The action of D-alpha-aminoadipate on excitatory amino acid receptors of rat thalamic neurones

Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Astrocytes have been promoted as a possible mechanistic target for anaesthetic hypnosis. The aim of this study was to explore this using the neocortical brain slice preparation. The methods were in two parts. Firstly, multiple general anaesthetic compounds demonstrating varying in vivo hypnotic potency were analysed for their effect on "zero-magnesium" seizure-like event (SLE) activity in mouse neocortical slices. Subsequently, the effect of astrocyte metabolic inhibition was investigated in neocortical slices, and compared with that of the anaesthetic drugs. The rationale was that, if suppression of astrocytes was both necessary and sufficient to cause hypnosis in vivo, then inhibition of astrocytic metabolism in slices should mimic the anaesthetic effect. In vivo anaesthetic potency correlated strongly with the magnitude of reduction in SLE frequency in neocortical slices (R(2) 37.7 %, p = 0.002). Conversely, SLE frequency and length were significantly enhanced during exposure to both fluoroacetate (23 and 20 % increase, respectively, p < 0.01) and aminoadipate (12 and 38 % increase, respectively, p < 0.01 and p < 0.05). The capacity of an anaesthetic agent to reduce SLE frequency in the neocortical slice is a good indicator of its in vivo hypnotic potency. The results do not support the hypothesis that astrocytic metabolic inhibition is a mechanism of anaesthetic hypnosis.

Synthesis of C5-tetrazole derivatives of 2-amino-adipic acid displaying NMDA glutamate receptor antagonism

Five derivatives of 2-amino-adipic acid bearing a tetrazole-substituted in C5 position were synthesized. These compounds displayed selective antagonism towards N-methyl-D: -aspartate (NMDA) receptors compared with AMPA receptors, and they were devoid of any neurotoxicity. Among these five analogues, one exhibited a higher affinity for synaptic NMDA responses than the other four. Therefore, C5 tetrazole-substituted of 2-amino-adipic acid represent an interesting series of new NMDA receptor antagonists. This approach may be considered as a new strategy to develop ligands specifically targeted to synaptic or extra-synaptic NMDA receptors.

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.

Inhibition of the glutamate transporter and glial enzymes in rat striatum by the gliotoxin, alpha aminoadipate

1. The effect of the gliotoxic analogue of glutamate, alpha aminoadipate, on the high affinity transport of D-[3H]-aspartate into a crude striatal P2 preparation, and on the activity of two enzymes of which glutamate is the substrate has been examined. 2. The L-isomer of alpha aminoadipate competitively inhibited the transport protein, with a Ki value of 192 microM, whereas the D-isomer of alpha aminoadipate was ineffective. The potent convulsant, L-methionine-S-sulphoximine, was also without effect on the activity of the glutamate transport protein. 3. L-alpha Aminoadipate was a competitive inhibitor of both glutamine synthetase, and gamma-glutamylcysteine synthetase, with Ki values of 209 microM and 7 mM respectively. Once again, the D-isomer of alpha aminoadipate was a far weaker inhibitor of either enzyme. 4. The results are discussed in terms of the mechanism of action of alpha aminoadipate in causing toxicity of glial cells.

Rapid increase of intracellular Ca2+ concentration caused by aminoadipic acid enantiomers in retinal Müller cells and neurons in vitro

The ability of the gliotoxic compounds D,L-, D- or L-2-aminoadipic acid (AAA) to increase selectively the intracellular concentration of free calcium ion ([Ca2+]i) was examined in Müller cells cultured with or without retinal neurons. The monitoring of [Ca2+]i following exposure to 0.06 to 6 mM AAA was performed by a microfluorometry using a fluorescent Ca2+ indicator, Fura-2 acetoxymethyl ester. A rapid increase of [Ca2+]i occurred in the Müller cells following exposure to a relatively low concentration of the L-isomer. This is compatible with the known strong gliotoxicity of this isomer. The D,L- and D-forms of AAA activated neurons at low concentrations and activated the Müller cells at higher concentrations. The D-isomer appears to act selectively on retinal neurons and may be an agonist of an excitatory amino acid receptor. These results indicate that the ability of AAA to elevate cytosolic [Ca2+]i depends on the stereospecificity of the AAA and on cell type.

