Behaviorally evoked transient reorganization of hippocampal spines.

Dendritic spines, microstructures that receive the majority of excitatory synaptic inputs, are fundamental units to integrate and store neuronal information. The morphological reorganization of spines accompanies the functional alterations in synaptic strength underlying memory-relevant modifications of network connectivity. Here we report the rapid dynamics of cell population-selective spine reorganizations related to behavioral experiences. In Thy1-GFP transgenic mice, hippocampal CA1 pyramidal neurons that were putatively activated during environmental explorations were detected with their post hoc immunoreactivity for Arc, an activity-dependent immediately-early gene. Immediately after a 60-min exposure to a familiar environment, the spine densities of Arc-positive and Arc-negative neurons were differently distributed. This density imbalance was due exclusively to changes in the number of small, rather than large, spines. The change disappeared within 60 min after mice were returned to the home cages. Thus, spines possess the ability to rapidly and reversibly alter their morphology in response to a brief environmental change. We propose that these transient spine dynamics represent a latent preliminary stage for longer-term plasticity on demand.