Patterns of excitation of thalamic neurones by amino-acids and by acetylcholine

Integrated Brain Metabolomics and Network Pharmacology Analysis to Reveal the Improvement Effect of Bai Chan Ting on Parkinson's Disease

BACKGROUND

Baichanting (BCT),a traditional Chinese medicine prescription, is a combination of Acanthopanax senticosus, Paeonia lactiflora, and Uncaria three herbs, which has the effect of benefiting the kidney and calming the liver. The study was aimed at investigating the protective effect of BCT against Parkinson's disease using an integrated strategy of network pharmacology and brain metabolomics.

MATERIALS AND METHODS

By integrating network pharmacology with metabolomic research, the protective effect of BCT against PD was investigated using a transgenic mouse model for α-synuclein. The metabolite level and gene level components of BCT that might be anti-PD were separated out. Indicators of behavior and pharmacodynamics were employed to gauge the effectiveness of BCT on PD in the preliminary stages. Network pharmacology, which may be the target of BCT, screened the active substances and target genes. The use of metabolomics to identify potential biomarkers of PD, and then through network pharmacology and metabolic pathways to determine their regulatory enzymes and regulatory genes, improve the pathway mechanism of the disease, has important guiding significance for the in-depth study of the pathogenesis of PD.

RESULTS

101 putative target genes were identified by the network pharmacology analysis in relation to the treatment of PD with BCT. According to the functional enrichment analysis, the proposed mechanism was primarily related to the transport of neurotransmitters, the metabolism of arachidonic acid, dopamine, and alpha-amino acids, as well as the transport of dopamine and the negative regulation of amino acid transport. 25 distinct endogenous metabolites were shown to be potential biomarkers for the BCT for treating PD based on metabolomics. These metabolites were mostly implicated in the important methionine and cysteine, tyrosine, histidine, and arginine and proline metabolic pathways. These results somewhat agreed with those of the network pharmacology analysis.

CONCLUSIONS

In conclusion, this study showed that BCT could delay the occurrence and development of PD by improving the brain metabolic disorder of α-Syn mice, which revealed the mechanism of BCT through multitarget and multipathway treatment, and provided a new explanation for the mechanism of anti-PD action. Our research, on the other hand, demonstrated that the network pharmacology-integrated metabolomics approach was a potent tool for discovering the active ingredients and mechanisms underlying the pharmacological effects of traditional Chinese medicine.

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.

Update on the glutamatergic neurotransmitter system and the role of excitotoxicity in amyotrophic lateral sclerosis

Excitotoxicity may play a role in certain disorders of the motor system thought to be caused by environmentally acquired toxins, including lathyrism and domoic acid poisoning. Motor neurons appear to be particularly susceptible to toxicity mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-kainate receptors. There is a body of evidence implicating glutamatergic toxicity as a contributory factor in the selective neuronal injury occurring in amyotrophic lateral sclerosis (ALS). Interference with glutamate-mediated toxicity is so far the only neuroprotective therapeutic strategy that has shown benefit in terms of slowing disease progression in ALS patients. Biochemical studies have shown decreased glutamate levels in central nervous system (CNS) tissue and increased levels in the cerebrospinal fluid (CSF) of ALS patients. CSF from ALS patients is toxic to neurons in culture, apparently via a mechanism involving AMPA receptor activation. There is evidence for altered expression and function of glial glutamate transporters in ALS, particularly excitatory amino acid transporter 2 (EAAT2). Abnormal splice variants of EAAT2 have been detected in human CNS. Mitochondrial dysfunction may contribute to excitotoxicity in ALS. Induction of neuronal nitric oxide synthase and cyclooxygenase 2 in ALS may also lead to significant interactions with regulation of the glutamate transmitter system. Certain features of motor neurons may predispose them to the neurodegenerative process in ALS, such as the cell size, mitochondrial activity, neurofilament content, and relative lack of certain calcium-binding proteins and molecular chaperones. Motor neurons appear vulnerable to toxicity mediated by calcium-permeable AMPA receptors. The relatively low expression of the glutamate receptor 2 (GluR2) AMPA receptor subunit and the high current density caused by the large number and density of cell surface AMPA receptors are potentially important factors that may predispose to such toxicity.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

The role of the frontal cortex in the mouse in behavioral sensitization to amphetamine

