Papers of the Week

As treatment for postsurgical pain (PSP) remains a major unmet medical need, the emergence of safe and innovative non-opioid drugs has been strongly coveted. Tetrahydrobiopterin (BH4) is an interesting molecule for gaining a better understanding the pathological mechanism of neuropathic pain. However, whether BH4 and its pathway are involved in the pathogenesis of PSP remains unclear. In this study, we found that early in a rat paw incision model, the gene expression of GTP cyclohydrolase 1 (GTPCH) and sepiapterin reductase (SPR), BH4-producing enzymes in the de novo pathway, were significantly increased in incised compared with naïve paw skin. Although a significant increase in GTPCH protein levels was observed in incised paw skin until only 1 day after incision, a significant increase in BH4 levels was observed until 7 days after incision. In vivo, Spr-knockout mice showed an antinociceptive phenotype in the hind paw incision compared with the wild-type and Spr heterozygote groups. Furthermore, QM385, the SPR inhibitor, showed a significant dose-dependent, antinociceptive effect, which was supported by a reduction in BH4 levels in incised skin tissues, with no apparent adverse effects. Immunohistochemical analysis demonstrated that macrophages expressing GTPCH protein were increased around the injury site in the rat paw incision model. These results indicate that BH4 is involved in the pathogenesis of PSP, and that inhibition of the BH4 pathway could provide a new strategy for the treatment of acute PSP.