The differential sensitivity to L-glutamate and L-aspartate of spinal interneurones and Renshaw cells

Team Evans and Watkins: Excitatory Amino Acid research at Bristol University 1973-1981

A series of Special Issues of Neuropharmacology celebrates the 40th anniversary of a seminal review on excitatory amino acid (EAA) receptors by two pioneers of the field - Dick Evans and Jeff Watkins. Brought together in the Department of Pharmacology at the University of Bristol in the 1970s, they forged a partnership that, through the synthetic chemistry prowess of Jeff Watkins, which provided novel agonists and antagonists for EAA receptors for Dick Evans's deft experimental studies, generated enormous insight into the multitude of actions of EAAs in the nervous system. Among many achievements from this time was not just the naming of the N-methyl-d-aspartate (NMDA) receptor, but also the demonstration of its antagonism by magnesium ions. Here, Dick and Jeff reflect upon those early halcyon days of EAA research, which, as these six¹ Special Issues of Neuropharmacology demonstrate, is very much alive and kicking. Bruno G. Frenguelli, Editor-in-Chief, Neuropharmacology.

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.

Patterns of Innovation in Alzheimer's Disease Drug Development: A Strategic Assessment Based on Technological Maturity

PURPOSE

This article examines the current status of translational science for Alzheimer's disease (AD) drug discovery by using an analytical model of technology maturation. Previous studies using this model have demonstrated that nascent scientific insights and inventions generate few successful leads or new products until achieving a requisite level of maturity. This article assessed whether recent failures and successes in AD research follow patterns of innovation observed in other sectors.

METHODS

The bibliometric-based Technology Innovation Maturation Evaluation model was used to quantify the characteristic S-curve of growth for AD-related technologies, including acetylcholinesterase, N-methyl-d-aspartate (NMDA) receptors, B-amyloid, amyloid precursor protein, presenilin, amyloid precursor protein secretases, apolipoprotein E4, and transactive response DNA binding protein 43 kDa (TDP-43). This model quantifies the accumulation of knowledge as a metric for technological maturity, and it identifies the point of initiation of an exponential growth stage and the point at which growth slows as the technology is established.

FINDINGS

In contrast to the long-established acetylcholinesterase and NMDA receptor technologies, we found that amyloid-related technologies reached the established point only after 2000, and that the more recent technologies (eg, TDP-43) have not yet approached this point. The first approvals for new molecular entities targeting acetylcholinesterase and the NMDA receptor occurred an average of 22 years after the respective technologies were established, with only memantine (which was phenotypically discovered) entering clinical trials before this point. In contrast, the 6 lead compounds targeting the formation of amyloid plaques that failed in Phase III trials between 2009 and 2014 all entered clinical trials before the respective target technologies were established.

IMPLICATIONS

This analysis suggests that AD drug discovery has followed a predictable pattern of innovation in which technological maturity is an important determinant of success in development. Quantitative analysis indicates that the lag in emergence of new products, and the much-heralded clinical failures of recent years, should be viewed in the context of the ongoing maturation of AD-related technologies. Although these technologies were not sufficiently mature to generate successful products a decade ago, they may be now. Analytical models of translational science can inform basic and clinical research results as well as strategic development of new therapeutic products.

The history of the pharmacology and cloning of ionotropic glutamate receptors and the development of idiosyncratic nomenclature

In this article, the beginnings of glutamate pharmacology are traced from the early doubts about 'non-specific' excitatory effects, through glutamate- and aspartate-preferring receptors, to NMDA, quisqualate/AMPA and kainate subtypes, and finally to the cloning of genes for these receptor subunits. The development of selective antagonists, crucial to the subtype classification, allowed the fundamental importance of glutamate receptors to synaptic activity throughout the CNS to be realised. The ability to be able to express and manipulate cloned receptor subunits is leading to huge advances in our understanding of these receptors. Similarly the tortuous path of the nomenclature is followed from naming with reference to exogenous agonists, through abortive early attempts at generic schemes, and back to the NC-IUPHAR system based on the natural agonist, the defining exogenous agonist and the gene names.

