Pulsed Radiofrequency Upregulates Serotonin Transporters and Alleviates Neuropathic Pain-Induced Depression in a Spared Nerve Injury Rat Model

Pulsed radiofrequency on DRG inhibits hippocampal neuroinflammation by regulating spinal GRK2/p38 expression and enhances spinal autophagy to reduce pain and depression in male rats with spared nerve injury

Evidence indicates that microglial G protein-coupled receptor kinase 2 (GRK2) is a key regulator of the transition from acute to chronic pain mediated by microglial products via the p38 mitogen-activated protein kinase (MAPK) pathway in the spinal cord dorsal horn (SCDH). Increasing studies have shown that autophagic dysfunction in the SCDH and neuroinflammation in the hippocampus underlie NeP. However, whether GRK2/p38MAPK and autophagic flux in the SCDH and hippocampal neuroinflammation are involved in NeP and depression comorbidity has not been determined. Here, we explored the effects of high-voltage pulsed radiofrequency (PRF) (85 V-PRF; HV-PRF) to the dorsal root ganglion (DRG) on pain phenotypes in Wistar male rats with spared nerve injury (SNI) and the underlying mechanisms. The exacerbation of pain phenotypes was markedly relieved by PRF-DRG. The SNI-induced reduction in GRK2 expression, elevation of p-p38 MAPK levels in the SCDH, and increase in IL-1β and TNF-α levels in the hippocampus were reversed by PRF, which was accompanied by an increase in autophagic flux in spinal microglia. The beneficial effect of 85 V-PRF was superior to that of 45 V-PRF. In addition, the improvements elicited by 85 V-PRF were reversed by intrathecal injection of GRK2 antisense oligonucleotide, and these changes were accompanied by GRK2 downregulation and p-p38 upregulation in the SCDH, increased pro-inflammatory factor levels in the hippocampus, and excessive autophagy in spinal microglia. In conclusion, our data indicate that the application of HV-PRF to the DRG could serve as an excellent therapeutic technique for regulating neuroimmunity and neuroinflammation to relieve pain phenotypes.

Involvement of 5-HT1A receptors of the thalamic descending pathway in the analgesic effect of intramuscular heating-needle stimulation in a rat model of lumbar disc herniation

Background

Intramuscular (IM) heating-needle therapy, a non-painful thermal therapy, has been found to exert an analgesic effect via the thalamic ventromedial (VM) nucleus, solely by reducing the triggering threshold for descending inhibition; this could be modulated by intracephalic 5-hydroxytryptamine-1A (5-HT1A) receptors, rather than via the regular analgesia pathway. In this study, the effect and the potential serotonergic mechanism of IM heating-needle stimulation at 43°C were explored in the case of the pathological state of lumbar disc herniation (LDH).

Methods

A modified classic rat model of LDH, induced via autologous nucleus pulposus implantation, was utilized. IM inner heating-needles were applied at the attachment point of skeletal muscle on both sides of the L4 and L5 spinous processes. WAY-100635 and 8-OH-DAPT, 5-HT1A receptor antagonist and agonist, were separately injected into the bilateral thalamic mediodorsal (MD) and VM nucleus via an intrathalamic catheter. Nociception was assessed by bilateral paw withdrawal reflexes elicited by noxious mechanical and heat stimulation.

Results

IM heating-needle stimulation at a temperature of 43°C for 30 or 45 min significantly relieved both mechanical and heat hyperalgesia in the rat model of LDH (P < 0.05). Heat hyperalgesia was found to be significantly enhanced by administration of WAY-100635 into the thalamic VM nucleus, blocking the effect of heating-needle stimulation in a dose-dependent manner (P < 0.05), while no effects were detected after injection into the thalamic MD nucleus (P > 0.05). Injection of 8-OH-DAPT into the thalamic MD nucleus exerted no modulating effects on either mechanical or heat hyperalgesia (P > 0.05).

Conclusion

IM heating-needle stimulation at 43°C for 30 min may activate 5-HT1A mechanisms, via the thalamic VM nucleus, to attenuate hyperalgesia in a rat model of LDH. This innocuous form of thermal stimulation is speculated to selectively activate the descending inhibition mediated by the thalamic VM nucleus, exerting an analgesic effect, without the involvement of descending facilitation of the thalamic MD nucleus.

The Clinical Application of Pulsed Radiofrequency Induces Inflammatory Pain via MAPKs Activation: A Novel Hint for Pulsed Radiofrequency Treatment

Pulsed radiofrequency (PRF) works by delivering short bursts of radiofrequency to a target nerve, thereby affecting nerve signal transduction to reduce pain. Although preliminary clinical investigations have shown that PRF treatment can be used safely as an alternative interventional treatment in patients with refractory pain conditions, unexpected damage to a normal nerve/ganglion is still one of the possible complications of using the PRF strategy. Noxious pain may also be triggered if PRF treatment accidentally damages an intact nerve. However, few studies in the literature have described the intracellular modifications that occur in neuronal cells after PRF stimulation. Therefore, in this study, we evaluated the effects of PRF on unimpaired nerve function and investigated the potential mechanisms of PRF-induced pain. Wistar rats were stimulated with 30-60 V of PRF for 6 min, and mechanical allodynia, cold hypersensitivity, cytokine and matrix metalloproteinase (MMP) production, and mitogen-activated protein kinase activity (p38 MAPK, ERK1/2, JNK/SAPK) were analyzed. The results indicated that PRF stimulation induced a significant algesic effect and nociceptive response. In addition, the protein array and Western blotting analyses showed that the clinical application of 60 V of PRF can induce the activation of MAPKs and the production of inflammatory cytokines and MMPs in the lumbar dorsal horn, which is necessary for nerve inflammation, and it can be suppressed by MAPK antagonist treatment. These results indicate that PRF stimulation may induce inflammation of the intact nerve, which in turn causes inflammatory pain. This conclusion can also serve as a reminder for PRF treatment of refractory pain.