SUMMARYThe search for `the how and the where' of memory formation in the brain,the engram, is still one of the unattained `Holy Grails' of neuroscience. Over the years, various paths have been trodden in attempts to attain this goal,and while tantalizing glimpses appear now and then on the scientific horizon,the Grail still has not been grasped. One of the paths that investigators have walked is the invertebrate `model system' approach. Some invertebrates possess relatively simple nervous systems that mediate relatively simple behaviours that are both interesting and trainable. In this commentary, we would like to shed light on a relatively new player, the pond snail Lymnaea stagnalis L., that is being used in the quest to illuminate `the how and the where' the nervous systems encode and store memory. We will show that it is possible to demonstrate that a single neuron is a site of memory formation and storage for a form of associative learning in this lowly snail. It may be that the Grail is a little closer to being grasped.

Two causes of forgetting have been promulgated: memory trace decay and retroactive interference. The authors show that forgetting is an active process requiring both new learning and memory. In the present Lymnaea model system, prevention of new learning of a conflicting association, inhibition of memory consolidation, or Right Pedal Dorsal 1 soma ablation, which blocks LTM formation, are all potent means to prevent forgetting. Thus procedures that alter the ability to learn or form memory of a new conflicting aerial respiratory association prevent forgetting of a learned associative behavior. These results are the 1st demonstration in any model system that forgetting requires the soma of a single neuron.

Adult snails are capable of learning associatively not to perform aerial respiration and then to consolidate the acquired behaviour into long-term memory (LTM). Juvenile Lymnaea, however, perform aerial respiration significantly less often and the three-neuron circuit that drives this behaviour operates significantly differently than in it does in adults. We asked whether these ontogenic behavioural and neurophysiological differences are manifested as an altered ability of juveniles to learn and/or form LTM. We found that juvenile snails learn significantly less well than adults and are, as a group, incapable of forming LTM. To control for the possibility that the poor learning and inability to form memory were the result of juvenile's receiving on average fewer reinforcing stimuli because they perform aerial respiration less often than adults we subjected juveniles to an enforced period of hypoxia to 'motivate' juveniles. Motivated juveniles perform aerial respiration as often as adults; yet these 'motivated' juveniles continue to be poor learners and still cannot form LTM. Additionally, a small percentage of juveniles perform aerial respiration as often as adults (i.e. high responders). When these 'highresponders were trained they still exhibited poorer learning ability compared with adults and could not form LTM. We conclude that juvenile snails have a more difficult time learning and remembering to suppress aerial respiratory activity than do adults.

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