An evaluation of D-alpha-aminoadipate and D-(and DL-)alpha-aminosuberate as selective antagonists of excitatory amino acids in the substantia nigra and mesencephalic reticular formation of the rat

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

The induction of N-methyl-D-aspartate receptor-dependent long-term potentiation

The role of N-methyl-D-aspartate (NMDA) receptors in the induction of long-term potentiation (LTP) was established during the 1980s. In this article I present a personal reflection upon the role that my colleagues and I played in the discovery of the mechanism of induction of NMDA receptor-dependent LTP.

Excitatory amino acid binding sites in the basal ganglia of the rat: a quantitative autoradiographic study

Quantitative receptor autoradiography was used to determine the distribution of excitatory amino acid binding sites in the basal ganglia of rat brain. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-D-aspartate, kainate, quisqualate-sensitive metabotropic and non-N-methyl-D-aspartate, non-kainate, non-quisqualate glutamate binding sites had their highest density in striatum, nucleus accumbens, and olfactory tubercle. Kainate binding was higher in the lateral striatum but there was no medial-lateral striatal gradient for other binding sites. N-Methyl-D-aspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding sites were most dense in the nucleus accumbens and olfactory tubercle. There was no dorsal-ventral gradient within the striatal complex for the other binding sites. Other regions of the basal ganglia had lower densities of ligand binding. To compare binding site density within non-striatal regions, binding for each ligand was normalized to the striatal binding density. When compared to the striatal complex, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and metabotropic binding sites had higher relative density in the globus pallidus, ventral pallidum, and subthalamic nucleus than other binding sites. Metabotropic binding also had a high relative density in the substantia nigra. Non-N-methyl-D-aspartate, non-kainate, non-quisqualate glutamate binding sites had a high relative density in globus pallidus, ventral pallidum, and substantia nigra. N-Methyl-D-aspartate binding sites had a low relative density in pallidum, subthalamic nucleus, substantia nigra and ventral tegmental area. Our data indicate heterogeneous distribution of excitatory amino acid binding sites within rat basal ganglia and suggest that the character of excitatory amino acid-mediated neurotransmission within the basal ganglia is also heterogeneous.

Increased expression of glutamic acid decarboxylase mRNA in rat substantia nigra after an ibotenic acid lesion in the caudate-putamen

In situ hybridization histochemistry and RNA blots were used to study expression of glutamic acid decarboxylase (GAD) mRNA in rat caudate-nucleus and substantia nigra. In situ hybridization combined with computerized image analysis revealed that in the intact substantia nigra reticulata the cross-section area of GAD mRNA positive neurons were 25% larger in the dorsolateral part as compared with the ventromedial part. A unilateral ibotenic acid injection in caudate-putamen lesioned neurons, some of which project to the ipsilateral substantia nigra. An increased level of GAD mRNA was observed in substantia nigra ipsilateral to the lesion. Computerized image analysis of sections from in situ hybridization revealed an increase in the number of silver grains over GAD mRNA positive neurons in the dorsolateral substantia nigra reticulata ipsilateral to the lesion. However, no change was observed in the ventromedial part suggesting that GAD mRNA expression in this part of the nigra is less sensitive to inhibition by caudate-putamen afferents. In agreement with in situ experiments, RNA blots showed a 2-fold increased level of GAD mRNA in substantia nigra ipsilateral to the lesion. The increased GAD mRNA expression in the deafferented substantia nigra suggests a disinhibition of nigral GABA neurons, resulting in an increased utilization of GABA in these substantia nigra neurons.

