Pharmacological distinction between the excitatory junctional potential and the glutamate potential revealed by concanavalin A at the crayfish neuromuscular junction

AMPA-preferring receptors mediate excitatory synaptic inputs to retinal ganglion cells

Pharmacological studies were performed to determine whether alpha-amino-3-hydroxy-5-methyl-4-isoazoleprionic acid (AMPA)- and/or kainate (KA)-preferring receptors mediate excitatory synaptic inputs to tiger salamander retinal ganglion cells. Excitatory postsynaptic currents (EPSCs), evoked either by light or by stimulating bipolar cells with puffs of K+, were measured using whole cell recording techniques in the tiger salamander retinal slice. The AMPA/KA component of the EPSCs was isolated by including antagonists of glycine-, gamma-aminobutyric acid (GABA)- and NMDA-receptors in the bath. The AMPA-preferring receptor antagonists, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI-52466) and 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4 - dihydro-5H-2,3-benzodiazepine (GYKI-53665), reduced light-evoked EPSCs and K+ puff-evoked EPSCs amplitudes in a concentration-dependent manner. The IC50 values for GYKI-52466 were 3.6 and 4.2 microM for the light- and puff-evoked responses, respectively. The more potent GYKI-53665 had IC50 values of 0.7 microM for both the light- and puff evoked responses. KA activates both KA- and AMPA-preferring receptors. KA-evoked currents were completely blocked by 10-40 microM GYKI-53665, indicating that little or no excitatory synaptic current was mediated by KA-preferring receptors. Concanavalin A, a compound that preferentially potentiates responses mediated by KA-preferring receptors, did not enhance either EPSCs or glutamate-evoked responses. By contrast, cyclothiazide, which selectively enhances AMPA-preferring receptor mediated responses, was found to enhance both EPSCs and glutamate-evoked currents. Our results indicate that the non-NMDA component of ganglion cell EPSCs is mediated by AMPA-preferring receptors and not significantly by KA-preferring receptors.

Characterization of the oligosaccharide side chains on kainate binding proteins and AMPA receptors

The amino acid sequences of the kainate binding proteins (KBPs) from frog and chicken brain are homologous with the carboxy terminal half of the rat brain AMPA receptors. In this study, we have characterized the oligosaccharide side chains present on the KBPs from chicken and frog brain, and the AMPA receptors (GluR1, GluR2, and GluR3) from rat brain. Deglycosylation of the asparagine-linked carbohydrates present on the chicken, frog, and rat receptor subunits with N-glycanase, resulted in decreases in the relative molecular weights (M(r)) of 3.4, 3.4, and 5.1 kDa respectively. Thus the percent of asparagine linked carbohydrate (based on M(r) values derived from SDS polyacrylamide gels) of the 49 kDa chicken, the 48 kDa frog, and the 107 kDa receptor rat subunits is 6.9, 7.1, and 4.8 percent respectively. No shifts in the M(r) were detected after treatment with neuraminidase indicating that sialic acid does not appear to be a major component of these receptors. Lectin binding studies demonstrated that both asparagine-linked and serine/threonine-linked oligosaccharides were present in the chicken, frog, and rat proteins. The data indicate that at least one of the asparagine linked oligosaccharide side chains appear to be of the complex or non-bisected hybrid type in all three species. The similarities in the glycosyl moieties of the chicken and frog kainate KBPs and the rat brain AMPA receptors suggests that the homology in the amino acid sequences between these proteins may extend to homology in their oligosaccharide sides chains as well.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Primary structure and expression of the gamma 2 subunit of the glutamate receptor channel selective for kainate

The presence and primary structure of a novel subunit of the mouse glutamate receptor channel, designated as gamma 2, have been revealed by cloning and sequencing the cDNA. The gamma 2 subunit has structural characteristics common to the neurotransmitter-gated ion channel family and shares a high amino acid sequence identity with the rat KA-1 subunit, thus constituting the gamma subfamily of the glutamate receptor channel. Expression of the gamma 2 subunit together with the beta 2 subunit in Xenopus oocytes yields functional glutamate receptor channels selective for kainate.

