Noradrenaline and microglia maintain plasticity-rigidity balance to safeguard rapid emotional learning in the prefrontal cortex
Balancing plasticity and rigidity is essential for brain function. While the neocortex typically shifts toward rigidity during development to preserve circuit integrity, certain forms of socially acquired emotional learning involve rapid remodeling in the medial prefrontal cortex (mPFC), raising the question of how such flexibility is achieved without compromising pre-existing representations. We found that activity-dependent dendritic spine enlargement is actively suppressed by microglia via a humoral factor in the adolescent mPFC. Noradrenaline lifted this suppression through cAMP signaling in microglia, permitting spine enlargement. Disruption of this noradrenaline-dependent pathway in the mPFC impaired the rapid acquisition of socially learned fear. Conversely, localized microglial ablation enhanced learning efficiency but disrupted emotional boundaries, inducing social fear learning to alter pain sensitivity. These findings demonstrate that noradrenaline and microglia regulate the plasticity-rigidity balance within an optimal range in the mPFC, enabling rapid social learning while safeguarding the functional segregation of emotional circuits.