Synapse-specific structural plasticity that protects and refines local circuits during LTP and LTD

Area of Science:

• Neuroscience
• Cell Biology
• Synaptic Plasticity

Background:

• Synapses are crucial for brain function, forming trillions of connections.
• Long-term potentiation (LTP) and long-term depression (LTD) are key cellular mechanisms for learning and memory.
• Synaptic strength and structure are dynamically modified by these processes.

Purpose of the Study:

• To investigate the structural changes at synapses during LTP and LTD.
• To understand the role of different synaptic zones in plasticity.
• To propose a model for synapse modification during learning and memory consolidation.

Main Methods:

• Three-dimensional reconstruction using serial section electron microscopy.
• Analysis of pre- to post-synaptic arrangements, including active zones (AZs) and nascent zones (NZs).
• Observation of vesicle docking and postsynaptic density formation.

Main Results:

• Three distinct synaptic arrangements were identified: strong AZs, weak AZs, and NZs.
• LTP onset involves vesicle recruitment to NZs, converting them to AZs.
• During LTP recovery, new NZs form, particularly on spines with enlarged AZs; sentinel spines with SER play a role.
• LTP and LTD saturation may protect recent memories.

Conclusions:

• A model is proposed where NZ plasticity drives synapse-specific AZ expansion during LTP and weak AZ loss during LTD.
• Spine clusters are functionally engaged or disassembled based on plasticity.
• Saturation of plasticity likely protects newly formed memories, explaining benefits of spaced learning.