It was recently shown that perisomatic GABAergic inhibitory postsynaptic potentials
(IPSPs) originating from basket and chandelier cells can be recorded as population
IPSPs from the hippocampal pyramidal layer using extracellular electrodes (eIPSPs).
Taking advantage of this approach, we have investigated the recruitment of perisomatic
inhibition during spontaneous hippocampal activity in vitro. Combining intracellular
and extracellular recordings from pyramidal cells and interneurons, we confirm that
inhibitory signals generated by basket cells can be recorded extracellularly, but
our results suggest that, during spontaneous activity, eIPSPs are mostly confined
to the CA3 rather than CA1 region. CA3 eIPSPs produced the powerful time-locked inhibition
of multi-unit activity expected from perisomatic inhibition. Analysis of the temporal
dynamics of spike discharges relative to eIPSPs suggests significant but moderate
recruitment of excitatory and inhibitory neurons within the CA3 network on a 10 ms
time scale, within which neurons recruit each other through recurrent collaterals
and trigger powerful feedback inhibition. Such quantified parameters of neuronal interactions
in the hippocampal network may serve as a basis for future characterisation of pathological
conditions potentially affecting the interactions between excitation and inhibition
in this circuit.
