Most of them might be called planarians (e.g., “land planarians” for Geoplanidae species), but the term planaria or planarians in research models mostly refers to freshwater species, especially from the Dugesiidae family. Triclads include marine ( Maricola), land ( Geoplanidae) and freshwater flatworms. They used to be classified as Turbellaria due to their gliding movement using ventral cilia but this class is now considered paraphyletic ( de Vries and Sluys, 1991). The other three out of the four major classes of flatworms – Trematoda, Cestoda, and Monogenea – contain mostly parasitic species but Tricladida are free-living organisms. The term planaria (or planarians) encompasses flatworms (platyhelminthes phylum) within the Tricladida order. In regards to past pharmacological studies, this review finally discusses the opportunities given by the model to extensively screen for novel antinociceptive drugs. It also highlights the convenience and relevance of this invertebrate model for nociceptive tests and suggests further lines of research. The aim of this review is therefore to aggregate the ancient and the most recent evidence for a true nociceptive behavior in planarians. However, among the few authors that tried to implement a full behavior analysis, none reached a consensual use of the terms used to describe planarian gaits yet, nor did they establish a comprehensive description of a potential planarian nociceptive system. It has also been shown that some antinociceptive drugs modulate these behaviors. This research field recently led to the discovery of causal relationships between the expression of Transient Receptor Potential ion channels in planarians and their behavioral responses to noxious stimuli such as heat, cold or pharmacological analogs such as TRP agonists, among others. Many researchers used planarians as a model system for the study of drug abuse because they display high similarities with the nervous system of vertebrates at cellular and molecular levels (e.g., neuronal morphology, neurotransmitter ligands, and receptor function). In addition to being studied for their exceptional regeneration abilities, planarians (i.e., flatworms) have also been extensively used in the context of pharmacological experiments during the past century. Institut des Neurosciences Cellulaires et Intégratives, UPR 3212, Centre National de la Recherche Scientifique, Strasbourg, France.Guillaume Reho, Vincent Lelièvre and Hervé Cadiou *
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