In the mammalian hippocampus different behavioral states are associated with distinct oscillatory rhythms. These are supposed to mediate specific stages of memory-related assembly formation: in active exploratory behavior theta (5–10 Hz) and superimposed gamma (30 - 100 Hz) oscillations are predominantly facilitating assembly formation. During slow-wave sleep or inactivity neuronal assemblies are re-activated by sharp-wave ripple (SPW-R) oscillations (~200 Hz) which are therefore suspected to contribute to memory retrieval and consolidation to neocortical brain areas.

Due to these underlying hypotheses we were interested to see in how far stability of neuronal assemblies can be observed during changing network states or whether network constitution and -properties change by the switching of oscillatory rhythms.

We performed tetrode recordings on mouse hippocampal slices to analyze coupling of active units in CA1 and CA3 to the underlying field potential. A transient period of gamma oscillation was introduced by carbachol and afterwards SPW-R oscillations were re-installed by atropine.

In these experiments we observed that units couple precisely to the local field potential during SPW-R and gamma. Units which are active during SPW-R can be retrieved after gamma with stable phase-coupling to SPW-R. Furthermore, SPW-related firing was even enhanced after the gamma period which is also reflected in distinct changes in field-potential properties. The observed increase in SPW-amplitude but not in SPW-shape underline stability of assembly identity but at the same time consolidation and potentiation of the existing neuronal assemblies.

As a consequence, the composition of hippocampal neuronal assemblies presents striking stability throughout alternating network states. On top of this, transient gamma oscillation leads to significant enhancement of these assemblies in subsequent SPW-R states.