Multiple sensory neurons mediate starvation-dependent aversive navigation in Caenorhabditis elegans.

Animals demonstrate flexible behaviors through associative learning based on their experiences. Deciphering the neural mechanisms for sensing and integrating multiple types of sensory information is critical for understanding such behavioral controls. The soil nematode Caenorhabditis elegans avoids salt concentrations it has previously experienced under starvation conditions. Here, we identify a pair of sensory neurons, the ASG neuron pair, which in cooperation with the ASER salt-sensing neuron generate starvation-dependent salt avoidance. Animals whose sensory input is restricted to only ASER failed to show learned avoidance due to inappropriately directed navigation behaviors. However, their navigation through a salt concentration gradient was improved by allowing sensory inputs to ASG in addition to ASER. Detailed behavioral analyses of these animals revealed that input from ASG neurons is required not only for controlling the frequency of initiating a set of sharp turns (called pirouettes) based on detected ambient salt concentrations but also adjusting the migration direction during pirouettes. Optogenetic activation of ASER by ChR2 induced turning behaviors in a salt concentration-dependent manner where presence of intact ASG was important for the starvation-dependent responses. Calcium imaging of the activity of ASG neurons in freely moving worms revealed that ASG is activated upon turning behavior. Our results indicate that ASG neurons cooperate with the ASER neuron to generate destination-directed reorientation in starvation-associated salt concentration avoidance.