在不均匀夸张的大鼠隔肌标本上串终板电位初始段衰减的突触前性质(英文)

在大白鼠离体不均匀牵张膈肌标本(INSMP)上可选择接近神经肌接头正常生理状态的终板,同时记录同一终板的小终板电位(MEPP),终板电位(EPP)和微电泳外源性乙酰胆碱(ACh)产生的乙酰胆碱电位(AChP);以AChP为直接的突触后膜兴奋性大小指标,以量子释放参数:量子含量(M)及二项统计参数为突触前递质释放量大小的指标,研究了串终板电位初始段衰减现象的性质.发现在2,10,50和100 Hz间接刺激下,INSMP上串EPPs都发生初始衰减现象;刺激频率增高,衰减更为明显.50 Hz时,在第1,3,5,7,11,25,50,75和100个EPP后20 ms的AChP幅度与EPP之前的AChP幅度无明显差别.100 Hz时,在产生第25,50,75和100个EPP后10 ms的AChP与EPP之前的AChP幅度也无明显差别.直接证明了在产生串EPPs衰减时无突触后膜兴奋性的改变.刺激频率增高,第1个EPP的量子含量(M_1)无变化,EPPs平台期的量子含量(M_x)减小,即刻可释放的量子贮存(n)无变化,平均量子释放机率(P)减少.表明串EPPs初始段的衰减是正常神经肌接头传递的一种自身调节,它完全由突触前量子释放减少引起的,其大小与刺激频率有关.突触前ACh受体可能参与了这种自身调节.     

箭毒对神经肌肉接头的突触前和突触后作用

<正> 已知箭毒(d—tubocurarine,d—TC)既有典型的突触后抑制作用,又有明显的影响突触前递质释放的作用。但在同一标本上同时定量比较其突触前和突触后的作用则尚未见报道,对d—TC突触前作用的性质也无统一看法。我们在大白鼠离体不均匀牵张膈肌标本上,用细胞内微电极同时记录运动终板区的微终板电位(MEPP)、终板电位(EPP) 和微电泳外源性乙酰胆碱(ACh)诱发的乙酰胆碱电位(AChP),并以AChP幅     

新霉素及链霉素对神经肌肉接头的作用及其对抗梭曼的作用(英文)

在不均匀牵拉的离体大鼠膈肌标本上观察了氨基甙类抗菌素新霉素和链霉对终板电位(EPP)和微终板电位(MEPP)的影响和对不可逆胆碱酯酶抑制剂索曼的对抗作用。新霉素减低EPP量子含量,抑制MEPP幅度和减慢MEPP放电频率的浓度相应为0.03,0.1和0.32 mmol/L。链霉素抑制EPP和MEPP相应比新霉素强50和30倍。不影响量子含量的阈下剂量的新霉素(0.015mmol/L)和链霉素(0.8 mmol/L)能部分预防梭曼减少量子含量的作用。再增加此两药的浓度不能提高它们对抗梭曼的效用.同样,不影响MEPP频率的阈下剂量的新霉素和链霉素能完全预防梭曼加快MEPP频率的作用。认为新霉素和链霉素对神经肌接头抑制作用的主要作用部位是突触前递质诱发释放系统。其机理可能是它们竞争性地减少梭曼引起的向突触前膜内Ca~(2+)内流的增加。它们预防梭曼增加MEPP频率的机理也可能是如此。     

在不均匀夸张的大鼠隔肌标本上串终板电位衰减的突触前性质

<正> 一般认为在神经肌肉标本上,高频间接强直刺激诱发的串终板电位(train of endplate potentials,EPPs)的衰减是一种突触前现象。药物对衰减率R(第20个以后的平均EPP幅度与第1个EPP幅度的比值)的影响可作为判定药物对突触前作用的指标。但尚无直接实验证据排除突触后因素的参与。我们在不均匀牵张的膈肌标本(isolated nonuniform stretched muscle preparation,INSMP)上,排除任何药物和异常离子的影响,膜电位水平为-60±2mV,温度控制在31±1℃,接近正常生理状态情况下,用玻璃微电极在同一个运动终板上记录了串EPPs及微终板电位(MEPP),同时还记录了只反映突触后膜兴奋性大小的外源性乙酰胆碱诱发的乙酰胆碱     

