Alanna J. Watt, Hermann Cuntz, Masahiro Mori, P. Jesper Sjostrom, and Michael Hausser
Wolfson Institute for Biomedical Research and Department of Physiology, University College London, London WC1E 6BT, UK
Although Purkinje cell axon collaterals were first postulated by Cajal to synapse onto other Purkinje cells, the function of this connection in the cerebellar cortex has been unknown. We used two-photon laser-scanning microscopy of GFP-expressing Purkinje cells in acute sagittal slices from young transgenic mice (P4-14) to trace Purkinje cell axon collaterals and identify candidate pre- and postsynaptic cells. Multiple whole- cell patch-clamp recordings were then used to characterize functional monosynaptic connections between targeted Purkinje cells. Anatomical analysis revealed that Purkinje cell axon collaterals project asymmetrically in the sagittal plane away from the lobule apex, and that they form synapses onto somata of other Purkinje cells. Based on our physiological and anatomical characterization of this connection, we constructed a network model that exhibited robust waves of activity traveling along chains of connected Purkinje cells. The direction of the waves depended on the GABA reversal potential, which is depolarizing in very young animals and then gradually becomes hyperpolarizing over the second postnatal week of development. We validated the model experimentally by making direct recordings of traveling waves of activity in Purkinje cells in sagittal slices from juvenile mice. The direction of the wave was consistent with the depolarizing GABA reversal potential at these ages (P3-6). In several brain regions, similar waves of activity are observed early in development and are essential for proper circuit formation, suggesting that the novel traveling waves we discovered may play an important role in the establishment and function of the cerebellar cortical circuitry.