Reactivation of a dormant developmental plasticity program drives synaptic rejuvenation and antidepressant-like responses

Antidepressants are widely used to treat mood and anxiety disorders, yet the cellular and molecular mechanisms underlying their therapeutic effects remain poorly understood. Here, we show that the selective serotonin reuptake inhibitor (SSRI) fluoxetine rejuvenates the dentate gyrus (DG) by reactivating a latent developmental plasticity program in mature neurons, which is required for re-engaging critical period-like synaptic remodeling and antidepressant-like behavioral effects. Single nuclei transcriptomic profiling across all hippocampal cell types revealed a strikingly selective response in the DG, where mature neurons re-engaged a developmental-like transcriptional state, an effect distinct from adult neurogenesis. Chronic fluoxetine treatment induced expression of SOX11, a developmental transcription factor involved in neurodevelopment and axonal regeneration that is normally silenced in the adult brain. Reactivation of SOX11 in mature DG neurons was necessary for both the antidepressant-like behavioral effects of fluoxetine and the re-engagement of critical period-like synaptic remodeling. Notably, SOX11 reactivation was also induced by physiological stimuli, including environmental enrichment and peripheral axon regeneration, suggesting that mature neurons retain access to a conserved developmental plasticity program throughout adulthood. We further identified evidence of this form of developmental plasticity in the adult human brain, an effect distinct from adult neurogenesis. At the structural level, fluoxetine remodeled novel extracellular matrix (ECM) structures within the DG distinct from canonical perineuronal nets, and targeted ECM degradation was sufficient to reactivate SOX11 in mature neurons, identifying the ECM as a key regulator of neuronal rejuvenation. Altogether, these findings demonstrate that mature neurons retain an intrinsic capacity to re-engage dormant developmental programs, and that antidepressants harness this regenerative plasticity to drive neural circuit remodeling and antidepressant-like effects.