Calcium dependence of quantal release triggered by graded depolarization pulses to nerve terminals on crayfish and frog muscle

α-Conotoxin M1 (CTx) blocks αδ binding sites of adult nicotinic receptors while ACh binding at αε sites elicits only small and short quantal synaptic currents

In ‘embryonic’ nicotinic receptors, low CTx concentrations are known to block only the αδ binding site, whereas binding of ACh at the αγ‐site elicits short single channel openings and short bursts. In adult muscles the αγ‐ is replaced by the αε‐site. Quantal EPSCs (qEPSCs) were elicited in adult muscles by depolarization pulses and recorded through a perfused macropatch electrode. One to 200 nmol L⁻¹ CTx reduced amplitudes and decay time constants of qEPSCs, but increased their rise times. CTx block at the αδ binding sites was incomplete: The qEPSCs still contained long bursts from not yet blocked receptors, whereas their average decay time constants were reduced by a short burst component generated by ACh binding to the αε‐site. Two nanomolar CTx applied for 3 h reduced the amplitudes of qEPSCs to less than half with a constant slope. The equilibrium concentration of the block is below 1 nmol L⁻¹ and lower than that of embryonic receptors. CTx‐block increased in proportion to CTx concentrations (average rate 2 × 10⁴ s⁻¹·mol⁻¹ L). Thus, the reactions of ‘embryonic’ and of adult nicotinic receptors to block by CTx are qualitatively the same. – The study of the effects of higher CTx concentrations or of longer periods of application of CTx was limited by presynaptic effects of CTx. Even low CTx concentrations severely reduced the release of quanta by activating presynaptic M2 receptors at a maximal rate of 6 × 10⁵ s⁻¹·mol⁻¹ L. When this dominant inhibition was prevented by blocking the M2 receptors with methoctramine, activation of M1 receptors was unmasked and facilitated release.

When CTx blocks the αδ binding site of adult nicotinic receptors, very small and short quantal synaptic currents (qEPSCs) are generated by binding of ACh quanta at the αε‐site, This is very similar to the effects of CTx at embryonic receptors where the short qEPSCs are generated by binding at the αγ site. CTx also activates presynaptic muscarinic M1 and M2 receptors.

Historical view and physiology demonstration at the NMJ of the crayfish opener muscle

Here we present some of the key important discoveries made with the opener neuromuscular (NMJ) preparation of crustaceans and illustrate that there is still much to learn from this model preparation. In understanding the history one can appreciate why even today this NMJ still offers a rich playground to address questions regarding pre- and post-synaptic function and plasticity. The viability and ease of access to the terminal for intracellular as well as extracellular electrophysiology and imaging are significant advantages. The mechanisms behind the modulation of vesicular kinetics and fusion within the high- and low-output terminals are begging for investigation. The preparation also offers a testable model system for computational assessments and manipulations to examine key variables in theoretical models of synaptic function, for example calcium dynamics during short-term facilitation. The synaptic complexity of active zone and statistical nature of quantal release is also an open area for future investigation both experimentally and computationally.

Depolarization amplitude and Ca2+ -inflow control the time course of quantal releases at murine motor nerve terminals

In order to test whether the time courses of quantal releases after a depolarization pulse are affected by the depolarization amplitude, time courses were measured for small depolarization pulses that elicited close to threshold releases and for large depolarizations that elicited releases approaching saturation level. Diaphragms of young mice were excised and superfused with Bretag's solution at 18 degrees C. Synaptic currents were elicited and recorded through a perfused macropatch pipette. Releases elicited by threshold depolarizations rose earlier than releases elicited by saturation depolarizations. The short delays in the rising phases of release after large depolarizations may be due to the shift of Ca(2+) currents flowing during the pulse to tail currents. After its peak, release decayed with a time constant tau. For saturation depolarizations tau was about 0.3 ms, and for threshold depolarizations tau increased up to 0.8 ms. In order to differentiate between the effects of variations in Ca(2+) inflow and in depolarization, the amplitudes of large depolarization pulses were held constant while the amount of release was depressed by halving the Ca(2+) concentration at the terminal. The time course of the lowered releases was slightly delayed while tau remained at 0.3 ms as typical for saturation depolarizations. Double pulse facilitation unexpectedly revealed a short phase of depression of release after the pulse. This depression may contribute to the rapid decay (tau) of release after large depolarizations. The dependence of tau on depolarization amplitude indicates that the final phase of the time course of release is largely controlled by the amplitude of the preceding depolarization.

The time course of transmitter release in mouse motor nerve terminals is differentially affected by activation of muscarinic M1 or M2 receptors

At endplates of mouse diaphragms the effects of activation of presynaptic muscarinic M1 and M2 autoreceptors on the time courses of monoquantal releases have been investigated at 20 degrees C. Quantal excitatory postsynaptic currents (qEPSCs) were elicited and recorded with a perfused macropatch electrode, through which control- and drug-containing solutions were applied to 10 microm phi regions of a neuromuscular junction. M2 receptors were activated with muscarine, while the M1 receptors were blocked by pirenzepine. M2 activation presented a slight, but highly significant augmentation of early releases. Analogously, M1 receptors were activated with muscarine, while M2 receptors were blocked by methoctramine. M1 activation elicited a highly significant small shift of the time course of release towards longer delays. In controls, the number of late releases decayed with a time constant of 0.3 ms. This time constant did not change appreciably when methoctramine or methoctramine + muscarine were applied. However, methoctramine + muscarine reduced the amplitude of qEPSCs and shortened their decay by a partial block of postsynaptic channels. Double blocks with pirenzepine + methoctramine allowed no presynaptic effect of muscarine, showing that the blocker concentrations were sufficient. Neither the addition of methoctramine to pirenzepine, nor the further addition of muscarine changed the time constant of decay of the number of late releases. The results are very similar to that of autoreceptor activations in the glutamatergic crayfish synapse: activation of inhibitory receptors augmented early releases, and that of facilitatory receptors depressed early releases [J. Dudel (2006a) Eur. J. Neurosci., 23, 2695-2700], which may suggest a general presynaptic mechanism.

Anti-ganglioside antibodies alter presynaptic release and calcium influx

Acute motor axonal neuropathy (AMAN) variant of Guillain-Barré syndrome is often associated with IgG anti-GM1 and -GD1a antibodies. The pathophysiological basis of antibody-mediated selective motor nerve dysfunction remains unclear. We investigated the effects of IgG anti-GM1 and -GD1a monoclonal antibodies (mAbs) on neuromuscular transmission and calcium influx in hemidiaphragm preparations and in cultured neurons, respectively, to elucidate mechanisms of Ab-mediated muscle weakness. Anti-GM1 and -GD1a mAbs depressed evoked quantal release to a significant yet different extent, without affecting postsynaptic currents. At equivalent concentrations, anti-GD1b, -GT1b, or sham mAbs did not affect neuromuscular transmission. At fourfold higher concentration, an anti-GD1b mAb (specificity described in immune sensory neuropathies) induced completely reversible blockade. In neuronal cultures, anti-GM1 and -GD1a mAbs significantly reduced depolarization-induced calcium influx. In conclusion, different anti-ganglioside mAbs induce distinct effects on presynaptic transmitter release by reducing calcium influx, suggesting that this is one mechanism of antibody-mediated muscle weakness in AMAN.

