A monolayer preparation of innervated skeletal muscle fibres of the m. cutaneus pectoris of the frog

Introducing a mammalian nerve-muscle preparation ideal for physiology and microscopy, the transverse auricular muscle in the ear of the mouse

A new mammalian neuromuscular preparation is introduced for physiology and microscopy of all sorts: the intrinsic muscle of the mouse ear. The great utility of this preparation is demonstrated by illustrating how it has permitted us to develop a wholly new technique for staining muscle T-tubules, the critical conductive-elements in muscle. This involves sequential immersion in dilute solutions of osmium and ferrocyanide, then tannic acid, and then uranyl acetate, all of which totally blackens the T-tubules but leaves the muscle pale, thereby revealing that the T-tubules in mouse ear-muscles become severely distorted in several pathological conditions. These include certain mouse-models of muscular dystrophy (specifically, dysferlin-mutations), certain mutations of muscle cytoskeletal proteins (specifically, beta-tubulin mutations), and also in denervation-fibrillation, as observed in mouse ears maintained with in vitro tissue-culture conditions. These observations permit us to generate the hypothesis that T-tubules are the "Achilles' heel" in several adult-onset muscular dystrophies, due to their unique susceptibility to damage via muscle lattice-dislocations. These new observations strongly encourage further in-depth studies of ear-muscle architecture, in the many available mouse-models of various devastating human muscle-diseases. Finally, we demonstrate that the delicate and defined physical characteristics of this 'new' mammalian muscle are ideal for ultrastructural study, and thereby facilitate the imaging of synaptic vesicle membrane recycling in mammalian neuromuscular junctions, a topic that is critical to myasthenia gravis and related diseases, but which has, until now, completely eluded electron microscopic analysis. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

alpha-Latrotoxin releases calcium in frog motor nerve terminals

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

Non-myelin-forming perisynaptic schwann cells express protein zero and myelin-associated glycoprotein

Perisynaptic Schwann cells (PSCs) envelop axonal terminals and are physiologically distinct from the nearby myelinating Schwann cells (MSCs), which surround the same innervating motor axons. PSCs have special functions at the neuromuscular synapse, where they detect and can modulate neurotransmitter release. Although PSCs are similar to non-myelinating Schwann cells in that they do not form multiple myelin wrappings around nerve terminals, they do wrap around single nerve terminals. These differences, as well as others, lead us to question whether PSCs are truly of Schwann cell origin. We thus characterized the expression of molecules, classically associated with myelin and Schwann cells, in PSCs at the frog neuromuscular junction. We wondered whether PSCs express the Schwann cell marker protein zero (P(0)) and whether their lack of myelination was related to an absence of myelin-associated glycoprotein (MAG), a protein found in myelinating cells that is considered important in myelination. Instead, we found that PSCs express both P(0) and MAG, and other myelinating glial markers such as galactocerebroside and 2',3'-cyclic nucleotide 3'-phosphodiesterase. In denervated preparations, P(0) and MAG expression persisted, including at newly formed PSC extensions. Because PSCs do not myelinate, it is clear that expression of these proteins alone is not sufficient for myelin formation. It is possible that factors present at synapses may prevent myelination, while P(0) and MAG may mediate adhesion between nerve terminals and the surrounding PSCs. The results indicate that PSCs are of Schwann cell origin.

Muscarinic control of cytoskeleton in perisynaptic glia

Similar to astrocytes at CNS synapses, perisynaptic Schwann cells (PSCs) surround nerve terminals at the neuromuscular junction (NMJ). These special teloglial cells are sensitive to neurotransmitters and upregulate glial fibrillary acidic protein (GFAP) when deprived of synaptic activity. We found that activation of muscarinic acetylcholine receptors (mAChRs) at PSCs, but not purinergic (ATP and adenosine) or peptidergic [substance P (SP) and calcitonin gene-related peptide (CGRP)] receptors, prevented this upregulation. When applied onto single PSCs, muscarine evoked Ca2+ responses that fatigued but prevented upregulation of this glial cytoskeletal protein. Application of ATP onto single PSCs evoked Ca2+ signals that showed little fatigue, and GFAP upregulation occurred. Thus, Ca2+ signals alone cannot prevent GFAP upregulation in the PSCs. After blockade of cholinergic receptors by gallamine, neuronal activity was not effective in maintaining low GFAP levels in the perisynaptic glia. Last, immunohistochemistry disclosed mAChRs on PSCs and nearby fibroblasts. Thus, acetylcholine secreted by the nerve terminal acts on the PSCs via mAChRs to regulate GFAP. Cytoskeletal changes may influence perisynaptic glial functions, including growth, remodeling, and modulation of the synapse.

Synchronous release of ATP and neurotransmitter within milliseconds of a motor nerve impulse in the frog

1. It has been suggested that ATP is released together with the neurotransmitter acetylcholine (ACh) and, after hydrolysis to adenosine, is the primary physiological mediator of prejunctional neuromuscular depression. To evaluate whether ATP is released with sufficient rapidity to mediate prejunctional depression, outside-out patches containing both ATP-gated and ACh-gated ion channels were made from acutely dissociated guinea-pig sympathetic neurons and used to detect the co-release of nucleotide and neurotransmitter in frog cutaneous pectoris nerve-muscle preparations. 2. In a normal bathing solution in which muscle nicotinic receptors were blocked, a single stimulus to the motor nerve produced channel openings in the detector patch characteristic of both ATP and ACh. 3. In the remaining experiments, preparations were treated with sufficient hexamethonium (200 microM) to block nicotinic responses in the detector patch. In these experiments, a single temporally isolated nerve impulse caused the synchronous opening of ATP-gated channels in the detector patch with a latency of < 5 ms when patches were placed within 10 microns of the motor nerve ending. This multichannel phasic response was followed by trail of discrete channel openings characteristic of ATP-gated channels. 4. The selective ATP antagonist suramin (50 microM) reversibly eliminated the response to nerve stimulation. 5. The results suggest that ATP is released synchronously together with the neurotransmitter ACh in response to an individual nerve impulse and with a brief latency characteristic of quantal release from synaptic vesicles.

