Isolation and characterization of postsynaptic densities from various brain regions: enrichment of different types of postsynaptic densities.

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Abstract
Postsynaptic densities (PSD) were isolated from cerebral cortex, midbrain, cerebellum and brain stem of dogs by the Triton X-100 method previously used in the isolation of cerebral PSD (Cohen et al.). These PSD were compared in protein composition, protein phosphorylation and morphology. Thin-section EM revealed that cerebral cortex and midbrain PSD were identical, being .apprx. 57 nm thick and composed of apparent aggregates 20-30 nm in diameter. Isolated cerebellar PSD appeared thinner (33 nm) than cerebral cortex PSD and lacked the apparent 20-30 nm aggregates, but had a latticelike structure. In unidirectional and rotary-shadowed replicas, the cerebrum and midbrain PSD were circular in shape with a large central perforation or hole in the center. Cerebellum PSD 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 20-30 nm aggregates in cerebrum PSD and were also observed radiating from 1 side of the PSD. Both cerebral cortex and midbrain PSD exhibited identical protein patterns on SDS gel electrophoresis. In comparison, cerebellar PSD lacked the major 51,000 Mr [relative molecular weight] protein, contained 2 times less calmodulin and contained a unique protein at 73,000 Mr. Ca plus calmodulin stimulated the phosphorylation of the 51,000 and 62,000 Mr bands in both cerebral cortex and midbrain PSD. In cerebellar PSD, only the 58,000 and 62,000 Mr bands were phosphorylated. In the PSD from all brain regions, cAMP stimulated the phosphorylation of Protein Ia (73,000 Mr), Protein Ib (68,000 Mr) and a 60,000 Mr protein, although cerebrum and midbrain PSD contained very much higher levels of phosphorylated protein than did the cerebellum. Morphological criteria, indicate that PSD isolated from cerebrum and midbrain were derived from the Gray type I, or asymmetric, synapses, whereas cerebellum PSD 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.