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

Endogenous excitotoxic agents

Although glutamate and aspartate are among the most likely compounds to function as central neurotransmitters, and both can produce cell death in neonatal animals, the efficient uptake systems for these amino acids mean that exceptionally high concentrations are required for toxicity in adults. A better candidate for an endogenous neurotoxin is quinolinic acid, which produces cell death via activation of the N-methyl-aspartate receptors. Several differences of detail between the activity of quinolinate and N-methyl-aspartate may indicate the existence of subpopulations of the N-methyl-aspartate receptor. Another compound in the same 'kynurenine' pathway as quinolinate, kynurenic acid, is an antagonist of the excitatory and neurotoxic actions of quinolinate, and the overall excitability of the central nervous system and the occurrence of cell death may therefore result from a balance between the concentrations of quinolinate and kynurenate.

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.

Toxicity induced by a polyglutamated folate analog is attenuated by NAALADase inhibition

Folates have been shown to be neurotoxic and convulsive. Endogenously, folates exist in the brain in a polyglutamated form with 1-7 terminal glutamates (approx. 1 microM). The brain enzyme N-acetylated alpha-linked acidic dipeptidase (NAALADase) has been shown to remove sequentially the gamma-linked glutamates from folic acid polyglutamates. We report that, at high concentrations (300 microM-30 mM), a folic acid hexaglutamate analog is dose-dependently toxic to dissociated rat cortical cultures and that this toxicity is reversed by 2-PMPA, a potent and selective NAALADase inhibitor. These data suggest a new mechanism for folic acid toxicity.

Mapping of the canine lumbosacral spinal cord neurons by Nauta method at the end of the early phase of paraplegia induced by ischemia and reperfusion

The Nauta impregnation method was used to map the neuronal changes in the canine lumbosacral segments following ischemia and reperfusion. The early perikaryal changes ensuing during the first phase after 30 min of thoracic aorta cross-clamping alone or followed by 30 min of reperfusion were mapped. During the second phase (one to six postischemic reperfusion days) the dendritic, preterminal and synaptic degeneration developed. The influence of 30 min cross-clamping immediately followed by perfusion fixation is characterized by the occurrence of flocculent argyrophilic clusters in the cytoplasm of middle-sized and large neurons of L3-S1 segments. Declamping of the thoracic aorta followed by 30 min of reperfusion basically modifies the susceptibility of lumbosacral neurons to Nauta impregnation promoting somatic and dendritic argyrophilia mainly of small (less than 15 microns) neurons, localized mostly in the fifth, sixth and seventh layers, respectively. This early appearing somatic and dendritic argyrophilia is not abolished by a pretreatment of sections with acetone in which cholesterol and its esters are highly soluble, or chloroform-methanol which extracts total lipid. After 24 h of reperfusion the somatic and dendritic argyrophilia is lost but the first signs of drop-like degeneration are detected in all but three superficial dorsal horn layers. At the end of the third reperfusion day, an atypical form of bouton degeneration was found, consisting of massive occurrence of enlarged (greater than 4 microns) boutons encircled by a clear halo. Laminar distribution of enlarged degenerating boutons coincides with laminar quantitative distribution of small argyrophilic neurons detected 30 min after reperfusion. The basic orientation of the many terminal fibres attached to enlarged boutons suggests that they belong to the axons localized mainly in the lateral and anterior columns. Despite a dense argyrophilic network pervading the gray matter of lumbosacral segments only pale shadows of middle-sized and large neurons were found at the end of the sixth reperfusion day and neither somatic nor vessel wall argyrophilia could be detected. All animals surviving one, three and six days postoperatively suffered from fully developed paraplegia.

Folates: epileptogenic effects and enhancing effects on [3H]TBOB binding to the GABAA-receptor complex

The biochemical mechanism responsible for the convulsive effects of folates was investigated. The epileptogenic effects of folates were determined in vivo by quantification of the seizures following intracortical application in rats. The rank order of epileptogenic effects is: folic acid greater than or equal to 5-HCO-H4 folate greater than H2 folate greater than 5-CH3-H4 folate. This sequence of epileptogenicity in vivo is compared to the rank order of the effects of folates on radioligand binding to the GABAA-receptor complex in vitro. The inhibitory potencies of folates on [3H]muscimol and [3H]diazepam bindings did not correlate with their epileptogenic effects. However, folates reverse the inhibiting effect of GABA on the binding of the cage convulsant [3H]TBOB [( 3H]t-butylbicycloorthobenzoate). The rank order of this in vitro effect (folic acid greater than 5-HCO-H4 folate greater than H2 folate = 5-CH3-H4 folate) resembles the rank order of epileptogenicity determined in vivo. A relationship between the in vivo and in vitro effects is therefore suggested.

