Anti-neuropathic pain activity of Ajugarin-I via activation of Nrf2 signaling and inhibition of TRPV1/TRPM8 nociceptors in STZ-induced diabetic neuropathy.

This study aimed to investigate the anti-neuropathic pain activity and its underlying molecular mechanism of Ajugarin-I (Aju-I) in a rat model of diabetic neuropathic pain. The rats were given a single injection of 60 mg/kg of streptozotocin (STZ) intraperitoneally (i.p.) to induce diabetic neuropathic pain. After two weeks, rats were given Aju-I (1 and 5 mg/kg/day) i.p. for four consecutive weeks. The results demonstrated that in diabetic rats, treatment with Aju-I decreased STZ-induced hyperglycemia. It reduced the pain hypersensitivity (mechanical, thermal, and cold nociception) caused by STZ. It effectively restored STZ-associated pathological changes in the pancreas. In the sciatic nerve and spinal cord, it attenuated STZ-associated histopathological alterations and DNA damage. It suppressed oxidative stress by increasing the expression of nuclear factor-erythroid factor 2-related factor 2 (Nrf2), thioredoxin (Trx), and heme oxygenase (HO-1), but decreasing the immunoreactivity of Kelch-like ECH-associated protein 1 (Keap1). Additionally, TRPV1 (transient receptor potential vanilloid 1) and TRPM8 (transient receptor potential melastatin 8) expression levels were considerably reduced by Aju-I treatment. It enhanced antioxidant levels and suppressed inflammatory cytokines production. Taken together, this research suggests that Aju-I treatment reduces pain behaviors in the STZ model of diabetic neuropathy via modulating Nrf2/Keap-1/HO-1 signaling and TRPV1/TRPM8 nociceptors.

Suppression of TRPV1/TRPM8/P2Y Nociceptors by Withametelin via Downregulating MAPK Signaling in Mouse Model of Vincristine-Induced Neuropathic Pain.

Vincristine (VCR) is a widely used chemotherapy drug that induced peripheral painful neuropathy. Yet, it still lacks an ideal therapeutic strategy. The transient receptor potential (TRP) channels, purinergic receptor (P2Y), and mitogen-activated protein kinase (MAPK) signaling play a crucial role in the pathogenesis of neuropathic pain. Withametelin (WMT), a potential Phytosteroid isolated from datura innoxa, exhibits remarkable neuroprotective properties. The present investigation was designed to explore the effect of withametelin on VCR-induced neuropathic pain and its underlying molecular mechanism. Initially, the neuroprotective potential of WMT was confirmed against hydrogen peroxide (H2O2)-induced PC12 cells. To develop potential candidates for neuropathic pain treatment, a VCR-induced neuropathic pain model was established. Vincristine (75 µg/kg) was administered intraperitoneally (i.p.) for 10 consecutive days (day 1-10) for the induction of neuropathic pain. Gabapentin (GBP) (60 mg/kg, i.p.) and withametelin (0.1 and 1 mg/kg i.p.) treatments were given after the completion of VCR injection on the 11th day up to 21 days. The results revealed that WMT significantly reduced VCR-induced pain hypersensitivity, including mechanical allodynia, cold allodynia, and thermal hyperalgesia. It reversed the VCR-induced histopathological changes in the brain, spinal cord, and sciatic nerve. It inhibited VCR-induced changes in the biochemical composition of the myelin sheath of the sciatic nerve. It markedly downregulated the expression levels of TRPV1 (transient receptor potential vanilloid 1); TRPM8 (Transient receptor potential melastatin 8); and P2Y nociceptors and MAPKs signaling, including ERK (Extracellular Signal-Regulated Kinase), JNK (c-Jun N-terminal kinase), and p-38 in the spinal cord. It suppressed apoptosis by regulating Bax (Bcl2-associated X-protein), Bcl-2 (B-cell-lymphoma-2), and Caspase-3 expression. It considerably attenuated inflammatory cytokines, oxidative stress, and genotoxicity. This study suggests that WMT treatment suppressed vincristine-induced neuropathic pain by targeting the TRPV1/TRPM8/P2Y nociceptors and MAPK signaling.