Release of gamma-[3H]aminobutyric acid from rat olfactory bulb and substantia nigra: differential modulation by glutamic acid

We have studied the glutamate modulation of gamma-[3H]aminobutyric acid ([3H]GABA) release from GABAergic dendrites of the external plexiform layer of the olfactory bulb and from GABAergic axons of the substantia nigra. In the olfactory bulb, [3H]GABA release was induced by high K+ and kainate, and not by aspartate and glutamate alone. However, when the tissue was conditioned by a previous K+ depolarization, glutamate and aspartate caused [3H]GABA release. The effect of glutamate was significantly enhanced when the GABA uptake mechanism was blocked by nipecotic acid. N-Methyl-D-aspartate and quisqualate did not cause [3H]GABA release under the same conditions. The acidic amino acid receptor antagonist 2-amino-4-phosphonobutyric acid and the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid significantly inhibited the K+-glutamate- and the kainate-induced [3H]GABA release. Mg2+ (5 mM), which blocks the N-methyl-D-aspartate receptors, significantly inhibited the K+-glutamate-induced but not the kainic acid-induced [3H]GABA release. The K+-glutamate-stimulated release, but not the K+-stimulated [3H]GABA release, was strongly inhibited by Na+-free solutions or by 300 nM tetrodotoxin. Apparently the glutamate-induced release of [3H]GABA occurs through an interneuron because it is dependent on the presence of nerve conduction. In the substantia nigra no [3H]GABA release was elicited by any of the glutamate agonists tested. The present results clearly differentiate between the effects of glutamate on the release of [3H]GABA from the substantia nigra and from the olfactory bulb.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of dendritic release of dopamine by N-methyl-D-aspartate receptors in rat substantia nigra

A superfusion system was used to study the effects of excitatory amino acids (EAA) on release of [3H]dopamine ([3H]DA) previously taken up by rat substantia nigra (SN) slices. The EAA tested (20-250 microM), with the exception of quisqualate and kainate, markedly evoked [3H]DA release from nigral slices when Mg2+ ions were omitted from the superfusion medium. The EAA receptor agonists exhibited the following relative potency in stimulating [3H]DA release: L-glutamate (L-Glu) greater than N-methyl-D-aspartate (NMDA) greater than NM(D,L)A greater than D-Glu much greater than quisqualate = kainate. D-2-Amino-5-phosphonovalerate (100-200 microM), an antagonist for NMDA receptors, substantially reduced [3H]DA release evoked by L-Glu or NMDA. In contrast, L-Glu diethyl ester (100-200 microM) produced a lesser blocking effect on [3H]DA release evoked by the EAA. Further experiments showed that the NMDA-mediated release of [3H]DA was totally suppressed by the omission of Ca2+ or by the addition of tetrodotoxin (0.1 microM) to the superfusion medium. In addition, strychnine, an antagonist for glycine (Gly) receptors, significantly decreased NMDA (100 microM)-evoked as well as glycine (100 microM)-evoked release of [3H]DA from nigral slices. The results shown support the idea that activation of NMDA subtype receptors in SN may trigger a Ca2+-dependent release of DA from dendrites of nigro-striatal DA-containing neurons. Furthermore, a transsynaptic mechanism that may partially involve Gly-containing interneurons is proposed to account for some of the events mediating NMDA receptor activation and DA release in SN.

Effects of excitatory amino acid receptor antagonists and agonists on suprachiasmatic nucleus responses to retinohypothalamic tract volleys

A slice preparation of the mouse hypothalamus that includes the suprachiasmatic nuclei (SCN), the optic chiasm and the optic nerves was used for pharmacologic investigations of the nature of the receptors mediating the excitation of SCN neurons by input from the retinohypothalamic tract (RHT). Bath application of cis-2,3-piperidinedicarboxylic acid, a non-selective antagonist of excitatory amino acid receptors, reversibly blocked the postsynaptic component of the field potentials evoked in the dorsolateral SCN by stimulation of the optic nerve. The selective antagonist of N-methyl-D-aspartate receptors, 2-amino-5-phosphonovaleric acid, had no effect on SCN responses. Glutamic acid diethyl ester and 2-amino-4-phosphonobutyric acid also were without effect, but gamma-D-glutamylglycine caused a small decrease in the amplitude of the postsynaptic wave. Addition of the agonists, kainate and N-methyl-D,L-aspartate, to the superfusate also blocked the postsynaptic response. Kainate was the most potent agonist. L-Glutamate was without effect at up to 100 microM. These results indicate that postsynaptic responses in the SCN to retinohypothalamic tract volleys are mediated by a non-NMDA class of excitatory amino acid receptors.

