Brief occurrence of a population of presynaptic intramembrane particles coincides with transmission of a nerve impulse.

Abstract

Small pieces of Torpedo electric organ were cryofixed at 1-ms time intervals in a liquid medium at -190°C before, during, and after the passage of a single nerve impulse. In contrast to studies using this or other preparations, these experiments were done without 4-aminopyridine or other drugs that potentiate transmitter release. Freeze-fracture replicas were made from the most superficial layers of the tissue, where the rate of cooling was rapid enough to retain ultrastructure in the absence of chemical fixation. We found that the transmission of an impulse was accompanied by the momentary appearance of a population of large intramembrane particles in both the protoplasmic (P) and the external (E) leaflets of the presynaptic plasma membrane. The change was very brief, appearing soon after the stimulus artifact. It lasted for 2-3 ms. Large pits denoting vesicle openings at the presynaptic membrane were found in a small proportion of nerve terminals; their number did not increase during transmission of the nerve impulse. Reducing the temperature from 16 to 5°C slowed the time course of both the electrophysiological response and the change in intramembrane particles. The number of large particles did not increase when stimulation was applied in a low-Ca medium, a condition where the nerve terminals were still depolarized by the action potential but did not release the neurotransmitter. From these and other observations, we conclude that this transient change of intramembrane particles is closely linked to the mechanism of acetylcholine release at the nerve-electroplaque junction.Transmission of a nerve impulse at synapses takes only a few milliseconds. This makes it difficult to distinguish, with biochemical or morphological techniques, the changes that accompany transmission of the nerve impulse and from the changes that take place soon thereafter. The difficulty can be overcome by using rapid-freezing methods and by treating the synapse with drugs, such as 4-aminopyridine, which potentiate and prolong the release of acetylcholine. This strategy was used by Heuser et al. (1) in their freeze-fracture work on the frog neuromuscular junction. They found that during and soon after the passage ofan impulse there is a large increase in the number of vesicle openings in the active zones of the motor nerve terminals. Active zones are specialized areas of this membrane that are near a double row of synaptic vesicles (2). This was followed by the appearance, at the same place, of large intramembrane particles (IMPs; ref. 3). Vesicle openings were also found by in cryofixed neuromuscular junctions that had been stimulated in the presence of 4-aminopyridine and a high-Ca concentration. These authors used cryosubstitution since they found that freeze-fracture did not preserve the ultrastructure of nerve terminal membranes [due to the large size of the myofibers, only muscle fragments with nerve terminals against the coolant are expected to give well-preserved presynaptic membranes, and the protoplasmic (P)-face of th...

Botulinum toxin type A blocks the morphological changes induced by chemical stimulation on the presynaptic membrane of Torpedo synaptosomes.

“…Similar results have been described in Torpedo electric organ (29,30) and synaptosomes (10)(11)(12). When ACh release is inhibited by BoNTx, there is a blockade of the rearrangement of IMPs at the PF and EF, as occurs during chemical stimulation in a calcium-free medium (12,30). Since calcium enters into intoxicated synaptosomes (9), our results show that IMP rearrangement is related to the ACh release process triggered by chemical stimulation, and it is not linked to the influx of calcium into the nerve terminals or to depolarization.…”

“…studies, using glutaraldehyde fixation (27,28), reported that electrical stimulation induced IMP rearrangement at the PF according to their density and size distribution. Similar results have been described in Torpedo electric organ (29,30) and synaptosomes (10)(11)(12). When ACh release is inhibited by BoNTx, there is a blockade of the rearrangement of IMPs at the PF and EF, as occurs during chemical stimulation in a calcium-free medium (12,30).…”

Momentary alteration of the postsynaptic membrane during transmission of a single nerve impulse.

“…No drug was present, and the tissue was not treated by chemical fixatives or cryoprotectants. The data were obtained from recent experiments where we found that a characteristic change occurs abruptly, for 2-3 ms, in the membrane of axon terminals when acetylcholine is released (4,5). In addition to the presynaptic membrane pictures, the freeze-fracture replicas often exposed areas of the postsynaptic membranes that were easily identified by their high density of intramembrane particles (IMPs) (see Fig.…”

“…In a given experiment, [10][11][12][13][14][15][16][17][18][19][20] prisms were fixed in their gold rings at different places under the freezing head. Before the experiment, all specimens were kept at rest at 16-18'C for at least 20 min in standard physiological medium containing 280 mM NaCI, 7 mM KCI, 4.4 mM CaC12, 1.3 mM MgCl2, 5 Freeze-fracture replica of a Torpedo electric organ synapse frozen 2 ms after stimulus. In this replica the plane of cleavage has passed through the presynaptic membrane exposing the external leaflet (at right) and then through the postsynaptic membrane exposing the P face (highly particulated membrane at left).…”

“…METHODS Stimulation, rapid-freezing, and cryofracture of tissue specimens were done as described (4)(5)(6). The electric rays (Torpedo marmorata) were supplied by the Station de Biologie Marine (Arcachon, France).…”