An evaluation of selected brain constituents as putative excitatory neurotransmitters

Localization and Synaptic Release of N-acetylaspartylglutamate in the Chick Retina and Optic Tectum

The neuropeptide, N-acetylaspartylglutamate (NAAG), was identified in the chick retina (1.4 nmol/retina) by HPLC, radioimmunoassay and immunohistochemistry. This acidic dipeptide was found within retinal ganglion cell bodies and their neurites in the optic fibre layer of the retina. Substantial, but less intense, immunoreactivity was detected in many amacrine-like cells in the inner nuclear layer and in multiple bands within the inner plexiform layer. In addition, NAAG immunoreactivity was observed in the optic fibre layer and in the neuropil of the superficial layers of the optic tectum, as well as in many cell bodies in the tectum. Using a newly developed, specific and highly sensitive (3 fmol/50 microl) radioimmunoassay for NAAG, peptide release was detected in isolated retinas upon depolarization with 55 mM extracellular potassium. This assay also permitted detection of peptide release from the optic tectum following stimulation of action potentials in retinal ganglion cell axons of the optic tract. Both of these release processes required the presence of extracellular calcium. Electrically stimulated release from the tectum was reversibly blocked by extracellular cadmium. These findings suggest that NAAG serves an extracellular function following depolarization-induced release from retinal amacrine neurons and from ganglion cell axon endings in the chick optic tectum. These data support the hypothesis that NAAG functions in synaptic communication between neurons in the visual system.

Baclofen in the treatment of polymyoclonus and ataxia in a patient with homocystinuria

A patient with homocystinuria due to cystathionine beta-synthase deficiency developed severe progressive polymyoclonus and ataxia. To our knowledge, this is the first time polymyoclonus and ataxia have been reported in association with homocystinuria. Although cerebrovascular thrombosis is usually thought to be responsible for neurologic dysfunction in homocystinuric patients, no infarctions were demonstrated on magnetic resonance imaging scans in our case. We have previously reported that baclofen dramatically improved the polymyoclonus and ataxia in a patient with Unvericht-Lundborg disease. Baclofen given to our patient reversed the polymyoclonus and the ataxia as well. This suggests that patients with polymyoclonus and ataxia, no matter what the etiology, may benefit from the use of baclofen.

N-Acetyl-aspartylglutamate (NAAG) in human cerebrospinal fluid: Determination by high performance liquid chromatography, and influence of biological variables

NAAG is one of the neuropeptides found in highest concentrations in the CNS. The presence of micromolar concentrations of NAAG in human CSF was demonstrated by using two different and complementary analytical approaches: 1) isocratic separation of endogenous NAAG by reverse-phase high performance liquid chromatography (HPLC) with dual wavelength detection and 2) derivatization of endogenous NAAG with acidic methanol and subsequent HPLC analysis of the derivative NAAG-trimethyl ester. The NAAG concentration was between 0.44µmol/l and 7.16µmol/l (mean of 2.19 ± 1.53µmol/l) in CSF samples from forty neuropsychiatric patients. Endogenous NAAG or [(3)H]NAAG added to CSF samples were not significantly degraded when the CSF was incubated at 37°C during one hour, suggesting that the peptide is a highly stable metabolite in the subarachnoid space. In addition, evidence is provided that NAAG does not present a concentration gradient along the lower subarachnoid space.

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.

Excitatory action of some aspartate- and glutamate-containing dipeptides after intracerebroventricular injection in mice

The effects of some dipeptides, analogues of N-acetyl-alpha-L-aspartyl-L-glutamate, were studied after i.c.v. administration into mice in acute experiments. N-Acetyl-alpha-L-aspartyl-L-glutamate itself did not induce seizures in animals, but prevented glutamate-induced convulsions. All other dipeptides possessed excitatory glutamate-like actions. Some structural requirements for the excitatory effects of the dipeptides are discussed.

Antibodies to glutamate and aspartate recognize non-endogenous ligands for excitatory amino acid receptors

Antisera raised against glutaraldehyde conjugates of glutamate (Glu) and aspartate (Asp) with hemocyanin proved highly specific for their respective unconjugated amino acid haptens when tested in immunocytochemical blocking experiments on sections of the rat spinal cord. In addition, immunocytochemical staining by the Glu antiserum was effectively blocked by quisqualate but not by kainate or N-methyl-D-aspartate (NMDA); staining with the Asp antiserum was effectively blocked by kainate, to a lesser extent by quisqualate, and was not affected by NMDA. These results may be explained by assuming that the specific binding regions of the antibodies tested share certain recognition characteristics with endogenous binding sites or receptors for excitatory amino acids and their agonists.

