Factors affecting long-term habituation in Caenorhabditis elegans
Creators: name: Beck, Christine Daily O’Brien
Publisher: name: University of British Columbia
Year: 2009

description: The objective of these experiments was to explore long-term memory in Caenorhabditis elegans. This examination of memory in a simple organism with accessible genetics and a well understood biology may permit later work to define the cellular processes that underlie long-term memory. Habituation training with a vibrational stimulus was administered on Day 1, and the retention test of a block of stimuli was given 24 h after the end of training on Day 2. Long-term retention of habituation was evident as a lower level of responding on Day 2 relative to the level of responding on Day 2 of untrained controls or the initial level of responding of worms on Day 1. In Experiments 1 and 2, a habituation training protocol that produced long-term retention of habituation was established, and the effects of stimulus number, interstimulus interval (ISI), and distribution of training on both short-term and long-term habituation were examined. In Experiment 1 (10-s ISI), there appeared to be a floor effect which resulted in a low level of responding regardless of training on Day 1; thus no evidence for long-term habituation after training at a 10-s ISI could be found. In Experiment 2 (60-s ISI), worms that received distributed and massed habituation training with 60 stimuli showed a significantly lower level of responding relative to untrained controls. The distributed habituation training appeared to be more effective at inducing long-term habituation and was used in the subsequent experiments. To characterize the effects of heat shock treatments used in the behavioral experiments that follow, the effects of heat shock on two assays, the induction of a heat shock protein gene, hsp16, and the rate of egg-laying were measured in Experiment 3. All heat shock treatments used caused the induction of hsp16. In addition, the number of eggs laid during a fixed interval after heat shock was sensitive to the heat shock treatments given in Experiments 4 through 8. In Experiments 4 through 8, the effects of heat shock on short- and long-term habituation were examined. Heat shock, which acts as a general cellular stressor, was administered at different times before, during and after training. In Experiment 4, heat shock (45 min, 32°C) was administered, ending 2 h before training on Day 1. Heat shock before training did not affect the initial level of responding on Day 1, habituation during training, short-term retention of habituation between blocks of training or long-term retention of habituation. In Experiment 5, heat shock (45 min, 32°C) was administered during the rest periods of distributed training in the 1-h interval after each training block. While heat shock during training had no significant effect on responding on Day 1, long-term habituation was blocked. In Experiment 6, the possibility that heat shock before training would prevent the disruption of long-term habituation by heat shock during training by inducing thermal tolerance was examined. This was tested by administering heat shock (45 min, 32°C) that ended 2 h before training and heat shock during training. It was found that heat shock before training did not prevent the disruption of long-term habituation by heat shock during training. In Experiment 7, the effect of heat shock that ended 2 h before the retention test on Day 2 on the retention of long-term habituation was examined. It was found that heat shock on Day 2 did not disrupt the retention of habituation. Finally, in Experiment 8, the effect of brief heat shock (15 min, 32°C) at different intervals in the rest period following the training blocks was examined in an attempt to more narrowly define a critical period for consolidation of long-term habituation. Although there was no significant effect of brief heat shock on retention of habituation, the pattern of the data suggests that there may be a period of greater vulnerability worth further investigation. In summary, heat shock given before training or before the retention test did not affect long-term habituation, while heat shock during training disrupted consolidation of long-term habituation. Taken together, these behavioral results provide the foundation for an investigation of the cellular processes underlying long-term memory in C. elegans. By exploring the dynamics of the formation of long-term habituation, intervals of time critical to the formation of long-term habituation were defined. This in turn will help to focus attention on the cellular processes whose activity during those intervals of time may be important to the consolidation of long-term memory.