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

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)

L-glutamate activates excitatory and inhibitory channels in Drosophila larval muscle

Muscle fibers from Drosophila larvae show an L-glutamate-sensitive membrane potential. Bath-applied L-glutamate depolarizes the muscle in the range from 0.5 to 20 microM. Greater concentrations of the agonist repolarize the fibers. The repolarizing effect disappears if chloride is replaced by sulfate in the external medium. Intracellular recordings show the occurrence of depolarizing and hyperpolarizing spontaneous miniature postsynaptic potentials (smpp). Patch-clamp studies indicate the presence of two types of receptor channels: (i) an anion-selective channel activated by both L-glutamate and GABA. In outside out-patches, bathed in symmetrical 140 mM Cl- and 200 microM GABA, the channel displays conductance substates of 40, 80 and 110 pS. In the presence of 200 microM L-glutamate only the 40 and 80 pS substates are observed; (ii) a cation-selective channel activated only by L-glutamate that has a conductance of 104 pS in cell-attached patches (128 mM Na+ outside). The presence of these two types of receptor channels in Drosophila muscle may explain the effect of bath-applied L-glutamate on membrane potential and the presence of inhibitory and excitatory smpp.

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).

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.

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.

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.

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.

Glutamate-operated postsynaptic channels and spontaneous excitatory postsynaptic currents in crayfish claw opener muscle

In opener muscle fibres of the crayfish claw excitatory postsynaptic currents activated by glutamate (1 . 10(-4)-5 . 10(-4) mol/l) were investigated by means of the noise analysis technique. For the apparent mean open time of glutamate-activated channels, tau noise = 1.15 ms +/- 0.16 (S.D., n = 24) resulted at membrane potentials between E = -60 mV and E = -100 mV, T = 20-23 degrees C. No significant voltage dependence for tau noise was observed, most likely due to the pretreatment of the fibres with 1 mumol/l concanavalin A. For the conductance, gamma, of these channels, gamma = 23.6 pS +/- 5.2 (S.D., n = 24) was found. These characteristics for glutamate-operated channels differ significantly from those observed recently in opener muscle fibres of crayfish first walking leg [13]. Similarly, different characteristics were found also for the decay time constants tau (sEPSCs) of spontaneous excitatory postsynaptic currents in the respective muscles. On average, tau (sEPSCs) was 1.3 ms +/- 0.3 (S.D., n = 11) in the claw. In the first walking leg tau (sEPSCs) was shorter by a factor of about 2.4 and in the second walking leg shorter by a factor of about 2.9, than in the claw.

Glutamate activated postsynaptic channels in crayfish muscle investigated by noise analysis

Excitatory synaptic channels in crayfish muscle were investigated under various experimental conditions. Small muscle fibres of length l less than or equal to 0.6 mm were voltage clamped, spatial control of the voltage being sufficient up to at least 500 Hz. Excitatory synaptic current was induced by superfusion of glutamate. The power density spectra of this current could be fitted by single component Lorentz curves. The analysis revealed a mean open time tau noise = 0.93 ms and a conductance gamma = 32.3 pS of the glutamate operated ion channels (membrane potential E = -60 mV, temperature T = 8 degrees C). Both the conductance gamma and the channel closing rate alpha = tau -1 noise increased significantly with temperature (Q10 approximately 2). The temperature dependence of gamma and alpha could be described by Arrhenius equations with the temperature independent activation energies E gamma = 42.3 kJ/mol and E alpha = 50.2 kJ/mol. alpha also dependent on the membrane potential, increasing about e-fold when the membrane was hyperpolarized by 120 mV. The potential dependence varied considerably from fibre to fibre. The mean channel open time tau noise agreed with the time constant of decay tau (sEPSC) of spontaneous excitatory postsynaptic currents (sEPSCs).