Neurotoxic action of methyltetrahydrofolate in rat cerebellum unrelated to direct activation of kainate receptors

Reduced pteridine derivatives induce apoptosis in human neuronal NT2/HNT cells

Elevated concentrations of the pteridine compound neopterin, usually accompanied by 7,8-dihydroneopterin were found in cerebrospinal fluids of patients with neurodegenerative diseases and central nervous system infections. Here, the potential of pteridines to induce apoptosis of the human neuronal cell line (NT2) was investigated. Reduced neopterin, biopterin- and folate derivatives led to a time-dependent increase of apoptosis of cells. In contrast, non-reduced pteridines did not significantly alter cell survival. After differentiation of neuronal precursor cells to neurons and astrocyte-like cells, similar effects were detected. Antioxidants partly protected NT2 from pteridines-induced apoptosis, suggesting the involvement of reactive oxygen intermediates. In vitro experiments using dichlorofluorescin-diacetate further indicated a direct formation of reactive oxygen species in cells. Results implicate that high concentrations of reduced pteridines, might contribute to the loss of neuronal cells in neurodegenerative diseases.

AMPA receptor-mediated slow neuronal death in the rat spinal cord induced by long-term blockade of glutamate transporters with THA

Excitotoxicity secondary to the loss of glutamate transporters (GluT) has been proposed as a possible pathogenetic mechanism for neuronal degeneration in amyotrophic lateral sclerosis. We therefore investigated whether prolonged in vivo pharmacologic inhibition of GluT would result in neuronal damage in the rat. DL-Threo-beta-hydroxyaspartate (THA), a potent GluT inhibitor, and glutamate were continuously infused into the rat spinal subarachnoid space by using a mini-osmotic pump. Animals that received both THA and glutamate, but not those received either singly, displayed tail paralysis with or without hind-limb paralysis and urinary incontinence after the third postoperative day. Pathologically, symptomatic animals exhibited neuronal loss with a variable extent of gliosis preferentially involving the dorsal horn of the lumbosacral cord. In the rostral spinal segments adjacent to those regions of intense pathologic changes, small neurons in the dorsal horn were selectively destroyed, a pattern similar to the late-onset neuronal damage induced by continuous intrathecal administration of 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) [R. Nakamura et al., Brain Res. 654 (1994) 279-285]. These behavioral and pathologic changes were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that pharmacologic blockade of GluT causes selective neuronal damage in vivo by AMPA receptor activation.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Folic acid protects chick retinal neurons against the neurotoxic action of excitatory amino acids

In this study we have examined the neurotoxic effects of folic acid (FA), alone or in combination with selected excitotoxins using in vitro preparations of chick retina. Folic acid alone at concentrations of up to 10 mM had no effect. Co-incubation of 10 mM FA with 2 microM kainic acid (KA) protected all cell types susceptible to KA toxicity, namely amacrine and bipolar cells. At lower concentrations the protective effect of FA to susceptible cell types was found to be dose-dependent. The rank order of cells which are protected by FA, in order of the lowest concentration of FA required, was amacrine, inner bipolar and outer bipolar cells. The effect of FA against KA is a very weak one, as a 130-400-fold concentration of FA is required to protect amacrine cells from KA and a 1000-5000-fold dose of FA required to protect bipolar cells. However, FA (2 mM) also protects susceptible retinal neurons from the neurotoxic effects of 40 microM N-methyl-DL-aspartic acid (NMDLA) and 60 microM quinolinic acid (QUIN); only requiring respectively 50 and 33 times the concentration of FA. Interestingly 10 mM FA had little effect against 40 microM quisqualic acid (QUIS). Thus FA antagonizes the effects of KA, NMDLA, QUIN and to a small extent QUIS. Although its action may be mediated through several receptor types, FA appears to be a more potent antagonist of the N-methyl-D-aspartic acid (NMDA)-preferring than the KA-preferring or QUIS-preferring receptor.

