Aversive Associative Learning and Memory Formation by Pairing Two Chemicals in Caenorhabditis elegans

The nematode Caenorhabditis elegans is an attractive model organism to study learning and memory at molecular and cellular levels because of the simplicity of its nervous system, whose chemical and electrical wiring diagrams were completely reconstructed from serial electron micrographs of thin sections. Here, we describe detailed protocols for the conditioning of C. elegans by massed and spaced training for the formation of short-term memory (STM) and long-term memory (LTM), respectively. By pairing 1-propanol and hydrochloric acid as conditioned and unconditioned stimuli, respectively, C. elegans was successfully trained to form aversive associative STM and LTM. While naïve animals were attracted to 1-propanol, the trained animals were no longer or very weakly attracted to 1-propanol. Like in other organisms such as Aplysia and Drosophila, "learning and memory genes" play essential roles in memory formation. Particularly, NMDA-type glutamate receptors, expressed in only six pairs of interneurons in C. elegans, are required for the formation of both STM and LTM, possibly as a coincidence factor. Therefore, the memory trace may reside among the interneurons.