Synaptic regulation of glial protein expression in vivo

We investigated signaling between individual nerve terminals and perisynaptic Schwann cells, the teloglial cells that cover neuromuscular junctions. When deprived of neuronal activity in vivo, either by motor nerve transection or tetrodotoxin injection, perisynaptic Schwann cells rapidly up-regulated glial fibrillary acidic protein. Addition of transcription or translation inhibitors to excised muscles prevented this increase. Stimulation of cut nerves prevented glial fibrillary acidic protein increases even when postsynaptic nicotinic receptors were blocked, but not when neurotransmitter release was blocked with omega-conotoxin GVIA. We conclude that there is a nerve terminal to glial signal, requiring presynaptic neurotransmitter release, which regulates perisynaptic Schwann cell genes. This may be a general principle since many types of glial are sensitive to transmitters applied in vitro or released in situ.

Transmitter release increases intracellular calcium in perisynaptic Schwann cells in situ

Glial cells isolated from the nervous system are sensitive to neurotransmitters and may therefore be involved in synaptic transmission. The sensitivity of individual perisynaptic Schwann cells to activity of a single synapse was investigated, in situ, at the frog neuromuscular junction by monitoring changes in intracellular Ca2+ in the Schwann cells. Motor nerve stimulation induced an increase in intracellular Ca2+ in these Schwann cells; this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of the cotransmitters acetylcholine and ATP evoked Ca2+ responses even in the absence of extracellular Ca2+. Successive trains of nerve stimuli or applications of transmitters resulted in progressively smaller Ca2+ responses. We conclude that transmitter released during synaptic activity can evoke release of intracellular Ca2+ in perisynaptic Schwann cells. This Ca2+ signal may play a role in the maintenance or modulation of a synapse. These data show that synaptic transmission involves three cellular components with both postsynaptic and glial components responding to transmitter secretion.

Effects of charybdotoxin, a blocker of Ca2+-activated K+ channels, on motor nerve terminals

1. The contribution of Ca2+-activated K+ currents (IK,Ca) to the control of electrical excitability of motor nerve terminals and the control of acetylcholine release was assessed by studying the effects of the specific K(Ca) channel blocking toxins charybdotoxin and apamin. Electrical activity of the terminal regions of motor nerves was assessed by extracellular recording from an electrode placed in the perineural sheaths of nerves in the mouse triangularis sterni and frog cutaneous pectoris preparations. Acetylcholine release was monitored by intracellular recording of endplate potentials (e.p.ps). 2. Charybdotoxin (20-300 nM), but not apamin (10 nM-2.5 microM), selectively reduced the amplitude of an IK,Ca unmasked by prior blockade of the delayed rectifier K+ current with 3,4-diaminopyridine (3,4-DAP). 3. In the combined presence of 3,4-DAP and charybdotoxin, large Ca2+-dependent plateau responses developed, but only moderate and transient increases in acetylcholine release occurred. 4. In the absence of 3,4-DAP, charybdotoxin did not alter the electrical activity of, or the transmitter release from motor nerve terminals. 5. A possible role of the charybdotoxin-sensitive IK,Ca in the control of transmitter release is discussed.

Potassium channel blocking actions of beta-bungarotoxin and related toxins on mouse and frog motor nerve terminals

1. beta-Bungarotoxin and other snake toxins with phospholipase activity augment acetylcholine release evoked from mouse motor nerve terminals before they produce blockade. This action of the toxins is independent of their phospholipase A2 activity, but the underlying mechanism for the facilitation of release is unclear. To determine whether the toxins affect ionic currents at motor nerve terminals, extracellular recordings were made from perineural sheaths of motor nerves innervating mouse triangularis sterni muscles. 2. Perineural waveforms had a characteristic shape, with two major negative deflections, the first being associated with nodal Na+ currents and the second with terminal K+ currents. Block of the K+ currents revealed a Ca2+-dependent component. 3. During the facilitatory phase of its action, beta-bungarotoxin (150 nM) reduced the second negative component of the perineural waveform by 30-50%. 4. The reduction could be a consequence of a decreased K+ ion contribution or of an increase in the current carried by Ca2+. As beta-bungarotoxin had similar effects in solutions which contained no added Ca2+, it is unlikely to be acting on the Ca2+ current. Also, it is unlikely to be blocking the Ca2+-activated K+ current, which is suppressed in zero Ca2+ conditions. 5. Other prejunctionally active snake toxins (taipoxin, notexin and crotoxin) had similar effects to those of beta-bungarotoxin, but a similar basic phospholipase of low toxicity from cobra venom had no effect. 6. Thus, beta-bungarotoxin and related toxins block a fraction of the K+ current in the motor nerve terminals of mouse preparations. Such an effect could explain the facilitation of acetylcholine release caused by these toxins before the onset of presynaptic blockade. 7. In frog cutaneous pectoris preparations, f-bungarotoxin reduced endplate potential amplitude but had little effect on perineural waveforms. Therefore, the consequences of toxin binding must be different in frog terminals.

Effects of the potassium channel blocking dendrotoxins on acetylcholine release and motor nerve terminal activity

1. The effects of the K+ channel blocking toxins, the dendrotoxins, on neuromuscular transmission and motor nerve terminal activity were assessed on frog cutaneous pectoris, mouse diaphragm and mouse triangularis sterni nerve-muscle preparations. Endplate potentials (e.p.ps) and miniature e.p.ps were recorded with intracellular microelectrodes, and nerve terminal spikes were recorded with extracellular electrodes placed in the perineural sheaths of motor nerves. 2. Dendrotoxin from green mamba (Dendroaspis angusticeps) venom and toxin I from black mamba (D. polylepis) venom increased the amplitude of e.p.ps by increasing quantal content, and also induced repetitive e.p.ps. 3. Perineural recordings revealed that dendrotoxins could decrease the component of the waveform associated with K+ currents at the nerve terminals, and induce repetitive activation of nerve terminals. 4. In frog motor nerves, dendrotoxins are known to block the fast f1 component of the K+ current at nodes of Ranvier. Blockade of a similar component of the K+ current at motor nerve terminals may be responsible for the effects of these toxins on neuromuscular transmission. 5. Similar conclusions can be drawn from the results obtained from mouse neuromuscular junctions.

