The action of six antagonists of the excitatory amino acids on neurones of the rat spinal cord

The effects of D-alpha-aminoadipic acid on long-term potentiation in the hippocampus of the rat in vitro

Many studies on long-term potentiation (LTP) in hippocampal region CA1 focus on receptor-mediated events that are often presumed to be linked to postsynaptic processes. Whereas it is now well-known that LTP consists of multiple components involving increases in postsynaptic responsiveness as well as enhanced presynaptic release of transmitter, little specific information has accrued on the nature of the presynaptic receptor-linked events. In the course of a series of experiments examining the actions of several antagonists of N-methyl-D-aspartate (NMDA) receptors on LTP, we made certain observations that suggested the role of a novel type of amino acid receptor which possibly was located presynaptically and that seemed to contribute to the induction of LTP. LTP evoked in region CA1 following high frequency stimulation (HFS) of the Schaffer collateral-commissural pathway measured 20-30 min after HFS always was attenuated incompletely when induced during administration of DalphaAA at doses ranging from 50 mu M to as high as 1000 mu M, whereas 2-amino-5-phosphonopropionate (AP5), at a concentration of 30 mu M, always abolished the process completely. 6,7-Dinitroquinoxaline-2,3-dione (DNQX) (10 mu M) administered alone also did not block LTP completely unless delivered in combination with DalphaAA. These non-AP5-like effects of DalphaAA could not be attributed to incomplete antagonism of postsynaptic NMDA receptors, since DalphaAA (200 mu M) completely and reversibly blocked the membrane depolarising effects of NMDA, as assessed through intracellular recording. Furthermore, the pharmacologically isolated NMDA-receptor-mediated component of the low-frequency, stimulus-evoked synaptic response was always abolished reversibly by DalphaAA (200 mu M). The most parsimonious explanation of these data is that a receptor which is only activated during HFS, is sensitive to the antagonising actions of AP5 and possibly also to DNQX but not to DalphaAA, and which could conceivably exist on terminals of the Schaffer collateral-commissural fibres, makes a significant contribution to LTP.

The effect of trans-ACPD, a metabotropic excitatory amino acid receptor agonist, on the responses of primate spinothalamic tract neurons

The responses of primate spinothalamic tract (STT) neurons to innocuous and noxious mechanical stimuli applied to the skin can be enhanced for more than an hour following prolonged noxious stimulation. This increased responsiveness is thought to reflect sensitization of dorsal horn neurons and may help account for secondary hyperalgesia and mechanical allodynia. The proposal that central sensitization is due to the activation of second messenger system was tested in this study by examining the effect of trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid), an agonist of metabotropic excitatory amino acid (EAA) receptors, introduced into the dorsal horn by microdialysis. A low dose of trans-ACPD resulted in an increase in the responses of STT cells to an innocuous mechanical stimulus (BRUSH), but no increase in the responses to noxious mechanical and thermal stimuli or in the excitation produced by iontophoretically applied EAAs. A high dose of trans-ACPD caused a transient increase in background activity, but no change in the responsiveness of spinothalamic cells to any of the test stimuli. It is concluded that low doses of trans-ACPD can selectively enhance transmission through interneuronal pathways mediating tactile inputs to spinothalamic cells.

Pharmacological characterization of non-NMDA subtypes of glutamate receptor in the neonatal rat hemisected spinal cord in vitro