Stereospecific effects of the alpha-aminoadipic acid on the retina: a morphological and electrophysiological study

In both frog and chicken an intravitreal injection of the dextrorotatory (D)-isomer of alpha-aminoadipic acid (alpha-aaa) leads to a progressive disappearance of the ERG b-wave without affecting a and c components. Tectal evoked potentials (TEP) are no longer recorded. These physiological effects are concomitant with a specific glial cell damage, without any apparent damage to neurons. The levorotatory (L)-isomer at low concentrations is more gliotoxic than the D-isomer, the ERG b-wave is suppressed, while the amplitude of both a and c components is increased. TEPs are always recorded, i.e., a visual message is still generated in the retina and transmitted to the optic tectum when the Müller cells have been damaged and the b-wave is abolished. At higher concentrations the L-isomer suppresses TEPs and damages both glial and neuronal cells. Thus alpha-aaa appears to be a good tool for analyzing ERG components, especially subcomponents of the c-wave.

The effect of D-alpha-aminoadipate on excitatory amino acid responses recorded intracellularly from motoneurones of the frog spinal cord

The excitatory responses to amino acids were recorded intracellularly from motoneurones in the isolated frog spinal cord and the effect of the antagonist D-alpha-aminoadipate examined. An unusual profile of antagonism was obtained in that with 50-100 microM D-alpha-aminoadipate the depolarization to quisqualate was unaffected (or slightly potentiated) while those to L-glutamate, L-aspartate and N-methyl-D-aspartate were all considerably attenuated. D-alpha-aminoadipate did not influence passive membrane properties although a small hyperpolarization was sometimes evident. Dorsal root evoked excitations of motoneurones, particularly those using low strength stimulation, were also susceptible to antagonism by D-alpha-aminoadipate. These data suggest a separate neuroeffector/receptor mechanism for quisqualate compared to L-glutamate, L-aspartate and N-methyl-D-aspartate.

Responses of solitary retinal horizontal cells to L-glutamate and kainic acid are antagonized by D-aspartate

Solitary horizontal cells dissociated from goldfish retinas depolarized when exposed to micromolar doses of either L-glutamate or kainic acid. The responses to both of these agonists were antagonized by D-aspartate, and unaffected by L-aspartate, L-glutamic acid diethyl ester and folic acid. the results of the present study thus suggest that L-glutamate and kainic acid may produce depolarizations of horizontal cells by interacting with pharmacologically similar membrane receptors.

Effect of D, L-alpha-aminoadipate on the mediobasal hypothalamus and endocrine function in the rat

Using the glutamate analog, D,L-alpha-aminoadipic acid (D,L-alpha AA), experiments were conducted to examine the nature, extent, and specificity of its toxicity in the mediobasal hypothalamus and to determine its effect on endocrine homeostasis. Neonatal rats received daily injections of D,L-alpha AA (4 g/kg BW) on postnatal days 5-10 and were killed at various post-treatment intervals. Sex-matched littermates were given equimolar amounts of NaCl and served as controls. Treated rats killed 18 days post injection weighed slightly less than controls and had reduced testicular, ovarian, and uterine weights, but the differences were not statistically significant. In D,L-alpha AA treated rats serum and pituitary levels of TSH and PRL were comparable to control values. Pituitary content of LH (male's and female's) and FSH (female's), however, was lower (P less than 0.05) in D,L-alpha AA treated rats than in controls, but serum levels were not significantly different. Distinct cytopathologic changes were evident in the arcuate nucleus and median eminence of D,L-alpha AA-treated rats killed at 2 and 6 h post injection only. By 12 h evidence of acute damage had largely disappeared. Both glial and ependymal cells underwent edematous swelling and necrosis, but neurons were largely unaffected. Evidence of reactive changes, such as gliosis, infiltration of microglia, and removal of debris, however, were not very conspicious. A random sample of mediobasal hypothalami of rats killed 18 days post injection failed to show any detectable lesion or residual effects of earlier pathology. Age at the time of exposure to the gliotoxin was found to be an important variable affecting both extent and duration of injury. The most deleterious effects were observed when the gliotoxin was administered in the form of a single injection on postnatal day 5 only. The results suggest that normal neuronal activity and endocrine homeostasis, specifically gonadotropin, may be irreversibly altered as a consequence of transient disruption of the glial compartment.

