Morphological changes in presynaptic terminals of the chick ciliary ganglion after stimulation in vivo. A stereological study showing a net loss of total membrane

Incorporation of vesicular antigens into the presynaptic membrane during exocytosis at the frog neuromuscular junction: a light and electron microscopy immunochemical study

We have studied the incorporation of vesicular membrane antigens into the presynaptic membrane during exocytosis of neurotransmitter at the frog neuromuscular junction. In a preliminary series of experiments, we first confirmed by electron microscopy that the synaptic vesicles are labelled following incubation with rabbit antisynaptic vesicle antibody of neuromuscular junction cross sections (cytoplasm and organelles reached by the antibodies). In a second series of experiments, intact neuromuscular junctions were stimulated with black widow spider venom and fixed with paraformaldehyde. The presence or absence of vesicular antigens in the presynaptic membrane was monitored with rabbit antisynaptic vesicle antibody and revealed with a second antibody coupled to peroxidase. In light microscopy, the labelled neuromuscular junctions are almost completely restricted to muscles stimulated with black widow spider venom and incubated with rabbit antisynaptic vesicle antibody. Only a few control muscles (not stimulated with black widow spider venom, but incubated with rabbit antisynaptic vesicle antibody) had labelled neuromuscular junctions. All control neuromuscular junctions, not incubated with rabbit antisynaptic vesicle antibody were unlabelled. Electron microscopy indicated that it is the presynaptic membrane of intact stimulated neuromuscular junctions which is labelled. In these intact neuromuscular junctions, the synaptic vesicles are usually unlabelled. Electron microscopy also indicated that the presynaptic membrane of only one type of control junctions (not stimulated with black widow spider venom, but incubated with rabbit antisynaptic vesicle antibody) is rarely and weakly labelled. Other types of controls (not incubated with rabbit antisynaptic vesicle antibody) are never labelled. Therefore our results are consistent with the incorporation of vesicular antigens into the plasma membrane during exocytosis produced by the black widow spider venom. The low level of labelling of unstimulated neuromuscular junctions suggest a rather complete retrieval of the vesicular proteins during endocytosis.

Non monotonic morphometric changes produced at mouse neuromuscular junctions following in vivo stimulation at various frequencies

Morphometric changes have been studied at the mouse neuromuscular junction (NMJ) as a function of the frequency of stimulation. In vivo stimulation was made at either 10, 25, 50 or 100 Hz. No significant change in the area of NMJ profiles is observed after stimulation. This indicates that stimulation does not change the volume of the nerve terminal. There is also no change of the volume density of mitochondria. However, there are significant reductions of the numerical density on area (NA, number/micron2) of the clear vesicles following stimulation at 25, 50 or 100 Hz. The NA of clear vesicles is not further reduced following stimulation at 50 or 100 Hz than after the stimulation at 25 Hz. The numerical density on area of the coated vesicles was significantly increased following the stimulation at 25 Hz and 100 Hz. It is not increased more following stimulation at 50 Hz or 100 Hz than following stimulation at 25 Hz. One possible interpretation of the clear vesicle and the coated vesicle results is that less vesicles are liberated per sec by the NMJ with stimulation at 50 and 100 Hz than at 25 Hz because the action potential may fail to invade some terminal arborizations. The total surface density (i.e. surface of membrane in micron2 per micron3 of nerve terminal) of the clear vesicles, the coated vesicles, the vacuoles and the presynaptic plasma membrane is not changed significantly following stimulation at any frequency. This indicates that there is no net loss of membrane at any of the frequencies investigated.

The effect of nerve activity on the distribution of synaptic vesicles

Certain gymnotid fish (apteronotids) continuously emit a high-frequency electric-organ discharge and thus continuously drive their electroreceptor afferents at high rates. Electroreceptor afferents terminate in one lamina of the electrosensory lateral line lobe (ELL) and can be readily sampled. Normally these terminals have many small vesicles clustered adjacent to the presynaptic membrane. When afferent activity is blocked for 24 hr by an injection of tetrodotoxin (TTX) into the electroreceptor nerve, the density of vesicles adjacent to the synaptic membrane declines; the volume of the remaining vesicles increases. If the nerve of a TTX-treated fish is stimulated proximal to the injection site, these changes can be reversed. These results imply that the migration of vesicles toward the presynaptic membrane is influenced by the level of activity in the nerve.

Veratridine-stimulated central synapses in culture: a quantitative ultrastructural analysis

Synapses in explant cultures of fetal rat neocortex at day 18 in vitro were stimulated by veratridine (10(-4)M) for 20 min. The cultures were subsequently processed for electron microscopy and the synapses were analyzed by quantitative techniques, incorporating set mathematical treatment. The mean values of area, perimeter, and form factor of the presynaptic elements significantly increased following veratridine stimulation, compared to the values of control synapses. The length of the postsynaptic thickening also increased, while synaptic curvature did not change significantly in the veratridine group. A fivefold reduction was observed in the mean number of synaptic vesicles per presynaptic element and in the vesicle-terminal area ratio, following veratridine stimulation. The cytoplasm-terminal area ratio and the occurrence of vacuoles/cisternae significantly increased after veratridine application. Planar measurement of membranes (boundary length) of different presynaptic organelles revealed that the total membrane did not change significantly in the veratridine group. The data indicated an increase in volume and swelling of the pre- and postsynaptic elements, considerable depletion of synaptic vesicles, and preservation of the total presynaptic membrane following veratridine stimulation in nerve tissue culture.