Experimental studies upon a bundle of tonic fibres in the locust extensor tibialis muscle

The bundle of tonic fibres situated at the proximal end of the locust metathoracic extensor tibialis muscle is innervated by the dorsal unpaired median neurone (DUMETi) as well as by the slow excitatory (SETi)) and common inhibitor (CI) neurones. It is not innervated by the fast excitatory neurone (FETi).These fibres contract spontaneously and rhythmically. The myogenic rhythm can be modified by neural stimulation.Spontaneous slow depolarizing potentials resembling the pacemaker potentials of insect cardiac muscle were demonstrated in these fibres.The actions of glutamate on the tonic muscle fibres are not compatible with its being a specific excitatory transmitter. Glutamate can stimulate weak contractions of the muscle, but this action is inhibited when chloride ions are removed from the saline.10^?6 M Octapamine hyperpolarizes the tonic fibre membrane. Octopamine, GABA and glutamate all inhibit the myogenic contractions and reduce the force of the neurally evoked contractions.The tonic muscle is very responsive to proctolin. At 5 × 10^?11 M proctolin enhances the force and increases the frequency of myogenic contractions. At 10^?9 M it depolarizes the muscle membrane potential, and at that and higher concentrations it causes the muscle to contract. At 2 × 10^?7 M proctolin induces contractures which resemble those evoked by sustained high-frequency neural stimulation. Iontophoretic experiments show that proctolin receptors occur at localized sites on the tonic fibre membrane.     

Further pharmacological study on the cholinergic and glutaminergic properties of the radula muscles of a mollusc,Rapana thomasiana

1. In the radula protractor of the prosobranch mollusc Rapana thomasiana, both twitch contractions and acetylcholine contractions were markedly depressed or blocked by propantheline (10⁻⁵ M) and strychnine (10⁻⁵ M), but in the radula retractor, only acetylcholine contraction was markedly affected by the antagonists,2. Glutamate contractions of both of the muscles were little or slightly affected by the drugs.3. Twitch contraction of the protractor was slowly depressed when the muscle was immersed in concanavalin A (0.3 mg/ml), while that of the retractor was first potentiated and then slowly depressed in it.4. In both of the muscles, glutamate contractions were markedly enhanced by the lectin, but acetylcholine contractions were not affected.5. These results support the notion that the principal excitatory neurotransmitter in the protractor is acetylcholine, whereas that in the retractor is glutamate.     

Inhibiton and gamma-aminobutyric acid-induced conductance on locust (Schistocerca gregaria) extensor tibiae muscle fibres

• 1.1. Increases in membrane conductance (g_m) were induced by GABA in distal bundles 32, 33 and 34 of extensor tibiae muscles of the locust (Schistocerca gregaria).
• 2.2. Bath application of GABA (10⁻⁵−5 × 10⁻³ M) induced reductions in muscle fibre space constant (λ).
• 3.3. GABA (5 × 10⁻³ M) induced additional membrane conductance of 2.21 ± 0.03 × 10⁻⁶ S/mm, 0.38 ± 0.03 × 10⁻⁶ S/mm and 0.29 ± 0.06 × 10⁻⁶ S/mm on muscle bundles 34, 33 and 32 respectively. The greater sensitivity of muscle fibres in bundle 34 to GABA is due at least in part to a larger number of GABA receptors on bundle 34 muscle fibres.
• 4.4. The decrement of electrotonic potentials in the presence of GABA were measured over distances of both half fibre length and whole fibre length. Good agreement was obtained between changes in space constant produced by GABA using half fibre length and whole fibre length data.
• 5.5. By taking into account changes in space constant induced by GABA it was possible to demonstrate that presynaptic GABA receptors were involved in the inhibition of slow excitatory postsynaptic potentials by GABA.
• 6.6. “Slow” excitatory postsynaptic potentials recorded under current clamp were inhibited in a dose-dependent manner by GABA. This inhibition was not dependent on muscle-fibre GABA sensitivity and could not be completely accounted for by GABA-induced changes in the cable properties of the muscle fibres.