Specific binding of L-[3H]-glutamic acid to rat substantia nigra synaptic membranes

The specific binding of L-[3H]-glutamic acid (GLU) was investigated in synaptic membranes from rat substantia nigra. L-[3H]-GLU binding to the membrane preparations occurred in a reversible and saturable way. The specific binding was stimulated by the presence of CaCl2 and was reduced by freezing and thawing the membranes. Scatchard analysis of the saturation isotherms yielded a non-linear plot suggesting that the binding reaction does not occur through a simple bimolecular association. Assuming non-interacting binding sites, a high (KD1, 139 nM; Bmax1, 3.5 pmoles/mg protein) and a low (KD2, 667 nM; Bmax2, 15.1 pmoles/mg protein) affinity L-[3H]-GLU binding site were obtained. The kinetics of dissociation of bound L-[3H]-GLU was biphasic; the respective dissociation rate constant (k-1) being 0.20 min-1 and 0.013 min-1. A series of amino acid receptor agonists and antagonists were tested as inhibitors of L-[3H]-GLU specific binding. Quisqualic acid, L-GLU and D-alpha-aminoadipate (D-alpha-AA) were the most potent inhibitors. DL-2-amino-4-phosphonobutyrate (APB), N-Methyl-D-aspartate (NMDA) and D-GLU were moderate inhibitors, whereas diaminopimelic acid (DAPA) and glutamate diethyl ester (GDEE) exhibited the lowest relative potency. Kainic acid (KA), gamma-aminobutyric acid (GABA) and bicuculline were not able to modify at any concentration used the specific binding of L-[3H]-GLU. These data demonstrate the presence of specific GLU binding sites in synaptic structures at substantia nigra level and support the idea that excitatory amino acids may play a role in synaptic transmission in this brain region.

Peptides derived from kainic acid as antagonists of N-methyl-D-aspartate-induced neuroexcitation in rat brain

The effects of dipeptides and an amide chemically derived from kainic acid (KA) on the response of rat striatal slices to excitatory amino acids were studied. Some of the gamma-peptides of KA were found to antagonize the response to N-methyl-D-aspartate (NMDA) more than that to quisqualate and glutamate and not to have any effect on the response to kainate. The least potent antagonists among the tested compounds were the gamma-amide of KA and the peptides of KA with beta-alanine and gamma-aminobutyric acid, whereas the gamma-kainyl peptides of the alpha-amino acids glycine, tyrosine, glutamate and KA were more active. The latter are the best blockers of the response to NMDA among the tested compounds.

The cortico-nigral projection: reduced glutamate content in the substantia nigra following frontal cortex ablation in the rat

Unilateral ablation of the frontal cortex results in a significant reduction (19.5%) of glutamic acid in the ipsilateral compared with the contralateral substantia nigra. The GABA level in the substantia nigra and both the glutamate and GABA levels in the hippocampus remain unchanged. The results suggest that glutamate is a transmitter in the cortico-nigral fibre tract.

Antagonism of the hypermotility response induced by excitatory amino acids in the rat nucleus accumbens

