Methylene Blue Application to Lessen Pain: Its Analgesic Effect and Mechanism

Abstract

Methylene blue (MB) is a cationic thiazine dye, widely used as a biological stain and chemical indicator. Growing evidence have revealed that MB functions to restore abnormal vasodilation and notably it is implicated even in pain relief. Physicians began to inject MB into degenerated disks to relieve pain in patients with chronic discogenic low back pain (CDLBP), and some of them achieved remarkable outcomes. For osteoarthritis and colitis, MB abates inflammation by suppressing nitric oxide production, and ultimately relieves pain. However, despite this clinical efficacy, MB has not attracted much public attention in terms of pain relief. Accordingly, this review focuses on how MB lessens pain, noting three major actions of this dye: anti-inflammation, sodium current reduction, and denervation. Moreover, we showed controversies over the efficacy of MB on CDLBP and raised also toxicity issues to look into the limitation of MB application. This analysis is the first attempt to illustrate its analgesic effects, which may offer a novel insight into MB as a pain-relief dye.

Keywords: anti-inflammation; denervation; methylene blue; pain; sodium current.

FIGURE 1

MB is involved in anti-inflammation by suppressing iNOS/NO-NF-κB pathway. (A) Basically, MB downregulates both eNOS and sGC that are major factors converting GTP to cGMP, ultimately leading to vasoconstriction. (B) Upon tissue injury, iNOS functions as a strong inflammatory mediator in different types of cells. It inhibits Sirt1 activation by NO-mediated S-nitrosylation, which in turn activates NF-κB and p53 to facilitate inflammatory cytokine expression and apoptosis, respectively. Of note, NF-κB activation intensifies these events by activating iNOS/NO-NF-κB pathway. Conversely, MB directly abates iNOS expression and moreover decreases the binding of NF-κB to iNOS promoter, which consequently interrupts this inflammatory signaling. (C) Meanwhile, NMDA receptors are activated during nerve injury and induces Ca²⁺ influx, which then results in the excessive expression of nNOS and markedly activates nNOS/NO signaling. The increased NO production stimulates NMDA receptors and triggers NO/cGMP/PKG cascade, which promotes the subsequent BNDF upregulation and neurotransmitter release and ultimately induces long-term hyperexcitability and central sensitization. Notably, BDNF and peroxynitrite potentiate NMDA receptors, which stimulate nNOS expression again. However, MB weakens these responses by inhibiting nNOS and sGC activation, thus may prevent the development of persistent pain.LTH, long-term hyperexcitability; STZ, sensitization; NT, neurotransmitter; CP, chronic pain.

FIGURE 2

MB significantly attenuates sodium currents by blocking VGSCs. (A) In general, VGSCs allow sodium ions to flow into the cell in the activated state. (B) However, early researchers found that the gate and sodium currents of the channels were markedly suppressed post-MB treatment. And notably, this event was maintained even after pronase treatment. Thus, they interpreted this event as MB functions as a pore blocker rather than an inactivation enhancer.

FIGURE 3

MB contributes to pain reduction via three major routes. (A) First of all, MB is deeply involved in anti-inflammation. MB application blocks iNOS/NO signaling by downregulating iNOS, and suppresses P2 × 3R and lncRNA expression, NF-κB activation, and inflammasome formation, which thereby decreases inflammatory cytokine levels. These events are ultimately followed by pain reduction with the prevention of tissue degradation and swelling. (B) In addition, MB application attenuates neuronal excitability by decreasing I_NA and firing rates. These altered electrophysiological properties may contribute to pain relief by blocking synaptic transmission. (C) Lastly, MB application improved chronic PA and LBP. An electron microscopic experiment demonstrated that such efficacy was due to the death of nerve endings.