Thermal avoidance in Caenorhabditis elegans: An approach to the study of nociception

Upon perception of a noxious stimulus, an organism executes defense mechani sms, such as escape responses. The molecular basis of these mechanisms is p oorly understood. In this paper we show that upon exposure to noxious tempe rature, Caenorhabditis elegans reacts by a withdrawal reflex. To analyze th is thermal avoidance behavior, we developed a laser-based assay to quantify the response. The escape reflex can be observed in 98% of the adult animal s, but is not executed in animals in diapause. The thermal avoidance respon se differs significantly from the thermotaxis behavior that is based on the perception of physiological temperature. It involves different neurons and is influenced by mutations in distinct genes. As in mammals, the strength of the thermal avoidance response is increased by application of capsaicin, the pungent ingredient in chili peppers. We find that thermal avoidance is strongly reduced in mutants affecting the neural transmission modulated by glutamate and neuropeptides as well as in mutants affecting the structure and function of sensory neurons. We suggest that the study of this nocicept ive behavior in C. elegans can be used to understand the genetic and molecu lar basis of thermal nociception.