The flow of information in the cerebral cortex is accompanied by a tight temporal precision of firing patterns that are often embodied in oscillations. Different types of network oscillations occur at different frequencies and are correlated with specific behaviour.
Theta oscillations (4-8 Hz) represent the ?on-line? state of the hippocampus and are observed exploring the environment or during movement and REM (rapid eye movement) sleep. Sharp wave-associated ripples (120-200 Hz) are observed during consummatory behaviour, behavioural immobility and slow-wave sleep. These ripples are connected to temporally compressed replay of information.
Even within a small cortical area a rich diversity of distinct neurons contributes differentially to information processing. Cortical neurons can be divided into excitatory pyramidal cells, which use glutamate as a neurotransmitter and give both local and long-range axonal projections, and inhibitory interneurons, which are GABAergic and control the activity and timing of pyramidal cells mainly through local axons.
Both types of neuron can be further subdivided on the basis of their distinct axo-dendritic arborisations, subcellular post-synaptic targets, in vitro firing patterns and by their differential expression of signalling molecules.
In this research programme we aim to determine how identified neurons of the hippocampus and prefrontal cortex contribute to the generation of network oscillations. For this we record the activity of single neurons during network oscillations in the hippocampus and/or medial prefrontal cortex. After the recording of the cell during spontaneously occurring network oscillations, the recorded cell is selectively filled with neurobiotin using the juxtacellular labelling method. This allows the unequivocal identification of the recorded neuron using several histological techniques. The soma, dendrites and axons are visualised with light microscopy and digitally reconstructed with neurolucida. The post-synaptic targets of the cell are identified with electron microscopy. Furthermore we use immunofluorescence microscopy to test the expression of different molecules by the recorded cells.
So far, we have demonstrated that different types of interneurons fire with distinct temporal patterns and these patterns change during different types of network oscillations. Our observations indicate that different types of interneuron have evolved to control different subcellular compartments of pyramidal cells through differential GABAergic input in time to produce network oscillations and brain states.
