Abstract
Multisynaptic boutons (MSBs) are presynaptic boutons in contact with multiple postsynaptic partners. Although MSB synapses have been studied with static imaging techniques such as electron microscopy (EM), the dynamics of individual MSB synapses have not been directly evaluated. It is known that the number of MSB synapses increases with synaptogenesis and plasticity but the formation, behavior, and fate of individual MSB synapses remains largely unknown. To address this, we developed a means of live imaging MSB synapses to observe them directly over time. With time lapse confocal microscopy of GFP-filled dendrites in contact with VAMP2-DsRed-labeled boutons, we recorded both MSBs and their contacting spines hourly over 15 or more hours. Our live microscopy showed that, compared to spines contacting single synaptic boutons (SSBs), MSB-contacting spines exhibit elevated dynamic behavior. These results are consistent with the idea that MSBs serve as intermediates in synaptic development and plasticity.
Highlights
Axonal boutons of excitatory neurons frequently synapse with multiple postsynaptic partners
Electron microscopy (EM) studies have shown that the number of multisynaptic boutons (MSBs) synapses as a percentage of all synapses can vary from approximately 14% in layer I of the adult mouse neocortex [1] to 19–25% in adult rat hippocampal area CA1 [4,5]
MSBs in dissociated neuron cultures Dissociated neuron cultures provide an optimal system for studying the dynamics of intact synaptic contacts in live neurons because they allow simultaneous visualization of presynaptic boutons in direct contact with their postsynaptic partners, which is prohibitively difficult with slice or in vivo preparations
Summary
Axonal boutons of excitatory neurons frequently synapse with multiple postsynaptic partners. These have been alternatively termed multisynaptic boutons (MSBs), multisynapse boutons, multiple synapse boutons, and multiple synapses. Electron microscopy (EM) studies have shown that the number of MSB synapses as a percentage of all synapses can vary from approximately 14% in layer I of the adult mouse neocortex [1] to 19–25% in adult rat hippocampal area CA1 [4,5]. Some studies have suggested that MSBs most often contact spines from different dendrites [4,6], while others have shown that both same- and different-dendrite MSB synapses are found in significant numbers [7,8]. Primarily same-dendrite MSBs are formed following LTP induction in rat hippocampal slices [8]
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