Fate of presynaptic afferents to Purkinje cells in the adult nervous mutant mouse: a model to study presynaptic stabilization.

The hemispheric cerebellar cortex from 174- to 420-day-old nervous mice has been studied with Golgi, Cajal-reduced silver and electron microscopic techniques. In all mice, the existence of a continuous process of Purkinje cell death, indicated by the presence of few of these neurons in early stages of degeneration, has been established. The morphology of such degenerating cells, as well as the removal of the necrotic debris and the consecutive isolation of presynaptic fibers by glia are described. In the molecular layer, and in spite of the important Purkinje cell loss, numerous varicosities belonging to parallel fibers persist. They can be either synapsing on interneurons or concentrated in small clusters of 2-10 boutons covered by astrocytic processes. The synaptic investment of stellate and basket cells, as well as the length and the branching pattern of their dendritic fields are not significantly changed, besides the increase in afferent terminals deprived of their postsynaptic targets. Basket formations, as numerous as in control cerebellum, are forming a discontinuous row at the superficial third of the granular layer. Over 90% of these baskets are devoid of Purkinje cells, however, their ultrastructural features mimic those of the same terminals in control mice, with the exception that few of them can either establish heterologous synapses with granule cell dendrites, or develop gap junctions between them. The significance of the persistence of normal looking presynaptic elements long time after deprivation of their postsynaptic targets is discussed in relation to the role that function can play in the stabilization of synaptic contacts.