Effect of D- and L-alpha-aminoadipate on the efflux of L-aspartate, L-glutamate and gamma-aminobutyrate from superfused rat brain slices

D-alpha-Aminoadipate (D-AA) and L-alpha-aminoadipate (L-AA) were found to significantly reduce spontaneous efflux of [14C]L-aspartate from preloaded rat brain slices. Only D-AA significantly reduced spontaneous efflux of [14C]L-glutamate and [3H]gamma-aminobutyric acid (GABA); L-AA reduced but not significantly the efflux of these 2 labeled amino acids. D-AA reduced K+-stimulated release of [14C]L-aspartate and [14C]L-glutamate significantly, and L-AA that of [3H]GABA significantly. Since both D-AA and L-AA inhibit the uptake of L-aspartate, L-glutamate and GABA, their effects on the efflux of these amino acids are more specific. These results also suggest that it is unlikely that the depressant effect of D-AA, and the excitant effect of L-AA on neurons when applied locally by iontophoresis are secondary to the accelerated or decelerated release of more specific transmitter amino acids from neighboring cells.

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)

Effects of D-alpha-aminoadipate on excitation of afferent fibers in the lateral line of Xenopus laevis

The effects of D-alpha-aminoadipate (D alpha AA) on excitation of afferent nerve fibers in the Xenopus laevis lateral line were studied in vitro. D alpha AA reversibly suppressed spontaneous activity and excitation induced by water motion at concentrations as low as 0.25-0.5 mM. Higher concentrations (up to 10 mM) caused a greater suppression that was rapidly and fully reversible. L-alpha-Aminoadipate at 0.25-1.0 mM had no suppressive effects. Responses elicited by NMDA (1.0-2.0 mM) were the most sensitive to D alpha AA (0.25-0.5 mM), those elicited by L-aspartate and L-glutamate (1.0-2.0 mM) were less sensitive and similar, and those elicited by kainate (5-15 muM) were the least sensitive. The results provide evidence that the transmitter released by hair cells in the Xenopus lateral line interacts postsynaptically with NMDA-preferring receptors and that the transmitter is an excitatory amino acid, possibly L-glutamate or L-aspartate.

Differential activation and blockade of excitatory amino acid receptors in the mammalian and amphibian central nervous systems

1. Experiments were conducted in vitro on isolated spinal cords of frogs and immature rats and in vivo on cat spinal neurones. 2. The concept of two major types of excitatory amino acid receptors present in these preparations is summarized, one type (NMDA receptors) being activated specifically by N-methyl-D-aspartate (NMDA) and blocked by specific antagonists such as D(-)-2-amino-5-phosphonovalerate (APV), and a second type (non-NMDA receptors) characterized by insensitivity to specific NMDA antagonists. This second type may be comprised of two sub-types activated selectively by the agonists quisqualate and kainate. The putative transmitters L-glutamate and L-aspartate have mixed action on both NMDA and non-NMDA receptors. The major action of both transmitter candidates is considered to be on non-NMDA receptors, but the proportion of the composite responses mediated by NMDA receptors (at least for spinal neurones) appears to be greater for L-aspartate than for L-glutamate. 3. The preference of NMDA and non-NMDA receptors for a range of agonists is discussed. Some newer agonists are considered, in addition to several known agonists not previously discussed in terms of NMDA- and non-NMDA-receptor preference. Structure-activity relations of agonists are discussed. 4. The actions of some new amino acid antagonists are reported. Some of these have useful kainate and quisqualate blocking activity, in addition to their ability to block NMDA induced responses. 5. Evidence is presented suggesting that excitatory amino acid receptors are involved in both polysynaptic and monosynaptic excitation in the spinal cord, NMDA receptors mediating polysynaptic excitation and non-NMDA receptors monosynaptic excitation. 6. The unusual effect is reported of L-2-amino-4-phosphonobutyrate, which potently blocks spinal synaptic excitation in the absence of depressant action on excitatory amino acid-induced responses.