Glutamatergic autoinhibition of quantal release augments the early phase of releases after a depolarization pulse

At the crayfish neuromuscular junction, glutamatergic autoinhibition of quantal excitatory postsynaptic current (qEPSC) release is mediated by a presynaptic DL-glutamate transporter and its associated Cl- conductance. I investigated whether it also affects the time course of release. qEPSCs were recorded with a perfused macroelectrode through which depolarization pulses and D- or L-glutamate could be applied to a terminal. In order to represent the time course of release, cumulative delays of qEPSCs were determined and scaled to a common final value. At 10 degrees C, on the application of D- or L-glutamate, release increased relative to the controls especially during its first millisecond, taking the mean of 20 experiments (P < 0.01). Also, in many single experiments the respective shifts in the time courses of release were highly significant. The relative surplus of early releases decreased with time constants tau1 of 86 micros and tau2 of 0.75 ms. At 0 degrees C, in the presence of glutamate, the surplus of early delays was increased relative to the controls to a significantly greater extent and for a longer time than at 10 degrees C. The tau1 of 240 micros was almost three times larger than at 10 degrees C. Autoinhibition was inactivated in Cl(-)-free solution. In such solutions the surplus of early releases also disappeared and the shortening of early delays reverted to a lengthening. Interaction of the inhibitory autoreceptor and its associated Cl- flow with the release machinery is discussed.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.

Glutamatergic chloride currents associated to glutamate transport?

Especially in arthropod glutamatergic synaptic systems, microM l-glutamate (Glu) concentrations often elicit Cl- currents, in addition to the excitatory cationic currents that are triggered by much higher Glu concentrations. In crayfish, Ibotenate (Ibo) is a specific agonist of the Glu-ergic Cl- currents. Application of Glu to Glu-transporters opens associated Cl- currents that inhibit quantal release presynaptically and by occupying the transporter prevents removal of released Glu. The latter prolongs the decay of postsynaptic EPSCs. It was tested whether the Ibo-elicited Cl- currents show the same pre- and post-synaptic effects as the transporter elicited ones, suggesting that also this current component arises through transporter activation. Indeed, Ibo applied to single synaptic junctions produced inhibition of quantal release and prolongation of EPSCs, very similar to the effects of Glu. It seems probable, therefore, that at least in crayfish Glu-ergic Cl- currents are generated by activation of transporters. Since generally such transporters are located around Glu-ergic synapses, this is likely to be a general mechanism. The toxin Ivermectin also elicits Cl- currents. However, while Ivermectin inhibits release too, it does not prolong the decay of EPSCs and is probable to activate GABAergic channels.

Presynaptic effects of immunoglobulin G from patients with Lambert-Eaton myasthenic syndrome: Their neutralization by intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) treatment improves muscle strength in Lambert-Eaton myasthenic syndrome (LEMS), but its specific mode of action is unknown. We have delineated its mode of action on neuromuscular blocking properties of LEMS IgG. The effect of sera and purified IgG from six patients with LEMS on evoked quantal release was investigated after direct application to the motor nerve terminal by the perfused macro-patch-clamp electrode in mouse hemidiaphragms. The effect of LEMS IgG was analyzed alone and after coincubation with different concentrations of IVIg or its Fab fragments. All LEMS sera and purified LEMS IgG fractions taken before IVIg treatment inhibited evoked quantal release in a dose-dependent manner. When LEMS IgG was coincubated with a therapeutic IVIg preparation, presynaptic inhibitory activity of LEMS IgG was diminished in a dose-dependent fashion. Monovalent Fab fragments were as effective in neutralizing the activity of LEMS IgG as whole IVIg. These direct neutralizing effects of IVIg may explain its therapeutic efficacy.

Both d- and l-glutamate induce transporter-mediated presynaptic autoinhibition of transmitter release

In crayfish motor nerve terminals l-glutamate (Glu) is the excitatory transmitter and low l-Glu concentrations exert autoinhibition by inhibiting release of Glu quanta from the terminals. This autoinhibition has been shown to be mediated by binding and transport of l-Glu by Glu transporters in the presynaptic membrane. Activated transporters open an associated Cl(-) channel and inhibit release [J. Dudel & M. Schramm (2003) Eur. J. Neurosci., 18, 902-910]. The excitatory, glutamatergic synaptic transmission is specific for the l-Glu isomer. However, transporters are non-selective for the stereoisomers. It is shown here that low concentrations (5 micro m) of d- as well as l-Glu inhibit quantal release on average to 55 and 68%, respectively. The power of inhibition varies widely at different terminals but the local sensitivity to d-Glu is seen to be the same as that for l-Glu. l-Glutamate has been reported to reduce the mean amplitude of nerve terminal action currents (excitatory nerve terminal currents) by about 10%, presumably due to the opening of Cl(-) channels. Evidence is given that d-Glu also inhibits this by an average of 10% (P < 0.001), as expected if both l- and d-Glu activate a transporter-associated Cl(-) conductance. The results give further support for this novel mechanism of regulation of synaptic strength.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

A receptor for presynaptic glutamatergic autoinhibition is a glutamate transporter

Monoquantal excitatory postsynaptic currents were recorded by means of a perfused macropatch electrode from 9 to 15 micro m stretches of crayfish neuromuscular junctions. The excitatory transmitter l-glutamate superfused to a terminal inhibits quantal release by activating autoreceptors [Parnas et al. (1996) Eur. J. Neurosci., 8, 116-126]. Substances related to glutamate that do not activate glutamatergic postsynaptic channels, but are substrates of glutamate transporters, elicited analogous inhibitions, e.g. l- and d-aspartate and some other glutamate transport blockers. As expected, all transport blockers prolonged synaptic currents. Blockers that bind to the transporter receptors but are not transported did not inhibit release, but prevented inhibition by the transport substrates. It appears that autoinhibition is elicited by transport of glutamate or its analogues. Transport into cells is powered by symport of three Na+. To block the transport step electrochemically, extracellular Na+ concentration was lowered to one-quarter, but this surprisingly left the inhibition of release by glutamate unaffected, showing inhibition to be associated to a step between binding and transport. After binding a substrate, glutamate transporters open a parallel Cl- channel. Replacement of extracellular Cl- prevented Cl- current, and release inhibition by glutamate or aspartate was blocked. It is suggested that the flow of Cl- across the cell membrane, after binding a transport substrate, mediates autoinhibition. We measured a related reduction of presynaptic action potentials.

