1Information processing in the brain depends on synaptic transmission and plasticity, which in 2 turn require specialized organization of neurotransmitter receptors and scaffolding proteins 3 within the postsynaptic density (PSD). However, how these molecules are organized in situ 4 remains largely elusive, limiting our mechanistic understanding of synaptic formation and 5 functions. Here, we have developed template-free classification of over-sampled sub-6 tomograms to analyze cryo-electron tomograms of hippocampal synapses, enabling us to 7 identify type-A γ-aminobutyric acid receptor (GABA A R) in inhibitory synapses and determine 8 its in situ structure at 19 Å resolution. We found that these receptors are organized 9 hierarchically: from GABA A R super-complexes with a fixed 11-nm inter-receptor distance but 10 variable relative angles, through semi-ordered two-dimensional receptor networks with 11 reduced Voronoi entropy, to mesophasic assembly with a sharp phase boundary. This 12 assembly aligns with condensates of postsynaptic scaffolding proteins and putative 13 presynaptic vesicle release sites. Such mesophasic self-organization may allow synapses to 14 achieve a "Goldilocks" state with a delicate balance between stability and flexibility, enabling 15both reliability and plasticity in information processing. 16 hierarchical organization of GABA A Rs within the PSD, establishing the structural basis for 44 synaptic transmission and plasticity. 45 46
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