Reversal potentials of the excitatory transmitter and L-glutamate at the crayfish neuromuscular junction

Two excitatory motoneurons differ in quantal content of their junctional potentials in abdominal muscle fibers of the cricket, Gryllus bimaculatus

In abdominal muscles 202 and 203 of the cricket, Gryllus bimaculatus, large and small excitatory junctional potentials (l- and s-EJPs) with similar durations can be recorded from the same muscle fibers. At the normal extracellular calcium ion concentration ([Ca(2+)](o)) of 5mM, the amplitudes of l-EJPs in both muscles were larger than the threshold membrane potential for muscle action potentials, which is about -40mV. Below 0.75mM [Ca(2+)](o), the amplitudes became much smaller and were below the firing level for the action potentials. At 0.5mM, they fluctuated and decreased to 10.3 and 1.9mV in muscles 202 and 203, respectively, and at 0.25mM frequent failures occurred. The amplitudes of s-EJPs at 5mM [Ca(2+)](o) were 13.3 and 5.1mV in muscles 202 and 203, respectively, and the fluctuating amplitudes were far below the threshold for muscle action potentials. Below 0.75mM, s-EJPs were rarely observed. The relation between log(EJP amplitude) and log([Ca(2+)](o)) was linear within a certain range of [Ca(2+)](o) and the slopes of the lines for l-EJPs were about twice as steep as those for s-EJPs in both muscles. In muscle 202, the amplitude distribution of l-EJPs obtained at 0.25mM and that of s-EJPs at 0.75mM both showed peaks at once and twice the voltage at the first peak, which were coincident with the voltages at the peaks of amplitude distributions of miniature EJPs recorded simultaneously. The reversal potentials for l- and s-EJPs in muscle 202 were +1.02 and +0.22mV, respectively. In muscle 202, the decreases in amplitude of both EJPs by L-glutamate were similar and concentration-dependent. The results suggest that the difference in amplitude between l- and s-EJPs is attributable mainly to the difference in quantal contents.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

Pharmacological characteristics and distributions of sigma- and phencyclidine receptors in the animal kingdom

The phylogenetic distributions of sigma- and phencyclidine receptors in neural tissues of 13 species and the pharmacological characteristics of these receptors in whole sea anemone and neural tissues of the guinea pig, chicken, and frog were studied. Specific binding of [3H]haloperidol and [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine, ligands that bind with high affinity to sigma- and phencyclidine receptors, respectively, was detected in all organisms examined. The order of potencies of various ligands to inhibit 1 nM [3H]haloperidol binding in brains of frogs and guinea pigs or 1 nM [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine in chicken or guinea pig brain homogenates was very similar. However, the characteristics and stereospecificity of binding of the two radioligands in sea anemone were different than in higher organisms. The results suggest that sigma- and phencyclidine binding sites are evolutionarily old, as the characteristics of the two sites are well preserved over a range of vertebrate phyla.

Postsynaptic effects of nickel ions at the insect neuromuscular junction

The effect of extracellular nickel on the excitatory postsynaptic response at the insect neuromuscular junction was studied in the segmental muscle of the larval mealworm Tenebrio molitor. The response to L-glutamate applied iontophoretically (glutamate potential, GP) was potentiated in the presence of Ni2+ though the excitatory postsynaptic potential (EPSP) was reduced. It seems unlikely that Ni2+ acts at the same binding site as L-glutamate does since the value of the limiting slope of double logarithmic plots for the action of glutamate was increased in the presence of Ni2+. The potentiation of GP in the presence of Ni2+ cannot be ascribed to competition between Ni2+ and Ca2+ since GP amplitude did not show any dependence on the concentration of Ca2+. Nickel ions did not alter the reversal potential of excitatory postsynaptic current (EPSC) and glutamate current (GC) under the voltage clamp condition, whereas the amplitude of GC was potentiated in the presence of Ni2+. The time constant of the decay of EPSC showed a weak voltage dependency: the more depolarized the membrane, the more prolonged the time constant. In the presence of 1 mM Ni2+ the amplitude of miniature EPSCs (MEPSCs) increased and the half decay time was prolonged significantly. These results suggest that Ni2+ interacts with charged groups near the glutamate receptor-channel complex so that the kinetics of the channel are altered.

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.

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)

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.