Differential blocking effects of the monoclonal anti-GQ1b IgM antibody and alpha-latrotoxin in the absence of complement at the mouse neuromuscular junction

Numerous in vitro electrophysiological studies found different effects of anti-ganglioside antibodies on neuromuscular transmission. Since a complement-dependent, latrotoxin-like effect has been described for the mouse monoclonal anti-GQ1b IgM antibody (termed CGM3), we here investigated the effect of CGM3 and alpha-latrotoxin by means of the perfused macro-patch clamp electrode in mice hemidiaphragms in the absence of complement. The CGM3 mab depressed evoked quantal release dose-dependently, whereas the rate of spontaneous releases and the amplitude of postsynaptic currents was not significantly affected. Alpha-latrotoxin induced an increase in spontaneous releases followed by a blockade of evoked quantal release, this was not altered by CGM3. The complement-independent presynaptic block by CGM3 was different from the action of alpha-latrotoxin.

Quantal endplate currents from newborn to adult mice and the switch from embryonic to adult channel type

Quantal endplate currents (qEPCs) were recorded extracellularly by a macropatch electrode from excised diaphragms of mice. During the first 3 days after birth, the mean rise time t(r) was 0.5 ms (0.1-0.9 peak, 20 degrees C). Double-exponential, amplitude-weighted fits of the decay discerned almost equally abundant components tau(1)' approximately equal to 6 ms and tau(2)' approximately equal to 9 ms. Beginning on day 3, on days 4 and 5 after birth both t(r) and the tau' dropped. Further decreasing slowly, adult values were reached at day 8, with t(r) approximately equal to 0.3 ms, tau(1)' approximately equal to 2 ms and a very weak tau(2)' approximately equal to 6 ms component. When compared to the kinetics of fetal channels, the tau(1)' and tau(2)' of up to 3 day qEPCs could correspond to the short and long splice variants of the fetal channel type. The tau(1)' of adult muscles of 2 ms agrees well with the burst durations of adult channels while a weak longer tau(2)' component may represent 'extrasynaptic' channels. The long t(r) of very young mice may correspond to the relatively slow rise of channel currents elicited by ACh pulses in mouse myoballs.

Intravenous immunoglobulins neutralize blocking antibodies in Guillain-Barré syndrome

Intravenous immunoglobulin (IVIg) treatment ameliorates the course of Guillain-Barré syndrome (GBS), but its specific mode of action is unknown. We attempted to delineate the effect of IVIg on neuromuscular blocking antibodies in GBS. A total of seven GBS serum samples were examined for blocking antibodies and the effect of IVIg with a macro-patch-clamp technique in mouse hemidiaphragms. First, serum was tested before and after treatment with IVIg. Second, we investigated with coincubation experiments whether the IVIg was capable of neutralizing neuromuscular blocking antibodies in GBS serum or affinity-purified immunoglobulin G (IgG) fractions. Finally, the mechanism of the neutralizing effect was studied by the coincubation of active blocking GBS IgG with Fab and Fc fragments prepared from IVIg. All GBS sera (two adults and two children) and GBS IgG fractions (three adults) taken before treatment with IVIg blocked evoked quantal release by approximately 90%. Blocking activity was markedly reduced in sera obtained after treatment with IVIg. Coincubation of the pretreatment blocking serum with the posttreatment serum, or with the IVIg preparation used for treatment, reduced the blocking activity of the pretreatment GBS serum. When GBS IgG was coincubated with IVIg, the blocking activity of GBS IgG was diminished dose-dependently. Monovalent and divalent Fab fragments prepared from the IVIg were as effective as whole IVIg, but Fc fragments were ineffective. Therapeutic IVIg is capable of neutralizing neuromuscular blocking antibodies in GBS by a dose-dependent, antibody-mediated mechanism. This may, in part, explain its therapeutic efficacy.

Myofibrillar protein isoform expression is correlated with synaptic efficacy in slow fibres of the claw and leg opener muscles of crayfish and lobster

In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T1 (TnT1), a 55 kDa isoform expressed in slow-tonic (S2) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT1, with lower levels in distal fibres; central fibres lacked TnT1, which is characteristic of slow-twitch (S1) fibres. In addition, differences in troponin-I isoforms correlated with TnT1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.

Pertussis toxin does not affect the time course of quantal release in crayfish and mouse muscle, but has other post- and presynaptic effects, especially on adenosine autoreceptors

While G-proteins are involved in the synaptic release machinery and also can mediate inhibition of presynaptic Ca2+ channels, we find that pertussis toxin (PTX) does not affect the amount and the time course of quantal release from motor nerve terminals on crayfish or mouse muscle. Monoquantal excitatory currents (qEPSCs) were recorded that were elicited by constant depolarisation pulses to a terminal by means of a perfused macro-patch electrode. Although presynaptic effects of PTX on output and time course of release of quanta were absent, postsynaptically the rise time of qEPCs was increased and their decay time constant reduced. Adenosine (Ad) is known to inhibit quantal release in vertebrate motor nerve terminals via PTX sensitive G-proteins, and Ad is generated during nicotinic synaptic transmission by breakdown of the co-transmitter adenosine triphosphate (ATP). As reported by others, we found in mouse muscle inhibition of quantal release after application of Ad, but in addition late facilitation. Both these effects of Ad were blocked when the muscle was pre-incubated with PTX.

Influence of serotonin on the kinetics of vesicular release

The mechanisms by which synaptic vesicles are transported and primed to fuse with the presynaptic membrane are important to all chemical synapses. Processes of signal transduction that affect vesicular dynamics, such as the second-messenger cascades induced by neuromodulators, are more readily addressed in assessable synaptic preparations of neuromuscular junctions in the crayfish. We assessed the effects of serotonin (5-HT) through the analysis of the latency jitter and the quantal parameters: n and p in the opener muscle of the walking leg in crayfish. There is an increase in the size of the postsynaptic currents due to more vesicles being released. Quantal analysis reveals a presynaptic mechanism by an increase in the number of vesicles being released. Latency measures show more events occur with a short latency in the presence of 5-HT. No effect on the frequency or size of spontaneous release was detected. Thus, the influence of 5-HT is presynaptic, leading to a release of more vesicles at a faster rate.

Effects of mobile buffers on facilitation: experimental and computational studies

Facilitation is an important form of short-term plasticity that occurs in most synapses. At crayfish neuromuscular junctions, basal transmission and facilitation were significantly reduced after presynaptic introduction of "fast" high-affinity calcium buffers, and the decay of facilitation was accelerated. The existence of residual calcium during facilitation was also demonstrated. Computational modeling of three-dimensional buffered Ca(2+) diffusion and binding to secretory and facilitation targets suggest that the facilitation site is located away from a secretory trigger mediating exocytosis; otherwise, the facilitation site would be saturated by each action potential. Our simulations account for many characteristics of facilitation and effects of exogenous buffer, and suggest that facilitation is caused by residual calcium gaining access to a site distinct from the secretory trigger through restricted diffusion.

