This laboratory previously discovered that the post synaptic density increases in thickness in minutes with stimulation, and recovers rapidly upon cessation of stimulation. Addition of CaMKII contributes to the increase in thickness;dephosphorylation is necessary for recovery;and and recovery is blocked in LTP. Technical advances and new detailed knowledge of the detailed molecular structure of the post synaptic density (PSD) emerging from our Synaptic Architecture Project now permit progress on further questions related to post synaptic dynamics, such as what other components of the PSD participate in rapid structural changes, how exactly they are incorporated into the PSD, and what is the role phosphorylation of these proteins. It appeared essential first to examine PSD structure in intact brains, which at the present state of the art precludes direct freezing to immobilize and stabilize them. A major effort in our laboratory has resulted in large improvements in the brain perfusion technique which, as typically used, directly stimulates the brain. Now that the basal level synaptic structure can be determined, many subtle effects of activity are becoming apparent, for example that stimulation induces direct interaction of pre- and post synaptic membranes at spinules. Identification of other players in the dynamic structural changes at synapses is being pursued with pre-embedding immunolabeling of hippocampal cultures, where it has become apparent that the scaffolding protein SHANK, but not its binding partner GKAP, reversibly moves to the PSD during activity. Our application of quantitative mass spectrometric techniques has revealed new CaMKII-mediated phosphorylation sites on the key PSD scaffolds, Shanks and GKAPs, showing that phosphorylation-induced re-organization of the PSD may follow stimulation.