Figure 5. Synaptic connectivity and distinct in vivo firing patterns of pyramidal cells and four types of interneurone embedded in the hippocampal network.

The schematic drawing summarizes the main synaptic connections in the CA1 area of pyramidal cells (blue), parvalbumin expressing basket, axo-axonic, bistratified and O-LM cells. The cells have differential temporal firing patterns during theta and ripple oscillations (mean of several cells). For clarity two theta cycles are shown in the firing probability histograms. The Y axis of the spike probability plots was constructed by including all events and cycles in the analysed period irrespective of whether the individual recorded cell fired or not. The phase relationship of the extracellularly recorded field potential (schematic white wave) used in the spike alignments and the phase shifted oscillation in the membrane potential oscillation of pyramidal cells reported from intracellular studies (blue waves) is shown schematically. For the ripples, time was normalized to the beginning, highest amplitude and end of ripple episode. The spike probability plots show that during different network oscillations representing two distinct brain states, interneurones of the same connectivity class show different firing activities and therefore modulate their specific postsynaptic target-domain in a brain-state-dependent manner. Interneurones belonging to different connectivity classes fire preferentially at distinct time points during a given oscillation. Because the different interneurones innervate distinct domains of the pyramidal cells, the respective compartments will receive GABAergic input at different time points. This suggests a role for interneurones in the temporal structuring of the activity of pyramidal cells and their inputs via their respective target domain in a co-operative manner, rather than simply providing generalized inhibition. (Firing probability histograms modified from Klausberger et al. 2003, 2004.)