Permeability changes produced by L-glutamate at the excitatory post-synaptic membrane of the crayfish muscle

Characterization of postsynaptic Ca2+ signals at the Drosophila larval NMJ

Postsynaptic intracellular Ca(2+) concentration ([Ca(2+)](i)) has been proposed to play an important role in both synaptic plasticity and synaptic homeostasis. In particular, postsynaptic Ca(2+) signals can alter synaptic efficacy by influencing transmitter release, receptor sensitivity, and protein synthesis. We examined the postsynaptic Ca(2+) transients at the Drosophila larval neuromuscular junction (NMJ) by injecting the muscle fibers with Ca(2+) indicators rhod-2 and Oregon Green BAPTA-1 (OGB-1) and then monitoring their increased fluorescence during synaptic activity. We observed discrete postsynaptic Ca(2+) transients along the NMJ during single action potentials (APs) and quantal Ca(2+) transients produced by spontaneous transmitter release. Most of the evoked Ca(2+) transients resulted from the release of one or two quanta of transmitter and occurred largely at synaptic boutons. The magnitude of the Ca(2+) signals was correlated with synaptic efficacy; the Is terminals, which produce larger excitatory postsynaptic potentials (EPSPs) and have a greater quantal size than Ib terminals, produced a larger Ca(2+) signal per terminal length and larger quantal Ca(2+) signals than the Ib terminals. During a train of APs, the postsynaptic Ca(2+) signal increased but remained localized to the postsynaptic membrane. In addition, we showed that the plasma membrane Ca(2+)-ATPase (PMCA) played a role in extruding Ca(2+) from the postsynaptic region of the muscle. Drosophila melanogaster has a single PMCA gene, predicted to give rise to various isoforms by alternative splicing. Using RT-PCR, we detected the expression of multiple transcripts in muscle and nervous tissues; the physiological significance of the same is yet to be determined.

Glutamate is a transmitter that mediates inhibition at the rectifying electrical motor giant synapse in the crayfish

Spike transmission at the electrical synapse between the giant fibres (GFs) and motor giant neurone (MoG) in the crayfish can be blocked by depolarising postsynaptic chemical inhibition, which has previously been shown to be mediated in part by gamma-aminobutyric acid (GABA). The authors show that glutamate applied to the synaptic region of the MoG mimics the depolarisation of the chemical input and can also block spike transmission from the GFs. The glutamate induces an inward current mediated by a conductance increase that is 30-40% of that induced by GABA and that is blocked substantially by picrotoxin. Glutamate has no effect on the presynaptic GF, and the effects in the MoG are maintained in the presence of cadmium, indicating that the glutamate is acting directly on the MoG. Both GABA and glutamate have similar effects on the cell body, where the response reverses 10-20 mV positive to resting potential, is dependent on chloride concentration, and is inhibited by picrotoxin. Joint application of glutamate and GABA induces a nonadditive current under voltage clamp, suggesting that the transmitters can activate the same postsynaptic receptors. Immunocytochemical staining shows that, whereas some synaptic profiles impinging on the MoG contain pleomorphic agranular vesicles and are immunoreactive to GABA and not glutamate (as previously reported), there are at least as many other profiles that contain round, agranular vesicles and that are immunoreactive to glutamate and not to GABA. Thus, the authors conclude that some of the interneurones mediating inhibition of the electrical synapse use glutamate as their neurotransmitter.

Reduced facilitation and vesicular uptake in crustacean and mammalian neuromuscular junction by T-588, a neuroprotective compound

Bath application of compound T-588, a neuroprotective agent, reduced paired-pulse and repetitive-pulse facilitation at mammalian and crustacean neuromuscular junctions. In addition, it reduced voltage-gated sodium and potassium currents in a use-dependent fashion, but had only a small effect on the presynaptic Ca(2+) conductance. By contrast, it blocked FM 1-43 vesicular uptake but not its release, in both species. Postsynaptically, T-588 reduced acetylcholine currents at the mammalian junction in a voltage-independent manner, but had no effect on the crayfish glutamate junction. All of these effects were rapidly reversible and were observed at concentrations close to the compound's acute protective level. We propose that this set of mechanisms, which reduces high-frequency synaptic transmission, is an important contributory factor in the neuroprotective action of T-588.

Calcium currents, transmitter release and facilitation of release at voltage-clamped crayfish nerve terminals

1. The presynaptic terminals at crayfish (Procambarus spp.) opener neuromuscular junctions were voltage clamped. Calcium currents were measured during (ICa) and following (tail ICa) presynaptic depolarizations; EPSPs or IPSPs were simultaneously recorded from the (postsynaptic) muscle fibre directly beneath the presynaptic impalement. 2. For short (< or = 6 ms) presynaptic depolarizations, most of the transmitter release occurred during the tail ICa. EPSP or IPSP amplitudes at the end of the 6 ms pulse (end EPSP or end IPSP) increased monotonically with the integral of the ICa ([symbol: see text]ICa). The suppression potential for transmitter release was near the apparent reversal potential for ICa. 3. When the end EPSP or end IPSP amplitude was plotted against the peak ICa elicited during a presynaptic pulse (peak ICa), large and small depolarizations which evoked the same peak ICa evoked different amounts of transmitter release. The differences in transmitter release were eliminated when end EPSP amplitude was plotted against [symbol: see text] ICa, suggesting that transmitter release during a depolarization depends only upon calcium current and not upon a subsequent voltage-dependent step. 4. The synaptic transfer function of various measurements of EPSP or IPSP amplitude vs. [symbol: see text]ICa evoked during a presynaptic depolarization was a power function having an exponent of about 3. Similar measurements of EPSP amplitude vs. [symbol: see text]tail ICa evoked following a presynaptic depolarization had an exponent of about 2. 5. Facilitation of an EPSP or IPSP was not due to increases in calcium current at the test depolarization. 6. When the conditioning depolarization was increased and the test depolarization remained constant, EPSP amplitude at the test depolarization and facilitation increased . When the conditioning depolarization remained constant and the test depolarization was increased, EPSP amplitude at the test depolarization increased, while facilitation decreased. 7. Our data suggested that transmitter release at crayfish neuromuscular junctions is a non-linear function of calcium influx, and that facilitated release utilizes intracellular calcium differently from non-facilitated release. These data contradict simple models of facilitation which combine the residual calcium hypothesis with the calcium co-operativity hypothesis of non-facilitated release.

