Three types of membrane modulations during transmitter release in rat spinal cord synapses.

Protoplasmic fracture faces of vesicle attachment sites (VSA) in the presynaptic active zone of rat spinal motoneurons were classified morphologically on the basis of particle-to-membrane relationships. Intramembranous particles are absent in type 1, located around the edges of type 2 and covering the center of type 3. Application of 4-aminopyridine (4-AP), which is known to enhance transmitter release, affected strikingly the total number of VAS (P less than 0.001) but not the percentage of the three types. It is postulated that the particle-free VAS (types 1 and 2) sharing 80--85% of VAS within the active zone, are related to exocytosis. The predominance of type 3 outside the active zone and its similarity to particle-loaded indentations possibly corresponding to coated vesicle formation, suggested that type 3 may represent endocytosis.