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.