An ultrastructural study of the microfilaments in rat brain by means of E-PTA staining and heavy meromyosin labeling. II. The synapses.

To identify structures involved in the translocation of the synaptic vesicles towards the presynaptic membrane, an ultrastructural study has been undertaken by means of (1) the E-PTA stain and (2) the HMM-labeling procedure. Using serial sections of E-PTA stained nervous tissue, especially those made in transversal and tangential planes, the geometric order of the presynaptic grid and of its constituents has been described in detail. It consisted of dense projections having the shape of small truncated pyramids cut parallel to their hexagonal bases which rested on the electron-lucent presynaptic membrane. The dense projections were arranged at the points of equilateral triangles. Around each dense projection, six asymmetric hexagonal holes were seen to be arrayed in an hexagonal pattern, forming thus the presynaptic sieve. From the spiny tops of the dense projections, which appeared as specialized structures of the dense material coating the inner surface of the plasma membrane at the level of the synaptic cleft, fine filaments, 40--60 A in diameter, radiated and formed a three-dimensional meshwork pervading the presynaptic bag. The dense cytoplasmic coating delineating the plasma membrane served as anchor points for these microfilaments. Upon incubation with rabbit skeletal muscle HMM the microfilaments underwent specific structural changes, consisting of: (1) a striking increase in diameter; (2) the association of periodic and polarized substructures with their surfaces. The synaptic vesicles and mitochondria were seen to be attached to the numerous HMM-decorated filaments or enmeshed in the network formed by these filaments. The actin-like filaments were anchored to the plasma membrane at many points and to the presynaptic dense projections. Following incubation in the buffer alone or in buffer HMM solutions containing Na+ pyrophosphate or ATP, no arrowheaded structures were observed. Thus, a network consisting of actin-like filaments was demonstrated in the presynaptic bag. Of particular interest was the structural relation of the actin-like filaments with the occasional, tapered myosin-like filaments. The role of the presynaptic actin-like network in the transport of synaptic vesicles towards the presynaptic membrane by a mechanism of chemomechanical transduction is discussed. In the postsynaptic dendrite or dendritic spine, a filamentous network was observed to be attached to the subsynaptic web by means of the E-PTA stain and of the HMM-labeling procedure. The occurrence of an actin-like meshwork in the postsynaptic region is suggested to produce changes in the macromolecular configuration of the postsynaptic membrane by a "mechanoenzyme" system similar to that described in the mitochondrial membrane.