Early life experiences selectively mature learning and memory abilities

The study investigates how early-life (infant) learning experiences shape the maturation of hippocampus-dependent learning and memory abilities, a process that is not well understood and is characterized behaviorally by rapid forgetting (infantile amnesia). The authors ask whether episodic learning during infancy induces distinctive molecular and synaptic changes in the hippocampus compared to later developmental stages (e.g., PN24 and adults), whether these changes are required for infant memory formation, and whether subsequent experiences can capture these changes to promote the functional competence necessary for long-term memory expression. They further test whether such maturation is selective to the type of experience (learning domain) encountered or generalizes across different hippocampus-dependent tasks.

Background work indicates infancy is a critical period for learning and memory development, often exhibiting infantile amnesia despite intact learning. Prior studies implicate immediate early genes (IEGs; Arc, c-Fos, Zif268) in memory trace formation, BDNF/TrkB signaling in synaptic maturation and critical period plasticity, and synaptic proteins such as PSD-95 and synaptophysin in excitatory synapse formation and stabilization. AMPA receptor trafficking and phosphorylation (GluA1 Ser831/Ser845) regulate synaptic plasticity. Enriched environment and experience-dependent plasticity literature suggests experiences can shape neural circuit structure and function, but the selectivity of experience-driven maturation in early life has remained unclear. This work builds on findings that infant memories can be reinstated later (savings) and that hippocampal molecular profiles change over development, testing whether early experiences selectively mature domain-specific memory competence.

- Subjects: Infant and juvenile rats (primarily PN17 and PN24; Long Evans) and mice. Male and female rats were housed pre-weaning; electrophysiology used three male rats per group. - Behavioral paradigms: • Inhibitory avoidance (IA) in rats at PN17, PN18/19, PN24; latency to enter shock compartment measured for acquisition and retention; reminder shock (RS) used for reinstatement/savings. • Contextual variants (contexts A and B) to test context specificity and transfer within IA. • Novel object location (nOL) at PN17/PN18 in rats and mice; exploration time of moved vs stationary object measured (percent time on moved object). • Contextual fear conditioning (CFC) in mice at PN17 (three unsignaled footshocks) with freezing measured at tests. - Molecular assays: Western blot analyses of hippocampal (dorsal hippocampus, dHC) extracts at multiple time points after training (30 min to 7 days). Targets included IEGs (Arc, Zif268, c-Fos), PSD-95, synaptophysin, AMPAR subunits GluA1/GluA2 and phospho-GluA1 (Ser845 and Ser831). Actin served as loading control. Sample sizes typically n=4–10 rats per group; multiple naive age-matched controls (e.g., PN17/PN19; PN24/PN26). - Pharmacological/ODN manipulations: • Bilateral dHC injections of anti-BDNF antibody vs control IgG 30 min before training to test BDNF dependence of synaptic protein changes. • PSD-95 antisense ODN (AS-ODN) vs scrambled ODN (SCR-ODN), injected immediately after training and 6 h later, to block de novo PSD-95 synthesis; assessed effects on molecular changes, electrophysiology, and memory. • c-Fos antisense ODN (two injections: immediately and 6 h after first learning) to reduce learning-induced neuronal activation; assessed impact on the ability of the second learning to achieve long-term memory. • BDNF infusion into dHC immediately after nOL to enhance nOL retention. - Electrophysiology: Whole-cell recordings in hippocampal slices (from rats trained at PN17 or PN24 and age-matched naive) to assess AMPAR-mediated EPSP amplitude and decay kinetics in CA1 pyramidal neurons; paired-pulse ratios measured as a proxy for presynaptic release; intrinsic properties assessed. Effects of PSD-95 AS-ODN vs SCR-ODN on AMPAR responses were tested in slices near cannula sites. Typical sample sizes: ~14 cells per condition for training-age comparisons; ~18–23 cells for ODN experiments. - Chemogenetics in mice: c-fos/tTA/tetO-hM3Dq double-transgenic mice trained at PN17 in CFC; CNO administered systemically 30 min before a 7-day test to reactivate cells tagged at learning and assess memory reinstatement/maturation. Control mice carried only one of the transgenes. - Savings/Reinstatement: Low-intensity footshock in training context given 7 days after CFC to test reinstatement of ‘forgotten’ infant memory. - Cross-domain tests: Sequential training across IA and nOL (in both orders) spaced by 24 h, with or without BDNF or chemogenetic reactivation, to test whether maturation of functional competence transfers across learning domains or remains experience-selective. - Statistics: One- and two-way ANOVA with appropriate post hoc tests (Dunnett’s, Tukey’s, Bonferroni), repeated-measures where applicable; one-sample t tests vs chance for nOL; significance threshold p<0.05. No a priori power calculations; no randomization reported; blinding used for behavioral scoring and some assays.