L-Glutamate effects on electrical potentials of synaptic plasma membrane vesicles

The electrogenic nature of the L-glutamate-stimulated Na+ flux was examined by measuring the distribution of the lipophilic anion [35S]thiocyanate (SCN-) into synaptic membrane vesicles that were incubated in a NaCl medium. Concentrations of L-glutamate from 10(-7) to 10(-4) M added to the incubation medium caused an enhanced intravesicular accumulation of SCN-. Based on the SCN- distribution in synaptic membrane vesicles it was calculated that 10 microM L-glutamate induced an average change in the membrane potential of + 13 mV. L-Glutamate enhanced both the Na+ and K+ conductance of these membranes as determined by increases in SCN- influx. Other neuroexcitatory amino acids and amino acid analogs (D-glutamate, L-aspartate, L-cysteine sulfinate, kainate, ibotenate, quisqualate, N-methyl-D-aspartate, and DL-homocysteate) also increased SCN- accumulation in synaptic membrane vesicles. These observations are indicative of the activation by L-glutamate and some of its analogs of excitatory amino acid receptor ion channel complexes in synaptic membranes.

L-proline as a glutamate antagonist at a crustacean neuromuscular junction

The fast as well as the slow contractions of the adductor muscle in the claw of Procambarus clarkii are inhibited by L-proline. This inhibition is dose dependent and decreases with increasing frequency of stimulation of the "slow" fiber. Contractions caused by perfusing the adductor muscle with L-glutamate solutions are also inhibited by L-proline. The inhibiting potency of L-proline is small; the effective concentration of this amino acid is 50--100 times that of the L-glutamate applied. It was postulated that the inhibitory effect of L-proline is based on competition for excitatory receptor sites of L-glutamate, which causes depolarization and contraction, and L-proline, which lacks these actions. Theoretical considerations suggested a linear relationship between the stimulating L-glutamate and the just-inhibiting L-proline concentrations. Experimental evidence supported this model.

A comparative study of the effects of glutamate and kainate on the lobster muscle fibre and the frog spinal cord

1 The depolarizing actions of glutamate and its conformationally restricted analogue kainate were investigated on the lobster muscle fibre and the frog spinal cord using intracellular and extracellular recordings, respectively. 2 Bath-applied kainate was less potent than glutamate on the lobster fibre but more potent on the frog cord. From the log-log transformation of dose-response curves it was proposed that more than one glutamate molecule was necessary to activate both the lobster and the frog receptor sites. In the frog, at least three kainate molecules were thought to be required for receptor activation. 3 The ionic dependence of glutamate and kainate responses appeared different for the two tissues. 4 Some possible explanations of the differential tissue sensitivity to kainate are discussed.

Ionic mechanisms associated with the depolarization by glutamate and aspartate on human and rat spinal neurones in tissue culture

The action of glutamate and aspartate was studied on the membrane potential of human and rat spinal neurones in tissue culture. Both amino acids caused a depolarization of the cell membrane, the size of which was dependent on the concentration of the amino acids in the bathing fluid. In order to study ionic mechanisms associated with the amino acid depolarization, the ionic composition of the extracellular fluid was altered. Removal of sodium ions from the bathing solution reversibly reduced or abolished the depolarization produced by glutamate and aspartate suggesting that the action of these amino acids is associated with an increased sodium permeability. Substituting lithium for sodium ions also reversibly abolished the depolarization by glutamate indicating that in contrast to the effect of lithium on the action potential, this ion cannot replace sodium for the glutamate depolarization. These experiments show that the method of tissue culture is a suitable model to study ionic mechanisms underlying the action of neurotransmitters in the mammalian and especially in the human CNS.

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

1. Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting 'glutamate current'. 2. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39-1 +/- 3-6 mV (mean +/- S.E. of mean) in normal solution and +16-5 +/- 2-0 mV in solutions made twice as hypertonic by the addition of sucrose. 3. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. 4. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. 5. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. 6. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. 7. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. 8. The results indicate that L-glutamate increases the membrane permeability mainly to Na+ and slightly to Ca2+. 9. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.

Ionic mechanism of the excitatory synaptic membrane of the crayfish neuromuscular junction

1. The reversal potential for the excitatory neuromuscular junction of the crayfish (Cambarus clarkii) was measured using the voltage clamp method. The potential change was recorded with an intracellular microcapillary and the negative phase of the output of the feed-back amplifier was connected to the stainless-steel wire which was inserted longitudinally into the muscle fibre. 2. When the excitatory nerve was stimulated, a transient feed-back current flowed inwardly through the membrane. This current was called the excitatory junctional current (e.j.c.). 3. Reversal potentials were determined by extrapolating the e.j.c.s measured at different membrane potentials. They were about 10-20 mV positive with respect to the bath solution (11-5 +/- 1-2 mV, mean +/- S.E.). 4. The reversal potential for the iontophoretically applied glutamate was identical with that for the e.j.c. 5. In hypertonic solutions, the reversal potentials for e.j.c. and glutamate became more negative. 6. When the sodium concentration of the bath solution was decreased, the reversal potential became more negative. 7. When the chloride and potassium concentration were altered, little, if any, change was observed in the reversal potential. 8. It was concluded that the e.j.c. was carried mainly by sodium ions. Contribution of other ions, possibly calcium ions, was discussed.

