Voltage clamp the cells and you'll see that the neurons are far from
silent. mIPSCs make up about half of the 2-4hz of spontaneous IPSCs
recieved by deep layer neurons, and although I've never made a
systematic count, I suspect action potential dependent Excitatory
events also impact on cells at about 1-2Hz.
Block excitation and I suspect this will start to increase massively.
Also, I've noted a marginal tonic GABA current in pyramidal neurons, so
removing that is going to slightly tonically depolarize neurons.