Abstract: 1. In experiments examining the possibility that an excitatory amino acid may be an optic nerve transmitter in mammals, excitatory amino acid antagonists have been ionophoretically applied to cells in layers A and A1 of the cat dorsal lateral geniculate nucleus and their effect on the excitatory response to visual stimulation of the receptive field centre has been assessed. 2. The antagonists used were D-α-aminoadipate (D-α-AA), DL-α-e-diaminopimelic acid (DAP), 1-hydroxy-3-amino-2-pyrrolidone (HA-966) and L-glutamate diethyl ester (GDEE). The antagonist effects on the visual response were compared with their effect on similar magnitude responses evoked by ionophoretic pulses of selected agonists and a control excitant, generally acetylcholine. 3. Both D-α-AA and HA-966 would selectively block or depress the visual response with respect to the response to the control excitant. At the stage the visual input was blocked, responses to the agonists N-methyl-D-aspartate (NMDA), DL-homocysteic acid (DLH) and glutamate were also greatly reduced or blocked. At dose levels below those causing a significant reduction in the visual response, D-α-AA and HA-966 would selectively depress responses to NMDA and DLH with respect to the response to glutamate. 4. GDEE was relatively ineffective in blocking either agonist responses or the visual response and only produced a significant reduction in either at dose levels that caused a similar depression in the response to acetylcholine. DAP would block responses to DLH but produced no significant effect on the visual response or the responses to glutamate and acetylcholine. 5. The cholinergic antagonists atropine and dihydro-β-erythroidine (DHβE) blocked responses to acetylcholine without significantly reducing either visual driving or the response to DLH. 6. The effects were the same for X and Y cells in the dorsal lateral geniculate nucleus (dLGN). There was also no distinctions between ‘on’ and ‘off’ centre types of each of the two groups. 7. The significance of these results is discussed. It is argued that they reintroduce the possibility that either L-aspartate, L-glutamate or a similar substance may be the transmitter mediating the optic nerve input to the cat dLGN.

Abstract: 1. In an effort to identify the neurotransmitter released from terminals of the lateral olfactory tract (LOT) we have studied excitatory amino acid agonist and antagonist actions on population and single-unit responses in submerged and perfused slices of rat prepyriform cortex. Previous studies suggest that the transmitter at this synapse is either aspartate (Asp) or glutamate (Glu). 2. The field potential reflecting the monosynaptic activation of pyramidal neurons after stimulation of the LOT was reversibly depressed by bath perfusion of agonists, with an order of potency being kainic acid (KA) greater than N-methyl-DL-aspartate (NMDA) greater than homocysteic acid (HC) greater than Asp = Glu. 3. The synaptic field potential was essentially unaffected by DL-alpha-aminoadipic acid (AA), 2-amino-3-phosphonopropionic acid (APP), and DL-alpha-diaminopimelic acid (DAPA), all presumed to be selective for Asp receptors, and by L-glutamic acid diethyl ester (GDEE), presumed to be specific for Glu receptors. The field potential was depressed or abolished by 2-amino-4-phosphonobutyric acid (APB), an agent known to block Glu responses in insect muscle. 4. The effects of ionophoretic application of agonists were studied on single neurons recorded extracellularly. While there was some variability among neurons in relative agonist potency, all neurons were excited by the five agonists with relative potencies in general similar to those observed for the field potentials. 5. Responses to Glu and Asp were unaffected by AA, GDEE, and APB at concentrations up to 5 X 10(-3) M. Responses to KA, NMDA, and HC were often depressed by APB but were unaffected by the other antagonists. The excitation on stimulation of the LOT was consistently, rapidly, and reversibly blocked by APB. 6. These observations are not consistent with either Glu or Asp being the neurotransmitter of the LOT.

Abstract: The differential susceptibility to blockade by six antagonists of the excitatory actions produced by a number of acidic amino acids has been determined. Four of these, DL-α-aminoadipate, 2-amino-5-phosphonovalerate, 2-amino-6-phosphonocaproate and γ-D-glutamylglycine block the action of N-methyl-D-aspartate (NMDA) most effectively and those of L-glutamate, trans-1-amino-1,3-dicarboxy-cyclopentane (trans-ADCP) and kainate the least. Cis-2,3-piperidine dicarboxylate resembles the other four, save in that kainate excitations are also powerfully blocked. L-glutamate diethylester was most effective against L-glutamate and least against NMDA: kainate and trans-ADCP excitations were also relatively unaffected. The data support the hypothesis of the existence of at least three classes of amino acid receptor (NMDA-, L-glutamate- and kainate-excited), and the possibility of a fourth category activated by trans-ADCP must be considered.

