Journal: bioRxiv

Article Title: The Reinstatement of a Forgotten Infantile Memory

doi: 10.1101/2025.09.27.678956

Figure Lengend Snippet: (A) Schematic of the behavioral protocol to trigger reinstatement of a forgotten infantile memory. Mice underwent contextual fear conditioning (cFC) training in infancy (P19), followed in adulthood by exposure to reminders of the CONTEXT and the AVERSIVE stimulus experienced during cFC training. The recall of the reinstated memory (REINSTATED recall) was tested in the CONTEXT one day after the final reminder. All behavioral sessions in adulthood were spaced by one day. (B) Freezing at CONTEXT reminder in adulthood was low in mice fear conditioned at P19, similar to animals that never experienced conditioning (Naϊve animals. Dunnett’s test, p = 0.634), and significantly higher in adult-conditioned animals (p < 0.001), indicating infantile amnesia. One-way ANOVA, F(2, 38) = 51.93, p < 0.001, r2 = 0.7321. n 2 11 mice per group. (C) At REINSTATED recall, mice conditioned at P19 expressed robust freezing (Dunnett’s test, p = 0.007), significantly higher than naϊve animals and comparable to animals conditioned during adulthood (> P60) only when both CONTEXT and AVERSIVE reminders were given. No reinstatement was observed in animals lacking either the CONTEXT (Dunnett’s test, p = 0.970) or the AVERSIVE (p > 0.999) reminder. One-way ANOVA, F(25, 226) = 11.18, p < 0.001, r2 = 0.5529; Brown-Forsythe test, F(25, 226) = 2.622, p < 0.001; Bartlett’s test, x2 = 81.17, p < 0.001. n 2 9 mice per group. (D) AAV-mediated expression of excitatory DREADD (hM3Dq-mCherry, yellow) in PV interneurons of dorsal hippocampus. Representative histological image showing mCherry expression in PV+ neurons in dorsal CA3 and CA1 (single-plane image, 10x objective; scale bar = 300 µm). (E, F) Activation of hippocampal PV interneurons via CNO injection during either the CONTEXT (E) or AVERSIVE (F) reminder prevented memory reinstatement, resulting in low freezing at REINSTATED recall. Student’s t -test against saline injected controls: CONTEXT, t = 3.515, p = 0.004, Cohen’s d = 0.507; AVERSIVE, t = 4.04, p = 0.003, Cohen’s d = 0.645. n = 6 for CONTEXT, n 2 4 for AVERSIVE.

Article Snippet: For bilateral delivery of DREADDs, PV-Cre mice received injections of pAAV-hSyn-DIO-hM3D(Gq)-mCherry (Addgene #44361-AAV1; titer: 2.2 x 1013 vg/mL) into the dorsal hippocampus (AP: −1.75 mm; ML: ±1.95 mm; DV: −1.85 mm; 350 nL per hemisphere) using a Nanoject III (Drummond).

Techniques: Expressing, Activation Assay, Injection, Saline

Journal: bioRxiv

Article Title: The Reinstatement of a Forgotten Infantile Memory

doi: 10.1101/2025.09.27.678956

Figure Lengend Snippet: We propose that reinstating forgotten infantile memories requires a carefully orchestrated hippocampus-centered network process that unfolds in three stages. First, exposure to a contextual reminder of the original experience primes the hippocampal network, increasing the recruitment of neurons associated with the forgotten infantile memory (iEngram) during subsequent experiences associated with the forgotten infantile memory (“ Priming ”, A ). Then, a reminder of the aversive stimulus experienced during infancy selectively tags iEngram neurons for offline reactivation in the following hours (“ Tagging ”, B ). Increased iEngram activity during offline network reactivation events binds previously latent infantile memory with novel neuronal ensembles, thereby reinstating behavioral responses consistent with the original experience (“ Binding ”, C ).

Article Snippet: For bilateral delivery of DREADDs, PV-Cre mice received injections of pAAV-hSyn-DIO-hM3D(Gq)-mCherry (Addgene #44361-AAV1; titer: 2.2 x 1013 vg/mL) into the dorsal hippocampus (AP: −1.75 mm; ML: ±1.95 mm; DV: −1.85 mm; 350 nL per hemisphere) using a Nanoject III (Drummond).

Techniques: Activity Assay, Binding Assay