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Stinging nettle, or Urtica dioica, is a perennial herbaceous flowering plant found worldwide, most commonly in northern Europe and Asia[1], where it is not limited by its need for moisture. The common name refers to the plant’s stinging hairs (trichomes or spicules), whose breakable ends become like a needle that release a variety of chemicals when touched, causing a painful sting or paresthesia. This defense mechanism was likely developed to avoid herbivory by larger animals[2], as it is still a primary food source for several species of butterfly and moths.

Stinging nettle defense works by impaling the skin via globular tips of trichomes on the plant’s surface and releasing various metabolites into the herbivore. These biochemical menaces include histamine, serotonin, acetylcholine, tartaric & oxalic acid, among others. Histamine, acetylcholine and serotonin lead to inflammation and pain, while several of the injected acids are linked with extended pain duration[3].

The role of oxalic and tartaric acid as pain-inducing toxins in rats was explored by Dr. Han Yi Fu, using high-performance liquid chromatography to determine the contents of chemical mixtures isolated from stinging hairs. HPLC results revealed that the main chemicals in the hairs were histamine, oxalic acid and tartaric acid. Following injection, behavioral tests showed that 10% formic acid and 2% serotonin injections elicited moderate pain sensation, while rats exposed to 2% oxalic acid and 10% tartaric acid experienced dramatic and persistent pain sensations in rats. This lead to the conclusion that oxalic acid and tartaric acid are major long-lasting, pain-inducing toxins in the stinging hairs of nettles.

Additionally, Dr. AJ Cummings investigated the hypothesis that the mechanism of stinging nettle defense is both biochemical and mechanical, due to its relevance regarding treatment for acute exposure[4]. The involvement of a mechanical defense mechanism was evaluated by brushing the skin of genetically modified hairless mice under the leaf and stem of a stinging nettle plant. Skin samples from these mice were then compared microscopically to samples not exposed to the nettles. Skin samples exposed to nettle leaves had many nettle spicules piercing the surface, indicating that spicules are the cause for the mechanical irritation and chemical effects of stinging nettles.

Although stinging nettle can cause painful responses among those who are exposed to it, this extraordinary plant has a long history of medicinal benefits. It was commonly used in the past to treat painful muscles and joints, as well as eczema and insect bites[5]. Even today, some people use it to treat urinary tract infections and benign prostatic hyperplasia[6].

References

1.     ''"Urtica dioica L.". Plants of the World Online. Royal Botanic Gardens, Kew.'' Retrieved 26 May 2019.

2.     Lambers, H. et al. Plant physiological ecology. New York, NY: Springer Science, 1998.

3.      Fu, Han Yi, et al. “Identification of Oxalic Acid and Tartaric Acid as Major Persistent Pain-Inducing Toxins in the Stinging Hairs of the Nettle, Urtica Thunbergiana.” Annals of Botany, Oxford University Press, July 2006.

4.      Cummings, Alexander J, and Michael Olsen. “Mechanism of Action of Stinging Nettles.” Wilderness & Environmental Medicine, U.S. National Library of Medicine, June 2011,

5.      Chrubasik JE, Roufogalis BD, Wagner H, Chrubasik S. A comprehensive review on the stinging nettle effect and efficacy profiles. Part II: urticae radix. Phytomedicine. 2007;14:568-579.

6.      Schneider T, Rubben H. Stinging nettle root extract (Bazoton-uno) in long term treatment of benign prostatic syndrome (BPS). Results of a randomized, double-blind, placebo controlled multicenter study after 12 months. Urologe A. 2004;43:302-306.