Pharmacological studies have shown that a variety of neuroeffectors are involved in behavioral sensitization to amphetamine-induced stereotypy. In the present work, the effect of some of these drugs on sensitization was studied after intracortical administration in order to determine the role of the cortex in mediating their systemic effects. The dopamine antagonists sulpiride and spiperone were both ineffective against the acute response to amphetamine; nevertheless, both blocked the induction of sensitization, suggesting that the mesocortical dopamine pathway is not involved in the acute response but is necessary for the induction of sensitization. Both CPP, an NMDA receptor antagonist, and THIP, a GABA(A) agonist, blocked the acute response and the induction of sensitization to amphetamine. On the other hand, mecamylamine, the nicotinic cholinergic antagonist, failed to affect either the acute response or the induction of sensitization, which suggests that the cortex is not a locus of its activity. Anisomycin, an inhibitor of protein synthesis, and diltiazem, a calcium-channel blocker, were both ineffective against the acute response, but both blocked induction. All of the drugs, except CPP and THIP, were ineffective against the expression of sensitization; therefore, the ability of the other drugs to block expression must reside within another locus. Bicuculline injected intracortically in non-convulsant doses produced a stereotypy indistinguishable from that induced by amphetamine; and the effect was readily antagonized by CPP administered either systemically or intracortically. In contrast, sulpiride by either route of administration failed to block the bicuculline-induced stereotypy; we conclude, therefore, that the stereotypic effect of bicuculline is not mediated by dopamine. These results imply that amphetamine-induced stereotypy is mediated in the cortex by the removal of the inhibitory control of the excitatory system. The data also suggest that cortical dopamine, as well as the NMDA and GABA(A) systems, is important in sensitization to amphetamine. In general the data demonstrate that different neuroeffectors involved in sensitization exert their effects at different brain loci.

Physiological release of excitatory amino acids

The contribution of in vivo monitoring to the study of glutamate release is reviewed. Physiological stimulation increases both glutamate and aspartate in the extracellular compartment of the brain and both amino acids show Ca(2+)-dependent K(+)-evoked release. However, the finding that only glutamate is stored in synaptic vesicles implies that glutamate is the excitatory transmitter. Released glutamate is taken up into both neurones and glia by glutamate transporters. Uptake of glutamate, in addition to clearing the synapse, has a number of additional functions. Uptake into glia leads to the release of glutamine, which is involved in the recycling of transmitter glutamate; uptake into both neurones and glia leads to the release of ascorbate; uptake into glia leads to an increase glycolysis and export of lactate, an energy substrate for neuronal metabolism. Reversal of the glutamate transporter accounts for the parallel release of glutamate and aspartate from the cytoplasmic compartment. The basal concentration of extracellular glutamate is in the micromolar range. Such levels could lead to desensitisation of both NMDA and non-NMDA receptors. The functional implications of the level of basal glutamate are difficult to assess at present in view of the existence of multiple glutamate receptor subunits with different functional properties and distributions.

Antagonists of the NMDA receptor-channel complex and motor coordination

Many structurally different, centrally active antagonists of the NMDA receptor-channel complex induce phencyclidine-like side effects in mammals which include head weaving, body rolling, sniffing and disturbances of motor coordination. The ability of these compounds to cause disturbances of motor coordination correlates directly with their ability to antagonize the NMDA receptor-channel complex in vivo. Although noncompetitive antagonists increase motility in rodents, whereas competitive antagonists do not, both classes of compounds appear to induce schizophrenia-like psychosis in human beings, and cause similar changes in a variety of different biogenic amine neurotransmitter systems in the limbic and motoric areas of the brain. The complex spectrum of behavioural effects observed after the administration of antagonists of the NMDA receptor-channel complex probably reflects the intricate nature of the interaction with positive and negative feedback loops of the motor circuit. Recent research indicates that the site of integration of this interaction could be the striatal medium spiny GABAergic neuron.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Amino acids modify thalamo-cortical response transformation expressed by neurons of the ventrobasal complex

