The termination of transmitter action at the crustacean excitatory neuromuscular junction.

1. Excitatory junctional currents (e.j.c.s) and miniature excitatory junctional currents (mine.e.j.c.s) have been followed by recording the focal extracellular potential at excitatory neuromuscular junctions of Maia squinado, the Spider Crab. 2. If L-aspartate (concentrations less than or equal to 1 mM) is present in the saline, the average e.j.c. is prolonged by an increase in the duration of its falling phase. 3. No change occurs in the time course of the probablility of release of quanta in the e.j.c. as determined from the histogram of first quantal latencies. 4. The min.e.j.c. is also prolonged by L-aspartate (concentrations less than or equal to 0-5 mM). The rise time increases slightly, the pre-exponential period of the decay phase almost doubles, and the time constant of the final exponential decay increases to a value larger than the average lifetime of the elementary conductance event produced by L-glutamate. 5. Changes in e.j.c.s and min.e.j.c.s produced by aspartate show a striking similarity to the action of neostigmine on the time course of the vertebrate end-plate current. It is proposed that aspartate exerts its action by blocking a process normally reponsible for clearing the synaptic cleft of transmitter.     

Inhibition of acetylcholinesterase accelerates axon terminal withdrawal at the developing rat neuromuscular junction

Summary In developing skeletal muscles, the rate at which superfluous innervation is lost from the endplates depends on the general level of neuromuscular activity. Whether it is activity of the presynaptic or postsynaptic structures (or both) that is critical is not well established. In this work, we transitorily inhibited the AChE of soleus muscle in postnatal rats, in order to increase postsynaptic activity, without directly altering activity of the nerve terminals. We then followed the time course of disappearance of axon terminals from the endplates of treated and normal muscles, using electron-microscope techniques. Three hours after inhibition of AChE, the muscle fibres exhibited local supercontracture and ultrastructural damage in the region of the endplate, consistent with local elevation of Ca²⁺ levels. At the same time, small electron-opaque vesicles, apparently of muscular origin, appeared in the synaptic cleft. The nerve terminals, however, were entirely normal in number and appearance. One day after treatment, endplates of esteraseinhibited muscles showed accelerated loss of nerve terminals, compared to endplates of normally developing muscles. No further loss of nerve terminals occurred, once AChE activity returned at the endplate. These results suggest that the rate at which superfluous nerve terminals retract from the developing neuromuscular junction is regulated by the level of activation of the muscle. It seems likely that activity of postsynaptic sites may similarly regulate changes in innervation patterns, in other developing or adapting neuro-neuronal or neuro-effector systems.     

Excitatory and inhibitory postsynaptic potentials in alpha-motoneurons produced during fictive locomotion by stimulation of the mesencephalic locomotor region

We tested the hypothesis that stimulation of the mesencephalic locomotor region (MLR) activates polysynaptic pathways that project to lumbar spinal motoneurons and are involved in the initiation of locomotion. Fictive locomotion was produced by MLR stimulation, and intracellular records of evoked postsynaptic potentials (PSPs) in alpha-motoneurons were computer analyzed. Stimulation of sites in the MLR that were maximally effective for the initiation of locomotion produced excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in all the motoneurons examined. The amplitudes of the PSPs increased as locomotion commenced. The EPSPs were largest during the depolarized phase of the step cycle, and in 17 of our 22 cells the EPSP was replaced by an IPSP of slightly longer latency during the hyperpolarized phase. The mean latency of the EPSPs measured from the stimulus artifact produced by stimulation of the MLR was 5.1 ms (3.0-7.0 ms). In all cases, the IPSP occurred 0.6 ms or more after the onset of the EPSP in the same cell. Later PSPs were sometimes observed as well. The effects of constant current injection on the membrane potential oscillations associated with fictive locomotion (locomotor drive potentials) were examined. The results showed that the amplitudes of the locomotor drive potentials (LDPs) could be affected by depolarizing and hyperpolarizing current injection. The data is consistent with the LDP having a predominant inhibitory component, which is more readily altered by current injection than is the excitatory component. The effect of constant current injections on the MLR-evoked PSPs was also examined, and it was observed that both EPSPs and IPSPs could be affected by the injected currents. The EPSPs increased in amplitude with constant hyperpolarizing current injection, and this fact rules out the possibility that the EPSP is actually a reversed IPSP. The IPSP was decreased in amplitude by hyperpolarizing current injection. Combined stimulation of the MLR and the ipsilateral high-threshold muscle or cutaneous afferents produced facilitation of both short- and long-latency MLR-evoked PSPs, suggesting that the two pathways share common interneurons. The possibility that the long-latency PSPs are produced by rapid oscillation in the locomotor central pattern generator is discussed. We concluded that MLR stimulation that evokes fictive locomotion produces both excitation and inhibition of spinal motoneurons. Spinal interneuronal systems are implicated and may be those involved in the initiation and control of locomotion. The probable relay sites for the descending pathway from the MLR to motoneurons are discussed.     