     

Glutamatergic motoneurons in the stomatogastric ganglion of the mantis shrimp Squilla oratoria

1. Transmitters of motoneurons in the stomatogastric ganglion (STG) of Squilla were identified by analyzing the excitatory neuromuscular properties of muscles in the posterior cardiac plate (pcp) and pyloric regions. 2. Bath and iontophoretic applications of glutamate produce depolarizations in these muscles. The pharmacological experiments and desensitization of the junctional receptors elucidate the glutamatergic nature of the excitatory junctional potentials (EJPs) evoked in the constrictor and dilator muscles. The reversal potentials for the excitatory junctional current (EJC) and for the glutamate-induced current are almost the same. 3. Some types of dilator muscle show sensitivity to both glutamate and acetylcholine (ACh) exogenously applied. The pharmacological evidence and desensitization of the junctional receptors indicate the glutamatergic nature of neuromuscular junctions in these dually sensitive muscles. The reversal potentials for the EJC and for the ACh-induced current are not identical. 4. Glutamate is a candidate as an excitatory neuro-transmitter at the neuromuscular junctions which the STG motoneurons named PCP, PY, PD, LA and VC make with the identified muscles. Kainic and quisqualic acids which act on glutamate receptors are potent excitants of these muscles. Extrajunctional receptors to ACh are present in two types of the muscle innervated by LA and VC. 5. Neurotransmitters used by the STG motoneurons of stomatopods are compared to those of decapods.     

Actions of acromelic acid on nervous system l-glutamate receptors

Acromelic acid, a naturally occurring kainoid, isolated from the mushroom Clitocybe acromelalga, is a weak displacer of [³H]L-glutamate binding to cockroach (Periplaneta americana) nerve cord membranes. Acromelic acid (1 mM) displaces ～?60% of specifically bound [³H]L-glutamate. When applied by bath perfusion to the cell body membrane of the cockroach fast coxal depressor motor neurone, acromelic acid generated slow, prolonged, dose-dependent depolarizations at concentrations of 0.3 μM and above. Thus acromelic acid is among the most potent of the excitatory amino acids tested to date on insect neurones. © 1994 Wiley-Liss, Inc.     

The action of iontophoretically applied L-glutamate on an insect visceral muscle

1) lontophoretic application of L-glutamate was employed to study the distribution of glutamate receptors in the superior longitudinal (SL) muscles of the locust (Locusta migratoria) hindgut, in which spontaneous activity was inhibited using normal saline containing 5 mM MgCl_2. 2) Junctional glutamate potentials with a rise time of 50–100 ms (peak) and a decay time of 250–400 ms were recorded at localized sites using ejection pulses in the range 5–10 nC. Most active sites were found in interfiber clefts and were spaced at about 250–300 μm intervals. 3) Desensitization of glutamate receptors occurred using ejection frequencies > 0.2 Hz. Desensitization could be irreversibly blocked using the lectin concanavalin A. 4) Depolarizing (D-) and biphasic depolarizing/hyperpofarizing (DH -) extrajunctional glutamate potentials were observed using ejection pulses > 15 nC. 5) δ-Philanthotoxin (δ-PTX) at concentrations > 0.3 Uml^?1 inhibited junctional glutamate potentials in a dose-dependent manner, 50% inhibition was achieved using 0.45 Uml^?1 δ-PTX. 6) Subthreshold concentrations of proctolin (up to 5 × 10^?10M) had no visible effect on glutamate potentials, suggesting that proctolin possibly does not act by modulating glutamate activity. 7) It is proposed that glutamate plays a transmitter role in SL muscles, while the role of proctolin is still unclear.     

Proctolin: a peptide transmitter candidate in insects.

