Consecutive clusters of GluR1 were categorized in a series of two separate platings as either associated or unassociated with presynaptic staining for the synaptic vesicle proteins synaptophysin or synapsin 1, as described in Materials and Methods. A lack of synaptic vesicle accumulation could be attributable to either a complete absence of nearby synaptic vesicles (the majority of cases) or a clear misalignment of postsynaptic clusters and presynaptic vesicles (see Fig. 6 for examples). Cultures stained for synapsin 1 were also stained with an antibody to tau to identify the presence of axons at these early GluR1 clusters.