Effects of tubocurarine on end-plate current rundown and quantal content during rapid nerve stimulation in the snake

Homeostatic Plasticity of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) is an ideal preparation to study synaptic plasticity. Its simplicity- one input, one postsynaptic target- allows experimental manipulations and mechanistic analyses that are impossible at more complex synapses. Homeostatic synaptic plasticity attempts to maintain normal function in the face of perturbations in activity. At the NMJ, 3 aspects of activity are sensed to trigger 3 distinct mechanisms that contribute to homeostatic plasticity: Block of presynaptic action potentials triggers increased quantal size secondary to increased release of acetylcholine from vesicles. Simultaneous block of pre- and postsynaptic action potentials triggers an increase in the probability of vesicle release. Block of acetylcholine binding to acetylcholine receptors during spontaneous fusion of single vesicles triggers an increase in the number of releasable vesicles as well as increased motoneuron excitability. Understanding how the NMJ responds to perturbations of synaptic activity informs our understanding of its response to diverse neuromuscular diseases.

Muscle Nicotinic Acetylcholine Receptors May Mediate Trans-Synaptic Signaling at the Mouse Neuromuscular Junction

Block of neurotransmitter receptors at the neuromuscular junction (NMJ) has been shown to trigger upregulation of the number of synaptic vesicles released (quantal content, QC), a response termed homeostatic synaptic plasticity. The mechanism underlying this plasticity is not known. Here, we used selective toxins to demonstrate that block of α1-containing nicotinic acetylcholine receptors (nAChRs) at the NMJ of male and female mice triggers the upregulation of QC. Reduction of current flow through nAChRs, induced by drugs with antagonist activity, demonstrated that reduction in synaptic current per se does not trigger upregulation of QC. These data led to the remarkable conclusion that disruption of synaptic transmission is not sensed to trigger upregulation of QC. During studies of the effect of partial block of nAChRs on QC, we observed a small but reproducible increase in the decay kinetics of miniature synaptic currents. The change in kinetics was correlated with the increase in QC and raises the possibility that a change in postsynaptic nAChR conformation may be associated with the presynaptic increase in QC. We propose that, in addition to functioning in synaptic transmission, ionotropic muscle nicotonic nAChRs may serve as signaling molecules that participate in synaptic plasticity. Because nAChRs have been implicated in a number of disease states, the finding that nAChRs may be involved in triggering synaptic plasticity could have wide-reaching implications.SIGNIFICANCE STATEMENT The signals that initiate synaptic plasticity of the nervous system are still incompletely understood. Using the mouse neuromuscular junction as a model synapse, we studied how block of neurotransmitter receptors is sensed to trigger synaptic plasticity. Our studies led to the surprising conclusion that neither changes in synaptic current nor spiking of the presynaptic or postsynaptic cell are sensed to initiate synaptic plasticity. Instead, postsynaptic nicotinic acetylcholine receptors (nAChRs), in addition to functioning in synaptic transmission, may serve as signaling molecules that trigger synaptic plasticity. Because nAChRs have been implicated in a number of disease states, the finding that they may mediate synaptic plasticity has broad implications.

Homeostatic synaptic plasticity at the neuromuscular junction in myasthenia gravis

A number of studies in the past 20 years have shown that perturbation of activity of the nervous system leads to compensatory changes in synaptic strength that serve to return network activity to its original level. This response has been termed homeostatic synaptic plasticity. Despite the intense interest in homeostatic synaptic plasticity, little attention has been paid to its role in the prototypic synaptic disease, myasthenia gravis. In this review, we discuss mechanisms that have been shown to mediate homeostatic synaptic plasticity at the mammalian neuromuscular junction. A subset of these mechanisms have been shown to occur in myasthenia gravis. The homeostatic changes occurring in myasthenia gravis appear to involve the presynaptic nerve terminal and may even involve changes in the excitability of motor neurons within the spinal cord. The finding of presynaptic homeostatic synaptic plasticity in myasthenia gravis leads us to propose that changes in the motor unit in myasthenia gravis may be more widespread than previously appreciated.