Effects of DL-alpha-amino adipic acid on Müller cells in frog and chicken retinae in vivo: relation to ERG b wave, ganglion cell discharge and tectal evoked potentials

In both frog and chicken, an intravitreal injection of DL-alpha-amino-adipic acid, (DL-alpha aaa) provoked a progressive depression and eventually the disappearance of the ERG b wave that was concomitant with severe damage to the Müller cells without any apparent damage to retinal neurons. Ganglion cell discharges as well as tectal evoked potentials were still recorded, i.e. a visual message was still generated in the retina and transmitted to the optic tectum, when the Müller cells had been damaged to as to provoke an abolition of the ERG b-wave. The whole of the drug-induced effects proved to be reversible.

Excitatory effect of L-aspartate and L-glutamate on Purkinje cells in rat cerebellum

L-aspartate and L-glutamate were microiontophoretically applied onto Purkinje cells and unidentified cerebellar neurons of urethane-anesthetized rats. Both amino acids produced a dose-dependent increase in the spontaneous firing rate of all cells tested. Fifty-three percent (8 of 15 cells) of the dose-response relationships for L-aspartate as compared to those for L-glutamate on Purkinje cells were not parallel, implying different mechanisms of action (suggesting different receptors). On these 8 Purkinje cells, L-glutamate was three times more potent than L-aspartate. Only thirty-three percent of the dose-response relationships (8 of 24 cells) for the two agents on the unidentified cerebellar cells were not parallel. There was no statistical difference in the potency of L-aspartate as compared to L-glutamate on these particular cells. Tests for antagonism on Purkinje cells revealed L-glutamic acid diethyl ester (GDEE) to be a more effective blocker of L-aspartate than of L-glutamate while DL-alpha-aminoadipic acid (DL alpha AA)) was not selective in antagonizing the action of either amino acid. These data are discussed in terms for L-aspartate functioning as a neurotransmitter in the cerebellum of rat and possessing receptor sites distinct from those for L-glutamate.

Glutamate and aspartate: alteration of thresholds and response patterns of auditory neurons

Iontophoretic application of the excitant amino acids glutamate and aspartate onto neurons in the chinchilla cochlear nucleus results in a lowering of the threshold of response to auditory stimuli. Neurons that display 'on'-type phasic responses to toneburst stimuli may become tonic, sustained responders with iontophoretic application of glutamate or aspartate. The ability of either glutamate or aspartate to effect changes in thresholds and response patterns of cochlear nucleus neurons is further evidence that one of these amino acids may be the afferent transmitter of the auditory nerve. The effects seen with these excitant amino acids may also provide insight into the underlying synaptic events involved in the generation of a particular response pattern.

Uptake, release, and metabolism of D- and L-alpha-aminoadipate by rat cerebral cortex

Accumulation of L-alpha-aminoadipate by rat cerebral cortical slices is a stereospecific and Na+-dependent process. The uptake of this compound is also temperature-dependent, with a Km of 1.6 X 10(-4)M for the high-affinity system. D-alpha-Aminoadipate has characteristics similar to those displayed by the L-isomer but to a lesser degree. L-Glutamate and L-aspartate inhibit the uptake of L-alpha-aminoadipate. D- and L-alpha-Aminoadipate are, respectively, weak uncompetitive and weak competitive inhibitors for the uptake of L-glutamate and L-aspartate. Both enantiomers inhibit GABA uptake but in quite different ways. The release of L-alpha-aminoadipate from the cerebral cortical slices is stimulated by a high concentration of K+ ions in the presence of Ca2+ in the perfusion buffer; the D-isomer displays this property to a lesser degree. The omission of Ca2+ markedly reduces the release of these two compounds. Less than 10% of the preloaded D- and L-alpha-aminoadipate are metabolized by the cerebral cortex during 40 min of superfusion. The possibility of L-alpha-aminoadipate as a neurotransmitter candidate is discussed.