1. A grease-gap technique was used to record depolarizing responses to alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) in the hemisected spinal cord of the neonatal rat. The pharmacology of non-NMDA subtypes of glutamate receptor was investigated with the novel quinoxalinedione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo (F)-quinoxaline (NBQX) and with a series of barbiturates. 2. NBQX antagonized AMPA- and kainate-, but not NMDA- induced depolarizations. The near parallel shifts of the major part of the dose-response curves for AMPA and kainate by NBQX gave pA2 values (+/- s.e.) of 6.7 +/- 0.2 and 6.8 +/- 0.2 respectively, consistent with a common site of action for these two agonists. 3. Below the 50% level at which these pA2 values were calculated, however, an NBQX-resistant plateau was seen within the kainate, but not the AMPA, dose-response curve. 4. In decreasing order of potency, methohexitone, secobarbitone, thiopentone, pentobarbitone and phenobarbitone preferentially reduced kainate-, rather than AMPA- and NMDA-, induced depolarizations. Methohexitone was also the most selective with IC50S against kainate, AMPA and NMDA of 31 +/- 7, 172 +/- 47 and greater than 200 microM respectively. 5. The NBQX-resistant plateau seen within the kainate dose-response curve was reduced by methohexitone. Kainate antagonism by methohexitone was not reduced by 50 microM picrotoxin. 6. We conclude that, while mixed agonist actions may hamper demonstration of antagonist selectivity, depolarizations induced by the non-NMDA ionotropic agonists, AMPA and kainate, are mediated in part via distinct receptors.

Excitatory Actions of the Metabotropic Excitatory Amino Acid Receptor Agonist, trans-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate (t-ACPD), on Rat Thalamic Neurons In Vivo

The metabotropic excitatory amino acid receptor agonist trans-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate (t-ACPD) was applied to rat ventrobasal thalamic neurons by iontophoresis. This agonist typically evoked an excitatory response which was slower in onset and of longer duration than responses to the other excitatory amino acid agonists, N-methyl-aspartate, kainate or (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. Responses to t-ACPD were resistant to the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and kynurenate. These results suggest that t-ACPD may exert its effects via the so-called 'metabotropic' excitatory amino acid receptor. The putative antagonists at this receptor, d-2-amino-4-phosphono-butyrate (d-AP4), l-2-amino-4-phosphono-butyrate (l-AP4) and l-2-amino-3-phosphono-propionate (l-AP3), were able to reduce responses to t-ACPD under certain circumstances. However, such antagonism was always accompanied by similar reductions in excitatory responses to other agonists. These non-selective effects would appear to limit the usefulness of AP4 and AP3 as antagonists of t-ACPD.

Trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid) elicited oscillation of membrane potentials in rat dorsolateral septal nucleus neurons recorded intracellularly in vitro

Glutamate receptors coupled to phosphoinositol turnover have been identified recently and named 'metabotropic' receptors. However, the exact functional roles of these receptors are still unknown. Trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid) is suggested to be the only selective agonist for metabotropic glutamate receptors. Here we report that trans-ACPD elicits membrane potential depolarization with oscillation of dorsolateral septal nucleus neurons recorded intracellulary in vitro. Our experiments also suggested that there may be multiple interactions between ionotropic quisqualate receptors and metabotropic glutamate receptors. The burst firing induced by high concentrations of trans-ACPD suggests that excessive activation of metabotropic glutamate receptors may lead to cellular toxicity or be associated with clinical disorders such as epilepsy.

Pharmacological and functional characteristics of metabotropic excitatory amino acid receptors

Until recently the metabotropic excitatory amino acid receptor could only be distinguished from ionotropic receptors by the nature of its second messenger system--phosphoinositide hydrolysis. However, the advent of new pharmacological tools, in particular the selective agonist trans-ACPD, has now allowed this receptor to be distinguished pharmacologically. Darryle Schoepp, Joel Bockaert and Fritz Sladeczek analyse the new data which can be correlated to functional responses and linked with physiological and pathological conditions.

Selective activation of phosphoinositide hydrolysis by a rigid analogue of glutamate

Ibotenate and trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) are rigid analogues of glutamate. Ibotenate has been shown to activate phosphoinositide hydrolysis in rat brain slices. We now report that trans-ACPD also stimulates phosphoinositide hydrolysis but with much higher potency and efficacy than ibotenate. The pharmacological profiles, regional distributions, and developmental regulation of the responses to ibotenate and trans-ACPD are similar, suggesting that these agonists act at the same site. However, trans-ACPD is the first agonist described that is selective for this receptor relative to other excitatory amino acid receptor subtypes.