Modulation of excitatory synaptic transmission by drugs that reduce desensitization at AMPA/kainate receptors

Desensitization at AMPA/kainate receptors has been proposed to contribute to the decay of excitatory synaptic currents. We examined the action of aniracetam, wheat germ agglutinin (WGA), and concanavalin A (Con A), drugs that act via separate mechanisms to reduce desensitization evoked by L-glutamate in rat hippocampal neurons. The decay of excitatory synaptic currents, and sucrose-evoked miniature excitatory postsynaptic currents (EPSCs) was slowed 2- to 3-fold by aniracetam. In contrast, WGA increased the EPSC decay time constant only 1.3-fold and Con A had no effect. Aniracetam increased the magnitude of stimulus-evoked EPSCs 1.9-fold; variance analysis suggests a postsynaptic mechanism of action. WGA and Con A reduced EPSC amplitude via a presynaptic mechanism. Aniracetam increased the burst length of L-glutamate-activated single-channel responses. Simulations suggest that aniracetam either slows entry into a desensitized state or decreases the closing rate constant for ion channel gating.

Concanavalin A: a tool to investigate neuronal plasticity

Neuronal plasticity is the ability of neurons to alter their cellular properties in response to changes in their environment. These changes are typically triggered by the binding of specific ligands, such as neurotransmitters, growth factors or other neuromodulators, to receptors on the neuronal membrane surface. Since the extracellular domains of many of these receptors are glycosylated, they can also be bound by lectins--proteins with high affinity binding sites for polysaccharides. Different lectins have different affinities for various sugar residues. This feature has made lectins useful in the investigation of the regional localization and relative mobility of different classes of glycosylated membrane receptors, and in the subsequent purification of the receptors. This article reviews some of the different kinds of neuronal plasticity produced by the plant lectin concanavalin A (Con A), such as enhancement of neurite outgrowth, modulation of neurotransmitter responses, and alteration in the specificity and strength of synaptic connections.

Solubilization and purification of a putative quisqualate-sensitive glutamate receptor from crustacean muscle

Two high-affinity glutamate-binding proteins have been solubilized and purified from crustacean muscle membranes. L-[3H]Glutamate binding to intact membranes gave IC50 values (concn. giving 50% inhibition) of 0.35 microM for glutamate. 9.0 microM for quisqualate and 36.2 microM for ibotenic acid. Kainate, domoate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, D-glutamate and L-aspartate were poor inhibitors of glutamate binding. The two protein were purified approx. 600-fold from muscle membranes by use of concanavalin A and gel-filtration column chromatography. One protein showed an apparent Stokes radius of 5.4 nm, as determined by gel filtration, with a subunit of Mr 74000 as shown by SDS/polyacrylamide-gel electrophoresis. Analysis of glutamate binding to this protein revealed a KD of 2.4 microM and a Bmax, of 15.5 nmol/mg, and a k1 for quisqualate inhibition of glutamate binding of 1.0 microM. The second protein was eluted anomalously from the gel-filtration column and showed a subunit of Mr 65000 on SDS/polyacrylamide gels. Glutamate binding to this protein showed a KD of 1.3 microM and a Bmax, of 14.5 nmol/mg, and was only slightly sensitive to the presence of quisqualate (Ki = 0.8 mM). Glutamate binding to both proteins was insensitive to kainate. It is concluded that the Mr-74000 protein is a prime candidate for the quisqualate-sensitive receptor of the crustacean neuromuscular junction.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Glutamate receptor desensitization and its role in synaptic transmission