Differential facilitation of high- and low-output nerve terminals from a single motoneuron

In the crayfish opener neuromuscular preparation, regional differences in synaptic transmission are observed among the terminals of a single motoneuron. With a single stimulus, the high-output terminals of the proximal region of the muscle produce a larger excitatory postsynaptic potential than do the low-output terminals of the central region of the muscle. We tested the hypothesis that the low-output terminals exhibit more facilitation than do high-output terminals for twin-pulse, train, and continuous-stimulation paradigms. Previous studies have not employed several stimulation paradigms to induce facilitation among high- and low-output terminals of a single motoneuron. We found that the high-output terminals on the proximal fibers facilitate more than the low-output terminals on the central muscle fibers, in contrast with previous studies on similar muscles. The difference in measured facilitation is dependent on the stimulation paradigm. These results are important because ultrastructural differences between these high- and low-output terminals are known and can be used for correlation with physiological measurements. Short-term facilitation is a form of short-term memory at the synaptic level, and the processes understood at the crayfish neuromuscular junction may well be applicable to all chemical synapses.

Evaluation of the number of agonist molecules needed to activate a ligand-gated channel from the current rising phase

We propose a new method for calculating the number of agonist binding sites (n) in ligand-gated receptor channels from the initial phase of the current. This method is based on the fact that the relation between the current (I) and its first-time derivative (I') at the beginning of the current reflects the number of transitions that lead to channel opening. We show that, for constant agonist concentration, the above relationship at t --> 0 provides the number of steps leading to channel opening. When the agonist concentration is not constant but rather increases linearly with time, the corresponding value can be obtained using a slightly modified procedure. The analytical results were compared with computer simulations and a good match between the two was obtained. The theoretical procedure was then validated experimentally using the nicotinic receptor, because, for this receptor, the number of binding sites is well established. Indeed, the expected number of two binding sites was obtained. The method was then tested for the quisqualate-type glutamate receptor channel from the opener muscle of crayfish. The number of this receptor's binding sites is not fully resolved. Our results suggest that, for this glutamate receptor as well, two binding sites must be occupied to open the channel.

Allatostatin modulates skeletal muscle performance in crustaceans through pre- and postsynaptic effects

Allatostatins, originally identified in insects as peptide inhibitors of juvenile hormone biosynthesis, are regarded as potent inhibitory regulators of intestinal muscles in insects and crustaceans. However, accumulating data indicate that allatostatins might also be involved in modulation of skeletal neuromuscular events. We show that most ganglia of two isopod crustaceans (Idotea baltica and I. emarginata) contain pairs of large, allatostatin-immunoreactive motor neurons which supply several segmental muscles. Among them are the dorsal extensor muscles, of which some fibres receive immunoreactive, varicose innervation. We demonstrate, on identified muscle fibres, that allatostatin exerts a twofold inhibitory effect: it reduces contractions of single voltage-clamped fibres, and it decreases the amplitude of evoked excitatory junctional currents recorded from individual release boutons. No change in excitation-contraction threshold or in passive membrane parameters was observed. As the amplitude of miniature currents generated by spontaneously released single transmitter quanta was not changed, the inhibitory effect of the peptide on junctional currents must be of presynaptic origin. Supportive results were obtained on leg muscles of the crab Eriphia spinifrons, where allatostatin decreased evoked synaptic currents by reducing the mean number of transmitter quanta released by presynaptic depolarization without affecting the amplitudes of currents generated by single quanta. This effect of allatostatin was similar for two functionally different neurons, the slow and the fast closer excitor. The data show that allatostatin occurs in identified motor neurons of Idotea and exerts complementary pre- and postsynaptic modulatory effects which reduce muscle responses. Thus, allatostatin counteracts the effects of another neuropeptide, proctolin, which is also present in Idotea and causes potentiating effects on the same muscle fibres.

Block of quantal end-plate currents of mouse muscle by physostigmine and procaine

of quantal end-plate currents of mouse muscle by physostigmine and procaine. Quantal endplate currents (qEPCs) were recorded from hemidiaphragms of mice by means of a macro-patch-clamp electrode. Excitation was blocked with tetrodotoxin, and quantal release was elicited by depolarizing pulses through the electrode. Physostigmine (Phys) or procaine (Proc) was applied to the recording site by perfusion of the electrode tip. Low concentrations of Phys increased the amplitude and prolonged the decay time constants of qEPCs from approximately 3 to approximately 10 ms, due to block of acetylcholine-esterase. With 20 microM to 2 mM Phys or Proc, the decay of qEPCs became biphasic, an initial short time constant taus decreasing to <1 ms with 1 mM Phys and to approximately 0.3 ms with 1 mM Proc. The long second time constant of the decay, taul, reached values of </=100 ms with these blocker concentrations. The blocking effects of Phys and Proc on the qEPC are due to binding to the open channel conformation. A method is described to extract the rate constants of binding (bp) from the sums 1/taus + 1/taul, and the rates of unbinding (b-p) from tau0. taus-1. taul-1 (tau0 is the decay time constant of the control EPC). For Phys and Proc bp of 1.3 and 5. 10(6) M-1 s-1 and b-p of 176 and 350 s-1, respectively, were found. Using these rate constants and a reaction scheme for the nicotinic receptor together with the respective rate constants determined before, we could model the experimental results satisfactorily.

Functional and immunocytochemical identification of glutamate autoreceptors of an NMDA type in crayfish neuromuscular junction

Functional and immunocytochemical identification of glutamate autoreceptors of an NMDA type in crayfish neuromuscular junction. J. Neurophysiol. 80: 2893-2899, 1998. N-Methyl--aspartate (NMDA) reduces release from crayfish excitatory nerve terminals. We show here that polyclonal and monoclonal antibodies raised against the mammalian postsynaptic NMDA receptor subunit 1 stain specifically the presynaptic membrane of release boutons of the crayfish neuromuscular junction. In crayfish ganglionic membranes, the polyclonal antibody recognizes a single protein band that is somewhat larger (by approximately 30 kD) than the molecular weight of the rat receptor. Moreover, the monoclonal (but not the polyclonal) antibody abolishes the physiological effect of NMDA on glutamate release. The monoclonal antibody did not prevent the presynaptic effects of glutamate, which also reduces release by activation of quisqualate presynaptic receptors. Only when 6-cyano-7-nitroquinoxatine-2,3,dione (CNQX) was added together with the monoclonal antibody was the presynaptic effect of glutamate blocked. These results show that presynaptic glutamate receptors of the crayfish NMDA type are involved in the regulation of neurotransmitter release in crayfish axon terminals. Although the crayfish receptor differs in its properties from the mammalian NMDA receptor, the two receptors retained some structural similarity.