在不均匀牵拉的离体大鼠膈肌标本上梭曼的突触前和突触后作用

在离体大鼠膈肌上制备了一种不均匀牵拉肌肉以制动,便于用微电极记录终板电位(EPP)的标本(INSMP),避免了常规制动方法带来的制动药物、台氏液Ca~(2+),Mg~(2+)浓度改变或钳压肌肉导致膜电位(MP)下降的种种干扰。INSMP的MP和小终板电位(MEPP)正常,EPP幅度高达30mV以上,是研究药物对接头作用的较好标本。在INSMP上,梭曼(5.5μM)使MEPP频率加快,串刺激(50Hz)诱发的平均串EPPs幅度及其平均ACh 量子含量减少80％和77％。小剂量三碘季铵酚和六烃季铵可部分对抗梭曼的作用。梭曼引起强直收缩抑制主要原因为终板区蓄积的ACh作用于突触前N受体、负反馈地抑制ACh 量子释放;次要原因为作用突触后N受体、使其对ACh敏感性降低。     

梭曼引起神经肌接头传递阻断的电生理学机理新探

<正> 梭曼(soman)不可逆地抑制乙酰胆碱酯酶(ACHE),造成突触间隙乙酰胆碱(ACh)大量蓄积,引起神经肌接头传递阻断,但其中毒的单突触电生理学机理尚不十分清楚。本文在接近正常生理水平的不均匀牵张膈肌标本(INSMP)上,以细胞内微电极记录技术,首次观察了梭曼中毒后以终板电位(EPP)消失为特征的突触后膜再生性去极化(regenerative depolarization,RD)现象。 结果表明,在正常情况下,高至1000 Hz的间接串刺激(持续1 s)也不能使串EPPs融合而消失。浴槽给5.5μmol·L~(-1)梭曼10 min后,微终板电位(MEPP)的频率增高,幅度增大,时程加宽;单个EPP幅度增     

梭曼引起大鼠神经肌接头突触后膜持续性去极化的机制

目的 研究梭曼引起神经肌接头突触后膜持续性去极化（SD）的产生机制。方法 不均匀牵张肌肉标本上,采用传统的微电极细胞内记录技术。结果 随着梭曼浓度（3.0-0.05)μmon/L的降低,在其所引起的SD过程中,小终板电位(mEPP)的出现逐渐提前,并且频率也逐渐增高。当梭曼浓度降到0.05μmol/L时,在SD的平台期、复极期及复极后的一段时间内出现频率显著增高的mEPP,反复给予膈神经短暂的串刺激或给予一个长时程的串刺激,当刺激落到SD的复极期,均可产生小幅度的终板电位（EPP）。结论 梭曼引起的SD过程中,突触后膜烟碱受体没有完全失敏,SD的产生可能主要与突触前递质翻译因素有关。     

三七总皂甙对小白鼠运动神经末梢递质释放的影响

胞内记录神经—终板电位及微终板电位,证明三七总皂甙不影响神经末梢去极化时Ca~(2+)及Ba~(2+)的内流,对同步、非同步及自发性的量子释放均无减弱。由于Cd~(2+)能明显减弱量子释放,而PNS与nifedipine却无作用,提示胆碱能运动神经末梢上参与递质释放的Ca~(2+)通道属于N型Ca~(2+)通道。     

石杉碱甲对离体小鼠神经肌肉接头递质传递的易化作用(英文)

目的:研究石杉碱甲在小鼠神经肌肉接头处对递质传递的作用。方法:用胞内记录的方法在小鼠的膈神经肌肉标本上研究了石杉碱甲(Hup-A),他克林(tacrine)和E2020对自发释放的小终板电位的作用。结果:Hup-A,tacrine和E2020均可增强小终板电位的振幅、上升相和半下降相,有剂量依赖关系,其抑制强度为E2020>Hup-A>tacrine。结论:Hup-A在胆碱能突触处对乙酰胆碱酯酶的抑制作用强于tacrine。     

梭曼对神经肌肉接头突触前乙酰胆碱释放的影响

<正> 本文研究了至今尚不完全清楚的胆碱酯酶抑制剂梭曼引起肌麻痹的机理。建立了大鼠高体不均匀牵拉的膈肌标本。排除了文献上用箭毒或高镁离子制动的干扰和切压肌肉制动引起的创伤。该标本膜电位正常、终板电位(EPP)高达30-36mV,还可同时记录到频率和幅度正常的小终板电位(MEPP)。此标本用来研究药物对接头的作用优于其他标本。 当给予5.5μM梭曼后,串刺激(50Hz,1.0s)膈神经时,第一个EPP幅度加大,半降期延长。第10个及     