Responses of excitatory amino acid receptors to rapidly applied glutamate were measured in outside-out membrane patches from chick spinal neurons. The peak current varied with glutamate concentration, with a half-maximal response at 510 microM and a Hill coefficient near 2. Currents activated by 1 mM glutamate desensitized and recovered in two phases. The faster time constant was identical to the time constant of decay of synaptic currents, suggesting that glutamatergic synaptic currents are terminated, in part, by receptor desensitization. Steady-state desensitization was evident following application of only 2-3 microM glutamate, concentrations comparable to levels in the extracellular space in the intact brain. Thus, glutamate receptor desensitization can affect synaptic efficacy in two ways: at high concentrations, rapid desensitization of receptors may curtail synaptic currents; at low concentrations, there is a significant reduction in the number of activatable receptors.

Concanavalin A selectively reduces desensitization of mammalian neuronal quisqualate receptors

A fast perfusion system was used to apply excitatory amino acids to embryonic hippocampal neurons grown in dissociated culture and voltage clamped in the whole-cell recording configuration. Responses to quisqualic acid and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA; a potent quisqualate-like agonist) showed rapid desensitization: at 100 microM the peak inward current declined to a plateau response on average 0.2 times the peak response (mean time constant, 30 ms). Responses to L-aspartic acid and N-methyl-D-aspartic acid also showed desensitization: at 100 microM, when recorded in Mg-free solution with 0.3 microM glycine, the peak inward current declined to a plateau value 0.5 times the peak, but with a time constant of desensitization (average, 248 ms) one order of magnitude slower than desensitization of responses to quisqualate. Responses to kainate and domoate (agonists at kainic acid receptors) did not show appreciable desensitization. Responses to L-glutamate and 5-Br-willardine (a potent non-NMDA receptor agonist), recorded in glycine-free solution with 1 mM Mg to suppress N-methyl-D-aspartic acid receptor activity, showed similar rapid desensitization to AMPA and quisqualate, but occurred with less depression of the peak current. The lectin concanavalin A (Con A) reduced desensitization at quisqualate receptors, with no effect on responses to kainate or N-methyl-D-aspartic acid. The effect of Con A developed slowly (average time constant at 2.5 microM, 250 s) but at steady state Con A increased the plateau current evoked by 100 microM quisqualate to 13 times control. Succinyl-Con A produced only a small reduction of desensitization to quisqualate, approximately 10% of that produced by native Con A. Con A did not change the decay time constant of fast excitatory synaptic currents evoked by stimulation of presynaptic neurons, although the peak synaptic current decreased after treatment with lectin. Con A was also without effect on the block of responses to kainate produced by coapplication of quisqualate.

The glutamate-induced chloride current in Aplysia neurones lacks pharmacological properties seen for excitatory responses to glutamate

The pharmacological properties of the L-glutamate (Glu)-induced chloride current (ICl) in enzymatically isolated Aplysia neurones were examined using the 'concentration clamp' technique. The Glu-ICl did not cross-desensitize with the ICl evoked by gamma-aminobutyric acid or acetylcholine. Quisqualate, kainate (one out of eight) and N-methyl-D-aspartate (one out of nine) induced a small, non-desensitizing ICl in Glu-responding neurones. The quisqualate- and kainate-ICl did not cross-desensitize with the Glu-ICl. L-Aspartate did not induce a ICl in 11 neurones tested, which showed a Glu-ICl. Glutamate diethyl ester, Joro Spider toxin and ketamine did not suppress the Glu-ICl. Concanavalin A had no effect on the time course of desensitization. These results suggest that the Glu receptor-Cl channel complex in Aplysia neurones has pharmacological properties which differ from those of the excitatory Glu receptor-channel complexes in the crustacean muscle fibres and in the central neurones of vertebrates.

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.