- Infant learning induces prolonged neuronal activation and synaptic maturation: IA training at PN17 produced slow, lasting induction of IEGs (Arc, Zif268, c-Fos) peaking ~24–48 h and returning to baseline by ~72 h; this contrasted with PN24, which showed rapid, transient IEG induction peaking at 30 min. - Synapse formation/maturation markers increased after PN17 learning: PSD-95 levels rose significantly (peak ~24 h, elevated at 48 h), synaptophysin increased from ~9 h to 48 h, and phospho-GluA1 Ser845 increased from 30 min to 48 h; Ser831 increased more gradually, significant by 24 h. These changes were not observed after training at PN24. Many measures returned to control by ~72 h. - BDNF dependence: Anti-BDNF injections into dHC before PN17 training blocked learning-induced increases in PSD-95 and synaptophysin at 24 h but did not affect pGluA1(Ser845/Ser831), indicating dissociable regulation (structure vs receptor phosphorylation). - PSD-95 is required for infant memory and AMPAR maturation: PSD-95 AS-ODN blocked the PN17 learning-induced increase in PSD-95, prevented reinstatement of IA memory at 7 days (that was recoverable in controls by reminder shock), and blocked learning-induced increases in AMPAR EPSP amplitude and decreases in decay time. PSD-95 AS-ODN had no effect on memory when training occurred at PN24. - Postsynaptic AMPAR response maturation at PN17: Training at PN17 increased AMPAR EPSP amplitude and decreased decay time in CA1 pyramidal neurons; paired-pulse ratios were unchanged (postsynaptic locus). These electrophysiological changes required PSD-95. - Two spaced learning events (24–48 h) mature functional memory competence: Rats trained at PN17 followed by a second IA trial at 24 or 48 h exhibited robust, long-lasting memory, persisting to PN24 and detectable at PN47 (though reduced), whereas single trials at PN17 or PN19 were rapidly forgotten. Tight timing mattered (two trials both at PN17 or both at PN18 did not produce long-term memory). - PSD-95 induction is necessary for the second learning to capture maturation: Blocking PSD-95 induction after the first PN17 training prevented the second PN18 training from producing long-term memory; hippocampal function per se remained intact as animals could relearn upon retraining. - Experience selectivity within domain: IA in context A followed 24 h later by IA in context B produced long-lasting memory for both contexts, indicating transfer within the same learning domain but context specificity (no generalization without training). - Cross-domain non-transfer: IA followed by nOL (or nOL followed by IA) did not confer maturation of functional competence across domains; nOL memory did not mature after prior IA, and IA memory competence did not transfer from nOL, even when nOL was enhanced with hippocampal BDNF. - Neuronal ensemble activation is required for functional competence: c-Fos AS-ODN after the first learning reduced c-Fos induction at 24 h and prevented the second learning from yielding long-term memory, indicating the necessity of the first-learning-activated ensemble for capturing maturation. - Artificial reactivation matures competence selectively: Chemogenetic reactivation (CNO) of cells tagged during PN17 CFC reinstated and matured CFC memory at remote tests in double-transgenic mice, but this artificial maturation did not transfer to nOL memory. A low-intensity ‘savings’ shock reinstated CFC memory at later tests in pre-trained mice. - Statistics/sample notes: Western blots typically n=4–10 per group; electrophysiology generally 14 cells per group (3 rats/group) or 18–23 cells across ODN conditions; multiple significant effects with ANOVA and post hoc tests at p<0.05 to p<0.001.

The findings demonstrate that infantile learning engages a distinct, protracted molecular and synaptic program in the hippocampus compared with older juveniles/adults. This program includes sustained IEG induction, BDNF-dependent increases in structural synaptic proteins (PSD-95, synaptophysin), and phosphorylation-driven modulation of AMPARs, culminating in postsynaptic AMPAR response maturation. These changes are necessary for infant memory formation and enable a temporal window during which a subsequent, similar learning event can capture the induced maturation to produce long-term, adult-like memory expression (functional competence). Crucially, the maturation of functional competence is experience-selective: it transfers within the same learning domain (e.g., IA across contexts) but does not generalize across distinct hippocampal learning domains (e.g., IA to nOL). Blocking ensemble activation (c-Fos) after the first learning prevents this competence, whereas chemogenetic reactivation of the tagged ensemble is sufficient to mature memory expression, reinforcing the role of specific cellular ensembles. Overall, early-life experiences sculpt hippocampal memory competence in a domain-specific manner, providing a mechanistic framework for how individual developmental histories shape selective cognitive abilities.

Early-life episodic experiences drive BDNF- and PSD-95-dependent excitatory synapse formation/maturation and AMPAR response maturation in the infant hippocampus, enabling subsequent, similar experiences within a defined temporal window to produce long-term memory expression. This functional maturation is selective to the learning domain engaged and depends on reactivation of the cellular ensemble formed by the initial learning. The work identifies PSD-95 and ensemble activation (c-Fos-marked) as necessary elements and shows that chemogenetic reactivation can artificially induce maturation of competence for the same domain. These insights suggest that targeted early-life experiences—or controlled reactivation of specific ensembles—could shape selective memory abilities. Future research should map the precise ensembles and circuits involved, test generalization across additional hippocampal tasks and species, examine long-term structural changes, and explore translational strategies for enhancing or correcting developmental trajectories of memory competence.

- Species and developmental stage constraints: Findings in infant rats and mice may not directly generalize to humans. - Task/domain scope: Only certain hippocampus-dependent tasks (IA, CFC, nOL) were tested; conclusions about experience selectivity may not extend to all learning domains. - Chemogenetic approach: Systemic CNO administration could affect broader circuits; while controls mitigate this concern, precise spatial specificity in infants is technically challenging. - Experimental design/stats: Methods note no a priori power calculations and no randomization for data collection; some experiments used small sample sizes. Electrophysiology used only male rats, potentially limiting sex generalizability. - Temporal windows: Spacing effects were tested at specific intervals (24–48 h); other timings were not comprehensively explored. - Molecular scope: While PSD-95, synaptophysin, and AMPAR phosphorylation were examined, broader receptor trafficking and other synaptic components were not exhaustively profiled.