Golgi-like demonstration of "dark" neurons with an argyrophil III method for experimental neuropathology

A silver method is proposed for the selective, well-contrasted and reproducible demonstration of "dark" neurons in frozen, vibratome and paraffin sections cut at a thickness of 5 to 200 microns from aldehyde-fixed brains. The Golgi-like staining of the dendrites enables assorting of "dark" neurons according to characteristic neuron classifications. The staining procedure includes an esterification with 1-propanol, a treatment with diluted acetic acid and development. The esterification strongly increases the argyrophilia of both "dark" neurons and mitochondria. Unwanted co-staining of mitochondria is suppressed by the acetic acid treatment, while a special developer is used to render the staining controllable. The applicability of the method to experimental neuropathology is demonstrated by Golgi-like staining of "dark" neurons in rat brains exposed, before transcardial perfusion-fixation and delayed autopsy, to various pathological conditions including ischemia, hypoglycemia, trauma, status epilepticus, deafferentation and poisoning with kainic acid, colchicine and sodium azide, respectively.

High-dose leucovorin reverses acute high-dose methotrexate neurotoxicity in the rat

Intravenous high-dose methotrexate (HD-MTX) reduces cerebral glucose metabolism and produces behavioral abnormalities and electroencephalographic slowing in an animal model of acute HD-MTX neurotoxicity and in cancer patients undergoing HD-MTX chemotherapy. We used our model of HD-MTX neurotoxicity in the rat to determine if leucovorin (5-formyltetrahydrofolate) reduces this neurotoxicity, and extended our characterization of this model to identify regional as well as global HD-MTX treatment effects and to investigate HD-MTX-induced alterations in regional brain pH. Intravenous high-dose leucovorin reversed the HD-MTX-induced decrease in cerebral glucose metabolism and associated behavioral and electroencephalographic abnormalities in the rat, but low-dose leucovorin was ineffective. The major effect of HD-MTX on cerebral glucose metabolism was a global reduction; however, smaller region-specific treatment effects were identified in auditory, thalamic, and white matter structures. HD-MTX did not alter regional brain pH. These findings suggest a potential clinical role for high-dose leucovorin in severe or prolonged acute HD-MTX neurotoxicity and provide an important justification for the role of positron emission tomography in the early detection of clinical HD-MTX neurotoxicity.

The measurement of gamma-glutamyl hydrolase (conjugase) activity in rat brain

An assay using the artificial substrate, 2,4-diamino-10-methyl-pteroylglutamyl-gamma-glutamate (MTX-G1), was developed to measure gamma-glutamyl hydrolase (conjugase), which hydrolyzes folylpolyglutamates. This assay allows us to: 1) measure conjugase for the first time in rat brain and 2) measure conjugase in a reliable, sensitive and inexpensive manner. The MTX-binding assay results were compared to samples analyzed by HPLC and found to vary by only 13%. The artificial substrate, MTX-G1, had a lower rate of hydrolysis than pteroylglutamyl-gamma-glutamate (Pte-G2), 70.7 +/- 0.64 and 92.6 +/- 0.22 nmoles/hr/mg protein respectively. Conjugase was semi-purified 24 fold in H2O and found to have a pH optimum of 5.0.

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.

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.

Injections of picrotoxin and bicuculline into the amygdaloid complex of the rat: an electroencephalographic, behavioural and morphological analysis

Bicuculline methiodide (0.5-3 nmol) and picrotoxin (0.5-4 nmol) were injected uni- or bilaterally into the rat amygdala and the resulting behavioural, electroencephalographic and morphological alterations were studied. In rats treated unilaterally with lowest doses of either bicuculline or picrotoxin (0.5 and 1 nmol) increase in the locomotor activity, occasional myoclonus of the hindlimbs and wet dog shakes were observed. At doses of 2-3 nmol, both gamma-aminobutyrate antagonists produced a sequence of repetitively occurring behavioural alterations including limbic gustatory automatisms, tremor and myoclonus of the forelimbs, head nodding and rearing, that developed over 15-30 min and built up progressively into the recurrent motor limbic seizures lasting for 1-6 h. In animals injected bilaterally with either bicuculline (0.5-3 nmol) or picrotoxin (0.5-3 nmol) motor limbic seizures rapidly developed into the status epilepticus lasting for several hours. Bicuculline and picrotoxin produced both ictal and interictal epileptiform activity in the electroencephalogram. A spectrum of electroencephalographic changes consisted of high voltage fast activity, slow and fast voltage spiking, paraoxysmal bursts and periods of postictal depression. The earliest electrographic alterations appeared in the amygdala and then rapidly spread to cortical areas. Electrographic seizures started 1-10 min after unilateral injections of large doses of bicuculline and pictrotoxin (2-4 nmol). Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the electrographic activity. Bilateral injections of large doses of both gamma-aminobutyrate antagonists (2-3 nmol) resulted in the status epilepticus. Morphological examination of frontal forebrain sections with light microscopy revealed a widespread damage to the amygdala, olfactory cortex, substantia nigra, thalamus, hippocampus and neocortex. Pretreatment of animals with diazepam prevented the build-up of convulsive activity and brain damage produced by bicuculline or picrotoxin. Muscimol retarded the appearance and shortened the duration of convulsive activity, but did not alter the sequence and intensity of seizures. The results indicate that gamma-aminobutyrate antagonists, bicuculline and picrotoxin when directly applied to the amygdala can elicit in rats motor limbic seizures, epileptic changes in the electroencephalogram indicative of repetitive limbic seizures, and status epilepticus accompanied by seizure-related brain damage.(ABSTRACT TRUNCATED AT 400 WORDS)