Behavioral classification of excitatory amino acid receptors in mouse spinal cord

Intrathecal injections of excitatory amino acid (EAA) agonists to the spinal cord of mice produces behavioral activation manifest as biting and scratching of the hindquarters. The dose-response relationship of EAA (N-methyl-D-aspartate (NMDA), kainate, quisqualate and glutamate)-induced activation revealed qualitative and quantitative differences in their pattern of action, suggesting that these agonists act at distinct receptors. Evaluation of the blockade of EAA-induced bites by a series of antagonists: DL-2-amino-5-phosphonovalerate (APV), gamma-D-glutamyl glycine (DGG), kynurenate and glutamylaminomethylsulphonate (GAMS), indicated that selective activation of the NMDA, quisqualate and kainate receptors can be demonstrated using this behavior. The NMDA receptors could be subdivided on the basis of different sensitivity to kynurenate and APV. Antagonist-resistant components of both kainate and quisqualate action were also shown. Thus, the biting behavior induced by the administration of intrathecal EAA agonists can be used as a relatively selective behavioral tool for assessing the pharmacological profile of action of excitatory amino acid agonists and antagonists in the spinal cord.

The N-methyl-D-aspartate receptor and burst firing of CA1 hippocampal pyramidal neurons

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological study of the cortical-induced excitation of subthalamic nucleus neurons in the rat: evidence for amino acids as putative neurotransmitters

Extracellular records were made from subthalamic nucleus neurons during microiontophoretic application of drugs and stimulation of the corticosubthalamic nucleus pathway. In 87% of the subthalamic nucleus cells, cortical stimulation induced a powerful excitation, consisting of a burst of 1-7 spikes. This projection must arise from a large area of the cortex since stimulation of nearly all the ipsilateral cortex and the rostral two-thirds of the contralateral cortex was found to influence the activity of subthalamic nucleus neurons. Experiments were undertaken in order to determine the identity of the neurotransmitter involved in the corticosubthalamic nucleus pathway. Glutamic acid diethyl ester reversibly suppressed subthalamic nucleus excitations induced by ipsi- or contralateral cortical stimulation or microiontophoretically applied glutamate. On the same cells, this compound had no effect on acetylcholine-evoked excitation and gamma-aminobutyric acid-evoked inhibition and subthalamic excitation induced by stimulation of the tegmenti pedunculopontine nucleus. Atropine at doses which antagonized the acetylcholine response, flupenthixol at dose which antagonized the dopamine response, and bicuculline at doses which antagonized the gamma-aminobutyric acid response failed to block excitations evoked by cortical stimulation and by glutamate. These experiments excluded a role for acetylcholine, dopamine and gamma-aminobutyric acid in the cortically evoked excitation of subthalamic nucleus cells. Since an amino acid seemed to play a role as neurotransmitter of the corticosubthalamic nucleus pathway, further experiments were designed to confirm these data and to determine the contribution of each amino acid receptor type in the cortical-induced excitation of subthalamic cells. All the subthalamic cells recorded were also excited by microiontophoretically applied N-methyl-D-aspartic, quisqualic and kainic acids. The cortical-evoked activation of subthalamic nucleus neurons was reversibly suppressed by kynurenic acid and cis-2,3-piperidine dicarboxylic acid, two broad-spectrum antagonists of excitatory amino acids, microiontophoretically applied at doses which also blocked excitations induced by N-methyl-D-aspartic, quisqualic and kainic acids. Application of 2-amino-5-phosphonovaleric acid inhibited excitation induced by N-methyl-D-aspartic acid but not those elicited by quisqualic or kainic acid, while glutamate excitation was only slightly affected. This compound had no effect on the cortically evoked excitation of subthalamic nucleus neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Magnesium ions inhibit the stimulation of inositol phospholipid hydrolysis by endogenous excitatory amino acids in primary cultures of cerebellar granule cells