The hypothesis has been tested that inhibitory mechanisms, active spatially and temporally between the input and the output of thalamic neurons, determine the nature of the information transmitted to the cerebral cortex. To enable this assessment, in barbiturate-anesthetized cats and urethane-anesthetized rats juxtacellular recordings were performed together with microiontophoretic ejection of transmitter agonists and antagonists. The effects of these drugs were studied on responses evoked by mechanical stimulation of cutaneous receptive fields (RFs) of neurons in the thalamic ventrobasal complex (VB). Neurons from different parts of the VB were investigated: 29 units were located medially, in the ventral posteromedial nucleus (VPM; facial RFs), and 11 units were located laterally, in the ventral posterolateral nucleus (VPL; forepaw and body RFs). A further eleven VB units had no detectable RF. Twenty-six neurons were tested with electrical stimulation of the somatosensory cortex (SI), 17 of these being identified as thalamo-cortical relay neurons and 5 being classified as presumed interneurons; the remaining 4 could not be activated. Four additional recordings were from trigemino-thalamic or thalamo-cortical fibers. For the quantitative assessment of the neurons' input and output, neuronal activity was induced by feedback-controlled, mechanical trapezoidal and/or sinusoidal stimuli applied to sinus hairs, fur or skin and the numbers of prepotentials and soma spikes were compared in peristimulus time histograms (PSTHs) generated simultaneously for both types of signal from 'DC' recordings. Iontophoretic administration of excitatory amino acids (EAAs) or bicuculline methiodide (BMI) increased output-input ratios in 87% of the cases tested, due to a higher rate of conversion of prepotentials into soma spikes taking place. In cases of neurons exhibiting a sustained-to-transient response pattern, changes to sustained-to-sustained patterns were demonstrated. Tests with gamma-aminobutyric acid (GABA) produced decreased output-input ratios in 90% of the neurons, due to a lower conversion rate of prepotentials into soma spikes taking place. In cases of neurons exhibiting high output-input ratios (sustained-to-sustained type), the responses changed to the sustained-to-transient pattern. For cortically evoked antidromic spikes of VB neurons, GABA produced a failure of the initial segment (IS-) spike to invade the soma, whereas BMI and glutamate (Glu) facilitated soma depolarization. When ejected with relatively higher currents than those needed to alter output-input ratios, EAAs decreased prepotential amplitudes while GABA produced increases in 16 of 18 neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

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.

Cholinergic responsiveness of neurons in the ventroposterior thalamus of the anesthetized rat

Acetylcholine has been implicated as an important neurotransmitter in the mechanisms of thalamic activation. Cholinergic mechanisms are thought to directly underlie the high level of excitability observed in thalamic relay neurons during waking and rapid eye movement sleep. We sought to determine if the cholinergic responsiveness of neurons in the ventroposterior nuclei of the thalamus in rat is consistent with this view. Neurons in the chloral hydrate-anesthetized rat were studied with extracellular recording and microiontophoretic application of cholinergic agents. In most cases (63% of 63 cells), the ejection of the agonist, carbachol, had no observable effect on spontaneous activity. Facilitation (25%), inhibition (8%) and inhibition followed by facilitation (3%) were also observed. Carbachol ejections that by themselves were ineffective in altering spontaneous activity proved capable, in 93% of 28 cells, of antagonizing the uniformly facilitatory responses produced by glutamate ejection. The putative M1-selective, cholinergic agonist, McN-A-343, was also ineffective alone in altering spontaneous activity in the majority of cases (74% of 27 cells) and produced only inhibitory responses in the remaining seven neurons studied. Interacting applications of McN-A-343 and glutamate resulted, in all cases, in antagonism of glutamate facilitation (N = 12). The various responses to applied cholinergic agonists were all capable of being antagonized by muscarinic receptor-blocking agents. Both the high proportion of inhibitory responses and the antagonism of glutamate facilitatory responses suggest that ventroposterior neurons in the rat differ from other thalamocortical relay neurons in the rat and cat with regard to cholinergic responsiveness. Additionally, the lack of predominantly facilitatory responding renders it unlikely that cholinergic mechanisms directly underlie increases in excitability of ventroposterior neurons observed during waking and rapid eye movement sleep.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Excitatory amino acid receptor-mediated transmission of somatosensory evoked potentials in the rat thalamus

