Most synapses in the central nervous system exhibit a prominent electron-opaque specialization of the postsynaptic plasma membrane called the postsynaptic density (PSD). We have developed a procedure for the isolation of PSDs which is based on their buoyant density and their insolubility in N-lauroyl sarcosinate. Treatment of synaptic membranes with this detergent solubilizes most plasma membranes and detaches PSDs from the plasma membrane so that they can be purified on a density gradient. Isolated PSDs appear structurally intact and exhibit those properties which characterize them in tissue. The isolated PSDs are of the size, shape, and electron opacity of those seen in tissue; they stain with both ethanolic phosphotungstic acid and bismuth iodide-uranyl lead and the fraction contains cyclic 3',5'-phosphodiesterase activity. Quantitative electron microscope analysis of the PSD fraction gives an estimated purity of better than 85%. Inasmuch as the PSD is associated primarily with dendritic excitatory synapses, our PSD fraction represents the distinctive plasma membrane specialization of this specific synaptic type in isolation.

A fraction enriched in synaptic complexes has been isolated from rat brain . The major structural elements of synaptic complexes after isolation are a sector of pre-and postsynaptic plasma membranes joined together by a synaptic cleft and a postsynaptic density (PSD) located on the inner surface of the postsynaptic membrane . On its outer surface, the postsynaptic membrane has a series of projections which extend about halfway into the cleft and which occur along the entire length of the PSD . Proteolytic enzymes at high concentrations remove the PSD and open the synaptic cleft ; at low concentrations the PSD is selectively destroyed . By contrast, the structural integrity of the PSD is resistant to treatment with NaCl, EGTA, and low concentrations of urea . Pre-and postsynaptic membranes also remain joined by the synaptic cleft after NaCl, EGTA, or mild urea treatment . High concentrations of urea cause the partial dissociation of the PSD . We conclude that polypeptides are probably one of the major components of the PSD and that the structural integrity of the PSD depends on polypeptides because disruption of the covalent or hydrophobic bonding of these polypeptides leads to a progressive loss of PSD structure .

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In immature animals, ablation of the entorhinal cortex elicited a rapid intensification of acetylcholinesterase (EC 3.1.1.7) staining in the outer one-quarter of the molecular layer of the dentate gyrus. Subsequent lesions of the septum eliminated this acetylcholinesterase intensification. Electron-microscopic histochemical analysis demonstrated a 30-fold increase in the number of acetylcholinesterase-positive synaptic endings in the intensification zone. The acetylcholinesterase augmentation thus appears attributable, in part at least, to an increase in the number of acetylcholinesterase-rich synaptic endings established by septo-hippocampal fibers. Observations in a comparative study -of immature and adult rats point to the animal's developmental state as a major determinant of differences in these lesion-induced neuronal adjustments.Both the anatomical and functional effects of brain injury are generally attributed solely to the loss of damaged or ablated neurons. Findings in recent experiments indicate that this view of the effects of brain lesions needs to be reexamined. Under some conditions, axons from intact neurons grow into areas vacated by the axons of damaged neurons and form new synapses that reoccupy the vacated synaptic territories (1-9).If such capacity for neural reorganization is a general property of brain tissue, these findings have several important implications for understanding the consequences of brain damage. First, in efforts to explain the physiological and behavioral effects of brain lesions it may become necessary to consider not only the net loss of neurons caused by the lesion, but also a possible secondary effect: the formation of new connections by intact neurons. Second, the reorganization of neuronal circuitry after brain damage could explain the frequently observed, but poorly understood, phenomenon of partial recovery of function after brain damage. Third, these findings have demonstrated that neuronal organization is much more dynamic than was heretofore realized; in fact, it has already been suggested that synaptic rearrangements are continuously taking place in the brain even under normal conditions (10). Thus, the induction of reactive synaptogenesis by a brain lesion provides a method of investigating what might well be one of the bases of functional plasticity in normal animals.We examined changes in the organization of the synapse population in the dentate gyrus of the rat's hippocampal formation after lesions that selectively destroy the massive Abbreviation: AChE, acetylcholinesterase. afferent system originating in the entorhinal cortex. In studies of synaptic reorganization, the dentate gyrus offers several advantages: (a) its normal synaptic organization is simple and well defined; (b) it is suitable for electrophysiological recording (11) so that the functional significance of any known structural changes eventually may be assessed; (c) the afferents of the dentate gyrus are accessible to experimental manipulation (e.g., surgical lesion); and (d) the syst...

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The expression of the calcium-binding protein calretinin (CR) in the chick brainstem auditory nuclei angularis (NA), laminaris (NL), and magnocelularis (NM) was studied during normal development and after deafening by surgical removal of the otocyst (embryonic precursor of the inner ear) or columella (middle ear ossicle). CR mRNA was localized by in situ hybridization by using a radiolabeled oligonucleotide chick CR probe. CR immunoreactivity (CR-IR) was localized on adjacent tissue sections. CR mRNA signal in the auditory nuclei was expressed at comparable levels at embryonic day (E)9 and E11 and increased thereafter to reach the highest levels in posthatch chicks. CR-IR neurons were apparent in NM and NA at E11 and in NL by E13, and CR-IR increased in all three auditory nuclei thereafter. Neither unilateral nor bilateral otocyst removal caused detectable changes in the intensity of CR mRNA expression or CR-IR in the auditory nuclei at any of the several ages examined. Similarly, columella removal at posthatching day 2 or 3 failed to significantly affect CR mRNA or CR-IR levels at 3 hours, 1 day, or 3-4 days survival times. We conclude that cochlear nerve input is not necessary for expression of either calretinin mRNA or protein and that the profound decrease in sound-evoked activity caused by columella removal does not affect the maintenance of CR expression after hatching.