Plasticity of synaptic size with constancy of total synaptic contact area on Purkinje cells in the cerebellum.

Reduction of Purkinje cell afferents during development or after maturation resulted in a large change in the size of synaptic contacts on dendritic spines. Ultrastructural studies revealed enlarged Purkinje cell spines and elongation of the synapses with parallel fibers in three models of parallel fiber reduction: 1) developmental malnutrition, 2) developmental virus-induced granule cell reduction, and 3) parallel fiber sectioning in the adult. In developmental malnutrition, five groups of animals having different numbers of parallel fiber afferents to Purkinje cells, but the same total number of these cells, were quantitatively compared. As the number of synaptic junctions on Purkinje cells decreased, average area of synaptic contact increased. From this determination, it was further established that the total contact area on each Purkinje cell remained constant for groups of animals having different levels of afferent reduction. This inverse relationship was the result of Purkinje cells having a constant total synaptic contact area for parallel fibers. These studies show that the total postsynaptic area for each Purkinje cell is intrinsically determined and that synapses with the remaining parallel fibers change their size in a plastic response to interactions between pre- and postsynaptic elements.