Neuromuscular blockade by IgG antibodies from patients with Guillain-Barré syndrome: a macro-patch-clamp study

Guillain-Barré syndrome (GBS) is often associated with serum antibodies to glycoconjugates such as GM1 and GQ1b. The pathogenic role of these antibodies and other serum factors has not yet been clarified. We have investigated the effect of serum, plasma filtrate, and highly purified IgG and IgM from 10 patients with typical GBS on motor nerve terminals in the mouse hemidiaphragm. Quantal endplate currents were recorded by means of a perfused macro-patch-clamp electrode. The plasma filtrate of all GBS patients led to a 5- to 20-fold reduction of evoked quantal release within 7 to 15 minutes of continuous superfusion. In 4 patients, the amplitudes of single quanta were clearly reduced (by 10-66% of control values), indicating an additional postsynaptic action. Blocking effects could be reversed to a variable degree within 15 to 18 minutes after washout. Purified IgG was as effective as native serum, whereas a purified GBS IgM fraction did not block transmission. Sera from convalescent patients and IgG from healthy subjects were without blocking effect. The effects were complement independent and there was no link to the presence (in 6 patients) or absence (in 4 patients) of detectable antibodies to GM1 or GQ1b. In GBS, antibodies to an undetermined antigen depress the presynaptic transmitter release and, in some cases, the activation of postsynaptic channels. We suggest that weakness in the acute stage of GBS may be caused in part by circulating antibodies.

Evoked quantal currents at neuromuscular junctions of wild type Drosophila larvae

Evoked excitatory postsynaptic currents (EPSC) were recorded with an extracellular macropatch electrode from glutamatergic neuromuscular junctions of Drosophila larvae. At 20 degrees C quantal current amplitude was about -400 pA and the 10-90% rise time was slightly below 0.2 ms for the fastest rising events and on average 0.3+/-0.1 ms in our best recordings. The quantal currents often had 'shoulders' but decayed approximately monoexponentially from half amplitude. The time constant of the exponential fit varied with mean values ranging from 2.5 ms to 7.7 ms in 13 experiments and an average value of 4.4+/-1.6 ms. Comparison of these results with data obtained earlier with outside-out patches of larval muscle membrane (Heckmann, M. and Dudel, J., Biophys. J., 72 (1997) 2160-2169) leads us to conclude that glutamate has to reach a saturating peak concentration of at least 10 mM in the synaptic cleft to allow the observed short quantal current rise times. To account for the time course of the quantal current decay one has to assume that the glutamate concentration in the synaptic cleft remains in the millimolar range for more than a millisecond and that the time course of the decay of the quantal currents is in part due to desensitization of the postsynaptic receptor channels.

Pre- and postsynaptic blockade of neuromuscular transmission by Miller-Fisher syndrome IgG at mouse motor nerve terminals

Miller-Fisher syndrome, a variant of an acute inflammatory neuropathy is often associated with serum antibodies to the ganglioside GQ1b, but the pathogenic role of these antibodies and other serum factors is unclear. We here investigated the effect of highly purified immunoglobulin G (IgG) from patients with typical Miller-Fisher syndrome, recording quantal endplate currents by means of a perfused macro-patch-clamp electrode on hemidiaphragms of adult mice. The GQ1b-positive and the GQ1b-negative Miller-Fisher IgG as well as its monovalent Fab-fragments depressed evoked quantal release in a fast and fully reversible, concentration and voltage dependent manner. The time-course of quantal release was changed with the late releases becoming more frequent. The extent of depression of release followed a Michaelis-Menten kinetic and depended on the extracellular calcium concentration. In addition the amplitude of quanta was reduced postsynaptically. IgG and sera from healthy subjects had no effect. Our results indicate that in Miller-Fisher syndrome, IgG antibodies to an undetermined antigen depress the release process, most likely by interfering with the presynaptic Ca2+ inflow or by interacting with proteins of the exocytotic apparatus, and prevent the activation of postsynaptic channels. Antibodies thus seem to be one pathogenic factor for muscle weakness in Miller-Fisher syndrome and our findings may explain why muscle strength recovers rapidly after therapeutical plasmapheresis.

Metabotropic glutamate autoreceptors on nerve terminals of crayfish muscle depress or facilitate release

In lobster and crayfish neuromuscular junctions superfusion of the excitatory transmitter L-glutamate (L-Glu) has been shown to depress release from motor axon terminals. In crayfish the effect depended on the level of depolarization of the terminal, Glu facilitating release when large depolarizing stimuli were applied. The presynaptic inhibition by Glu could be blocked only with a combination of blockers of Glu channels of the AMPA and N-methyl-D-aspartate (NMDA) types. We report that the effects of Glu can be mimicked by superfusion of (1S,3R)-1-aminocyclo-pentane-1.3-dicarboxylic acid (t-ACPD), an agonist of vertebrate metabotropic Glu-receptors. However, in the recent run of experiments the majority of terminals reacted with facilitation of release on superfusion by Glu or t-ACPD, even when the depolarizing stimuli to the terminal or the control rate of release were low. Average facilitation by 5 microM Glu was by a factor of 4.5. Facilitory responses were not blocked by the combination of APV and CNQX, and thus two types of metabotropic receptors seem to be present, their proportion varying for unknown reasons.

The amplitude of quantal currents is reduced during short-term depression at neuromuscular synapses in Drosophila

Focal extracellular excitatory postsynaptic currents were recorded to investigate short-term depression at glutamatergic Drosophila neuromuscular synapses. The amplitudes of quantal excitatory postsynaptic currents (qEPSCs) elicited before and after depolarizations eliciting large release were compared. Depression reduced the amplitude of the qEPSCs to 0.65 +/- 0.14 of control. Recovery from depression and of the receptor channels from desensitization follow a similar time course. Thus receptor desensitization seems to be involved in short-term depression at Drosophila neuromuscular junctions.

Pharmacologic inhibition of twin-pulse facilitation of release of transmitter quanta at the mouse neuromuscular junction

1. The frequency (F,s-1) of miniature endplate potentials and the quantal content (m) of endplate potentials were simultaneously measured intracellularly at mouse diaphragm endplates in a bath solution that contained 0.6 mM Ca2+ ions and 5 mM Mg2+ ions. 2. Twin pulses at 4-ms intervals gave the quantal contents of the first (m1) and second (m2) responses. The ratio of m2/m1 was taken as an indicator of the temporal facilitation of the release of transmitter. 3. Lead ions (Pb2+; 10 microM), bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; loaded for 60 min at 200 microM), and chlortetracycline (CTC; loaded for 30 min at 80 microM) reduced the values of F and m2/m1. Pb2+ ions and CTC reduced the value of m, whereas BAPTA did not.omega-Agatoxin (omega AGT; 10 ng/ml) reduced the value of m without affecting F or m2/m1. 4. These results suggest that synaptic facilitation is modifiable by agents that can affect systems which buffer intracellular levels of Ca2+ ions.

