The action of cobalt ions on neuromuscular transmission in the frog

1. Cobalt ions, in concentrations of 0.05-2 mM, block neuromuscular transmission in the frog sartorius muscle.2. The reduction in the e.p.p. amplitude produced by Co(2+) is due to a decrease in the amount of transmitter released by a nerve impulse (mean quantum content). This reduction is associated with little change in the resting membrane potential of the muscle fibre or in the mean amplitude of spontaneous m.e.p.p.s.3. The reduction in evoked transmitter release produced by Co(2+) may be antagonized by elevation of the external Ca(2+) concentration. It is suggested that the antagonism between Co(2+) and Ca(2+) is competitive in nature.4. The mean dissociation constant for Co(2+) and its hypothetical membrane complex was found to be 0.18 mM. On this basis, it is concluded that Co(2+) is about 20 times more potent than Mg(2+) in suppressing evoked transmitter release.5. In contrast to the inhibitory action on evoked release, Co(2+) increases spontaneous transmitter release. However, concentrations of Co(2+) 20-60 times greater than those which decrease the e.p.p. amplitude are needed to produce a significant increase in m.e.p.p. frequency.     

Muscle-specific decrease in presynaptic calcium dependence and clearance during neuromuscular transmission in aged rats

1. Calcium regulation in the vicinity of synaptic release sites was measured indirectly at the neuromuscular junction of diaphragm, soleus, and extensor digitorum longus (EDL) muscles of rats 10 (mature adult) and 25-27 mo of age. 2. The rate of miniature end-plate potentials (MEPPs) per nerve terminal increased by 79% with age in EDL but did not change significantly in diaphragm or soleus; since MEPP rate depends, in part, on resting steady-state intracellular Ca2+ levels, ([Ca2+]i), it was inferred that these levels may be elevated by at least 16% in the aged EDL tissue. 3. The double-logarithmic relationship between extracellular Ca2+ ([Ca2+]e), and quantal release (m) was determined for [Ca2+]e between 0.5 and 1.2 mM. The slope (n) of this relationship was 4.1 and 3.2 in EDL muscles from the younger and the older animals, respectively; this difference was significant statistically. 4. A static model of the saturable cooperative relationship between [Ca2+]e and m was used to evaluate possible causes of the age-related change in this relationship. Changes resembling those seen in EDL during aging could be produced by relatively slight variations in the amount of Ca2+ entering the cell, intracellular buffering capacity, and several other related aspects of Ca2+ availability. The observed changes could not, however, be attributed quantitatively to increased steady-state [Ca2+]i. 5. The decay rates of synaptic facilitation and of posttetanic augmentation were prolonged by 164 and 227%, respectively, in EDL during aging. Since both of these phenomena have been attributed to residual Ca2+, it was inferred that rates of Ca2+ clearance from synaptic release sites were correspondingly slower in aged EDL. 6. Each of these age-associated changes in Ca2+ regulation was observed only in EDL and not in diaphragm or soleus. This specificity may be related to progressive disuse of EDL (a fast-twitch leg muscle), and consequently decreased expression of Ca2+-regulatory enzymes, during aging.     

Effect of calcium on excitatory neuromuscular transmission in the crayfish

1. The effects of varying the external Ca concentration from 1.8 to 30 mM/l. ((1/8)-2 times normal) have been studied at the in vitro crayfish excitatory neuromuscular junction. Electrophysiological techniques were used to record transmembrane junctional potentials from muscle fibres and extracellular junctional currents from the vicinity of nerve terminals.2. The excitatory junctional potential amplitude was proportional to [Ca](0) (n), where n varied between 0.68 and 0.94 (mean 0.82) when [Ca](0) was varied from 1.8 to 15 mM/l.3. The increase in junctional potential amplitude on raising [Ca](0) resulted primarily from an increase in the average number of quanta of excitatory transmitter released from the presynaptic nerve terminal by the nerve impulse.4. The size of the quanta, synaptic delay, presynaptic potential and electrical properties of the muscle membrane were little affected by changes in calcium concentration in the range studied.     

Kinetic analysis of the action of chemical modulators on neuromuscular transmission

The kinetics of the direct action of 4-aminopyridine (4AP) and streptomycin (SM) on the mechanism of transmitter release were studied by recording the endplate potentials from curarized frog muscles using conventional microelectrode techniques, and by calculating the fractional release from the store of available acetylcholine quanta from the experiments with tetanic rundown during short train stimulations. To explain the tremendous facilitatory effect of 4AP on the fractional release and the antagonistic interaction of SM thereupon, it was postulated that 4AP and SM modify the transmitter output by combining with the Ca-dependent process X; i.e., 4AP and SM compete for the occupancy of the X site. A combination of 4AP or SM presumably modifies allosterically the action of the Ca-X complex, resulting in a profound augmentation of the evoked release of the available transmitter with the former and its depression with the latter. The theoretically derived equations from the above assumptions agreed reasonably well with the results obtained.     