Anion interaction at the inhibitory post-synaptic membrane of the crayfish neuromuscular junction

1. (24)Na, (36)Cl and (35)S thiourea were infused I.V. in rabbits according to schedules designed to yield approximately level activity in plasma for periods up to 5 hr. Cerebrospinal fluid was sampled before ending the experiment by decapitation and the radioactivities in cerebrospinal fluid and in homogenized brain were compared in each case to a time weighted mean value for plasma.2. The results are considered in terms of a simplified model which specifically acknowledges the continuity of the extracellular and cerebrospinal fluids and thus the coupling between processes which occur at the interfaces bordering those fluids.3. From the rate constants for exchange across the blood-brain interface that were necessary for simulation of the observed behaviours, permeability coefficients for that interface were estimated for the materials studied and, from experiments of others, for (42)K.     

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

Summary 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.     

Facilitation at the frog neuromuscular junction during and after repetitive stimulation

Summary 1. E.p.p.s and min.e.p.p.s were recorded intracellularly from Mg-blocked nerve muscle preparations (M. sartorius and M. cutaneous pectoris) of summer and winter frogs in vitro.2. Prolonged repetitive stimulation at frequencies above 5/sec induced synaptic facilitation (measured as e.p.p. increase) which continued to increase throughout the longest periods of stimulation tested (40 sec at 20/sec). For a given number of stimuli the facilitation was the greater the higher the frequency of stimulation.3. Increasing the release of ACh per impulse by reducing the Mg-concentration of the bathing solution caused a levelling off and even a depression of the e.p.p.s in the course of tetanic stimulation, i.e. the pattern of the e.p.p. response shifted towards that found in curarized preparations.4. After stimulation the e.p.p.s remained enlarged for periods from 100 to several hundreds of msec, depending on the number and frequency of the conditioning volleys. At frequencies below 100/sec the frequency of stimulation significantly influenced the amount and duration of the e.p.p. facilitation whereas at frequencies above 100/sec the duration of the e.p.p. facilitation was mainly determined by the number of conditioning volleys.5. After stimulation the frequency of the spontaneous min.e.p.p.s was increased. This increase decayed to the control level with a time course similar to that of the e.p.p. facilitation. The possibility is discussed that these parallel changes of e.p.p. amplitude and min.e.p.p. frequency are probably due to a mobilization of transmitter from its presynaptic stores to the release sites.

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Zusammenfassung 1. An Mg-blockierten Nerv-Muskel-Präparaten in vitro (M. sartorius und M. cutaneous pectoris) von Sommer- und Winterfröschen wurden intracellulär e.p.p. und min.e.p.p. gemessen.2. Lange tetanische Reizung mit Frequenzen von mehr als 5 Hz verursacht synaptische Bahnung (gemessen als e.p.p.-Vergrößerung). Diese Bahnung nimmt auch am Ende des längsten untersuchten Tetanus (40 sec bei 20 Hz) noch zu. Für eine gegebene Zahl von Reizen steigt die Bahnung mit der Reizfrequenz.3. Bei Vergrößerung der pro Impuls freigesetzten Menge ACh durch eine Reduktion der Mg-Konzentration der Badelösung erreicht die e.p.p. Amplitude im Verlauf des Tetanus ein Plateau oder beginnt sogar abzunehmen, d.h. die e.p.p. verhalten sich mehr wie in curarisierten Präparaten.4. Nach Reizung bleibt das e.p.p. gebahnt. Je nach Anzahl und Frequenz der Reize dauert die Bahnung hundert bis einige hundert Millisekunden an. Bei Reizfrequenzen von weniger als 100 Hz beeinflußt die Frequenz sowohl die Dauer als auch das Ausmaß der Potenzierung beträchtlich, während bei Frequenzen von mehr als 100 Hz die Dauer hauptsächlich durch die Zahl der Reize bestimmt wird.5. Nach Reizung ist die Frequenz der spontanen min.e.p.p. erhöht. Mit einem Zeitverlauf ähnlich dem der e.p.p.-Potenzierung kehrt die Frequenz der min.e.p.p. wieder zum Ausgangswert zurück. Diese parallelen Veränderungen der e.p.p. Amplitude und der min.e.p.p. Häufigkeit sind wahrscheinlich durch eine vermehrte Bereitstellung (Mobilisation) von Überträgersubstanz am synaptischen Spalt aus den präsynaptischen Speichern verursacht.

