  Experiments of pain with human subjects are difficult, subjective, and
ethically constrained. Since the molecular mechanisms of pain transduction are
reasonably conserved among different species, these problems are partially
solved by the use of animal models. However, animals cannot easily communicate
to us their own pain levels. Thus progress depends crucially on our ability to
quantitatively and objectively infer the perceived level of noxious stimuli
from the behavior of animals. Here we develop a quantitative model to infer the
perceived level of thermal nociception from the stereotyped nociceptive
response of individual nematodes Caenorhabditis elegans stimulated by an IR
laser. The model provides a method for quantification of analgesic effects of
chemical stimuli or genetic mutations in C. elegans. We test the nociception of
ibuprofen-treated worms and a TRPV (transient receptor potential) mutant, and
we show that the perception of thermal nociception for the ibuprofen treated
worms is lower than the wild-type. At the same time, our model shows that the
mutant changes the worm's behavior beyond affecting nociception. Finally, we
determine the stimulus level that best distinguishes the analgesic effects and
the minimum number of worms that allow for a statistically significant
identification of these effects.