The slow, striated muscles of the proctodeum (hindgut) of the cockroach, $\text{Periplaneta americana}$ (L.), were examined pharmacologically with reference to the responses evoked by nerve stimulation, glutamate, 5-HT, and proctolin, a myotropic peptide from $\text{Periplaneta}$ recently isolated and identified. The graded contractions evoked by repetitive nerve stimulation were simulated by 5-HT and proctolin at threshold concentrations of about 10⁻⁷ and 10⁻⁹ M respectively; responses to glutamate (∼10⁻⁴ M) were not similarly graded. The 5-HT receptors are distinct from other receptors, including the post-synaptic receptors, since they were specifically blocked by bromolysergic acid diethylamide. Proctolin was fully active on TTX-treated or surgically denervated muscle indicating that the proctolin receptors are located on the muscle fibre membrane. Tyramine, at threshold levels 5×10⁻⁸ M, reversibly antagonized the responses evoked by proctolin and by nerve stimulation but was without effect on the 5-HT and glutamate responses. Neurally evoked responses were potentiated by subthreshold concentrations of proctolin but not by glutamate. Pharmacologically, the proctolin and post-synaptic receptors appear to be identical and distinct from the glutamate and 5-HT receptors. Since proctolin is known to be a constituent of an efferent pathway of the proctodeal nerves, the evidence suggests that it may function as an excitatory transmitter substance. Peptidergic transmission is discussed in relation to the ultrastructural organization of the proctodeal nerve terminals which contain neurosectory granules in addition to electron-lucent, synaptic vesicles.     

Distribution and density of α-bungarotoxin binding sites on innervated and noninnervated Xenopus muscle cells in culture

The distribution and density of α-bungarotoxin (α-BT) binding sites on Xenopus muscle cells in culture by autoradiography using ¹²⁵I-α-BT were examined. In muscle cells grown alone α-BT binding sites were fairly uniformly distributed over the entire surface with a mean density of 104/μm² (background density). Occasionally, spots of higher density were observed (“hot spots”) where the mean density was 890/μm². The addition of neural tube cells did not change the background density. Similarly in the majority of cases medium contained with neural tube cells did not affect the density of α-BT binding sites. Previous findings that the background acetylcholine sensitivity of muscle cells increased in the presence of neural tube cells (by approximately 50%) or in conditioned medium (by approximately 70%), therefore, are not likely due primarily to an increase in the acetylcholine receptor (AChR) density. In cocultures of nerve and muscle cells regions of high α-BT binding sites were occasionally associated with the path of neurites. In such regions the density of α-BT binding sites was estimated to be approximately 1000/μm². However, even in these cells the density at non-nerve contacted regions was not different from that in muscle cells cultured alone. Whether the increase in AChR density at the junctional area is sufficient to explain a previous observation of a fivefold increase in the amplitude of spontaneous synaptic potentials during the process of AChR accumulation is discussed.     

Electrical properties of skeletal muscle fibres of the flour moth larva Ephestia kuehniella

The electrical properties of the ventral longitudinal muscle fibres in the flour moth larva Ephestia kuehniella were investigated at rest and during electrical activity. The membrane resting potential was only partially dependent on the K-concentration gradient across the muscle membrane. The electrical constants λ, τ, R_m, R_i, and C_m were determined according to the equations for ‘short cables’ (Table 1). Current-voltage relationships of the muscle membrane were measured: they revealed anomalous as well as delayed rectification of the membrane. Stimulation of the muscle fibres with intracellular current pulses elicited graded action potentials in most fibres; in some fibres ‘all-or-none’ action potentials were generated. In contrast to graded action potentials these ‘all-or-none’ action potentials were propagated without decrement along the muscle fibre. Indirect stimulation of the muscle fibres resulted in large excitatory junction potentials which generally gave rise to action potentials.     

Neuromuscular block in locust skeletal muscle caused by a venom preparation made from the digger wasp Philanthus triangulum F. from Egypt.

A preparation from P. triangulum F., made by extracting abdomens and purified by Sephadex filtration, does not affect potassium ion-induced contractions of the retractor unguis muscle of S. gregaria, but the reduction of the glutamate contractions is at least as pronounced as the effect on the neurally-evoked twitch. Glutamate potentials are affected at a lower venom dose than are the neurally evoked excitatory postsynaptic potentials (EPSPs). The half-decay-time of the glutamate potentials starts to decrease just before the decrease in amplitude is initiated.In the retractor unguis muscle the resting plasma membrane is slightly depolarized at high venom concentrations, but this effect cannot explain the effects on neuromuscular transmission. It is concluded that the venom preparation of P. triangulum affects the glutamate or transmitter-induced transient permeability change, possibly by blocking the open ion-channels.     

