Potentiation of glutamate-induced depolarization by kainic acid in the crayfish opener muscle

The Discovery and Characterization of Targeted Perikaryal-Specific Brain Lesions With Excitotoxins

The neurotoxic action of glutamic acid was first described by Lucas and Newhouse, who demonstrated neural degeneration in the inner layers of the neonatal mouse retina after systemic treatment with L-glutamate. Olney extended these findings by showing that neuronal degeneration affected other brain structures including neurons within the arcuate nucleus of the hypothalamus and the area postrema, that the lesion spared axons passing through these areas, and that the neurotoxic potency of glutamate analogs correlated with their excitatory potency, resulting in the designation “excitotoxins.” As this method affected only a small number of brain regions, it was not suitable for targeted brain lesions. The Coyle laboratory showed that direct injection of the potent glutamate receptor agonist, kainic acid, into the rat striatum caused a rapid degeneration of intrinsic neurons while sparing axons of passage or termination including the unmyelinated dopaminergic terminals. Kainic acid also exhibited this perikaryal-specific and axon-sparing profile when injected into the cerebellum, hippocampus and eye. However, neuronal vulnerability was highly variable, with hippocampal CA3, pyriform cortex and amygdala neurons exhibiting great sensitivity due to kainate’s high convulsive activity. In a comparison study, ibotenic acid, a potent glutamatergic agonist isolated from the amanita muscaria mushroom, was found to have excitotoxic potency comparable to kainate but was far less epileptogenic. Ibotenate produced spherical, perikaryal-specific lesions regardless of the site of injection, and experiments with specific glutamate receptor antagonists showed that its effects were mediated by the N-methyl-D-aspartate receptor. Because of this uniform neurotoxicity and near ubiquitous efficacy, ibotenic acid became the excitotoxic lesioning agent of choice. The discovery of the excitotoxic properties of the tryptophan metabolite quinolinic acid and of the anti-excitotoxic, neuroprotective effects of the related metabolite kynurenic acid in the Schwarcz laboratory then gave rise to the concept that these endogenous compounds may play causative roles in the neuropathology of a wide range of neurological and psychiatric disorders.

Kainate Receptors, Homeostatic Gatekeepers of Synaptic Plasticity

Extensive research over the past decades has characterized multiple forms of synaptic plasticity, identifying them as key processes that allow the brain to operate in a dynamic manner. Within the wide variety of synaptic plasticity modulators, kainate receptors are receiving increasing attention, given their diversity of signaling mechanisms and cellular expression profile. Here, we summarize the experimental evidence about the involvement of kainate receptor signaling in the regulation of short- and long-term plasticity, from the perspective of the regulation of neurotransmitter release. In light of this evidence, we propose that kainate receptors may be considered homeostatic modulators of neurotransmitter release, able to bidirectionally regulate plasticity depending on the functional history of the synapse.

Kainic acid: insights into excitatory mechanisms causing selective neuronal degeneration

Kainic acid, an acidic pyrolidine isolated from the seaweed Digenea simplex, is the most potent of the commonly used exogenous excitotoxins. The neurotoxic threshold of kainic acid is nearly two magnitudes lower than that of the other receptor-specific agonists, N-methyl-D-aspartic acid and quisqualic acid. Neurophysiological and ligand-binding studies indicate that the neurotoxic action of kainic acid is mediated by a specific receptor which exhibits a remarkably broad phylogenetic distribution in the nervous system of vertebrates and invertebrates. The mechanism of neurotoxicity of kainic acid appears to be indirect and requires the functional integrity of excitatory afferents to vulnerable neurons. Consistent with the excitotoxin hypothesis, kainic acid depletes high-energy phosphates and glucose at sites of neurotoxic action; nevertheless, the proximate cause of neurotoxicity may involve increases in intraneuronal calcium levels and the activation of calcium-dependent proteases. Kainic acid neurotoxicity provides a useful animal model for selective neuronal vulnerability that may shed light on the pathophysiology of a number of neurodegenerative disorders, including Huntington's disease and temporal lobe epilepsy.