FURTHER COMPARISON OF THE EFFECTS OF PHYSOSTIGMINE AND NEOSTIGMINE ON FROG NEUROMUSCULAR TRANSMISSION

1. The effects of physostigmine and neostigmine were compared at frog neuromuscular junctions using intracellular microelectrodes. 2. Both drugs increased the amplitudes of miniature endplate potentials (MEPP) in a dose-dependent manner because of their ability to competitively inhibit cholinesterase activity. 3. In addition, physostigmine (1.0-20.0 mumol/l) decreased by approximately 20% the quantal content (determined by two methods), indicating a decrease in the amount of transmitter released upon nerve stimulation, whereas neostigmine did not alter quantal content. 4. Because the endplate potential (EPP) amplitude is a composite of pre- and post-junctional effects, the dose-response curve for neostigmine on EPP amplitude approximately paralleled that obtained on MEPP amplitude, whereas the effect of physostigmine on EPP amplitude was depressed because of its action to decrease transmitter release. 5. Upon washout of neostigmine, the increase in EPP amplitude reversed more rapidly than effects on MEPP amplitude, a difference dependent on the magnitude of quantal content and explained by an action on the postjunctional membrane. Similar results were obtained with physostigmine. 6. Neither drug significantly affected MEPP frequency although small decreases were sometimes observed. 7. These results suggest that at the frog neuromuscular junction the effects of neostigmine are explained primarily by its inhibition of cholinesterase, whereas physostigmine are explained primarily by its inhibition of cholinesterase, whereas physostigmine has an additional action on nerve terminals of decreasing nerve-stimulated release of acetylcholine.     

Miniature endplate potentials as a tool in neurotoxicology

The toxic effect of thallium added to the bath solution was studied with intra- and extracellular recordings from mammalian nerve-muscle preparations. To elucidate the target region, 3 different functional parameters were studied: (1) Post-synaptic endplate potentials (EPPs) resulting from evoked transmitter release; (2) Post-synaptic miniature endplate potentials (MEPPSs) resulting from spontaneous transmitter release; and (3) Presynaptic ion currents at the nerve terminal. At a concentration of 0.5 mM/l thallium acetate, EPP amplitudes were irreversibly decreased while MEPP amplitudes remained unaffected. MEPP frequencies were reversibly increased, indicating a presynaptic rather than a post-synaptic target site of thallium toxicity. The subpopulation of small MEPPs (sub-MEPPs) behaved like the MEPP population, except that upon addition of 4-AP, the sub-MEPP population was augmented at the cost of the MEPP population. In view of the slow time course of the toxic effects (30 min for a 10-fold increase of MEPP frequency, 100-180 min for a 50% reduction of EPP amplitudes), it is concluded that thallium needs to be transported across the cell membrane before it finally interferes with release mechanisms. It is hypothesised that thallium reduces the number of active sites recruited by one action potential (reduced EPP amplitude), while at the same time the probability of transmitter liberation is enhanced (increased MEPP frequency). The rather indirect mode of action of thallium was also found when presynaptic ion currents were recorded using extracellular electrodes. In proportion to the decrease of the EPP amplitudes, a reduction of all inward and outward currents was observed. This effect was also irreversible. It is concluded that in spite of some similarities, thallium behaves quite differently from bivalent heavy metals like cobalt and cadmium, which act as competitive calcium antagonists at the presynaptic nerve terminal. In these toxic substances, the time course of intoxication is much faster, the required concentration is much lower, and the inhibition of the slow calcium current is reversible.     

Effects of emetine and dehydroemetine at the frog neuromuscular junction

Emetine and dehydroemetine caused concentration-dependent reduction of the quantal content of the endplate potential (EPP) at the frog neuromuscular junction. At lower concentrations, the drugs had presynaptic action only, as they decreased the amplitude of the EPP without significantly affecting that of the spontaneous miniature EPP (MEPP). At higher concentrations, the drugs had postsynaptic effects as well. Studies of low frequency facilitation indicated that although at low concentrations the effect of emetine on quantal content was independent of frequency of stimulation, at higher concentrations the effect became markedly frequency-dependent and many of the stimuli (at 4 and 8 Hz) failed to evoke EPPs. In the voltage-clamped transected cutaneous pectoris muscle emetine (10(-4) M) depressed the peak amplitude of the endplate current (EPC). Emetine markedly shortened the time constant of decay of the EPC (tau) at more negative holding potentials but did not change the single exponential character of the decay. The results indicate that the acute effects of emetine involve blocking neuromuscular transmission by acting prejunctionally at low concentrations and pre- as well as postjunctionally at higher concentrations. The prejunctional effect is due to interference with evoked release and probably, mobilization of acetylcholine. The postjunctional effect is due to blockade of the acetylcholine-activated ionic channel.     

