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.