[Freeze fracture study of vesicle attachment sites during mediator liberation in spinal motoneuron synapses].

Freeze-fractured appearances of vesicle attachement sites (VAS) in presynaptic active zone were studied in the spinal cord of 4-aminopyridine (4-AP) treated and nembutalized rats. The VAS were classified into: type 1 containing no intramembranous particles, type 2 with particles around the edge of openings, type 3 containing several particles within the VAS. The application of 4-AP, which is known to enhance transmitter release, affected strikingly the total number of VAS (p less than 0.001) but not the proportion of the three types. 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. It is likely that the remaining two types represent exocytosis.