Crowding, an environmental stressor, blocks long-term memory formation in Lymnaea.

Crowding is an environmental stressor. We found that this stressor altered (i.e. prevented) the ability of Lymnaea to form long-term memory (LTM) following operant conditioning of aerial respiratory behaviour. The ability to form LTM was compared between snails that had been crowded (20 snails per 100 ml of pond water) and those maintained in uncrowded conditions (two snails per 100 ml of pond water). Crowding either immediately before or after two different operant conditioning procedures - the traditional training procedure and the memory augmentation procedure - blocked LTM formation. However, if crowding is delayed by more than 1h following training or if crowding stops 1h before training, LTM results. If memory is already formed, crowding does not block memory recall. Pond water from a crowded aquarium or crowding with clean shells from dead snails, or a combination of both, is insufficient to block LTM formation. Finally, crowding does not block intermediate-term memory (ITM) formation. Since ITM is dependent on new protein synthesis whereas LTM is dependent on both new protein synthesis and altered gene activity, we hypothesize that crowding alters the genomic activity in neurons necessary for LTM formation.