1. To examine the role of excitatory amino acid receptors in the rat ventrobasal thalamic nucleus (v.b.t.n.) for the transmission of cortical somatosensory evoked potentials (s.e.p.s), potentials were recorded from the somatosensory cortex of barbiturate-anaesthetized and of unanaesthetized awake rats. The effects of microapplications of the selective N-methyl-D-aspartate (NMDA) antagonist (-)-2-amino-7-phosphono-heptanoate ((-)AP7) and the broad-spectrum excitatory amino acid antagonist 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (pCB-PzDA) into the thalamus on the amplitudes and latencies of cortical potentials were measured. 2. To define the receptor specificity of local microinjections of (-)AP7 and pCB-PzDA electroencephalogram (e.e.g.) recordings were made from the immediate vicinity of the injection cannula within the thalamus. (-)AP7 selectively antagonized epileptic discharges induced by NMDA, but not those by kainate, whereas pCB-PzDA antagonized epileptic discharges induced by both. 3. In both anaesthetized and unanaesthetized rats, microapplications of pCB-PzDA into the thalamus suppressed transmission of cortical potentials as indicated by a decrease of their amplitudes and an increase of their peak latencies. Further experiments in anaesthetized rats showed that pCB-PzDA exerted its effects in a dose-dependent and site-specific way. 4. In both anaesthetized and unanaesthetized rats, microapplications of (-)AP7 into the ventrobasal thalamus did not affect cortical potentials. 5. These results are consistent with the assumption that an excitatory amino acid serves as transmitter at thalamic synapses mediating transmission of cortical potentials, and that this transmitter interacts preferentially with non-NMDA receptors.

The origins of cholinergic and other subcortical afferents to the thalamus in the rat

The origins of the cholinergic and other afferents of several thalamic nuclei were investigated in the rat by using the retrograde transport of wheat germ agglutinin conjugated-horseradish peroxidase in combination with the immunohistochemical localization of choline acetyltransferase immunoreactivity. Small injections placed into the reticular, ventral, laterodorsal, lateroposterior, posterior, mediodorsal, geniculate, and intralaminar nuclei resulted in several distinct patterns of retrograde labelling. As expected, the appropriate specific sensory and motor-related subcortical structures were retrogradely labelled after injections into the principal thalamic nuclei. In addition, other basal forebrain and brainstem structures were also labelled, with their distribution dependent on the site of injection. A large percentage of these latter projections was cholinergic. In the brainstem, the cholinergic pedunculopontine tegmental nucleus was retrogradely labelled after all thalamic injections, suggesting that it provides a widespread innervation to the thalamus. Neurons of the cholinergic laterodorsal tegmental nucleus were retrogradely labelled after injections into the anterior, laterodorsal, central medial, and mediodorsal nuclei, suggesting that it provides a projection to limbic components of the thalamus. Significant basal forebrain labelling occurred only with injections into the reticular and mediodorsal nuclei. Only injections into the reticular nucleus resulted in retrograde labelling of the cholinergic neurons in the nucleus basalis of Meynert. The results provide evidence for an organized system of thalamic afferents arising from cholinergic and noncholinergic structures in the brainstem and basal forebrain. The brainstem structures, especially the cholinergic pedunculopontine tegmental nucleus, appear to project directly to principal thalamic nuclei, thereby providing a possible anatomical substrate for mediating the well-known facilitory effects of brainstem stimulation upon thalamocortical transmission.

Choline acetyltransferase immunoreactivity in the rat thalamus

The distribution of choline acetyltransferase immunoreactivity in the rat thalamus was investigated by using a specific monoclonal antibody and was compared with the pattern of acetylcholinesterase staining. The only choline acetyltransferase-immunoreactive cell bodies in the thalamus were in the medial habenula. A wide range of densities of immunoreactive fibers and varicosities was found. The highest densities of stained varicosities were in the anteroventral, reticular, lateral mediodorsal, and intralaminar nuclei. At the other extreme, the anterodorsal, ventroposteromedial, and paraventricular nuclei were almost devoid of immunoreactive varicosities. A light density of fibers was observed in several medial nuclei, including parataenial, reuniens, and gelatinosus. Most other nuclei contained moderately dense regions of varicose fibers that were often heterogeneous or patchy. The pattern of choline acetyltransferase immunoreactivity in the thalamus was in general similar to that of acetylcholinesterase. A marked discrepancy, however, was found in the anterodorsal nucleus, which was intensely stained for acetylcholinesterase but contained no apparent choline acetyltransferase immunoreactivity. Numerous physiologic studies have demonstrated striking effects of acetylcholine on thalamic activity. The present study provides a description of choline acetyltransferase-immunoreactive structures in the thalamic nuclei, providing a first step toward elucidating the anatomical basis for the physiologic and functional importance of cholinergic transmission in the thalamus.

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.

