The molecular infrastructure of glutamatergic synapses in the mammalian forebrain

Glutamatergic synapses form the vast majority of connections within neuronal circuits but how these subcellular structures are molecularly organized within the mammalian brain is poorly understood. Conventional electron microscopy using chemically fixed, metal-stained tissue has identified a proteinaceous, membrane-associated thickening called the postsynaptic density (PSD). Here, we combined mouse genetics and cryo-electron tomography to determine the 3D molecular architecture of fresh synapses in the adult forebrain. The native glutamatergic synapse lacked a PSD. Instead, a concentrated synaptoplasm consisting of cytoskeletal elements, macromolecular complexes and membrane-bound organelles extended throughout the pre- and post-synaptic compartments. Snapshots of active processes gave insights into the architectural basis for synaptic remodeling. Clusters of 4-60 ionotropic glutamate receptors were positioned inside and outside the synaptic cleft. Together, these information-rich tomographic maps present a detailed molecular framework for the coordinated activity within mammalian brain synapses.