Absence of neurotoxic effects in leopard sharks, Triakis semifasciata, following domoic acid exposure

Domoic acid (DA), a potent neurotoxin produced by select species of algae and diatoms, kills neurons bearing kainic acid-type glutamate receptors. Studies have shown that DA bioaccumulates in invertebrates and fish that consume the diatoms. In every vertebrate species tested or observed in the wild, dietary or systemic DA causes neuronal damage or clinical signs of neurotoxicity. Sharks, like marine birds and mammals, are exposed to DA through their diet; however, no research has demonstrated the effect of DA on shark behavior or physiology. In this study, juvenile leopard sharks (Triakis semifasciata) were given DA by intracoelomic injection at doses of 0, 1, 3, 9, and 27 mg/kg and observed for 7 days. The sharks failed to demonstrate behavioral or histological changes in response to the toxin. We identified putative brain glutamate receptors by probing western blots with an antibody specific for kainic acid-type glutamate receptors and demonstrated receptor localization in the cerebellum with immunohistochemistry. Blood levels of DA in three sharks dosed at 9 mg/kg fell rapidly within 1.5h of injection. We show that leopard sharks possess the molecular target for DA but are resistant to doses of DA known to be toxic to other vertebrates.

Somato-, branchio- and viscero-motor neurons contain glutaminase-like immunoreactivity

Immunocytochemistry combined with a fluorescent dye tracer method revealed that somatic, branchial and visceral motoneurons in the brainstem and spinal cord of the rat contain phosphate-activated glutaminase (PAG). An excitatory neurotransmitter glutamate is synthesized mainly through this enzyme. Among these motoneurons, neurons in the dorsal motor nucleus of the vagus nerve (dmnX), autonomic preganglionic neurons in the spinal cord and urethral sphincter motoneurons (DL) were most intensely immunostained. PAG is co-expressed with choline acetyltransferase, calcitonin gene-related peptide or galanin in these neurons. These findings, together with the findings that motor endplates in urethral sphincter muscle contain PAG and PAG-like immunostaining in dmnX motoneurons was decreased after axotomy, suggest that glutamate is a co-transmitter of acetylcholine in motoneurons. Brainstem motoneurons were moderately stained, while somatic motoneurons in the spinal cord other than DL, showed very weak staining for PAG. However, they showed intense PAG-like immunoreactivity at their premature stage, suggesting that glutamate has some effects on the maturation of these neurons. A variety of functional roles of glutamate in motoneurons is discussed.

L-[3H]glutamate binding to a membrane preparation from crayfish muscle

The binding of L-[3H]glutamate to an isolated membrane preparation from crayfish tail muscle has been studied. The muscle homogenate was osmotically shocked, frozen and thawed, and thoroughly washed before incubation with L-[3H]glutamate. The preparation showed high specific binding of L-glutamate with a KD of 0.12 microM and Bmax of 4.7 pmol/mg protein measured in Tris/HCl pH 7.3 and at 4 degrees C. Nonspecific binding was 5-10% of total binding. The glutamate binding was highly stereospecific [K0.5 (D-glutamate), 270 microM] and showed a high degree of discrimination between L-glutamate and L-aspartate [K0.5 (L-aspartate), 54 microM]. In mammalian CNS preparations potent agonists of L-glutamate such as kainate and N-methyl-D-aspartate had no effect at 1 mM, and quisqualate was a weak inhibitor of L-glutamate binding [K0.5 (quisqualate), 162 microM]. Ibotenate was the most potent inhibitor [K0.5 (ibotenate), 0.27 microM], and various esters of L-glutamate were of intermediate potency as displacers of L-[3H]glutamate binding (K0.5 values from 6 to 60 microM). The glutamate binding site from crayfish muscle is clearly different from any of the subclasses of glutamate receptors in mammalian CNS. A possible physiological function of the binding site is a postsynaptic receptor for glutamate, either an extra-junctional or a junctional receptor.

Acromelic acid is a much more potent excitant than kainic acid or domoic acid in the isolated rat spinal cord

Excitatory actions of acromelic acid were investigated in the isolated newborn rat spinal cord. Test compounds were added to the perfusing fluid and the responses were recorded from the ventral root extracellularly. Acromelic acid caused a depolarizing response in a dose-dependent manner and the depolarizing activity of acromelic acid was superior to that of other kainoids such as kainate and domoate. The depolarization induced by acromelate was not affected by NMDA antagonists at all. Acromelic acid was proved to be one of the most potent agonists of excitatory amino acids in both vertebrates and invertebrates.