Study of the inhibitor of the crayfish neuromuscular junction by presynaptic voltage control

The inhibitor of the crayfish opener muscle was investigated by a presynaptic voltage control method. Two microelectrodes were inserted into the inhibitor and the amplitude and duration of presynaptic depolarization were controlled by a voltage-clamp amplifier. The inhibitory postsynaptic potential (IPSP) was measured from a muscle fiber located near the presynaptic voltage electrode. Nonlinear summation of IPSP amplitudes was corrected after chloride equilibrium potential was measured. With the use of 5-ms presynaptic pulses, the depolarization-release coupling (D-R) curve constructed from IPSP peak amplitudes (IPSPcor) had a threshold of about -35 mV and reached its maximal level at -5 to -10 mV. Depolarization beyond the maximum led to a suppression of neurotransmitter release. When transmitter release during a presynaptic pulse was completely suppressed, IPSPs activated by tail current could be identified with an average synaptic delay of 2.5 ms. Transmitter secretion triggered by a calcium current activated during the 5-ms pulses (IPSPon) was also measured on the rising phase of an IPSP, at 2.5 ms after the end of the 5-ms pulses. D-R coupling plots measured from IPSPon exhibited a more pronounced suppression than that obtained from IPSPcor. The effect of presynaptic pulse duration on the level of transmitter release was analyzed. Transmitter release increased with increasing duration and was nearly saturated by 20-ms pulses depolarized to 0 mV. The following conditions were identified as necessary to obtain a consistent D-R curve with a clear suppression: 1) small animals, 3.8 cm head to tail, 2) 15 degrees C, 3) 40 mM tetraethylammonium and 1 mM 4-aminopyridine, 4) an extracellular calcium concentration of < or = 10 mM. In addition, a consistent correlation was found among the branching pattern of the inhibitor, the placement of the presynaptic electrode, and the characteristics of the D-R curves. An ideal presynaptic electrode configuration involved placing the voltage electrode in a secondary branch, approximately 100 microns from the main branch point, and placing the current electrode at the branch point. Postsynaptically, optimal recordings were obtained from muscle fibers innervated by a single branch of the inhibitor that originated from a point near the presynaptic voltage electrode. A cable-release model was constructed to evaluate the relationship between the shape of the D-R coupling curves and the space constants of the presynaptic terminals. A comparison between the model and the D-R coupling curves suggested that the space constant of an inhibitor branch on a muscle fiber is > or = 8 times longer than its actual length. Therefore the upper limit estimate of the space constant of a typical preparation is approximately 3 mm. Results reported here outline morphological and physiological conditions needed to achieve optimal control of the presynaptic branch of the crayfish inhibitor. The cable-release model quantitatively defines the extent of presynaptic voltage control.

Disorganisation of quantal acetylcholine release by zinc at the Torpedo nerve-electroplate junction

The effects of zinc (Zn2+) on quantal acetylcholine release at the Torpedo nerve-electroplate junction were analysed by using loose patch electrodes designed to record evoked and spontaneous electroplate currents in a delimited area (electrode diameter of 10-15 microm) of the synaptic region. Zn2+ reduced the amplitude, prolonged the synaptic delay and slowed down the rising phase of all-or-none electroplate currents (EPCs) generated in response to activation of Na+ channels in a preterminal nerve branch. In graded EPCs (generated in response to direct activation of terminal Ca2+ channels), Zn2+ caused a reduction of quantal content but no change in the quantal size or in the minimum synaptic delay. The rise time of graded EPCs was prolonged but their half-decay time was not affected. Miniature EPCs (MEPCs) in control preparations had a widely distributed amplitude distribution but a homogeneous and rapid time course. Conversely, MEPCs in Zn2+-treated tissue exhibited a homogeneous and small amplitude, but a prolonged and more variable time course. Zn2+ at 1 mM caused, by itself, a high occurrence of MEPCs under conditions (flat-edged electrodes) when MEPCs are normally very infrequent. It is concluded that Zn2+ can both activate and inhibit the release mechanism and Zn2+-induced quanta exhibit an abnormal time course. The activation of the release process by Zn2+ or by Ca2+ may result in the production of quanta with different kinetics.

Dependence on temperature of the effect of dinitrophenol on the release of transmitter quanta at neuromuscular junctions in the mouse diaphragm

1. The frequencies (F, s-1) of miniature endplate potentials and the quantal content (m) of endplate potentials were measured intracellularly and simultaneously at mouse diaphragm endplates in a bathing solution that contained 0.6 mM Ca2+ ions and 5 mM Mg2+ ions. 2. Twin pulses at 4 ms intervals gave the quantal contents of the first (m1) and second (m2) responses. The ratio of m2/m1 was taken as an indicator of the temporal facilitation of the release of transmitter. 3. Dinitrophenol (DNP, 10 microM) increased the values of F and m at 36 degrees C. This effect did not depend on extracellular Ca2+ ions. 4. The potentiating effect of DNP disappeared at 24 degrees C but the value of m2/m1 remained constant. 5. These results suggest that the effect of DNP is modifiable by temperature which can affect systems that control the intracellular metabolism of Ca2+ ions.

Glutamate depresses release by activating non-conventional glutamate receptors at crayfish nerve terminals

The present study shows that release of glutamate from crayfish nerve terminals is inhibited at low depolarizing current pulses by glutamate, N-methyl-D-aspartate (NMDA) and quisqualate. These agonists elicit inhibitory effects at concentrations as low as 10(-8) M (quisqualate) and 10(-7) M (glutamate and NMDA). The NMDA-mediated inhibition is blocked by (+/-)-2-amino-5-phosphonovaleric acid (APV). The quisqualate-mediated inhibition is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Both CNQX and APV are needed to block glutamate-mediated inhibition. The inhibition of release is not accompanied by a detectable change in presynaptic membrane conductance at the secondary branch. Using fura-2, Ca2+ accumulation during repetitive stimulation (100 Hz) was monitored in single release boutons. Inhibition of release, elicited by 10(-4) M glutamate, was not associated with a reduction in the accumulation of Ca2+. We show that the glutamate released from a single or a few release boutons during normal activity acts similarly to glutamate added externally, i.e. it inhibits its own release.

Immunoglobulin G from a patient with Miller-Fisher syndrome rapidly and reversibly depresses evoked quantal release at the neuromuscular junction of mice

A neuromuscular blocking factor has been described in the serum of patients with Miller-Fisher syndrome (MFS). We here examined the effect of immunoglobulins (Ig) on neuromuscular transmission in mice recording quantal endplate currents by means of a perfused macro-patch-clamp electrode. Ig and IgM- and IgG-fractions from an anti-GQ1b-positive patient with typical MFS were highly purified. After application of MFS-IgG, quantal release decreased 1000-fold within 2 min. Returning to control solution the average release came back to the baseline level within 4 min. In contrast, control-IgG and MFS-IgM did not cause any blocking effect. The very fast and fully reversible presynaptic blockade of release caused by the highly purified IgG-fraction may be one factor producing muscle weakness in MFS.