The glutamate story

Glutamatergic synaptic transmission in the mammalian central nervous system was slowly established over a period of some 20 years, dating from the 1950s. Realisation that glutamate and like amino acids (collectively known as excitatory amino acids (EAA)) mediated their excitatory actions via multiple receptors preceded establishment of these receptors as synaptic transmitter receptors. EAA receptors were initially classified as N-methyl-D-aspartate (NMDA) and non-NMDA receptors, the latter subdivided into quisqualate (later AMPA) and kainate receptors after agonists that appeared to activate these receptors preferentially, and by their sensitivity to a range of differentially acting antagonists developed progressively during the 1970s. NMDA receptors were definitively shown to be synaptic receptors on spinal neurones by the sensitivity of certain excitatory pathways in the spinal cord to a range of specific NMDA receptor antagonists. Importantly, specific NMDA receptor antagonists appeared to be less effective at synapses in higher centres. In contrast, antagonists that also blocked non-NMDA as well as NMDA receptors were almost universally effective at blocking synaptic excitation within the brain and spinal cord, establishing both the existence and ubiquity of non-NMDA synaptic receptor systems throughout the CNS. In the early 1980s, NMDA receptors were shown to be involved in several central synaptic pathways, acting in concert with non-NMDA receptors under conditions where a protracted excitatory postsynaptic potential was effected in response to intense stimulation of presynaptic fibres. Such activation of NMDA receptors together with non-NMDA receptors led to the phenomenon of long-term potentiation (LTP), associated with lasting changes in synaptic efficacy (synaptic plasticity) and considered to be an important process in memory and learning. During the 1980s, it was shown that certain glutamate receptors in the brain mediated biochemical changes that were not susceptible to NMDA or non-NMDA receptor antagonists. This dichotomy was resolved in the early 1990s by the techniques of molecular biology, which identified two families of glutamate-binding receptor proteins (ionotropic (iGlu) and metabotropic (mGlu) receptors). Development of antagonists binding to specific protein subunits is currently enabling precise identification of discrete iGlu or mGlu receptor subtypes that participate in a range of central synaptic processes, including synaptic plasticity.

Ionotropic glutamate receptor activated by N-methyl-D-aspartate: a key molecule of conscious life

We want to propose that basic stereotyped integrative functions are the result of sequentially built neuronal circuits in the primitive regions of the brain in whose build-up a particular subunit composition iGluR-NMDA would play a central role. iGluR-NMDA is a multiregulated heteromeric glutamate receptor-ion channel found in plasma membranes of neurons and other cells. iGluR-NMDA may be composed of up to five subunits, depending on the type of cell involved and its location. There are three major types of subunits and there are variations within each type allowing for up to 13 possible subunits, at least in the rat, which differ from each other in amino acid sequence and thus, in tertiary structure. The actual iGluR-NMDA heteropolymer involved in a given function may thus have a great number of subunit composition possibilities which would be the result of the particular genes expressed in a given type of cell. The iGluR-NMDA is an ion channel that opens in response to glutamate in a highly regulated fashion in which different molecules and ions present in the interstitial fluid determine whether or not the channel opens upon glutamate binding. The original function of iGluR-NMDA may have been that of allowing calcium influx to cells. As the brain receives external stimuli through the senses, new circuits will be formed stepwise in the neocortex in which a particular subunit composition iGluR-NMDA will be involved. Differentiation between external stimuli generated circuits and those governing internal functions will allow distinction between self and non-self, thus generating conscious awareness. The role of the particular subunit composition iGluR-NMDA proposed would be that of providing a means of calcium influx to the synapses to be formed and, if the same stimulus is forthcoming, allowing permanent synapses formation through the membrane incorporation of calcium dependent adhesion molecules such as cadherins and cytoskeleton reorganization promoted by nectins.

Effect of excitatory amino acid neurotransmitters on acid secretion in the rat stomach

Excitatory amino acids (EAAs), in particular, L-aspartate (L-Asp) neurons and their processes, were localized in the rat stomach using a immunohistochemical method with specific antibodies against either L-Asp or its synthesizing enzyme, aspartate aminotransferase (AAT). Myenteric ganglia and nerve bundles in the circular muscle and in the longitudinal muscle were found to be AAT- or L-Asp-positive. In addition, AAT- or L-Asp-positive cells were also found in the muscle layer and the deep mucosal layer. The distribution of AAT- or L-Asp-positive cells in both the mucosal and muscle layers was heterogeneous in the stomach. In addition, L-Asp at 10(-6) M negligibly influenced acid secretion in an everted preparation of isolated rat stomach. However, according to our results, L-Asp markedly inhibited the histamine-stimulated acid secretion, but not the oxotremorine- or the pentagastrin-stimulated acid secretion. Furthermore, L-Asp also inhibited histamine-induced elevation of cAMP. L- Asp itself did not affect the cAMP level although it elevated the cGMP level in the stomach. Moreover, either (+)2-amino-5-phosphonovaleric acid or (+/-)3-(2-carboxypiperazin-4-yl)prophyl-1-phosphonic acid, i.e. two specific antagonists for N-methyl-D-aspartic acid (NMDA) receptors, blocked the inhibitory effect of L-Asp on histamine-stimulated acid secretion or histamine-induced elevation of cAMP. Since cAMP has been strongly implicated as the second messenger involved in histamine-induced acid secretion, we believe that L-Asp regulates acid secretion in the stomach by inhibiting histamine release through the NMDA receptors, subsequently lowering the level of cAMP and ultimately reducing acid secretion.