The margin of safety of neuromuscular transmission

1. The margin of safety for neuromuscular transmission in the tibialis and sartorius muscles of the cat has been determined by measuring the ratio by which end-plate depolarization produced by succinylcholine, decamethonium, octamethonium or iodocholine is antagonized, in the presence of neuromuscular block produced by tubocurarine, gallamine or DF-596. The estimate of the margin of safety was independent of the particular drugs chosen for the measurement.2. To produce threshold block to indirect stimulation once every 10 sec, a fractional occupancy by the antagonist of 0.76 +/- 0.05 (S.D.) was required; for nearly complete block, an occupancy of 0.917 +/- 0.16 (S.D.) was required. These figures correspond to factors of safety of 4.1 and 12 for the most sensitive and the most resistant groups of fibres respectively.3. The interaction between the agonists and the antagonists, when tested over a wide range of dosage, did not conform with the conditions of full competitive equilibrium. It was concluded that this arose, not because of some interfering non-competitive process, but because, during the relatively brief exposure to agonist, the equilibrium between the antagonist and the receptors is not significantly disturbed. An analysis of this condition of quasi-equilibrium is given. A correction downwards of the direct estimates of the margin of safety is required, but this proves to be small, about 8%, and may not be significant.4. The safety factor diminished when the motor nerve had been cut more than 5 hr; it is suggested that this represents an early sign of nerve degeneration.5. With dog sartorius muscle, results similar to those in the cat were obtained. But for deep block in the rabbit, the safety factor was only about 4.6. The existence of a substantial margin of safety influences considerably the interpretation of the time course of action of blocking drugs, and of comparisons between responses to nervous excitation and drug injection.     

The effect of procaine on neuromuscular transmission

1. The mechanism of procaine action on post-synaptic receptors for acetylcholine was studied by recording the end-plate current at membrane potentials ranging from about +30 to about -140 mV.

2. It has been found that at resting membrane potential of about -60 to -80 mV the end-plate current has a fast initial and a slow late component. During hyperpolarization of the muscle fibre the amplitude of the slow component is depressed and its half-time lengthened. When the membrane potential is inverted the difference in the time course of both components is much less pronounced or absent.

3. It is suggested that procaine modifies the receptor response induced by acetylcholine, and that this modification is dependent on membrane potential.

     

The effect of colchicine on neuromuscular transmission in the frog during repetitive stimulation

When colchicine 10^–4 mol·l^–1 was applied at the beginning of repetitive stimulation of the nerve at 10·s^–1, the facilitation was markedly inhibited. The quantal content showed a very slight increase and its maximum value was reached later. The maximum frequency of spontaneous release was also reached later in the presence of colchicine than in the absence of the drug. In addition, the synaptic delay was much more pronounced in the presence of colchicine than in the control experiment.The results suggest that a partial block by colchicine of the release process in the nerve terminal occurs. This effect may be due to the action of the drug on the nerve terminal membrane. The results cannot exclude the possibility that colchicine interferes with the transport of vesicles towards the sites of release located on the membrane.     

Maturation of neuromuscular transmission during early development in zebrafish.

We have examined the rapid development of synaptic transmission at the neuromuscular junction (NMJ) in zebrafish embryos and larvae by patch-clamp recording of spontaneous miniature endplate currents (mEPCs) and single acetylcholine receptor (AChR) channels. Embryonic (24-36 h) mEPCs recorded in vivo were small in amplitude (<50 pA). The rate of mEPCs increased in larvae (3.5-fold increase measured by 6 days), and these mEPCs were mostly of larger amplitude (10-fold on average) with (相似文献   

The timing and frequency of hybrid formation in African trypanosomes during cyclical transmission

The frequency of hybrid formation between twoTrypanosoma brucei clones during cyclical transmission throughGlossina morsitans centralis was analyzed. In two independent experiments, teneralG. m. centralis were infected with an equal mixture of twoT. brucei clones showing different homozygous isoenzyme patterns for isocitrate dehydrogenase (ICD; E.C. 1.1.1.42) and alkaline phosphatase (AP; E.C. 3.1.3.1). Trypanosomes were cyclically transmitted to mice from 23 infective flies and the subsequent blodstream-form populations were characterized by isoenzyme electrophoresis. Heterozygous patterns for ICD and AP indicated that hybrid formation occurred in at least 9 of the 23 vectors. There was further evidence that extrusion of hybrid parasites with saliva from a single fly was not necessarily continuous but could alter over time with the occurrence of either of both of the homozygous parental clones.     