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: II. Mechanism of action

The specificity and mechanism of the neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). Of 21 compounds tested as potential mimics of KA neurotoxicity in the OPL, only the enantiomers of glutamate and aspartate mimicked KA, inducing a low-level neurotoxic effect at concentrations 5,000-10,000-fold higher than concentrations of KA giving comparable effects. Further, of 22 compounds tested as potential blockers of KA neurotoxicity in the OPL, only D-gamma-glutamylglycine, D,L-alpha-amino pimelic acid, sodium pentobarbital, D,L-alpha-amino adipic acid, L-glutamate, and L-aspartate blocked KA neurotoxicity (IC50 values of 0.1, 0.3, 0.3, 2, 5, and 15 mM, respectively). In ionic substitution experiments, KA-induced vacuolization was found to require sodium and chloride ions but not calcium ions in the extracellular medium. These findings support the hypothesis that KA combines with specific receptors in the membrane of susceptible neurons in the retinal OPL, leading to prolonged opening of membrane channels permeable to sodium and potassium ions. An accompanying equilibrating chloride influx may result in intracellular ion excess, leading to osmotic swelling and vacuolization. The membrane receptors involved in mediating the action of KA in the OPL are likely to be a class of postsynaptic or extrasynaptic glutamate receptor.

Effects of kainic acid and other excitotoxins in the rat superior colliculus: relations to glutamatergic afferents

In this study we have performed surgical, chemical and combined surgical/chemical lesions in order to elucidate neurotransmitter mechanisms in the superior colliculus (SC) of albino rats. Visual cortex (VC) ablation reduced high affinity (HA) uptake of D-Asp by 32% in the deafferented SC. Local injection of kainic acid (KA) into SC reduced HA D-Asp uptake selectively in the lower dose range (less than 1 nmol) by 50-60%. The GABAergic marker glutamate decarboxylase (GAD) was decreased by maximally 60% only at doses exceeding 2 nmol. Choline acetyltransferase (ChAT), however, was not affected at any of the doses administered. VC ablation provided an almost complete protection against 1 nmol KA. When KA was injected 2 days prior to VC ablation an additive effect on HA D-Asp uptake of the two lesions was observed. From these observations we infer that the notion of a glutamatergic projection from VC to SC has been strengthened. Moreover, local neurons in intermediate layers account for about 60% of the HA D-Asp uptake in SC, and these are most likely impinged upon by the glutamatergic afferents. The neurotoxic effects of KA were compared with those of some suspected endogenous excitotoxins, i.e. N-methyl tetrahydrofolic acid (Me-THF), other folates and the tryptophan metabolite quinolinic acid (QA). N-methyl tetrahydrofolic acid, Me-THF (4 and 10 nmol) reduced HA D-Asp uptake by about 50%, only when coinjected with ascorbic acid. GAD and ChAT were not affected at either of the doses. QA was about 100-fold less potent than KA on a molar basis, and the maximal reduction of GAD was similar in QA and KA injected animals, whereas the maximal reduction of HA D-Asp was only 40% after QA injection in SC. We conclude that Me-THF, QA and KA exert their neurotoxic actions by different mechanisms as judged by the behavioral, histopathological and biochemical sequelae seen after local injections of the respective substances in intermediate layers of SC and corroborate data obtained from other brain areas.

Folate induced-hypermotility response after bilateral injection into the nucleus accumbens of the rat. Possible mediation through dopaminergic mechanisms

Folic acid (FA) and certain of its reduced congeners produce excitatory effects when applied to neuronal tissue. Recent evidence has suggested that folates have other biological properties in common with the excitatory amino acids. The purpose of this study was to determine the activity of folate compounds in a system sensitive to excitatory amino acids. Bilateral injection of folic acid into the nucleus accumbens resulted in a marked increase in locomotor activity at doses of 2.5 and 5 micrograms. Larger doses resulted in behavioral responses, such as body tremor and labored breathing, which interfered with the locomotor response. Similarly, 5-formyltetrahydrofolic acid (FTHF) produced a marked hypermotility response after bilateral injection into the nucleus accumbens (2.5-25 micrograms), while dihydrofolic acid, tetrahydrofolic acid, and 5-methyltetrahydrofolic acid were ineffective. Pretreatment with reserpine (10 mg/kg, i.p.) markedly reduced the hypermotility response elicited by folic acid and FTHF as did pretreatment with haloperidol in both peripheral (0.8 mg/kg) and direct (5 micrograms) injection into the nucleus accumbens. In addition, injection of muscimol (30 ng), which depresses hypermotility induced by dopamine and amphetamine, produced a significant decrease in the hypermotility response produced by folic acid. In contrast, pretreatment with phentolamine (5 mg/kg, i.p.) or propranolol (4 mg/kg, i.p.) did not decrease folic acid or FTHF-induced responses. These results suggest that folic acid and FTHF produce an increase in locomotor activity by facilitating dopaminergic neurotransmission in the nucleus accumbens, possibly by inducing the release of dopamine from the nerve terminals. Thus, these folates have effects similar to those of the excitatory amino acids when injected into the nucleus accumbens.

