Burst and Tonic Spinal Cord Stimulation in the Mechanical Conflict-Avoidance System: Cognitive-Motivational Aspects

Spinal Cord Stimulation Paradigms and Alleviation of Neuropathic Pain Behavior in Experimental Painful Diabetic Polyneuropathy

OBJECTIVES

Spinal cord stimulation (SCS) is an alternative treatment option for painful diabetic polyneuropathy (PDPN). Differential target multiplexed (DTM)-SCS is proposed to be more effective than conventional (Con)-SCS. Animal studies are essential for understanding SCS mechanisms in PDPN pain relief. Although the Von Frey (VF) test is the gold standard for preclinical pain research, it has limitations. Operant testing using the conditioned place preference (CPP) test provides insights into spontaneous neuropathic pain relief and enhances the translatability of findings. This study aims to 1) use the CPP test to evaluate Con- and DTM-SCS effects on spontaneous neuropathic pain relief in PDPN animals and 2) investigate the correlation between mechanical hypersensitivity alleviation and spontaneous neuropathic pain relief.

MATERIAL AND METHODS

Diabetes was induced through streptozotocin injection in 32 rats; 16 animals developed PDPN and were implanted with a quadripolar lead. Rats were conditioned for Con-SCS (n = 8) or DTM-SCS (n = 7), and a preference score compared with sham was determined. After conditioning, a 30-minute SCS protocol was conducted. Mechanical sensitivity was assessed using VF before, during, and after SCS.

RESULTS

There were no significant chamber preference changes for DTM-SCS (p = 0.3449) or Con-SCS (p = 0.3632). Subgroups of responders and nonresponders were identified with significant increases in preference score for responders for both DTM-SCS (-266.6 to 119.8; p = 0.0238; n = 4) and Con-SCS (-350.7 to 88.46; p = 0.0148; n = 3). No strong correlation between SCS-induced spontaneous neuropathic pain relief and effects on mechanical hypersensitivity in PDPN animals is noted.

CONCLUSIONS

The CPP test is a valuable tool to test the efficacy of the pain-relieving potential of various SCS paradigms in PDPN animals. The results of this study show no differences in spontaneous neuropathic pain relief between DTM- and Con-SCS in PDPN animals. Furthermore, there is no correlation between the effect of SCS in spontaneous pain relief and hind paw mechanical hypersensitivity.

Ninety-Hz Spinal Cord Stimulation-Induced Analgesia Is Dependent on Active Charge Balance and Is Nonlinearly Related to Amplitude: A Sham-Controlled Behavioral Study in a Rodent Model of Chronic Neuropathic Pain

BACKGROUND

Ninety-Hz active-recharge spinal cord stimulation (SCS) applied at below sensory-threshold intensity, as used with fast-acting subperception therapy spinal cord stimulation, has been shown clinically to produce significant analgesia, but additional characterization is required to better understand the therapy. This preclinical study investigates the behavioral effect of multiple 90-Hz SCS variants in a rodent model of neuropathic pain, focusing on charge balance and the relationship between 90-Hz efficacy and stimulation intensity.

MATERIALS AND METHODS

Rats (n = 24) received a unilateral partial sciatic nerve ligation to induce neuropathic pain and were implanted with a quadripolar lead at T13. Mechanical hypersensitivity was assessed before, during, and after 60 minutes of SCS. After a prescreen with 50-Hz SCS 67% motor threshold ([MT], the positive control), rats underwent a randomized-crossover study including sham SCS and several 90-Hz SCS paradigms (at 40% MT or 60% MT, either using active or pseudopassive recharge) (experiment 1, n = 16). A second, identical experiment (experiment 2) was performed to supplement data with 90-Hz SCS at 20% and 80% MT (experiment 2, n = 8).

RESULTS

Experiment 1: At 40% MT, 90-Hz active-recharge SCS produced a significantly larger recovery to baseline than did 90-Hz pseudopassive SCS at both tested intensities and sham SCS. Experiment 2: Only the 90-Hz SCS active recharge at 40% MT and 50-Hz SCS positive control caused mean recovery to baseline that was statistically better than that of sham SCS.

CONCLUSIONS

The degree to which 90-Hz SCS reduced mechanical hypersensitivity during stimulation depended on the nature of charge balance, with 90-Hz active-recharge SCS generating better responses than did 90-Hz pseudopassive recharge SCS. In addition, our findings suggest that the amplitude of 90-Hz active-recharge SCS must be carefully configured for efficacy.