Several compounds have been shown to antagonize the excitation of single neurons produced by excitatory amino acids. This study was designed to determine the effectiveness of these compounds in antagonizing the hypermotility response to excitatory amino acids after intra-accumbens administration. Of the putative antagonists tested, D-aminoadipic acid, diaminopimelic acid and glutamic acid diethyl ester all showed significant inhibitory effects on excitatory amino acid-induced hypermotility while 2-amino-5-phosphonovaleric acid, gamma-D-glutamylglycine, 2-amino-4-phosphonobutyric acid and cis-2,3-piperidine dicarboxylic acid were ineffective. D-Aminoadipic acid decreased N-methyl-aspartic acid-induced hypermotility while having no significant effect on the hypermotility responses induced by kainic or quisqualic acids. Diaminopimelic acid markedly decreased N-methyl-aspartic acid- and kainic acid-induced hypermotility but was totally ineffective on quisqualic acid-induced hypermotility. In contrast to D-aminoadipic acid, glutamic acid diethyl ester antagonized the increase in motility produced by kainic and quisqualic acids but not that produced by N-methyl-aspartic acid. The above data suggests that N-methyl-aspartic acid and quisqualic acid may produce their motor effects through the activation of two different receptors in the nucleus accumbens while kainic acid may mediate its hypermotility response through both N-methyl-aspartic acid and quisqualic acid receptors. However, a third receptor type activated solely by kainic acid cannot be excluded at this time.

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.

Reversible effects of tetanus toxin on striatal-evoked responses and [3H]-gamma-aminobutyric acid release in the rat substantia nigra

1 The effects of sublethal doses of tetanus toxin on gamma-aminobutyric acid (GABA)-mediated synaptic transmission and [3H]-GABA release were studied in the rat substantia nigra. 2 Intranigral injections of tetanus toxin at 1-5 times the mouse LD50 dose produced ipsiversive circling behaviour which was maximal after 1 week and lasted 2-3 weeks. Rats then displayed normal behavior suggesting that the effects of the toxin were fully reversible. 3 In the treated nigra of circling rats there was a reduction in the striatal-evoked inhibition of compacta and reticulata neurones, but no change in their spontaneous firing rates. Some forms of striatal-evoked excitation were also reduced. Once rats had recovered from circling no alterations in the synaptic responses were detected. 4 In circling rats there were no differences in the sensitivities of neurones in the treated and untreated nigra to GABA or to other inhibitory neurotransmitters. 5 The Ca2+-dependent, K+-evoked release of [3H]-GABA from slices prepared from the treated nigra of circling rats was less than that from the untreated nigra of circling rats. No differences in nigral [3H]-GABA release were observed once rats had recovered from the circling behaviour. 6 The results demonstrate that doses of tetanus toxin which produce reversible behavioural effects can interfere reversibly with GABA-mediated synaptic transmission by a presynaptic mechanism which probably involves a reduction in transmitter release.

Actions of substance P and opiates in the rat substantia nigra

The actions of iontophoretically applied substance P, morphine and met-enkephalin on single neurones in the substantia nigra and overlying reticular formation were investigated. None of these agents affected nigrostriatal compacta neurones, however, reticulata and reticular formation neurones were often excited by morphine and substance P, and the latter inhibited by met-enkephalin. These results suggest that activation of the dopaminergic nigrostriatal pathway by these substances may be mediated indirectly, possibly via reticulata or reticular formation neurones.

(RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid: wet dog shakes, catalepsy and body temperature changes in rats

(RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) was microinjected into the lateral brain ventricle of conscious rats in order to evaluate its pharmacological effects. Microinjection (5 microliter) were made unilaterally and the effects of AMPA were assessed for 6 hr. AMPA produced generalized myoclonic seizures, short lasting hypoactivity followed by hyperactivity and hyperthermia when low doses were injected (0.25-1.0 microgram). When AMPA was injected at higher doses (1.5-5.0 microgram) it produced generalized myoclonic seizures, a hypoactive phase and hypothermia rapidly followed by hyperthermia. As the seizure activity and hypoactive phase receded, AMPA at doses of less than 2.5 microgram produced hyperactivity and wet dog shakes in a dose-related manner. After receiving AMPA at doses of 2.5 and 5.0 microgram, rats developed transient catalepsy. High quantities (5.0 microgram) evoked a spectrum of generalized convulsive seizures lasting for 2-3 hr (1 seizure every 15 min). Biochemical assays showed that AMPA had complex effects on brain aminergic systems. AMPA decreased brain NA while brain DA concentration was slightly increased in a dose dependent manner. Moreover, AMPA increased brain 5-HT and 5-HIAA concentration in a dose- and time-related manner.