The action of thallium acetate on phasic transmitter release in the mouse neuromuscular junction

Endplate potentials (EPP's) and miniature endplate potentials (MEPP's) were recorded from neuromuscular junctions of the mouse phrenic nerve-diaphragm preparation, blocked by high Mg⁺⁺ (12×10^–3 mol/l)-Ringer. Superfusion of the preparations with Mg⁺⁺-Ringer solutions containing thallium acetate (5×10^–4 mol/l Tlac) decreased phasic transmitter release as judged by EPP amplitudes as well as average quantal content, until total synaptic blockade (within about 300 min) occurred. Simultaneously MEPP amplitudes remained unchanged, whereas the frequency of MEPP's increased. When EPP amplitudes and/or quantal content were reduced by 50% (usually within about 180 min), superfusion with Mg⁺⁺-Ringer solution without Tlac did not restore phasic transmitter release. However, the increase in spontaneous transmitter release was reversible, as MEPP frequencies returned to normal values. 4-Aminopyridine (5×10^–4 mol/l 4-AP) as added to the bath solution in the state of 50%-reduced phasic release temporarily restored EPP amplitudes and average quantal content, whereas MEPP amplitudes remained unchanged.It is concluded that thallium irreversibly blocks phasic transmitter release, whereas spontaneous transmitter release is reversibly enhanced.     

Effects of synthetic omega-conotoxin, a new type Ca2+ antagonist, on frog and mouse neuromuscular transmission

A new type Ca2+ antagonist, synthetic omega-conotoxin (omega-CgTX) decreased the peak height of the endplate potential (EPP) in frog muscle but had no effect on the mouse neuromuscular junction. The reduction of endplate potential in frogs was due to a decrease in transmitter release, since the mean quantal content estimated by variance of EPPs (m) and from the peak heights of EPPs and miniature EPPs (m1) was reduced by omega-CgTX, but the postsynaptic sensitivity to ACh was unaltered. The decrease of mean quantal content caused by omega-CgTX was reversed by 4-aminopyridine, guanidine and Bay K 8644. Also, the effect of omega-CgTX was weakened in the presence of 10 mM Ca2+ or 12 mM Mg2+. Statistical analysis revealed that omega-CgTX decreased the number of quanta available (n) whereas the probability of release (p) remained unaffected.     

Interactions of lead and cadmium on acetylcholine release at the frog neuromuscular junction

The interactive effects of Cd2+ and Pb2+ on evoked and spontaneous transmitter release were studied in the sciatic nerve-sartorius muscle preparation of the frog (Rana pipiens). Either Pb2+ or Cd2+ competitively inhibited the actions of Ca2+ in bringing about evoked release, as measured by the endplate potential (EPP) amplitude. Combinations of Pb2+ and Cd2+ were additive in their effects on the EPP. The rate of spontaneous transmitter release was measured as the miniature endplate potential (MEPP) frequency. In contrast to their effects on the EPP, exposure of preparations to combinations of Pb2+ and Cd2+ actually increased the MEPP frequency less than exposures of the preparations to Pb2+ alone. The degree of reduction in MEPP frequency produced by Cd2+ depended upon the relative ratio of Pb2+ and Cd2+ ions. These results suggest that Pb2+ and Cd2+ ions competed for a common presynaptic receptor site during evoked release. They also suggest that Pb2+ ions may enter the nerve terminal, possibly through the Ca2+ channel, and that this entry is inhibited by Cd2+.     

Ephedrine: a postsynaptic depressant drug at the mammalian neuromuscular junction

The actions of ephedrine were studied using intracellular recording techniques in guinea pig intercostal muscle fiber. After application of ephedrine (10^?5 ?10^?4 M), miniature end-plate potential (MEPP) amplitude was decreased in a concentration-dependent manner. A greater depression in amplitude of end-plate potentials (EPP) was observed in preparations immobilized with d-tuboeurarine. In addition, ephedrine depressed the transient depolarization produced by iontophoretically applied acetylcholine (ACh).In normal Krebs solution and high Mg²⁺ solution, 10^?4 M ephedrine increased MEPP frequency but did not alter the quantal content of EPPs in high Mg²⁺ blocked muscle. In these preparations, EPP amplitude did not change significantly. These results suggested that in the absence of a postsynaptic receptor blocking agent, a slight presynaptic action of ephedrine may offset the decreased responsiveness of the postsynaptic membrane. In addition, no significant alterations of resting membrane potential or the electrical characteristics of the membrane were recorded.Pretreatment of a mouse phrenic nerve-diaphragm preparation with ephedrine did not protect against the irreversible blockade by α-bungarotoxin. These results suggested that ephedrine may depress ACh potentials, MEPPs, and EPPs in dTc-blocked muscle by acting postsynaptically at a site on or associated with the ACh receptor other than the ACh recognition site.