DYNAMICS OF PRESYNAPTIC ENDOSOMES PRODUCED DURING TRANSMITTER RELEASE

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.