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This work was supported by the Deutsche Forschungsgemeinschaft.     

Relative distribution of Ca2+ channels at the crayfish inhibitory neuromuscular junction

We investigated the Ca(2+) channel-synaptic vesicle topography at the inhibitor of the crayfish (Procambarus Clarkii) neuromuscular junction (NMJ) by analyzing the effect of different modes of Ca(2+) channel block on transmitter release. Initial identification of Ca(2+) channels revealed the presence of two classes, P and non-P-type with P-type channels governing approximately 70% of the total Ca(2+) influx. The remaining Ca(2+) influx was completely blocked by Cd(2+) but not by saturating concentrations of omega-conotoxins MVIIC and GVIA, or nifedipine and SNX-482. To examine the relative spatial distribution of Ca(2+) channels with respect to synaptic vesicles, we compared changes in inhibitory postsynaptic current amplitude and synaptic delay resulting from different spatial profiles of [Ca(2+)](i) around release sites. Specifically, addition of either [Mg(2+)](o), which decreases single-channel current, or omega-Aga IVA, which completely blocks P-type channels, prolonged synaptic delay by a similar amount when Ca(2+) influx block was <40%. Because non-P-type channels are able to compensate for blocked P-type channels, it suggests that these channels overlap considerably in their distribution. However, when Ca(2+) influx was blocked by approximately 50%, omega-Aga IVA increased delay significantly more than Mg(2+), suggesting that P-type channels are located closer than non-P-type channels to synaptic vesicles. This distribution of Ca(2+) channels was further supported by the observations that non-P-type channels are unable to trigger release in physiological saline and EGTA preferentially prolongs synaptic delay dominated by non-P-type channels when transmitter release is evoked with broad action potentials. We therefore conclude that although non-P-type channels do not directly trigger release under physiological conditions, their distribution partially overlaps with P-type channels.     

Excitatory, inhibitory and biphasic synaptic potentials mediated by an identified dopamine-containing neurone.

1. A giant dopamine-containing cell, situated in the left pedal ganglion of the water snail Planorbis corneus, was identified in isolated living preparations of the central nervous system. Spectrophotofluorimetric analysis confirms that the cell contains dopamine, whereas noradrenaline appears to be absent. The cell is unique in being a repeatedly identifiable dopamine-containing neurone. 2. Stimulation of the giant dopamine-containing cell resulted in excitatory, inhibitory or biphasic (depolarizing-hyperpolarizing) synaptic potentials in a number of follower neurones. The duration of the e.p.s.p.s and i.p.s.p.s was 0-3-5 sec; they ranged from barely detectable responses to ones 7 mV in amplitude in different cells. The depolarizing phase of a biphasic synaptic potential (b.p.s.p.) was usually less than 1 mV in amplitude (max. 3mV) and lasted 40-400 msec. The latency of i.p.s.p.s was long (70-120 msec) compared with that of e.p.s.p.s and b.p.s.p.s (20 msec). Abolition of the depolarizing phase of b.p.s.ps. by tubocurarine left a long-latency (70-120 msec) i.p.s.p. All responses showed summation and marked facilitation. 3. Evidence is presented that the post-synaptic potentials are produced by direct connections from the giant cell and result from a release of dopamine. Of eight putative transmitter substances tested on these different groups of neurones, only dopamine produced a potential change which in each case was of the same polarity as the post-synaptic potential when this was monophasic. However, generally applied dopamine produced only a hyperpolarization in follower cells showing b.p.s.p.s. This result is probably partly due to rapid desensitization of the receptors mediating the depolarization and also to a masking of the depolarization by the more effective hyperpolarizing response. 4. Erogometrine and 6-hydroxydopamine specifically antagonized the i.p.s.p.s and dopamine receptors mediating inhibition. Neither the e.p.s.p.s nor the excitatory dopamine response were blocked by high concentrations of hexamethonium. Hexamethonium was also ineffective in blocking the depolarizing phase of a b.p.s.p., which was, however, selectively eliminated by tubocurarine. 5. It is suggested that dopamine is the transmitter released from the giant cell and that it can mediate excitatory, inhibitory or biphasic responses in different follower neurones.     