Turning behaviour and catalepsy after injection of excitatory amino acids into rat substantia nigra

Unilateral injection of AMPA ((R,S)-alpha-3-hydroxyl-5-methyl-4-isoxazolepropionic acid), 4-bromohomoibotenic acid, kainic acid and N-methylaspartic acid into the caudal part of substantia nigra induced ipsilateral turning behaviour. AMPA and kainic acid were most potent. Ibotenic acid produced short-lasting ipsilateral turning followed by long-lasting contralateral turning behaviour which was antagonized by the GABA antagonist bicuculline methiodide. The turning induced by AMPA was not blocked by previous 6-hydroxydopamine lesion of the nigrostriatal dopaminergic pathway and was increased by systemic apomorphine. High doses of bilateral intranigral AMPA induced rigid catalepsy. The results are consistent with an involvement of glutamate or aspartate receptors in the regulation of dopamine output systems.

The influence of striatal stimulation and putative neurotransmitters on identified neurones in the rat substantia nigra

The response of two populations of neurones in the substantia nigra (nigro-striatal compacta neurones and reticulata neurones) to microelectrophoretically administered putative neurotransmitters and stimulation of the ipsilateral striatum has been investigated in anaesthetized rats. There were marked differences between compacta and reticulata neurones in respect to their action potential configurations, spontaneous firing rates and their responses to striatal stimulation. However, both compacta and reticulata neurones were excited and/or inhibited by striatal stimulation, although inhibition was usually the predominant response in both neuronal populations. Compacta neurones were strongly inhibited by noradrenaline (NA) and dopamine (DA) but were unaffected by acetylcholine (ACh) and 5-hydroxytryptamine (5-HT). Reticulata neurones were excited by ACh and showed mixed responses to 5-HT, DA and NA. Excitant amino acids overdepolarized compacta neurones preventing them from firing rapidly, but induced large increases in reticulata neurone firing rate; effects that were readily antagonized by D-alpha-aminoadipate. Compacta neurones were less sensitive than reticulata neurones to GABA and glycine. The action of these inhibitory amino acids were selectively and reversibly antagonized by bicuculline methochloride and strychnine, respectively. The striatal-evoked inhibition of both compacta and reticulata neurones was reversibly reduced by bicuculline methochloride and irreversibly reduced by tetanus toxin, but was unaffected by strychnine. These results demonstrate that nigrostriatal-compacta neurones and reticulata neurones are physiologically and pharmacologically distinct neuronal populations and both receive inhibitory GABAergic and excitatory striatal inputs.

Lactones derived from kainic acid: novel selective antagonists of amino acid-induced Na+ fluxes in rat striatum slices

The effects of amino acid lactones chemically derived from the neuroexcitant kainic acid on the response of rat striatal slices to excitatory amino acids, were studied. These compounds antagonize to varying extents the effects of kainic acid and N-methyl-D-aspartic acid but have no effect on the responses to glutamic or quisqualic acid. Some of the lactones antagonize preferentially the effects of kainic acid. This study further confirms the existence of heterogenous populations of excitatory amino acid receptors in the rat striatum.

L-Aspartate binding sites in rat cerebellum: a comparison of the binding of L-[3H]aspartate and L-[3H]glutamate to synaptic membranes