Factors influencing the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction

1. The effects of several conditions and agents on the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction in low-Ca2+ high-Mg2+ bathing solutions were examined. 2. Twin-pulses gave two endplate potential (epps) with first (m2) and second (m2) quantal contents. The ratio of m2/m1 was taken as a measure of the degree of facilitation. 3. The mean value of this ratio was > 1. Individual ratios fluctuated widely at junctions with smaller values of m1 but were focused around 1 at junctions with larger values of m1. Thus, some populations of junctions with smaller values of m1 contributed to an increment in the mean ratio. 4. The mean ratio was virtually constant irrespective of changes in the spontaneous and evoked release of transmitter at temperatures between 20 and 36 degrees C and at external concentrations of Ca2+ from 0.4 to 0.8 mM. 5. 4-Aminopyridine(4-AP) slightly but significantly increased this ratio with increases in m1 and m2 at temperatures of 24 and 36 degrees C. Ouabain slightly but significantly reduced the ratio, with increases in m1 and m2. The steadiness of the ratio was maintained in the presence of caffeine, high K+, neomycin or omega-conotoxin irrespective of changes in m1 and m2, except in the case of omega-conotoxin. 6. Spontaneous output at 36 degrees C increased in the presence of 4-aminopyridine, ouabain, caffeine, high K+ or neomycin. 7. These results indicate that maintenance of a stable value of the ratio of m2 to m1 is a dominant feature.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of a reduction in temperature on the quantal release of transmitter at the mouse neuromuscular junction

1. The effects of a reduction in temperature were examined on evoked and spontaneous release of transmitter quanta and on presynaptic negative signals, blocked by Cd2+, measured externally at neuromuscular junctions in mouse diaphragm muscles in low-Ca2+, high-Mg2+ Krebs-Ringer solutions. 2. The evoked release was enhanced with lowering of the temperature, whereas the extent of spontaneous release was reduced. Cooperativity of Ca2+ in the evoked release was slightly reduced by lowering the temperature. 3. The presynaptic negative signals increased in duration with lowering of the temperature. 4. These results support the hypothesis that the effect of a reduction in temperature reflects the improved efficacy of the calcium-mediated mechanism of transmitter release, manifested as a prolongation of the inflow of Ca2+. The process involved in the evoked release is probably attributable to an almost passive mechanism.

A two-step model of secretion control in neuroendocrine cells

Recent experiments on a variety of neuroendocrine cells indicate that intense stimuli readily depress the secretory response. The most likely explanation for this depression is that a pool of release-ready granules is depleted. We present a two-step model of secretion that allows one to simulate the dynamics of such a pool for different time courses of free intracellular Ca concentration [Ca2+]i. We derive rate constants of the model from two types of experiment and find that, for the simplest type of model, not only the rate of consumption (exocytosis) but also the rate of vesicle supply to the pool of release-ready granules must be made Ca-dependent. Given these functional dependences a variety of results from the literature can be simulated. In particular, the model predicts the occurrence of secretory depression and augmentation under appropriate conditions.

Spatial facilitation and depression within one motor nerve terminal of frogs

1. Perfused macropatch electrodes were used to stimulate and simultaneously measure release from two sites on the same terminal of the frog cutaneous pectoris muscle. 2. It was found that release occurring at one site often affected release at an adjacent site 50 microns away, either enhancing it ('spatial facilitation') or depressing it ('spatial depression'). Spatial facilitation (or depression) was defined as the release produced by a test pulse at the second site (test electrode) when preceded by a pulse at the first site (prepulse electrode) divided by the release produced by the test pulse alone. 3. Spatial facilitation varied with the time interval between the prepulse and the test pulse. Peak spatial facilitation, which on the average was 2.14, occurred with an interval of 1-3 ms. With longer intervals spatial facilitation decayed with a time constant between 3-6 ms. When the time interval between the prepulse and the test pulse was zero (no delay), the release after the test pulse was always depressed. 4. When Ca2+ was omitted from the perfusate of the prepulse electrode, spatial facilitation was abolished. When a brief hyperpolarizing pulse followed the depolarizing prepulse with zero delay spatial facilitation was also abolished. 5. Electrotonic spread or Ca2+ diffusion within the axon terminal are excluded as coupling agents for spatial facilitation. It is suggested that the coupling agent may possibly be related to a hypothetical release-promoting factor.

Contribution of Ca2+ inflow to quantal, phasic transmitter release from nerve terminals of frog muscle

Evoked quantal release from sections of frog endplates contained in an extracellular electrode has been investigated with Ca2+ inflow prevented by superfusing the extracellular space with a Ringer's solution containing Cd2+e or with an "intracellular", EGTA-buffered solution containing less than 0.1 microM Ca2+e. Pulse application and recording were by a perfused macro-patch-clamp electrode. The muscle outside the electrode (bath) was superfused with Ringer's solutions containing Cd2+b to block Ca2+ inflow and normal (1.8 mM) or elevated (10 mM) Ca2+b. The depolarization level of the terminal during current pulses that generated maximal Ca2+ inflow was used as unit relative depolarization. Starting from a threshold above 0.5 relative depolarization, the average release increased by a factor of about 1000 with increasing depolarization, reaching a plateau above 1.2 relative depolarization. The high level of plateau release extended to at least a relative depolarization of 4, i.e. to about +200 mV. When Ca2+ inflow was prevented in the section of the terminal within the electrode, release was depressed strongly for relative depolarizations around 1, i.e. at potentials at which Ca2+ inflow is high. However, for large depolarizations (> 1.5 relative units), the depression of release by block of Ca2+ inflow was weak or absent. The time course of release, measured in distributions of the delays of quanta after the depolarizing pulse, was unaffected by block of Ca2+ inflow. If the extra-electrode superfusion of Ca2+b of the muscle was elevated to 10 mM and Cd2+b was 0.1 mM or 0.5 mM, perfusion of the electrode with solutions below 0.1 microM Ca2+e raised the average release paradoxically. With 0.5 mM Cd2+b this paradoxical increase of release was, on average, 4-fold at 6 degrees C, and 19-fold at 16 degrees C. Quantal endplate currents recorded in less than 0.1 microM Ca2+e had slightly increased amplitudes, and decay time constants were prolonged by about 50%. The results are interpreted to support the Ca2+/voltage theory of release, which proposes that evoked, phasic release is controlled by both intracellular Ca2+ concentration and another membrane-depolarization-related factor. If the resting intracellular Ca2+ concentration is sufficiently high, large depolarizations can elicit release independent of the presence or absence of Ca2+ inflow.

Evoked phasic release in frog nerve terminals obtained after block of Ca2+ entry by Cd2+

Cutaneous pectoris muscles of frogs were isolated, mounted in a chamber and superfused with Ringer's solution. With a macro-patch-clamp electrode placed on a section of a motor nerve terminal, quantal synaptic currents were elicited by depolarizing pulses and recorded. The electrode tip and the section of the terminal recorded from were perfused rapidly by Ringer's solution alone or containing 20-500 microM Cd2+ to block Ca2+ inflow. Separate superfusion of the muscle and the rest of the terminal with normal or elevated Ca2+ Ringer's solution provided a sufficiently high resting Ca2+ concentration in the terminal even when Ca2+ was blocked by Cd2+. The depolarization level of maximal Ca2+ inflow into the terminal was found by measuring maximal test pulse facilitation, Fc. In control solution as well as in the case of Cd2+ block, the rate of phasic release after depolarizing pulses rose further when depolarization was increased past the level of Fc, and reached a saturation level which was maintained at estimated depolarizations up to +200 mV. Block of Ca2+ inflow by Cd2+ decreased release substantially, but did not suppress it. The depression of release was greater in the range of large Ca2+ inflow (around Fc) than for very large depolarizations. The time course of phasic release was unaltered by blockage of Ca2+ inflow. It is concluded that Ca2+ inflow contributes to the promotion of evoked release only in the depolarization range in which Ca2+ inward current is large.(ABSTRACT TRUNCATED AT 250 WORDS)