Quantal secretion and loss of vesicles induced by veratridine at the crayfish neuromuscular junction

Experiments were conducted in nerve-muscle preparations of small young crayfish (Austropotamobius torrentium, Astacus astacus). Application of veratridine in the superfusate induced strong quantal release of transmitter. After about 5 min when quantal release had declined to a low level preparations were fixed for electron microscopy. Unlike control preparations, veratridine-treated preparations revealed nerve terminals which were largely depleted of their synaptic vesicles. Our findings suggest that in the presence of veratridine the decline of quantal secretion results from the loss of vesicles caused by tonic nerve terminal depolarization. Moreover, our results indicate that during or after excessive quantal release triggered by veratridine synaptic vesicles may fuse with both the presynaptic membrane and each other.     

An investigation of the post-tetanic potentiation of end-plate potentials at a mammalian neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded intracellularly from neuromuscular junctions in curarized or Mg-paralysed rat diaphragm-phrenic nerve preparations in vitro. In Mg-paralysed preparations after 1000 impulses at 100/sec the amplitude of e.p.p.s elicited at 1/sec before and after the tetanus was on average greater than the control amplitude for 120 +/- 30 sec.2. The post-tetanic potentiation (P.T.P.) of e.p.p. amplitudes was not thought to be dependent upon post-tetanic hyperpolarization (P.T.H.) of nerve terminals as it lasted longer than the hyperpolarization generated by an identical tetanus; was unaffected by hyperpolarizing currents which reduced P.T.H. or depolarizing currents which prolonged P.T.H.; and was diminished in solutions containing 30% of the normal NaCl concentration or 1% ethyl alcohol, both of which procedures prolong P.T.H. The magnitude and duration of P.T.P. were influenced by the pH of the bathing solution in the range 7-7.5 although there was no change in P.T.H. under these conditions. The inability of polarizing currents to influence P.T.P. was also thought inconsistent with the hypothesis that P.T.P. is due to an increase in available transmitter.3. P.T.P. was not thought to be due to sodium accumulation in nerve terminals, for P.T.P. was reduced or abolished by procedures which would be expected to increase the intraterminal sodium ion concentration. These procedures were: exhibition of metabolic inhibitors (1.8 x 10(-6)M antimycin A, 3-5 mM sodium azide or 1 mM sodium iodoacetate), exhibition of cardiac glycosides (7.7 x 10(-6)M digoxin or 0.42 mM ouabain), and omission of glucose or potassium ions from the bathing solution. Abolition of P.T.P. by potassium-free solutions was also thought to be inconsistent with the hypothesis that P.T.P. is due to a reduction in the potassium concentration in nerve terminals.4. P.T.P. was not thought to be due to terminal volume changes, for no consistent effect upon the quantal content of e.p.p.s could be detected in hypo- or hyperosmotic solutions.5. It was concluded that the only hypothesis for P.T.P. not excluded by our experiments was that P.T.P. is due to some change in ionized calcium at a membrane site important in transmitter release.     

On the effect of calcium on the frequency of miniature end-plate potentials at the frog neuromuscular junction.

1. The effect of the extracellular Ca concentration on the frequency of miniature end-plate potentials (min. e.p.p.s) at the frog neuromuscular junction was studied. 2. In saline containing elevated K (5 or 11 mM), the frequency of min. e.p.p.s increased as Ca concentration was increased from 0-1 to 1-3 mM. However, with further increases of Ca concentration up to 10 mM, min. E.P.P. frequency declined. 3. In saline containing the normal concentration of K (2 mM), increasing Ca concentration from 0-1 to 10 mM produced a slight, monotonic increase in min. e.p.p. frequency. 4. The non-monotonic effect of Ca on min. e.p.p. frequency in preparations depolarized by elevated K is consistent with the existence of two opposing effects of Ca on transmitter release. Firstly, raising the external concentration of Ca increases the electrochemical potential for Ca entry, which tends to increase Ca influx and transmitter release. Secondly, increasing external Ca concentration increases electrostatic screening of fixed negative charges on the outer surface of the nerve terminal membrane. Such an increase in screening of charges near voltage-sensitive Ca gates would produce a hyperpolarization across the gates and they would tend to close, an effect which would tend to decrease Ca influx. The monotonic increase in min. e.p.p. frequency with increasing Ca concentration in 2 mM-K is consistent with the voltage insensitivity of the Ca gates at potentials close to the normal resting potential.     