Excitatory action of a plant extract, stizolobic acid, in the isolated spinal cord of the rat

Actions of stizolobic acid, stizolobinic acid and their derivatives were examined on the isolated spinal cord of the newborn rat. The responses were recorded from the ventral root. Stizolobic acid and its bromo-derivatives caused a depolarizing response in a dose-dependent manner. Stizolobinic acid was considerably less potent than stizolobic acid. Depolarizing responses to stizolobic acid and its bromo-derivatives were not affected by the existence of Mg2+ and specific N-methyl-D-aspartate (NMDA) antagonists. Kynurenate depressed responses to stizolobic acid. These results suggest that stizolobic acid is a new excitatory amino acid in the mammalian central neurons which binds preferably to other receptors than the NMDA-type receptor.

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.

Stizolobic acid, a competitive antagonist of the quisqualate-type receptor at the crayfish neuromuscular junction

Stizolobic acid and stizolobinic acid are amino acids isolated from a plant, Stizolobium hassjoo. Both amino acids reduced responses to glutamate and quisqualate in a competitive manner at the crayfish neuromuscular junction, without affecting responses to GABA and acromelic acid. Excitatory junctional potentials were decreased in the presence of stizolobic or stizolobinic acid in a concentration dependent manner. Stizolobinic acid was about 5 times less potent than stizolobic acid.

Acromelic acid, a novel excitatory amino acid from a poisonous mushroom: effects on the crayfish neuromuscular junction

A novel amino acid, acromelic acid, which is one of kainoids isolated from a poisonous mushroom, markedly depolarizes the crayfish opener muscle fiber in a dose-dependent manner, its potency being much greater than that of kainoids such as kainic or domoic acid. Moreover, acrolemic acid markedly potentiates the glutamate response, in spite of the fact that it reduces the quisqualate response in a dose-dependent manner. The amplitude of excitatory junctional potentials was slightly reduced by acromelic acid.

Evidence for L-glutamate as the neurotransmitter of the squid giant synapse

The effect of a spider toxin (JSTX), a specific blocker of glutamate receptors, was studied in the squid stellate ganglion. JSTX irreversibly blocked the excitatory postsynaptic potential in a dose-dependent manner. The toxin neither affected the spike in the postsynaptic nor the presynaptic fibers. Spontaneous miniature potentials recorded from thin stellate nerves were suppressed by the toxin. Iontophoretically applied L-glutamate depolarized the postsynaptic membrane of the giant axon and this potential was also blocked by JSTX. Kainic acid also depolarized the postsynaptic membrane but this was partially blocked by JSTX, indicating that JSTX differentiated kainate receptor from glutamate one. The results strongly suggest that L-glutamate is the neurotransmitter in the giant synapse of squid.

Inhibitory actions of tuberostemonine on the excitatory transmission at the crayfish neuromuscular junction

At the crayfish neuromuscular junction, tuberostemonine, an alkaloid from Stemona japonica, reduced the amplitude of both the excitatory junctional potential (e.j.p.) and the glutamate response in a dose-dependent manner at concentrations above 0.1 mM. Tuberostemonine acted presynaptically on the crayfish neuromuscular junction to reduce a quantal content of extracellularly recorded e.j.p.s, and postsynaptically to reduce their unit size. The decay of the excitatory synaptic current was accelerated by tuberostemonine. The gradual decline of the successive glutamate currents induced by a train was facilitated by the presence of tuberostemonine even in the muscle fibre pre-treated with concanavalin A. The rate of recovery from the refractory form of the glutamate receptor to the free reactive one was slightly affected by tuberostemonine when it was determined by using a paired pulse method. The inhibitory action of tuberostemonine on glutamate responses was voltage-dependent and hyperpolarization increased the drug action. These results indicate that tuberostemonine acts in part as an open-channel blocker at the crayfish neuromuscular junction.

Developmental changes in the glutamate receptor at the insect neuromuscular synapse

Developmental changes in the glutamate receptors were studied using insect (Tenebrio molitor) neuromuscular synapses. Bath-applied L-glutamate produced depolarization of the postsynaptic membrane of larva muscle, whereas in imago muscle L-glutamate produced both depolarization and hyperpolarization with conductance increase. Iontophoretically applied glutamate distinguished two types (depolarization and hyperpolarization) of receptors in the imago muscle. Quisqualic acid and ibotenic acid mimic depo- and hyper-type response by L-glutamate respectively. Kainic acid blocks the excitatory postsynaptic potentials in both larva and imago, with little effect on the resting conductance of the postsynaptic membrane. As the nerve terminal spike was abolished by kainate it is suggested that kainate mainly acts on the nerve terminal.

