Changes in TRPV1 Receptor, CGRP, and BDNF Expression in Rat Dorsal Root Ganglion with Resiniferatoxin-Induced Neuropathic Pain: Modulation by Pulsed Radiofrequency Applied to the Sciatic Nerve.

Pulsed radiofrequency (PRF) is a safe method of treating neuropathic pain by generating intermittent electric fields at the needle tip. Resiniferatoxin (RTX) is an ultrapotent agonist of transient receptor potential vanilloid subtype-1 (TRPV1) receptors. We investigated the mechanism of PRF using a rat model of RTX-induced neuropathic pain. After administering RTX intraperitoneally, PRF was applied to the right sciatic nerve. We observed the changes in TRPV1, calcitonin gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) in the dorsal root ganglia by western blotting. Expressions of TRPV1 and CGRP were significantly lower in the contralateral (RTX-treated, PRF-untreated) tissue than in control rats (p<0.0001 and p<0.0001, respectively) and the ipsilateral tissues (p<0.0001 and p<0.0001, respectively). BDNF levels were significantly higher in the contralateral tissues than in the control rats (p<0.0001) and the ipsilateral tissues (p<0.0001). These results suggest that, while TRPV1 and CGRP are decreased by RTX-induced neuronal damage, increased BDNF levels result in pain development. PRF may promote recovery from neuronal damage with concomitant restoration of TRPV1 and CGRP, and exert its analgesic effect by reversing BDNF increase. Further research is required to understand the role of TRPV1 and CGRP restoration in improving mechanical allodynia.

Extracellular signal-regulated kinase phosphorylation enhancement and NaV1.7 sodium channel upregulation in rat dorsal root ganglia neurons contribute to resiniferatoxin-induced neuropathic pain: The efficacy and mechanism of pulsed radiofrequency therapy.

Pulsed radiofrequency (PRF) therapy is one of the most common treatment options for neuropathic pain, albeit the underlying mechanism has not been hitherto elucidated. In this study, we investigated the efficacy and mechanism of PRF therapy on resiniferatoxin (RTX)-induced mechanical allodynia, which has been used as a model of postherpetic neuralgia (PHN). Adult male rats were intraperitoneally injected with a vehicle or RTX. Furthermore, PRF current was applied on a unilateral sciatic nerve in all RTX-treated rats. On both ipsilateral and contralateral sides, the paw mechanical withdrawal thresholds were examined and L4-6 dorsal root ganglia (DRG) were harvested. In the DRG of rats with RTX-induced mechanical allodynia, NaV1.7, a voltage-gated Na+ channel, was upregulated following the enhancement of extracellular signal-regulated kinase phosphorylation. Early PRF therapy, which was applied 1 week after RTX exposure, suppressed this NaV1.7 upregulation and showed an anti-allodynic effect; however, late PRF therapy, which was applied after 5 weeks of RTX exposure, failed to inhibit allodynia. Interestingly, late PRF therapy became effective after daily tramadol administration for 7 days, starting from 2 weeks after RTX exposure. Both early PRF therapy and late PRF therapy combined with early tramadol treatment suppressed NaV1.7 upregulation in the DRG of rats with RTX-induced mechanical allodynia. Therefore, NaV1.7 upregulation in DRG is related to the development of RTX-induced neuropathic pain; moreover, PRF therapy may be effective in the clinical management of patients with PHN via NaV1.7 upregulation inhibition.

Selective optogenetic activation of NaV1.7-expressing afferents in NaV1.7-ChR2 mice induces nocifensive behavior without affecting responses to mechanical and thermal stimuli.

In small and large spinal dorsal root ganglion neurons, subtypes of voltage-gated sodium channels, such as NaV1.7, NaV1.8, and NaV1.9 are expressed with characteristically localized and may play different roles in pain transmission and intractable pain development. Selective stimulation of each specific subtype in vivo may elucidate its role of each subtype in pain. So far, this has been difficult with current technology. However, Optogenetics, a recently developed technique, has enabled selective activation or inhibition of specific neural circulation in vivo. Moreover, optogenetics had even been used to selectively excite NaV1.8-expressing dorsal root ganglion neurons to induce nocifensive behavior. In recent years, genetic modification technologies such as CRISPR/Cas9 have advanced, and various knock-in mice can be easily generated using such technology. We aimed to investigate the effects of selective optogenetic activation of NaV1.7-expressing afferents on mouse behavior. We used CRISPR/Cas9-mediated homologous recombination to generate bicistronic NaV1.7-iCre knock-in mice, which express iCre recombinase under the endogenous NaV1.7 gene promoter without disrupting NaV1.7. The Cre-driver mice were crossed with channelrhodopsin-2 (ChR2) Cre-reporter Ai32 mice to obtain NaV1.7iCre/+;Ai32/+, NaV1.7iCre/iCre;Ai32/+, NaV1.7iCre/+;Ai32/Ai32, and NaV1.7iCre/iCre;Ai32/Ai32 mice. Compared with wild-type mice behavior, no differences were observed in the behaviors associated with mechanical and thermal stimuli exhibited by mice of the aforementioned genotypes, indicating that the endogenous NaV1.7 gene was not affected by the targeted insertion of iCre. Blue light irradiation to the hind paw induced paw withdrawal by mice of all genotypes in a light power-dependent manner. The threshold and incidence of paw withdrawal and aversive behavior in a blue-lit room were dependent on ChR2 expression level; the strongest response was observed in NaV1.7iCre/iCre;Ai32/Ai32 mice. Thus, we developed a non-invasive pain model in which peripheral nociceptors were optically activated in free-moving transgenic NaV1.7-ChR2 mice.