The impact of chronic network hyperexcitability on developing glutamatergic synapses

The effects recurring seizures have on the developing brain are an important area of debate because many forms of human epilepsy arise in early life when the central nervous system is undergoing dramatic developmental changes. To examine effects on glutamatergic synaptogenesis, epileptiform activity was induced by chronic treatment with GABAa receptor antagonists in slice cultures made from infant rat hippocampus. Experiments in control cultures showed that molecular markers for glutamatergic and GABAergic synapses recapitulated developmental milestones reported previously in vivo. Following a 1-week treatment with bicuculline, the intensity of epileptiform activity that could be induced in cultures was greatly diminished, suggesting induction of an adaptive response. In keeping with this notion, immunoblotting revealed the expression of NMDA and AMPA receptor subunits was dramatically reduced along with the scaffolding proteins, PSD95 and Homer. These effects could not be attributed to neuronal cell death, were reversible, and were not observed in slices taken from older animals. Co-treating slices with APV or TTX abolished the effects of bicuculline suggesting that effects were dependent on NMDA receptors and neuronal activity. Neurophysiological recordings supported the biochemical findings and demonstrated decreases in both the amplitude and frequency of NMDA and AMPA receptor-mediated miniature EPSCs (mEPSCs). Taken together these results suggest that neuronal network hyperexcitability interferes with the normal maturation of glutamatergic synapses, which could have implications for cognitive deficits commonly associated with the severe epilepsies of early childhood.