Progression of activity and structural changes in the anterior cingulate cortex during remote memory formation

• Conditioned mice show strong freezing on post-training days 1, 5, 7, 14, 21 and 36.
• C-fos activation in anterior cingulate cortex augments regularly from day 5 to day 21.
• Spine density on neurons in the same region augments at day 7 and then remains stable.
• Single neuron spine density scores become more homogeneous as memory matures over time.

The progression of activity and structural changes in the anterior cingulate cortex during remote contextual fear memory formation was measured by imaging c-fos expression and dendritic spines following retrieval tests administered at six post-training time points (days 1, 5, 7, 14, 21, 36). Here we report that conditioned mice exhibit robust freezing at each time point. C-fos expression starts to augment on day 5, showing a monotonic increase over the successive time points, and then stabilized in relation to the higher freezing scores. The first significant increase in mean spine density emerges on day 7. By day 14, the net number of spines remained stable, yet the distribution of single neuron spine density becomes progressively more homogeneous. Our findings reveal that activity changes precede structural remodeling of neurons in the neocortex while remodeling coherence develops gradually in cortical neuron ensembles.