We have demonstrated that, after permeation with saponin and decoration with S-1 myosin subfragment, the cytoplasmic actin is organized in filaments in dendritic spines, dendrites, and axon terminals of the dentate molecular layer. The filaments are associated with the plasma membrane and the postsynaptic density with their barbed ends and also in parallel with periodical cross bridges. In the spine stalks and dendrites, the actin filaments are organized in long strands. Given the contractile properties of actin, these results suggest that the cytoplasmic actin may be involved in various forms of experimentally induced synaptic plasticity by changing the shape or volume of the pre-and postsynaptic side and by retracting and sprouting synapses.Recent discoveries are implicating the cytoskeleton and the cytoplasmic ground substance in a number of intracellular functions. The cytoplasmic ground substance consists of a protein-rich polymerized phase forming a complex microtrabecular lattice of contractile, ready solubilized proteins (28) that are in dynamic equilibrium with monomers of their subunits and therefore are ready to undergo phase transitions (18). Actin is the main component of the cytoskeletal microfllaments, and actin and myosin were shown to be part of the ground substance in a number of nonmuscle cells (for review see references 5 and 10) including the neurons (for review see reference 32). By analogy with muscles, it is assumed that the prime function of contractile proteins in neurons is transduction of chemical to mechanical energy, which may be essential for neuronal physiology.In our previous work on synaptic plasticity in the visual cortex (6) and in the dentate fascia (7, 8), we observed changes in dendritic spines and synaptic contacts, the mechanism of which might involve microfilamems and microtrabeculae of the cytoplasmic ground substance. Likewise, various other experimental interventions or even physiological activity per se could induce changes in the cytoskeletal system of neurons which might be the underlying mechanism of what are, in general terms, known as plastic reactions of the central nervous system (CNS). With this assumption in mind, we have investigated the organization of actin fdaments in dendritic spines, dendrites, and axon terminals in the dentate fascia of the hippocampus, a region which is known to react with distinct plastic morphological and physiological changes to increased electrical activation. Actin can be identified ultrastructurally by decorating with the myosin S-1 subfragment. This method was introduced by Ishikawa (14) and was substantially improved by adding tannic acid to the fixative (1). MATERIALS AND METHODSAll animals used were 25-g mice of the HS/IBG strain from the Institute for Behavioral Genetics, University of Colorado, Boulder, CO. Under urethane anesthesia, mice were perfused transcardially under constant pressure of 3 lb/in 2 with 0.1% glutaraldehyde in stabilization buffer (0. I M PIPES; 5 mM MgC12; 0.1 mM EDTA at pH 6.9), followed by...

Stimulation of the perforant path induces a long-lasting increase in the area of dendritic spines, which are sites of termination of the stimulated pathway in the distal third of the dentate molecular layer. No enlarged spines were found in the proximal third of the dentate molecular layer, where the commissural afferents terminate. Following a single tetanic stimulus of 30 sec duration at 30/sec, spines became significantly larger by 15%, 38%, 35% and 23% within poststimulation intervals of 2-6 min, 10-60 min, 4-8 h, and 23 h, respectively. Axon terminals decreased their area by 15% within the 2-6 min interval and the vesicle density was decreased by 19% within the 10-60 min interval. Both changes were reversible and terminals resumed their prestimulation condition at longer intervals (greater than 4 h). The initial enlargement of spines was interpreted as being due to a glutamate-induced increase in the sodium permeability of the spine membrane, whereas for the long-lasting enlargement an increase in protein synthesis was postulated. The long-lasting enlargement of dendritic spines in the dentate molecular layer following a short train of stimuli delivered to the perforant path, supports the postulate which links such a change to the mechanism of long-lasting postactivation potentiation observed in this pathway.

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