Acetylcholine activates a chloride channel as well as glutamate and GABA

Summary In conventional two microelectrode experiments, acetylcholine had qualitatively the same effect as GABA and glutamate on membrane potential and input resistance of muscle fibres of the opener and intrinsic stomach muscles of crayfish (Austropotamobius torrentium). In patch-clamp experiments, acetylcholine occasionally elicited single channel openings in cell-attached patches on these muscles. If outside-out patches were excised and the Cl^– concentration was high on both sides of the membrane, acetylcholine at concentrations of 1 nM regularly elicited single channel currents. The amplitude of single channel currents depended strongly on the intracellular concentration of Cl^–. The reversal potential of the channel, determined after replacing intracellular K⁺ with Cs⁺, corresponded to the Nernst potential for Cl^–. The voltage dependence and the reversal potential of single channel current amplitudes elicited by ACh, glutamate and GABA were identical. The distribution of life times of openings (>1 ms) elicited by ACh and glutamate could be fitted by a single exponential with a time constant of about 2.5 ms, corresponding to the mean open time. ACh and glutamate applied to the same outside-out patch showed cross-desensitization, and thus ACh and glutamate activate the same channels. An excitatory, cationic ACh-activated channel could not be identified. Permeabilities of the chloride channel were calculated according to the Goldman-Hodgkin-Katz equation at different membrane potentials. Negative single channel current amplitudes (inward currents) could be fitted with a permeability of ₂= 3.9×10^–14 cm³s^–1. For positive currents (outward) the channel had a permeability of ₁= 1.4× 10^–14 cm³s^–1. The permeability of the channel declined from 16×10^–14 cm³s^–1 to 2.3×10^–14 cm³s^–1 if the intracellular Cl^–-concentration was raised from 6 to 257 mM. The activation elicited by acetylcholine was inhibited by extracellular Ca⁺⁺. The mean current activated by ACh was reduced by a factor of 50 if the extracellular concentration of Ca⁺⁺ was raised from 0.1 mM to the physiological concentration of 13.5 mM.     

Nicotine-induced and depolarisation-induced glutamate release from hippocampus mossy fibre synaptosomes: two distinct mechanisms

Hippocampus mossy fibre terminals activate CA3 pyramidal neurons via two distinct mechanisms, both quantal and glutamatergic: (i) rapid excitatory transmission in response to afferent action potentials and (ii) delayed and prolonged release following nicotinic receptor activation. These processes were analysed here using rat hippocampus mossy fibres synaptosomes. The relationships between synaptosome depolarisation and glutamate release were established in response to high-KCl and gramicidin challenges. Half-maximal release corresponded to a 52 mV depolarisation step. KCl-induced release was accompanied by transient dissipation of the proton gradient across synaptic vesicle membrane. Nicotine elicited a substantial glutamate release from mossy fibre synaptosomes (EC₅₀ 3.14 μM; V _max 12.01 ± 2.1 nmol glutamate/mg protein; Hill's coefficient 0.99). However, nicotine-induced glutamate release was not accompanied by any change in the membrane potential or in the vesicular proton gradient. The effects of acetylcholine (200 μM) were similar to those of nicotine (25 μM). Nicotinic α7 receptors were evidenced by immuno-cytochemistry on the mossy fibre synaptosome plasma membrane. Therefore, the same terminals can release glutamate in response to two distinct stimuli: (i) rapid neurotransmission involving depolarisation-induced activation of voltage-gated Ca²⁺ channels and (ii) a slower nicotinic activation which does not involve depolarisation or dissipation of the vesicular proton gradient.     

Neurotransmitters of motor neurons in the stomatogastric ganglion of an isopod crustacean, Ligia exotica

Neurotransmitters of motor neurons in the foregut muscles of an isopod Ligia exotica were identified by recording changes in membrane potential to exogenously applied glutamate and acetylcholine. The effects of antagonists, tubocurare and joro spider toxin, on excitatory junctional potentials evoked by nerve stimulation and by iontophoretic application of glutamate and acetylcholine provided additional evidence for identification. The junctional receptors were desensitized by putative neurotransmitters. Glutamate is a candidate as an excitatory neurotransmitter at the neuromuscular junctions in intrinsic muscles of the gastric mill and pylorus, and acetylcholine is a candidate in the extrinsic muscles of the gastric mill and cardiopyloric valve.     

