Although C. elegans exhibits several types of behavioral plasticity (ex. Rankin, 2004 Curr. Biol., 14, R617), we know little about plasticity of its avoidance behaviors. Because avoidance cues are toxic and/or hazardous signals, we speculated that the animals may exhibit plasticity other than adaptation in avoidance behaviors. We studied changes in avoidance behaviors of N2 animals after persistent stimulation using higher osmolarity, 1-octanol or 2-nonanone, which are sensed mainly by different sensory neuron classes respectively. For osmolarity assay, we used quadrant plates to obtain temporally more stable stimulus than the traditional "ring" assay; For repellent odor assay, we used 9 cm round plates and scored the average relative distance of the animals from the odor source. Osmolarity: Avoidance behavior to higher osmolarity was suppressed after starvation as well as after pre-exposure to the same stimulus for 1 hr. This suppression after pre-exposure is likely due to the adaptation of ASH neuron (Hilliard et al., 2005 EMBO J., 24, 63). 1-octanol: After starvation, avoidance behavior to 1-octaol (6 - 600 nL) was also suppressed, as similarly reported by Chao et al. (2004 PNAS, 101, 15512). However, after animals had been pre-exposed to 90 nL of the same odor during starvation, the avoidance behavior was not suppressed, and its magnitude was similar to that of naive animals. This modulation of avoidance behavior by pre-exposure could be explained either by suppression of the starvation-induced suppression of avoidance behavior, or by enhancement of the suppressed avoidance behavior. 2-nonanone: In contrast to 1-octanol, avoidance behavior to 2-nonanone was not suppressed by starvation. Still, pre-exposure to 30 nL of 2-nonanone during starvation significantly enhanced the avoidance behavior: Magnitude of the avoidance behavior to 200 nL of 2-nonanone of the pre-exposed N2 animals was enhanced (p < 0.001) and comparable to that of the behavior to 600 nL of 2-nonanone of unexposed animals. This result suggests that the pre-exposure may enhance the sensitivity of the sensory neuron to 2-nonanone by about 3-fold. Alternatively, the enhancement could be due to modulation of the locomotion. The pre-exposure in the presence of food also enhanced the avoidance behavior although it was not statistically significant. We are interested in these pre-exposure-induced modulations of avoidance behaviors because the plasticity of avoidance behaviors may be much more complicated than we expected. We are currently studying the plasticity in the following aspects: (1) its specificity and persistence, (2) responsible sensory neurons, and (3) genes required for the phenomenon.

Affiliations:
- Str. Biol. Ctr., Nat. Inst. Genet., Mishima, Japan
- Dept. Genet., Grad. Univ. Adv. Stud., Mishima, Japan