Omission of Mg2+ from the incubation buffer results in a six- to eightfold increase in [3H]inositol-1-phosphate ([3H]Ins-1-P) accumulation in primary cultures of cerebellar granule cells at 7-9 days in vitro. This increase is reversed by low concentrations of 2-amino-5-phosphono-valerate (APV), a result indicating that the absence of Mg2+ facilitates the activation of a specific receptor by the endogenous excitatory amino acids (presumably L-glutamate and L-aspartate) released from the granule cells. The absence of Mg2+ also potentiates the action of exogenously applied N-methyl-D-aspartate (NMDA), L-glutamate, L-aspartate, and kainate. In contrast, the action of quisqualate is virtually unaffected by Mg2+ and is resistant to APV inhibition. Addition of the depolarizing agent veratridine enhances the accumulation of [3H]Ins-1-P also in Mg2+-containing buffer. The action of veratridine is antagonized by APV, a result suggesting that, under depolarized conditions, the NMDA receptor can be activated by the endogenously released excitatory amino acids, despite the presence of Mg2+. Accordingly, in the presence of Mg2+, veratridine potentiates the action of exogenously applied NMDA but does not facilitate the action of quisqualate.

A novel spinal cord slice preparation from the rat

A spinal cord slice preparation is described. The lumbar enlargement of weanling rats is exposed by laminectomy, the dorsal and ventral roots cut and a portion of cord consisting of about 4 segments removed and chilled to 3 degrees C. A modified tissue chopper is used to produce 400 micron dorsoventral longitudinal slices. The slices are maintained in a low-volume, continuously perfused tissue chamber at the interface between warm moist 95% O2/5% CO2 and oxygenated artificial cerebrospinal fluid, and remain viable for over 8 h. An extracellular recording of excitatory amino acid-induced activity of a dorsal horn lamina IV cell is shown. Excitatory amino acid antagonists applied iontophoretically and in the perfusate have actions similar to those in the adult spinal cord in vivo. This in vitro preparation of the cord has permitted stable extracellular recordings from single cells of 2 h or more, and has the potential for intracellular investigation of spinal cord neurones.

Long-term potentiation in the dentate gyrus: induction and increased glutamate release are blocked by D(-)aminophosphonovalerate

D(-)Aminophosphonovalerate, a specific antagonist of the N-methyl-D-aspartate subtype of glutamate receptor, was perfused through a push-pull cannula into the dentate gyrus of rats anaesthetized with urethan in order to observe its effect on the induction and maintenance of long-term potentiation and on the increase in release of endogenous glutamate associated with long-term potentiation. The amplitude of the population spike evoked by single test shocks to the perforant path was significantly depressed by 100 microM D(-)aminophosphonovalerate, but there was a minimal effect on the slope of the population excitatory postsynaptic potential, or on the concentration of glutamate released into the perfusate. A brief high-intensity tetanus given to the perforant path while D(-)aminophosphono-valerate was being perfused failed to induce long-term potentiation or the sustained increase in glutamate release associated with long-term potentiation. Short-term post-tetanic potentiation was not affected. After wash-out of D(-)aminophosphonovalerate, a second high-frequency train produced both long-term potentiation and an increase in glutamate release which was sustained for the subsequent 1 h period of observation. D(-)Aminophosphonovalerate did not suppress long-term potentiation once it had been induced. D(-)Aminophosphonovalerate (100 microM) did not itself affect in vivo release of glutamate. However, in a separate series of in vitro experiments, D(-)aminophosphonovalerate at concentrations of 50 microM and above was found to depress the Ca2+ -dependent, K+-stimulated release of preloaded [14C]-glutamate from dentate slices.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate and Co2+ selectively block depolarization and burst firing of rat hippocampal CA1 pyramidal neurones by N-methyl-D-aspartate