The pharmacological and physiological profile of glutamate receptors at the Drosophila larval neuromuscular junction

Abstract.  Drosophila larval muscles are commonly used for developmental assessment in regard to various mutations of synaptically relevant molecules. In addition, the molecular sequence of the glutamate receptors on the muscle fibre have been described; however, the pharmacological profiles to known agonists and antagonists have yet to be reported. Here, the responses of N -methyl- d -aspartic acid, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), l -glutamate, kainate, quisqualic acid, NBQX, AP5 and DNQX are characterized with regard to synaptic transmission and direct effects on the muscle fibres. The muscle fibres depolarize to application of glutamate or quisqualate and the excitatory postsynaptic potential (EPSP) amplitudes are diminished. Kainate does not alter the muscle membrane potential but does reduce the EPSP amplitude. The known antagonists NBQX, AP5 and DNQX have no substantial effect on synaptic transmission at 1 m m , nor do they block the response of quisqualate. Kainate may be acting as a postsynaptic antagonist or via autoreceptors presynaptically to reduce evoked transmission.     

Physiological and structural properties of colchicine-treated chick skeletal muscle cells grown in tissue culture.

Chick muscle cells grown in tissue-culture medium containing colchicine developed into rounded cells called “myosacs.” Electron micrographs of myosacs were similar to untreated myofibers except that microtubules were absent and the contractile material was disorganized. Most of the electrophysiological characteristics of myosacs were similar to those of untreated myofibers. Thus, both cellular types had similar resting potentials, nonlinear current voltage curves, three types of action potentials (Na⁺, Ca²⁺, and Cl^? spikes), and acetylcholine sensitivity with “hot spots.” Both demonstrated contraction with electrical or chemical (acetylcholine, caffeine) stimuli. The one significant difference was that myosacs, unlike myofibers, were always isopotential throughout their intracellular space.     

Glutamate potential : differences from the excitatory junctional potential revealed by diltiazem and concanavalin A in crayfish neuromuscular junction.

1. The effect of diltiazem and concanavalin A (Con A) on the crayfish neuromuscular junction was investigated in order to compare the action of L-glutamate with that of the excitatory transmitter. 2. When diltiazem (0.3 nM) was added to the perfusion fluid, the iontophoretic glutamate potential was reduced to about half, whereas the amplitude of excitatory junctional potentials (EJPs) increased by about two times. 3. Dose-response curves of L-glutamate suggested that diltiazem acted in a non-competitive manner. The decrease in amplitude of the glutamate potential caused by diltiazem was not due to the acceleration of desensitization of the glutamate receptor. 4. The increase in amplitude of EJPs caused by diltiazem was due to the increase in membrane resistance. The quantal content and size of extracellular EJPs were not affected by diltiazem. 5. In normal saline, bath application of glutamate decreased the amplitude of both glutamate potentials and EJPs because of desensitization of the glutamate receptor. The decrease in amplitude of the glutamate potential was completely prevented by previous application of Con A (10(-6) M). On the other hand, Con A had no influence on the decrease in amplitude of EJPs. 6. Some possible explanations of these pharmacological differences between glutamate potentials and EJPs revealed by diltiazem and Con A are considered.     

The mechanism of block of glutamate synapses by dipicolinic acid

The effect of dipicolinic acid (2,6-pyridine dicarboxylic acid) on the mealworm neuromuscular junction was studied using conventional microelectrode recording techniques. Dipicolinic acid (10^?5-10^?3 M) added to the bathing solution reversibly blocked neuromuscular transmission. The depolarization in response to iontophoretically applied L-glutamate (glutamate potential) was not affected by dipicolinic acid even when the neurally evoked excitatory postsynaptic potential (EPSP) was totally abolished. Focal extracellular recordings from single synaptic sites revealed that in the presence of 1 x 10^?4 M dipicolinic acid the presynaptic spike was unchanged, but the quantal content for evoked transmitter release was reduced. The calcium-dependent action potential elicited by direct stimulation of the muscle fiber was not impaired by dipicolinic acid. These results suggest that dipicolinic acid interferes with the transmitter-releasing mechanism from the presynaptic terminal.     

