Abstract
The morphology of monoaminergic synaptic vesicles of rat pineal nerves was investigated after fixation with calcium-containing solutions of control and electrically stimulated glands. Stimulated and nonstimulated glands were fixed either in glutaraldehyde or in glutaraldehyde-osmium tetroxide with and without calcium chloride and processed for electron microscopy. It was observed that an electron dense particle, located in the vesicle core, appeared within the small and large vesicles when calcium was present in the fixation fluid. The size and electron density of these particles, as well as the proportion of vesicles showing these particles, depended on the concentration of calcium in the fixative, and the particle could be discerned in unosmicated tissues indicating that the electron opacity was produced by the presence of calcium ions. Although postfixation with OsO4 enhanced the electron density of these particles, it could be established that osmium and calcium affinities for the central core of monoaminergic vesicles are independent of each other and primarily due to different core components. Following electrical stimulation, these electron dense particles become significantly reduced or abolished. A decrease in calcium deposits after stimulation has also been described in cholinergic synaptic vesicles. The hypothesis is formulated that depolarization induces a change in the calcium-binding site apparently resulting in a loss of the calcium ions normally trapped by the vesicle. Calcium leaving the vesicle would then increase the calcium concentration in the cytoplasm thus contributing to the release of neurotransmitter, and would be recaptured by synaptic vesicles under the conditions of relatively mild stimulation at the resting state. Thus synaptic vesicles could act as one of the systems of sequestration for calcium ions instrumental in process of depolarization and release of neurotransmitter.
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