Infant cognitive training preshapes learning-relevant prefrontal circuits for adult learning: learning-induced tagging of dendritic spines.

Work in various animal models has demonstrated that cognitive training in infancy has a greater effect on adult cognitive performance than pretraining in adulthood. Since the underlying synaptic mechanisms are unclear, the aim of this study was to test the working hypothesis that associative training "preshapes" synaptic circuits in the developing infant brain and thereby improves learning in adulthood. Using a two-way active avoidance (TWA) paradigm, we found that avoidance training during infancy, even though the infant rats were not capable to learn a successful avoidance strategy, improves avoidance learning in adulthood. On the neuroanatomical level we show here for the first time that infant TWA training in the ventromedial prefrontal cortex suppresses developmental spine formation. In contrast in the lateral orbitofrontal cortex, developmental spine pruning is suppressed, possibly by "tagging" activated synapses, which thereby are protected from being eliminated. Moreover, we demonstrate that infant TWA training alters learning-induced synaptic plasticity in the adult brain. The synaptic and dendritic changes correlate with specific behavioral parameters. Taken together, these results support the working hypothesis that infant cognitive training interferes with developmental reorganization and maturation of dendritic spines and thereby "optimizes" prefrontal neuronal circuits for adult learning.