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

Abstract: A series of piperidine dicarboxylates (PDA) have been tested for excitatory amino acid agonist and antagonist activity and for synaptic depressant properties in the spinal cords of frogs and immature rats in vitro and of cats in vivo. The substances tested comprised (±)-cis-2,3-PDA, (±)-cis-2,4-PDA, (±)-cis-2,5-PDA, (±)-cis-2,6-PDA, (±)-trans-2,3-PDA, (±)-trans-2,3-PDA and both (+) and (−) forms ofcis-2,3-PDA. Peak excitatory amino acid agonist activity was observed with (±)-trans-2,3- and (±)-trans-2,4-PDA. Excitatory amino acid antagonism and synaptic depressant activity was observed only withcis-dicarboxylates, this activity being greatest in the 2,3-analogue. The agonist actions of piperidine dicarboxylates were effectively depressed by the specific NMDA receptor antagonist, (−)-2-amino-5-phosphonovalerate and, where tested, also byd-α-aminoadipate and low concentrations of Mg2+. It was concluded that the major part of these agonist actions were mediated by NMDA receptors. The main structural feature of the NMDA agonist actions of these substances was considered to be their close relationship to N-alkyl-aspartic and glutamic acid molecules, with thetrans arrangement of the respective 2,3- and 2,4-situated carboxyl groups promoting most effective interaction with the active sites of the NMDA receptor. (±)-Cis-2,3-PDA depressed excitatory responses induced by NMDA, kainate, quisqualate, (±)-trans-2,3-PDA and (±)-trans-2,4-PDA, or evoked by dorsal root stimulation. Both monosynaptic and polysynaptic excitation were susceptible to the depressant action of this substance. The (−) isomer ofcis-2,3-PDA carried both excitatory amino acid agonist and antagonist activity and also the synaptic depressant properties observed with the racemic form of this substance. The (+) isomer showed little pharmacological activity. It is proposed that the structure-activity features of these heterocyclic amino acids indicate some of the conformational requirements for interaction with physiological excitatory amino acid receptors.

Abstract: 1 Synaptic potentials and the responses of frog spinal cord to various acidic amino acids were examined by means of the sucrose gap recording technique. 2 Divalent cations (50-250 microM) specifically antagonized responses evoked at N-methyl-D-aspartate (NMDA) receptors by N-methyl D,L aspartic acid (NMDLA). The rank order of potency was Ni2+ greater than Co2+ greater than Mg2+ greater than Mn2+. Responses to glutamate and aspartate were relatively insensitive to these concentrations of divalent cations. 3 The rank order of potency for divalent ions (1 mM) for antagonism of synaptic transmission in bullfrog sympathetic ganglia was Mn2+ greater than Co2+ greater than Ni2+ greater than Mg2+. Thus synaptic transmission in ganglia was especially sensitive to Mn2+ whereas NMDLA responses were especially sensitive to Co2+ and Mg2+. 4 It was possible to depress selectively the dorsal root-dorsal root potential (DR-DRP) and dorsal root-ventral root potential (DR-VRP) of frog spinal cord using low doses of Co2+ or Mg2+ which did not affect VR-DRP (ventral root-dorsal root potential). It was not possible to produce this selective depression of DR-DRP and DR-VRP with Mn2+, as this cation non-selectively depressed all responses. 5 These results suggest that: (i) divalent cations do not antagonize NMDLA responses by blocking Ca2+ channels which may mediate the response; (ii) postsynaptic NMDA receptors are activated by a neurotransmitter involved in the DR-DRP and DR-VRP pathways but not by any neurotransmitters involved in the VR-DRP pathway; (iii) the neurotransmitter activating NMDA receptors in amphibian spinal cord may be an aspartate-like substance rather than aspartate itself or glutamate.

Abstract: The possible role of excitatory amino acids in the striatum in mediating tonic activity in the electromyogram (EMG) was studied. Glutamate diethylester (GDEE) (100-400 nmoles) induced a tonic activity in the EMG in a dose-dependent way when injected into the striatum. This effect was well antagonized by intrastriatal injection of quisqualic acid (5 and 25 nmoles), less by kainic acid (5 nmoles) and not significantly by N-methyl-D-aspartate (NMDA) (30 nmoles). Systemic administration of naloxone (2 mg/kg IP) did not inhibit the GDEE-induced activity in the EMG. The tonic activity in the EMG, induced by systemic administration of morphine (15 mg/kg IP) was not significantly influenced by injection of GDEE (200 nmoles) into the striatum, but was first decreased and then slightly enhanced by intrastriatal injection of quisqualic acid (25 and 50 nmoles), not affected by kainic acid (5 nmoles) and first slightly decreased and then strongly enhanced by NMDA (15 and 30 nmoles). Injection of kainic acid (5 nmoles), quisqualic acid (5 or 25 nmoles) or NMDA (30 nmoles) alone into the striatum did not produce any tonic activity in the EMG. Our results support the assumption that quisqualic acid, kainic acid and NMDA react with different types of receptors for excitatory amino acids in the striatum. Both quisqualicacid and NMDA showed a biphasic action, whereas kainic acid was ineffective. Furthermore, the activity in the EMG induced by morphine might be at least partly due to a functional antagonism of morphine against glutamate in striatal neurons.

Abstract: Several aspects on regulation of rat brain glutamate dehydrogenase when the enzyme catalyses the reaction of glutamate degradation have been studied. The 2-oxo-glutarate is a competitive inhibitor against glutamate and NADH competes with NAD. The enzyme seems to have three sites of binding for glutamate, two of them bind the alpha and gamma carboxylic groups and the other the NH2-group of glutamate. Between the binding of two -COO- groups, the site on the enzyme which binds the gamma -COO- group of glutamate seems to be more important than the alpha one.

