain pathways in the nervous system using animal and computational models is a fruitful approach to improve this promising therapy.

Differential effects on KCC2 expression and spasticity of ALS and traumatic injuries to motoneurons

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease manifested by progressive muscle atrophy and paralysis due to the loss of upper and lower motoneurons (MN). Spasticity appears in ALS patients leading to further disabling consequences. Loss of the inhibitory tone induced by downregulation of the potassium chloride cotransporter 2 (KCC2) in MN has been proposed to importantly contribute to the spastic behavior after spinal cord injury (SCI). The aim of the present study was to test whether the alterations in the expression of KCC2 are linked to the appearance of spasticity in the SOD^G93A ALS murine model. We compared SOD^G93A mice to wild type mice subjected to SCI to mimic the spinal MN disconnection from motor descending pathways, and to sciatic nerve lesion to mimic the loss of MN connectivity to muscle. Electrophysiological results show that loss of motor function is observed at presymptomatic stage (8 weeks) in SOD^G93A mice but hyperreflexia and spasticity do not appear until a late stage (16 weeks). However, KCC2 was not downregulated despite MN suffered disconnection both from muscles and upper MNs. Further experiments revealed decreased gephyrin expression, as a general marker of inhibitory systems, accompanied by a reduction in the number of Renshaw interneurons. Moreover, 5-HT fibers were increased in the ventral horn of the lumbar spinal cord at late stage of disease progression in SOD1^G93A mice. Taken together, the present results indicate that spasticity appears late in the ALS model, and may be mediated by a decrease in inhibitory interneurons and an increase of 5-HT transmission, while the absence of down-regulation of KCC2 could rather indicate an inability of MNs to respond to insults.

The effect of spinal cord stimulation frequency in experimental painful diabetic polyneuropathy

BACKGROUND

Spinal cord stimulation (SCS) has been shown to be an effective treatment for painful diabetic polyneuropathy (PDP). An increase of efficacy is needed since only 67% of patients benefit from SCS. This study aimed to develop an animal model for SCS in PDP and study the effect of various stimulation frequencies on the functional outcome. As the pathophysiology of PDP is complex, including vasoconstriction and nerve injury, the frequency of SCS may result in different outcomes.

METHODS

Diabetes mellitus was induced by an intraperitoneal injection of streptozotocin in 8-week-old female Sprague-Dawley rats (n=76; glucose >15 mmol/L; n=51). A SCS device was implanted at level Th13 4 weeks later. SCS of the dorsal columns was applied for 30 min and the effect on mechanical hypersensitivity was evaluated.

RESULTS

Mechanical hypersensitivity developed in 26 rats, which were included (low-frequency, n=6; mid-frequency, n=8; high frequency, n=9; and sham, n=3). SCS of the dorsal columns was applied for 40 min, and the effect on mechanical hypersensitivity was evaluated. In all treatment groups, SCS resulted in reversal of mechanical hypersensitivity and a clinically relevant reduction was achieved in 70% of animals. No differences in efficacy were found between the different treatment groups.

CONCLUSIONS

The pain-relieving effect of SCS in PDP was studied in an experimental model. Our study shows that SCS on mechanical hypersensitivity in PDP rats is equally effective when applied at low, mid and high frequency.