Spontaneous and nerve-evoked quantal transmission in regenerated motor terminals

Anti-GQ1b antibodies and evoked acetylcholine release at mouse motor endplates

Miller Fisher syndrome (MFS) is clinically characterized by ataxia, areflexia, and ophthalmoplegia, and is associated with serum anti-GQ1b-ganglioside antibodies. We have previously shown that anti-GQ1b antibodies induce complement-dependent, alpha-latrotoxin-like effects at mouse neuromuscular junctions (NMJs) in vitro. This effect comprises a massive increase in spontaneous quantal acetylcholine (ACh) release, accompanied by block of evoked release and muscle paralysis. This mechanism may contribute to the motor features of MFS. Whether the block of evoked ACh release is a primary effect of anti-GQ1b antibodies or occurs secondary to massive complement-dependent spontaneous release is unknown. Using conventional micro-electrode methods, we measured in detail ACh release evoked with low- and high-rate nerve stimulation, and studied the effect on it of a purified MFS IgG and a mouse monoclonal anti-GQ1b IgM (without added complement). We found that evoked transmitter release was unaffected. Control experiments proved binding of anti-GQ1b antibody at the NMJ. We conclude that the block of nerve-evoked ACh release at the NMJ is not a primary effect of anti-GQ1b antibodies, but is dependent on antibody-mediated complement activation. It remains to be determined whether the block of nerve-evoked ACh release is the consequence of massive spontaneous ACh release or occurs as a concomitant event.

Synchronization of evoked secretion of quanta of mediator as a mechanism facilitating the action of sympathomimetics

Experiments on frog neuromuscular junction preparations with extracellular recording of nerve terminal action potentials and single-quantum end-plate currents (EPC) were used to assess the time course of evoked quantum secretion of mediator by analyzing histograms of the distribution of true synaptic delays. These studies showed that noradrenaline, isoproterenol, and dobutamine change the kinetics of secretion of quanta, leading to synchronization of the process of mediator release; substances blocking beta-adrenoceptors (atenolol, propranolol) blocked this effect. Clonidine and phenylephrine, which activate alpha-receptors, had no effect on the kinetics of secretion, while the alpha-blocker phentolamine had no effect on the synchronizing action of noradrenaline. Reconstruction of multiquantum EPC from changes in the level of synchronization in the release of individual quanta, showed that EPC amplitude increased in response to noradrenaline by 17%, and that this was due only to alterations in the time course of secretion. These data led to the conclusion that there is a special presynaptic mechanism which facilitates the action of sympathomimetics, acting via beta-adrenoceptors.

Noradrenaline synchronizes evoked quantal release at frog neuromuscular junctions

1. Noradrenaline (NA) increases synaptic efficacy at the frog neuromuscular junction. To test the hypothesis that one of the actions of NA is to shorten the period over which evoked quanta are released, we measured the latencies of focally recorded uniquantal endplate currents (EPCs). 2. NA shortened the release period for evoked quantal release. The interval between the time when responses with minimal delay appeared and the point at which 90 % of all latencies had occurred was shortened in the presence of 1 x 10-5 M NA by about 35 % at 20 C and by about 45 % at 8 C. Inhibitor and agonist experiments showed that NA acts on a beta-adrenoreceptor. 3. The better synchronization of release significantly increased the size of reconstructed multi- quantal EPCs. This suggests that NA facilitates synaptic transmission by making the release of quanta more synchronous. 4. The synchronizing action of NA might potentiate neuromuscular transmission during nerve regeneration, transmitter exhaustion and other extreme physiological states where the quantal content is reduced, such as survival in cold and hibernation.

Miller Fisher anti-GQ1b antibodies: alpha-latrotoxin-like effects on motor end plates