Intracellular recordings from pyramidal neurones during microiontophoretic ejection of N-methyl-D-aspartate and quisqualate into the pyramidal cell layer of the CA1 region of the rat hippocampal slice showed that both amino acids caused depolarization and evoked spike activity. Whereas quisqualate evoked tetrodotoxin-sensitive spikes, those produced by N-methyl-D-aspartate consisted of bursts of tetrodotoxin-sensitive action potentials superimposed on an underlying depolarizing shift of membrane potential. Both membrane depolarization and the superimposed depolarizing shifts associated with N-methyl-D-aspartate excitation were selectively and reversibly antagonized by the D(-) isomer of 2-amino-5-phosphonovalerate and Co2+. Both amino acids caused an increase in membrane conductance when small ejection currents were used, and the depolarizing response to these compounds was prevented by current injection. However, only the increase by N-methyl-D-aspartate was blocked by 2-amino-5-phosphonovalerate and Co2+. These results provide evidence to support the suggestion that different mechanisms underlie the excitatory response to N-methyl-D-aspartate and quisqualate in CA1 pyramidal neurones.

Synaptic excitation in cultures of mouse spinal cord neurones: receptor pharmacology and behaviour of synaptic currents

Fast monosynaptic excitatory post-synaptic potentials between spinal cord neurones in cell culture (s.c.-s.c. e.p.s.p.s) were studied with current-clamp and two-electrode voltage-clamp methods. The reversal potential, response to acidic amino acid antagonists, and behaviour of the synaptic current were examined. The amplitude of the e.p.s.p. increased with membrane potential hyperpolarization and decreased with depolarization. The reversal potential of the e.p.s.p. was +3.8 +/- 2.5 mV (mean +/- S.E. of mean). The reversal potential for responses to ionophoretically applied L-glutamate and L-aspartate was also near 0 mV. The acidic amino acid antagonist, cis-2,3-piperidine dicarboxylic acid (PDA, 0.25-1.0 mM) reversibly antagonized the monosynaptic e.p.s.p.s as well as responses to kainate (KA) or quisqualate (QA). The selective N-methyl-D-aspartate antagonist, (+/-) 2-amino-5-phosphonovaleric acid (APV), had little effect on either the monosynaptic e.p.s.p.s or responses to QA or KA at concentrations that abolished responses to L-aspartate. Under voltage clamp, the peak synaptic current (e.p.s.c.) was linearly related to the membrane potential, increasing in amplitude with hyperpolarization and decreasing with depolarization from the resting potential. The decay of a somatic e.p.s.c. was well fitted by a single exponential function with a time constant of 0.6 ms at 25 degrees C. E.p.s.c.s which had proximal dendritic locations had decay time constants of 1-2 ms. The decay time constant was voltage-insensitive between -80 and +10 mV. We suggest that an acidic amino acid receptor other than that for NMDA mediates excitatory transmission at the s.c.-s.c. synapse; and that the underlying conductance mechanism is voltage insensitive with an estimated mean channel lifetime of less than 1 ms.

Blocking action of pentobarbital on receptors for excitatory amino acids in the guinea pig hippocampus

The actions of pentobarbital sodium (Pent) on receptors for glutamate (Glu) and related compounds were studied in thin sections of the guinea pig hippocampus. Depolarizations induced by Glu and quisqualate (Quis) in CA3 neurons were reduced in amplitude during iontophoretic administration of Pent. This action of Pent was not accompanied by any noticeable changes in membrane potential or neuron input resistance. Depolarizations induced by N-methyl-D-aspartate were less sensitive to Pent. The fast kainate (KA) response was as susceptible as the Glu response, whereas the slow KA response was unaffected by Pent in three quarters of the neurons examined. Pent suppressed the Glu response at lower concentrations than required to potentiate responses to gamma-amino butyric acid. Excitatory postsynaptic potentials (EPSPs) elicited by stimulation of mossy fibers were suppressed by Pent. The EPSPs were a little more resistant to Pent than were the Glu responses. These results indicate that Pent blocks receptors for excitatory amino acids in the hippocampus. Of the three different populations of the receptors, Quis receptors are the most sensitive to Pent and KA receptors are the least sensitive. The suppression of the EPSPs is in accordance with the notion that Glu is the transmitter released from mossy fibers.

