Sharp wave-like activity in the hippocampus in vitro in mice lacking the gap junction protein connexin 36

Bath application of kainate (100-300 nM) induced a persistent gamma-frequency (30-80 Hz) oscillation that could be recorded in stratum radiatum of the CA3 region in vitro. We have previously described that in knockout mice lacking the gap junction protein connexin 36 (Cx36KO), gamma-frequency oscillations are reduced but still present. We now demonstrate that in the Cx36KO mice, but not in wild-type (WT), large population field excitatory postsynaptic potentials, or sharp wave-burst discharges, also occurred during the on-going gamma-frequency oscillation. These spontaneous burst discharges were not seen in WT mice. Burst discharges in the Cx36KO mice occurred with a mean frequency of 0.23 +/- 0.11 Hz and were accompanied by a series of fast (approximately 60-115 Hz) population spikes or "ripple" oscillations in many recordings. Intracellular recordings from CA3 pyramidal cells showed that the burst discharges consisted of a depolarizing response and presumed coupling potentials (spikelets) could occasionally be seen either before or during the burst discharge. The burst discharges occurring in Cx36KO mice were sensitive to gap junctions blockers as they were fully abolished by carbenoxolone (200 microM). In control mice we made several attempts to replicate this pattern of sharp wave activity/ripples occurring with the on-going kainate-evoked gamma-frequency oscillation by manipulating synaptic and electrical signaling. Partial disruption of inhibition, in control slices, by bath application of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline (1-4 microM) completely abolished all gamma-frequency activity before any burst discharges occurred. Increasing the number of open gap junctions in control slices by using trimethylamine (TMA; 2-10 mM), in conjunction with kainate, failed to elicit any sharp wave bursts or fast ripples. However, bath application of the potassium channel blocker 4-aminopyridine (4-AP; 20-80 microM) produced a pattern of activity in control mice (13/16 slices), consisting of burst discharges occurring in conjunction with kainate-evoked gamma-frequency oscillations, that was similar to that seen in Cx36KO mice. In a few cases (n = 9) the burst discharges were accompanied by fast ripple oscillations. Carbenoxolone also fully blocked the 4-AP-evoked burst discharges (n = 5). Our results show that disruption of electrical signaling in the interneuronal network can, in the presence of kainate, lead to the spontaneous generation of sharp wave/ripple activity similar to that observed in vivo. This suggests a complex role for electrically coupled interneurons in the generation of hippocampal network activity.