Differentiation of kainate and quisqualate receptors in the cat spinal cord by selective antagonism with gamma-D(and L)-glutamylglycine

The D and L forms of the dipeptide, gamma-glutamylglycine depress NMDA-, L-aspartate- and kainate-induced excitation in cat spinal cord while little or nor effect on L-glutamate- and quisqualate-induced responses. The dipeptides also depress dorsal root-evoked excitation of Renshaw cells, but not acetylcholine- or ventral root-evoked excitation of these cells. The D form of the dipeptide is more potent than the L form. The results are interpreted in favour of three types of receptors for excitatory amino acids on spinal neurones, these being sensitive to the selective agonist action of NMDA, kainate and quisqualate.

Stereospecificity of the gliotoxic and anti-neurotoxic actions of alpha-aminoadipate

The glutamate (Glu) analog, DL-alpha-aminoadipate (DL-alpha AA), and the separate D and L isomers of alpha AA, were administered subcutaneously to infant mice and histopathological effects on the arcuate hypothalamic (AH) nucleus were studied. L-alpha AA induced striking gliotoxic and neurotoxic changes; D-alpha AA and DL-alpha AA respectively induced mild and extreme gliotoxic but not neurotoxic changes. The neurotoxicity of L-alpha AA is of interest in view of its known neuroexcitatory potential. The non-neurotoxicity of DL-alpha AA implies effective antagonism by D-alpha AA of the neurotoxicity of L-alpha AA, which is of interest in that D-alpha AA is recognized as an effective antagonist of amino acid excitants and is thought to block specifically at the excitatory receptor.

Effects of D,L-alpha-aminoadipate on postsynaptic amino acid responses in cultured mouse spinal cord neurons

The effects of the dicarboxylic amino acid, DL-alpha-aminoadipate (DLAA) on amino acid responses have been investigated using intracellular recordings from mouse spinal cord neurons grown in dissociated cell culture. DL-alpha-Aminoadipate markedly antagonized postsynaptic responses to iontophoretically appllied aspartate; antagonism of glutamate was much less prominent. DL-alpha-Aminoadipate altered the affinity of aspartate for its receptor while having no observed effects on aspartate-receptor cooperativity. No direct effects of DLAA on membrane potentials or passive membrane properties were seen at the currents used for antagonism. Responses to the inhibitory amino acids GABA and glycine were unaffected by DLAA.

Pharmacological identification of acetylcholine and glutamate excitatory systems in the dentate gyrus of the rat

The responses of dentate granule cells to medial septum (MS) and perforant path (PP) stimulation were examined in urethane anaesthetized rats. MS and PP stimulation evoked an orthodromic activation of granule cells which was correlated with the negative transient of the characteristic field potential elicited from each site. The effects of electrophoretic application of acetylcholine (ACh) and glutamate (Glu) were examined on granule cells identified in this manner. The excitatory action of ACh but not that of Glu was antagonized by atropine. Glutamate diethylester (GDEE) blocked the excitation produced by Glu but not ACh. The synaptically evoked excitation elicited by MS was blocked by atropine but unaltered by GDEE whereas the PP excitatory response was blocked by GDEE and unaltered by atropine. The results of this study indicate that two discrete excitatory systems are present in the dentate gyrus of the rat: a cholinergic system originating in the medial septum and a glutamate mediated system originating in the entorhinal cortex.

Selective antagonist activity of 5-aminohex-2-enedioic acid on amino acid excitation of cat spinal neurones

D- and L-5-aminohex-2-enedioic acid (AHED) and the DL-2-bromo derivative, which are conformationally restricted analogues of the excitatory amino acid antagonist D-alpha-aminoadipic acid (D alpha AA), were tested microelectrophoretically on cat spinal neurones. D-AHED reduced the excitatory action of N-methyl-D-aspartate (NMDA) to a greater extent than that of L-glutamate and the potency of this action was a half to a third that of D alpha AA. The DL-2-bromo derivative excited neurons, an effect greater than that of L-glutamate and not blocked by either D alpha AA or L-glutamic acid diethyl ester.