Anion permeability of motor nerve terminals

Motor nerve terminals in mouse and frog display behavior consistent with an appreciable permeability of the nerve terminal membrane to chloride. In mouse diaphragm, in the presence of 15 mM K+ and 2 mM or 8 mM Ca2+, replacement of Cl- by NO3-, Br- or acetate causes a transient increase in the quantal release of acetylcholine, measured as the frequency of spontaneously occurring miniature end plate potentials (FMEPP); a rapid rise in FMEPP is followed by a slow decline, with a half-time of about 4 min, to an equilibration level close to the control level. After equilibration in a solution in which the Cl- is replaced by another anion, return to Cl- -containing solution causes a transient decrease in FMEPP with a subsequent slow recovery. The data are consistent with transient nerve terminal depolarization or hyperpolarization, reflecting a nerve terminal permeability to anions in the sequence Cl- greater than Br- greater than NO3- greater than acetate. In 5 mM K+, changes in nerve terminal excitability, determined using focal stimulation, are also consistent with alteration of nerve terminal membrane potential as a consequence of anion substitution. The time course of relaxation of FMEPP after a change from Cl- to an anion of lower permeability, or vice versa, is considerably slower than that expected if Cl- permeability of nerve terminals is similar to that of skeletal muscle fibres, and if the nerve terminal behaves as a single compartment. In frog cutaneous pectoris, transient changes in FMEPP produced by substitution of anions in the bathing solution were similar to those produced in mouse diaphragm, but more rapid in time course.

Agonist and blocking effects of choline at the neuromuscular junction

The effects of choline chloride were studied at the voltage-clamped frog neuromuscular junction by measuring miniature endplate currents and equilibrium dose response curves for acetylcholine applied by microionophoresis. Choline reduced the amplitude and shortened the time constant of miniature endplate currents in a dose dependent manner. Dose response curves carried out in the presence of low doses of choline (200 microM) were shifted to the right and the apparent dissociation constant for ACh was increased without affecting the Hill coefficient or the maximum conductance at the endplate. Higher doses of choline shifted the curve even further to the right but reduced the Hill coefficient and maximum conductance. Choline ionophoretic dose response curves were carried out but the conductance response was only about 1% of the response to comparable concentrations of ACh. In the presence of ethanol, which reduces the agonist dissociation constant, choline responses were increased and the dose response curve analysis revealed that the efficacy of choline was about 17% in comparison to ACh. Similar effects were measured at rat endplates. Rat nerve-muscle preparations were used to investigate the effects of choline upon neuromuscular transmission.

Miniature endplate potential amplitudes corrected for spatial decay are not normally distributed

Miniature endplate potentials (MEPPs) were recorded simultaneously with two intracellular electrodes placed in the muscle fiber near the ends of the frog neuromuscular junction (nmj). The MEPP amplitudes are different in each electrode and are not normally distributed. A method is proposed to correct MEPP amplitudes for spatial decay. The MEPP amplitudes corrected for spatial decay are also not normally distributed. When the class interval of the distribution is small, multiple peaks are observed.

Control of quantal transmitter release at frog's motor nerve terminals. I. Dependence on amplitude and duration of depolarization

Motor terminals on the cutaneous pectoris muscle of the frog were depolarized by current pulses through the recording macro-patch-clamp electrode and the resulting quantal release was measured (excitation blocked with TTX). Above a threshold release increased very steeply with depolarization until saturation was approached. The dependence of release on duration of depolarization was even steeper: doubling pulse duration often produced more than 100-fold release ('early facilitation'). Distributions of delays of quantal release after the depolarization pulse were determined for wide ranges of depolarizations and pulse durations. The shape of these distributions was little affected by large changes in average release; increasing the temperature from 0 degrees C to 10 degrees C about halved the time scale of the distributions. Lengthening the depolarization from 0.5 to 6 ms produced a 'latency shift': the distributions of delays were shifted by almost the increase in pulse duration. At 5-6 ms pulse duration a few releases occurred during the final millisecond of the pulse. It is suggested that the time course of the phasic release is not controlled by the time course of changes in intracellular calcium concentration, but by an activator which is produced about proportional to supra-threshold pulse amplitude and duration, and that this activator effects release with a cooperativity of 6-7. An additional depolarization produced repressor is responsible for the minimum delay.

Distribution of spontaneous release along the frog neuromuscular junction

A new method, called the spatial decay method, is described. This method permits to localize the site of occurrence of the spontaneous release producing each miniature end-plate potential (MEPP) recorded at the frog neuromuscular junction (nmj). The method requires simultaneous intracellular recording at both distal ends of the nmj. The electrodes are positioned using Nomarksi optics. The assumptions that the MEPP amplitude is maximum in front of its release site and decays exponentially toward the recording electrodes placed distally, permit the calculation of the position of the release site producing each MEPP recorded intracellularly. The spatial decay method indicates that the occurrence of spontaneous release is not uniformly distributed along the frog nmj.