Fast and slow depolarizing potentials induced by short pulses of kainic acid in hippocampal neurons

Responses of hippocampal neurons to short pulses of alpha-kainic acid (KA) were studied intracellularly in thin brain slices of the guinea pig. A KA pulse induced a slow depolarizing potential either with or without a preceding much faster depolarization. Large fast responses were induced only at the center of glutamate-sensitive spots, where short pulses of L-glutamate (Glu) induced large depolarizations in the impaled neuron. The fast responses resembled Glu-induced depolarizations in time-course, in sensitivity to movement of the tips of amino acid-pipettes, in sensitivity to Glu-antagonists and in reversal potential. The slow response was much more resistant to movement of the amino acid-pipettes and to Glu-antagonists. Mn2+ was without effect on the fast as well as the slow responses. Glu-induced depolarizations super-imposed on the slow response were simply depressed. These results indicate that two different types of receptors are activated by administration of KA, and suggest that the slow response results from a direct action of KA and the fast response is produced as a consequence of either the direct action of KA on the Glu receptors or a calcium-independent release of Glu by KA.

The excitatory actions of kainic acid and some derivatives at the crab neuromuscular junction

The actions of kainic acid and its derivatives including the new tricarboxylic kainic acid were tested on the Hermit crab neuromuscular junction. Kainic acid had a direct excitatory effect producing a depolarization. Although not so potent, alpha-ketokainic acid and tricarboxylic kainic acid mimicked this effect whilst dihydrokainic acid did not. The slopes of the dose-response curves for these kainic acid analogues differed from that of L-glutamate. A Hill coefficient approximating one was obtained for kainic acid compared to a value of three for L-glutamate. Threshold concentrations of l-glutamate and kainic acid but not alpha-ketokainic acid and tricarboxylic kainic acid potentiated the neurally evoked excitatory junction potentials and the ionophoretic L-glutamate potential. The results indicate a possible interaction of the kainic acid derivatives with a heterogeneous receptor population and provide a comparison with other invertebrate and vertebrate systems.

Glutamate-containing dipeptides enhance specific binding at glutamate receptors and inhibit specific binding at kainate receptors in rat brain

The dipeptide, L-phenylalanyl-L-glutamate (PG), augments the specific binding of the excitatory amino acid receptor antagonist, [3H]2-amino-7-phosphonoheptanoic acid (APH), to rat forebrain membranes by 5-fold at 100 microM with an EC50 of 4.9 microM. The increase in the specific binding of [3H]AHP induced by PG results exclusively from an increase in Bmax. In contrast, PG inhibits the specific binding of [3H]kainic acid to forebrain membranes with a Ki of 6.8 microM. Of several related peptides examined, active ones affected the two receptor sites in a reciprocal fashion. The results suggest an allosteric interaction between [3H]APH and kainate receptors modulated by glutamate-containing peptides.

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

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

Specific antagonism of the glutamate receptor by an extract from the venom of the spider Araneus ventricosus

A venom sac extract selectively and irreversibly suppressed excitatory postsynaptic potentials in the lobster neuromuscular junction without affecting inhibitory postsynaptic potentials. Glutamate-induced depolarization of the postsynaptic membrane was blocked by the venom extract, whereas membrane depolarization by aspartate was unaffected.

gamma-D-glutamylglycine as an antagonist of kainic acid on leech Retzius neurones

Intracellular recordings were made from Retzius cells from the segmental ganglia of the leech, Hirudo medicinalis, gamma-D-Glutamylglycine, (50 microM), reversibly and preferentially antagonised the excitatory action of kainate, having no effect on actions of quisqualate or carbachol but at this concentration the antagonism was of short duration, reversing prior to washing off the gamma-D-glutamylglycine. However, higher concentrations of gamma-D-glutamylglycine (250-500 microM) reversibly antagonised both quisqualate and carbachol-induced excitatory responses, indicating a lack of specificity. The value of gamma-D-glutamylglycine in determining the possible site of action of kainate on leech Retzius cells is discussed.

