# Aging-independent decrease of complex multi-spine boutons in hippocampal area CA1 after contextual fear conditioning

## TL;DR

This study shows that the complexity of multi-spine boutons in the hippocampus decreases after a memory-forming event, and this change is not affected by aging.

## Contribution

The study reveals an aging-independent synaptic change in multi-spine bouton complexity after contextual fear conditioning.

## Key findings

- Complex multi-spine boutons decrease in the hippocampal CA1 region 24 hours after contextual fear conditioning.
- This decrease occurs in both young and aged mice, showing no effect of aging on the synaptic change.
- Reduced MSB complexity may support specific memory recall by altering neuronal connectivity.

## Abstract

Long-lasting synaptic changes enable memory storage and regulate recall in the brain. Our previous work established that generation of multi-innervated dendritic spines (MISs), spines with typically two excitatory presynaptic inputs, underlies hippocampal memory formation in aged, but not young mice. The identification of MIS generation was done by ultrastructural analysis in hippocampal CA1 stratum radiatum 24 h after contextual fear conditioning (CFC). However, our analysis did not consider multi-spine boutons (MSBs), which were recently shown to increase in complexity (complex MSBs are pre-synaptic boutons connecting with more than two post-synapses) at a later time point after CFC in young age. Therefore, we re-analyzed our three-dimensional electron microscopy images and show that, unexpectedly, MSB complexity, decreases in CA1 stratum radiatum 24 h after CFC. The decrease in MSB complexity occurred both in young and aged mice, indicating that aging has no impact on this synaptic change. Considering that complex MSBs link the activity of multiple postsynaptic neurons, we suggest that after CFC a decrease in MSB complexity may be required for specific memory recall.

The online version contains supplementary material available at 10.1186/s13041-025-01265-z.