Ultrastructural and morphometric investigations on the mechanism of neurotransmitter release from synaptic vesicles

The mechanism of neurotransmitter release from synaptic vesicles as the results of neuron excitation is not quite clear. It is believed, that this process may occur by way of exocytosis into the synaptic cleft directly from the cytosol to the cleft. Ultrastructural-morphometric analysis was attempted of the synaptic vesicles from the motor cortex of mice. As stimulation model an audiogenic epileptic seizure was used. The method of ZIO impregnation was applied, which according to Kawana (7) allowed to visualise the presence of a neurotransmitter in synaptic vesicles. Morphometric analysis consisted in counting the synaptic vesicles ZIO-positive, filled with neurotransmitter, and ZIO-negative, empty ones, in 3 successive zones of the synaptic bouton in the initial and end phases of seizures. As the result of excitation the number of ZIO-positive-synaptic vesicles diminished significantly whereas the number of ZIO-negative ones did not increase proportionally. This indicates that the "dark" vesicles do not change into the "clear" ones, but they disappear completely. The applied method did not allow us to precisely establish vesicle translocation toward the synaptic cleft. Nevertheless the considerable reduction of their number with the percentual distribution of vesicles remaining the same within the particular zones of bouton, may indicate, under the assumption of translocation of vesicles, that they disappear close to the cleft, probably by way of exocytosis.