Altered neurotransmitter release machinery in mice deficient for the deubiquitinating enzyme Usp14

Homozygous ataxic mice (ax(J)) express reduced levels of the deubiquitinating enzyme Usp14. They develop severe tremors by 2-3 wk of age, followed by hindlimb paralysis, and death by 6-8 wk. While changes in the ubiquitin proteasome system often result in the accumulation of ubiquitin protein aggregates and neuronal loss, these pathological markers are not observed in the ax(J) mice. Instead, defects in neurotransmission were observed in both the central and peripheral nervous systems of ax(J) mice. We have now identified several new alterations in peripheral neurotransmission in the ax(J) mice. Using the two-microelectrode voltage clamp technique on diaphragm muscles of ax(J) mice, we observed that under normal neurotransmitter release conditions ax(J) mice lacked paired-pulse facilitation and exhibited a frequency-dependent increase in rundown of the end plate current at high-frequency stimulation (HFS). Combined electrophysiology and styryl dye staining revealed a significant reduction in quantal content during the initial and plateau portions of the HFS train. In addition, uptake of styryl dyes (FM dye) during HFS demonstrated that the size of the readily releasable vesicle pool was significantly reduced. Destaining rates for styryl dyes suggested that ax(J) neuromuscular junctions are unable to mobilize a sufficient number of vesicles during times of intense activity. These results imply that ax(J) nerve terminals are unable to recruit a sufficient number of vesicles to keep pace with physiological rates of transmitter release. Therefore, ubiquitination of synaptic proteins appears to play an important role in the normal operation of the neurotransmitter release machinery and in regulating the size of pools of synaptic vesicles.

ATP released together with acetylcholine as the mediator of neuromuscular depression at frog motor nerve endings

1. The hypothesis that ATP released by presynaptic stimulation is hydrolysed to adenosine and mediates prejunctional neuromuscular depression was tested at vertebrate neuromuscular junctions. Electrophysiological recordings of evoked acetylcholine (ACh) release and perineural ionic currents at motor nerve endings were made using the frog cutaneous pectoris nerve-muscle preparation. Either tubocurarine or alpha-bungarotoxin was used to block muscle contractions. 2. Either alpha,beta-methylene ADP (which inhibits ecto-5'nucleotidases and thus prevents the degradation of ATP to adenosine) or selective adenosine receptor antagonists (8-cyclo-pentyl alkyl xanthines) prevented the inhibitory effects of exogenous ATP on ACh release in response to low-frequency nerve stimulation. These results confirm earlier findings that ATP must be hydrolysed to adenosine to inhibit ACh release. 3. The presence of alpha,beta-methylene ADP completely prevented neuromuscular depression in response to repetitive high-frequency nerve stimulation (0.5-1 Hz). alpha,beta-Methylene ADP had no effect on ACh secretion under conditions where ACh release is well maintained (low-frequency stimulation, 0.05 Hz). 4. Selective adenosine receptor antagonists completely eliminated neuromuscular depression produced by repetitive high-frequency nerve stimulation (1.0 Hz) but had no effect on ACh release at low frequencies of stimulation (0.05 Hz). 5. Exogenous adenosine deaminase (5 i.u. ml-1), which degrades adenosine to its inactive nucleoside inosine, also eliminated neuromuscular depression but had no significant effect on ACh release at frequencies of nerve stimulation too low to produce prejunctional depression. 6. During maximal neuromuscular depression, the effects of exogenous adenosine or 2-chloroadenosine, an adenosine agonist, were occluded. 7. The calcium-sensitive component of perineurial recordings of motor nerve terminal currents did not change during depression or during application of adenosine receptor antagonists and adenosine deaminase, suggesting that neuromuscular depression in this species was not associated with changes in presynaptic Ca2+ currents. 8. These results suggest that, under the conditions of these experiments, endogenous ATP, after hydrolysis to adenosine, causes prejunctional neuromuscular depression. This inhibitory effect of endogenous adenosine occurs at a site distal to the locus of Ca2+ entry in the frog.

Age-induced alteration of neuromuscular transmission: effect of halothane

Age-induced alteration in neuromuscular transmission and the effect of halothane were examined in 3- and 30-month-old rats with the voltage clamp technique. Significant pre- and post-junctional changes of synaptic transmission occur with advancing age as the frequency of spontaneous release, quantal content and mobilization rate increase and decay time (tau) of miniature end plate current (MEPC) and end-plate current (EPC) is prolonged and altered (bi-exponential). Effects of halothane (0.3-0.88 mM) appeared to be pre-junctional, as the anesthetic decreased the frequency of spontaneous release, end plate current amplitude, quantal content and mobilization rate, had little or no effect on decay time of miniature end plate current and end plate current, and produced no run-down of ionophoretically evoked trains of end plate current. Some of these effects of halothane are more prominent in 30-month-old rats.