Aliphatic alcohols increase the decay rate of glutamate-activated currents at the crayfish neuromuscular junction

Excitatory junction currents produced by glutamate were recorded with an extracellular electrode at the excitatory neuromuscular junction of the crayfish. The currents decayed more quickly as the membrane was hyperpolarized. The direction of the voltage sensitivity of the decay phase is thus opposite to that found for acetylcholine-activated currents at the amphibian endplate. The aliphatic alcohols ethanol to octanol all increased the rate of decay of the currents. The effects of the short chain alcohols were opposite to their actions at the toad endplate, where ethanol to pentanol prolong the currents. This observation was explained in terms of the opposite direction of the voltage sensitivity in the two preparations. For each alcohol, the relationship between the half-decay time of the currents (t 1/2) and alcohol concentration was exponential. The potency of each alcohol in decreasing t 1/2 was exponentially related to carbon chain length, which would be predicted if the effects of the alcohols were directly related to their concentration in the lipid phase of the membrane. These findings are consistent with the ideas that the alcohols may alter membrane polarizability or change membrane fluidity in the vicinity of the channels.

The piperidine analogues prevent desensitization of glutamate receptors on crustacean muscle

The effect of the four cis piperidine decarboxylate analogues on glutamate receptor desensitization was investigated. Intracellular recordings of the depolarization in response to ionophoretic application of L-glutamate were made from the opener muscle of the Hermit crab (Eupagurus bernhardus). Following bath application of any of the piperidine dicarboxylates (1 h) the decline in the amplitude of successive glutamate potentials in response to a train of ionophoretic pulses was prevented. This result indicated that the piperidine dicarboxylates inhibited the development of glutamate receptor desensitization. This effect was reversed on washing. During a prolonged ionophoretic glutamate application (5 s) the depolarization declined due to the onset of desensitization. This decline in the potential no longer occurred after exposure to a piperidine dicarboxylate analogue (30-60 min). Similarly, after application of the piperidine dicarboxylates the shape of the bath-applied glutamate potential was transformed from a rapid depolarization followed by repolarization to a maintained depolarization. These effects of the piperidine dicarboxylates are consistent with a loss of glutamate receptor desensitization.

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.

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.

Excitatory postsynaptic channels operated by quisqualate in crayfish muscle

Small crayfish muscle fibers were voltage clamped and membrane current noise elicited by bath application of quisqualate, a compound structurally related to the excitatory transmitter glutamate, was measured. It was found that quisqualate activates the same excitatory postsynaptic receptors as glutamate but its affinity for these receptors is about one hundred times larger. Noise analysis revealed that the high potency of quisqualate was attributable in part to an increased apparent mean channel open time, tau noise (quisqualate), which was about ten times larger than on activation by glutamate. The channel conductance gamma(quisqualate), however, was about three times smaller than gamma(glutamate). At T = 8 C and E = -60 mV, tau noise (quisqualate) = 9.3 +/- 1.8 ms and gamma(quisqualate) = 9.7 +/- 1.1 pS resulted. tau noise (quisqualate) decreased with hyperpolarization but it was much less voltage dependent than tau noise (glutamate): tau noise (quisqualate) = 6.0 ms . exp (E/362 mV). Both gamma(quisqualate) and alpha = tau noise (quisqualate) increased with temperature (Q10 approximately 1.6). This temperature dependence was characterized by the temperature independent activation energies E gamma = 29.1 +/- 1.7 kJ/mol and E alpha = 33.9 +/- 1.3 kJ/mol. Concanavalin A which blocks desensitization of the quisqualate/glutamate receptors did not influence tau noise (quisqualate) significantly.

Effects of concanavalin A on glutamate operated postsynaptic channels in crayfish muscle

Small crayfish muscle fibres were voltage clamped, and synaptic current noise induced by bath application of glutamate was measured. Desensitization of the glutamate receptors was blocked by preincubating the fibres with 0.3-1.0 mumol/l concanavalin A (Con A) for at least 30 min. The power density spectra of the glutamate current noise could be fitted by single component Lorentz curves. The lectin Con A did not influence significantly the conductance gamma of the glutamate channels but increased their mean open time, tau noise. The respective mean values found at T = 8 degrees C and E = -60 mV were gamma = 23.5 +/- 7.0 pS and tau noise = 1.5 +/- 0.2 ms. Both the conductance gamma and the closing rate alpha = tau -1 noise increased with temperature (Q10 approximately 1.9). This temperature dependence was characterized by the activation energies E gamma = 35.2 +/- 7.1 kJ/mol and E alpha = 46.9 +/- 2.1 kJ/mol. The potential dependence of tau noise was almost completely abolished by Con A.

