Most animals memorize environmental cues and change their behavior based on the memory. C. elegans shows this sort of response to NaCl by memorizing and moving towards particular salt concentrations at which they were cultivated. In this study, we identified a specific neural circuit and its dynamics required for salt concentration memory. We investigated the relationship between the locomotory behavior and the response of salt-sensing neural circuit. Animals responded to salt stimuli by changing their turning frequency depending on the difference of salt concentrations between current stimulus and previous cultivation: salt-concentration decrease induced turning after cultivation at a salt concentration higher than the stimulus, whereas salt-concentration increase induced turning after cultivation at a salt concentration lower than the stimulus. With regard to the neural response, we focused on the salt chemosensory neuron ASER and its downstream neurons, AIB, RIM, and AVA. The response of ASER always correlated with sensory input: it was activated by salt concentration decrease irrespective of cultivation conditions. On the other hand, the responses of downstream neurons correlated well with the behavioral output. These results indicate that the neural circuit ASER-AIB-RIM-AVA modulates experience-dependent salt chemotaxis, and the sensory information is processed and converted to a behavioral decision at the level between the sensory neuron and the interneurons. Cell-specific rescue of sensory function demonstrated that input from ASER generates the bidirectional AIB response as well as the behavior of worms. Furthermore, we revealed that glutamate signaling from ASER to AIB is necessary for generating responses to both increase and decrease in ambient salt concentration. There are several sets of interneurons other than AIB that act downstream of ASER. These neurons may play modulatory roles in the transmission from ASER to AIB. Therefore, we genetically ablated AIA and AIY, and monitored the responses of AIB as well as the behavior of the worms. These worms showed attenuated response to salt concentration decrease, but showed almost no response to salt concentration increase, indicating that AIA and AIY are involved in the responsivity of AIB.

Affiliations:
- Dept. of Syst. Inf. Sci., Grad. Sch. of Inf. Sci., Tohoku Univ, Sendai, JP
- Dept Biol Sci, Grad Sch Sci, The University of Tokyo, Tokyo, JP