Drebrin A is a postsynaptic protein that localizes in vivo to the submembranous surface of dendritic sites forming excitatory synapses

Drebrin A is a neuron-specific, actin binding protein. Evidence to date is from in vitro studies, consistently supporting the involvement of drebrin A in spinogenesis and synaptogenesis. We sought to determine whether drebrin A arrives at the plasma membrane of neurons, in vivo, in time to orchestrate spinogenesis and synaptogenesis. To this end, a new antibody was used to locate drebrin A in relation to electron microscopically imaged synapses during early postnatal days. Western blotting showed that drebrin A emerges at postnatal day (PNd) 6 and becomes progressively more associated with F-actin in the pellet fraction. Light microscopy showed high concentrations of drebrin A in the synaptic layers of the hippocampus and cortex. Electron microscopy revealed that drebrin A in these regions is located exclusively in dendrites both neonatally and in adulthood. In adulthood, nearly all of the synaptic drebrin A is within spines forming asymmetric excitatory synapses, verified by gamma-aminobutyric acid (GABA) negativity. At PNd7, patches of drebrin A immunoreactivity were discretely localized to the submembranous surfaces of dendrites forming slight protrusions-protospines. The drebrin A sites exhibited only thin postsynaptic densities and lacked axonal associations or were contacted by axons that contained only a few vesicles. Yet, because of their immunoreactivity to the NR2B subunit of N-methyl-D-aspartate receptors and immunonegativity of axon terminals to GABA, these could be presumed to be nascent, excitatory synapses. Thus, drebrin A may be involved in organizing the dendritic pool of actin for the formation of spines and of axospinous excitatory synapses during early postnatal periods.