Is a certain amount of cysteine prerequisite to produce brain damage in neonatal rats?

In this study, various manipulations were used to determine if certain amounts of cysteine are essential to damage the neonatal rat brain. The information gathered from this study indicated that concentration of free cysteine may be 0.6 mumol/g of wet brain weight or more to cause the toxicity to produce the brain damage, and the results were discussed in the light that free cysteine might itself be the cause of brain damage.

Release of N-acetylaspartylglutamate on depolarization of rat brain slices

In a great number of investigations, evidence in favor of a neurotransmitter role of the N-terminal-blocked, acidic dipeptide N-acetylaspartylglutamate (NAAG) has been accumulating. In fact, in some systems of the mammalian brain, almost all of the classical criteria for neurotransmitters have been fulfilled by NAAG except for the demonstration of its release from nervous tissue on depolarization. For quantification of NAAG in superfusates of brain slices, we have developed an analytical procedure consisting of an ion exchange prepurification, followed by a derivatization procedure and gas chromatography-mass spectrometry with chemical ionization and selected ion monitoring. Deuterated NAAG was used as an internal standard to provide a high degree of reliability for the analytical method. Detection limits of less than 1 pmol were achieved. A statistically highly significant increase of NAAG concentration in superfusates from rat neocortex, piriform cortex/amygdala, and hippocampus on depolarization with 50 mM K+ could be demonstrated and was shown to be largely Ca2+ dependent. These results support the hypothesis that NAAG is a neurotransmitter. Especially with respect to the piriform cortex, the present demonstration of NAAG release is consistent with electrophysiological and immunohistochemical evidence for its neurotransmitter function at terminals of the lateral olfactory tract.

Release of neuroactive substances: homocysteic acid as an endogenous agonist of the NMDA receptor

Sulfur containing amino acids such as homocysteic acid (HCA), cysteinsulfinic acid, homocysteinsulfinic acid are released by depolarization of slices from various rat brain regions in a Ca++-dependent manner. L-HCA excites caudate neurons through their N-methyl-D-aspartic acid (NMDA) receptor and potentiates their cortically evoked excitatory postsynaptic potentials. 35S-methionine can label the releasable pool of HCA, and thus appears as a precursor of HCA. Thus HCA is a transmitter candidate which acts predominantly on the NMDA receptor.

Behavioral classification of excitatory amino acid receptors in mouse spinal cord

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

Differential effects of N-methyl-D-aspartic acid and L-homocysteic acid on cerebellar Purkinje neurons

The effects of N-methyl-D-aspartic acid (NMDA) and L-homocysteic acid (LH) were measured on cerebellar Purkinje neurons. In urethane-anesthetized rats, iontophoretic application of NMDA elicited 3 different effects on the spontaneous activity of Purkinje cells: excitation, inhibition and biphasic responses consisting of excitation followed by inhibition. On the other hand, LH elicited excitation, only, regardless of the actions of NMDA on the same neurons. We also examined the effects of various excitatory amino acid antagonists on NMDA- and LH-mediated responses. Excitatory effects of NMDA were antagonized effectively by D.L-2-amino-5-phosphonovalerate (APV), ketamine, gamma-D-glutamylglycine (DGG), D,L-2-amino-7-phosphonoheptanoate (APH), and were not influenced significantly by L-glutamate diethylester (GDEE). Inhibitory responses of NMDA were antagonized by APV, APH and ketamine. LH-mediated excitations were influenced significantly by DGG and ketamine whereas GDEE, APV and APH failed significantly to attenuate the effects of LH. Based on the differential actions of LH and NMDA and the selectivity of NMDA antagonists for NMDA rather than LH-mediated excitations, it appears that the major actions of LH may not be mediated through NMDA receptor sites, at least in the cerebellum.

Dystonia in homocystinuria

Three patients with homocystinuria due to cystathionine beta-synthase deficiency who developed progressive generalised dystonia are described. Although cerebrovascular thrombosis is usually thought to be responsible for neurological dysfunction in homocystinuric patients, neuropathological studies in one case and clinical and radiological evidence in the other two suggested that dystonia was not caused by brain infarction. Movement disorder associated with homocystinuria may result from the neurochemical changes in the basal ganglia related to the inherited defect in sulphur amino acid metabolism.