Specific localization of the alpha-latrotoxin receptor in the nerve terminal plasma membrane

The receptor for alpha-latrotoxin, the major protein component of the black widow spider venom, was investigated by the use of the purified toxin and of polyclonal, monospecific anti-alpha-latrotoxin antibodies. Experiments on rat brain synaptosomes (where the existence of alpha-latrotoxin receptors was known from previous studies) demonstrated that the toxin-receptor complex is made stable by glutaraldehyde fixation. At saturation, each such complex was found to bind on the average five antitoxin antibody molecules. In frog cutaneous pectoris muscles, the existence of a finite number of high-affinity receptors was revealed by binding experiments with 125I-alpha-latrotoxin (Kd = 5 X 10(-10) M; bmax = 1.36 +/- 0.16 [SE] X 10(9) sites/mg tissue, dry weight). Nonpermeabilized muscles were first treated with alpha-latrotoxin, and then washed, fixed, dissociated into individual fibers, and treated with anti-alpha-latrotoxin antibodies and finally with rhodamine-conjugated sheep anti-rabbit antibodies. In these preparations, muscle fibers and unmyelinated preterminal nerve branches were consistently negative, whereas bright specific fluorescent images, indicative of concentrated alpha-latrotoxin binding sites, appeared in the junctional region. These images closely correspond in size, shape, and localization to endplates decorated by the acetylcholinesterase reaction. The presynaptic localization of the specific fluorescence found at frog neuromuscular junctions is supported by two sets of findings: (a) fluorescent endplate images were not seen in muscles that had been denervated; and (b) the distribution of fluorescence in many fibers treated with alpha-latrotoxin at room temperature was the one expected from swollen terminal branches. Swelling of terminals is a known morphological change induced by alpha-latrotoxin in this preparation. When muscles were treated with either proteolytic enzymes (trypsin, collagenase) or detergents (Triton X-100) before exposure to alpha-latrotoxin, the specific fluorescent endplate images failed to appear. Taken together these findings indicate that the alpha-latrotoxin receptor is an externally exposed protein highly concentrated in the nerve terminal plasma membrane. Its density (number per unit area) at the frog neuromuscular junction can be calculated to be approximately 2,400/micron2.

The effects of alcohols and diols at the nicotinic acetylcholine receptor of the neuromuscular junction

A series of straight chain aliphatic alcohols from ethanol to octanol were tested at voltage-clamped frog endplates. In the presence of high concentrations of ethanol (greater than 1 M) the individual current responses to ionophoretic pulses of ethanol were reduced in amplitude and the dose-response curve for acetylcholine was shifted to the right. All the alcohols tested had this effect and their potency increased with the length of the carbon chain. The results were interpreted to indicate that as the molecular weight of the alcohol increased, its potency as a channel blocker also increased. The diol derivative of ethanol, which is ethylene glycol (ethanediol), was totally inactive up to 400 mM. However, 1,3-propanediol was a more potent blocker than propanol. After dose-response curves were carried out in high doses of ethanol and propanediol, the number of receptors was found to be permanently reduced. This effect could be due to irreversible denaturation of the receptor and therefore reversible denaturation could account for some of the reversible blocking effects caused by such drugs. An additional effect on the receptor was observed in that low concentrations of ethanol and propanol reduced the apparent dissociation constant for acetylcholine, thus increasing the amplitude of individual responses and shifting the dose-response curve to the left.

On the mechanism by which adenosine receptor activation inhibits the release of acetylcholine from motor nerve endings

The process by which adenosine receptor agonists inhibit the evoked release of acetylcholine (ACh) was studied at motor nerve endings to frog skeletal muscle. Adenosine and 2-chloroadenosine were employed as agonists. Each agonist reduced the mean number of ACh packets released synchronously in response to a nerve impulse (m). Adenosine was from one to two orders of magnitude less potent than 2-chloroadenosine as an inhibitor of this release. Focally recorded nerve terminal action potentials were unaffected by either adenosine receptor agonist. In normal Ca (1.8 mM), addition of sufficient Mg to reduce m to less than half the control value did not alter the degree of inhibition produced by adenosine receptor agonists. ACh release evoked by methods that do not require Ca entry through Ca channels (Ca-containing liposomes, La) was inhibited by either 2-chloroadenosine or adenosine. In Ca-free solutions containing Ba, the magnitude of neurally evoked asynchronous ACh release (miniature end-plate potential frequency = m.e.p.p.f) was depressed by either adenosine receptor agonist without change in the rate constant of decay of m.e.p.p.f; the m.e.p.p.f decay is thought to reflect the rate of clearance of Ba from regions of ACh release. Agents which displace Ca from storage sites and also inhibit phosphodiesterases increased m.e.p.p.f in the virtual absence of extracellular Ca and increased the level of inhibition produced by adenosine receptor agonists. RMI 12,330A (7 X 10(-6) to 7 X 10(-5) M), an adenylate cyclase inhibitor, occluded the effects of adenosine receptor agonists on ACh release. The results are consistent with the hypothesis that activation of extracellular adenosine receptors on adenylate cyclase inhibits evoked ACh release by reducing the affinity for Ca of an intracellular component of the secretory apparatus.

Acetylcholine receptor channel ionic selectivity: ions experience an aqueous environment

Alkali metal and alkaline earth cations pass readily through the acetylcholine receptor channel. Monovalent cations with larger crystal radii are more permeant than ones with smaller radii. For divalent ions, this selectivity sequence is reversed: smaller ions are more permeant than larger ones. This reversal in selectivity sequence with change of valence from 1 to 2 can be naturally accounted for by electrostatic interactions between the ion and its environment in the selectivity region of the channel. For monovalent ions, ion-dipole interactions dominate, and, for divalent ions, ion-induced dipole interactions are more important. The sign of these two types of effects is opposite and produces the reversal in the selectivity sequence. The magnitude of electrostatic interactions can be estimated from experimental data and suggests that the permeating ion's environment in the selectivity region of the channel is essentially like that in free water.

