Inefficacy of a novel osmotic associative learning assay in C. elegans

OBJECTIVE: In this study, we developed and tested a novel osmotic associative assay for facilitating the study of associative learning circuits with constrained sensory inputs in Caenorhabditis elegans (C. elegans). RESULTS: Hyperosmolarity, the unconditioned stimulus, was paired with benzaldehyde, an attractive odorant here used as a conditioned stimulus, in order to train an association between hyperosmolarity and odor. It was hypothesized that animals conditioned in hyperosmotic conditions for 90 min would show reduced chemotaxis to benzaldehyde compared to control animals. Results suggest that, following exposure to a hyperosmolar 500mM fructose solution, C. elegans chemotaxis to benzaldehyde was impaired. At more moderate hyperosmolarity, worms failed to learn to associate hyperosmolarity with benzaldehyde - i.e., there was no difference between chemotaxis index values of conditioned and naive worms in any of the fructose concentrations. Thus, our results both suggest that 90-minute associative learning assays in C. elegans may not succeed with hyperosmolarity as an unconditioned stimulus, and provide a cautionary note that researchers using hyperosmotic solutions in worm behavior assays should be cognizant of potential effects on naïve chemotaxis behavior.