Regularity and differentiation within the structure of brain postsynaptic densities

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Abstract
Postsynaptic densities (PSDs) isolated from rat forebrain were examined in samples prepared for electron microscopy by negative staining. Two structurally distinct areas could be distinguished within each individual PSD, a peripheral zone composed of a planar array of spherical subunits with a mean diameter of 18 nm and one or more islands of fine granular material enclosed by the subunits. These enclosures correspond in distribution and size to ‘holes’ which are known to occur in PSDs in intact brain tissue. The spherical subunits can occur singly in the isolated PSD preparations showing that the structure of each individual subunit does not depend upon its interaction with others. These observations indicate that the PSD structure has both regular and differentiated features. These are considered in relation to the possible roles of the PSD in defining the postsynaptic locus and mediating the postsynaptic events of synaptic transmission.