N-methyl-D-aspartate R1 messenger RNA and [125I]MK-801 binding decrease in rat spinal cord after unilateral hind paw inflammation

Recent evidence suggests that N-methyl-D-aspartate receptors play an important role in the etiology and maintenance of chronic nociception. Previous studies have demonstrated that tissue injury or stimulation of nociceptive afferent projections results in the expansion of receptive fields, hyperalgesia and C-fiber-induced wind-up, events that can be inhibited by N-methyl-D-aspartate antagonists. This study examines the effect of unilateral hind paw inflammation on N-methyl-D-aspartate R1 messenger RNA and [125I]dizocilpine maleate binding in the L4-L5 segments of the lumbar spinal cord of rats. Spinal cords were examined at 7.5 h, three, seven and 20 days after injection of the left hind paw with 120 microliters of complete Freund's adjuvant. N-methyl-D-aspartate R1 messenger RNA, as measured with in situ hybridization, was observed to decrease bilaterally in laminae I, II and X of the lumbar spinal cord. This decrease was evident in laminae I and II at 7.5 h and three days after hind paw injection. In lamina X, a postinjection decrease in hybridization signal was observed at 7.5 h and seven days. A bilateral decrease in [125I]dizocilpine maleate binding was observed in laminae I and II at three, seven and 20 days after paw injection. This observed decrease in binding at the N-methyl-D-aspartate receptor suggests a compensatory mechanism by which N-methyl-D-aspartate-mediated nociceptive events may be modulated.

Analogue of aspartate and glutamate active at synapses are attractants for Escherichia coli

1. This research was carried out to compare Escherichia coli bacteria with animals in their response to L-aspartate and L-glutamate and their analogues. 2. Various analogues of aspartate and glutamate known to be neurotransmitters at synapses were shown to be attractants for E. coli. 3. The amino acid sequences of the animal receptors and the bacterial receptor, however, have no detectable relationship. Based on the amino acid sequence, evolutionarily the two systems appear not to be related.

Excitatory amino acid receptors: a gallery of new targets for pharmacological intervention

The excitatory amino acids (EAAs) L-glutamate and L-aspartate are the most abundant amino acids in brain and play a number of roles in maintaining neuronal function. Among these are their use as protein constituents, as key intermediates in ammonia metabolism, and as precursors for other neurotransmitters. Given the widespread distribution of EAA-containing neurons, these transmitters are likely to be involved in virtually all central nervous system functions, with abnormalities in neurotransmission contributing to the symptoms of a host of neurological and psychiatric disorders. Because of the importance of EAAs in maintaining the functional integrity of the central nervous system, efforts are underway to design agents capable of regulating the activity of these transmitters for therapeutic gain. Inasmuch as potential side effects preclude a generalized modification of this system, strategies must be found to alter EAA neurotransmission in selected brain regions. In this regard, pharmacological data suggest several functionally distinct EAA receptors, a finding confirmed by cloning studies which hint at an even larger family of sites. Moreover, it appears that some excitatory amino acid receptor complexes are composed of interacting sites which orchestrate receptor function, and there is evidence that EAA receptors may influence the activity of one another. Thus, there appear to be numerous sites that can be targeted to selectively modify excitatory amino acid neurotransmission in brain. Besides the agonist recognition site for each receptor subtype, other targets include regulatory subunits, ion channels and components of receptor-coupled second messenger systems.

Allosteric modulation of [3H]CGP 39653 binding by glycine in rat brain

D,L-(E)-2-Amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653), a new, high-affinity, selective NMDA receptor antagonist, interacts with rat cortical membranes in a saturable way and apparently to a single binding site, with a KD of 10.7 nM and a receptor density of 2.6 pmol/mg of protein. Displacement analysis of [3H]CGP 39653 binding shows a pharmacological profile similar to that reported for another NMDA antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). Glycine, however, is able to discriminate between the two ligands; in fact, it does not affect [3H]CPP binding but inhibits [3H]CGP 39653 binding in a biphasic way. D-Serine, another agonist at the strychnine-insensitive glycine binding site of the NMDA receptor complex, inhibits [3H]CGP 39653 binding in the same way as glycine, with a potency that correlates with its binding affinity at the glycine site. In addition, 7-chlorokynurenic acid, an antagonist at the glycine site, is able to reverse the displacement of [3H]CGP 39653 by glycine in a dose-dependent manner. Furthermore, the dissociation rate constant of [3H]CGP 39653 is enhanced in the presence of glycine, whereas the presence of NMDA receptor ligands does not modify the rate of dissociation of [3H]CGP 39653 from the receptor. These results indicate that part of the binding of the NMDA antagonist CGP 39653 can be potently modified by glycine through an allosteric mechanism, and suggest the existence of two antagonist preferring NMDA receptor subtypes that are differentially modulated through the glycine binding site.

