Cinnamaldehyde induces a TRPA1-mediated nociceptive behavior in planarians.

Nociception is defined as "the neural process of encoding noxious stimuli" by the International Association for the Study of Pain (IASP). Nociception relies on detecting noxious stimuli arising from a potentially or actually tissue-damaging event via specialized cells called nociceptors. In planarians, nociceptive behavior is often indicated by a 'scrunching' gait, in contrast to the usual gliding behavior displayed in normal conditions. The present study extends our previous study Reho et al. (2024) by testing a new potentially irritant molecule, Cinnamaldehyde (CA), which could induce scrunching gaits. We reproduced the nociceptive chemical tests from our previous study using CA instead of Allyl isothiocyanate (AITC) on Girardia dorotocephala (Gd) implementing an open field behavioral analysis. CA induced a dose-dependent increase in scrunching gait similar to the action of AITC and was expectedly partially suppressed by morphine and meloxicam. Knocking down the expression of the Gd-TRPA1 ion channel by RNA interference also suppressed the behavioral reaction to the molecule. In conclusion, we demonstrated that CA induced a nociceptive behavior in planarians through an action on the ion channel TRPA1. SIGNIFICANCE STATEMENT: In this article, we provide evidence that cinnamaldehyde induces a nociceptive behavior through a direct action in an invertebrate model (flatworm) much in the same way that in vertebrates.