Measuring Associative Learning in Chemotaxis of the Nematode <em>Caenorhabditis elegans</em>

To adapt their behavior, animals assign valence to sensory information through associative learning, and update this association when environmental conditions change. The behavioral responses of Caenorhabditis elegans to sodium chloride offer a well-defined paradigm for investigating the mechanisms governing associative learning. Exposing worms to specific NaCl levels paired with the presence or absence of food elicits chemotaxis towards or away from this NaCl concentration, respectively. The associative memory is formed within a few hours and can be modified in the same timeframe upon exposing the animals to a different NaCl concentration. Here, we describe how to assay behavioral plasticity and learning based on C. elegans salt chemotaxis. The assay strains are grown in synchronized cohorts until they reach adulthood and are then conditioned for several hours on plates with different NaCl concentrations, either in the presence or absence of food. Animals are then transferred to assay plates that contain regions of low, medium, or high NaCl concentrations and allowed to crawl for 30 min before counting the number of worms in each region. A chemotaxis index is then calculated from the worms' distribution on the plate. The assay can be used to test the effect of a range of factors affecting learning.