L-Glutamate-induced responses in OFF-type bipolar cells of the cat retina

L-Glutamate (Glu)-induced current responses were studied in 119 isolated OFF-type bipolar cells of the cat retina. Cells were recorded by the patch clamp technique in the whole-cell configuration. Glu induced a current carried by alkali metal ions and divalent cations with a permeability ratio of PNa:PK:PCs:PCa = 1:0.94:1.32:0.57. Sensitivity to Glu was highest in the dendritic region. Kainate and AMPA worked as potent agonists, but neither APB, L-aspartate, ACPD, nor NMDA (all at 100 microM) was effective. The Glu-induced response was antagonized by > 1 microM CNQX. We inferred that OFF-type bipolar cells have a non-NMDA receptor channel that is permeable to alkali metal ions with low selectivity, but not NMDA receptor or metabotropic Glu receptor.

Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons

In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

GABA-gated anion channels in intact crayfish opener muscle fibres and stretch-receptor neurons are neither activated nor desensitized by glutamate

The influence of glutamate on the GABA-activated Cl- conductance was studied in the slowly adapting stretch-receptor neuron and dactylopodite opener muscle fibre of the crayfish (Astacus astacus) using a two-microelectrode and a three-microelectrode voltage clamp, respectively. Glutamate (0.5-1.0 mM) had no effect on the GABA-activated conductance in either preparation. This indicates that the availability of the inhibitory channels for activation of GABA is not influenced by glutamate. The present results are in sharp contrast to those obtained by Franke et al. (J Comp Physiol A 159:591-609, 1986) in experiments on excised membrane patches, which suggested that glutamate is capable of both activating and desensitizing inhibitory postsynaptic channels in the crayfish opener muscle fibre.

Effects of a high molecular weight toxin from Physalia physalis on glutamate responses

The effects of a high molecular weight toxin from Physalia physalis (P3) were investigated on glutamate evoked potentials in snail (Zachrysia guanesis) neurons and in crayfish (Cambarus clarkii) neuromuscular junction. The glutamate evoked potentials of snail neurons were reversibly blocked by P3 in a dose-dependent manner (2-200 microM). A reversible blocking action was also found for P3 on excitatory junctional potentials and on glutamate potentials of crayfish at a concentration range of 6 nM-60 microM. Experiments carried out with independent stimulation of the excitatory and inhibitory nerves showed that the effect of P3 (60 nM-10 microM) was exerted predominantly on excitatory junctional potentials. However, at higher doses (greater than 10 microM) a slight reduction of the inhibitory potentials was also observed. These results suggest that P3 reversibly blocks glutamate receptors. Thus, it could be a promising tool for further studies on glutamatergic transmission.

Ionic permeabilities of L-glutamate activated, excitatory synaptic channel in crayfish muscle

Excitatory single channel currents triggered by L-glutamate were measured in outside-out excised patches of crayfish muscle membrane. If an 'intracellular' solution was present in the pipette and normal extracellular solution with added glutamate (10(-3) M) passed the outside of the patch, the single channel currents, i1, had amplitudes of -8 pA at a patch potential of -70 mV. If in the extracellular solution Na+ was replaced by Li+ or Ca2+, the amplitudes of single channel currents were reduced by about 30%. Only about 20% of the channel current amplitude remained on replacement of Na+ by choline. Replacement of Na+ reduced the variance of channel amplitude distributions to the level of the baseline. Presence of Na+ thus induces an additional variance of open channel current. When the proportions of Na+/choline were varied, the resulting channel currents could be separated in Na+, Ca2+ and choline components. The amplitude of the Na+ component, i1,Na, could be described by a constant channel permeability pi Na = 110 10(-15) cm3 s-1 according to the constant field equation. Ba2+ could replace Ca2+ without change in single channel current, while replacement of Ca2+ by Mg2+ reduced the channel currents by 20%. The following permeabilities of the single channel were estimated (in 10(-15) cm3 s-1): pi Na = 110, pi K = 86, pi Ca = 30, pi Mg = 24, pi Ba = 30, pi Li = 84 and pi choline = 11. These permeabilities were obtained inserting ionic concentrations. The respective permeabilities are listed also as calculated on the basis of ionic activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate opens Na+/K+ channels in cultured astrocytes

Glial cells from different brain regions and species are depolarized by the neurotransmitter glutamate. The depolarization or, if voltage-clamped at the resting membrane potential, the inward current induced by glutamate could be due either to activation of receptor-coupled ion channels or electrogenic uptake of the transmitter. In the present study we applied the patch-clamp technique in the whole-cell recording mode to analyze glutamate-induced currents in cultured astrocytes from rat cerebral hemispheres. At the resting membrane potential, glutamate induced an inward current ranging from 40 to 300 pA. This current decreased in size with depolarization and reversed at about 0 mV. The resulting current-to-voltage curve was linear and depended strongly on the transmembrane Na+ but not on the Ca++ or Cl- gradient. In the presence of glutamate, current noise increased at potentials positive or negative from the reversal potential indicating that ionic channels are activated by glutamate. Both kainate and quisqualate mimicked the effect of glutamate. We conclude that glutamate opens a Na+/K+ channel in cultured astrocytes because of activation of a receptor which shares many properties with the neuronal kainate/quisqualate receptor.

Calcium dependent gating of the L-glutamate activated, excitatory synaptic channel on crayfish muscle

Excitatory, glutamate-activated single channel currents were measured in outside-out patches of crayfish muscle. The open times of single channel openings, and the durations and rates of bursts were evaluated. These kinetic parameters were not appreciably affected by replacement of extracellular Na+ by Li+ or choline. Changes in extracellular Ca2+ concentration Cao also did not influence the duration of single openings. However the mean burst duration decreased for Cao less than 13.5 mM and the rate of bursts declined with a power of almost 2 in low Cao. At Cao less than 1 mM practically no channel openings were observed in presence of glutamate. In order to exclude more rapid desensitization of the glutamate receptors in low Cao as the cause of disappearance of channel openings, glutamate was applied in short pulses with a liquid-filament switch. In 0 Cao also a glutamate pulse did not trigger channel openings. In presence of 13.5mM Cao, the inorganic Ca-channel blockers La3+ and Cd2+ diminished the duration and rate of bursts of channel openings in a similar manner as low Cao. The effects of low Cao and of Cd2+ were tested also on quantal postsynaptic currents, EPSCs, which were recorded through a perfused macro-patch-clamp electrode. At 1.4 mM Cao in the perfused electrode tip, spontaneous EPSCs were reduced at least by a factor of 4, and elicited EPSCs by a factor of 16. Application of Cd2+ had similarly strong effects on the EPSCs. Also the decay of EPSCs was shortened substantially in 1.4 mM Cao or 5 mM Cd2+. The inhibitory Cl(-)-channel of crayfish muscle, activated by glutamate or GABA, also was studied in outside-out patches. The openings of this channel persisted in 0 Cao solutions; the block of channel openings in low Cao thus is a specific property of the excitatory channel. The action of Cao on the excitatory channel may be described as that of a cofactor to glutamate. A possible reaction scheme is proposed.

