Pulsed Radiofrequency Alleviated Neuropathic Pain by Down-Regulating the Expression of Substance P in Chronic Constriction Injury Rat Model
Neuropathic pain (NP) is a complex and debilitating condition characterized by pain initiated or caused by a lesion or dysfunction in the somatosensory system. Unlike other types of pain, NP poses a significant challenge to treatment due to the risk of permanent loss of function caused by neurological impairment. Traditional pharmacological treatments, such as anti-epileptic drugs, often provide limited relief and can lead to unintended side effects with long-term use. In cases where drug treatments are ineffective, interventional treatments or surgeries are often considered. Among these, pulsed radiofrequency (PRF) has emerged as a promising non-invasive and minimally invasive technique for the clinical treatment of NP.
PRF, first proposed by Sluijter, involves the application of short bursts of radiofrequency current to the nervous system. Unlike continuous radiofrequency, which generates heat that can damage nerve tissue, PRF delivers current in 20 ms pulses followed by a 480 ms interval, allowing the heat to dissipate and preventing nerve damage. Despite its growing popularity, the exact mechanism by which PRF alleviates NP remains unclear. Recent studies suggest that PRF may modulate the expression of pain-related neuropeptides, such as substance P (SP), which plays a crucial role in pain signal transmission. This study aimed to investigate the effects of PRF on NP in a chronic constriction injury (CCI) rat model, focusing on changes in SP expression in the spinal cord.
Experimental Design and Methods
The study utilized 96 healthy male Sprague-Dawley rats, aged 4 months and weighing between 200 to 220 g. The rats were randomly divided into four groups: the sham-surgery-sham-treatment group (S-S group), the sham-surgery-PRF group (S-P group), the CCI-sham-treatment group (C-S group), and the CCI-PRF group (C-P group). The CCI model was established in the C-S and C-P groups by ligating the sciatic nerve, while the S-S and S-P groups underwent a sham operation. Fourteen days after the surgery, PRF treatment was administered to the C-P and S-P groups at the site of sciatic nerve ligation or the corresponding site in the sham operation group. PRF was applied for 300 seconds using a therapy instrument set to a pulse frequency of 2 Hz, a temperature of 42°C, and an output voltage of 45 V.
Mechanical pain thresholds were assessed using the hindpaw withdrawal threshold (HWT) and thermal withdrawal latency (TWL) tests. These tests were conducted at baseline, before treatment (0 days), and at 1, 7, 14, and 28 days after treatment. The HWT was measured using von Frey hairs, while the TWL was assessed using an infrared radiant heat stimulus generator. Spinal cord tissues (L4 to L6) were collected at the same time points for quantitative polymerase chain reaction (qPCR) and Western blotting to measure SP mRNA and protein expression levels.
Results
The HWT and TWL values in the C-S and C-P groups were significantly lower than those in the S-S and S-P groups at 14 days after CCI, indicating successful induction of mechanical hyperalgesia. PRF treatment in the C-P group resulted in a significant increase in HWT and TWL values compared to the C-S group. Specifically, the HWT in the C-P group was significantly higher than in the C-S group at 7, 14, and 28 days post-treatment. Similarly, the TWL in the C-P group showed a significant increase at 7 and 28 days after PRF treatment. These results demonstrate that PRF effectively alleviates mechanical hyperalgesia in CCI model rats.
SP mRNA and protein expression levels in the spinal cord were significantly higher in the C-S and C-P groups compared to the S-S and S-P groups at 14 days after CCI. PRF treatment in the C-P group led to a gradual reduction in SP expression. At 7 days post-treatment, SP mRNA levels in the C-P group were significantly lower than in the C-S group, and this reduction continued through 28 days. Similarly, SP protein expression in the C-P group was significantly lower than in the C-S group at 14 and 28 days after PRF treatment. These findings suggest that the analgesic effect of PRF may be mediated by the down-regulation of SP expression in the spinal cord.
Discussion
The results of this study provide compelling evidence that PRF treatment can effectively alleviate mechanical hyperalgesia in a CCI rat model of NP. The gradual increase in HWT and TWL values following PRF treatment suggests that the therapeutic effect of PRF is not immediate but develops over time. This delayed onset of pain relief is consistent with clinical observations, where patients often experience gradual improvement after PRF treatment. The lack of significant changes in HWT and TWL in the S-P group indicates that PRF does not cause nerve damage or induce hyperalgesia in healthy nerves, supporting its safety as a non-destructive treatment modality.
The down-regulation of SP expression in the spinal cord of PRF-treated rats provides a potential mechanism for the observed analgesic effects. SP is a neuropeptide that plays a critical role in pain signal transmission, and its increased expression has been associated with the development of NP. The reduction in SP levels following PRF treatment suggests that PRF may modulate pain signaling pathways by decreasing the availability of SP in the spinal cord. This finding aligns with previous studies that have implicated SP in the pathophysiology of NP and highlights SP as a potential target for therapeutic interventions.
The electric field generated by PRF is thought to be the primary mechanism underlying its therapeutic effects. PRF-induced electric fields may lead to changes in neuronal activity and gene expression, resulting in the down-regulation of pain-related neuropeptides like SP. Further research is needed to explore the relationship between PRF electric field intensity and changes in SP expression, as well as to determine whether similar mechanisms are involved in other NP models.
Limitations and Future Directions
While this study provides valuable insights into the effects of PRF on NP, several limitations should be acknowledged. First, the study focused exclusively on the CCI model, and it remains unclear whether PRF exerts similar effects in other NP models. Second, the study did not include an antagonist group to further confirm the role of SP in PRF-mediated pain relief. Future studies could investigate the effects of SP receptor antagonists in conjunction with PRF treatment to better understand the underlying mechanisms. Additionally, the study only measured SP expression in the spinal cord, and future research could explore changes in SP levels in other parts of the nociceptive pathway, such as the dorsal root ganglia and sciatic nerve.
The long-term efficacy of PRF treatment also warrants further investigation. While the study demonstrated significant pain relief at 28 days post-treatment, longer follow-up periods are needed to assess the durability of PRF effects. Moreover, PRF treatment may involve the modulation of multiple neuropeptides and signaling pathways, and future studies could explore the role of other pain-related molecules in PRF-mediated analgesia.
Conclusion
In conclusion, this study demonstrates that PRF treatment can effectively alleviate mechanical hyperalgesia in a CCI rat model of NP. The therapeutic effects of PRF are associated with the down-regulation of SP expression in the spinal cord, suggesting that PRF may modulate pain signaling pathways by reducing the availability of this critical neuropeptide. These findings provide a mechanistic basis for the use of PRF in the treatment of NP and highlight SP as a potential target for future therapeutic interventions. Further research is needed to explore the broader implications of PRF treatment in different NP models and to optimize its clinical application for the management of chronic pain.
doi.org/10.1097/CM9.0000000000000619
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