A simple method for the preparation of D-alpha-aminoadipic acid

High quantity (1 g and more) of racemically and chromatographically pure D-alpha-aminoadipic acid was prepared by selective metabolism of the L-isomer of the commercially available DL-alpha-aminoadipate by Pseudomonas putida. The overall yield of this preparation averaged 40%. The final product has [a]25D value of -25 degrees. This procedure can be useful in the synthesis of high purity D-alpha-amino-adipate, a compound shown recently to be a useful tool in the study of neurotransmission mechanism mediating synaptic excitation.

Selective antagonism of amino acid-induced and synaptic excitation in the cat spinal cord

1. The effects of D-alpha-aminoadipate (DalphaAA), D-alpha-aminosuberate (DalphaAS) and other excitatory amino acid antagonists have been compared on the excitatory responses of neurones of the cat spinal cord to acetylcholine, a range of glutamate-related amino acids and stimulation of appropriate excitatory synaptic pathways. The ionophoretic technique was used for administration of excitants and antagonists. 2. DalphaAA and DalphaAS had little or no effect on acetylcholine-induced excitation of Renshaw cells. Responses of either Renshaw cells or dorsal horn neurones in the spinal cord to excitatory amino acids were depressed in the order: N-methyl-D-aspartate (NMDA), L-homocysteate, D-glutamate, ibotenate greater than D-homocysteate, L-aspartate, D-aspartate greater than L-glutamate, kainate and quisqualate. 3. These effects are consistent with the existence of different excitatory amino acid receptors, one type being sensitive to the actions of the antagonists, and activated predominantly by the NMDA group of excitants, with other receptors being relatively insensitive to DalphaAA and DalphaAS and activated predominantly by quisqualate and kainate. On this hypothesis, many amino acids are assumed to have mixed actions on DalphaAA-sensitive and -insensitive receptors. 4. 2-Amino-4-phosphonobutyrate (2APB) and L-glutamic acid diethyl ester (GDEE) produced different patterns of antagonism of excitatory amino acid-induced responses from those observed with DalphaAA and DalphaAS. Neither substance was as potent as DalphaAA or DalphaAS as an excitatory amino acid antagonist. 5. Both DalphaAA and DalphaAS selectively antagonized synaptic excitation of Renshaw cells evoked by dorsal root stimulation without affecting cholinergic excitation of these cells evoked by ventral root stimulation. These latter responses were selectively antagonized by dihydro-beta-erythroidine (DHbetaE). DalphaAA also antagonized synaptic excitation of unidentified dorsal horn neurones of the spinal cord evoked by dorsal root stimulation. Neither GDEE (particularly) nor 2APB were as effective as DalphaAA or DalphaAS as depressants of synaptic excitation. 6. Taken in conjunction with the results of in vitro studies on the specificity of action of Dalpha¿ and related substances, these observations suggest that certain synaptic excitations in the spinal cord are mediated by an excitatory amino acid transmitter, and that this transmitter interacts with receptors which are activated selectively by NMDA, less selectively by other amino acids, including L-aspartate, and probably only slightly by quisqualate, kainate and (exogenous) L-glutamate.

Amino acids as neurotransmitters of corticofugal neurones in the rat: a comparison of glutamate and aspartate

1 The relative sensitivities to aspartate and glutamate of neurones receiving a corticofugal innervation were examined by microiontophoresis, and compared with the relative sensitivities of neurones not appearing to receive such an input.2 On all the cells tested, glutamate appeared to be a more potent excitant than aspartate in terms of neuronal response size or effective dose.3 DL-alpha-Aminoadipate (alphaAA) reduced the excitatory amino acid responses on all the neurones tested. On many of these cells a control excitation could be produced, by acetylcholine or hydrogen ions, which was in most cases unaffected by doses of alphaAA producing antagonism of amino acid excitation.4 On 70% of the cells, aminoadipate showed no selectivity for aspartate compared with glutamate but a differential action, involving blockade of aspartate but not glutamate, was apparent on the other 30%.5 Doses of alphaAA which selectively reduced responses to aspartate had no effect on short latency evoked spikes, but doses which also reduced responses to glutamate reduced the short-latency synaptic excitation induced by electrical stimulation of either the surface of the cerebral cortex, or of the pyramidal tracts in the medulla.6 These findings suggest that corticofugal neurones having an excitatory action on cells in various parts of the brain may use an amino acid, probably glutamate, as a common neurotransmitter.7 As no significant difference could be demonstrated in the potency ratios of glutamate:aspartate on monosynaptically activated cells compared with other cells, doubt is cast on the validity of drawing conclusions about transmitter identity from potency ratios alone, without the support of antagonist studies.