Direct observations of synapses between L-glutamate-immunoreactive boutons and identified spinocervical tract neurones in the spinal cord of the cat

Four spinocervical tract cells in lumbosacral spinal cords of adult cats were physiologically characterized and intracellularly labelled with horseradish peroxidase. The neurones were examined with a light microscope and reconstructed. Selected regions were chosen for ultrastructural analysis. Thin sections were treated to reveal the presence of L-glutamate by using the postembedding immunogold method. Two antisera, which specifically recognise the presence of fixed glutamate in tissue, were used in the study. Somata, proximal, and distal dendrites of all four neurones received synaptic contacts from boutons which displayed an obvious immunogold reaction. These boutons formed between 35% and 48% of all synaptic contacts onto spinocervical tract cells. Glutamate-enriched boutons were associated with gold particle densities which were 2-3 times greater than the average densities associated with the surrounding neuropil. Their profiles had a mean diameter of 1.68 microns, contained round agranular synaptic vesicles, and formed asymmetrical synaptic junctions. However, not all boutons displaying these characteristics were enriched with glutamate. Immunogold studies of alternate thin sections, which were incubated with glutamate or GABA antiserum, demonstrated that synaptic boutons on spinocervical tract cells were either enriched with GABA or with glutamate and formed two separate populations which had distinct morphological characteristics. GABA-containing boutons contained irregularly shaped agranular vesicles and formed symmetrical synaptic junctions, whereas glutamate-enriched boutons corresponded to those described above. A further population of boutons, containing highly flattened vesicles, was not immunoreactive for GABA or glutamate. The evidence supports the idea that much of the excitatory transmission into the SCT is mediated by L-glutamate.

Different populations of parvalbumin- and calbindin-D28k-immunoreactive neurons contain GABA and accumulate 3H-D-aspartate in the dorsal horn of the rat spinal cord

The colocalization of parvalbumin (PV), calbindin-D28k (CaBP), GABA immunoreactivities, and the ability to accumulate 3H-D-aspartate selectively were investigated in neurons of laminae I-IV of the dorsal horn of the rat spinal cord. Following injection of 3H-D-aspartate into the basal dorsal horn (laminae IV-VI), perikarya selectively accumulating 3H-D-aspartate were detected in araldite embedded semithin sections by autoradiography, and consecutive semithin sections were treated to reveal PV, CaBP and GABA by postembedding immunocytochemistry. Perikarya accumulating 3H-D-aspartate were found exclusively in laminae I-III, and no labelled somata were found in deeper layers or in the intermediolateral column although the labelled amino acid clearly spread to these regions. More than half of the labelled cells were localized in lamina II. In this layer, 16.4% of 3H-D-aspartate-labelled perikarya were also stained for CaBP. In contrast to CaBP, PV or GABA was never detected in neurons accumulating 3H-D-aspartate. A high proportion of PV-immunoreactive perikarya were also stained for GABA in laminae II and III (70.0% and 61.2% respectively). However, the majority of CaBP-immunoreactive perikarya were GABA-negative. GABA-immunoreactivity was found in less than 2% of the total population of cells stained for CaBP in laminae I-IV. A significant proportion of the GABA-negative but PV-immunoreactive neurons also showed CaBP-immunoreactivity in laminae II and IV. These results show that out of the two calcium-binding proteins, CaBP is a characteristic protein of a small subpopulation of neurons using excitatory amino acids and PV is a characteristic protein of a subpopulation of neurons utilizing GABA as a transmitter. However, both proteins are present in additional subgroups of neurons, and neuronal populations using inhibitory or excitatory amino acid transmitters are heterogeneous with regard to their content of calcium-binding proteins in the dorsal horn of the rat spinal cord.

Central boutons of glomeruli in the spinal cord of the cat are enriched with L-glutamate-like immunoreactivity

