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In tissue from area CA1 of the rat hippocampus prepared for electron microscopic study by thin-sectioning, asymmetric synaptic junctions were found on dendritic spines, spiny dendritic shafts, and non-spiny dendritic shafts. In freeze-fractured preparations, aggregates of large particles were found on the extracellular half of the postsynaptic membrane at each of these synaptic junctions. Particle aggregate areas were measured and particle packing densities were computed at dendritic spine synapses and dendritic shaft synapses in area CA1, and compared to similar measures of particle aggregates on dendritic spines of cerebellar Purkinje cells. All of these CA1 and cerebellar synapses are excitatory and are thought to use glutamate as a neurotransmitter. There was a tendency for the dispersion of particles within individual aggregates to be less uniform in larger aggregates in both area CA1 and cerebellar cortex. Distinct particle-free zones could be distinguished in the center of particle aggregates on large "mushroom-shaped" spines in area CA1. There was no statistically significant difference between the particle densities at CA1 dendritic spines (2848 +/- 863 particles/micron2) and CA1 dendritic shafts (2707 +/- 718 particles/micron2). However, the average density of particles at cerebellar dendritic spine synapses (3614 +/- 1081 particles/micron2) was significantly greater than at dendritic spine or shaft synapses found in area CA1. Symmetric synaptic junctions were observed on the CA1 pyramidal cell somas and dendritic shafts in thin-sectioned preparations. These synapses typically exert an inhibitory action mediated by gamma-aminobutyric acid. In freeze-fracture preparations, large varicosities were found apposed to the pyramidal somal and dendritic membranes, but there were no specializations of particle distribution on either the extracellular or the cytoplasmic half of the fractured postsynaptic membranes. This finding parallels observations from freeze-fracture preparations of other GABAergic synapses in the central nervous system.

In tissue prepared by rapid freezing, freeze fracture, and shallow etching, the postsynaptic density of Purkinje cell dendritic spines has a substructure consisting of fine filaments and irregular, globular adherent proteins. The number and packing density of the globular proteins vary from region to region within a single density and are even more variable when different junctions are compared. Whereas actinlike microfilaments and spectrinlike filaments are juxtaposed to the postsynaptic density, they do not appear to be continuous with the constituent filaments of the density. We suggest that the postsynaptic density at this class of synapse is composed of fine filamentous proteins that insert on the postsynaptic membrane and serve as a supporting framework for a variety of globular proteins. The globular proteins may vary qualitatively and quantitatively from junction to junction, and are positioned in the region of the spine that has the greatest concentration of ionized calcium entering with the synaptic current, and the greatest extent of postsynaptic depolarization.

In the glomerular synapses of developing mouse cerebellar cortex, 2 components of the synaptic junctions assemble independently in the immature granule cell dendrites, and then combine. 'Initial' junctions between mossy fiber axons and immature granule cell dendrites have presynaptic and postsynaptic electron-dense fuzz and a widened synaptic cleft, but lack the aggregate of intramembrane particles associated with the extracellular half of the postsynaptic membrane which characterizes mature synaptic junctions. In the vicinity of 'initial junctions' there are particle aggregates which resemble those at mature synaptic junctions, but which are less densely packed and which are not associated with the other features of a junction. The constituent particles of these aggregates move to the sites of 'initial junctions' to combine with them and form 'immature synaptic junctions'. Many of these immature junctions are larger in area than mature synaptic junctions. The immature junctions accumulate a fairly uniform complement of intramembrane particles, which increase in packing density as the junctions decrease in area and attain smaller, adult size.

Three sets of filamentous structures were found to be associated with synaptic junctions in slices of cerebellar tissue prepared by rapid-freezing and freeze-etch techniques. The electron-dense fuzz subjacent to postsynaptic membranes corresponds to a web of 4-6-nm-diam filaments that were clearly visualized in rapid-frozen, freeze-etched preparations. Purkinje cell dendritic spines are filled with a meshwork of 5-7-nm filaments that were found to contact the spine membrane everywhere except at the synaptic junction, and extend through the neck of the spine into the parent dendrite. In addition, 8-10-nm microfilaments, possibly actin, were seen to be associated with the postsynaptic web and to extend into the body and neck of the spine. The arrangements and attachments of the filamentous elements in the Purkinje cell dendritic spine may account for its shape.

During postnatal development of cerebellar cortex, junctions are formed between parallel fiber axons and the shafts of Purkinje cell dendrites. These shaft junctions resemble synaptic junctions on spines in thin sections, in that the axon contains a cluster of synaptic vesicles, and the pre- and postjunctional membranes are lined by electron-dense material. The shaft junctions do not have the aggregate of particles arrayed on the extracellular half of the postjunctional membrane that is characteristic of mature spine synaptic junctions in freeze-fractured preparations, however, and so presumably have a different protein composition. Shaft junctions are transient specializations, present only in developing tissue, but do not appear simply to be intermediates in the formation of mature spine synaptic junctions. In normal development, spines are formed by the Purkinje cell dendrite at sites not occupied by shaft junctions. Moreover, in certain neurological mutant mice, shaft junctions form in the absence of spine synapses, and in other systems spines develop in the absence of shaft junctions. We suggest that shaft junctions are a class of synapse formed during development which is distinguished by its capacity to dissociate, and that only some fraction of the parallel fibers forming shaft synapses with a developing Purkinje cell will have established spine synapses in the adult.