Effect of kainic acid, glutamate, and aspartate on CO2 production by goldfish tectal slices

For a study of the excitatory effect of kainate, glutamate, and aspartate in the goldfish optic tectum, these substances were tested on the production of CO2 from radioactive glucose in tectal slices incubated in Krebs-Ringer medium for fish. Kainate increased the rate of CO2 production for up to 30 min in a dose-related manner, the effect being maximum at 0.1 mM concentration and decreasing at higher doses. The effect was blocked by ouabain (1 mM) as well as by the substitution of choline for Na+ in the incubation medium. Glutamate and aspartate exerted a less pronounced excitatory effect on CO2 production at higher concentration than kainate. This effect was also abolished by ouabain. Glutamate, added to the medium at a concentration at least 100-fold higher than kainate, partially reversed the increase in CO2 production induced by kainic acid. No similar effect was noticed for aspartate. The supposed glutamate antagonists glutamic acid diethylester (1 mM) and proline (5 mM) did not affect the excitatory action of kainic acid or exert an antagonistic effect towards glutamate. At higher concentration (10 mM) glutamic acid diethylester increased CO2 production, an effect that was, however, ouabain insensitive. Methyltetrahydrofolic acid (1 mM), a substance reported to compete for the kainate receptor, did not inhibit the effect of kainic acid or increase CO2 production.

The pharmacology of Limulus central neurons

1. Intracellular recordings have been made from neurons in the central nervous system of the horse-shoe crab, Limulus polyphemus. Neurons possess resting potentials between -40 and -60 mV, with action potentials ranging from 2-3 mV up to 60 mV in amplitude. Neurons also have excitatory and inhibitory postsynaptic potentials. 2. All the neurons studied are inhibited by GABA and excited by cholinomimetics. The GABA response is chloride mediated and reversibly antagonised by picrotoxinin but not by bicuculline or bicuculline methochloride or methoiodide. The cholinergic response is nicotinic and blocked by pentolinium, hexamethonium, chlorisondamine and dihydro-beta-erythroidine. 3. L-Glutamate can excite some cells, inhibit others and have a biphasic action, inhibition followed by excitation, on other cells. The inhibitory effect is chloride mediated and blocked by picrotoxinin. Ibotenate mimics the action of glutamate both in terms of inhibition and excitation but kainate and quisqualate only mimic the excitatory action of L-glutamate. 4. Dopamine, octopamine, 5-hydroxytryptamine and histamine excite some neurons while inhibiting others or have a biphasic action. Dopamine and octopamine normally have different effects on the same cell, suggesting they act via different receptors. Octopamine shows stereospecificity for the (-) isomer which is more than 100 times more active than the (+) isomer and octopamine is reversibly antagonised by phentolamine and cyproheptadine. 5. Proctolin has an excitatory action on these neurons and this effect is long lasting and can be potentiated by dibutyl cyclic AMP. 6. The pharmacology of Limulus central neurons is compared to the pharmacology of insect and crustacean central neurons. It is concluded that GABA and acetylcholine are central transmitters throughout the arthropods. It is also probable that L-glutamate and octopamine have a physiological role in the arthropod central nervous system. Proctolin appears to modify neuronal and muscle activity in the arthropods and has a modulatory or transmitter function.

Stimulated release of 3H-glycine from retina

The release of 3H-glycine was studied from rabbit retinas. Kainic acid (10(-5) M), a glutamate agonist, evoked prompt CA2+-dependent increased release of 3H-glycine. However, nuciferine (10(-4) M) could not suppress the effect of kainic acid. The effect of glutamic acid (10(-5) M) was significantly reduced in the presence of nuciferine (10(-4) M). The GABA analogues, muscimol and THIP, had no effect on the release of 3H-glycine. Aspartate and cystic acid were also without effect.

Ontogeny of receptor binding sites for [3H]glutamic acid and [3H]kainic acid in the rat cerebellum

The development of the specific binding sites for L-[3H]glutamic acid (KD = 370 nM) and for [3H]kainic acid (KD = 39 nM) was studied in the rat cerebellum. Specific binding at both sites remains low during the first week after birth but increases markedly during the second and third weeks after birth, when glutamatergic parallel fiber synaptogenesis occurs. The development of the kainate site lags behind that of the glutamate site, indicating their autonomy.