Several lines of evidence indicate that L-glutamate may be a neurotransmitter of fine myelinated and unmyelinated primary afferent fibres in the spinal cord. The aim of the present study was to determine if L-glutamate was enriched in the terminals of these fibres. We performed the post-embedding immunogold technique on sections taken from the superficial regions of the lumbar cord in two cats. An antiserum, raised against protein-conjugated L-glutamate, was employed. Several tests on tissue and on a model system indicated that the antiserum recognized a glutaraldehyde-fixed L-glutamate-like substance. Terminals of fine afferent fibres were identified in the substantia gelatinosa as central boutons of synaptic glomeruli. Central boutons were examined through serial sections following immunogold reactions and were found to be heavily labelled with gold particles in consecutive sections. Quantitative analysis indicated that central boutons were more than two and a half times as densely labelled with gold particles than the tissue average. It was concluded that this represents a genuine enrichment of L-glutamate in these structures. Comparisons were made between L-glutamate-immunoreactive properties of central terminals and immunoreactivity for GABA, aspartate and glutamine. Statistical analysis revealed that central boutons in sections incubated in GABA antiserum and glutamine antiserum were associated with significantly lower densities of gold particle labelling than the average for the same tissue. Particle densities of central boutons in sections incubated in aspartate antiserum were not significantly different from average tissue densities. It was concluded that central boutons were not enriched with these three amino acids. Central boutons of synaptic glomeruli were classified into three groups on morphological criteria: (1) dense sinusoidal boutons; (2) large dense-core vesicle-containing boutons; and (3) regular synaptic vesicle-containing boutons. Quantitative analysis revealed that all of these groups were enriched in glutamate immunoreactivity, however, there were differences between the groups; large dense-core vesicle-containing boutons were associated with significantly lower densities of particles than regular synaptic vesicle-containing and dense sinusoidal terminals. The evidence indicates that central boutons, which most probably originate from fine myelinated and unmyelinated primary afferent fibres, are enriched with L-glutamate which may serve as a neurotransmitter in such fibres.

Differential effects of excitatory amino acid antagonists on dorsal horn nociceptive neurones in the rat

The effects of two excitatory amino acid receptor antagonists gamma-D-glutamylglycine (DGG) and 2-amino-5-phosphonovaleric acid (APS), applied onto the spinal cord surface, were tested on the responses of dorsal horn nociceptive neurones in the anaesthetized rat. DGG is a non-selective antagonist at both the N-methyl-D-aspartate (NMDA) and non-NMDA receptors, whereas AP5 acid is selective for the NMDA receptor. DGG dose-dependently reduced the A and C fibre-evoked responses of neurones in all laminae of the dorsal horn and also inhibited the post-discharges of intermediate and deep neurones resulting from repeated C fibre stimulation. There was little difference in the effects of the antagonist on the intermediate neuronal population compared to superficial or deep cells in the dorsal horn. AP5 has little effect on C fibre-evoked activity in superficial cells but produced slight inhibitions of the C fibre-evoked responses and clear reductions in the post-discharge of the deep neurones. This contrasts with the excitatory effects of the antagonist on both types of responses in the intermediate cells. A fibre-evoked responses were unaffected by AP5. Taking into account the results with the two antagonists it appears that both A and C fibre-evoked responses of dorsal horn nociceptive neurones are mediated by non-NMDA receptors whilst the C fibre-evoked wind-up of deep dorsal horn cells involves the NMDA receptor which also seems to mediate, in a complex manner, C fibre responses of intermediate, presumed substantia gelatinosa neurones. The results are discussed with regard to nociceptive mechanisms in the dorsal horn.

An intracellular analysis of amino acid induced excitations of deep dorsal horn neurones in the rat spinal cord slice

The rat spinal cord slice preparation has been used to investigate the sensitivity of deep dorsal horn neurones to the excitatory amino acids N-methyl-D-aspartate (NMDA), quisqualate and L-glutamate. Intracellular recordings were made from 44 neurones in laminae III-VI of 14- to 16-day rats. Superfusion of quisqualate (30 microM) excited all neurones, NMDA (50 microM) excited 72% and L-glutamate (0.5-1 mM) 63% of the neurones. Depolarizations were retained after tetrodotoxin but with a reduced amplitude. The NMDA antagonist D-aminophosphonovalerate (D-APV, 10 microM) reduced NMDA and L-glutamate depolarizations by 66% and by 40%, respectively, while the quisqualate responses were enhanced by 27%. Dorsal root stimulation elicited two main patterns of activity; short-latency single/double spikes followed by subthreshold excitatory postsynaptic potentials (EPSPs) or a burst of spikes rising from a long duration composite EPSP. D-APV reduced the long-latency components of the first type and reduced the amplitude and duration of the composite EPSP of the second. These results support a specialized role for NMDA receptors in synaptic transmission in the dorsal horn.

