Actions of beta-bungarotoxin on spontaneous release of transmitter at muscle end-plates treated with botulinum toxin

A nerve clamp electrode design for indirect stimulation of skeletal muscle

A nerve clamp electrode was developed to indirectly stimulate skeletal muscle innervated by α motor neurons as an alternative to conventional electrodes. The stimulating electrode device consists of a spring coil-activated nerve clamp mounted inside a 1-mL syringe barrel. Supramaximal pulses were generated by a Grass stimulator and delivered to the nerve segment via the nerve clamp electrode. The salient feature of the electrode is its ability to produce muscle contractions indirectly through stimulation of the attached nerve. Indirect muscle stimulation is critical for studying the paralytic actions of presynaptic-acting toxins such as botulinum neurotoxins (BoNT), a potent inhibitor of acetylcholine (ACh) release from α motor neurons. This device enables stimulation of muscle contraction indirectly as opposed to contraction from direct muscle stimulation. The electrode is able to stimulate indirect muscle contraction when tested on ex vivo preparations from rodent phrenic nerve-hemidiaphragm muscle in similar fashion to conventional electrodes. In addition, the electrode stimulated external intercostal nerve-muscle preparations. This was confirmed after applying BoNT serotype A, a potent inhibitor of ACh release, to induce muscle paralysis. Alternative methods, including suction and bipolar loop electrodes, were unsuccessful in stimulating indirect muscle contraction. Therefore, this novel electrode is useful for physiological assessment of nerve agents and presynaptic actions of toxins that cause muscle paralysis. This electrode is useful for stimulating nerve-muscle preparations for which the length of nerve is a concern.

Effects of botulinum neurotoxin and Lambert-Eaton myasthenic syndrome IgG at mouse nerve terminals

The interaction between two presynaptically acting agents, Lambert-Eaton myasthenic syndrome (LEMS) immunoglobulin G (IgG) and purified botulinum neurotoxin (BoNT) type A, was studied. Intracellular microelectrode recordings were carried out on mouse muscles after injection with LEMS IgG. BoNT was either injected before recordings were made or applied in vitro. The time course of the in vitro actions of BoNT on miniature end-plate potential and end-plate potential parameters were not affected by pretreatment with LEMS IgG. After in vivo injection of BoNT, end-plate potential quantal content was reduced to less than 2% of control values, whether or not LEMS IgG had also been previously given. Quantitative electron-microscope autoradiographical analysis showed that neither the binding of 125I-BoNT to acceptors on the nerve terminal membrane nor the pattern of its internalisation were affected by pretreatment with LEMS IgG. We conclude that the effects of BoNT are not affected by LEMS IgG, suggesting different presynaptic binding sites for the two agents.

Zinc antagonizes the effect of botulinum type A toxin at the mouse neuromuscular junction

Zn2+ (10-100 microM) elevated the frequency of miniature end-plate potentials (MEPPs) in the mouse diaphragm. The effect did not depend on external Ca2+. Botulinum type A toxin (BTXA, 50 ng/ml) abolished MEPPs almost completely within 30 min. Zn2+ (100 microM) restored MEPPs and increased their frequency after they had been abolished by BTXA in Ca2+ -free solutions. The antagonistic effect of Zn2+ in the Ca2+ -free solution was reduced by exposing the diaphragm to the toxin in the Ca2+ -free solutions containing high K+. Thus, the action of BTXA is probably enhanced by depolarization of the motor nerve terminals.

Botulinum toxin A blocks glutamate exocytosis from guinea-pig cerebral cortical synaptosomes

The exocytotic release of L-glutamate from guinea-pig cerebral cortical synaptosomes can be extensively inhibited by preincubation with botulinum neurotoxin type A at 37 degrees C for 1-2 h. The toxin has no effect on synaptosomal respiratory control, respiratory capacity, ATP synthesis, plasma-membrane 86Rb+ permeability or plasma-membrane potential, does not inhibit the entry of 45Ca2+ into the synaptosome upon depolarization and does not alter the ability of intrasynaptosomal mitochondria to sequester Ca2+. The blockade of Ca2+-dependent glutamate release may be totally reversed by the Ca2+/2 H+-exchange ionophore ionomycin, but not by increasing extracellular Ca2+ concentration. It is suggested (a) that exocytosis is triggered by the penetration of Ca2+ into an intracellular hydrophobic milieu; (b) that this stage is blocked by the toxin and (c) that ionomycin is able to bypass this block and deliver Ca2+ to the exocytotic apparatus.

The effects of in vitro application of purified botulinum neurotoxin at mouse motor nerve terminals

1. Purified botulinum neurotoxin type A (10 nM) was applied in vitro to mouse diaphragm muscles. Intracellular micro-electrode recordings were made continuously in single fibres. 2. This treatment reduced end-plate potential (e.p.p.) amplitudes with a time to half-maximal effect of about 75 min at 22-25 degrees C. E.p.p. rise-times remained fast and unaffected by the toxin. 3. Miniature end-plate potential (m.e.p.p.) frequency was reduced by the toxin to less than 5% of control frequency, and followed a similar time course to the block of e.p.p. amplitudes. The m.e.p.p. rise-time and coefficient of variation (c.v.) of m.e.p.p. amplitude distributions both increased, but the time course of these increases lagged significantly behind the change in frequency. 4. A population of slow rise-time m.e.p.p.s was present in controls at low frequency. This population was found to be unaffected by the toxin. 5. The above-detailed in vitro changes could be explained by the toxin acting by a single common mechanism to inhibit the release process underlying both fast rise-time m.e.p.p.s and e.p.p.s. A distinct release process, which leads to slow rise-time m.e.p.p.s, was unaffected by the toxin.

Calcium-insensitive miniature endplate potentials at the neuromuscular junction

Several different types of acetylcholine secretion have been shown to coexist at the neuromuscular junction along with the Ca2+-dependent quantal release producing miniature endplate potentials (mepps) and endplate potentials. One of these, the Ca2+-insensitive, slow-rising mepps (slow mepps), is present in normal untreated muscles but is most prominent in many conditions where the Ca2+-dependent quantal release mechanism is not functioning properly. Slow mepps occur at a frequency of less than 0.1 Hz in normal muscles, with large variability between fibres and muscles, and can reach frequencies of 1-2 Hz in several pathological conditions. The potentials are also highly variable in size and shape, being generally of high amplitude (0.1-15 mV) and prolonged time course (1-15 ms rise time). Most importantly, slow mepps are not affected by procedures which increase the intraterminal Ca2+ concentration, including nerve stimulation, thus being unable to contribute to the function of synaptic transmission. The cellular source of the Ca2+-insensitive mepps has been determined to be the nerve terminal and not the Schwann cells or nerve sprouts. The release process producing slow mepps is generally insensitive to many drugs, ions, and procedures, stimulation being observed with vinblastine, cytochalasin B, and caffeine. Depression of this secretion is effected by uncouplers of oxidative phosphorylation and by a drug (AH5183) which inhibits the vesicular active acetylcholine transport system. It is concluded that the slow mepps are due to an exocytic fusion of unique synaptic vesicles with the plasma membrane near the active zones, in a process insensitive to many intracellular ions and regulators. Since slow mepps are prominent in many pathological conditions of nerve and muscle, it is speculated that they play some role in the recovery or development of synaptic function.