Excitatory and differential disinhibitory actions of acetylcholine in the lateral geniculate nucleus of the cat

Single neurones were recorded in the dorsal lateral geniculate nucleus (d.l.g.n.) of adult cats anaesthetized with a mixture of halothane, nitrous oxide and oxygen. The multibarrel-glass micro-electrodes were filled with sodium acetate, L-glutamate, acetylcholine (ACh), gamma-aminobutyric acid (GABA) and bicuculline. In normally innervated, spontaneously active d.l.g.n. cells, ACh and L-glutamate elicited increased firing rates. After elimination of the excitatory input from the retina by retinal photocoagulation, the effects of ACh and L-glutamate were similar. This proves that both drugs have direct excitatory effects on d.l.g.n. cells and that disinhibition is not the most prominent influence of ACh in the d.l.g.n. The excitatory action of ACh on relay cells in the d.l.g.n. was strongly influenced by barbiturates. Sub-narcotic levels of sodium pentobarbitone completely abolished the excitation by ACh while the response to L-glutamate remained unchanged. Excitation, centre-surround antagonism and periphery effects were elicited by spots of light and by large field phase-reversing gratings with and without central sparing of the receptive field area. Binocular inhibition was elicited with the phase-reversing grating presented to the non-dominant eye. After localized destruction of the retinal receptive field area, retinogeniculate excitation ceased and an isolated lateral inhibition was observed in the acutely deafferented d.l.g.n. cells. The time course and strength of this inhibition was disclosed by raising the background discharge with microiontophoretically applied L-glutamate. With increasing size of retinal lesions the strength of isolated lateral inhibition decreased exponentially. A maximal intrageniculate range of more than 1000 microns was derived from computations of the lateral extent of deafferentation in the d.l.g.n. The inhibition acted beyond the classic surround inhibition of d.l.g.n. cells and thus was named long-range lateral inhibition. Microiontophoretically applied GABA elicits a strong inhibitory effect at the d.l.g.n. cells which is antagonized by bicuculline. Centre-surround antagonism, binocular inhibition and long-range inhibition were blocked by bicuculline and thus proven to be GABAergic. Each class of inhibition was differentially influenced by microiontophoretically applied ACh. Long-range inhibition was disinhibited, centre-surround antagonism was enhanced, and binocular inhibition was not significantly changed. In contrast to ACh excitation, the disinhibitory action of ACh was not suppressed by pentobarbitone.(ABSTRACT TRUNCATED AT 400 WORDS)

A vasopressor cell group in the rostral dorsomedial medulla of the rabbit

Microinjections of sodium glutamate solution (which excites neuronal cell bodies but not axons of passage) into a circumscribed region within the dorsal reticular formation in the rostral medulla oblongata evoked a large increase in arterial pressure due to wide-spread vasoconstriction. In spontaneously breathing animals, glutamate stimulation of the pressor region did not affect respiratory activity and evoked only a very small and transient increase in electromyographic activity of axial skeletal muscles. The pressor response was not reduced by decerebration or decerebellation, indicating that the pathway connecting the dorsomedial pressor region to the spinal sympathetic outflow is intrinsic to the lower brainstem and spinal cord. Anatomical observations in the present study, combined with those from previous studies, indicate that neurons in this region do not project directly to the spinal cord and do not receive direct afferent inputs from the nucleus tractus solitarius. It is concluded that there exists a circumscribed group of vasopressor neuronal cell bodies within the rostral dorsomedial medulla. The anatomical connections of these neurons, however, are markedly different from those of a previously described group of vasopressor neurons in the rostral ventrolateral medulla, suggesting that the two groups may have different functional roles in cardiovascular regulation.

Quantitative autoradiography of binding sites for [3H]AMPA, a structural analogue of glutamic acid

Binding sites for the potent glutamate agonist [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were localized in rat brain frozen sections by quantitative autoradiography. Highest levels of binding were seen in stratum radiatum and stratum oriens of the CA1 hippocampal subfield and in the dorsal subiculum. Substantially less but still high amounts of [3H]AMPA binding occurred in other hippocampal subfields and in rostral forebrain structures. The heterogeneous nature of [3H]AMPA binding is discussed in relation to [3H]glutamate binding visualized by similar methods. From these data it is suggested that [3H]AMPA may label a particular subclass of the glutamate receptor population which exhibits a high affinity for quisqualic acid.

