REDISTRIBUTION OF SYNAPSIN-I AND SYNAPTOPHYSIN IN RESPONSE TO ELECTRICAL-STIMULATION IN THE RAT NEUROHYPOPHYSEAL NERVE-ENDINGS

To understand the dynamics of synaptic vesicles and synapsin I, we hav e studied the localization of synapsin I and synaptophysin in resting and stimulated nerve endings by ultracryomicrotomy and colloidal gold- immune cytochemistry. First, we characterized microvesicles in resting nerve endings of the rat neurohypophysis, which was chosen as the mod el of nerve ending in this study. Synaptophysin was localized in micro vesicles that were clustered beneath the plasma membrane. Quick-freeze deep-etching electron microscopy showed that short strands cross-link ed microvesicles to each other, which highly resembly the structures o bserved in our studies of the presynaptic nerve terminals of central a nd peripheral nervous system and in vitro reconstitution of synapsin I and synaptic vesicles. Immunocytochemistry showed that synapsin I was localized to the region of cluster of microvesicles. Second, using th is system, we examined localization of synapsin I and synaptophysin in nerve endings after electrical stimulation. Besides release of neuros ecretory granules, clusters of microvesicles disappeared and both micr ovesicles and synaptophysin were scattered over nerve endings. These c hanges were also confirmed by quick-freeze, freeze-substitution. Immun ocytochemistry of the stimulated sample revealed that synapsin I was a lso scattered. The results show that microvesicles in neurohypophysis have similar characteristics of typical synaptic vesicles and synapsin I has a role as a scaffold to cross-link microvesicles to be clustere d in resting nerve endings. This scaffold of synapsin I was disengaged after stimulation to redistribute microvesicles and synapsin I itself , which may be the mechanism of synapsin I to regulate the availabilit y of synaptic vesicles for release.