Glutamate receptors expressed in Xenopus oocyte by messenger RNA from invertebrate muscle

Glutamate induced responses in the surface membrane of Xenopus oocyte by injection with mRNA from crustacea and insect muscle. Glutamate (10(-5) to 10(-4) M) elicited smooth current responses which resembled those induced by kainate. This smooth current reversed direction at about 0 mV and was blocked by a spider toxin (JSTX). Higher concentrations (more than 1 x 10(-3) M) of glutamate induced JSTX-insensitive oscillatory current responses which may be caused by Cl- activation.

Calcium dependence of presynaptic calcium current and post-synaptic response at the squid giant synapse

1. Neurotransmitter release has a non-linear dependence upon the external Ca concentration, [Ca]o. This may be due to a 'co-operative' action of Ca in triggering release. The dependence of presynaptic Ca currents and post-synaptic currents (p.s.c.s) upon [Ca]o was examined at voltage-clamped 'giant' synapses of squid to determine whether this 'co-operativity' occurs during or after influx of Ca into the presynaptic terminal. 2. Presynaptic Ca current was proportional to [( Ca]o/(1 + [Ca]o/KD]n, where n, the order of the function, was roughly 1 and KD, the apparent dissociation constant for Ca, was approximately 80 mM. 3. P.s.c.s also could be described by the same function, but had an n of 3-4 and a lower KD. 4. These results suggest that the 'co-operative' action of Ca occurs at a step or steps beyond entry of Ca into the presynaptic terminal. 5. Synaptic transfer curves relating presynaptic Ca currents, elicited by depolarizations to different potentials, to resultant p.s.c.s were power functions whose exponent depended upon [Ca]o. Maximum exponents were as high as 4 at [Ca]o of 3 mM. The dependence of these curves upon [Ca]o helps to explain why previous determinations, which were performed at a variety of [Ca]o levels, yielded a variety of transfer curve exponent values. 6. Transfer curves generated from responses to constant presynaptic depolarizations, with Ca current varied by [Ca]o changes, also were power functions with exponents of approximately 4. Thus p.s.c.s were high-exponent power functions of Ca current regardless of whether Ca current was modified by changes in membrane potential or in [Ca]o.

A new potent channel blocker: effects on glutamate responses at the crayfish neuromuscular junction

The glutamate blocking action of 5-methyl-1-phenyl-2-(3-piperidinopropylamino)-hexane-1-ol (MLV-5860) was studied at the crayfish neuromuscular junction using electrophysiological techniques. The opener muscle of the dactyl in the first leg of the crayfish was used to examine the action of the drug on the glutamate response. MLV-5860 reduced the amplitude of repetitively-induced glutamate potentials in a dose-dependent manner at the crayfish neuromuscular junction and this reduction was time- and activity-dependent. The minimum effective concentration of MLV-5860 to reduce the glutamate response was estimated to be lower than 50 nM, and therefore MLV-5860 is the most powerful glutamate blocker known at the crayfish neuromuscular junction. Pretreatment of the muscle fiber with concanavalin A did not affect the action of MLV-5860. MLV-5860 reduced the amplitude of excitatory junctional potentials (EJPs) and increased the decay rate of extracellularly-recorded EJPs in a dose-dependent manner. Quisqualate responses were also reduced by this drug but the conductance increase of the muscle membrane induced by GABA was not affected. MLV-5860 did not cause a significant change in the input resistance of the opener muscle fiber at concentrations less than 10 microM. The action of the drug is possibly explained in part by the open channel block of the glutamate-activated ion channels. The forward rate constant for channel blockade was estimated from the difference between the decay rate constants of extracellular EJPs in the absence and presence of the drug and the estimated value was 6.5 +/- 1.4 X 10(7) M-1 s-1.

Effect of calcium ions on the sensitivities of cultured cerebellar neurons to glutamate and aspartate

Iontophoretically applied glutamate and aspartate induced depolarizations in immature (6-13 days in culture) and mature (25-45 days) cultured chick cerebellar neurons, immature neurons being less sensitive. The input resistances of the neurons were variously changed by these amino acids. Reversal potentials of the depolarizations induced by both amino acids were similar in either immature or mature neurons. The population of amino acid-sensitive neurons increased with maturation. In mature neurons, the amplitude of glutamate- or aspartate-induced depolarization was decreased by addition of 10 mM Ca2+ to normal Tyrode's solution, aspartate responses being decreased more greatly. In low-Na+ solution (2.7 mM), however, high Ca2+ significantly enhanced amino acid-induced depolarizations. In immature neurons, on the other hand, the amplitude of glutamate- or aspartate-induced depolarization was drastically and consistently increased when 10 mM Ca2+ was added either to normal solution or to the low-Na+ solution. These enhancing actions of Ca2+ were abolished by Mn2+, but only partially by 10 mM glutamic acid diethylester or 1 mM D-alpha-aminoadipate, though responses to both amino acids in normal solution were blocked by these antagonists at the same concentrations. These results suggest that calcium ions enhance the effect of glutamate and aspartate in immature neurons, possibly by interacting with the ionophores involved in amino acid responses.

Excitatory transmitter release induced by high concentrations of gamma-aminobutyric acid (GABA) in crayfish neuromuscular junctions

At the neuromuscular junction of very small crayfish (0.4-2 g) addition of gamma-aminobutyric acid (GABA) to the superfusing solution at concentrations exceeding 100 mmol/l elicited high frequency release of excitatory transmitter quanta. In seven experiments single application of 500 mmol/l GABA gave rise to instantaneous release of 70,000 to 130,000 quanta. These stores of transmitter were released by GABA in a first order process with time constants, tau q, of between 9 s and 20 s, the maximum rate of release, ñ0, reaching 10,000 quanta/s in some cases. After release had ceased in the presence of GABA, the preparation was allowed to recover for five minutes in normal solution. Subsequently, a second trial evoked about 50% of the release induced during the first application of GABA. Pretreatment of the preparation with 2 mumol/l serotonin (5-HT) facilitated GABA-induced transmitter release resulting in larger rates of release and consequently in a larger output of transmitter by a factor of about 3. The largest amount of transmitter released on a single application of GABA in the presence of serotonin comprised about 220,000 quanta with a maximum rate of release ñ0 approximately equal to 25,000 quanta/s. The release evoked by high GABA-concentrations did not depend markedly on extracellular Ca2+ or Mg2+, but required extracellular Na+. The effects induced by high concentrations of GABA on release of excitatory transmitter quanta were quantitatively similar to the effects of high glycine-concentrations on release of quanta from the inhibitory terminals (Finger 1983a, b).