The role of calcium in neuromuscular facilitation

1. The hypothesis is put forward that a residue of the ;active calcium' which enters the terminal axon membrane during the nerve impulse is responsible for short-term facilitation.2. This suggestion has been tested on the myoneural junction by varying the local calcium concentration so that during the first of two nerve impulses [Ca](o) is either much lower than, or raised to a level approaching that, during the second impulse. Facilitation is much larger in the latter case, which is in accordance with the ;calcium hypothesis'.3. A short pulse of depolarization focally applied to the junction is followed by a brief period of very intense facilitation. This can be seen in the tetrodotoxin-treated preparation, e.g. by lengthening the depolarization from 1 to 2 msec which can cause a more than fifty-fold increase in transmitter release. This large ;early facilitation' (which presumably occurs also during the course of a normal action potential) is discussed in relation to the ;calcium hypothesis'.     

Co-operative action a calcium ions in transmitter release at the neuromuscular junction

1. The quantitative dependence of transmitter release on external calcium concentration has been studied at the frog neuromuscular junction, using intracellular recording and taking the amplitude of the end-plate potential (e.p.p.) as an index of the number of packets released.2. The relation between [Ca] and the e.p.p. is highly non-linear. The initial part of this relation on double logarithmic co-ordinates gives a straight line with a slope of nearly four (mean 3.78 +/- 0.2 S.D. in 28 experiments). Addition of a constant amount of Mg reduces the e.p.p. without altering the slope of the log e.p.p./log Ca relation.3. The slope of this logarithmic relation diminishes as [Ca] is raised towards the normal level.4. The results are explained quantitatively on the hypothesis that Ca ions combine with a specific site X on the nerve terminal forming CaX, and that the number of packets of acetylcholine released is proportional to the fourth power of [CaX].5. The analysis suggests that a co-operative action of about four calcium ions is necessary for the release of each quantal packet of transmitter by the nerve impulse.     

Coupling between perception and action timing during sensorimotor synchronization

Time is an important parameter in behaviour, especially when synchronization with external events is required. To evaluate the nature of the association between perception and action timing, this study introduced pitch accented tones during performance of a sensorimotor tapping task. Furthermore, regularity of the pacing cues was modified by small (subliminal) or large (conscious) timing perturbations. A global analysis across the intervals showed that repeated accented tones increased the tap-tone asynchrony in the regular (control) and irregular (subliminal) trials but not in the irregular trials with awareness of the perturbations. Asynchrony variability demonstrated no effect of accentuation in the regular and subliminal irregular trials, whereas it increased in the conscious irregular trials. A local analysis of the intervals showed that pitch accentuation lengthened the duration of the tapping responses, but only in the irregular trials with large timing perturbations. These data underline that common timing processes are automatically engaged for perception and action, although this arrangement can be overturned by cognitive intervention. Overall, the findings highlight a flexible association between perception and action timing within a functional information processing framework.     

Triggering of BDNF facilitatory action on neuromuscular transmission by adenosine A2A receptors

Motor nerve terminals possess adenosine A(2A) receptors and brain derived neurotrophic factor (BDNF) TrkB receptors. In the present work we evaluated how BDNF actions on neuromuscular transmission would be influenced by adenosine A(2A) receptors activation. BDNF (20-100 ng/ml) on its own was devoid of effect on evoked endplate potentials (EPPs) recorded intracellularly from rat innervated diaphragms paralysed with tubocurarine. However, when BDNF was applied 45 min after a brief (2 min) depolarizing KCl (10 mM) pulse or when the adenosine A(2A) receptors were activated with CGS 21680 (10 nM), BDNF (20 ng/ml) increased EPPs amplitude without influencing the resting membrane potential of the muscle fibre. The action of BDNF was prevented by the adenosine A(2A) receptor antagonist, ZM 241385 (50 nM) as well as by the TrkB receptor phosphorylation inhibitor, K252a (200 nM). The PKA inhibitor, H-89 (1 microM), prevented the excitatory effect of CGS 21680 (10 nM) on EPPs as well as prevented its ability to trigger a BDNF effect. The PLCgamma inhibitor, U73122 (5 microM), did not prevent the excitatory action of CGS 21680 (10 nM) on neuromuscular transmission, but abolished the action of BDNF in the presence of the A(2A) receptor agonist. The results suggest the following sequence of events in what concerns cooperativity between A(2A) receptors and TrkB receptors at the neuromuscular junction: A(2A) receptor activates the PKA pathway, which promotes the action of BDNF through TrkB receptors coupled to PLCgamma, leading to enhancement of neuromuscular transmission.