Membrane associated particles of the presynaptic active zone in rat spinal cord. A morphometric analysis.

The distribution of membrane-associated particles in the presynaptic membrane was investigated in the spinal cord of unanesthetized and anesthetized rats by freeze etching electron microscopy. Both 'external' face (EF) and 'protoplasmic' face (PF) were examined. Particles were classified according to size in two categories: small particles (diameter 5.0-8.7 nm) and large particles (diameter 8.7-13.7 nm). The presynaptic region was subdivided into an active and a surrounding zone, depending on specific ultrastructural criteria. The density of large particles in the PF was found to be significantly higher in the active as compared with the surrounding zone in both unanesthetized and anesthetized rats. Thus, the presence of large particles represents an important feature of the active zone. Considerably more large particles were found in the waking than in the barbiturized state. This difference is paralleled by a vast increase of vesicle attachment sites in the presynaptic membrane of unanesthetized animals reported by Streit et al. and confirmed in the present study. It is suggested that the two phenomena could be interrelated and that the large particles may represent calcium channels and thus provide the morphological substrate for the mechanism of excitation-secretion coupling.