TI-233 as a glutamate channel blocker at the crayfish neuromuscular junction

Effects of TI-233 (4-isopropyl-1-[N2-(5,6-dimethyl-aminonaphthalene-1-sulphonyl)-L-arginyl ]- piperidine) on glutamate-induced responses and nerve-evoked synaptic responses were compared at the crayfish neuromuscular junction. Intracellularly recorded excitatory junctional potentials (e.j.ps) were markedly augmented by TI-233 when they were evoked at long intervals, whereas the unit size of extracellular e.j.ps was hardly affected by TI-233 and, at that stage, the glutamate-induced current was markedly reduced by TI-233. The decay rate of extracellular e.j.ps was slightly increased 3 min after the addition of TI-233 at concentrations higher than 0.05 mM. Repetitive stimulation of the excitatory axon at a high frequency caused a gradual decrease in the amplitudes of extracellular e.j.ps in the presence of TI-233. After prolonged application of TI-233 with repetitive nerve stimulation, the glutamate-induced response became significantly smaller than the control. TI-233 increased the input resistance of the crayfish muscle fibre and facilitated transmitter release at the excitatory neuromuscular junction. These two effects would entirely explain the augmentation of intracellular e.j.ps by TI-233. TI-233 (greater than 3 microM) reduced the amplitude of current responses to trains of glutamate pulses in a dose-dependent manner, but this reduction by TI-233 was time- and activity-dependent. The effect of TI-233 on glutamate-induced responses was voltage-dependent and hyperpolarization increased this effect. Pretreatment of the muscle fibre with concanavalin A did not affect the gradual decline, caused by TI-233, of the successive currents evoked by a train of glutamate pulses. The apparent differences between the glutamate-induced current and nerve-evoked synaptic response revealed by TI-233 can be explained by open-channel block of the glutamate-activated ion-channel, and do not confute the hypothesis that glutamate is the natural transmitter substance at this junction.

Pharmacological evidence for L-aspartate as the neurotransmitter of cerebellar climbing fibres in the guinea-pig

Climbing fibre responses (c.f.r.s) evoked by white matter stimulation and the depolarizations induced by iontophoretically applied L-glutamate and L-aspartate were recorded intracellularly from the proximal dendrites of Purkinje cells in in vitro slice preparations of the guinea-pig cerebellum. Short pulses of L-glutamate and L-aspartate dose-dependently depolarized the Purkinje cell dendrite. Even small doses of these amino acids reduced the input resistance. The maximum decrease in input resistance induced by L-glutamate was 36% and that by L-aspartate was 38%. Intracellular injection of Cs+ allowed Purkinje cell dendrites to be depolarized to a range of -15 to +30 mV. The mean reversal potential for the c.f.r. (Ec) was found to be +10.2 mV (n = 4). The mean reversal potentials obtained for L-glutamate (Eg) and for L-aspartate (Ea) were +7.3 mV (n = 7) and +5.6 mV (n = 7) respectively. When external Na+ concentration was reduced, Ec, Ea and Eg were linearly and similarly shifted in the negative direction, indicating that all these reversal potentials are determined primarily by a Na+ conductance. The effects of the glutamate antagonists 2-amino-5-phosphonovaleric acid (APV), gamma-D-glutamylglycine (gamma-DGG), N-methyl-DL-aspartic acid (NMDLA) and glutamic acid diethylester (GDEE) were compared as to the responses to L-glutamate and L-aspartate and Ca2+-activated focal climbing fibre responses (c.f.c.f.r.s) in order to investigate the receptor type at the synapses formed by the climbing fibres with Purkinje cell dendrites. The order of antagonistic potency to the c.f.c.f.r. was : APV (mean percentage blockade = 99%) greater than gamma-DGG (87%) greater than NMDLA (71%) greater than GDEE (28%). The order of antagonistic potency to the response to L-aspartate was: gamma-DGG (69%) greater than APV (66%) greater than NMDLA (60%) greater than GDEE (31%), and that to the response to L-glutamate was: GDEE (63%) greater than NMDLA (22%) greater than gamma-GDD (15%) greater than APV (14%). APV was found to be the most effective anatagonist of the c.f.c.f.r. Its action was reversible, selective for L-aspartate-induced depolarization and had no effect on the responses to L-glutamate. NMDLA, which has no activity as an agonist, was a greater suppressant of the responses to L-aspartate than those to L-glutamate. These electrophysiological and pharmacological findings suggest that the receptor for the transmitter at the synapses formed by climbing fibres with Purkinje cell dendrites is of the L-aspartate-preferring type, and are thus consistent with the bio-and histochemical findings that L-aspartate may be the endogenous transmitter at this synapse.

Permeability changes induced by L-glutamate in solitary retinal horizontal cells isolated from Carassius auratus

Solitary horizontal cells isolated from goldfish retinae are depolarized by L-glutamate (Glu) (Ishida, Kaneko & Tachibana, 1984), a possible candidate for the transmitter of photoreceptors. The underlying mechanisms were analysed under voltage-clamp conditions using 'giga-seal' suction pipettes in the whole-cell recording configuration. Glu induced an inward current at the resting membrane potential (ca. -57 mV). Membrane depolarization decreased the amplitude of Glu-induced current and reversed its polarity to outward beyond approximately -3 mV. Membrane hyperpolarization below the resting potential decreased the amplitude of the Glu-induced inward current. When a K current through the anomalous rectifier, which is activated by membrane hyperpolarization (Tachibana, 1983), was blocked by Cs ions, this phenomenon disappeared and the Glu-induced current increased in amplitude with hyperpolarization. Mg ions had no effect on the reduction of the Glu-induced current at hyperpolarized potentials. It was strongly suggested that Glu produced two types of conductance change; a conductance increase due to an activation of Glu channels and a conductance decrease due to a blockage of the K current through the anomalous rectifier. The latter effect is analysed in detail in the following paper (Kaneko & Tachibana, 1985b). The Glu-activated channel was permeable to cations (Na, K, Ca, Mg, Tris and choline ions) with low selectivity, but not to anions. The least effective dose of Glu was less than 10 microM. The relation between the Glu-induced current and the membrane potential curved upwards near the reversal potential, and this relation was not affected by Mg ions.

