Increased immunogold labelling of neural cell adhesion molecule isoforms in synaptic active zones of the chick striatum 5-6 hours after one-trial passive avoidance training.

An area of the chick striatum, the lobus parolfactorius plays an important role in one-trial passive avoidance learning tasks. In the present study we report evidence that 5-6 h post-training, a significantly higher proportion of synaptic active zones in this area contain labelled epitopes of the neural cell adhesion molecule, with the greatest occurrence of labels at the edges of active zone profiles (in both control and trained groups). This suggests that there is a period after training when expression of the neural cell adhesion molecule in synaptic membranes almost doubles, and that events at active zone edges may play a specific role in mechanisms of synaptic adhesion. Cellular mechanisms of long-term memory formation are believed to include alterations in neural circuitry at the synaptic level. The involvement of the neural cell adhesion molecule (NCAM) in functional synaptic modifications has been demonstrated using a number of physiological models. Performance of rats in the Morris water maze, a spatial learning paradigm which requires the hippocampus, is impaired by either intraventricular injection of NCAM antibodies, or injection into the hippocampus of an enzyme which increases homophilic adhesion of the molecule, due to the removal of polysialic acid residuals from extracellular NCAM domains. In addition, intraventricular injections of anti-NCAM antibodies 6-8 h post-training were shown to impair memory for a one-trial passive avoidance task in the rat. An avoidance training model in the one-day-old chick indicates a similar time window, 5-6 h post-training during which memory for the task can be impaired by intraventricular injection of NCAM antibodies. In the hyperstriatum ventrale, a chick forebrain area involved in the passive avoidance task. subtle changes in the distribution pattern, but not density of NCAM molecules in synaptic membranes were revealed 5-6 h post-training. However, on the basis of studies of synaptic morphometry, a region of striatum, the lobus parolfactorius (LPO), appears to play a more important role in longer term memory storage for the task.