Inhibitory and excitatory effects of CNS depressants on invertebrate synapses

(1) The effects of pentobarbital were studied on the membrane properties and synaptic activity of crustacean neuromuscular junction preparations and molluscan neurons. (2) Pentobarbital selectivity depressed in a dose-dependent, reversible manner the exciatory postynaptic potentials (EPSPs) recorded at crustacean neuromuscular junctions without altering either inhibitory postsynaptic potentials (IPSPs) or post-synaptic membrane properties. (3) Pentobarbital depressed cholinergic EPSPs recorded in an identified molluscan neuron and depressed the depolarizing phase of biphasic PSP without affecting the hyperpolarizing phase of the BPSP on the same cell. Facilitation of the EPSP was not affected. (4) Pentobarbital did not appreciably alter the reversal potentials of the EPSP and IPSP. (5) Low concentrations of pentobarbital did not alter the appearance of spontaneously occurring IPSPs, while high concentrations changed the pattern of regular IPSP input to an irregular, burst-like pattern. (6) Pentobarbital and 5 other CNS depressants (cholralose, chloroform, ethanol, and urethane) increased the excitability and altered the current--voltage relations of a cell whose membrane properties have been proposed as a model of presynaptic terminal membranes. The effects were dependent on the species of external divalent cation present. (7) The results in these invertebrate systems may provide insight into the cellular basis of the depressant and excitatory effects of these agents.

Divalent cations: effects on post-synaptic pharmacology of invertebrate synapses

(1) The effects of divalent cations (Ca++, Mg++, Sr++ and Co++) were studied on the post-synaptic responses of crustacean neuromuscular junctions and identified molluscan neurons to bath and iontophoretic application of putative transmitters. (2) The glutamate response of the crustacean muscle was parabolically dependent on [Ca++]0, while the ACh response of an identified molluscan neuron was inversely dependent on[Ca++]0. Elevated [Ca++]0 depressed both glutamate and ACh depolarizations in a concentration-dependent, reversible manner. Low concentrations of Co++ also depressed both depolarizations in a concentration-dependent, reversible manner. (3) Double-reciprocal plot analyses of the Ca++ and Co++ depressions indicate that these agents were apparently not acting to reduce the affinity of the receptor for the agonist. Elevated concentrations of both Ca++ and Co++ shifted the inversion potential of the ACh response in a hyperpolarizing direction, suggesting a preferential block of the receptor-coupled Na+ conductance. (4) Neither Ca++ nor Co++ depressed Cl- or K+-dependent responses coupled to the putative transmitters GABA, glutamate, dopamine or ACh. (5) The selective inhibition of the ACh and glutamate responses by the general anesthetic pentobarbital was examined as a function of[Ca++]0. Decreasing [Ca++]0 by 5-fold decreased the pentobarbital inhibition by about 50% while increasing [Ca++]0 by 5-fold produced an insignificant increase in the inhibition. (6) The data indicate that divalent cations, like general anesthetics, selectively depress post-synaptic excitatory responses that are primarily Na+-dependent. This selective depression by Ca++ could contribute to its anesthetic and anticonvulsant properties when present in elevated concentrations in the ventricular fluid. The mechanism by which divalent ions and general anesthetics selectively depress receptor-coupled conductances appear to be different: divalent ions preferentially attack the Na+ component while anesthetics block Na+ and K+ conductance equally (possibly by affecting the kinetics of the mechanism).

CNS depressants: effects on post-synaptic pharmacology

(1) The effects of 5 anesthetics (chloralose, chloroform, ethanol, pentobarbital and urethane) and one anticonvulsant (diphenylhydantoin) were studied on the membrane properties and post-synaptic responses of crustacean neuromuscular junction preparations and molluscan neurons to putative transmitters and peptides. (2) In crustacean preparations pentobarbital selectively depressed, in a dose-dependent, reversible manner, post-synaptic, Na+-dependent, depolarizing responses to the putative transmitter glutamate without altering post-synaptic, Cl(-)-dependent inhibitory responses to the putative transmitter gamma-aminobutyric acid. (3) The effects of all the agents on post-synaptic pharmacology of a molluscan neurosecretory cell were studied either by causing the cell to hyperpolarize to about--100mV through repeated application of acetylcholine (ACh) in a K+-free, Ca++-containing solution or by hyperpolarization through injection of intracellular current in a K+-free solution. Effects of these agents on post-synaptic responses on other molluscan neurons were studied using intracellular current injection to manipulate membrane potential. (4) All of the agents tested selectively depressed the depolarizing Na+-K+-dependent post-synaptic responses of the neurosecretory cell to ACh in a dose-dependent reversible manner without appreciably altering the membrane properties of the cell (over the potential range of the ACh responses). (5) Pentobarbital did not alter the inversion potential of the ACh response. (6) Reciprocal plot analysis of all of the agents tested revealed that the antagonism of the ACh response was primarily non-competitive. (7) None of the agents tested altered hyperpolarizing, K+-dependent responses to dopamine and glutamate on the neurosecretory cell, nor did they affect either the induction or enhancement of BPP activity by the vertebrate peptide vasopressin on this cell.