Relaxation after a voltage step of inhibitory synaptic current elicited by nerve stimulation (crayfish neuromuscular junction)

The clamped membrane potential of small crayfish muscle fibers was shifted in rapid steps between potentials of about –60 and –120 mV, and the clamp currents measured after de-and hyperpolarizing steps were averaged. In addition, the inhibitory nerver fiber was stimulated either synchronously or asynchronously with the averaging. Synchronous stimulation yielded the usual IPSCs, and asynchronous stimulation a steady state inhibitory current which relaxed to a new level after a voltage step. Fast relaxations were observed in all fibers. Their time constants =15 to 20 ms at –60 mV (10°C) decreased on hyperpolarization and agreed with those of the decay of the IPSC at the respective potential. The relaxations could be described quantitatively by a model in which the synaptic current depends on membrane potential due to (1) the potential dependence of the life time of a synaptic channel, (2) to a constant channel permeability, and (3) to the potential difference from the equilibrium potentialE _Cl.In many fibers slow relaxations of the inhibitory synaptic current were observed also, with time constants to 150 ms at –60 mV (10°C), which decreased at more negative potentials. These slow relaxations correspond to a reported slow current noise component induced by GABA. Interpretations of this slow synaptic current component are discussed.This investigation was supported by the Deutsche Forschungsgemeinschaft     

The effects of pH changes on the frequency of miniature end-plate potentials at the frog neuromuscular junction.

As reported by Landau & Nachshen (1975), a decrease in extracellular pH at the frog neuromuscular junction leads to an increase in min.e.p.p. frequency. 2. Decreasing the extracellular pH still increases the min.e.p.p. frequency when the bathing Ringer contains 10 mM-Ca2+, in place of the usual 2-5 mM. At the mammalian neuromuscular junction, the elevated Ca2+ blocks the effect of the pH change on the min.e.p.p. frequency (Hubbard, Jones & Landau, 1968). 3. In Cl--free solution (isethionate or methylsulphate substitution) min.e.p.p. frequency is no longer a monotonic function of decreasing pH. Instead there is an optimum pH for spontaneous release between pH 6-6 and 8-6. 4. This suggests that in Cl- containing Ringer min.e.p.p. frequency increases with increasing extracellular acidity because there is a change in the PCl of the nerve terminal leading to a depolarization. In agreement with this idea,in low Ca2+ Ringer, acid pH has little effect on the min.e.p.p. frequency. 5. Decreasing the intracellular pH by raising PCO2 produces substantial increases in the min.e.p.p. frequency. The effects are much greater than the effects of equal changes of H+ in the extracellular solution. 6. Possible explanations for the effects of increased PCO2 are discussed. Although release of Ca2+ from mitochondria or other unknown effects of intracellular pH change or molecular CO2 are possible, the results do give some support to the hypothesis that an important step in transmitter release involves an electrostatic repulsion between fixed membrane surface charges on the transmitter containing vesicles and the inner face of the nerve terminal. The surface charge density would be decreased by a lower pH in the axoplasm, and this would increase the rate of spontaneous transmitter release, in agreement with the observations.     

Substance P-like immunoreactivity at the frog neuromuscular junction.

It has been recently shown that frog motoneurons and their nerve terminals contain calcitonin gene-related peptide-like immunoreactivity in large dense-core vesicles (Matteoli et al., Proc. natn. Acad. Sci. U.S.A. 85, 7366-7370, 1988). We report here by an immunofluorescence approach that the same neurons and nerve terminals also contain substance P-like immunoreactivity. The demonstration of substance P-like immunoreactivity in the frog motor nerve endings supports previous data suggesting a physiological role for this peptide in the modulation of cholinergic transmission.     

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.     

The actions of 'selective' excitatory amino acid antagonists on the crustacean neuromuscular junction

Intracellular recordings of neurally evoked excitatory junction potentials were made from the hermit crab neuromuscular junction and the effects of a series of putative antagonists, including the phosphonic amino acids, were examined for their effectiveness. The most potent antagonists were (+/-)2-amino-5-phosphonohexanoic acid and (+/-) 2-amino-5-phosphonovaleric acid while glutamate diethyl ester, DL-alpha-aminoadipate, 2-amino-7-phosphonoheptanoic acid and gamma-D-glutamylglycine were less effective. None of the compounds produced a change in membrane input resistance, but pyridine-2,5-dicarboxylic acid depolarized the muscle membrane and this probably accounted for its apparent antagonist properties. All the compounds readily and reversibly, but not competitively, antagonized the ionophoretic L-glutamate-induced depolarization. Since none of the compounds was capable of producing complete antagonism it is concluded that a more potent and competitive antagonist is required.