Postsynaptic densities (PSDs) have been isolated from cerebral cortex, midbrain, cerebellum, and brain stem by the Triton X-100 method previously used in the isolation of cerebral PSDs. These PSDs have been compared in protein composition, protein phosphorylation, and morphology. Thin-section electron microscopy revealed that cerebral cortex and midbrain PSDs were identical, being ~57 nm thick and composed of apparent aggregates 20-30 nm in diameter. Isolated cerebellar PSDs appeared thinner (33 nm) than cerebral cortex PSDs and lacked the apparent 20- to 30-nm aggregates, but had a lattice like structure. In unidirectional and rotary-shadowed replicas, the cerebrum and midbrain PSDs were circular in shape with a large central perforation or hole in the center of them. Cerebellum PSDs did not have a large perforation, but did have numerous smaller perforations in a lattice like structure. Filaments (6-9 nm) were observed connecting possible 20- to 30-nm aggregates in cerebrum PSDs and were also observed radiating from one side of the PSD. Both cerebral cortex and midbrain PSDs exhibited identical protein patterns on SDS gel electrophoresis. In comparison, cerebellar PSDs (a) lacked the major 51,000 M(r) protein, (b) contained two times less calmodulin, and (c) contained a unique protein at 73,000 M(r). Calcium plus calmodulin stimulated the phosphorylation of the 51,000 and 62,000 M(r) bands in both cerebral cortex and midbrain PSDs. In cerebellar PSDs, only the 58,000 and 62,000 M(r) bands were phosphorylated. In the PSDs from all brain regions, cAMP stimulated the phosphorylation of Protein Ia (73,000 M(r)), Protein Ib (68,000 M(r)), and a 60,000 M(r) protein, although cerebrum and midbrain PSDs contained very much higher levels of phosphorylated protein than did the cerebellum. On the basis of the morphological criteria, it is possible that PSDs isolated from cerebrum and midbrain were derived from the Gray type I, or asymmetric, synapses, whereas cerebellum PSDs were derived from the Gray type II, or symmetric, synapses. Since there is some evidence that the type I synapses are involved in excitatory mechanisms while the type II are involved in inhibitory mechanisms, the role of the PSD and of some of its proteins in these synaptic responses is discussed.
|Original language||English (US)|
|Number of pages||13|
|Journal||Journal of Cell Biology|
|State||Published - 1980|
ASJC Scopus subject areas
- Cell Biology