Abstract
Excitatory synapses in the mammalian brain exhibit diverse functional properties in transmission and plasticity. Directly visualizing the structural correlates of such functional heterogeneity is often hindered by the diffraction-limited resolution of conventional optical imaging techniques. Here, we used super-resolution stochastic optical reconstruction microscopy (STORM) to resolve structurally distinct excitatory synapses formed on dendritic shafts and spines. The majority of these shaft synapses contained N-methyl-d-aspartate receptors (NMDARs) but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), suggesting that they were functionally silent. During development, as more spine synapses formed with increasing sizes and expression of AMPARs and NMDARs, shaft synapses exhibited moderate reduction in density with largely unchanged sizes and receptor expression. Furthermore, upon glycine stimulation to induce chemical long-term potentiation (cLTP), the previously silent shaft synapses became functional shaft synapses by recruiting more AMPARs than did spine synapses. Thus, silent shaft synapse may represent a synaptic state in developing neurons with enhanced capacity of activity-dependent potentiation.
Highlights
In the mammalian brain, excitatory communication between neurons is primarily mediated by glutamatergic synapses[1,2]
In the current study, we used low density culture of rat hippocampal neurons that formed synaptic connections starting from ~11 days in vitro (DIV)
With immunofluorescence labeling of presynaptic scaffolding protein bassoon and postsynaptic amino-3-hydroxy-5methyl-4-isoxazolepropionic acid receptors (AMPARs) subunit GluA1, many synapses were visible under conventional fluorescence microscopy as fluorescent puncta with overlapping bassoon and GluA1 signals and without much discernable substructures (Fig. 1), because these synapses were usually hundreds of nanometers in size, close to the diffraction limit of optical microscopy
Summary
Excitatory communication between neurons is primarily mediated by glutamatergic synapses[1,2]. Activity-induced plasticity of these synapses is believed to underlie learning and memory function of the brain[3,4,5,6]. An extreme case is the so-called silent synapse[9,10,11,12], which contains few α-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid receptors (AMPARs) and cannot carry out excitatory transmission upon presynaptic activation, but can be converted into the. Xu et al Cell Discovery (2020)6:8 of single molecule localization-based super-resolution fluorescence microscopy[22,23] and its quantitative capability, to investigate in cultured hippocampal neurons the morphology and receptor expression of different forms of excitatory synapses and their changes during development and plasticity
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