The effects of N-acetylaspartylglutamate and distribution of N-acetylaspartylglutamate-like immunoreactivity in the rat somatosensory thalamus

The ventrobasal thalamus and adjacent regions were stained for the presence of N-acetylaspartylglutamate-like immunoreactivity. Immunoreactive axonal terminals were observed in this area and also in certain non-specific thalamic nuclei, the reticular thalamic nucleus and the lateral geniculate nucleus. Stained somata were found in the habenula, centrolateral thalamic nucleus and reticular thalamic nucleus. Iontophoretically applied N-acetylaspartylglutamate had variable, although predominantly inhibitory, actions on ventrobasal thalamus neurons. These results indicate that N-acetylaspartylglutamate is unlikely to be the neurotransmitter of ascending somatosensory afferents, but do not rule out the possibility that it has some other neurotransmitter or neuromodulator role in the ventrobasal thalamus.

Immunohistochemistry and biosynthesis of N-acetylaspartylglutamate in spinal sensory ganglia

N-Acetylaspartylglutamate (NAAG) is a nervous system-specific dipeptide which has been implicated in chemical neurotransmission. Antisera were prepared against NAAG in order to study its cellular distribution. When these antisera were applied to tissue sections of rat spinal sensory ganglia, NAAG-like immunoreactivity was detected within a subpopulation of relatively large neuronal cell bodies in cervical, lumbar, and thoracic ganglia. In order to confirm the presence of NAAG within these neurons, the dipeptide was extracted and purified from spinal ganglia using high-performance liquid chromatography and its composition confirmed by amino acid analysis. Further, the biosynthesis of NAAG was studied in vitro by following the incorporation of either [3H]glutamine or [3H]glutamate into the glutamate residue of the purified dipeptide. [3H]Aspartate was not incorporated efficiently into NAAG under these conditions, suggesting a precursor role for the large N-acetylaspartate pool. The incorporation of radiolabeled amino acids into newly synthesized NAAG by spinal sensory ganglia was not inhibited by incubation of the cells with anisomycin or cycloheximide at concentrations which significantly inhibited protein synthesis. These data suggest that NAAG is present in a subpopulation of primary afferent spinal neurons and that its biosynthesis is mediated by a dipeptide synthetase.

Excitatory action of N-acetylaspartylglutamate on Purkinje cells in guinea pig cerebellar slices: an intrasomatic study

The action of N-acetylaspartylglutamate (NAAG) on Purkinje cell somata in guinea pig cerebellar slices was intracellularly investigated in comparison with L-aspartate (Asp) and L-glutamate (Glu). A synthetic NAAG sample used was confirmed not to be contaminated with free Glu and Asp. NAAG, applied by either iontophoresis or superfusion, dose-dependently depolarized Purkinje cell somata, and iontophoretically applied NAAG decreased the membrane resistance in voltage- and Mg2+-dependent manners, like Asp or Glu. Relative depolarizing potencies seemed to be Glu greater than NAAG not equal to Asp in the presence of external Mg2+ and Asp greater than NAAG greater than or equal to Glu in the absence of Mg2+. In addition to this selective blocking effect of Mg2+, 2-amino-5-phosphonovalerate (APV) antagonized the actions of NAAG and Asp more strongly than that of Glu, while 2-amino-4-phosphonobutyrate showed rather non-selective antagonisms. The reversal potential of the NAAG action was at about 10 mV and similar to that of the action of Asp or Glu. These results suggest that NAAG itself is excitatory to guinea pig cerebellar Purkinje cells and is likely to be acting on Asp- and APV-sensitive, Mg2+-dependent receptors on cerebellar Purkinje cells.

L-proline depolarizes rat spinal motoneurones by an excitatory amino acid antagonist-sensitive mechanism

1 Isolated spinal cords prepared from neonatal rats were used to examine the effects of L-proline (L-Pro). 2 L-Pro (1-8 mM) depolarized ventral and dorsal roots in a dose-dependent manner with one sixth of the potency of L-glutamate (L-Glu). L-Pro was four times more potent than D-Pro. Prolonged application of L-Pro produced a plateau depolarization of motoneurones with no apparent fade. 3 Omission of calcium ions from the medium potentiated the depolarizing actions of L-Pro, L-Glu and quisqualate. 4 L-Pro was antagonized by concentrations of 2-amino-5-phosphonovalerate (25 microM), gamma-D-glutamylglycine (100 microM) and Mg2+ ions (1 mM) that depressed responses to N-methyl-D-aspartate (NMDA). The NMDA receptor-mediated component of the response to L-Pro was estimated to be 60-70%. 5 These data suggest that L-Pro should be considered as a possible excitatory neurotransmitter and that, because L-Pro is a neutral compound, excitatory amino receptors may not require an agonist to possess two anionic groups and one cationic group.