Effects of caroverine and diltiazem on synaptic responses, L-glutamate-induced depolarization and potassium efflux in the frog spinal cord

The frog spinal cord was used to determine the characteristics of the actions of caroverine and diltiazem, two organic Ca2+-antagonists, on synaptic responses and L-glutamate-induced depolarization. Caroverine and diltiazem (10(-4)M) depressed the dorsal root potential (DR-DRP) induced by electrical stimulation of an adjacent dorsal root. Diltiazem also depressed the ventral root potential (DR-VRP), whereas caroverine augmented both the polysynaptic component in the ventral root reflex and the size of the DR-VRP. The root potentials induced by high frequency stimulation (20 Hz, for 1 s) were markedly depressed by these Ca2+-antagonists at a concentration of 10(-4)M. When the preparation was perfused with normal medium, the compounds depressed L-glutamate-induced depolarizations in ventral and dorsal roots. In preparations treated with tetrodotoxin (TTX) (2 X 10(-7)M), the antagonizing actions of the drugs against L-glutamate-induced depolarizations in the ventral root were markedly reduced or abolished, while significant antagonizing actions on the depolarization in the dorsal root were still observed. The increase in extracellular K+ activity induced by L-glutamate in the TTX-treated preparation was significantly reduced by the compounds. Caroverine and diltiazem had no effect on the presynaptic nerve spike and on the focal synaptic potential induced by a single stimulation of a dorsal root; however, the focal synaptic potential induced by high frequency stimulation (20 Hz, 1 s) was attenuated. Motoneuronal action potentials were abolished by the drugs, while the excitatory postsynaptic potential remained unaffected. 9 The present results suggest that caroverine and diltiazem are not specific L-glutamate antagonists in the frog spinal cord, but that they block the initiation of an action potential without affecting presynaptic nerve conduction, transmitter release or transmitter-receptor interactions. The inhibitory effects of these compounds on L-glutamate-induced K+-efflux are discussed with reference to their Ca2+-antagonizing actions.

The effect of calcium ions on the glutamate response and its desensitization in crayfish muscle fibres

The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponentials, are affected by the external Ca2+ concentration. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization. Con A causes a partial blockade of the glutamate response. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.

Modulation of neuronal responses to L-glutamate in Aplysia

Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp), similar to that which has been described at the crustacean neuromuscular junction, is observed in Aplysia neurons which are glutamate sensitive. Potentiation of the inhibitory responses to ionophoretically applied Glu in neurons preconditioned with Asp permits experiments which serve to differentiate among four hypotheses previously proposed to explain the underlying mechanism of the phenomenon. The potentiation is inhibited by cooling (Q10 = 1.3 +/- 0.2) and is blocked in Na+-free seawater, where the response to Glu applied alone is increased in both amplitude and duration. These results are most consistent with the view that Glu is normally removed from the extracellular medium through an active reuptake process which is Na+ dependent, is slightly temperature sensitive, and may be blocked by Asp. Potentiation of the excitatory response to L-glutamate (Glu) by L-aspartate (Asp) has been previously described at the crustacean neuromuscular junction (Kravitz et al., 1970; Nistri and Constanti, 1979). This potentiation has been attributed to an Asp-induced change in conformation of the Glu receptor, thereby increasing its affinity for Glu (Shank and Freeman, 1975); suppression of the rate of desensitization of the Glu receptor induced by Asp (Dudel, 1977); blockade by Asp of a Glu reuptake process (Crawford and McBurney, 1977); and release, triggered by Asp, of a bound store of Glu (Constanti and Nistri, 1978).(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic effect of streptomycin on the insect neuromuscular junction

The effect of streptomycin on the neuromuscular junction of the cockroach leg muscle was studied by means of intracellular electrodes. The miniature excitatory postsynaptic potential (MEPSP) frequency decreased in the presence of 1 mM streptomycin. Double logarithmic plots of the quantal content against the external calcium concentration in the absence and presence of streptomycin suggested the competition for a common site of presynaptic membrane at insect neuromuscular junctions.

Postsynaptic actions of ethanol and methanol in crayfish neuromuscular junctions

Actions of ethanol and methanol on excitatory postsynaptic channels activated by quisqualate were investigated in opener muscles from the first walking leg and the claw of crayfish. Both ethanol and methanol reduced the elementary currents [i] that flow through channels operated by quisqualate in a concentration-dependent manner but did not affect the apparent mean open time, tau noise, of the channels estimated from power spectra. 0.26 mol/l ethanol, or 1 mol/l methanol, respectively, reduced [i] e-fold. Ethanol also markedly decreased the size and the decay time constant tau (sEPSCs) of spontaneous excitatory postsynaptic currents (sEPSCs). At ten fibres, on the average, 0.26 mol/l ethanol decreased tau (sEPSCs) by a factor 1.56 +/- 0.24 (SD). tau (sIPSCs) and tau noise of inhibitory postsynaptic currents apparently were not affected by ethanol. Moreover the size of elementary inhibitory postsynaptic currents did not decrease in the presence of this alcohol. Thus, in crayfish opener muscles ethanol seems to selectively depress excitatory postsynaptic currents.

Ionic mechanism of a hyperpolarizing glutamate effect on two identified neurons in the buccal ganglion of Aplysia

The ionic mechanism of a membrane effect of L-glutamate on two identified neurons in the buccal ganglion of Aplysia kurodai was investigated with conventional microelectrode techniques and glutamate iontophoresis. Bath-applied and iontophoresed glutamate hyperpolarized the membrane and increased the membrane conductance. The hyperpolarizing glutamate response decreased in amplitude and finally reversed its polarity by conditioning hyperpolarization. The reversal potential of the hyperpolarizing glutamate response was close to the ECl (-60 mV). The reversal potential changed by 22.4 mV when the external chloride concentration was altered by a factor of 5. The relationship between the iontophoretically applied current and the membrane conductance changes was suggestive of two glutamate molecules reacting with a single receptor site. The hyperpolarizing glutamate response was essentially unaffected by 2-amino-4-phosphonobutyric acid (2-APB), L-proline, and quinuclidinyl benzilate (QNB). It was concluded that the hyperpolarizing glutamate response was generated by an activation of Cl- conductance.

Excitatory junctional responses and glutamate responses at the crayfish neuromuscular junction in the presence of chlorisondamine

At the crayfish neuromuscular junction chlorisondamine reduced the amplitude of both the excitatory junctional potential and the glutamate current in a dose-dependent manner in concentrations above 3 microM, and it is suggested that the drug is a powerful non-competitive antagonist for glutamate. Chlorisondamine did not act presynaptically on the crayfish neuromuscular junction. A double exponential decay of excitatory synaptic currents was observed in the presence of chlorisondamine, suggesting that this drug is an open channel blocker for the excitatory neurotransmitter. The glutamate current tail was prolonged in the presence of chlorisondamine. This prolongation increased with increasing iontophoretic current of glutamate. The rate of recovery from the refractory form of the glutamate receptor to the free reactive one was hardly affected by chlorisondamine. The inhibitory action of chlorisondamine on glutamate responses was voltage-dependent and hyperpolarization reduced the drug action. Chlorisondamine depressed the glutamate current even in Na-free, Ca-rich solution.