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P₃], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P₃. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent.A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P₃ generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive.The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10⁻⁵M. While 10⁻³M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P₃ levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P₃ levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect.Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P₃ levels in the tissues.     

Excitatory neurotransmitters in the tentacle flexor muscles responsible for space positioning of the snail olfactory organ

Recently, three novel flexor muscles (M1, M2 and M3) in the posterior tentacles of the snail have been described, which are responsible for the patterned movements of the tentacles of the snail, Helix pomatia. In this study, we have demonstrated that the muscles received a complex innervation pattern via the peritentacular and olfactory nerves originating from different clusters of motoneurons of the cerebral ganglia. The innervating axons displayed a number of varicosities and established neuromuscular contacts of different ultrastructural forms. Contractions evoked by nerve stimulation could be mimicked by external acetylcholine (ACh) and glutamate (Glu), suggesting that ACh and Glu are excitatory transmitters at the neuromuscular contacts. Choline acetyltransferase and vesicular glutamate transporter immunolabeled axons innervating flexor muscles were demonstrated by immunohistochemistry and in Western blot experiments. Nerve- and transmitter-evoked contractions were similarly attenuated by cholinergic and glutamatergic antagonists supporting the dual excitatory innervation. Dopamine (DA, 10^?5 M) oppositely modulated thin (M1/M2) and thick (M3) muscle responses evoked by stimulation of the olfactory nerve, decreasing the contractions of the M1/M2 and increasing those of M3. In both cases, the modulation site was presynaptic. Serotonin (5-HT) at high concentration (10^?5 M) increased the amplitude of both the nerve- and the ACh-evoked contractions in all muscles. The relaxation rate was facilitated suggesting pre- and postsynaptic site of action. Our data provided evidence for a DAergic and 5-HTergic modulation of cholinergic nerves innervating flexor muscles of the tentacles as well as the muscles itself. These effects of DA and 5-HT may contribute to the regulation of sophisticated movements of tentacle muscles lacking inhibitory innervation.     

Solubilisation of a Glutamate Binding Protein from Rat Brain

Rat brain synaptic plasma membranes were solubilised in either 1% Triton X-100 or potassium cholate and subjected to batch affinity adsorption on L-glutamate/bovine serum albumin reticulated glass fibre. The fibre was extensively washed, and bound proteins eluted with 0.1 mM L-glutamate in 0.1% detergent, followed by repeated dialysis to remove the glutamate from the eluted proteins. Aliquots of the dialysed extracts were assayed for L-[3H]glutamate binding activity in the presence or absence of 0.1 mM unlabelled L-glutamate (to define displaceable binding). Incubations were conducted at room temperature and terminated by rapid filtration through nitrocellulose membranes. Binding to solubilised fractions could be detected only following affinity chromatography. Binding was saturable and of relatively low affinity: KD = 1.0 and 1.8 microM for Triton X-100 and cholate extracts, respectively. The density of binding sites was remarkably high: approximately 18 nmol/mg protein for Triton X-100-solubilised preparations, and usually double this when cholate was employed. Analysis of structural requirements for inhibition of binding revealed that only a very restricted number of compounds were effective, i.e., L-glutamate, L-aspartate, and sulphur-containing amino acids. Binding was not inhibited significantly by any of the selective excitatory amino acid receptor agonists--quisqualate, N-methyl-D-aspartate, or kainate. The implication from this study is that the glutamate binding protein is similar if not identical to one previously isolated and probably is not related to the pharmacologically defined postsynaptic receptor subtypes, unless solubilisation of synaptic membranes resulted in major alterations to binding site characteristics. Since solubilisation with Triton X-100 is known to preserve synaptic junctional complexes, it seems likely that the origin of the glutamate binding protein may be extrajunctional, although its functional role is unknown.