Redistribution of synapsin I and synaptophysin in response to electrical stimulation in the rat neurohypophysial nerve endings.

To understand the dynamics of synaptic vesicles and synapsin I, we have studied the localization of synapsin I and synaptophysin in resting and stimulated nerve endings by ultracryomicrotomy and colloidal gold-immunocytochemistry. First, we characterized microvesicles in resting nerve endings of the rat neurohypophysis, which was chosen as the model of nerve ending in this study. Synaptophysin was localized in microvesicles that were clustered beneath the plasma membrane. Quick-freeze deep-etching electron microscopy showed that short strands cross-linked microvesicles to each other, which highly resemble the structures observed in our studies of the presynaptic nerve terminals of central and 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 this system, we examined localization of synapsin I and synaptophysin in nerve endings after electrical stimulation. Besides release of neurosecretory granules, clusters of microvesicles dissappeared and both microvesicles and synaptophysin were scattered over nerve endings. These changes were also confirmed by quick-freeze, freeze-substitution. Immunocytochemistry of the stimulated sample revealed that synapsin I was also 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 clustered 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 availability of synaptic vesicles for release.