Presence of both constitutive and inducible forms of heat shock protein 70 in the cerebral cortex and hippocampal synapses.

Heat shock proteins serve as molecular chaperones in a protein "holding and folding" system. Protein sequencing, extraction and immunoblot analyses indicate that Hsc70, a constitutive form, is a major component of the rat postsynaptic density (PSD) fraction, while Hsp70, an inducible form, is present at the basal level. Immunohistochemical studies show that expression of Hsc70 is high, but that of Hsp70 is low, in the cerebral cortex and hippocampal formation. In dissociated hippocampal neurons, both Hsp70 and Hsc70 immunoreactivities are distributed throughout the soma and dendrites. In dendrites, there are many stained puncta which are mostly co-localized with PSD-95, a postsynaptic marker. Interestingly, variation in staining intensity of the puncta is significantly larger for Hsp70 than for Hsc70 in 2-week-old cultures, but becomes less significant in 5(1/2)-week-old cultures. At the electron microscopic level, both Hsp70 and Hsc70 are mainly associated with asymmetrical PSDs. However, Hsc70 is also associated with amorphous subsynaptic structures and spine apparatus-like cisternae. Our data indicate that both Hsp70 and Hsc70 are present in PSDs but are differentially distributed at subsynaptic sites, and provide a potential candidate system for a "synaptic tag".