Morphometric characterization of the neuromuscular junction of rodents intoxicated with 2,4-dithiobiuret: evidence that nerve terminal recycling processes contribute to muscle weakness.
ABSTRACT 2,4-Dithiobiuret (DTB) causes ascending motor weakness when given chronically to rodents. In muscles of animals with DTB-induced weakness, quantal release of acetylcholine (ACh) is impaired. We examined in detail the structural changes that occurred at neuromuscular junctions and their associated Schwann cells of extensor digitorum longus (EDL) muscles of male rats treated with DTB to the onset of muscle weakness, 5-8 days. Our objective was to assess the involvement of the Schwann cells and to determine the most likely primary targets of DTB. At the onset of muscle weakness, nerve terminals exhibited some enlarged regions, but did not sprout. Terminal Schwann cells became flatter and expanded to cover most of the endplate. The extent of invasion of the synaptic cleft by Schwann cell processes was not significantly different from controls; extension of Schwann cell sprouts away from the junction was not seen. Thus, the morphology of the Schwann cells, although clearly affected by DTB, does not suggest that they contribute directly to the physiological defects of DTB-treated terminals. Abnormal tubulovesicular structures or tangles of neurofilaments were clustered in the centers of about 25% of treated terminals. Fewer synaptic vesicles occupied the region opposite the postsynaptic folds. Vesicle volumes were variable and included some very large vesicles, corresponding with the variable MEPP amplitudes reported previously for terminals of DTB-treated rodents. The postsynaptic area stained by rhodamine-labeled alpha-bungarotoxin expanded with terminal swelling, apparently by unpleating of the postsynaptic folds. No loss of ACh receptors or spread of ACh receptors beyond terminal boundaries was detected. Morphometric data are consistent with the conclusion that DTB affects, either directly or indirectly, vesicular release of ACh and the subsequent vesicular recycling process.
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ABSTRACT: (1) Intracellular micro-electrodes and electron-microscopy were used to study normal and denervated end-plates in rat diaphragm muscle fibres. (2) In normal muscles 84.5 to 100% of the micro-electrode insertions were sufficiently close to the neuromuscular junction to detect miniature end-plate potentials. The structure of the normal neuromuscular junction had the usual 3-cell arrangement: muscle with synaptic folds, axon and Schwann cell. (3) Within one day after section of the phrenic nerve, the axon disintegrated and miniature end-plate potentials ceased to occur. Subsequently, miniature potentials were not observed at denervated end-plates, except during the third week after denervation, at which time a low-frequency discharge was seen in eight out of 770 fibres. The miniature potentials at these end-plates resembled those at normal junctions, and were presumably also due to acetylcholine acting on the muscle membrane. (4) The synaptic folds remain for several months after denervation, and serve to identify electron-microscopically the denervated end-plate. After prolonged denervation (> 3 weeks), when miniature end-plate potentials were never observed, there was generally no cell overlying the synaptic folds. (5) During the first 3 weeks after denervation, a nucleated cell, presumably the Schwann cell, was in close contact with the muscle. 'Schwann-muscle' contacts were observed in muscle without miniature end-plate potentials. (6) Electron microscopy of a portion of denervated muscle, which included a fibre with miniature potentials, showed that the fibre had extensive 'Schwann-muscle' contacts. (7) It is concluded that the Schwann cell is the source of the packages of acetylcholine which evoke miniature end-plate potentials in denervated muscle. Since the Schwann cell was in contact with muscle fibres without miniature potentials, it appears that the presence of the Schwann cell is a necessary, but not a sufficient, condition for the production of miniature potentials at denervated end-plates.Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain) 03/1968; 169(16):289-306.
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ABSTRACT: The osmium-bridging properties of thiocarbohydrazide have been used to bind additional osmium to a variety of soft tissue surfaces before critical-point drying. The procedure gives satisfactory protection to specimens under a scanning electron beam at accelerating voltages between 5 and 25 kV and provides the microscopist with a simple, efficient, and economical alternative to evaporative coating with other heavy metals prior to analysis in the scanning electron microscope.Journal of Ultrastructure Research 12/1973; 45(3):254-8.
Article: Quantal release of serotonin.[show abstract] [hide abstract]
ABSTRACT: We have studied the origin of quantal variability for small synaptic vesicles (SSVs) and large dense-cored vesicles (LDCVs). As a model, we used serotonergic Retzius neurons of leech that allow for combined amperometrical and morphological analyses of quantal transmitter release. We find that the transmitter amount released by a SSV varies proportionally to the volume of the vesicle, suggesting that serotonin is stored at a constant intravesicular concentration and is completely discharged during exocytosis. Transmitter discharge from LDCVs shows a higher degree of variability than is expected from their size distribution, and bulk release from LDCVs is slower than release from SSVs. On average, differences in the transmitter amount released from SSVs and LDCVs are proportional to the size differences of the organelles, suggesting that transmitter is stored at similar concentrations in SSVs and LDCVs.Neuron 11/2000; 28(1):205-20. · 15.77 Impact Factor