Consolidation of memory for odour-reward association requires transient polysialylation of the neural cell adhesion molecule in the rat hippocampal dentate gyrus

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Abstract
Cell adhesion molecule function is involved in hippocampal synaptic plasticity and is associated with memory consolidation. At the infragranular zone of the dentate gyrus, neurons expressing the polysialylated form of the neural cell adhesion molecule (NCAM PSA) transiently increase their frequency at the 12‐hr posttraining time in behaviours elicited by stressful stimuli, such as those associated with conditioned avoidance, water maze, and fear conditioning paradigms. To determine whether learning‐induced modulation of NCAM polysialylation is limited to stressful paradigms, we employed a reward‐based odour discrimination task. Animals show a rapid acquisition and recall of this task in terms of latency to identify the food‐associated odour and the number of choice errors. Immunohistochemical procedures were employed to determine the change in NCAM PSA expression following task acquisition. NCAM PSA immunoreactivity in the hippocampal formation was most intense on the granule‐like neurons in the infragranular zone of the dentate gyrus, and their frequency transiently increased in the 12‐hr posttraining period. The nature of the transient increase in NCAM PSA‐immunoreactive neurons was indistinguishable from that observed following avoidance conditioning or spatial learning, in that it occurred at the same time. The transient increase in NCAM PSA expression is suggested to facilitate dendritic elaboration in response to the acquisition of novel behavioural repertoires.