Abstracts

Rationale: The neonatal period is characterized by enhanced susceptibility to seizures due to a physiologic imbalance between excitation and inhibition. Many studies of hippocampal or cortical networks have examined global alterations in randomly sampled neurons without the knowledge of their activation state during seizures, which may fail to detect heterogeneous activity dependent responses, especially in light of the fact that neuronal networks are heterogeneous. Hippocampus, while being a relatively simple structure, shows structural and physiological heterogeneity across the CA1 neurons, which is a critical factor determining hippocampal function. How these heterogeneous hippocampal neurons respond to early life seizures remains largely unclear. We hypothesize that early life seizures selectively activate subpopulations of neurons within the immature hippocampus.Methods: Seizures were induced in P10 c-Fos-GFP mice by one dose PTZ (60mg/kg, i.p.). Neuronal activation was monitered by immunohistochemistry. Neuronal excitability of activated GFP+ neurons and non-activated GFP- hippocampal CA1 neurons from 2h post-seizure mice and littermate controls was examined by whole-cell patch clamp recordings.Results: We revealed about 40-55% of CA1 neurons were labeled by GFP at 2h post-PTZ seizures (CA1: 41.38±2.89%; CA3: 52.20±4.26%; DG: 40.19± 5.18%, n=11), compared to a minimal baseline in control mice (CA1: 1.99±1.29%; CA3: 2.21±1.89%; DG: 3.08±2.12%, n=8, all p<0.05). The majority (>90%) of c-Fos+ neurons were co-labeled with GFP in both hippocampus and cortex (n=6). There was no co-labeling of GFP and GAD65/67 following PTZ seizures (n=6), confirming that a selective subpopulation of excitatory CA1 pyramidal neurons is activated during PTZ seizures. GFP+ neurons and GFP- neurons had similar intrinsic properties (p>0.05, n=9-11). However, AMPAR mEPSCs showed significantly larger amplitude (−13.64±0.39pA; n=15; p < 0.001) and higher frequency (1.29±0.14Hz; n=15; p < 0.001) in GFP+ neurons compared to GFP- neurons (amplitude: −8.47±0.67pA; n=11; frequency: 0.43±0.12Hz; n=11). Importantly, mEPSC amplitude and frequency in non-activated GFP- neurons were not different from controls (n= 10, p>0.05), suggesting that enhancement of AMPAR function only occurs in the selectively activated GFP+ neurons. Minimally evoked AMPAR eEPSCs through stimulating single fibers of Shaffer Collateral pathway showed significantly higher amplitudes in GFP+ neurons (22.64±2.91pA; n=9, p<0.05) compared to GFP- neurons (12.19±1.29pA; n=9). In addition, no significant changes of paired-pulse facilitation were observed between GFP+ and GFP- neurons (p>0.05, n=6-8), indicating no changes in the probability of presynaptic glutamate release onto postsynaptic GFP+ neurons.Conclusions: Our study describes a novel approach to examine heterogeneous responses to early life seizures, and target-specific synaptic alterations in CA1 neurons represent a novel mechanism mediating neuronal circuit reorganization in early life epilepsy. Supported by URF-2404, NS031718-02A1, NS080565-01A1, MFE-115462.