N-acetylaspartylglutamate identified in the rat retinal ganglion cells and their projections in the brain

N-Acetylaspartylglutamate like immunoreactivity (NAAG-L) was identified in retinal ganglion cell bodies and their axons. The presence of the dipeptide in ganglion cell projection areas, the lateral geniculate nucleus (LGN) and superior colliculus (SC), was confirmed following NAAG purification from these tissues by a high-performance liquid chromatographic method. NAAG-L was identified in the optic tract as well as within fibers and puncta in the LGN and SC. The hypothesis that NAAG is present within ganglion cell axons in the brain was tested by unilateral enucleation which resulted in loss of NAAG and NAAG-L within the contralateral LGN and SC.

In vitro release of endogenous excitatory sulfur-containing amino acids from various rat brain regions

Efflux of various amino acids from rat brain slices was determined under resting or depolarizing conditions. Slices of neocortex, hippocampus, striatum, cerebellum, mesodiencephalon, pons-medulla, and spinal cord were depolarized by K+ (50 mM) or veratrine (33 micrograms/ml). The 4-N,N-dimethylamino-azobenzene-4'-isothiocyanate (DABITC) derivatization method of Chang [Biochem. J. 199, 537-545 (1981)] for HPLC was adapted for analysis of amino acids and peptides in superfusion solutions. It allowed the separation and simultaneous detection of the sulfur-containing amino acids cysteine sulfinic acid (CSA), cysteic acid (CA), homocysteine sulfinic acid (HCSA), and homocysteic acid (HCA) at the picomole level. All four were shown to be released on depolarization in a Ca2+-dependent manner from brain slices. CSA and HCSA were released from cortex, hippocampus, mesodiencephalon, and, for HCSA only, striatum. HCA release, observed in all regions, was most prominent in cortex and hippocampus. CA was slightly increased by depolarization in hippocampus and mesodiencephalon. These sulfur-containing amino acids have been shown to exert an excitatory action on CNS neurons. The fact that these sulfur-containing amino acids are released as endogenous substances from nervous tissue supports the hypothesis that they play a role in CNS neurotransmission.

Chloride-dependent binding sites for L-[3H]glutamate on dendrodendritic synaptosomal membranes of rat olfactory bulb

Dendrodendritic synapses occur between granule cell dendrites and secondary dendrites of mitral cells within the olfactory bulb and are attainable in a subcellular fraction (DDS). Since the mitral cells are thought to utilize an excitatory amino acid as a neurotransmitter, we determined the pharmacologic specificity of Na+-independent L-[3H]glutamate binding to fresh membranes of DDS in 50 mM Tris-HCl, pH 7.1. Binding of L-glutamate to membranes of DDS was specific, Cl(-)-dependent, and saturable. Scatchard plots were analyzed by nonlinear regression analyses using the computer program LIGAND, and the data was best-fitted to a one-site model with KD of 0.56 +/- 0.04 microM and an apparent Bmax of 48 +/- 5 pmol/mg protein. Hill plots also indicated the presence of one site and no cooperativity (nH = 0.99 +/- 0.03). However, the relative effectiveness of several compounds in inhibiting L-glutamate binding to membranes of DDS clearly demonstrated the presence of more than one site. Electrophysiological studies suggest that 2-amino-4-phosphonobutyrate (APB) is a potent antagonist of evoked responses elicited by stimulation of mitral cell axons and that quisqualate is a potent agonist; both of these compounds were highly effective inhibitors of L-glutamate binding to DDS membranes. APB displaced about 70% of the sites labeled with 200 nM L-glutamate with a KI of 1.6 microM, whereas quisqualate inhibition of L-glutamate binding yielded a line that was curvilinear in the Scatchard plot and was resolved into two sites of relatively high affinity (KI values of 0.02 and 0.65 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant properties of 3-hydroxy-2-quinoxalinecarboxylic acid, a newly found antagonist of excitatory amino acids

Various aromatic acids have been investigated as putative ligands of excitatory amino acid receptors. 3-Hydroxy-2-quinoxalinecarboxylic acid (HQC), at 1 mM, was found to antagonize the 22Na+ efflux produced in 22Na+-preloaded brain slices by N-methyl-D-aspartate and kainate. The response to glutamate was also affected but not that to quisqualate. The apparent KI value of HQC for suppression of the N-methyl-D-aspartate response was 0.27 mM. The anticonvulsant action of HQC was investigated in mice and rats. HQC administered intracerebroventricularly caused a dose dependent delay in the occurrence of picrotoxin induced convulsions and afforded protection against picrotoxin induced death. These results confirm the proposition that antagonists of excitatory amino acids possess anticonvulsant properties.