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

Spinal Cord Stimulation for Spinal Cord Injury-Related Pain: A Pilot Study

BACKGROUND

Spinal cord stimulation (SCS) has emerged as an effective treatment for managing chronic pain that is unresponsive to traditional therapies. While SCS is well documented for conditions like failed back surgery syndrome (FBSS) and complex regional pain syndrome (CRPS), its effectiveness in managing pain related to spinal cord injuries (SCI) is less studied. This study aims to assess the efficacy of SCS in alleviating SCI-related pain and improving patients' quality of life, filling a gap in the existing literature.

METHODS

This cohort study included 15 adult patients with traumatic and non-traumatic SCIs, treated between 2016 and 2022. Patients received SCS implants after either a trial or direct implantation. Pain levels were assessed using visual analog scale (VAS) scores, while quality of life was evaluated using the EuroQol five-dimensional (EQ-5D) scale. The SCS devices were implanted at different spinal levels, with various stimulation protocols applied, including high-frequency stimulation (10 kHz).

RESULTS

In patients with traumatic SCI, the mean VAS score decreased from 8.6 to 4.5, with 71% reporting more than 50% pain relief. Non-traumatic SCI patients experienced a reduction from 8.5 to 2.5, with all showing more than 50% pain relief. EQ-5D scores improved in both groups. A 49% reduction in pain medication usage was also observed, though one patient required revision surgery due to an adverse event.

CONCLUSIONS

SCS significantly reduces pain and improves quality of life for SCI patients, particularly with high-frequency protocols. While promising, further research is needed to optimize patient selection and stimulation parameters for better long-term outcomes.

Visualizing neuroinflammation with fluorescence and luminescent lanthanide-based in situ hybridization

BACKGROUND

Neurokine signaling via the release of neurally active cytokines arises from glial reactivity and is mechanistically implicated in central nervous system (CNS) pathologies such as chronic pain, trauma, neurodegenerative diseases, and complex psychiatric illnesses. Despite significant advancements in the methodologies used to conjugate, incorporate, and visualize fluorescent molecules, imaging of rare yet high potency events within the CNS is restricted by the low signal to noise ratio experienced within the CNS. The brain and spinal cord have high cellular autofluorescence, making the imaging of critical neurokine signaling and permissive transcriptional cellular events unreliable and difficult in many cases.

METHODS

In this manuscript, we developed a method for background-free imaging of the transcriptional events that precede neurokine signaling using targeted mRNA transcripts labeled with luminescent lanthanide chelates and imaged via time-gated microscopy. To provide examples of the usefulness this method can offer to the field, the mRNA expression of toll-like receptor 4 (TLR4) was visualized with traditional fluorescent in situ hybridization (FISH) or luminescent lanthanide chelate-based in situ hybridization (LISH) in mouse BV2 microglia or J774 macrophage phenotype cells following lipopolysaccharide stimulation. TLR4 mRNA staining using LISH- and FISH-based methods was also visualized in fixed spinal cord tissues from BALB/c mice with a chronic constriction model of neuropathic pain or a surgical sham model in order to demonstrate the application of this new methodology in CNS tissue samples.

RESULTS

Significant increases in TLR4 mRNA expression and autofluorescence were visualized over time in mouse BV2 microglia or mouse J774 macrophage phenotype cells following lipopolysaccharide (LPS) stimulation. When imaged in a background-free environment with LISH-based detection and time-gated microscopy, increased TLR4 mRNA was observed in BV2 microglia cells 4 h following LPS stimulation, which returned to near baseline levels by 24 h. Background-free imaging of mouse spinal cord tissues with LISH-based detection and time-gated microscopy demonstrated a high degree of regional TLR4 mRNA expression in BALB/c mice with a chronic constriction model of neuropathic pain compared to the surgical sham model.

CONCLUSIONS

Advantages offered by adopting this novel methodology for visualizing neurokine signaling with time-gated microscopy compared to traditional fluorescent microscopy are provided.

Spinal Cord Stimulation Modulates Gene Expression in the Spinal Cord of an Animal Model of Peripheral Nerve Injury

BACKGROUND AND OBJECTIVES

Previously, we found that application of pulsed radiofrequency to a peripheral nerve injury induces changes in key genes regulating nociception concurrent with alleviation of paw sensitivity in an animal model. In the current study, we evaluated such genes after applying spinal cord stimulation (SCS) therapy.