The binding of L-[3H]aspartate to sonicated, extensively washed and preincubated cerebellar synaptic membranes was investigated. Binding was optimal under physiological conditions of pH and temperature, and attained equilibrium within 10 min. Binding was saturable, and Eadie-Hofstee analysis revealed interaction with a single population of binding sites (Kd = 874 nM and Bmax = 44 pmol/mg protein), which displayed no cooperativity (Hill coefficient approx. = 1). Specific [3H]aspartate was readily and reversibly displayed by unlabelled L-aspartate (the D-isomer being less than half as active) with a half-life of dissociation of 32 sec. Quisqualate, 4-fluoroglutamate and 2-amino-4-phosphonobutyrate, which are good displacers of [3H]glutamate binding, were only weakly active against the aspartate system. The excitatory amino acid antagonists, DL-alpha-aminoadipate, DL-alpha-aminosuberate and HA-966 were effective displacers, but the proposed aspartate receptor-preferring agonist, N-methyl-D-aspartate was inactive. Kainic acid exhibited negligible affinity for the aspartate binding site, in common with that for glutamate. While freezing or cold storage of membranes resulted in diminished [3H]-aspartate binding, lyophilization was not only able to confer substantial stability, but induced a marked increase in affinity of the binding site. Differential effects of various cations on [3H]aspartate binding were observed--monovalent cations reduced, while divalent cations enhanced L-[3H]aspartate binding.

Characterization of two [3H]glutamate binding sites in rat hippocampal membranes

The specific binding of L-[3H]glutamate was investigated in the presence and the absence of sodium ions in freshly prepared membranes from rat hippocampus. Sodium ions were found to have a biphasic effect; low concentrations induced a marked inhibition of the binding (in the range 0.5-5.0 mM), whereas higher concentrations resulted in a dose-dependent stimulation of binding (in the range 10-150 mM). These results permit the discrimination of two binding sites in hippocampal membranes. Both Na+-independent and Na+-dependent binding sites were saturable, exhibiting dissociation constants at 30 degrees C of 750 nM and 2.4 microM, respectively, with Hill coefficients not significantly different from unity, and maximal number of sites of 6.5 and 75 pmol/mg protein, respectively. [3H]Glutamate binding to both sites reached equilibrium between 5 and 10 min and was reversible. The relative potencies of a wide range of compounds, with known pharmacological activities, to inhibit [3H]glutamate binding were very different for the Na+-independent and Na+-dependent binding and suggested that the former sites were related to post-synaptic glutamate receptors, whereas the latter were related to high-affinity uptake sites. This conclusion was also supported by the considerable variation in the regional distribution of the Na+-dependent binding site, which paralleled that of the high-affinity glutamate uptake; the Na+-independent binding exhibited less regional variation.

Antidromic latency variations of nigral compacta neurones

Variations in the antidromic latency of substantia nigra compacta neurones were commonly observed following striatal stimulation. These results provide electrophysiological evidence for a branched unmyelinated nigrostriatal pathway and demonstrate that the antidromic criterion of constant latency is not valid for this type of pathway.

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.

Regulation of hippocampal glutamate receptors: evidence for the involvement of a calcium-activated protease

Specific [3H]glutamate binding to rat hippocampal membranes and the calcium-induced increase in this binding are markedly temperature-sensitive and are inhibited by alkylating or reducing agents as well as by various protease inhibitors. N-Ethylmaleimide, chloromethyl ketone derivatives of lysine and phenylalanine, and tosylarginine methyl ester decrease the maximum number of [3H]glutamate binding sites without changing their affinity for glutamate. Preincubation of the membranes with glutamate does not protect the glutamate "receptors" from the suppressive effects of these agents. The proteases trypsin and alpha-chymotrypsin increase the maximum number of [3H]glutamate binding sites. The effects of calcium on glutamate binding are different across brain regions. Cerebellar membranes are almost insensitive whereas hippocampal and striatal membranes exhibit a strong increase in the number of binding sites after exposure to even low concentrations of calcium. These results suggest that an endogenous membrane-associated thiol protease regulates the number of [3H]glutamate-associated thiol protease regulates the number of [3H]glutamate binding sites in hippocampal membranes and that this is the mechanism by which calcium stimulates glutamate binding. The possibility is discussed that the postulated mechanisms participate in synaptic physiology and in particular may be related to the long-term potentiation of transmission found in hippocampus under certain conditions.

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.