Voltage clamp analysis of the effect of cationic substitution on the conductance of end-plate channels

The effect of two commonly used sodium substitutes, tris and glucosamine, on the amplitude and kinetics of miniature end-plate currents (MEPCs), acetylcholine (ACh) induced end-plate currents (EPC) and EPC fluctuations was studied in voltage clamped single muscle fibres from a monolayer preparation of the cutaneous pectoris muscle. Total replacement of sodium with each substitute shifted the reversal potential from -4.7 mV (normal sodium solution) to -3.6 mV (tris) and -49.0 mV (glucosamine). In tris and glucosamine substituted solutions the current (MEPC or EPC) - voltage relation became markedly nonlinear, with peak current decreasing with membrane hyperpolarization. Peak current at +40 mV, was unaltered in tris solutions and reduced in glucosamine substituted solutions. MEPCs decayed with a single exponential time course and the EPC fluctuation spectra were characterized by single Lorentzian functions in both normal sodium solution and each substituted solution. Analysis of EPC fluctuations demonstrated that both tris and glucosamine decrease single channel conductance and increase channel lifetime. Both effects were enhanced by either membrane hyperpolarization or by increasing the concentration of each substitute. In the presence of each cationic substitute, single channel conductance increased and mean channel lifetime decreased with membrane depolarization. Analysis of the data according to the constant field assumptions (Goldman, Hodgkin, Katz equation) provided an inadequate description of experimental currents obtained at hyperpolarized membrane potentials with total ion substitution. Shifts in reversal potential with partial substitution were, however, adequately predicted by the GHK equation. These results suggest that tris and glucosamine ions interact with end-plate channels to reduce cation permeability and decrease channel closing rates. The dependence of this block on the level of membrane potential suggests that these cations bind to site(s) within open end-plate channels.

Effects of ethidium bromide on the nicotinic acetylcholine receptor

Ethidium bromide was tested for electrophysiological effects at the postsynaptic membrane of the frog muscle endplate. At low concentrations ethidium bromide blocked the open ion channel of the nicotinic acetylcholine receptor and reduced its open time. The rates for channel blocking and unblocking were calculated giving a dissociation constant of 139 nM at -80 mV membrane potential.

Effects of calcium and strontium in the process of acetylcholine release from motor nerve endings

1. The effects of Ca and Sr ions on synchronous acetyleholine (ACh) secretion (the impulsive, physiologically functional form of secretion which produces an end-plate potential in response to a single nerve impulse) and on asynchronous ACh secretion (the delayed, residual increase in miniature end-plate potential frequency evoked by repetitive nerve impulses or by accumulation of intracellular divalent cations) were studied at frog neuromuscular junctions.2. In a comparison of their extracellular effects, Ca was far more effective than Sr in supporting synchronous ACh secretion but less effective than Sr in mediating asynchronous release evoked by repetitive nerve impulses.3. In studies of their intracellular effects, Sr and Ca were delivered to the nerve terminal cytoplasm using liposomes as a vehicle. Ca-containing liposomes, although producing effects on asynchronous ACh secretion that were indistinguishable from those of equimolar Sr-containing liposomes, were more effective than Sr-containing liposomes in increasing synchronous release.4. Extracellular Ca behaved as a potent competitve inhibitor of asynchronous, neurally evoked release mediated by Sr. In contrast, intracellular Ca (i.e. liposomal Ca), whilst increasing synchronous ACh release, failed to antagonize evoked asynchronous release.5. The results demonstrate that synchronous and asynchronous secretion have different sensitivities to alterations in intracellular divalent cation concentrations. It is suggested that selectivity for Ca over Sr may occur at intraterminal sites responsible for synchronous ACh secretion but not at sites responsible for asynchronous ACh release. Furthermore, Ca appears to bind with high affinity as an antagonist at the external surface of the nerve ending. These results are discussed in conjunction with current theories of depolarization-secretion coupling.

On the calcium receptor that mediates depolarization-secretion coupling at cholinergic motor nerve terminals

1 The behaviour of the divalent cations Ca and Sr as agonists for receptors that mediate the synchronous evoked secretion of acetylcholine (ACh) was studied in the hope of determining whether the relationship between Ca binding and ACh secretion is determined only by the law of mass action or by the mathematical framework of receptor theory. Experiments were designed to evaluate the assumption that maximum effect requires occupation of all receptors by testing for the presence of spare Ca receptors on presynaptic terminals. Frog cutaneous nerve-muscle preparations were employed in conjunction with conventional electrophysiological methods.2 Curves of log [Ca] or log [Sr] against the mean number of ACh quanta released (m) were constructed to saturation. The log [Sr]-m relationship was shifted to the right and had a smaller maximum than the log [Ca]-m curve. This suggests that Ca has a higher efficacy than Sr and raises the possibility that spare binding sites are present for Ca.3 As a qualitative test for spare Ca receptors, La(3+) (>/=0.5 mum) or 2-chloroadenosine (25 mum) was employed as an irreversible antagonist of the effects of extracellular Ca on evoked ACh release. Despite the irreversible blockade of a proportion of receptors, increases in the [Ca] overcame this antagonism and produced a parallel shift in the log [Ca]-m relation to the right. This suggests an apparent receptor reserve for Ca. Antagonism of Sr-mediated ACh release by either La(3+) or 2-chloroadenosine could not be overcome by increasing the [Sr].4 As a quantitative test for spare Ca binding sites, the equilibrium affinity constant for Sr(K(Sr)) as a competitive inhibitor of Ca was determined and compared with values for K(Sr) calculated by two other methods which invoke the spare receptor assumption. All three methods produced comparable results. (K(Sr) = 0.24-0.27 mm(-1)).5 The equilibrium affinity constant for Ca (K(Ca)) was calculated by comparing reciprocal plots of the concentrations of Ca that produce equal levels of ACh release in the presence and absence of La(3+) (0.5 mum-3 mum). K(Ca) was estimated to be between 0.02 and 0.06 mm(-1).6 Efficacy (e), which is thought to reflect the ability of Ca or Sr once bound to receptors to support ACh release, was determined by the modified occupation theory of Stephenson (1956). The e(Ca) was estimated to be 9-20 and e(Sr) was 0.2-0.5.7 The experimentally determined values for K(Ca), K(Sr), e(Ca), e(Sr) along with the assumptions that spare Ca binding sites exist and that the non-linearities in the log [Ca] or log [Sr]-m curves are introduced beyond the sites of binding and efficacy were used to generate theoretical log [Me]-m curves. The theoretical relationships were similar to the experimental results.8 The results suggest that spare Ca receptors are present at motor nerve endings and that receptor theory provides an accurate quantitative description of the lumped events between Ca binding and ACh secretion. The possible physical correlates of affinity and efficacy are discussed.

