Assembly of new individual excitatory synapses: Time course and temporal order of synaptic molecule recruitment

Time-lapse microscopy, retrospective immunohistochemistry, and cultured hip pocampal neurons were used to determine the time frame of individual glutam atergic synapse assembly and the temporal order in which specific molecules accumulate at new synaptic junctions. New presynaptic boutons capable of a ctivity-evoked vesicle recycling were observed to form within 30 min of ini tial axodendritic contact. Clusters of the presynaptic active zone protein Bassoon were present in all new boutons. Conversely, clusters of the postsy naptic molecule SAP90/PSD-95 and glutamate receptors were found on average only similar to 45 min after such boutons were first detected. AMPA- and NM DA-type glutamate receptors displayed similar clustering kinetics. These fi ndings suggest that glutamatergic synapse assembly can occur within 1-2 hr after initial contact and that presynaptic differentiation may precede post synaptic differentiation.