Acute toxicity of folic acid in mice

The toxicity of folic acid (PGA) was studied in different inbred strains of mice. LD50 values of PGA by the i.p. route showed a unique toxicity pattern. In some strains, convulsions, ataxia and weakness were observed. Histopathological study in strains S/RVCri, BDF1, DBA/2 and DBA/2fNCri showed acute renal tubular necrosis.

Neurotoxicity of L-glutamate and DL-threo-3-hydroxyaspartate in the rat striatum

Destruction of the glutamatergic corticostriatal pathway potentiates the neurotoxic action of 1 mumol L-glutamate injected into the rat striatum, whereas the toxic effects of 10 nmol kainate are markedly attenuated. Injection of 170 nmol of the glutamate uptake inhibitor, DL-threo-3-hydroxyaspartate, into the intact striatum also causes neuronal degeneration, which is accompanied by a reduction in markers for cholinergic and GABAergic neurones. Prior removal of the corticostriatal pathway destroys the ability of DL-threo-3-hydroxyaspartate to cause lesions in the striatum. These results indicate that removal, or blockade, of uptake sites for glutamate increase the vulnerability of striatal neurones to the toxic effects of synaptically released glutamate.

Electroencephalographic, behavioral, and histopathologic features of seizures induced by intra-amygdala application of folic acid in cats

The effects of intracerebral injection of folic acid are still controversial. We studied the electroencephalographic, behavioral, and histopathologic consequences of the seizures induced by intra-amygdala administration of various doses of FA in freely moving cats. The severity of the seizures was dose-dependant. For doses of 25 and 50 nmol, single low-amplitude spikes appeared in the amygdala 15 to 20 min after injection and a typical amygdala symptomatology was observed. From doses of 100 nmol recurrent limbic seizures occurred 40 to 80 min after injection. Finally, from doses of 150 nmol secondarily generalized seizures were induced, which could be followed by death 4 to 6 h after injection. The severity of the cerebral lesions was related to both the dose and the paroxysmal manifestations. In cases with short survival time (6 h) and few seizures the pathology was restricted to a lymphocytic and glial reaction with some ischemic cells at the injected site. In cases with status epilepticus, edema and neuronal degeneration was observed in the hippocampus, amygdala, thalamic nuclei of the midline, entorhinal cortex, and cerebellum. No neuronal alteration at the injected site was observed. For longer survival times (8 days) edema was less severe, but hyperchromatic cells were still numerous. These results, compared with those of intra-amygdala administration of kainic acid, suggest that pathologic lesions induced in cats by folic acid more closely resemble those described in man after some status epilepticus.

Excitotoxic models for neurodegenerative disorders

In recent years, considerable interest has been shown in the neurotoxin properties of excitatory amino acids and their possible relevance for the study of human neurodegenerative disorders. The term "excitotoxin" has been coined for a family of acidic amino acids which are neuroexcitants and produce a characteristic type of "axon-sparing" neuronal lesion. Intracerebral infusions of kainic and ibotenic acids, the two most commonly used excitotoxins, result in a morphological and biochemical picture in experimental animals which resembles that observed in the brains of Huntington's disease and epilepsy victims. The emergence of such animal models for neurodegenerative disorders has led to the hypothesis that endogenous excitotoxins may exist which are linked to the pathogenesis of human diseases. The most promising candidate discovered so far is quinolinic acid, a hepatic tryptophan metabolite which has recently also been found to occur in brain tissue. The particular excitotoxic properties of quinolinic acid warrant a thorough investigation of its metabolic and synaptic disposition in normal and abnormal brain function. While little is known about the mechanisms by which excitotoxins cause selective neuronal death, most current speculations propose the participation of specific synaptic receptors for acidic amino acids. The recent development of selective antagonists of such receptors has aided in the elucidation of excitotoxic mechanisms. Although a biochemical link between endogenous excitotoxins and human neurodegenerative disorders remains elusive at present, pharmacological blockade of excitotoxicity may constitute a novel therapeutic strategy for the treatment of these disease states.