Organization of glutamate-like immunoreactivity in the rat superficial dorsal horn: light and electron microscopic observations

Glutamate has been shown to be a neurotransmitter in the central nervous system of vertebrates, and it has been hypothesized that glutamate is functional as a neurotransmitter in the spinal cord dorsal horn. A monoclonal antibody to fixative-modified glutamate was used in this study to examine the light microscopic and ultrastructural profiles of glutamate-like immunoreactivity in the superficial dorsal horn of the rat spinal cord. Glutamate-like immunoreactivity was observed in neurons, fibers, and terminals of both laminae I and II. Marginal zone immunoreactive neurons ranged from 10 to 30 micron in diameter and received many nonimmunoreactive somatic synapses. In substantia gelatinosa, immunoreactive neurons were observed in both inner and outer layers, ranged 5 to 10 micron in diameter, and received few nonimmunoreactive somatic synapses. Glutamate-like immunoreactive dendrites were observed in both laminae and were contacted primarily by nonimmunoreactive synaptic terminals that generally contained small clear vesicles. Both myelinated and unmyelinated immunoreactive axons were observed in Lissauer's tract. Immunoreactive terminals contained small (40 nm) clear vesicles and generally formed simple synaptic contacts with nonimmunoreactive dendrites in laminae I and II. The results of this study corroborate the importance of glutamate as a neurotransmitter in spinal sensory mechanisms.

Influence of excitatory amino acid receptor antagonists and of baclofen on synaptic transmission in the optic nerve to the suprachiasmatic nucleus in slices of rat hypothalamus

Electrical stimulation of the optic nerve evoked two positive waves with short latency, followed by a large negative wave in the suprachiasmatic nucleus of slices of hypothalamus of the rat. The latency to peak of the two positive waves and the large negative wave were 2.7 +/- 0.1, 6.1 +/- 0.1 and 10.3 +/- 0.5 msec, respectively. Only the large negative wave disappeared in low calcium Ca2+-high magnesium (Mg2+) Krebs solution and with the addition of tetrodotoxin (1 microM) all the waves disappeared. Baclofen inhibited the large negative wave in a dose-dependent manner but not the two positive waves. Excitatory amino acid antagonists also inhibited only the large negative wave, i.e. it was reduced to about 70% by 1 mM glutamic acid diethyl ester and to about 50% by both 1 mM 2-amino-4-phosphonobutyric acid and 1 mM DL-2-amino adipic acid. All waves were unaffected by 0.1 mM atropine, hexamethonium and curare. These results indicate that two positive waves, induced by stimulation of the optic nerve are attributed to nerve conduction and the large negative wave to the neurons of the suprachiasmatic nucleus, and that the neuronal pathway from the optic nerve to the suprachiasmatic nucleus may include aspartate and/or glutamate as an excitatory neurotransmitter.

Effect of 450191-S, a 1H-1,2,4-triazolyl benzophenone derivative, on cerebral content of neuroactive amino acids

The effect of 450191-S, one of the 1H-1,2,4-triazolyl benzophenone derivatives and possessing benzodiazepine-like anti-anxiety actions, on the content of various amino acids in the mouse brain was examined in comparison with that of nitrazepam. Among the various amino acids examined, only glycine, aspartic acid and alanine showed a statistically significant decrease following the oral administration of 450191-S. The oral administration of nitrazepam also induced a similar decline in the cerebral contents of glycine and alanine. Furthermore, it was found that the administration of Ro 15-1788, a benzodiazepine antagonist, eliminated the 450191-S-induced decline in the cerebral contents of glycine and aspartic acid. These results indicate that the administration of 450191-S induces the decrease of glycine and aspartic acid, neuroactive amino acids, in the brain, possibly via the activation of benzodiazepine receptors.

beta-N-Oxalylamino-L-alanine: action on high-affinity transport of neurotransmitters in rat brain and spinal cord synaptosomes

beta-N-Oxalylamino-L-alanine (BOAA) is a dicarboxylic diamino acid present in Lathyrus sativus (chickling pea). Excessive oral intake of this legume in remote areas of the world causes humans and animals to develop a type of spastic paraparesis known as lathyrism. BOAA is one of several neuroactive glutamate analogs reported to stimulate excitatory receptors and, in high concentrations, cause neuronal vacuolation and necrosis. The present study investigates the action of BOAA in vitro on CNS high-affinity transport systems for glutamate, gamma-aminobutyric acid (GABA), aspartate, glycine, and choline and in the activity of glutamate decarboxylase (GAD), the rate-limiting enzyme in the decarboxylation of glutamate to GABA. Crude synaptosomal fractions (P2) from rat brain and spinal cord were used for all studies. [3H]Aspartate transport in brain and spinal cord synaptosomes was reduced as a function of BOAA concentration, with reductions to 40 and 30% of control values, respectively, after 15-min preincubation with 1 mM BOAA. Under similar conditions, transport of [3H]glutamate was reduced to 74% (brain) and 60% (spinal cord) of control values. High-affinity transport of [3H]GABA, [3H]glycine, and [3H]choline, and the enzyme activity of GAD, were unaffected by 1 mM BOAA. While these data are consistent with the excitotoxic (convulsant) activity of BOAA, their relationship to the pathogenesis of lathyrism is unknown.

