MEMORY CONSOLIDATION INDUCES A TRANSIENT AND TIME-DEPENDENT INCREASE IN THE FREQUENCY OF NEURAL CELL-ADHESION MOLECULE POLYSIALYLATED CELLSIN THE ADULT-RAT HIPPOCAMPUS

Animals trained in a passive avoidance task exhibit a transient time-d ependent increase in hippocampal neural cell adhesion molecule (NCAM) polysialylation at 12-24 h following the initial learning trial. Using immunocytochemical techniques with a monoclonal antibody that specifi cally recognises NCAM-polysialic acid homopolymers, a distinct populat ion of granule-like cells, at the border of the granule cell layer and the hilus in the dentate gyrus of the adult rat hippocampus, has been demonstrated to exhibit time-dependent change in frequency at 10-12 h following the initial learning of a one-trial, step-through, passive avoidance response. These changes were paradigm specific as they faile d to occur in those animals rendered amnesic with scopolamine. These p olysialylated dentate neurons are not de novo granule cell precursors as administration of 5-bromo-2'-deoxyuridine every 2 h from the point of learning to the 12-h posttraining time showed no significant differ ence between trained and passive animals in the small number of hetero geneously distributed, labelled cells. These findings directly identif y a morphological substrate of memory, implied by previous correlative and interventive studies on NCAM function.