In 1998, it was hypothesized that gap junctions existed between the axons of hippocampal pyramidal cells (Draguhn et al. 1998). This hypothesis was suggested by two experimental observations: the occurrence of 200 Hz population oscillations in neuronal networks in which synaptic transmission was blocked, but where the oscillations required gap junctions; and the shape of putative coupling potentials in principal neurons, which were too fast to be generated by gap junctions located on somata or dendrites. There is now electrophysiological and dye-coupling evidence that such gap junctions exist, and are located roughly 100 µm from the soma (Schmitz et al. 2001). Modelling shows that gap junctions in this location can give rise to very fast oscillations in networks of principal neurons, as well as to 200 Hz ‘ripples’ (as seen in vivo, and consisting of IPSPs), when interneurons are also in the circuit (Traub et al. 1999; Traub & Bibbig, 2000). In addition, axonal gap junctions can underlie the generation of 40 Hz oscillations, in the presence of cholinergic agonists or of kainate (Traub et al. 2000). Modelling predicts, and experiments confirm, that in such conditions, the oscillation spectrum contains both 40 Hz and also very fast (> 80 Hz) components.

This work was supported by The Wellcome Trust and the DFG.