The movements of the presynaptic endocytotic structures produced durin
g tetanic stimulation at 10 Hz were examined morphometrically and cyto
chemically in the cat superior cervical ganglion in vivo. The longitud
inal profiles of the axon terminal and preterminal area were subdivide
d into five zones, I-V. Zone I, the area adjacent to the active zone,
was assigned a hemicircle with a diameter equivalent to the active zon
e width (2R). Zones II-IV were defined by subdividing successively the
presynaptic and preterminal areas within hemicircles with diameters e
quivalent to three-, five-, and sevenfold of 2R, respectively. Zone V
was composed of the rest of the preterminal profile. The endocytotic s
tructures, macropinocytotic endosomes and coated vesicles, observed in
each zone were morphometrically analyzed with the time course of stim
ulation. The lateral surface of zone II was shown to be the main site
for internalization of the terminal surface membrane during transmitte
r release. A large amount of the plasmalemma of zone II was rapidly re
trieved by macropinocytosis to produce early endosomes at an increased
rate of about three times that at rest. The population of coated vesi
cles, few in number at rest, increased to two- to threefold in zones I
I-V following the stimulation. Cytochemical examinations showed the in
corporation of HRP into synaptic vesicles, endosomes and multivesicula
r bodies. An antibody against synaptophysin labeled the presynaptic en
dosomes, multivesicular bodies, coated vesicles as well as synaptic ve
sicles. The results have suggested that a considerable part of these e
ndosomes was transported retrogradely at an increasing rate from zone
II to zone V via zones III and IV. On the other hand, synaptophysin wa
s observed to be distributed on the tubular protrusions of the presyna
ptic endosomes, suggesting the segregation and recycling of a part of
synaptic vesicle proteins from the early endosomes in the nerve ending
s of the cat superior cervical ganglion.