The effects of cyclic dicarboxylic acids on spontaneous and amino acid-evoked activity of rat cortical neurones

1 A series of cyclic dicarboxylic acids were applied by microiontophoresis to neurones in the cerebral cortex of rats anaesthetized with urethane. The object was to examine effects on spontaneous firing rates and any ability to antagonize responses to excitatory amino acids. 2 At relatively low ejecting currents (10-25 nA) cis-2,3-piperidine dicarboxylic acid (cis-2,3-PDA) had no effect on spontaneous firing but selectively antagonized the excitation evoked by n-methyl-D-aspartate (NMDA) without affecting responses to quisqualaife or kainate. At higher ejecting currents (60-100 nA) responses to all three agonists were reduced. 3 Other cis-piperidine dicarboxylic acids and piperazine-2,3-dicarboxylic acid had only weak and variable effects on cell firing and responses to NMDA, quisqualate, kainate, glutamate and aspartate. 4 2, 3-Pyridine dicarboxylic acid (quinolinic acid) produced excitation of all cortical neurones tested. 5 2-Amino-5-phosphono-valeric acid, an NMDA antagonist, reduced responses to quinolinate, implying that this compound can act at NMDA receptors. 6 It is suggested that quinolinic acid may be of physiological interest as a potential endogenous excitant in the nervous system and that cis-2,3-PDA and its N-methyl derivative may be of use in studies of receptor pharmacology and the identification of synaptic transmitters.

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 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.

The presynaptic effects of valproic acid in the isolated frog spinal cord

The effects of the anticonvulsant valproic acid (n-dipropylacetate, DPA) on frog primary afferent fibers was examined with sucrose gap recordings from the dorsal roots. Addition of DPA to the superfusate consistently reduced the amplitude and duration of the dorsal root potential. In contrast, DPA augmented the depolarization of dorsal roots produced by GABA, beta-alanine and taurine. It also decreased afferent fiber 'desensitization' to GABA. DPA depressed the ability of K+ and the excitatory amino acids glutamate and aspartate to depolarize afferent fibers. In addition, the compounds decreased the amount of K+ released by tetanic stimulation of the dorsal root. The K+-evoked release of tritiated GABA from cord slices was initially reduced by exposure to DPA, but was then unaffected after a longer application of the anticonvulsant. On the other hand, the high affinity uptake of tritiated GABA and glycine were almost totally blocked by the addition of DPA to the incubating medium. In sum, DPA has complex effects on neuronal membranes. Some of these effects may serve to explain the anticonvulsant actions of this drug.

The influence of concanavalin A on glutamate-induced current fluctuations in locust muscle fibres

1. The influence of the plant lectin Concanavalin A (ConA) on the properties of membrane channels opened by 25-200 microM-L-glutamate in voltage-clamped locust muscle fibres has been studied. Power spectral density plots of membrane current fluctuations were used to estimate the mean lifetime and conductance of glutamate-induced channels. 2. Exposure to ConA (10 micrograms/ml.) increased the amplitude of the mean current evoked by 200 microM-glutamate from 7.7 +/- 4.7 nA (mean +/- S.D.) to 81 +/- 57 nA (membrane potential -60 mV, temperature 16 degrees C). The mean conductance of glutamate-induced channels (65 +/- 16.8 pS at 16 degrees C) was negligibly affected by the lectin. 3. In normal saline the mean lifetime of glutamate-induced channels tau noise decreased on membrane hyperpolarization. After treatment with ConA (70 micrograms/ml.), tau noise become totally independent of membrane voltage when studied at potentials in the range -60 to -120 mV. The value of tau noise at the resting potential was little altered by the lectin.