Abstract: Weight Peptides [in Russian], Riga (1980), pp. 178-227. 4. A. Constantopoulos and V. A. Najjar, J. Biol. Chem., 248, 3819 (1973). 5. A. I. Cunningham, Nature, 207, 1106 (1965). 6. I. Florentin, M. Bruley-Rosset, N. Kiger, et al., Cancer Immunol. Immunother., ~, 211 (1978). 7. M. Fridkin, V. Stabinsky, V. Zakut, et al., Biochim. Biophys. Acta, 496, 203 (1977). 8. V. A. Najjer, M. K. Chandhuri, D. Konopinska, et al., in: Augmenting Agents in Cancer Therapy, E. M. Hersh et al., eds., New York (1981), pp. 459-478. 9. V. A. Najjer and K. Nishioka, Nature, 228, 672 (1970). I0. J. H. Phillips, G. F. Babcock, and K. Nishioka, J. Immunol., 126, 915 (1981). ii. Y. Stabinsky, M. Fridkin, V. Zakut, et al., Int. J. Peptide Protein Res., 12, 130 (1978). 12. L. Steinman, E. Tzehoval, I. R. Cohen, et al., Eur. J. Immunol., 8, 29 (1978). 13. E. Tzehoval, S. Segal, Y. Stabinsky, et al., Proc. Natl. Acad. 8ci. USA, 75, 3400 (1978).

Abstract: Several lines of evidence suggest that striatal cholinergic inter-neurones receive an excitatory input from the cerebral cortex which utilizes an excitatory amino acid, L-glutamate or L-aspartate, as its neurotransmitter. Cortical ablation reduces striatal high-affinity glutamate uptake1,2 and concomitantly decreases acetylcholine turnover3. The destruction of cholinergic, as well as other, neurones of the striatum by the rigid glutamate analogue kainic acid seems to require a functional excitatory amino acid innervation4,5. Furthermore, electron microscopic studies6 suggest the existence of a cortical input to the aspiny dendrites of neurones morphologically similar to the presumed striatal cholinergic interneurones7,8. The recent discovery of preferential antagonists of amino acid-induced excitation in the vertebrate central nervous system has led to the classification of excitatory amino acid receptors into three subtypes: the N-methyl-D-aspartate (NMDA), the quisqnalate-and the kainate-preferring receptors9. We have now attempted to characterize the receptor(s) mediating the excitatory amino acid influence on striatal cholinergic neurones by investigating the effects of specific excitatory amino acid receptor agonists and antagonists on the release of 3H-acetylcholine from slices of the rat corpus striatum. We find that excitatory amino acid analogues evoke a tetrodotoxin-sensitive release of 3H-acetylcholine from rat striatal slices superfused in Mg2+-free medium, NMDA and ibotenate being the most potent and kainate and quisqualate the least potent. This effect is antagonized by Mg2+ and by (−)2-amino-7-phosphonoheptanoate (−APHept) and (±)2-amino-5-phosphonopentanoate (±APPent) but not by glutamic acid diethyl ester (GDEE). These data suggest the involvement of a NMDA-type receptor in the excitatory amino acid influence on striatal acetylcholine release.

Abstract: With the exception of acetylcholine at the motoneurone–Renshaw cell synapse, the excitatory chemical transmitters of the major pathways in the vertebrate brain are unknown1. However, on the basis of biochemical and electrophysiological experiments at the extracellular level, the excitatory acidic amino acids glutamate and aspartate have long been suggested as the most likely candidates for this role2–4. Unfortunately, difficulties with recording intracellularly from postsynaptic cells at many of these synapses and the lack of selective antagonists have curtailed the criteria by which the identification of glutamate and aspartate as transmitters can be explored. We have now made intracellular recordings from granule cells of the dentate gyrus in vitro and report that γ-D-glutamylglycine (γ-DGG) is an effective antagonist of the excitatory postsynaptic potential (e.p.s.p.) evoked by stimulation of the medial perforant path (PP). During this antagonism the reversal level of the e.p.s.p. remains the same and the passive membrane properties of the postsynaptic membrane are unaltered. Quantal analysis shows that the reduction in the e.p.s.p. amplitude can be accounted for almost entirely by a reduction in quantal size while the quantal content remains unchanged, indicating a direct reduction in the postsynaptic sensitivity to the endogenous transmitter. (±)2-Amino-5-phosphonovalerate (APV) proved to be ineffective as a blocker of the PP-evoked e.p.s.p. and cis-2,3-piperidinedicarboxylate(PDA) caused excitation. The selectivity of γ-DGG and APV against the depolarization induced by the iontophoretic application of N-methyl-D-aspartate (NMDA), quisqualate and kainate closely resembled that in spinal cord. This constitutes the first evidence from a readily identifiable synapse in support of the view that excitatory amino acid receptors of the quisqualate/kainate type are involved in synaptic transmission and offers further support for the suggestion that the endogenous transmitter is aspartate or glutamate.