MORPHOGENESIS IN MEMORY FORMATION - SYNAPTIC AND CELLULAR MECHANISMS

We review some of the evidence for structural changes in synapses in r esponse to environmental stimulation. These include changes in synapse number, in distribution of presynaptic vesicles, in synaptic bouton s ize, and complex changes in the shape and size of synaptic contact zon es. Increased numbers of postsynaptic polyribosomal aggregates (PRA) a re correlated histologically with developmental plasticity. We discuss the role that dendritically targeted mRNAs and polyribosomes might pl ay in providing rapid, localized synthesis of proteins necessary for s tructural change. Using synaptoneurosomes, we have demonstrated that d epolarization leads to a rapid (1-2 min) increase in PRA and in [S-35] methionine incorporation into polypeptides. We have shown that this pr ocess is initiated by metabotropic glutamate receptors, which trigger phosphatidyl inositol hydrolysis, leading to release of internal Ca2stores and activation of protein kinase C. Entry of external Ca2+, how ever, seems to downregulate polyribosomal aggregation, via a calmoduli n-dependent mechanism, suggesting that translation may be controlled b y interaction of ionotropic receptors, voltage-dependent calcium chann els, and metabotropic receptors.