Identification of oxalic acid and tartaric acid as major persistent pain-inducing toxins in the stinging hairs of the nettle, Urtica thunbergiana

• Background and Aims: Once human skin contacts stinging hairs of Urtica spp. (stinging nettles), the irritant is released and produces pain, wheals or a stinging sensation which may last for >12 h. However, the existence of pain-inducing toxins in the stinging hairs of Urtica thunbergiana has never been systematically demonstrated. Experiments were therefore conducted to identify the persistent pain-inducing agents in the stinging hairs of U. thunbergiana. • Methods: The stinging hairs of U. thunbergiana were removed and immersed in deionized water. After centrifugation, the clear supernatants were then subjected to high-performance liquid chromatography (HPLC), enzymatic analysis and/or behavioural bioassays. • Key Results: The HPLC results showed that the major constituents in the stinging hairs of U. thunbergiana were histamine, oxalic acid and tartaric acid. However, the well-recognized pain-inducing agents, serotonin and formic acid, existed at a low concentration as estimated by HPLC and/or enzymatic analyses. The behavioural tests showed that 2 % oxalic acid and 10 % tartaric acid dramatically elicited persistent pain sensations in rats. In contrast, 10 % formic acid and 2 % serotonin only elicited moderate pain sensation in the first 10 min. Moreover, no significant pain-related behavioural response was observed after injecting 10 % acetylcholine and histamine in rats. • Conclusions: Oxalic acid and tartaric acid were identified, for the first time, as major long-lasting pain-inducing toxins in the stinging hairs of U. thunbergiana. The general view that formic acid, histamine and serotonin are the pain-inducing agents in the stinging hairs of U. dioica may require updating, since their concentrations in U. thunbergiana were too low to induce significant pain sensation in behavioural bioassays.