Some characteristics of pre- and post-synaptic inhibitory receptors at the hermit crab neuromuscular junction

1 The effects of gamma-aminobutyric acid (GABA), beta-guanidinopropionic acid (betaGP) and picrotoxin on the pre- and post-synaptic receptors of the hermit crab neuromuscular junction were studied quantitatively usine electrophysiological techniques. Reductions in excitatory junction potential (e.j.p.) amplitude and membrane resistance were measured simultaneously from the same cells. 2 The pre- and post-synaptic receptors were activated by the same order of concentration of GABA, whereas betaGP stimulated the pre-synaptic receptors at a concentration ten times lower than was required to affect the post-synaptic membrane. 3 Picrotoxin appeared to antagonize the pre-synaptic action of betaGP in a competitive manner. The affinity constants (+/- s.e. mean) for picrotoxin 5 times 10(-6)M and 2 times 10(-4)M were 6.80(+/-0.46) times 10(5)M-1 and 6.42(+/-1.8) times 10(5)M-1 respectively. 4 The effect of GABA on e.j.p. amplitude also appeared to be antagonized competitively by picrotoxin whereas the post-synaptic effect was antagonized in a non-competitive manner. 5 Possible differences in the nature of the pre- and post-synaptic receptors are discussed.

Dual effect of L-glutamate on excitatory postjunctional membranes of crayfish muscle

Iontophoretically applied glutamate produces different excitatory postjunctional permeability changes on separate muscle fibers in a single crayfish muslce. At junctions on some fibers glutamate appears to increase the conductance to both sodium and potassium whereas at others its effect is primarily on the sodium conductance. These results obtained by studying the reversal potential for the extracellularly recorded glutamate potential under conditions of varied extracellular sodium and potassium concentrations.

Cooperative interaction of glutamate and aspartate with receptors in the neuromuscular excitatory membrane in walking limbs of the lobster

When applied to lobster muscle fibers, L-glutamate, L-aspartate, and combinations of the two amino acids can induce membrane depolarization. Under normal conditions, a quantitative analysis of the depolarization response or change in membrane conductance was precluded by nonlinearities in the voltage-current relationship of the membrane. By including gamma-aminobutyrate (GABA) in the bathing medium, the voltage-current relationship was made linear in the depolarizing direction over a range of 15-20 mV from the resting potential. However, a meaningful examination of the increase in membrane conductance caused by glutamate and aspartate was still not possible. Therefore, the depolarization responses caused by the excitatory amino acids were taken as a quantitative reflection of receptor activation in the excitatory postsynaptic membrane. In the presence of GABA, aspartate by itself, at concentrations up to 10 mM, had little excitatory activity, whereas glutamate effected an appreciable membrane depolarization at concentrations of 0.1 to 0.2 mM. Aspartate, at concentrations which exhibited no activity alone, markedly enhanced the excitatory action of glutamate. Aspartate shifted the glutamate dose-response curve to the left, but did not appear to affect the maximum depolarization response elicited by glutamate. These observations are consistent with the concept that aspartate increases the affinity between glutamate and the glutamate binding sites. Limiting slopes of log-dose versus log-response curves for the excitatory action of glutamate suggest that the interaction of glutamate with excitatory receptors is a cooperative process. The possibility exists that individual receptors contain multiple and distinct glutamate and aspartate binding sites. These results support the view that neuromuscular excitation in the lobster is mediated by glutamate and aspartate functioning synergistically.

The accumulative properties of facilitation at crayfish neuromuscular synapses

1. Synaptic facilitation was measured with intracellular recording at two classes of neuromuscular synapses in the opener muscle of the crayfish dactyl by placing a test stimulus at various intervals after either a single conditioning stimulus or a short conditioning train.2. The facilitative effect of one stimulus reaches approximately the same level with both the superficial central and superficial distal synapses. The facilitation decreases smoothly in two phases after the conditioning stimulus at superficial central synapses and in a more complex fashion at superficial distal synapses.3. The two synaptic types differ in the manner in which they add up the facilitative effects produced by each of the stimuli in a short train. With superficial distal synapses the facilitative effects of conditioning stimuli add linearly, while with superficial central synapses the facilitative effects accumulate exponentially.4. The linear addition of facilitation at superficial distal synapses is not altered when the quantal content is lowered by decreasing the external Ca concentration from 13.5 to 3 mM.5. The rate of decay of facilitation is the same following both one and three conditioning stimuli, even though the facilitation is nearly six times larger in the latter case.6. The results are discussed in terms of mechanisms for synaptic facilitation.