Induced sharp wave-ripple complexes in the absence of synaptic inhibition in mouse hippocampal slices

The characteristic, behaviour-related network oscillations of the mammalian hippocampus ({theta}, {gamma} and ripples) are accompanied by strongly phase-coupled action potentials in specific subsets of GABAergic interneurones. It has been suggested that the resulting phasic, repetitive inhibition shapes rhythmic coherent activity of the neuronal network. Here, we examined whether synaptic inhibition entrains approximately 200 Hz network ripples by applying the GABA(A) receptor antagonist gabazine to CA1 minislices of mouse hippocampus. Gabazine blocked spontaneously occurring sharp wave-ripple (SPW-R) activity. However, local application of KCl to the dendritic layer elicited excitatory sharp waves on which approximately 200 Hz ripple oscillations were superimposed with equal temporal properties of native SPW-R. The activity also persisted in the additional presence of blockers of glutamatergic synaptic transmission. In contrast, synchrony was largely abolished after addition of gap junction blockers. Thus, GABAergic transmission appears to be involved in the generation of sharp waves but phasic inhibition is no prerequisite for the precise synchronization of hippocampal neurones during high-frequency oscillations at approximately 200 Hz. Gap junctions on the other hand seem to be necessary to orchestrate coordinated activity within the ripple frequency domain