Intracellular magnesium does not antagonize calcium-dependent acetylcholine secretion

1. The effects of intracellular application of Ca and Mg ions on evoked acetylcholine secretion at frog motor nerve terminals were studied. Ca and Mg were applied to the nerve-ending cytoplasm using liposomes as a vehicle. 2. Under conditions in which intracellular application of Ca produced many-fold increased in evoked acetylcholine release. 3. When Mg was applied to the nerve-ending cytoplasm concurrently with Ca, acetylcholine release was further increased above the level produced by introducing Ca alone. 4. The results suggest that intracellular Mg does not antagonize depolarization-secretion coupling and that antagonism of transmitter release by extracellular Mg occurs only at the external surface of the nerve ending.

An antiactin antibody that distinguishes between cytoplasmic and skeletal muscle actins

We elicited antibodies in rabbits to actin purified from body wall muscle of the marine mollusc, Aplysia californica. We found that this antiactin has an unusual specificity: in addition to reacting with the immunogen, it recognizes cytoplasmic vertebrate actins but not myofibrillar actin. Radioimmunoassay showed little or no cross-reaction with actin purified from either chicken gizzard or rabbit skeletal muscle. Immunocytochemical studies with human fibroblasts and L6 myoblasts revealed intense staining of typical cytoplasmic cables. Myofibrils were not stained after treatment of human and frog skeletal muscle with the antibody, although the distribution of immunofluorescence suggested that cytoplasmic actin is associated with membrane systems in the muscle fiber. The antibody may therefore be especially suited for studying the localization of cytoplasmic actin in skeletal muscle cells even in the presence of a great excess of the myofibrillar form.

Postsynaptic effects of ethanol at the frog neuromuscular junction

The molecular mode of action of alcohol in the central nervous system (CN) is unclear. The effects of ethanol on axonal action potentials can only be measured at concentrations which are very much higher than those required to produce central effects. At the frog neuromuscular junction similar concentrations increase the open time (tau) of the ion channel associated with the nicotinic acetylcholine (ACh) receptor. We have now investigated the effect of ethanol on the postsynaptic membrane of the frog neuromuscular junction by measuring equilibrium dose-response curves for the interaction between the neuro-transmitter (ACh) and the ACh receptors. Using this system, we found that ethanol produces significant changes in receptor function. Moreover, we found that an ethanol concentration which can be physiologically tolerated by man (0.2%) the dose-response curve is measurably affected.

Triton detergents and the frog neuromuscular end-plate: an electrophysiological and ultrastructural study

The effects of the nonionic detergents Triton X-45 and Triton X-100 were studied in the frog muscle end-plate, by intracellular recordings of spontaneous miniature end-plate potentials (m.e.p.p.'s) and the potential changes produced by iontophoretic application of acetylcholine (ACh-potentials). In addition, the ultrastructural changes produced by Triton X-100 were studied by transmission electron microscopic and freeze-fracture techniques. It was found that Triton X-45 and Triton X-100 caused a rapidly developing reduction of the amplitude of the m.e.p.p.'s. The response to iontophoretic application of acetylcholine was reduced by Triton X-100. Following return to normal Ringer solution the ACh-potentials recovered, although not completely. The dissociation constant calculated from the rate constants for onset and offset of the reaction (Kp=k2/k1) was 5--50 micron depending on the type of stoichiometric reaction presumed to occur between Triton X-100 and the cholinergic receptor. The ultrastructural changes observed indicate that the nerve terminal plasma membrane and mitochondria are affected by Triton X-100. Leakage of Ca2+ from the latter may therefore be the cause of the increase in m.e.p.p. frequency. It is concluded that the influence on the amplitude of the m.e.p.p.'s and the ACh-potentials can be attributed to a direct effect of the detergent upon the acetylcholine receptor protein.

Azidophenantridinium compounds as photoaffinity labels of cholinergic proteins

The synthesis of diazidopropidium and diazidoethidium is described. The applicability of these compounds as photoaffinity labels for cholinergic proteins has been investigated: diazidopropidium inhibits neuromuscular transmission. This inhibition is reversible if the compound is applied in the dark but becomes irreversible after irradiation with white light. Inhibition is accompanied by a disappearance of miniature endplate potentials. Electrophysiological analysis of this effect indicates that diazidopropidium acts postsynaptically by blocking the acetylcholine receptors. At the molecular level the action of diazidopropidium and diazidoethidium on acetylcholinesterase has been investigated: both compounds appear to bind to a peripheral acetylcholine binding site of this enzyme. Binding of 125I-labeled alpha-neurotoxin from Naja naja siamensis to purified membranes from Torpedo californica electric tissue rich in acetylcholine receptors is diminished after incubation and irradiation with diazidopropidium. About half of the toxin binding sites appear to be blocked by the photoaffinity label.

