Biphasic response of silent synapses: Differential remodeling of hippocampal synaptic plasticity by status epilepticus in juvenile versus adult mice

Abstract

To investigate age-dependent mechanisms of silent-synapse transformation and their contribution to hippocampal circuit reorganization following convulsive brain injury induced by status epilepticus (SE). Three-week-old (juvenile) and eight-week-old (adult) C57BL/6J mice were used. SE was induced by intraperitoneal kainic acid (KA). Hippocampal assessments were performed at 3 days (acute phase) and 28 days (chronic phase) post-SE. Dendritic spines in the dentate gyrus (DG) were visualized by Dil staining and confocal imaging to quantify total density and subtype distributions. Western blot was used to measure hippocampal expression of AMPA receptor subunits (GluA1-GluA4), NMDA receptor subunits (GluN1, GluN2A, GluN2B), and PSD95. Surface synaptic localization of GluA1 in CA1 was further evaluated by immunofluorescence staining and multi-threshold colocalization quantification with Synapsin. Whole-cell patch-clamp recordings were performed to determine the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in CA1. Correlation analyses were conducted to examine the association between DG filopodia density and CA1 sEPSC frequency during the acute phase. (1) After SE, juvenile mice exhibited increased densities of mushroom and stubby spines at both time points (P < 0.01) accompanied by a reduction in filopodia density (P < 0.01). In contrast, adult mice showed decreased mushroom spine density (P < 0.01) with increased filopodia formation during the acute phase (P < 0.01). (2) In the juvenile group, the expression levels of GluA1, GluA4, GluN2A, GluN2B, and PSD95 in the hippocampus were significantly elevated during the acute phase after SE. By the chronic phase, GluA3 expression remained upregulated while GluA4 was downregulated, and GluN2A/GluN2B remained persistently increased. In adults, GluA1 was decreased and GluA4 increased during the acute phase. GluN1 remained elevated across both phases, and GluN2A increased selectively in the chronic phase. PSD95 upregulation in adults was observed only in the chronic phase. Immunofluorescence analysis further corroborated the biochemical findings by showing increased surface synaptic GluA1 colocalization with Synapsin in CA1 at 3 days post-SE in juveniles, but decreased colocalization in adults. (3) Electrophysiological assessment revealed that the juvenile group showed increased sEPSC frequency (P < 0.05) and decreased amplitude (P < 0.01), whereas adults exhibited reduced sEPSC frequency (P < 0.05) without a significant change in amplitude. (4) Correlation analyses revealed a strong negative association between DG filopodia density and CA1 sEPSC frequency at 3 days post-SE in both age groups (juveniles: r = -0.908, p = 0.033; adults: r = -0.964, p = 0.036), indicating an age-dependent but coordinated structure-function coupling across hippocampal subregions during the acute post-SE period. SE bidirectionally modulates silent synapse transformation in an age-dependent manner. The juvenile brain adapts to pathological stimuli induced by SE through rapid conversion of silent synapses into functional mature synapses. In contrast, the adult brain exhibits reduced synaptic maturity and suppressed synaptic function, which may exacerbate post-epileptic neural network dysfunction.