Axonal and synaptic remodeling in the mature cerebellar cortex.

By blocking electrical activity in the cerebellar cortex the Purkinje cell dendrites become a uniform territory with a high density of spines all bearing the glutamate receptor delta2 subunit (GluRdelta2) and being mainly innervated by parallel fibers. Such a subunit, which is constitutively targeted specifically to the parallel fiber synapses, appears in the spines contacted by the climbing fibers before they disconnect from the target. A similar pattern of hyperspiny transformation and innervation occurs a few days after a subtotal lesion of the inferior olive, the source of climbing fibers. During the climbing fiber reinnervation process which follows the removal of the electrical block or by collateral sprouting of surviving inferior olive neurons, the new active climbing fibers establish synaptic contacts with proximal dendritic spines that bear GluRdelta2s. After, they repress these subunits and displace the parallel fibers to the distal dendritic territory. These findings suggest the following operational principle in the axonal competition for a common target. The Purkinje cells have an intrinsic phenotypic profile which is compatible with the parallel fiber innervation, this mode being operational in targets innervated by a single neuronal population, like the neuromuscular system. An additional input, the climbing fibers, in order to achieve its own territory on the proximal dendrite needs the ability to displace the competitor. Such an inhibition is activity-dependent and the activity needs to be present in order to allow the climbing fiber to maintain its territory, even when the developmental period is over.