Neurotoxicity of folates: implications for vitamin B12 deficiency and Huntington's chorea

Recent work has shown that several folates interact with excitatory kainic acid receptors in the mammalian brain and appear to have agonist activity at these receptors. Since kainic acid is a potent neurotoxin it is possible that folates share this toxicity and that high levels of folates result in neuronal damage. Levels of methyltetrahydrofolate are markedly elevated in vitamin B12 deficiency, a disease associated with neuronal destruction. We propose that this destruction occurs as a result of a neurotoxic action of methyltetrahydrofolate. Injection of kainic acid into the basal ganglia of experimental animals produces a pattern of damage similar to that found in patients dying of Huntington's chorea. It is possible that the underlying defect in this disease resides in the pathways of folate metabolism such that a neurotoxic excess of folates accumulates in the central nervous system. Such a disease process might be arrested by antifolate drugs.

Kainic acid stimulation of cerebellar cyclic GMP levels: potentiation by glutamate and related amino acids

A number of excitatory amino acids (including kainate and other "excitotoxic" amino acids, (+/-)-ibotenate, and N-methyl-D-aspartate) are able to increase levels of cyclic GMP in cerebellar slices from immature (8-day-old) rats. The response to kainate is markedly potentiated by glutamate and a restricted number of analogues. This phenomenon is not seen with either (+/-)-ibotenate or N-methyl-D-aspartate, and may have implications for the possible mechanisms of cytotoxicity for kainate.

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.

Phylogenetic distribution of [3H]kainic acid receptor binding sites in neuronal tissue

The phylogenetic distribution of specific binding sites for kainic acid was determined in 14 species including invertebrates and vertebrates. The highest level of binding was observed in brains of the frog (Xenopus laevis), followed by the spiny dogfish (Heterodontus francisci), the goldfish (Carasius auratus) and the chick (Gallus domesticus). Although significant specific binding was noted in some of the lowest forms tested (e.g. Hydra littoralis), this was not a consistent observation in the invertebrates. In most cases, specific binding to both high and low affinity sites was detected; notable exceptions were the cockroach brain (Periplaneta americana), which had negligible high affinity binding, and the crayfish brain (Procambarus) which had negligible low affinity binding. In the spiny dogfish, the smooth dogfish and the chick, the highest level of binding occurred in cerebellum with less in the forebrain and the least in the medulla; in the mammalian species, the highest level of binding occurred in the forebrain structures with less in the cerebellum and least in the medulla. Eadie plots of the saturation isotherms for [3H]kainic acid revealed similar kinetics of binding for frog whole brain, rat forebrain and human parietal cortex with two apparent populations of binding sites: KD1 = 25--50 nM and KD2 = 3--14 nM. While binding in the spiny dogfish forebrain and human caudate nucleus occurred exclusively at a high affinity component, the cerebella of chick, rat and man exhibited only a low affinity binding site. In the 3 species studied most extensively, frog, rat and man, unlabeled kainic acid was the most potent inhibitor of the specific binding of [3H]-kainic acid. L-Glutamic acid was 20--20-fold less potent than kainic acid, and D-glutamic acid was 4--2500-fold less potent than its L-isomer. Reduction of the isopropylene side chain of kainic acid to form dihydrokainic acid decreased the affinity of the derivative 115--30,000-fold. Hill coefficients derived from these displacement curves were 1.0 for unlabeled kainic acid but approximately 0.5 for L- and D-glutamic acids and dihydrokainic acid, which is compatible with negative cooperativity. In summary, these studies demonstrated a widespread distribution throughout the animal kingdom of specific binding sites for kainic acid in neural tissue; the characteristics of these receptor sites are remarkably similar from primitive vertebrates to man.

Actions of glutamate, kainate, dihydrokainate and analogues on leech neurone acidic amino acid receptors

Intracellular recordings have been made from leech Retzius cells. L-Glutamate and kainate both excite these cells, kainate being about 100 times more potent than glutamate. Cross desensitization was observed between glutamate and kainate. Dihydrokainate was found to be equipotent with kainate on these neurones. Esterification of either kainate or dihydrokainate rendered the compounds inactive as did the addition of a benzyloxycarbonyl group on the nitrogen of both compounds. These results suggest that the double bond in the side chain of kainate is not necessary for its potent activity on leech Retzius neurones. But, that free carboxyl groups and an unsubstituted nitrogen are essential for glutamate-like activity.