Permeability changes induced by L-glutamate at the crayfish neuromuscular junction

Two different methods are described which allow the reversal potential (Er) for the channels opened by L-glutamate at the voltage-clamped, crayfish neuromuscular junction to be measured accurately. In both cases the value of Er was found to be about +6 mV. Reversal potentials were also measured in solutions where Na+ was replaced by K+, Ca2+, or Mg2+; or in which Cl- was replaced by isethionate. In solutions where Na+ was partially replaced by K+, the measured reversal potentials were compared to theoretical values predicted by both the constant-field and equivalent-circuit equations. The experimental values were more accurately described by the constant-field equation. Permeability ratios (PX/PNa) for K+, Ca2+, Mg2+, and Cl- were calculated using the constant-field equation. K+ and Na+ were equally permeant while Ca2+ and Mg2+ were about half as permeant as the monovalent cations. Cl- was impermeant. The results of these experiments indicate that the L-glutamate activated channel is non-selective for cations. Furthermore, the value of the permeability ratios for the physiological cations tested are very similar to those obtained for the acetylcholine activated channel in vertebrate skeletal muscle.

Trimethaphan as a glutamate inhibitor at the crayfish neuromuscular junction

At the crayfish neuromuscular junction trimethaphan reduced the amplitude of both the glutamate-induced synaptic current and the excitatory junctional current in a dose-dependent manner at concentrations greater than 5 microM. These effects were dependent on membrane potential. Trimethaphan did not affect the inhibitory junctional potential and the input resistance of the opener muscle. The dose-response curves for inhibition of glutamate responses by trimethaphan suggest that trimethaphan is not a competitive glutamate antagonist. A quantum analysis of extracellularly recorded excitatory junctional potentials showed that trimethaphan decreased both quantum content and average unit size. Trimethaphan also prolonged the glutamate currents evoked by both short and prolonged ionophoretic currents, but the decay of nerve-evoked synaptic currents was accelerated by the drug. Three explanations worthy of consideration to explain the action of trimethaphan are the responses of extra-junctional receptors, the sudden release and short actions of the neurotransmitter in contrast with the progressive application and long exposure of exogenous agonists to receptors, and discrimination of glutamate and excitatory transmitter in the crayfish neuromuscular junction. The second of these possibilities is mainly discussed at length.

Effects of a spider toxin on the glutaminergic synapse of lobster muscle

We studied the effect of neurotoxin (JSTX) separated from spider venom on the lobster neuromuscular junction. JSTX selectively suppressed excitatory post-synaptic potentials (e.p.s.p.s) without affecting the inhibitory post-synaptic potentials (i.p.s.p.s). The effect of JSTX was dose-dependent. The threshold dose for suppressing e.p.s.p.s corresponded to a small fraction of the toxin amount in a venom gland. At high concentration, JSTX irreversibly blocked e.p.s.p.s. The reduction in amplitude of extracellularly recorded e.p.s.p.s after JSTX application followed an exponential time course. The rate of suppression increased proportionally with the toxin concentration. JSTX blocked the glutamate potential in the post-synaptic membrane but it failed to affect the aspartate-induced depolarization. Kainic acid potentiated the glutamate-induced depolarization but it was without effect in the presence of JSTX. Depolarization produced by quisqualic acid is suppressed by the toxin. Our results suggest that the spider venom contains specific blockers of glutamate receptors in crustacean neuromuscular junctions.

The ionic mechanism of the excitatory action of glutamate upon the membranes of motoneurones of the frog

Simultaneous intra- and extracellular recordings with K+, Na+, Ca2+, and Cl- sensitive microelectrodes were performed in motoneurones of the spinal cord of the frog during depolarizations mediated by glutamate (GLUT) and by experimentally increased extracellular K+. Depolarization resulting from increased K+ activity (alpha K+) in the bathing solution evoked a decrease of intracellular Na+ activity (alpha Na+i); a transient increase of alpha Na+i accompanied by a decrease of alpha Na+e was observed during the depolarization induced by GLUT. Both modes of depolarization led to an increase of alpha Cl-i and a concomitant decrease of alpha Cl-e. An experimental increase of alpha K+e led to a threshold dependent increase of alpha Ca2+i by at least one order of magnitude and to an equally threshold dependent strong decrease of alpha Ca2+e. The threshold of these changes of alpha Ca2+ was at a membrane potential of -25 mV. During a depolarization of half the amplitude induced by GLUT a comparable increase of alpha Ca2+i and a smaller decrease of alpha Ca2+e were observed. The GLUT mediated changes of alpha Ca2+ were not threshold dependent and occurred synchronously with the onset of depolarization. A transient decrease of alpha K+i and a parallel strong increase of alpha K+e occurred during the GLUT induced depolarization. Depolarization evoked by an experimental increase of alpha K+e led to an increase of alpha K+i. The observed changes in the ionic composition of the intra- and extracellular fluids indicate that GLUT evokes an increase in membrane permeability to Na+ and Ca2+ and a subsequent influx of these ions into motoneurones, while the inward shift of Cl- and the outward shift of K+ are presumably passive. A voltage dependent Ca2+ influx is triggered at -25 mV membrane potential.

Selective stimulation of neurotransmitter release from chick retina by kainic and glutamic acids

The excitatory action of kainic and glutamic acids in chick whole retina was demonstrated as an immediate stimulation of the release of labeled gamma-aminobutyric acid (GABA) and glycine in a superfusion system. This stimulatory effect was 3-10 times greater than that produced by a depolarizing K+ concentration; in addition, it was independent of Ca2+ in the medium, but notably inhibited when Na+ was omitted from the medium. Under identical experimental conditions, neither kainic nor glutamic acid had any effect on the release of labeled dopamine or alpha-aminoisobutyric acid, thus indicating that their effect is not unspecific or due to cell damage. Similar although less marked stimulation of labeled GABA and glycine release by kainic acid was obtained in subcellular retinal fractions, particularly in fraction P1, which contained photoreceptor terminals and outer segments. This stimulation was also Ca2+ independent and greatly reduced when Na+ was omitted from the medium. It is suggested that the stimulation of GABA release by kainic and glutamic acids is probably due to a Na+-dependent, carrier-mediated mechanism that responds to the entry of Na+ produced by the interaction of glutamic and kainic acids with retinal membranes. In cortical or striatal slices from mouse brain, these acids had a negligible stimulatory effect on GABA and dopamine release.

Concanavalin A blocks the Ca2+ -dependence of crayfish muscle fiber responses to glutamate

(1) The response of crayfish muscle fibers to bath-applied glutamate is strongly inhibited when the Ca concentration of the physiological solution is reduced. Other divalent cations cannot substitute for Ca. The trivalent impermeant cation La can at low concentration replace Ca. Moreover, decreasing the Ca concentration in the presence of La potentiates the glutamate response. (2) The time course of responses to ionophoretically applied glutamate suggests a faster desensitization in low Ca solutions. The lectin concanavalin A, which blocks desensitization, also eliminates the decrease of the glutamate response in low Ca solutions. (3) The above results are compared to available data concerning Ca-dependence, desensitization and effects of concanavalin A.