Amino acid neurotransmission between fimbria-fornix fibers and neurons in the lateral septum of the rat: a microiontophoretic study

We investigated the nature of the excitatory amino acid and the type of amino acid receptor involved in the projection of fimbria-fornix (fi-fx) fibers on neurons in the lateral septal complex (LSC) of the rat. It appeared that neurons which were strongly orthodromically activated (SOA) by stimulation of fi-fx fibers were excited by glutamate (GLU) and aspartate (ASP) at much lower ejecting currents than neurons which were only weakly orthodromically excited. In addition, GLU was a stronger agent than ASP, particularly in SOA septal cells. Two amino acid antagonists tested, glutamic acid diethylester (GDEE) and 2-amino-5-phosphonovaleric acid (2-APV), selectively antagonized responses to the amino acid agonists quisqualate (QUIS) and N-methyl-D-aspartate (NMDA), respectively. They also depressed GLU- and ASP-induced responses, although in that case the antagonists frequently had to be expelled with currents higher than those needed to block QUIS- and NMDA-evoked excitations. Furthermore, GDEE frequently antagonized GLU-induced responses better than ASP-evoked excitations, whereas 2-APV often blocked responses to ASP more effectively than those to GLU. It was observed that GDEE, ejected with currents that blocked responses to QUIS reversibly, decreased the number of synaptic responses induced in SOA cells by fi-fx stimuli. Synaptically induced excitation in these neurons was consistently unaffected by 2-APV, even when the antagonist was expelled with high currents. According to these results, LSC neurons, in particular the SOA neurons, are more readily activated by GLU than by ASP. Monosynaptic excitations elicited in SOA septal cells by fi-fx stimuli appear to be predominantly if not exclusively mediated by QUIS receptors. There are indications that GLU-induced responses in the LSC neurons are presumably mediated by the QUIS receptors. From these data it may be inferred that GLU rather than ASP is the transmitter involved in the projection of fi-fx fibers on LSC neurons.

Pharmacologic evidence for synaptic transmission mediated by non-N-methyl-D-aspartate receptors in the avian cochlear nucleus

The hypothesis that synaptic transmission between the auditory nerve and the cochlear nucleus is mediated by an excitatory amino acid acting through N-methyl-D-aspartate (NMDA) receptors was examined in an in vitro preparation of the chicken brainstem. The ability of various bath-applied excitatory amino acid receptor antagonists to inhibit synaptically-evoked responses was assessed by recording field potentials from nucleus magnocellularis (NM) following electrical stimulation of the cochlear nerve. Antagonists that selectively block responses mediated by NMDA receptors, such as D-alpha-aminoadipate and 2-amino-5-phosphonovalerate, were without effect on evoked transmission in NM. In contrast, antagonists that additionally act on non-NMDA receptors, such as cis-2,3-piperidine dicarboxylate and gamma-D-glutamylglycine, reversibly suppressed transmission. The results indicate that (1) transmission in the chicken auditory system is mediated by non-NMDA receptors, and (2) a substance(s) chemically akin to aspartate and glutamate may be the transmitter used by the auditory nerve in NM.

