We are interested in the C. elegans nociceptive response, and particularly in the involvement of neuropeptides, including opioids, in this response. C. elegans has five TRPV-type channels, osm-9 and ocr-1–4 (Xiao and Xu, 2011). Interestingly, we have previously noted that wild-type worms do not normally avoid capsaicin (consistent with previous findings, Tobin et al., 2002), yet worms DO avoid crude hot pepper extracts, the primary active ingredient of which is capsaicin. We also previously described that the worms avoid crude extracts of garlic, black pepper, hot mustard, and cloves, the putative active ingredients of which are allicin, piperine, allyl isothiocyanate, and eugenol, respectively (Sadaiappen et al., 2009). Of these, it had previously been determined that worms are neither attracted to or repelled by eugenol (Bargmann et al., 1993). We wished to test whether the other active components of our extracts were nociceptive agents for worms. We were particularly interested in the worms’ response to piperine, as this compound is known to act on the same TRPV1 receptor as capsaicin, and has been used medicinally as well as an insecticide (Szallasi, 2005).
As shown in Fig. 1, we were able to demonstrate that N2 significantly avoids piperine, at concentrations as low as 0.1%. We found especially interesting the wild-type response to piperine, which could be characterized as an “exaggerated backing” or “sneeze” response. Animals approach the piperine, withdraw with significant nose twitches/head shaking, and retreat quickly, often making at least one body length of backing, as shown in Fig. 2. They frequently repeat this behavior over minutes, to suggest that they are not adapting very quickly to the piperine.
We have also tested N2’s with allyl isothiocyanate and eugenol. (Allicin is highly unstable and not easily tested.) Worms appear not to avoid allyl isothiocyanate; however, they do robustly avoid eugenol—this latter result is in contradiction to previous results with this compound (Bargmann et al., 1993). Since eugenol is toxic to worms (Asha et al., 2001), it is reasonable to think that worms might avoid it, and we suspect that the previous result could have been due to the particular concentrations or conditions used.
In addition, we have begun testing the various TRPV mutants for their response to piperine. Since piperine functions on the same receptor as capsaicin, our prediction is that osm-9 and/or ocr-2 should show a specifically defective response.
We thank the CGC for providing strains.
Sadaiappen R, Augillard A, Kelly J, Preyan L and Barrett P. (2009). Toward a C. elegans-based model of nociception and analgesia. 17th International C. elegans Meeting.
Tobin D, Madsen D, Kahn-Kirby A, Peckol E, Moulder G, Barstead R, Maricq A and Bargmann C. (2002). Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons. Neuron 35, 307-318.
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