Spontaneous epileptiform activity in the CA1 region of Has3^−/− hippocampus. A, Extracellular field potential recordings in the stratum pyramidale of CA1 hippocampus in brain slices from WT and Has3^−/− mice. No activity was found in the WT slices, whereas rhythmic spontaneous field potentials were observed in the Has3^−/− slices. B, These spontaneous field potentials in the Has3^−/− slices were blocked by the AMPA/kainate glutamate receptor antagonist NBQX (10 μm), and they were restored in NBQX-free ACSF. C, Extracellular recordings were made in the stratum pyramidale of the CA3 region (CA3; n = 5) and the granule cell layer of the dentate gyrus (DG; n = 5) of the hippocampal formation in Has3^−/− brain slices. We found no or very small field potentials in CA3 and DG. In two of the slices, dual extracellular recordings were performed with one electrode in the stratum pyramidale of CA1 hippocampus and the other in either the stratum pyramidale of CA3 hippocampus (left) or the granule cell layer of the DG (right). Dual extracellular recordings showed rhythmic spontaneous field potentials in CA1 and much smaller field potentials, occurring synchronously with the CA1 field potentials, in CA3 and DG.