Identification and characterisation of L-[3H]aspartate binding sites on rat spinal cord synaptic membranes

The binding of L-[3H] aspartate to extensively-washed rat spinal cord synaptic membranes was investigated. Specific binding was enriched in synaptic membranes and was optimal under physiological conditions of temperature and pH. Equilibrium binding was established relatively slowly over a period of 30 min, and was totally reversible within 40 min. Saturation analysis revealed complex binding patterns. Two sites were clearly demonstrable, only one of which was shown to be saturable over the ligand concentration range employed in the study (0.1-10 microM). There was also some indication of the presence of a higher affinity site, although this was not investigated in any detail. Saturable binding demonstrated a KD = 1.4 microM and Bmax = 105 pmole/mg protein. Structure-activity studies with a range of amino acid analogues indicated that binding was stereospecific and was inhibited by a very restricted range of compounds. The most potent inhibitors of binding were L-glutamate and L-aspartate. There was no evidence for the involvement of NMDA receptors. Effects of possible endogenous modulators, including ions and guanosine nucleotides were investigated, and the chemical nature of the binding site probed with a number of protein-modifying agents.

N-Methyl-D-aspartate and L-aspartate activate distinct receptors in piriform cortex

The effects of ionophoretically applied N-methyl-DL-aspartate (NMDA) and aspartate on identified pyramidal neurons in rat piriform cortex were examined in isolated, submerged, and perfused brain slices. NMDA was more potent than aspartate in eliciting neuronal discharge. Perfusion of the acidic amino acid antagonists, DL-2-amino-5-phosphonovalerate (APV), 10(-6) or 10(-5) M, DL-2-amino-7-phosphonoheptanoate (APH), 10(-5) M, and gamma-D-glutamylglycine (gamma DGG), 10(-5) M, selectively blocked the response to NMDA without effect on the response to aspartate. At higher concentrations which blocked responses to both NMDA and aspartate, gamma DGG blocked kainate responses and depressed glutamate and quisqualate responses. These results suggest that in piriform neurons NMDA and aspartate act at distinct receptor sites, not a common receptor site, and that both of these sites are distinct from those that mediate responses to glutamate, quisqualate, and kainate.

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.

Responses of solitary retinal horizontal cells from Carassius auratus to L-glutamate and related amino acids

Effects of L-glutamate and its analogues on membrane potentials of solitary horizontal cells were studied by intracellular recording. L-glutamate depolarized these cells at micromolar concentrations (greater than or equal to 10 microM), while D-glutamate and L-alpha-amino adipic acid produced slight depolarizations only at millimolar concentrations. Neither L- nor D-aspartate, even at millimolar doses, produced any change in solitary horizontal-cell resting potential. Solitary horizontal-cell responses to L-glutamate did not desensitize detectably. Responses to pairs of brief, ionophoretic pulses of L-glutamate were nearly equal in amplitude at inter-pulse intervals as short as 50 ms. Responses to maintained applications of low doses of L-glutamate did not decline for as long as 2 min. Depolarizing responses were produced by ionophoretic applications of L-glutamate near cell somata as well as dendrites. The mean sensitivity was 1.4 +/- 1.5 mV/nC with a maximum of 5.1 mV/nC. Depolarizing responses to L-glutamate reversed in polarity at membrane potentials between 0 and -20 mV, were accompanied by a decrease in membrane slope resistance, and were suppressed by replacement of extracellular sodium ions with choline. These results demonstrate that chemosensitivity of retinal horizontal cells to acidic amino acids persists after dissociation protocols, and in several respects resembles that found in horizontal cells in situ. These findings are consistent with the notion that retinal horizontal cells receive a synaptic input involving L-glutamate or a similar substance.

Effects of glutamate and aspartate on sympathetic preganglionic neurons in the upper thoracic intermediolateral nucleus of the cat

Glutamate and aspartate excited all spontaneously active sympathetic preganglionic neurons (SPNs) tested in the intermediolateral nucleus of spinal segments T1-T3. Most silent neurons could be induced to discharge but the remainder showed only a decrease in antidromic spike amplitude. These effects were typically fast (on, off less than 1 s). D,L-Homocysteic acid also produced excitation; this effect was typically slower. Glutamate and aspartate were usually equipotent; 20% were differentially sensitive to aspartate, 10% to glutamate.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

Quinolinic acid: regional variations in neuronal sensitivity

The excitatory action of quinolinic acid has been examined on neurons in different parts of the rat CNS. When applied by microiontophoresis quinolinic acid excited cells in the cerebral cortex, hippocampus and neostriatum, but even when applied from electrodes which produced responses in these areas, quinolinic acid was ineffective in the cerebellum and spinal cord. Like most excitants tested in other studies, L-glutamate was excitatory to all cells examined. As an endogenous compound, therefore, quinolinic acid may merit special attention as a potential neurotransmitter in brain.