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.

Cooperative interactions at [3H]kainic acid binding sites in rat and human cerebellum

Inhibition of the specific binding of [3H]kainic acid was studied in membranes isolated from rat and human cerebellum; the sequence of potencies in both species were: kainic acid greater than L-glutamic acid greater than dihydrokainic acid greater than D-glutamic acid. Whereas the Hill coefficient for unlabelled Kainate was 1.0, dihydrokainic acid and D- and L-glutamic acids exhibited negative cooperativity with Hill coefficients of near 0.5. This allosteric interaction of glutamic acid at the kainic acid recognition site suggests a biochemical correlate for the synergistic effects of these compounds in vivo.

L-glutamate specific [3H]kainic acid binding in the rat neostriatum after degeneration of the cortico-striatal pathway

Four weeks after lesion of the cortico-striatal pathway, the specific kainic acid binding was reduced by 20% in the neostriatum. It is concluded that the binding sites are most likely not located on cortico-striatal neurons. The displacement of kainic acid by some chemical indicates that the kainic acid binding site is different from the glutamate binding site, as defined iontophoretically.

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

The effect of concanavalin A(Con A) on desensitization of the glutamate receptor was investigated in the crayfish opener muscle. The depolarization of the crayfish muscle fiber caused by bath-applied L-glutamate was greatly augmented by Con A. The time course of the appearance of the augmentation was slow. Con A completely prevented the development of desensitization of the glutamate receptor. When L-glutamate was applied iontophoretically with a constant current pulse, a decline of the depolarization was seen during the course of the drug application which was presumably due to desensitization of the glutamate receptor. The glutamate potential was slightly increased by Con A, though the increase was transient. On the other hand, the amplitude of excitatory junctional potentials (EJPs) was not increased but decreased by addition of Con A. In normal saline, the amplitudes of both glutamate potentials and EJPs remarkably decreased because of desensitization of the glutamate receptor, but the decrease in amplitude of the glutamate potential was completely prevented by previous application of Con A. On the other hand, Con A had no influence upon the decrease in amplitude of EJPs. These results show that there is a pharmacological difference between the glutamate potential and EJPs.

Further observations on the interaction between glutamate and aspartate on lobster muscle

1 The ability of bath-applied L-glutamate to enhance subsequent depolarizations produced by bath-applied L-aspartate on lobster muscle was further investigated by means of intracellular recording techniques. 2. Increasing the conditioning glutamate concentration or exposure time produced a greater enhancement of aspartate responses. Enhancement was also dependent on the time interval between glutamate and aspartate doses and was not prevented by overnight storage of preparations in vitro. 3. The dose-depolarization curve for enhanced aspartate responses (measured at a fixed time following a given dose of glutamate) was displaced to the left along the abscissa scale relative to control, with no detectable change in limiting log-log slope. 4. Conditioning doses of kainate or domoate (but not quisqualate, aspartate, or KCl) also enhanced aspartate responses; however, their conditioning effect was little affected by increasing the concentration, exposure time, or time interval before applying aspartate. The rate of onset and decline of the enhanced aspartate response always resembled that of the previous conditioning agonist. 5. D and L-Aspartate were approximately equieffective depolarizing agents whereas D-glutamate was approximately 1/40 as potent as L-glutamate. After a conditioning dose of D or L-glutamate, responses to D or L-aspartate were enhanced. 6. In a Na+-free (Li+) medium, both the glutamate depolarization and the conditioning effect towards aspartate were largely abolished. With kainate however, Na+ was not apparently important either for evoking the kainate response or for producing the conditioning effect. 7. Bath-applied glutamate greatly enhanced and prolonged the time course of the iontophoretic aspartate potential with only a small effect on the glutamate potential; however, these effects were not maintained after washout of glutamate. In contrast, bath-application of aspartate depressed the aspartate potential while enhancing the glutamate potential. Some sites that were insensitive to iontophoretically-applied aspartate became clearly responsive to this agent during a bath-application of glutamate. 8. It is proposed that during conditioning with bath-applied glutamate, kainate or domoate, some agonist is trapped by extrajunctional sites and is subsequently displaced by bath-applied aspartate to produce the long-term enhancement effect.