Changes in the structure of synaptic junctions during climbing fiber synaptogenesis.

Synaptogenesis between climbing fiber axons and Purkinje cells involves both an orderly translocation of synaptic junctions over the Purkinje cell surface and an elimination of all but one innervating axon. We used thin-section and freeze-fracture electron microscopic techniques to study structural changes in synaptic junctions during this interval of synapse translocation and elimination. In freeze-fractured preparations, virtually all climbing fiber synaptic junctions with the perisomatic processes and somatic spines lacked the particle aggregates that characterized the extracellular half of the postsynaptic membrane of mature synaptic junctions with dendritic spines. Some climbing fiber junctions with the dendritic shaft in the second postnatal week were associated with such aggregates, despite the fact that these junctions are transient. Thus, during the interval when Purkinje cells initially were innervated by multiple climbing fibers, and subsequently denervated of all but one climbing fiber afferent per cell, only a few of the transient synaptic junctions on the cell body and proximal dendrites have associated particles. The presence of a particle aggregate at a synaptic junction does not appear to be correlated with the permanence of that junction and probably is not correlated with the capacity to support synaptic transmission. The particle aggregates might be indicative of relatively long-lived junctions, or might occur only at junctions formed by the climbing fiber that will persist in synaptic contact with the mature Purkinje cell.