Blockade of amino acid-induced depolarizations and inhibition of excitatory post-synaptic potentials in rat dentate gyrus

Excitatory post-synaptic potentials (e.p.s.p.s) evoked by stimulation of the medial perforant path and depolarizations induced by excitatory amino acids were recorded from granule cells in the preparation of the hippocampal slice from the rat. The effects of (+/-)-2-amino-5-phosphonovalerate (APV), gamma-D-glutamylglycine (gamma DGG) and cis-2,3-piperidinedicarboxylate (PDA), antagonists of excitatory amino acids on these phenomena were compared. gamma DGG was the most effective antagonist of the e.p.s.p. Its action was reversible and not associated with any change in the passive membrane properties of the granule cells or in the apparent reversal potential of the e.p.s.p. Quantal analysis showed that the reduction in the e.p.s.p. paralleled the decrease in quantal size rather than quantal content, confirming a post-synaptic site of the action of gamma DGG. The potency of gamma DGG against the exogenous agonists was N-methyl-D-aspartate greater than kainate greater than or equal to quisqualate. APV had very little effect on the e.p.s.p. but was a selective antagonist of N-methyl-D-aspartate-induced depolarizations. PDA depolarized granule cells and increased their membrane input resistance. Although gamma DGG was a potent antagonist of both glutamate- and aspartate-induced depolarizations, no clear pattern of specificity could be found. The action of glutamate was unaffected by APV. These results indicate that the receptor for the transmitter at the synapses formed by the fibres of the perforant path with the granule cells is of the quisqualate and/or kainate type. The present data are consistent with the biochemical evidence that glutamate may be the endogenous transmitter at his synapse.

Effects of the glutamate analogue AMPA and its interaction with antagonists on cultured rat spinal and brain stem neurones

Glutamate and the glutamate analogue AMPA ((RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid) caused depolarizations of cultured rat spinal and brainstem neurones which were reversibly antagonized by the glutamate antagonist glutamic acid diethylester (GDEE) but not by 2-amino-5-phosphonovalerate (APV), an N-methyl-D-aspartate (NMDA) antagonist. In contrast, depolarizations by NMDA were blocked by APV but not by GDEE. These results suggest that the depolarization by AMPA is caused by the activation of glutamate/quisqualate-preferring receptors.

Receptors for the excitatory amino acids in the mammalian central nervous system

On the basis largely of neuropharmacological analysis, three different receptors mediating neuronal excitation can be identified. The first is activated by quisqualate and other "flexible" molecules including L-glutamate and appears to bind its ligands in a folded configuration. The second is excited by NMDA and has a more extended conformation, the spacing between the amino and the omega-carboxylate groups being the determinant of specificity. The third type accepts kainate and appears to possess a reactive site for the unsaturated side chain which is essential to the operation of this receptor. All three classes appear to be implicated in synaptic events [although some kainate receptors at least are certainly extra-synaptic (Watkins et al., 1981)] and each appears to activate different ionophores in neuronal membranes. Of the endogenous amino acids which may function as synaptic transmitters, L-glutamate and L-cysteate seem to react preferentially with quisqualate receptors (McLennan and Lodge, 1979), while L-aspartate is more of a mixed agonist capable of reaction both with quisqualate and with the NMDA types. Whether folate has a physiological role involving kainate receptors is unknown; and the same is true of any action possessed by quinolinate. The fact that there are amino acid excitants which are pharmacologically distinct from those reacting with any of the three best known receptors suggests that at least one more class of receptor may also exist, but no further information is available at the present time. Other sites with which the pharmacologically active acidic amino acids react are identifiable neurochemically in membrane preparations derived from tissues of the central nervous system. Kinetic studies and analysis of inhibition of sodium-independent binding indicate that there are sites which accept glutamate, others binding aspartate and a third which binds kainate. However, the first does not correspond completely to the quisqualate excitatory receptor, and NMDA does not react with any of the binding sites. It is difficult to conclude then that any of these binding sites can be fully identified with the excitatory receptors. Finally, there are a number of systems which in their patterns of activity again appear completely distinct, but which presumably mediate uptake of amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

The antagonism of amino acid-induced excitations of rat hippocampal CA1 neurones in vitro

