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Bidirectional changes in synapsin I phosphorylation at MAP kinase‐dependent sites by acute neuronal excitation in vivo

Synapsin I is a synaptic vesicle‐associated protein which is phosphorylated at multiple sites by various kinases. It has been proposed to play a role in the regulation of neurotransmitter release and the organization of cytoskeletal architecture in the presynaptic terminal. To better understand the physiological regulation of its phosphorylation in vivo, we induced acute, reversible neuronal excitation by electroconvulsive treatment (ECT) in rats, and studied its effects on synapsin I phosphorylation at sites 3, 4/5 and 6 by immunoblot analyses of homogenates from hippocampus and parietal cortex using phospho‐site‐specific antibodies. A decrease in phosphorylation at all sites was observed soon after the electrical stimulation, followed by a large increase in phosphorylation at site 4/5 peaking at 5 min and a moderate increase in phosphorylation at site 6 peaking at 20 min. Systemic injection of SL327, a mitogen‐activated protein kinase (MAPK) kinase inhibitor, prior to ECT, suppressed the increase in phospho‐site 4/5 level, as well as that in MAPK activity, but not that in phospho‐site 6 level. Thus, phosphorylation at site 4/5 of synapsin I has been shown to be regulated by MAPK in vivo.