Metformin attenuates diabetic neuropathic pain via AMPK/NF-κB signaling pathway in dorsal root ganglion of diabetic rats

Highlights

• Metformin obviously attenuates mechanical allodynia of diabetic rats.

• Activation of AMPK contributes to analgesic effect of Metformin on diabetic rats.

• Activation of AMPK decreased the expression of NF-κB in diabetic rats.

Abstract

Neuropathic pain is a common complication of diabetes mellitus with poorly relieved by conventional analgesics. Metformin, a first-line drug for type 2 diabetes, reduces blood glucose by activating adenosine monophosphate protein kinase (AMPK) signalling system. However, the effect of Metformin on diabetic neuropathic pain is still unknown. In the present study, we showed that Metformin was capable of attenuating diabetes induced mechanical allodynia, and the analgesia effect could be blocked by Compound C (an AMPK inhibitor). Importantly, Metformin enhanced the phosphorylation level of AMPK in L4-6 DRGs of diabetic rats but not affect the expression of total AMPK. Intrathecal injection of AICAR (an AMPK agonist) could activate AMPK and alleviate the mechanical allodynia of diabetic rats. Additionally, phosphorylated AMPK and NF-κB was co-localized in small and medium neurons of L4-6 DRGs. Interestingly, the regulation of NF-κB in diabetic rats was obviously reduced when AMPK was activated by AICAR. Notably, Metformin could decrease NF-κB expression in L4-6 DRGs of diabetic rats, but the decrease was blocked by Compound C. In conclusion, Metformin alleviates diabetic mechanical allodynia via activation of AMPK signaling pathway in L4-6 DRGs of diabetic rats, which might be mediated by the downregulation of NF-κB, and this providing certain basis for Metformin to become a potential drug in the clinical treatment of diabetic neuropathic pain.

Introduction

Diabetic neuropathic pain develops in about one third of patients with diabetic peripheral neuropathy (Alsaloum et al., 2019, Lee-Kubli et al., 2018, Ziegler and Fonseca, 2015). The patients often suffer from severe hyperalgesia and allodynia, which can be disabling and devastating (Didangelos et al., 2014), but the pharmacological treatment of chronic diabetic neuropathic pain remains difficult, partly because of the unclear molecular mechanisms underlying diabetic neuropathic pain (Zilliox and Russell, 2011). Therefore, it is imperative to look for effective drugs, especially with fewer side effects, for the management of diabetic neuropathic pain.

Metformin is an anti-diabetic drug that has been safely and widely used for the treatment of type 2 diabetes for decades (Yu et al., 2019b). Recent studies reported that Metformin might inhibit injury-induced neuropathic pain through AMPK pathway (Melemedjian et al., 2011). And AMPK pathway has been reported to play an important role in pain regulation by some researchers (Bullón et al., 2016, Ge et al., 2018). Additionally, the analgesia of Metformin was also efficient in neuropathic pain of mouse model induced by chemotherapy (Ludman and Melemedjian, 2019), and mechanical allodynia in lumbar radiculopathy pain caused by spinal nerve ligation or nerve injury (Inyang et al., 2019). However, the analgesia effect of Metformin on diabetic neuropathic pain and the underlying mechanism of pain relief remains unclear.

Nuclear factor-kappaB (NF-κB), a nuclear transcription factor, is considered to control numerous genes encoding inflammatory and nociceptive mediators and plays roles in the development of chronic pain (Elshamly et al., 2019). Previous researches showed that p65 (a subunit of NF-κB) expression was upregulated in the L4-6 DRGs of diabetic rats with neuropathic pain (Zhang et al., 2015), and selective blockade of p65 by PDTC or siRNA attenuates diabetes-induced painful behaviors (Zhang et al., 2015). Notably, researchers demonstrated that activation of AMPK could suppress NF-κB activation in inflammation and tumors (Jung et al., 2014). Additionally, it has been reported that activated AMPK could inhibit NF-κB in the procedure of relieving Chronic Constriction Injury-induced neuropathic pain (Gui et al., 2015). However, the interaction of NF-κB and AMPK in diabetic neuropathic pain remains unclear.

In this study, we observed the analgesic effects of Metformin and its potential effects on the AMPK/NF-κB signaling pathway in diabetic mechanical allodynia of diabetic rats. Our data reveal a potential mechanism underlying the analgesic effect of Metformin and may shed new light on the therapeutics for diabetic neuropathic pain.