Differential sensitivity of selected brain areas to excitatory amino acids

The 22Na+ efflux stimulated by selected agonists of the 4 excitatory amino acid receptors, previously detected in the striatum, has been studied on 22Na+-preloaded slices prepared from 10 major areas of the rat brain. All brain areas were found to be sensitive, albeit to varying extents, to excitatory amino acids. The cerebellum was exceptional in its high sensitivity to kainate and quisqualate and in the absence of effect of N-methyl-D-aspartate. These results support the suggestion that excitatory amino acids interact with heteregenous receptors which differ from each other not only in their pharmacological properties but also in their regional distribution.

A method for evoking physiological responses by stimulation of cell bodies, but not axons of passage, within localized regions of the central nervous system

A method for evoking physiological responses by microinjection of sodium glutamate solution into localized regions of the central nervous system (CNS) is described. The major advantage of this method is that the cell bodies or dendritic processes of neurones within the injection site are excited, whereas axons of passage are unaffected. It was demonstrated that injections of minute volumes (50-100 nl) of 0.5 M glutamate solution into selected sites within the medulla or midbrain of anaesthetized or conscious animals, respectively, elicited marked autonomic, somatomotor or behavioural responses, depending on the injection site. In contrast, glutamate microinjection into fibre tracts failed to elicit any response, whereas electrical stimulation applied at the same sites elicited marked responses. The degree of localization of the glutamate stimulus and the relation between glutamate concentration and magnitude of evoked response are described. It is concluded that this method is a very effective means of selectively stimulating cell bodies within highly localized regions of the CNS. Further, by using this method in combination with focal electrical stimulation, it is possible in some cases to provide evidence that a response arises from excitation of axons of passage rather than cell bodies.

Unit study in cat claustrum of the effects of iontophoretic neurotransmitters and correlations with the effects of activation of some afferent pathways

Glutamate (Glut), acetylcholine (ACh) and dopamine (DA) were iontophoretically applied on cat claustral neurons. Glut did not affect all the neurons; ACh had both excitatory and inhibitory effects, while DA was prevalently inhibitory. An analysis was made of the time-course of excitatory and inhibitory responses on the basis of the mean firing rate variations during and after ACh and DA release. Three types of responses are described for each drug: short lasting inhibition, long lasting inhibition and long lasting excitation. The experimental data were statistically elaborated. The effects of ACh and of DA were compared with those of activation obtained by sensorial peripheric and thalamic stimulations. ACh could be supposed to be the transmitter of most of the inhibitory terminals of these sensitive afferences to the claustrum.

Possible involvement of glutamate in electrocortical activity

Microinjections of the excitatory amino acid L-glutamic acid into the mesencephalic reticular formation lead to activation or desynchronization of the electrocorticogram, whereas its topical application to the exposed cortex induces an increase in the amplitude of electrocortical activity. The results indicate that L-glutamic acid may well be involved directly or indirectly with transmission of reticular cell populations related to the control of electrocortical activity and may also act directly on the cortical neurones.

Pharmacology of the brachium conjunctivum: red nucleus synaptic system in the baboon

Acetylcholine, biogenic amines, and certain amino acids were applied by microiontophoresis to parvicellular and magnocellular red nucleus (RN) neurons of baboon while recording brachium conjunctivum (BC)-evoked and amino acid-evoked unit discharge from these neurons. Glycine, gamma-aminobutyric acid, and beta-alanine were potent depressants of BC-RN synaptic transmission, amino acid-evoked firing, and spontaneous activity of all RN neurons studied. Glycine was clearly more potent than the other 2 depressant amino acids. L-Glutamic and DL-homocysteic acid were strong excitants of all RN neurons tested. Dopamine, noradrenaline, and 5-hydroxytryptamine depressed the excitability of both parvicellular and magnocellular RN neurons; no excitatory effects were observed with these biogenic amines on RN neurons. Acetylcholine increased the rate of firing of spontaneously discharging parvicellular RN neurons and facilitated the amino acid-induced firing of these same neurons. Acetylcholine did not facilitate BC-RN synaptic transmission nor could this transmission be blocked by cholinergic antagonists. Unlike parvicellular RN neurons, the responsiveness of magnocellular neurons was either unaltered by acetylcholine or slightly decreased. These experiments demonstrate a difference in the pharmacologic responsiveness of parvicellular and magnocellular RN neurons to acetylcholine but do not provide evidence for a cholinergic input to RN via the brachium conjunctivum.