Inhibitory action of ibotenic acid on the crayfish neuromuscular junction

The effect of ibotenic acid on the crayfish neuromuscular junction was investigated. Ibotenate reduced dose-dependently the amplitude of the excitatory junctional potentials (EJPs) elicited by repetitive stimulation of the excitatory axons. Ibotenate did not affect facilitation of successive EJPs. The decrease in the EJP amplitude caused by ibotenate was almost completely blocked by picrotoxin. A quantum analysis of extracellularly recorded EJPs demonstrated that the mean quantum content was reduced by ibotenate without remarkable change in unit size. Ibotenate increased the conductance change induced by ibotenate was observed even if the glutamate receptor was completely desensitized by the prolonged application of glutamate. From an analysis of the dose-response curve of GABA with or without ibotenate, it is suggested that ibotenate acts on the GABA receptor in the crayfish neuromuscular junction.

Distribution and pharmacological properties of synaptic and extrasynaptic glutamate receptors on crayfish muscle

1. The distribution of glutamate sensitivity was measured in the opener muscle in the walking legs of the crayfish (Cambarus clarkii). L-Glutamate was ionophoretically applied under visual control. 2. Bundles of a few muscle fibres were isolated and viewed with Nomarski optics. Two axons, presumably excitatory and inhibitory, branched widely over the surface of individual muscle fibres, forming numerous clusters of boutons or varicosities. 3. Glutamate sensitivity was measured from the slope of the dose-response curves obtained by ionophoretic application of L-glutamate and expressed as mV/nC. The highest sensitivities were about 100 mV/nC, obtained at the edge of synaptic boutons. The sensitivity declined to less than 5% about 2 micrometer away from the synaptic terminal. The time course of glutamate potentials was approximately the same as that of spontaneous synaptic potentials. 4. Glutamate depolarization started within 300 microsec after ionophoretic release of glutamate. This time lag was shorter than the synaptic delay of the nerve-evoked synaptic potential measured with an extracellular micro-electrode. This indicates that glutamate depolarization results from a direct action on the post-synaptic receptor. 5. Application of L-alpha-kainic acid decreased the amplitude of the glutamate potential produced at the synaptic region, whereas kainate increased the amplitude of the glutamate potential at the extrasynaptic region. It is suggested that the pharmacological properties of the extrasynaptic receptor differ from those of the synaptic receptor. Possible mechanisms for the different actions of kainate are discussed.

Differential effects of diltiazem on glutamate potentials and excitatory junctional potentials at the crayfish neuromuscular junction

1. The effects of diltiazem on glutamate potentials and excitatory junctional potentials (e.j.p.s) were investigated in the crayfish neuromuscular junction. 2. When diltiazem (0.3 mM) was added to the perfusion fluid, the ionophoretic glutamate potential was reduced to about half, whereas the peak amplitude of successive e.j.p.s elicited by a train of pulses of 100/sec increased by about 2 times. 3. It was suggested that diltiazem was a non-competitive inhibitor of L-glutamate. The reduction of the response to applied glutamate was not due to the acceleration of desensitization of the glutamate receptor. The rate of recovery from desensitization was delayzed by diltiazem. 4. The increase in amplitude of e.j.p.s caused by diltiazem was due to the increase in membrane resistance. The quantum content and size of extracellular e.j.p.s were not affected by diltiazem. 5. It was substantiated using the micro-electrode technique that the glutamate sensitive area coincided with the neuromuscular junctional area. 6. The pharmacological difference between glutamate potentials and e.j.p.s revealed in the present study is difficult to explain on the glutamate transmitter hypothesis. One explanation worthy to be considered is that there are two pharmacologically different kinds of receptors sensitive to L-glutamate.