Insulin can induce the expression of a memory-related synaptic protein through facilitating AMPA receptor endocytosis in rat cortical neurons

Learning and memory depend on long-term synaptic plasticity including long-term potentiation (LTP) and depression (LTD). Activity-regulated cytoskeleton-associated protein (Arc) plays versatile roles in synaptic plasticity mainly through inducing F-actin formation, underlying consolidation of LTP, and promoting AMPA receptor (AMPAR) endocytosis, underlying LTD. Insulin can also induce LTD by facilitating the internalization of AMPARs. In neuroblastoma cells, insulin induced a dramatic increase in Arc mRNA and Arc protein levels, which may underlie the memory-enhancing action of insulin. Thus, a hypothesis was made that, in response to insulin, increased AMPAR endocytosis leads to enhanced Arc expression, and vice versa. Primary cultures of neonatal Sprague-Dawley rat cortical neurons were used. Using Western-blot analysis and immunofluorescent staining, our results reveal that inhibiting AMPAR-mediated responses with AMPAR antagonists significantly enhanced whereas blocking AMPAR endocytosis with various reagents significantly prevented insulin (200 nM, 2 h)-induced Arc expression. Furthermore, via surface biotinylation assay, we demonstrate that acute blockade of new Arc synthesis after insulin stimulation using Arc antisense oligodeoxynucleotide prevented insulin-stimulated AMPAR endocytosis. These findings suggest for the first time that an interaction exists between insulin-stimulated AMPAR endocytosis and insulin-induced Arc expression.