Inhibitory effects of acetylcholine on neurones in the feline nucleus reticularis thalami

1. Short iontophoretic pulses of acetylcholine (ACh) inhibited almost every spontaneously active cell encountered in the nucleus reticularis thalami of cats anaesthetized with a mixture of halothane, nitrous oxide and oxygen. On 200 cells the mean current needed to eject an effective inhibitory dose of ACh was 67 +/- 2 nA. When the ACh-evoked inhibition was mimicked by gamma-aminobutyric acid (GABA) or glycine on the same cell, the current required to release ACh was found to be approximately twice as great as that required to release an equally effective dose of GABA or glycine. 2. ACh inhibitions developed with a latency which was very much shorter than that for ACh excitation in cells of the ventrobasal complex. The latency of the ACh-evoked inhibition was as rapid as the onset and offset of the excitation of the same cells glutamate and their inhibition by GABA or glycine. 3. The firing pattern of ACh-inhibited neurones in the nucleus reticularis was characterized by periods of prolonged, high frequency bursts, and their mean firing frequency was 22 Hz. Raster dot displays and interspike interval histograms showed that whereas ACh suppressed the spikes that occurred between bursts much more readily than those that occurred during bursts, all spikes were equally sensitive to the depressant action of GABA and glycine. Large doses of ACh provoked or exaggerated burst activity. 4. ACh-evoked inhibition was extremely sensitive to blockade by short iontophoretic applications of atropine, which had no effect on the inhibitions evoked on the same cell equipotent doses of GABA or glycine. The ACh-evoked inhibitions were also antagonized by dihydro-beta-erythroidine released with slightly larger currents. When tested on the same cell, small iontophoretic applications of picrotoxin and bicuculline methoiodide blocked the inhibition evoked by GABA but had no effect on that evoked by ACh. Iontophoretic strychnine only rarely affected the inhibition evoked by ACh, while readily blocking the inhibition evoked on the same cell by an equipotent dose of glycine. In two cats, intravenous strychnine (1-2 mg/kg) had no effect on the ACh-evoked inhibition, while greatly reducing the sensitivity of the cell under study to glycine. 5. Only four out of forty-eight ACh-inhibted cells tested were inhibited by iontophoretic applications of either guanosine or adenosine 3':5'-phosphate. 6. Cells of the nucleus reticularis have been shown to have an inhibitory action on the thalamic relay cells, which are excited by ACh. It is suggested that the presence of both ACh excited and inhibited cells in different nuclei of the thalamus could be of considerable functional significance in gating sensory transmission through the thalamus.

The interaction of four putative glutamate antagonists with glutamate and their effects on the toad spinal cord

1. Four putative glutamate antagonists (L-glutamate diethyl ester, L-glutamate dimethyl ester, L-proline and 1-hydroxy-3-amino-pyrrolidone-2) were tested on the isolated hemisected toad spinal cord. 2. 1-hydroxy-3-amino-pyrrolidone-2 (10(-3)-10(-2) M) selectively antagonized the depolarizations evoked in both dorsal and ventral roots by applications of L-glutamate (5 X 10(-4) M). 3 1-hydroxy-3-amino-pyrrolidone-2 also antagonized the depolarizations evoked in both dorsal and ventral roots by stimulation of the adjacent dorsal root. 4. The dimethyl and diethyl esters of L-glutamate and L-proline had their own depolarizing actions on the dorsal and ventral roots, and neither potentiated nor antagonized the effects of L-glutamate. 5. The results with 1-hydroxy-3-amino-pyrrolidone-2 offer further evidence for the involvement of L-glutamate and L-aspartate in synaptic transmission in the amphibian spinal cord.

Effects of glutamate, aspartate, and two-presumed antagonists on feline rubrospinal neurones

The actions of dicarboxyl amino acids (GLUT, ASP, DLH) and of presumed amino acid antagonists (GDEE, HA 966) were studied in rubrospinal neurones with microelectrophoretic techniques. ASP and GLUT depolarized reversibly the cell membrane and increased its conductance. ASP had slightly stronger actions than GLUT. DLH had strong depolarizing actions without a clearcut change in membrane conductance; this may be due to the fact that DLH effects do not involve synaptic receptors. A specific action as antagonists for GLUT could not be shown for GDEE or HA 966. HA 966 effects were often accompanied by apparent postsynaptic actions. GDEE antagonized DLH effects sometimes even stronger than GLUT effects. The effects of GLUT and ASP are in principal agreement with a function as excitatory transmitters. The demonstration of a role as transmitter substances in excitatory (e.g. corticorubral or interpositorubral) pathways, however, still awaits the specific pharmacological antagonist.