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BACKGROUND:Responding to noxious stimuli by invoking an appropriate escape response is critical for survival of an organism. The sensations of small and large changes in temperature in most organisms have been studied separately in the context of thermotaxis and nociception, respectively. Here we use the nematode C. elegans to address the neurogenetic basis of responses to thermal stimuli over a broad range of intensities.RESULTS:C. elegans responds to aversive temperature by eliciting a stereotypical behavioral sequence. Upon sensation of the noxious stimulus, it moves backwards, turns and resumes forward movement in a new direction. In order to study the response of C. elegans to a broad range of noxious thermal stimuli, we developed a novel assay that allows simultaneous characterization of multiple aspects of escape behavior elicited by thermal pulses of increasing amplitudes. We exposed the laboratory strain N2, as well as 47 strains with defects in various aspects of nervous system function, to thermal pulses ranging from DeltaT = 0.4degreesC to 9.1degreesC and recorded the resulting behavioral profiles.CONCLUSIONS:Through analysis of the multidimensional behavioral profiles, we found that the combinations of molecules shaping avoidance responses to a given thermal pulse are unique. At different intensities of aversive thermal stimuli, these distinct combinations of molecules converge onto qualitatively similar stereotyped behavioral sequences.
When the temperature suddenly increases, C. elegans worms respond by stopping, backing up, turning around 180 degrees, and then moving forward. The authors quantify several different components of the response, for different temperature increases. They then test these parameters on candidate mutant strains of worms. They find that most of the mutants can still respond to large changes in temperature, even though many of them don't respond to smaller changes. This could suggest that multiple parallel pathways are used to sense temperature changes, and that at high temperatures some of these pathways become dispensable.
Methodologically, the authors determine whether the mutants are different from WT in a couple ways. I preferred the method where (I'm simplifying wildly here) the authors determined whether any of the quantified traits were mathematically indistinguishable from any of the others (ie, used Principal component analysis), and then used the quantifications of the most distinguishing traits to determine, overall, how different individual mutants were from WT. A problem with this method is that one has to select an arbitrary cutoff after which a mutant is "different enough" to call different from WT, whereas their alternate method didn't have this concern.
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