Epidural Laterality and Pain Relief With Burst Spinal Cord Stimulation

INTRODUCTION

Burst spinal cord stimulation (SCS) can achieve excellent clinical reduction of pain, alongside improvements in function, quality of life, and related outcomes. Good outcomes likely depend on good lead placement, thereby enabling recruitment of the relevant neural targets. Several competing approaches exist for lead implantation, such as the use of single vs bilateral leads and leads lateralized vs placed at midline. The objective of this study was to examine the relationship between paresthesia locations and pain relief with burst SCS in a prospective double-blind crossover design.

MATERIALS AND METHODS

All participants had bilateral back and leg pain, with more intense pain experienced on one side of the body. A trial SCS system was placed, during which brief intraoperative mapping with conventional stimulation was used to characterize paresthesia locations. Two programs for subperception burst SCS treatment were then applied for two days each, in random order: bilateral paresthesia coverage vs unilateral paresthesia coverage contralateral to the side of the body with more intense pain. Pain ratings (visual analog scale [VAS]) and pain reductions (scaling pain relief [SPR]) were reported for each.

RESULTS

Of the 30 participants who completed the study, 24 (80%) had good pain relief with at least one program. A baseline VAS score of 8.75 was reduced to 5.98 with contralateral stimulation and to 2.88 with bilateral stimulation; with SPR, this equated to 31.25% and 67.50% improvement, respectively. The incremental benefit of bilateral stimulation over contralateral stimulation was statistically significant (p < 0.001). Of the 24 participants, 87.5% preferred bilateral stimulation, whereas 12.5% preferred unilateral stimulation. The six participants who failed the trial had no preference.

DISCUSSION

When burst stimulation is delivered to spinal targets that can generate paresthesias contralateral to the side of worst pain, suboptimal therapy is achieved. Thus, attention to laterality and pain coverage is critical for successful therapy, and it may be important to carefully consider lead implantation techniques.

Intrathecal non-viral interleukin-10 gene therapy ameliorates neuropathic pain as measured by both classical static allodynia and a novel supra-spinally mediated pain assay, the Two-Arm Rodent Somatosensory (TARS) task

Intrathecal delivery of interleukin-10 (IL-10) gene therapy has been reported to be effective in suppressing pain enhancement in a variety of rodent models. However, all publications that have tested this treatment have relied upon measures of static allodynia (von Frey test) and thermal hyperalgesia (Hargreaves test). As this plasmid DNA IL-10 (pDNA-IL10) therapeutic approach is now in human clinical trials for multiple pain indications, including intrathecal delivery for human neuropathic pain, it is important to consider the recent concerns raised in the pain field that such tests reflect spinal rather than supraspinal processing of, and responsivity to, noxious stimuli. Consequently, this raises the question of whether intrathecal pDNA-IL10 can reverse established neuropathic pain when assessed by a test requiring supraspinal, rather than solely spinal, mediation of the behavioral response. The present study utilizes the rat sciatic chronic constriction injury (CCI) model of neuropathic pain to compare the expression of static allodynia with that of cognitively controlled choice behavior in a two-arm maze, adapted from Hayashida et al. (2019). This modification, termed the Two-Arm Rodent Somatosensory (TARS) task, provides rats free choice to reach a desired goal box via a short "arm" of the maze with tactile probes as flooring versus a longer "arm" of the maze with a smooth surface. Here we demonstrate that static allodynia and avoidance of the nociceptive flooring in TARS develop in parallel over time, and that both behaviors also resolve in parallel following intrathecal pDNA-IL10 gene therapy. Details for the construction and use of this new maze design are also provided. Together, this study documents both: (a) the important finding that intrathecal IL-10 gene therapy does indeed resolve neuropathic pain as measured by a supraspinally-mediated behavioral task, and (b) a new, supraspinally-mediated task that allows behavioral assessments across weeks and allows the analysis of both development and resolution of neuropathic pain by therapeutic interventions. As such, the TARS operant behavior task is an improvement over other approaches such as the mechanical conflict-avoidance system which have difficulties demonstrating development and reversal of pain behavior in a within-subject design.

Conventional, high frequency and differential targeted multiplexed spinal cord stimulation in experimental painful diabetic peripheral neuropathy: Pain behavior and role of the central inflammatory balance