In the Miller Fisher syndrome (MFS) variant of the Guillain-Barré syndrome, weakness is restricted to extraocular muscles and occasionally other craniobulbar muscles. Most MFS patients have serum antibodies against ganglioside type GQ1b of which the pathophysiological relevance is unclear. We examined the in vitro effects of MFS sera, MFS IgG, and a human monoclonal anti-GQ1b IgM antibody on mouse neuromuscular junctions (NMJs). It was found that anti-GQ1b antibodies bind at NMJs where they induce massive quantal release of acetylcholine from nerve terminals and eventually block neuromuscular transmission. This effect closely resembled the effect of the paralytic neurotoxin alpha-latrotoxin at the mouse NMJs, implying possible involvement of alpha-latrotoxin receptors or associated downstream pathways. By using complement-deficient sera, the effect of anti-GQ1b antibodies on NMJs was shown to be entirely dependent on activation of complement components. However, neither classical pathway activation nor the formation of membrane attack complex was required, indicating the effects could be due to involvement of the alternative pathway and intermediate complement cascade products. Our findings strongly suggest that anti-GQ1b antibodies in conjunction with activated complement components are the principal pathophysiological mediators of motor symptoms in MFS and that the NMJ is an important site of their action.

Involvement of protein kinases in the upregulation of acetylcholine release at endplates of alpha-bungarotoxin-treated rats

1. ACh release from motor nerve endings in diaphragms of rats treated chronically with alpha-bungarotoxin (alpha-BuTX) is upregulated at the level of the individual endplate. Involvement of protein kinases in this mechanism of synaptic adaptation was investigated. 2. Miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were recorded after mu-conotoxin treatment, which prevents muscle action potentials. The quantal content at endplates was calculated 'directly', i.e. by dividing the EPP amplitude by the MEPP amplitude. 3. Incubation of muscles from control and alpha-BuTX-treated rats with H-7, a protein kinase C (PKC) inhibitor, reduced MEPP amplitudes but had no clear effect on quantal contents. Polymyxin B, another PKC inhibitor, had a similar effect on muscles from alpha-BuTX-treated rats. 4. Incubation of muscles from alpha-BuTX-treated rats with K252a, a broad-spectrum protein kinase inhibitor of, amongst others, PKC, Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and neurotrophin receptor tyrosine kinases, resulted in a 30% decrease of the quantal content. However, K252a did not change the quantal content of controls. Incubations with the closely related compound K252b, which has an exclusively extracellular action, had a similar effect. 5. KN62, a specific inhibitor of CaMKII, decreased the mean quantal content of muscles from alpha-BuTX-treated rats by 18%. 6. Tyrphostin 51, a selective tyrosine kinase inhibitor, had no effect on quantal contents of muscles from alpha-BuTX-treated and control rats. However, it increased the frequency and amplitude of MEPPs in muscles from alpha-BuTX-treated rats, leaving those of controls unchanged. 7. The extent of reduction of quantal content, caused by K252a, K252b and KN62, varied between endplates of individual muscles from alpha-BuTX-treated rats; quantal contents at endplates with small MEPPs were more sensitive than those at endplates with large MEPPs. 8. It is concluded that PKC does not play a role in the mechanism of upregulation of ACh release at endplates of alpha-BuTX-treated rats. Instead, CaMKII and some tyrosine kinases in the presynaptic membrane, as well as in the cytoplasm, might be involved.

Acetylcholine release in myasthenia gravis: regulation at single end-plate level

In myasthenia gravis, loss of acetylcholine receptors at motor end-plates is induced by antireceptor autoantibodies. At end-plates in rats in which myasthenia gravis-like symptoms are induced by chronic treatment with alpha-bungarotoxin, acetylcholine release is increased. Within muscles from such rats there is a strong correlation between the increase of acetylcholine release at an end-plate and the loss of postsynaptic acetylcholine receptors, caused by the toxin. The question is whether upregulation of acetylcholine release is a clinically relevant compensatory mechanism in myasthenia gravis or only a feature of the animal model using alpha-bungarotoxin. We investigated electrophysiologically the in vitro acetylcholine release at end-plates of muscles from patients with myasthenia gravis and rats with experimental autoimmune myasthenia gravis where acetylcholine receptor reduction is caused by autoantibody attack. In both human and rat autoimmune myasthenic muscle, the mean quantal content was considerably increased compared with control levels. At each individual myasthenic end-plate, the increase in quantal content appeared to be correlated with the reduction of the amplitude of the miniature end-plate potential. This finding suggests the existence of an important compensatory mechanism in myasthenia gravis, in which retrograde acting factors (i.e., from muscle fiber to nerve terminal) upregulate acetylcholine release.

