Specific higher cognitive functions are linked to distinct multi-neuronal activity patterns that can be recorded as oscillating electrical fields in the electroencephalogram (EEG) or local field potential (lfp). Recent evidence indicates that such network oscillations orchestrate the activity of the projecting, excitatory neurons and are thus involved in the mechanisms underlying the formation, consolidation and read-out of neuronal assemblies. During sleep and resting immobility, the hippocampus generates a certain oscillation pattern, called sharp-wave ripple complexes, which are accompanied with selective re-activation of previously active place cells. It has been shown that disruption of these oscillations impairs spatial memory consolidation. However, it is unknown how individual neurons 'know' that they are part of a certain assembly and are co-activated during sharp-wave ripple associated memory consolidation.

Here, we show that CA1 pyramidal neurons do indeed fall into two distinct classes - cells that take part in the network oscillation and cells that are excluded from it. Moreover, participating neurons are activated by a peculiar mechanism resulting in antidromic action potentials. Pharmacological experiments, studies on mutant animals as well as computer modeling suggest a dual role of GABA: i) perisomatic inhibition suppresses background activity of neurons increasing 'signal-to-noise' ration in the network. ii) GABA has a facilitating effect on action potential generation when acting on axonal compartments. These findings provide a new mechanism by which selected neurons are recruited into neuronal assemblies.