Endogenous Antinociceptive Ligands 485 be recognized as dynamic. Thus the specific response of a cell to a ligand will be determined as the sum of many variables, and net effect of different ligands depends upon all of these factors. That is why signal transduction studies performed in trans- fected cells ultimately must be validated contextually in cells, organs, and intact animals in which the different receptors are endogenously expressed. Not all of the endogenous ligands have been discovered, and even fewer data are available about their interactions, therefore researchers need to continue identifying key cellular and molecular factors affected in sensory pathways during different pain syndromes to characterize potential targets for new drugs and drug combinations. Acknowledgments This work was supported by a grant of the National Research and Development Office, Hungary (OMFB-0066/2005/DNT) and a Hungarian R esearch Grant (OTKA, K60278). 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