Serotonin-induced depolarization of rat facial motoneurons in vivo: comparison with amino acid transmitters

Intracellular recordings were obtained from facial motoneurons in anesthetized rats. The effects of iontophoretically applied serotonin were compared to those of the excitatory amino acids glutamate and DL-homocysteic acid (DLH), and the inhibitory amino acids, glycine, GABA and muscimol, under various conditions of membrane polarization and intracellular chloride concentration. Iontophortically applied serotonin caused a depolarization of facial motoneurons which was accompanied by increased input resistance and increased neuronal excitability. Experiments comparing the response to serotonin with those of glycine, GABA, and muscimol demonstrated that the serotonin effect does not involve changes in membrane conductance to chloride. Comparisons of serotonin with glutamate and DLH at varying levels of membrane hyperpolarization indicated that the serotonin-induced depolarization is not caused by increased conductance to sodium or calcium, and differs in its underlying ionic mechanism from depolarizations induced by glutamate and DLH. Results were consistent with the hypothesis that serotonin causes depolarization, increased input resistance, and increased excitability in rat facial motoneurons by decreasing resting membrane conductance to potassium ions. Such changes in motoneurons in the brain stem and spinal cord probably account for some of the physiological and behavioral effects observed during pharmacological activation of serotonin receptors.

Properties of miniature excitatory junctional currents at the locust nerve-muscle junction

1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na(+) (i.e. in normal medium, Ca(2+)-free medium and Cl(-)-free medium). M.e.j.c.s were also examined in isotonic Ca(2+) where the inward post-synaptic current was carried mainly by Ca(2+).2. In normal medium, mean m.e.j.c. amplitude = 2.34+/-0.05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) was tau(m.e.j.c.) = 2.62+/-0.11 msec (V(m) = -80 mV, T = 22 degrees C). Decay-time was not markedly changed in Ca(2+)-free or Cl(-)-free medium. tau(m.e.j.c.) approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. tau(m.e.j.c.) decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182.+/-24.8 mV change in V(m).5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca(2+)-free and Cl(-)-free solutions.6. The kinetics of junctional channels are altered in isotonic Ca(2+). M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j.c. = 0.74+/-0.03 nA. The decay time becomes markedly briefer, tau(m.e.j.c.) = 1.01+/-0.08 msec, indicating a reduction in mean channel life-time (V(m) = -80 mV, T = 22 degrees C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca(2+), thus producing a prolongation in mean tau(m.e.j.c.). This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of tau(m.e.j.c.) of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca(2+). The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

Glutamate inhibitors in the crayfish neuromuscular junction

1. The effects of chlorisondamine and TI-233 on the crayfish neuromuscular junction were investigated in order to compare the action of glutamate with that of the excitatory transmitter. 2. The glutamate-induced synaptic current was inhibited by both of these two drugs. Excitatory junctional potentials were significantly reduced by chlorisondamine, whereas they were increased by TI-233. 3. It is suggested that chlorisondamine and TI-233 are powerful non-competitive antagonists for glutamate. 4. A quantum analysis of extracellular EJPs demonstrated that chlorisondamine did not possess presynaptic action in the crayfish neuromuscular junction. Chlorisondamine shortened the decay phase of extracellular EJPs, and the decay was frequently fitted by a double exponential in relatively low concentrations. 5. Semilogarithmic plots of the decay phase of the glutamate current evoked by a short glutamate pulse were nearly linear, but they shifted from linearity to some extent in the presence of chlorisondamine, showing prolongation of the glutamate current tails. 6. When TI-233 was added to the bathing solution at a concentration of 0.1 mM, the quantum content of extracellular EJPs was increased by about two times, but the average unit size was not changed. 7. There was no change in the rise time and the decay phase of the glutamate potential in the presence of TI-233. 8. Pharmacological difference between glutamate responses and EJPs was revealed in the presence of chlorisondamine and TI-233. Unless this difference can be explicated with a reasonable explanation on the glutamate transmitter hypothesis, it is difficult to confirm that glutamic acid is an excitatory transmitter at the crayfish neuromuscular junction.

The action of serotonin on excitatory nerve terminals in lobster nerve-muscle preparations

1. The action of serotonin on excitatory transmission in the opener muscle of the dactyl of the lobster walking leg was examined by intracellular recording techniques. 2. Serotonin, at concentrations as low as 5 x 10(-9) M, caused a sustained increase in the size of the excitatory junctional (synaptic) potential (e.j.p.). When serotonin was washed out of the bath the e.j.p. declined in two steps (T 1/2 approximately equal to 1-2 min; T 1/2 approximately equal to 30 min) to the control size. The increased e.j.p. size was predominantly due to a serotonin-induced increase in the release of quanta of excitatory transmitter with nerve stimulation. 3. The increase in transmitter release did not require nerve stimulation or the presence of Na+ or Ca2+ ions in the bathing medium during the period of serotonin treatment. 4. Three types of experiments suggested that a part of the action of serotonin on excitatory nerve terminals might involve a long-term metabolic change within terminals, possibly involving the buffering or storage of Ca2+ ions. First, serotonin increased the frequency of spontaneous release of transmitter in both normal saline (26 mM-Ca2+) and saline with very low levels of Ca2+ (less than 10(-8) M). Secondly, serotonin greatly potentiated increases in miniature excitatory junctional potential frequency induced by the loading of the nerve terminal with Na+ either by veratridine or by inhibition of the Na+ pump or by the addition of the Na-ionophore monensin in low-Ca2+ salines. Thirdly, in some experiments, serotonin treatment produced a partial restoration of the nerve-evoked release of transmitter in the low-Ca2+ medium (less than 10(-8) M).

The role of calcium ion in the L-glutamate-induced depolarization in the frog spinal cord

1. The role of Ca2+ in L-glutamate-induced depolarization was investigated in the isolated frog spinal cord. 2. The size of a depolarization induced by L-glutamate (3 mM) was inversely related to the extracellular Ca2+ concentration, but was reduced in a Ca2+-free medium containing EGTA (0.3 mM). 3. L-Glutamate caused a marked depolarization in both ventral and dorsal roots, even in a NaCl-deficient medium (Ca2+, 2.0 mM). The size of the depolarization was attenuated by a prolonged or repeated application of L-glutamate. Ca2+ can be replaced by Sr2+ or Mg2+. 4. Concanavalin A (1 microM) prevents the development of desensitization to L-glutamate. 5. Present results suggest that Ca2+ plays the role of a charge carrier for L-glutamate-induced depolarization and of a regulator of modulator for L-glutamate-receptor sensitivity. The roles are exaggerated in NaCl-free medium.

