Synapsin I plays a key role in synaptic plasticity in brain. This effect is due in large part to the ability of the synapsins to regulate the availability of synaptic vesicles for release. In addition to its role in plasticity, the e, Xenopus/Amphibian,pression of synapsin I is a precise indicator of synapse formation. Thus, synapsin I immunocytochemistry provides a valuable tool for the study of synaptogenesis. The role of synapsin in synaptic plasticity and in synaptogensis is regulated by phosphorylation. Ser 549 along with Ser 62 and Ser 67 are the sites of synapsin I that are phosphorylated by MAP kinase. Phosphorylation and subsequent dephosphorylation of these sites are thought to play a key role in synaptic vesicle trafficking.