Enhancement by an antagonist of transmitter release from frog motor nerve terminals

1 The effect of Ba2+ on the synchronous release of acetylcholine from frog motor nerve terminals was studied by conventional electrophysiological techniques. 2 When Ca2+ and Ba2+ were the only divalent cations in the bathing fluid, Ba2+ caused a presynaptic reduction in the amplitude of the endplate potential (e.p.p.). This effect was surmountable by increasing the Ca2+ concentration. 3 The affinity constant (KA) for Ba2+, calculated on the assumption that Ba2+ is a competitive inhibitor of the agonist, Ca2+, was 1.1 +/- 0.4 mM-1 (mean +/- s.e. mean, n = 8). 4 When e.p.ps were depressed by the addition of 1 mM Mg2+, addition of Ba2+ (1 to 3 mM) caused either a further presynaptic depression of moderate magnitude or had no additional effect. 5 When e.p.p.s were depressed with [Mg2+] greater than or equal to 2 mM, addition of Ba2+ greater than or equal to 0.9 mM enhanced the e.p.p. amplitude by a presynaptic mechanism. 6 The interaction of the divalent cation antagonists Mg2+ and Ba2+ with the agonist, Ca2+ is discussed. It is demonstrated that a model which considers the nonequilibrium, kinetic properties of binding can be used to describe interactions between divalent cations at the external surface of the motor nerve ending.

The effect of the CO2/HCO3- buffer system on the membrane potential of frog skeletal muscle

1. The membrane potential of frog skeletal muscle was measured in various solutions, in the presence and in the absence of the CO2/HCO3- buffer. 2. The CO2/HCO3- buffer (PCO2 = 38-593 mm Hg; [HCO3-] = 5-25 mM/1) generally induced a reversible depolarization. 3. In the presence of C1-, there was a slowly developing but marked depolarization. 4. In the absence of C1-, there was an early depolarization which increased in high-PCO2 or low-K+ solutions, and decreased in low-PCO2, high-K+ or Na+-free solutions. Changing the HCO3- concentration did not modify the depolarization. 5. The early depolarization and contractions observed in C1--free Ringer persisted in presence of tubocurarine chloride (2.5-10(-5) M/1). 6. Possible mechanisms for the depolarization are discussed.

Suppression by elevated calcium of black widow spider venom activity at frog neuromuscular junctions

Frog neuromuscular junctions were treated with both concentrated black widow spider venom (BWSV) and elevated extracellular calcium (5-50 mM). This procedure causes a dramatic increase in the frequency of spontaneous miniature endplate potentials (mepps) which persists for only a few minutes. In contrast, BWSV-induced mepp activity, the venom effect (VE), continues for 20 min-1 h at junctions in elevated calcium Ringer solutions treated with doses of dilute venom or at junctions in normal calcium (1.91 mM) Ringer solution treated with concentrated venom. Following the disappearance of the VE in elevated extracellular calcium, only a few normal amplitude mepps and a few giant amplitude mepps are observed. The disappearance of the VE in these preparations is irreversible and occurs whether exposure to elevated extracellular calcium precedes or follows exposure to BWSV. Electron microscopy indicates that the major structural alterations produced by exposure to concentrated BWSV and 20 mM calcium Ringer solution are the swelling of nerve terminal mitochondria and the clumping of synaptic vesicles, large numbers of which remain in the terminals. Exposure to 20 mM calcium Ringer solution alone produces no ultrastructural modifications in these preparations. These observations can best be explained if one of the effects of BWSV is to increase the permeability of the nerve terminal membrane to calcium. Only doses of concentrated venom can sufficiently elevate intracellular calcium to a concentration at which synaptic vesicles clump together, thus interruping the transmitter release process.

Intra- and extracellular measurements of frog neuromuscular transmission upon stretch of the muscle at different stimulus frequencies

Flexible intracellular micro-electrodes were used to study the effect of changes in muscle length on the end-plate potential in the isolated m. cutaneus pectoris for different frequencies of stimulation (1/60-5Hz). A 20% step-wise increase in muscle length within the physiological range increases the end-plate potential immediately by about 50% (range 0-120%) at all frequencies tested. At stimulus frequencies lower than 1/5 Hz this increase is sustained during a period of 15 min stretch. At 1 Hz, however, the initial increase in the end-plate potential amplitude on the average declines within a few minutes to a steady-state value about 35% higher than the steady-state end-plate potential before stretch. At 5 Hz, the initial amplitude increase is followed by a decline of about 15 min duration and the final amplitude is not increased in comparison with the pre-stretch amplitude. The amplitude of the compound muscle action potential of the gastrocnemius muscle with intact circulation shows a similar time dependent increase upon stretch at different stimulus frequencies. It is concluded that stretch of a frog muscle gives both an immediate and a sustained increase in transmitter release from the nerve terminals during prolonged stimulation at frequencies up to about 5 Hz. This effect of stretch on transmitter release can improve in vivo neuromuscular impulse transmission.

Dual end-plate potentials at the single neuromuscular junction of the adult frog

Electrophysiological evidence is presented that at least 30 percent of sartorius muscle fibres of adult frogs are innervated by two or more axons at a single end-plate zone. In these fibres, increasing stimulation of the common sartorius nerve led to the appearance of two or more distinct levels of end-plate potentials (e.p.p.) (or currents, measured by the voltage clamp technique). They had an identical time course, reversal potential and delay to nerve stimulation. When the recording microelectrode was moved along the same fibre and reinserted, both components of e.p.p. decreased proportionally. This indicated that both components of e.p.p. originated very closely to each other on the muscle fibre, presumably in one end-plate zone. Many fibres of the sartorius muscle of adult frogs therefore possess polyneural innervation of a single end-plate zone, which is otherwise typical for early stages of ontogenesis.

The M. omohyoideus of the mouse as a convenient mammalian muscle preparation. A study of junctional and extrajunctional acetylcholine receptors by noise analysis and cooperativity

Muscles from cats, rats, guinea pigs and mice have been investigated as preparations for visualizing mammalian neuromuscular junctions with the aid of Nomarski interference optics. The M. omohyoideus of the mouse was found to be most convenient. Electrophysiological investigations showed that an endplate is normally surrounded by a population of perijunctional receptors. For junctional receptors in the endplate, a Hill coefficient of nH = 2.6 for acetylcholine was determined at 38 degrees C, decreasing to a value of 2.3 at room temperature. For both perijunctional and extrajunctional receptors (the latter occurring after denervation), the coefficient nH was 1.9. Noise analysis revealed a channel conductance gamma which changed abruptly from 22.4 +/- 1.0 pS (10--23 degrees C) to 45.6 +/- 3.9 pS (34--39 degrees C) in a very small temperature range around 25.5 degrees C. The mean channel lifetime tau was 0.3 ms at 39 degrees C and 1.0 ms at 23 degrees C.