The upregulation of acetylcholine release at endplates of alpha-bungarotoxin-treated rats: its dependency on calcium

1. The presynaptic component of an adaptive feedback mechanism leading to increased acetylcholine (ACh) release was studied in endplates of diaphragms from rats treated chronically with alpha-bungarotoxin (alpha BTX). 2. Quantal contents were calculated 'directly' from the amplitude of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) which were recorded after mu-conotoxin treatment to prevent muscle action potentials. 3. In vitro application of the Ca2+ channel blockers nifedipine (10 microM) or omega-conotoxin (40 nM) had no significant effect on the increased quantal content of endplates from alpha BTX-treated rats. 4. At control endplates, in vitro block of presynaptic K+ channels by 5 microM 3,4-diaminopyridine did increase the quantal content to a level which was similar to that found in endplates of alpha BTX-treated rats but also induced a broadening of EPPs, which was not found at endplates after alpha BTX treatment. 5. The difference between quantal contents of alpha BTX-treated and control rats was highly dependent on the [Ca2+]o/[Mg2+]o ratio when [Mg2+]o was fixed at 1 mM. At low [Ca2+]o, the quantal content of endplates from alpha BTX-treated rats was lower than that of controls while at [Ca2+]o in the normal and high range this was reversed. However, changing the [Ca2+]o/[Mg2+]o ratio by means of [Mg2+]o, at a fixed [Ca2+]o of 2 mM, did not influence the relative increase of quantal contents at endplates from alpha BTX-treated rats. Double logarithmic plots of the 'toxin-induced' myasthenia gravis (TIMG) and control quantal content versus [Ca2+]o had an approximately linear part between 0.2 and 1.5 mM [Ca2+]o. The slopes of the TIMG and control lines were 1.81 and 0.96, indicating that the ACh release in TIMG muscles was more sensitive to changes of [Ca2+]o than controls. 6. At normal [Ca2+]o and [Mg2+]o, the depression of EPP amplitude during stimulation of the phrenic nerve at 30-50 Hz was somewhat larger at endplates from alpha BTX-treated rats than at control endplates. At low [Ca2+]o, the potentiation of EPP amplitudes during a stimulus train was much larger at endplates from alpha BTX-treated rats than from controls. 7. The results do not support the idea that the increased release of ACh is caused via regulatory effects on the presynaptic Ca2+ or K+ channels. Instead, the anomalous dependency of ACh release on Ca2+ in muscles of alpha BTX-treated rats suggests that a cytoplasmic, Ca(2+)-dependent, component is involved in the adaptive change of transmitter release.

Adaptation of quantal content to decreased postsynaptic sensitivity at single endplates in alpha-bungarotoxin-treated rats

1. Rats were injected once every 48 h with alpha-bungarotoxin (alpha BTX) for periods up to 6 weeks. Injections caused weakness of facial muscles which lasted about 8 h. Hemidiaphragms were dissected for biochemical and electrophysiological measurements. 2. In muscles from animals treated for 2-3 weeks with toxin, the binding of 125I-alpha BTX was reduced to 58%, and the ACh content to 81% of control values. Choline acetyltransferase activity was unchanged. ACh release evoked by 3 Hz nerve stimulation was increased to 175% of control values. 3. The use of mu-conotoxin, which specifically blocks muscle action potentials, enabled the recording of full-sized endplate potentials (EPPs) and miniature endplate potentials (MEPPs) at normal muscle membrane potentials (-70 to -80 mV). The amplitude of MEPPs was decreased to 57% in muscles from animals treated for 3 weeks with alpha BTX. The mean of the quantal contents, calculated from the ratio of the corrected EPPs and the MEPPs, was increased to 154%. 4. Within individual muscles of both alpha BTX-treated and control rats, there was an inverse relationship between the quantal content of an endplate and its MEPP amplitude. 5. The MEPP frequency of endplates from control muscles was positively correlated with the quantal content. However, this correlation was not found in alpha BTX-affected muscles. 6. Three hours after a single injection of alpha BTX the amplitude of the MEPPs was reduced to about 60% of control values but no increase of the quantal content was found. During the first few days of alpha BTX treatment the quantal content gradually increased; it reached a plateau between 20 and 30 days. 7. The results suggest the existence of an adaptive mechanism, operating at individual endplates, in which retrograde signals at the motor nerve terminals modulate ACh release when neuromuscular transmission is endangered by block of acetylcholine receptors.