L-histidine: effects on sensitivity of cat spinal neurones to amino acids

L-Histidine enhanced the inhibitory actions of GABA, muscimol and beta-alanine rather than that of glycine on spinal neurones in pentobarbitone-anaesthetised cats. L-Histidine also enhanced the excitatory action of L-glutamate, D- and L-aspartate, L-homocysteate and especially that of quisqualate, whereas the actions of acetylcholine, kainate, N-methyl-D-aspartate and D-homocysteate were more commonly reduced. These actions of L-histidine are best ascribed to an effect on amino acid transport systems, probably of the low affinity type, which therefore appear to be partially responsible for the inactivation of exogenously administered amino acids.

The antagonism of amino acid-induced excitation of spinal neurones in the cat

DL-alpha-Aminoadipate (DLalphaAA) and L-glutamic acid diethylester (GDEE) were compared as antagonists of the excitatory effects of a number of amino acids on interneurones, and also of acetylcholine on Renshaw cells of the cat spinal cord. The excitants could be ranked in order of their sensitivity of the two antagonists. Ibotenate and the optical isomers of N-methylaspartate were the most readily antagonised by DLalphaAA and the least by GDEE; L-glutamate and quisqualate were most affected by GDEE and least by DLalphaAA; the actions of L-cysteate, L-aspartate and D- and L-homocysteate were reduced to some extent by both; and kainate was relatively insensitive to both antagonists. Acetylcholine excitations of Renshaw cells were often reduced by GDEE and occasionally by DLalphaAA. The results indicate the existence on spinal neurones of at least two populations of amino acid receptors which have differing sensitivities to the antagonists, and which do not precisely conform to the "glutamate-perferring" and "aspartate-preferring" categories which have been proposed on other grounds.

Glutamate as the neurotransmitter of cerebellar granule cells in the rat: electrophysiological evidence

1 Glutamate and the excitatory aminoacid antagonist, alpha-aminoadipic acid (alphaAA), have been applied by microiontophoresis to Purkinje cells in the rat cerebellum. 2 Glutemate produced excitation of Purkinje cells and alpha AA selectively reduced that excitation without affecting responses to acetylcholine or hydrogen ions. 3 Monosynaptic spikes were evoked in Purkinje cells by stimulating the parallel fibres. alpha AA had little effect on these spikes when applied alone. 4 When the Purkinje cell excitability was reduced by the iontophoresis of gamma-aminobutyric acid, alpha AA then produced railure of the monosynaptic spike on 10 of 13 Purkinje cells, in doses shown to be selectively antagonistic towards aminoacids. 5 These results support neurochemical evidence that glutamic acid may be the neurotransmitter released by granule cell parallel fibres.

Specific antagonism of excitant amino acids in the isolated spinal cord of the neonatal rat

The specificity of the neurodepressant actions of D-alpha-aminoadipate, alpha,epsilon-diaminopimelic acid, HA-966 (HAP) and Mg2+ has been investigated. On the isolated spinal cord of the neonatal rat, ventral root depolarizations produced by kainate, substance P, carbachol and noradrenaline were relatively unaffected by the same concentrations (0.25--1 mM) of the agents as those which reduced synaptic activity and ventral root depolarizations produced by N-methyl-D-aspartate (especially), L-aspartate and L-glutamate. The same or higher concentrations of the agents did not affect excitatory transmission in the isolated rat superior cervical ganglion. It is proposed that the agents specifically block synaptic transmission mediated by an excitatory amino acid.