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Tale of the sting
I can still remember very vividly an incident from my childhood when I fell off a fence that I was climbing, straight into a bed of stinging nettles (which I now know as Urtica doica). Of course, I knew nothing of the chemistry of the process but I certainly knew that it was very painful! The stinging power of nettles has been known for a very long time, but strangely there is still no consensus on what makes the sting so painful. Han Yi Fu et al. (Taipei and Taoyuan, Taiwan, pp. 57–65) have analysed the exudate from the stinging hairs of U. thunbergiana. The main components were histamine, oxalic acid and tartaric acid, with a trace of formic acid. Several compounds that had previously been thought to be involved in generating the persistent pain, including serotonin and acetylcholine, were not detected in these assays. The next phase of the work was to record pain-related behaviour in rats injected with these compounds. Of the compounds present in the nettle-sting exudate, both oxalic acid and tartaric acid caused pain with a similar persistence to that caused by the exudate itself. Histamine and formic acid (and acetylcholine) were ineffective. Interestingly, serotonin, not detected in the extracts, was very effective in pain induction. The authors conclude that in U. thunbergiana, the pain inducers are oxalic and tartaric acids, two compounds widely distributed in the plant kingdom. It would be interesting to know whether in nettle stings they act synergistically and also whether histamine, although not on its own having any effect, may increase the pain-inducing properties of the acids. The inflammatory response to wasp stings, which are much more complex chemically than nettle stings, is at least partly caused by histamine, serotonin and acetylcholine; furthermore, other compounds in the venom induce the production of more histamine from the victim’s mast cells.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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