Characterization of spontaneous network-driven synaptic activity in rat hippocampal slice cultures

A particular characteristic of the neonatal hippocampus is the presence of spontaneous network-driven oscillatory events, the so-called giant depolarizing potentials (GDPs). GDPs depend on the interplay between GABA and glutamate. Early in development, GABA, acting on GABAA receptors, depolarizes neuronal membranes via a Cl- efflux. Glutamate, via AMPA receptors, generates a positive feedback needed for neuronal synchronization. The depolarizing action of GABA ensures calcium entry through NMDA receptors and voltage-gated calcium channels. The whole cell configuration of the patch clamp technique (in current and voltage clamp) was used to assess whether GDPs were present also in hippocampal slice cultures. This would constitute a good model to be used for imaging and genomic approaches. GDP-like activity started appearing during the second week in culture and disappeared during the third postnatal week. GDPs occurred at the frequency 0.14 ± 0.03 Hz (n = 15). GDPs were synchronous in pair recordings from pyramidal neurons (CA1 and CA3, CA3 and CA3) and pyramidal neurons and interneurons. They reversed polarity at –35 ± 2 mV (n = 9) and were blocked by bicuculline (10 µM) or DNQX (20 µM). In the presence of bicuculline, interictal burst occurred at the frequency of 0.013 ± 0.003 Hz (n = 6). They reversed polarity at -6 ± 3 mV (n = 6) and they were blocked by DNQX (20 µM), suggesting that they were mediated by AMPA/kainate receptors. Bicuculline also blocked spontaneously occurring spike-like events observed in cell attach recordings. In additional experiments performed with gramicidin-perforated patch, the reversal potential of GDPs was –42 ± 3 mV (n = 3). These data suggest that GDP like activity recorded from organotypic slice cultures displays a pattern very similar to that observed in acute slices obtained from neonatal rats and suggest that, at least in the majority of neurons, the action of GABA is still depolarizing and excitatory.