Junctional and extrajunctional acetylcholine receptors in normal and denervated frog muscle fibres. Noise analysis experiments with different agonists

Ionic channel properties of acetylcholine receptors located in, in the vicinity of, or far away from a frog neuromuscular junction were investigated by noise analysis of drug induced current fluctuations. For drugs applied to the junction, in certain cases two Lorentzian curves were necessary to describe the data. It is postulated that the reason for this observation is that a contribution from perijunctional receptors was being observed. The conductance of a single channel in the junction was independent of the nature of the agonist and had an average value of 17.9 pS (temperature range 8-25 degrees C, solution buffered with Tris). After denervation for 21 days the conductance gamma was 7.5 pS at extrajunctional locations. In the close neighbourhood of the junction (peri-junctional receptors) values were found between 4 and 19 pS. The mean value of the open channel life-time tau in the endplate exposed to acetylcholine was 2.4 ms at 8-11 degrees C. This value was 0.90 ms with carbachol, 0.50 ms with succinylcholine, 0.28 ms with decamethonium and 0.45 ms with nicotine. The receptors outside the endplate exhibited tau-values which at a given temperature were 2-3 times larger than those at the endplate. Raising the temperature to 23 degrees C reduced all tau-values by factors of 2-3. It is concluded that at least two types of ACh-receptors with different properties exist in the muscle membrane, possibly produced by ACh-receptive units in different states of aggregation.

Local anesthetic alteration of miniature endplate currents and endplate current fluctuations

The effect of the local anesthetic QX222 on the kinetics of miniature endplate currents and acetylcholine induced endplate current fluctuations was studied in voltage clamped cutaneous pectoris muscle of Rana pipiens. Both the endplate current fluctuation spectra and the miniature endplate current decay consisted of two or three components depending upon the holding potential and local anesthetic concentration. The cutoff frequency of each spectral component was equal to the decay rate of its corresponding constituent of the miniature endplate current. Comparison of the relative amplitudes of the spectral and miniature endplate components indicated that QX222 did not act by creating two kinetically distinct populations of acetylcholine receptors. QX222 action could be explained by alteration of the acetylcholine receptors such that they sequentially change conformation form one open state to another. A specific case in which QX222 binds to the open state of the acetyl-choline receptor creating a blocked state, was found to account for the observed relationship between the relative amplitudes of the miniature endplate current and spectral components, as well as the previously observed voltage and concentration sensitivity of the decay rates of endplate current components.

Current-voltage relation and reversal potential at junctional and extrajunctional ACh-receptors of the frog neuromuscular junction

The relationship between synaptic current and membrane potential has been examined at junctional and extrajunctional end-plate receptors of the frog. At junctional receptors, the synaptic current-membrane potential relationship is linear for close range iontophoretic ACh application and non-linear when it is delivered from some distance. At extra-junctional receptors the current-voltage relationship is always non-linear. These non-linearities can be related to the fact that in both cases [ACh] on membrane outlasts the mean life-time of the synaptic channels. When their mean life-time is varied, the current-voltage relationship obtained at junctional receptors is no longer linear and the peak synaptic conductance increases or decreases with hyperpolarization as the channel life time is shortened or lengthened, respectively.

Voltage dependence of agonist effectiveness at the frog neuromuscular junction: resolution of a paradox

1. End-plate currents produced by nerve-released acetylcholine and iontophoretically applied acetylcholine and carbachol have been recorded from voltage-clamped frog cutaneous pectoris neuromuscular junctions made visible with Nomarski differential interference contrast optics. 2. The effectiveness of agonists - that is, the end-plate conductance change produced by a given dose-has been determined as a function of post-junctional membrane potential. 3. As the post-junctional membrane potential is made more negative, nerve-released acetylcholine becomes less effective whereas iontophoretically-applied agonists become more effective. 4. This voltage dependence of agonist effectiveness is mediated neither by end-plate current iontophoresis of agonist into the cleft nor through electric field effects on the esterase. 5. Influences of membrane potential on the opening and closing of end-plate channel gates can account quantitatively for the voltage-dependent effectiveness of both nerve-released and iontophoretically applied agonist.

Acetylcholine receptor: modification of synaptic gating mechanism after treatment with a disulfide bond reducing agent

Reduction of a 'reactive' disulfide bond in the postsyraptic membrane of the frog neuromuscular junction by dithiothreitol (DTT) decreases both the sensitivity of the membrane to applied acetylcholine (ACh) and the amplitude of the single 'shot effect'. Analysis of ACh induced conductance fluctuations in voltage clamped frog endplates indicates that DTT reduces both the amplitude gamma and duration tau of the elementary conductance events. The mean control value of gamma was 18.5-10(-12) omega-1 with no significant dependence on temperature. The mean control values pi were 2.3 msec at 7-9 degrees C and 0.94 msec at 20-22 degrees C. At 7-9 degrees C 1m7 DTT (20-50 min after application) reduced gamma to 61% of the control value and at 20-22 degrees C to 39%, while tau was reduced to 70% at both temperature ranges. The dose-response curve for iontophoretically applied ACh indicates that neither the total number of ionic channels nor the cooperativity within the receptors are changed. However, the affinity of ACh for the receptor sites was reduced. All effects of DTT were fully reversed by the oxidizing agent 5,5'-dithie-bis-(2-nitro-benzoic acid) (DTNB).