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The comparative localization of two prominent synaptic proteins, synapsin-I (Syn-I) and PSD-95, was investigated in slices of developing (P3–P21) rat cerebellar cortex using double- or triple-label fluorescence immunohistochemistry and confocal microscopy. During the first postnatal week, Syn-I and PSD-95 immunoreactive (IR) puncta were strongly concentrated in the Purkinje cell layer (PCL) where they circumscribed irregularly shaped PC somata, forming pericellular nests that likely correspond to early climbing fiber synapses. PSD95 and Syn-I puncta also were found along the shafts and at the tips of growing PC dendrite branches labeled with calbindin. During the second postnatal week, synaptic puncta were lost from the PC layer, while many new puncta were added to the molecular layer (ML). At P10, about half of the PCs were circumscribed by PSD-95 or Syn-I puncta, whereas at P14 no PCs were circumscribed. By P14, PSD95 and Syn-I became most strongly localized to many small puncta in the ML and to large clusters at mossy fiber rosettes in the glomerular layer (GL) where PSD-95 often encircled Syn-I clusters. Some large clusters in the GL contained only PSD-95 or Syn-I, but not both, suggesting differential growth or remodeling of pre- and post-synaptic structures. No PSD-95 staining of pre-synaptic terminal
pinceau was observed during the first 3 weeks of postnatal development. Thus, in relation to PCs, there is a developmental shift in PSD-95 localization whereby, first, it is concentrated on PC cell bodies and short dendrites (P3–P7), then it is lost on PC cell bodies (P7–14) and becomes localized almost exclusively to PC dendrites (P14–P21).