Nicotinic agonists antagonize quantal size increases and evoked release at frog neuromuscular junction

1. Previous studies at the frog neuromuscular junction showed that quantal size can be increased two- to fourfold by a variety of treatments, including prior exposure to hypertonic solution (which activates protein kinase A) and insulin (which acts via an unknown pathway). Size increases largely because quanta contain more acetylcholine (ACh). 2. Now the effects of cholinergic agonists on the increases in quantal size have been studied. One muscle from a frog was kept for 2 h in hypertonic sodium gluconate solution. The miniature endplate potential (MEPP) sizes were measured in saline: they had increased about fourfold. The paired muscle went through the same experimental sequence, except that an agonist was added to the hypertonic gluconate solution. Again MEPP sizes were measured in saline. The increase in quantal size was significantly depressed by 0.2 microM 1,1-dimethyl-4-phenyl- piperazinium (DMPP). The sequence of effectiveness of agonists was: DMPP > carbachol > or = ACh = cytisine > oxytremorine. This sequence suggests that the receptor belongs in the nicotinic class. 3. Quantal size is doubled after 1 h in insulin. One micromolar carbachol largely blocked the size increase. 4. The effects of cholinergic antagonists were tested by keeping the experimental muscle in hypertonic gluconate solution containing 1 microM carbachol plus an antagonist. The controls were paired muscles kept in hypertonic gluconate solution (without carbachol or antagonist). MEPP sizes were measured in saline. The depressing action of carbachol on the increase in MEPP size was blocked by 0.2 microM neuronal-bungarotoxin (nBTX). The sequence of effectiveness of antagonists was: nBTX > trimethaphan > d-tubocurarine (dTC). Ten micromolar atropine (without carbachol) depressed the increase in quantal size. Therefore, the antagonist potency of atropine could not be adequately tested. Carbachol action was not blocked by 10 microM hexamethonium or 10 microM mecamylamine. 5. Once quanta are made large they can be converted back to normal size by cholinergic agonists. Muscles in which quantal size had been enlarged were exposed to hypertonic solutions containing the agonist. Quantal size was reduced to a fraction of its former value when the hypertonic solution contained 1 microM carbachol- or 1 microM DMPP. One micromolar oxytremorine had no effect. Carbachol still reduced quantal size when applied in low-Ca2+ solutions, so it does not appear to act by elevating intracellular [Ca2+]. 6. Previous work suggested that the treatments produce a subpopulation of large quanta that are positioned for release.(ABSTRACT TRUNCATED AT 400 WORDS)

Run-down of neuromuscular transmission during repetitive nerve activity by nicotinic antagonists is not due to desensitization of the postsynaptic receptor

1. Whether the function of the postsynaptic acetylcholine receptor is use-dependently affected by repetitive nerve stimulation in the presence of competitive antagonists was studied in the mouse phrenic nerve-hemidiaphragm preparation. 2. For electrophysiological experiments, the preparation was immobilized by synthetic mu-conotoxin, which preferentially blocks muscular Na-channels causing neither depolarization of the membrane potential, inhibition of quantal transmitter release, nor depression of nicotinic receptor function. 3. High concentrations of cobratoxin depressed indirect twitches and endplate potentials (e.p.ps) without inducing waning of contractilities or run-down of trains of e.p.ps evoked at 10-100 Hz. However, waning and run-down were accelerated after washout of the toxin despite diminished postsynaptic receptor blockade. Once the run-down of e.p.ps was produced by washout or low concentrations of cobratoxin, further depression of e.p.p. amplitude with high concentrations of cobratoxin did not attenuate the e.p.p. run-down. 4. The degrees of waning of tetanus and trains of e.p.ps produced by a very high concentration of tubocurarine (20 microM) were also less than that caused at a 100 fold lower concentration, albeit the amplitudes of twitches and the first e.p.p. were depressed more rapidly and markedly. 5. Tubocurarine, like cobratoxin, depressed the amplitude of miniature endplate potentials (m.e.p.ps) more than e.p.ps. 6. In contrast to the steepened run-down of successive e.p.ps in the presence of low concentrations of either nicotinic antagonists, the amplitude of m.e.p.ps observed during repetitive stimulation was uniform and was not different from that before stimulation. 7. The results suggest that the e.p.p. run-down and tetanic fade induced by nicotinic antagonists are due to a slow kinetic blockade of presynaptic receptors and confirm that the e.p.p. run-down is not produced by a use-dependent failure of postsynaptic nicotinic receptors. The roles of the presynaptic nicotinic receptor in positive or negative feedback modulations of transmitter release are discussed.

Influence of extracellular calcium in the effects of dibekacin and diltiazem on indirectly elicited tetanic responses in the rat phrenic-hemidiaphragm preparation

1. Dibekacin (70 microM-3 mM) produced a decrease of peak tetanic tension in a concentration-dependent manner and this effect was dependent on extracellular calcium (0.3-2.5 mM Ca2+). Only minimal fade was observed and it was not related with extracellular calcium concentrations. 2. Diltiazem (30-300 microM) decreased peak tetanic tension and produced tetanic fade. Both effects were independent of extracellular calcium, although a significant potentiation was observed at 0.3 mM calcium. 3. It is concluded that tetanic parameters are related differently to extracellular calcium.