A multidisciplinary study of folic acid neurotoxicity: interactions with kainate binding sites and relevance to the aetiology of epilepsy

Folic acid has been injected unilaterally into the amygdaloid complex of awake chronically implanted rats, or in rats under anaesthesia. Clinical, electrographic, and metabolic changes (estimated by means of the 2-deoxyglucose method) have been studied in relation to subsequently demonstrated neuropathology using Fink-Heimer and Nissl stains. The observations are compared to the corresponding effects of intra-amygdaloid application of kainic acid. Major differences were noted between the folate and the kainate induced seizure/brain damage syndrome. Thus: folate produced essentially stereotypies, alternating with myoclonic unilateral jerks of head and limbs. In contrast, limbic motor seizures which are characteristically produced by kainic acid, were extremely rare. Folate did not produce the preferential and sequential electrographic activation of limbic structures as observed after kainate. 2-Deoxyglucose autoradiography revealed an enhanced metabolic activity in the injected amygdala and in the overlying piriform and entorhinal cortices. The most conspicuous rise in labelling, however, occurred in the entire fronto-parietal cortex (ipsilaterally) up to the cingulate region, as well as in the ventral thalamic complex and the globus pallidus, i.e. in structures which are not labelled after kainate treatment. Some extent of local damage was observed 1-8 days after the injection; distant from the injection site, we found massive anoxic-ischemic type of damage in the superficial layers of the fronto-parietal cortex, a complete necrosis of the piriform lobe, and neuronal cell loss in the ventral thalamus and several extrapyramidal structures. The full range of limbic damage associated with kainate was never produced by folate. The CA3 region of the hippocampus, most susceptible to kainate, was only mildly affected by folate. These differences between kainate and folate prompted us to re-evaluate the recently reported high affinity of folates for kainic acid membrane binding sites. We found that folic acid competed only very weakly with [3H]kainic acid for binding sites on striatal, cortical, hippocampal, amygdaloid, and cerebellar membranes. It is thus concluded, that folate is not a good candidate for an endogenous kainate-like substance. We propose intracerebral injections of folic acid as a useful tool to study the vulnerability of brain structures to anoxic-ischemic conditions.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Effects of folic and kainic acids on synaptic responses of hippocampal neurones

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.

The ionic mechanisms associated with the excitatory response of kainate, L-glutamate, quisqualate, ibotenate, AMPA and methyltetrahydrofolate on leech Retzius cells

Intracellular recordings were made from Retzius cells from segmental ganglia of the leech, Hirudo medicinalis. The ionic mechanisms of the following compounds were examined: L-glutamate, ibotenate, quisqualate, AMPA, kainate, methyltetrahydrofolate and carbachol. All these compounds depolarise and excite Retzius cells. In sodium-free Ringer, the responses to L-glutamate, kainate, ibotenate and AMPA were greatly reduced, the response to quisqualate was reduced, the response to methyltetrahydrofolate was normal while the response to carbachol was abolished. In sodium-free high calcium Ringer the responses to L-glutamate, ibotenate and carbachol were absent, the responses to quisqualate and AMPA greatly reduced, the responses to methyltetrahydrofolate and kainate were normal. The methyltetrahydrofolate and kainate responses in sodium-free high calcium Ringer were greatly reduced on addition of cobalt. All the responses are associated with an increase in conductance, the increase being the largest in the case of kainate. It is concluded that the response to L-glutamate, ibotenate and carbachol are dependent on sodium, the responses to quisqualate and AMPA are mainly sodium dependent, possibly with a small calcium component. The kainate response in normal Ringer is largely sodium dependent but in sodium-free Ringer calcium can completely substitute for sodium. The methyltetrahydrofolate response appears to be sodium independent but at least partly calcium dependent. These studies provide further evidence that L-glutamate and ibotenate act on a common receptor on leech Retzius cells while kainate acts on a separate receptor which can activate a calcium ionophore. It is probable that methyltetrahydrofolate acts on a different ionophore system to kainate. N-Methyl-D-aspartate has no agonist activity on any of these receptors.