Spinal cord stimulation (SCS) is a last resort treatment for pain relief in painful diabetic peripheral neuropathy (PDPN) patients. However, the effectivity of SCS in PDPN is limited. New SCS paradigms such as high frequency (HF) and differential target multiplexed (DTM) might improve responder rates and efficacy of SCS-induced analgesia in PDPN patients, and are suggested to modulate the inflammatory balance and glial response in the spinal dorsal horn. The aim of this study was to research the effects of Con-, HF- and DTM-SCS on pain behavior and the spinal inflammatory balance in an animal model of PDPN. Streptozotocin-induced PDPN animals were stimulated for 48 hours with either Con-SCS (50Hz), HF-SCS (1200Hz) or DTM-SCS (combination of Con- and HF-SCS). Mechanical hypersensitivity was assessed using Von Frey (VF) test and the motivational aspects of pain were assessed using the mechanical conflict avoidance system (MCAS). The inflammatory balance and glial response were analyzed in the dorsal spinal cord based on RNA expression of pro- and anti-inflammatory cytokines (Tnf-α, Il-1ß, Il-4, Il-10), a microglia marker (Itgam), an astrocyte marker (Gfap), a T-cell marker (Cd3d), microglia proliferation markers (Irf8, Adgre1) and P2X4, p13-MAPK, BDNF signaling markers (P2x4, Mapk14, Bdnf). The results show that Con-, HF-, and DTM-SCS significantly decreased hypersensitivity after 48 hours of stimulation compared to Sham-SCS in PDPN animals, but at the same time did not affect escape latency in the MCAS. At the molecular level, Con-SCS resulted in a significant increase in spinal pro-inflammatory cytokine Tnf-α after 48 hours compared to DTM-SCS and Sham-SCS. In summary, Con-SCS showed a shift of the inflammatory balance towards a pro-inflammatory state whilst HF- and DTM-SCS shifted the balance towards an anti-inflammatory state. These findings suggest that the underlying mechanism of Con-SCS induced pain relief in PDPN differs from that induced by HF- and DTM-SCS.

Mammalian Sterile 20-Like Kinase 1 Mediates Neuropathic Pain Associated with Its Effects on Regulating Mitophagy in Schwann Cells

Myelin degradation initiated by Schwann cells (SCs) after nerve injury is connected to the induction and chronicity of neuropathic pain (NP). Mitophagy, a selective clearance of damaged mitochondria via autophagy, contributes to the maintenance of normal function in SCs. Mitochondrial function and mitophagy activity are highly modulated by mammalian ste20-like kinase1 (Mst1). However, whether Mst1 can regulate mitophagy in SCs to play a role in NP remains poorly understood. In the present study, Sprague-Dawley rats were subjected to chronic constriction injury (CCI) on the sciatic nerve to induce NP. Small interfering RNA of Mst1 was applied to the injured sciatic nerve to knockdown Mst1. Behavioral tests were performed to evaluate NP, and myelin degeneration was assessed by transmission electron microscope and immunofluorescence. Autophagy and mitophagy were detected in the injured sciatic nerve and cultured SCs (RSC96 cells) by Western blot. ROS level, mitochondria membrane potential, and apoptosis were assessed in vitro via flow cytometry and Western blot. Mst1 knockdown alleviated mechanical allodynia and thermal hyperalgesia in the CCI-induced NP model and rescued myelin degeneration of the injured nerve. Meanwhile, CCI-increased levels of Parkin and p62 were reversed by Mst1 knockdown. In vitro RSC96 cells were subjected to starvation to induce mitophagy. Protein levels of mitochondrial Parkin and mitochondrial p62 significantly increased after Mst1 knockdown, while those in the cytosol diminished indicate that the translocation of Parkin and p62 from the cytosol to the mitochondria was promoted by the knockdown of Mst1. In addition, Mst1 knockdown reduced ROS level and apoptosis activity, while enhancing mitochondria membrane potential in RSC96 cells. The study showed that Mst1 knockdown alleviated CCI-induced NP, associated with enhanced Parkin recruitment to mitochondria and subsequent mitophagy degradation, thus preserving mitochondrial function and myelin integrity.

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.

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.

All bursts are equal, but some are more equal (to burst firing): burstDR stimulation versus Boston burst stimulation

Introduction: Since the introduction of burst spinal cord stimulation for neuropathic pain, several companies have developed their own version of burst stimulation, which is confusing the marketplace and clinicians of what burst stimulation truly is, the value and utilization of the therapy.Areas covered: We review those two burst stimulation designs and notice important differences. The original burstDR^TM stimulation tries to mimic physiologic burst firing, which involves closely spaced high frequency sodium spikes nested on a calcium mediated plateau. This is realized by generating a train of 5 monophasic spikes of increasing amplitude with passive charge balance after the last spike, in contrast to the other burst designs which involve a version of cycling 4-5 spikes each being individually actively charge balanced spikes.Expert opinion: Based on the neurobiology of burst firing as well as abductive reasoning we like to clarify that burstDR^TM is a true physiologic burst stimulation, and that other versions being called burst stimulation are essentially clustered tonic stimulation. This differentiating terminology will prevent confusion for healthcare providers, regulators, and the marketplace of what burst stimulation is.

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.