Analyses of habituation in Caenorhabditis elegans

Although the nonassociative form of learning, habituation, is often describ ed as the simplest form of learning, remarkably little is known about the c ellular processes underlying its behavioral expression. Here, we review res earch on habituation in the nematode Caenorhabditis elegans that addresses habituation at behavioral, neural circuit, and genetic levels. This work hi ghlights the need to understand the dynamics of a behavior before attemptin g to determine its underlying mechanism. In many cases knowing the characte ristics of a behavior can direct or guide a search for underlying cellular mechanisms. We have highlighted the importance of interstimulus interval (I SI) in both short- and long-term habituation and suggested that different c ellular mechanisms might underlie habituation at different ISIs. Like other organisms, C. elegans shows both accumulation of habituation with repeated training blocks and long-term retention of spaced or distributed training, but not for massed training. Exposure to heat shock during the interblock intervals eliminates the long-term memory for habituation but not the accum ulation of short-term habituation over blocks of training. Analyses using l aser ablation of identified neurons, and of identified mutants have shown t hat there are multiple sites of plasticity for the response and that glutam ate plays a role in long-term retention habituation training.