Glutamate-induced cellular injury in isolated chick embryo retina: Müller cell localization of initial effects

The neurotoxic and gliotoxic effects of glutamate and several glutamate analogues were studied in isolated chick embryo retinas. To facilitate examination of initial pathological events, a short-term incubation system was developed and used for light microscopic and autoradiographic investigation. Low-dose, short-term glutamate treatment of 12-day retinas resulted in a glial-specific lesion in the Müller cells, characterized by extensive cellular edema; at higher concentrations and/or longer treatment times, neurotoxic as well as gliotoxic effects were seen. The early glial damage was identical in appearance to that seen after incubation with DL-alpha-aminoadipate and other reported gliotoxins. No evidence of a similar glial-specific action was seen after administration of kainic acid, although extensive neuronal degeneration did result. Incubation of retinas with tritiated glutamate (3H-glu) revealed a selective uptake of the label by Müller cells. Autoradiographic grains were localized over Müller foot processes at the inner limiting membrane, and by 30 minutes labeled the entire glial system. Prior treatment with neurotoxic levels of glutamate did not alter the autoradiographic localization to glial cells. Possible glial-neuronal interactions and their effect on cytotoxic patterns are discussed.

Effect of motor and premotor cortex ablation on concentrations of amino acids, monoamines, and acetylcholine and on the ultrastructure in rat striatum. A confirmation of glutamate as the specific cortico-striatal transmitter

At 1, 2, and 4 weeks after unilateral premotor and motor cortex ablation in rats, a significant and lasting decrease in glutamate levels in the ipsilateral versus contralateral striatum was observed. A significant corresponding fall in aspartate was seen only after 1 week. In contrast, there was a large increase in the striatal concentrations of lysine, threonine, alanine, and glutamine 1 week after the cortical ablation. This correlates with the extensive glial proliferation in the deafferented ipsilateral striatum. Four weeks after cortical ablation the GABA concentration was significantly increased. There was no decrease in other putative transmitters (dopamine, serotonin, acetylcholine, glycine and taurine), nor was a glutamate decrease observed in the hippocampus or in the hypothalamus, which do not receive direct premotor and motor cortical inputs. Both biochemical and morphological evidence for a minor contralateral cortico-striatal projection was obtained. Correlating with the fall in glutamate, ultrastructural observations indicated the degeneration of two types of striatal synapses, i.e., those of the axo-spinous type III and of the axo-dendritic type VII. Frontal cortex ablation clearly affects, in opposite directions, the metabolism of various striatal amino acids but not that of acetylcholine and the monoamine transmitters. The results strongly support the view that glutamate is the transmitter of the cortico-striatal fibers.

The responses of neurones of the superficial dorsal horn to ionophoretically applied glutamate ion

Extracellular spikes were recorded from neurones in the marginal zone and substantia gelatinosa of the cat lumbar spinal cord using low-noise carbon fibre microelectrodes. Ionophoretic ejection of glutamate ion was used to distinguish soma/dendrite from axon spikes. The receptive fields of the units and their response to percutaneous electrical stimulation was investigated. The majority of units had low-threshold receptive fields on the hair and skin of the toes and foot. The receptive fields indicated a convergence of different modality afferents onto single cells. Ionophoretic glutamate acted synergistically with the action of the natural transmitter from the low-threshold afferents onto the substantia gelatinosa cells.

L-glutamate and L-aspartate bind to separate sites in rat brain synaptic membranes

The specific binding of L-glutamate (L-Glu) and L-aspartate (L-Asp) was measured in rat brain synaptic plasma membranes (SPMs). A distinction between the binding sites for these amino acids was made on the basis of the kinetics, ion effects, pharmacology and chemical susceptibility of the binding. The existence of distinct binding sites for L-Glu and L-Asp is consistent with electrophysiological data that mammalian neurons possess separate receptors for these amino acids.

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.

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.

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.

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.

Effects of excitatory and inhibitory amino acids on phasic respiratory neurons

The responsiveness of phasically active brainstem respiratory neurons to several amino acids was investigated in cats under Dial anesthesia. Four-barreled microelectrodes were used to extrude iontophoretically the putative neurotransmitters L-glutamate, L-asparatate, glycine, and gamma-aminobutyric acid (GABA), L-glutamate and L-aspartate caused increased activity when applied to either inspiratory or expiratory neurons and appeared to be equal in efficacy. Likewise, GABA and glycine depressed ongoing phasic neural activity of both inspiratory and expiratory units. In this case, however, the dosage of GABA required to produce a given depression was significantly less than the required dosage of glycine. These findings support the hypothesis that L-glutamate and/or L-aspartate may act as excitatory neurotransmitter agents at the synapses of brainstem respiratory neurons and conversely, GABA may act as the natural inhibitory neurotransmitter.