Abstracts

Rationale: Neonatal seizures are often refractory to conventional antiepileptic drugs, and can result in later life epilepsy, cognitive deficits and autism. We have shown that early-life seizures (HS) lead to acute and long-term enhancement of AMPAR function and impairment in LTP. In addition to Hebbian plasticity, homeostatic synaptic scaling calibrates neuronal excitability by adjusting synaptic strengths during prolonged changes in synaptic activity. HS itself can evoke PLK2-mediated homeostatic modulation of seizure-induced enhancement of AMPAR function (Sun et al., 2013). Here we hypothesize that HS may alter homeostatic synaptic scaling in epileptic neurons in response to further chronic activity changes. Methods: HS were induced by acute global hypoxia in P10 rats. Ex vivo slices were maintained in vitro for 48-72h using an acute culture procedure. Whole-cell current and voltage clamp recordings were made in cortical pyramidal neurons of somatosensory cortex from post-HS 48h rats and littermate controls. Human epilepsy tissues were collected and handled following surgical resection for medically intractable epilepsy in accordance with the guidelines of the hospitals IRB. AMPAR mEPSCs were isolated by blocking GABAA receptors and NMDA receptors.Results: Neurons in acute cultured slices did not show significant difference in intrinsic membrane properties and function of AMPAR mEPSCs compared to those in rat acute cortical slices (all p>0.05, n=8-9). We found that chronic activity blockade by 1 M TTX for 24-48h induced significant increases in amplitude and frequency of AMPAR mEPSCs in neurons from both HS rats (Amplitude: 12.3 0.5pA vs 16.6 0.8pA, n=6-8, p<0.05; Frequency: 100 15.2% vs 151.2 18.2%, n=6-8, p=0.0507) and normoxic controls (Amplitude: 10.3 0.7pA vs 12.8 0.6pA, n=7-8, p<0.05; Frequency: 100 13.2% vs 161.2 21.4%, n=7-8, p<0.05) with no changes in AMPAR mEPSC rising and decay time (all p>0.05, n=7-8). However, the decreases in synaptic strength in response to elevated synaptic activity by application of 100 M Picrotoxin 24-48h as identified in controls (Amplitude: 8.26 0.59pA, n=7, p<0.05; Frequency: 83.59 15.67%, n=7, p=0.433) was occluded in neurons from HS rats (Amplitude: 13.29 0.49pA, n=8, p=0.179; Frequency: 93.57 20.95%, n=8, p=0.807). Human epilepsy biopsy cortical slices were successfully maintained in vitro for 48-72h in a good quality comparable to acute slices. We found that synaptic scaling down in response to 48h elevated synaptic activity by application of Picrotoxin was abolished (AMPAR mEPSC amplitude: 9.4 0.3pA vs 9.7 0.4pA, n=6-8, p>0.05; Frequency: 100 10.2% vs 163.1 13.2%, n=6-8, p<0.05), while scaling up in response to 48h synaptic activity blockade by TTX was remained (AMPAR mEPSC amplitude: 11.8 0.6pA, p<0.05; Frequency: 112.6 12.9%, p>0.05, n=7).Conclusions: These data suggest an impaired homeostatic synaptic scaling down in cortical neurons following HS, which may, at least in part, contribute to neonatal seizure-induced long-term neuronal hyperexcitability and epileptogenesis.