METHODS

Male Sprague-Dawley rats (n = 6 per group) were randomized into test and control groups. The spared nerve injury model was used to simulate a neuropathic pain state. A 4-contact microelectrode was implanted at the L1 vertebral level and SCS was applied continuously for 72 hours. Mechanical hyperalgesia was tested. Spinal cord tissues were collected and analyzed using real-time polymerase chain reaction to quantify levels of IL1β, GABAbr1, subP, Na/K ATPase, cFos, 5HT3ra, TNFα, Gal, VIP, NpY, IL6, GFAP, ITGAM, and BDNF.

RESULTS

Paw withdrawal thresholds significantly decreased in spared nerve injury animals and stimulation attenuated sensitivity within 24 hours (P = 0.049), remaining significant through 72 hours (P = 0.003). Nerve injury caused up-regulation of TNFα, GFAP, ITGAM, and cFOS as well as down-regulation of Na/K ATPase. Spinal cord stimulation therapy modulated the expression of 5HT3ra, cFOS, and GABAbr1. Strong inverse relationships in gene expression relative to the amount of applied current were observed for GABAbr1 (R = -0.65) and Na/K ATPase (R = -0.58), and a positive linear correlations between 5HT3r (R = 0.80) and VIP (R = 0.50) were observed.

CONCLUSIONS

Continuously applied SCS modulates expression of key genes involved in the regulation of neuronal membrane potential.

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.

The reduction of intraepidermal P2X3 nerve fiber density correlates with behavioral hyperalgesia in a rat model of nerve injury-induced pain

Skin biopsies from patients with neuropathic pain often show changes in epidermal innervation, although it remains to be elucidated to what extent such changes can be linked to a particular subgroup of nerve fibers and how these changes are correlated with pain intensity. Here, we investigated to what extent behavioral signs of hyperalgesia are correlated with immunohistochemical changes of peptidergic and non-peptidergic epidermal nerve fibers in a rat model of nerve injury-induced pain. Rats subjected to unilateral partial ligation of the sciatic nerve developed significant mechanical and thermal hyperalgesia as tested by the withdrawal responses of the ipsilateral footpad to von Frey hairs and hotplate stimulation. At day 14, epidermal nerve fiber density and total epidermal nerve fiber length/mm² were significantly and consistently reduced compared to the contralateral side, following testing and re-testing by two blinded observers. The expression of calcitonin gene-related peptide, a marker for peptidergic nerve fibers, was not significantly changed on the ipsilateral side. In contrast, the expression of the P2X₃ receptor, a marker for non-peptidergic nerve fibers, was not only significantly reduced but could also be correlated with behavioral hyperalgesia. When labeling both peptidergic and non-peptidergic nerve fibers with the pan-neuronal marker PGP9.5, the expression was significantly reduced, albeit without a significant correlation with behavioral hyperalgesia. In conjunction, our data suggest that the pathology of the P2X₃ epidermal nerve fibers can be selectively linked to neuropathy, highlighting the possibility that it is the degeneration of these fibers that drives hyperalgesia.

Probing pain pathways with light

We have witnessed an accelerated growth of photonics technologies in recent years to enable not only monitoring the activity of specific neurons, while animals are performing certain types of behavior, but also testing whether specific cells, circuits, and regions are sufficient or necessary for initiating, maintaining, or altering this or that behavior. Compared to other sensory systems, however, such as the visual or olfactory system, photonics applications in pain research are only beginning to emerge. One reason pain studies have lagged behind is that many of the techniques originally developed cannot be directly implemented to study key relay sites within pain pathways, such as the skin, dorsal root ganglia, spinal cord, and brainstem. This is due, in part, to difficulties in accessing these structures with light. Here we review a number of recent advances in design and delivery of light-sensitive molecular probes (sensors and actuators) into pain relay circuits to help decipher their structural and functional organization. We then discuss several challenges that have hampered hardware access to specific structures including light scattering, tissue movement and geometries. We review a number of strategies to circumvent these challenges, by delivering light into, and collecting it from the different key sites to unravel how nociceptive signals are encoded at each level of the neuraxis. We conclude with an outlook on novel imaging modalities for label-free chemical detection and opportunities for multimodal interrogation in vivo. While many challenges remain, these advances offer unprecedented opportunities to bridge cellular approaches with context-relevant behavioral testing, an essential step toward improving translation of basic research findings into clinical applications.