Are convulsant and toxic properties of folates of the kainate type?

Intra-amygdaloid injections of folic acid (FA) in rats induce behavioural, metabolic (assessed using the 2-deoxyglucose method) and neuropathological changes which, however, differ considerably from those produced by kainic acid (KA). Thus FA, in contrast to KA, does not readily induce limbic motor seizures, fails to activate the entire limbic system and does not readily reproduce the local and distant damage induced by KA, notably in the Ammon's horn of the hippocampus. The results argue against the hypothesis that KA acts at folate receptors to induce its limbic epileptic/brain damage syndrome.

Quinolinic acid: an endogenous metabolite that produces axon-sparing lesions in rat brain

A current hypothesis links the neuroexcitatory properties of certain acidic amino acids to their ability to cause selective neuronal lesions. Intracerebral injection of the neuroexcitatory tryptophan metabolite, quinolinic acid, has behavioral, neurochemical, and neuropathological consequences reminiscent of those of exogenous excitotoxins, such as kainic and ibotenic acids. Its qualities as a neurotoxic agent suggest that quinolinic acid should be considered as a possible pathogenic factor in neurodegenerative disorders.

Folic acid derivatives do not reproduce the neurotoxic effects of kainic acid on chicken retina

Methyltetrahydrofolic acid, formyltetrahydrofolic acid, folic acid and dihydrofolic acid were injected intravitreally into the eyes of chickens. No short-term or long-term signs of neurotoxicity were observed, even when doses 100-200 times those at which kainic acid produces clear neurotoxic effects were injected. The folic acid derivatives neither inhibited nor potentiated the neurotoxic effects. Thus no support is given to the suggestion that folic acid and its derivatives may act as kainic acid agonists or antagonists, even though the receptors involved in kainic acid-induced neurotoxicity appear to be of the kainic acid rather than quisqualic acid or N-methyl-D-aspartic acid type.

Methyltetrahydrofolate as an antagonist of excitatory amino acids on spinal neurones

Folate and N5-methyl-5,6,7,8-tetrahydrofolate (MTHF) have been reported to have excitatory effects upon cortical neurones, possibly due to interaction with kainate receptors. On spinal neurones these compounds have been found inactive as excitants; however MTHF is a weak antagonist of kainate and N-methyl-D-aspartate excitations, and less effectively blocks quisqualate also. It thus resembles the antagonist 2,3-piperidine dicarboxylate.

An ionophoretic study of the responses of rat caudal trigeminal nucleus neurones to non-noxious mechanical sensory stimuli

1. Extracellular recordings of the responses of single caudal trigeminal nucleus neurones to non-noxious and noxious facial stimuli and to ionophoretically applied L-glutamate, L-aspartate and acetylcholine were made in urethane anaesthetized rats. 2. Neurones excited by non-noxious mechanical stimuli were located primarily in the magnocellular part of nucleus caudalis, whereas neurones excited by both noxious and non-noxious stimuli were located either ventromedially to the magnocellular part of nucleus caudalis or superficially to the substantia gelatinosa. 3. Both L-aspartate and L-glutamate were found to excite all neurones tested in nucleus caudalis. In contrast, however, acetylcholine was found to excite only 31% of the neurones tested. 4. Responses of nucleus caudalis neurones to non-noxious sensory stimulation were not antagonized by the excitatory amino acid antagonist D-alpha-aminoadipate, but were antagonized by cis-2, 3-piperidine dicarboxylate and gamma-D-glutamylglycine, two excitatory amino acid antagonists with a broader spectrum of action. 5. It is concluded that the chemical synaptic transmitter of non-nociceptive mechanoreceptive primary afferent fibres to nucleus caudalis may be a ligand for an excitatory amino acid receptor other than a D-alpha-aminoadipate-sensitive receptor. The synaptic receptor may thus be of the kainate or quisqualate type, and the transmitter possibly L-glutamate, L-aspartate or an as yet unidentified substance.