1. The effects of the ionophoretic application of a number of excitatory amino acids and antagonists to the dendrites of CA1 neurones of rat hippocampal slices maintained in vitro were examined. Cells were excited by N-methyl-DL-aspartate (NMA), kainate, quisqualate, L-aspartate and L-glutamate; NMA was unique in causing cells to fire in bursts of repetitive discharges in contrast to the sustained firing seen with the other compounds. 2. D-(-)-alpha-aminoadipate (DAA) and (+/-)-2-amino-5-phosphonovalerate (APV) were selective NMA antagonists, the latter appearing to be the more potent; in addition both compounds potentiated the responses to kainate and quisqualate. L-glutamate excitations were affected less by APV than were those of L-aspartate. The antagonist properties of APV appeared to reside with the D-(-)-isomer. 3. gamma-D-glutamylglycine (DGG) in low ionophoretic doses inhibited NMA-, kainate- and aspartate-induced cell firing but at higher doses the quisqualate and glutamate responses were also decreased. 4. Kainate and NMA responses were blocked by D-(-)-2-amino-4-phosphonobutyrate (D-APB) which also had some action against the excitatory effects of L-aspartate. L-APB had no antagonistic effects, but often produced potentiation of amino acid excitations or was itself an excitant. 5. The effects of NMA and those of kainate and quisqualate were blocked by (+/-)-cis-2,3-piperidine dicarboxylate (PDA), but this compound itself had a direct excitatory effect. L-glutamate diethylester (GDEE) did not show specific antagonism of any amino acid excitations. 6. DGG and APV did not affect ACh excitations and these selective antagonists should be of value in studying the involvement of the excitatory amino acids in synaptic transmission in the hippocampus. Because they are less potent and/or have complicating direct effects PDA, GDEE, D- and L-APB may be less useful in this regard.

Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus

1. The effects of excitatory amino acids and some antagonists applied by ionophoresis to stratum radiatum in the CA1 region of rat hippocampal slices were examined on the locally recorded field e.p.s.p. evoked by stimulation of the Schaffer collateral-commissural projection. 2. L-glutamate, L-aspartate and the more potent and selective excitatory amino acids quisqualate, kainate and N-methyl-DL-aspartate (NMA) depressed the e.p.s.p., presumably through depolarization and/or a change in membrane conductance. 3. The depression induced by kainate considerably outlasted the period of ejection whereas NMA depressions were rapidly reversible and were often followed by a potentiation of the e.p.s.p. In higher doses NMA also depressed the presynaptic fibre volley. The possible involvement of these effects in neurotoxicity and synaptic plasticity is raised. 4. The selective NMA antagonist, DL-2-amino-5-phosphonovalerate (APV) applied in doses which abolished responses to NMA, had no effect on the e.p.s.p. but prevented long term potentiation (l.t.p.) of synaptic transmission evoked by high frequency stimulation of the Schaffer collateral-commissural pathway. Other antagonists which had little or no effect on normal synaptic transmission included D-alpha-aminoadipate (DAA), the optical isomers of 2-amino-4-phosphonobutyrate (APB) and L-glutamate diethylester (GDEE). 5. In contrast, gamma-D-glutamylglycine (DGG), applied in amounts which affected quisqualate and kainate actions as well as those of NMA, was an effective synaptic antagonist whilst having no effect on the presynaptic fibre volley. 6. These results indicate that the synaptic receptor in the Schaffer collateral-commissural pathway may be of the kainate or quisqualate type. Although NMA receptors do not appear to be involved in normal synaptic transmission in this pathway they may play a role in synaptic plasticity. The interaction of L-glutamate and L-aspartate with these receptors is discussed.

Methyltetrahydrofolate as an antagonist of excitatory amino acids on spinal neurones

Folate and N5-methyl-5,6,7,8-tetrahydrofolate (MTHF) have been reported to have excitatory effects upon cortical neurones, possibly due to interaction with kainate receptors. On spinal neurones these compounds have been found inactive as excitants; however MTHF is a weak antagonist of kainate and N-methyl-D-aspartate excitations, and less effectively blocks quisqualate also. It thus resembles the antagonist 2,3-piperidine dicarboxylate.