Depolarizing pulses to neuromuscular terminals of frogs can elicit graded, phasic transmitter release in the absence of Ca influx

Quantal synaptic currents were recorded by means of a macro-patch-clamp electrode, through which the terminal could be also depolarized by current pulses. The tip of the electrode was perfused rapidly, applying either Ringer's solution or an EGTA-buffered less than 0.1 microM Ca Ringer's. Muscle and nerve outside the electrode were superfused with normal or 10 mM Ca Ringer which served to keep the resting intracellular Ca concentration, Cair, in the terminal below the electrode relatively high. When Ca inflow was prevented by decreasing the Ca concentration to less than 0.1 microM, release was depressed, but still measurable, for low depolarizations, and much less or not at all depressed for large depolarizations to positive membrane potentials. The time course of the depressed release without Ca-inflow was the same as that in the controls with Ca inflow. It appears that the voltage dependent activator proposed in the Ca-voltage theory of release can elicit maximal release in the absence of Ca inflow, provided Cair is sufficiently high. The voltage dependencies of this activator as well as that of Ca inflow can be estimated from the results.

Inhibition of Ca2+ inflow at nerve terminals of frog muscle blocks facilitation while phasic transmitter release is still considerable

Action potentials were triggered in the motor nerve by a suction electrode and calcium currents (iCa) in the nerve terminals were measured by means of a perfused macro-patch-clamp electrode on the distal portion of the end-plates. Postsynaptic currents were blocked by adding d-tubocurarine, whereas presynaptic Na+ (iNa) and K+ (iK) currents were blocked by adding tetrodotoxin (TTX), tetraethylammonium and 3,4-diaminopyridine, respectively, to the perfusate of the electrode. The current components which could be suppressed by addition of Cd2+ to the perfusate were taken as presynaptic iCa. The observed effects on the presynaptic current components were very similar to those reported previously. If the electrode was perfused with Ringer's solution containing the blockers for iNa and iK, the same, obviously complete block of iCa was obtained by 50 and 100 microM Cd2+, an average of 96% block by 20 microM Cd2+ and 50% block by about 5 microM Cd2+. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (Fd) were elicited by variable-duration (0.5-3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20 microM Cd2+ added to the perfusate reduced the rate of phasic release of quanta insignificantly for short depolarizing pulses and by a factor of about 10 for longer pulses. Fd was blocked almost completely. Addition of 50 microM Cd2+ to the perfusate had a greater depressive effect on release after short depolarizing pulses and reduced release after longer pulses by a factor of about 100.(ABSTRACT TRUNCATED AT 250 WORDS)

Twin pulse facilitation in dependence on pulse duration and calcium concentration at motor nerve terminals of crayfish and frogs

Phasic release from motor-nerve terminals of crayfish and frogs was elicited and recorded by means of a macro-patch-clamp electrode through which the terminal was depolarized in graded pulses. The tip of the electrode was perfused and the Ca concentration around the terminal, Cae, was controlled independent from that in the superfusion of the muscle, Cab. Release increased with pulse duration with a double-logarithmic slope of 5 to 9 in crayfish and frogs, which represents a form of "early facilitation" (Katz and Miledi 1968). In crayfish, this relation was shifted to longer pulse durations on lowering Cae, while in frogs, in addition, the saturation level of release was suppressed at low Cae. Responses to twin pulses with intervals of 7-10 ms showed facilitation, Fd. When pulse duration of the twin pulses was increased, starting from about 0.5 ms, Fd increased to a maximum, but declined for longer pulses which elicited release approaching the saturation range. On lowering Cae, the maximum of Fd, Fd, increased in amplitude and was shifted to larger pulse durations. Also reduction of Cab increased Fd. The effects of pulse duration and of Cae and Cab on Fd are predicted by the residual Ca theory of facilitation, if it is assumed that changes of Cae produce corresponding changes in Ca inflow during depolarization, and if the resting intracellular Ca concentration is influenced by the extracellular Ca concentration. The large values of early facilitation can not be explained by the residual Ca theory of facilitation and may indicate the action of another depolarization dependent factor which joins in the control of release.

Calcium and depolarization dependence of twin-pulse facilitation of synaptic release at nerve terminals of crayfish and frog muscle

Transmitter quanta were elicited from nerve terminals of crayfish and frog muscle by depolarization pulses through a macro-patch-clamp electrode. The rates of quantal release for twin pulses and their ratio, twin pulse facilitation, Fd, were determined. When the electrode was perfused with normal Cae (13.5 mM for crayfish, 1.8 mM for frog), Fd was low for threshold depolarizations, increased to a maximum at medium depolarizations, decreased when the rate of release due to the first pulse approached saturation, and increased again for larger depolarizations. If under these conditions the superfusion of the muscle outside the electrode was changed from normal to 0 Cab and high Mgb solution, Fd increased. When the Ca concentration around the terminal, Cae, was reduced to levels at which release did not reach the saturation level for large depolarizations, Fd in dependence on depolarization did not show the minimum at higher depolarizations. The amplitude of Fd measured for large, constant depolarization pulses showed a maximum at a Cae below that of the normal solution. The maximum of Fd was much higher if the superfusion of the bath contained 0 Cab and high Mgb than when normal bathing solution was superfused. The maxima of Fd at a low value of Ca inflow are predicted by the "residual Ca" theory of facilitation, if release is influenced by a resting low internal Ca concentration, Cair, and reaches a saturation level for large Ca-inflow. It is also predicted that decreasing Cair (as in low Cab) will increase Fd.

Shifts in the voltage dependence of synaptic release due to changes in the extracellular calcium concentration at nerve terminals on muscle of crayfish and frogs

The rate of release of transmitter quanta, elicited by variable depolarization pulses applied to a nerve terminal by means of a macro-patch-clamp electrode, was measured in muscles of crayfish and frogs. The electrode was perfused with solutions containing different Ca concentrations, Cae. The bath was superfused separately, usually with solutions containing nominally no Cab and elevated Mgb. A fixed depolarization pulse followed the variable test pulse within 7-10 ms, and facilitation, Fc, of release after the fixed pulse was determined as a measure of Ca-inflow during the test pulse. As described before, Fc always showed a peak, Fc, at depolarization amplitudes of the test pulse below the saturation level of release. When Cae was changed, the depolarization levels generating Fc shifted in a negative direction if Cae was lowered, and in a positive direction if Cae was increased. These shifts agreed with the known dependence of the effective membrane potential (controlling e.g. Ca inward current) on Cae which is due to shielding of surface changes by Ca2+ (cf. Hille 1984). Changes of Cab, at constant Cae, did not affect the depolarization dependence of Fc. It is concluded that Ca inflow is not the only factor controlling quantal release, and that at least in depolarizations beyond those eliciting Fc another potential dependent factor increases release while Ca inflow presumably falls.