Excitation of hippocampal pyramidal cells by glutamate in the guinea-pig and rat

1. The mechanism by which L-glutamic acid depolarizes hippocampal CA1 pyramidal neurones was investigated by using the in vitro slice and ionophoretic techniques. 2. Two types of responses were seen. One (in 85% of cells) consisted of spike discharges that outlasted the glutamate-induced depolarization. In the other (the rest of the cells), spikes were produced only during the rising phase of the depolarization. 3. The effect was highly localized; it disappeared when the ionophoretic electrode was moved vertically by as little as 20 micrometers. 4. The effect of glutamate persisted after synaptic transmission was blocked; this probably was due to a direct effect of glutamate on the cell membrane. 5. Small doses of glutamate produced either no change or an apparent increase in input resistance. With larger doses, the input resistance invariably decreased. The apparent increase in input resistance was not seen in cells treated with Mn2+ and TTX and is believed to be an effect of the depolarization rather than a direct effect of glutamate. 6. By extrapolation, the reversal potential for the glutamate response (EGlu) was found to -3.6 mV. 7. Following intracellular injection of Cs+, neurones could be depolarized to a range of +20 to +50 mV. The glutamate response could then be reversed. EGlu in these cells was -1.5 mV. 8. Using the Cs+-injection technique, it was also possible to reverse the e.p.s.p. E.e.p.s.p. was similar to EGlu. 9. When the external sodium concentration was reduced, the size of the glutamate response decreased, and EGlu became more negative.

The effect of reduced calcium on quantal unit current and release at the crayfish neuromuscular junction

Excitatory postsynaptic currents (EPSCs) were recorded extracellularly from large muscle fibers by means of 'patch clamp' electrodes. Compared to usual extracellular recordings, better signal/noise ratio and temporal stability were achieved. In the range of extracellular calcium concentrations [Ca]0 between 2.7 and 13.5 mmol/l (normal), the average amplitude of the EPSC increased more than proportional to [Ca]0. The unit quantum current, C1, and the average release rate, m, were determined from EPSCs and also from spontaneous sEPSCs, using both Poisson and binomial statistics. The main effect of [Ca]0 was on m: at different synaptic sites m depended on the second to fourth power of [Ca]0. In terms of binomial parameters, the release probability p is the [Ca]0-dependent one. In addition, reduction of [Ca]0 from 13.5 to 2.7 mmol/l decreased the unit quantum C1 consistently to 60%; simultaneously the rise and decay of EPSCs and sEPSCs were shortened by 10-20%. [Ca]0 thus has strong presynaptic effects on the release probability, but in addition smaller ones on the postsynaptic channel characteristics.

Synaptic excitation may activate a calcium-dependent potassium conductance in hippocampal pyramidal cells

In hippocampal CAl pyramidal cells, orthodromic synaptic excitation is followed by an early hyperpolarization mediated by gamma-aminobutyric acid (GABA) and a late non-GABA-mediated hyperpolarization that has properties consistent with an increase in potassium conductance. Depolarizations produced by iontophoretically applied glutamate are followed by hyperpolarizations that have features in accordance with an increase in potassium conductance. The hyperpolarizations are independent of chloride and resistant to tetradotoxin but are blocked by a low-calcium, high-cobalt medium. Voltage clamping the glutamate depolarization does not reduce the subsequent hyperpolarization, indicating that the hyperpolarization results from a direct increase in calcium conductance produced by glutamate, rather than from activation of voltage-sensitive calcium channels. A single transmitter, possibly acting on one type of receptor and channel, may initiate both excitation and inhibition in the same postsynaptic cell.

A glutamate-activated chloride conductance on a crustacean muscle

A glutamate-activated inhibitory response on a 'crustacean neuromuscular preparation that receives cholinergic excitatory innervation is described. Glutamate produced a dose-dependent conductance increase to C1-ions. The response was mimicked by ibotenic acid, but not by quisqualic acid, and was blocked by picrotoxin.

Determination of glutamic acid and gamma-aminobutyric acid in Ringer's solution without desalination at the femtomole level by gas chromatography chemical ionization mass spectrometry

For the quantification of glutamic acid in Ringer's solution, pentafluoropropionic methyl ester was the most sensitive derivative. The detectable concentration was 0.01 microM glutamic acid in Ringer's solution; the amount of the preparation was 1 pmol and the injection into a gas chromatograph mass spectrometer was 10 fmol. For the quantification of gamma-aminobutyric acid in Ringer's solution, the trifluoroacetal-hexafluoropropionyl ester was quantification of gamma-aminobutyric acid in Ringer's solution, the trifluoroacetal-hexafluoropropionyl ester was detectable at a concentration of 0.01 microM. Ringer's salts facilitated acylation in the order heptafluorobutyric anhydride greater than pentafluoropropionic anhydride greater than trifluoroacetic anhydride. The effect depended on esterification of carboxy groups in the order methyl ester greater than hexafluoropropionyl ester greater than butyl ester. Sodium carbonate, sodium acetate and sodium citrate also facilitated acylation with pentafluoroproionic anhydride, while sodium phosphate inhibited the acylation and sodium sulfate inhibited it slightly. The pentafluoropropionic methyl ester of glutamic acid was stable for up to 10 days, when it was dissolved in acetone and stored at -18 degrees C.

Release of glutamate from the crayfish neuromuscular junction

1. The superficial abdominal flow flexor muscle was isolated from the crayfish (Cambarus clarkii) and placed in a bath solution of 100 microliters. The concentration of glutamate in this solution was measured by mass fragmentography using a gas chromatograph-mass spectrometer. 2. The excitatory post-synaptic potential (e.p.s.p.) of the slow flexor muscle and its sensitivity to L-glutamate were similar to those observed in the opener muscle of the dactyl in the walking leg or claw of the crayfish. 3. The background efflux of glutamate during control rest periods was about 20 p-mole/10 min. Nerve stimulation caused a significant increase in the efflux of glutamate. The net release of glutamate above the background was 11.9 p-mole/100 microliters. at 10 Hz stimulation and 21.1 p-mole/100 microliters. at 20 Hz stimulation. 4. When the amplitude of e.p.s.p. was decreased by streptomycin, thereby reducing the muscle contraction, the net release of glutamate by nerve stimulation was not changed. Streptomycin depressed the e.p.s.p. by its action on the post-synaptic membrane. 5. When the external concentration of Ca was lowered, the amplitude of e.p.s.p. and the net release of glutamate were decreased. 6. It is concluded that L-glutamate is released from the nerve terminals of the crayfish neuromuscular junction.