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

Contemporary issues surrounding folic Acid fortification initiatives

The impact of folate on health and disease, particularly pregnancy complications and congenital malformations, has been extensively studied. Mandatory folic acid fortification therefore has been implemented in multiple countries, resulting in a reduction in the occurrence of neural tube defects. However, emerging evidence suggests increased folate intake may also be associated with unexpected adverse effects. This literature review focuses on contemporary issues of concern, and possible underlying mechanisms as well as giving consideration the future direction of mandatory folic acid fortification. Folate fortification has been associated with the presence of unmetabolized folic acid (PteGlu) in blood, masking of vitamin B12 deficiency, increased dosage for anti-cancer medication, photo-catalysis of PteGlu leading to potential genotoxicity, and a role in the pathoaetiology of colorectal cancer. Increased folate intake has also been associated with twin birth and insulin resistance in offspring, and altered epigenetic mechanisms of inheritance. Although limited data exists to elucidate potential mechanisms underlying these issues, elevated blood folate level due to the excess use of PteGlu without consideration of an individual's specific phenotypic traits (e.g. genetic background and undiagnosed disease) may be relevant. Additionally, the accumulation of unmetabolized PteGlu may lead to inhibition of dihydrofolate reductase and other enzymes. Concerns notwithstanding, folic acid fortification has achieved enormous advances in public health. It therefore seems prudent to target and carefully monitor high risk groups, and to conduct well focused further research to better understand and to minimize any risk of mandatory folic acid fortification.

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.

The influence of folic acid on the picrotoxin-sensitive site of the GABAa-receptor complex

The site of action responsible for the convulsive effect of folic acid was investigated in vitro. Folic acid (ECmax 5 x 10(-4) M) enhances the binding of the cage convulsant [3H]t-butylbicycloorthobenzoate ([3H]TBOB) to rat brain membranes, namely to 130% of control in the absence of GABA and to over 300% of control in the presence of physiological concentrations of GABA. Analysis of the binding parameters reveals that folic acid increases the apparent number of [3H]TBOB binding sites.

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.

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.

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.