Long-term effects of excitatory amino acid antagonists NBQX and MK-801 on the developing brain

Talampanel suppresses the acute and chronic effects of seizures in a rodent neonatal seizure model

PURPOSE

To test the efficacy of the novel candidate anticonvulsant talampanel (GYKI 53773) in a rodent model of hypoxic neonatal seizures. Talampanel is a noncompetitive antagonist of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid subtype of the glutamate receptor (AMPAR). We have previously shown that AMPARs play a critical role in the generation of acute seizures and later-life seizure susceptibility in this model of neonatal seizures.

METHODS

Seizures were induced in postnatal day (P) 10 Long-Evans rat pups by a 15 min exposure to global hypoxia. Acute seizure activity at P10 and subsequent susceptibility to seizure-induced neuronal injury with a "second-hit" kainate-induced seizure at P30-31 were compared between animals receiving talampanel (1, 5, 7.5, or 10 mg/kg) intraperitoneally (i.p.) versus saline vehicle treatment.

RESULTS

Talampanel treatment suppressed seizures in a dose-dependent manner, with maximal effect at 7.5 and 10 mg/kg. In addition, talampanel treatment 30 min before hypoxia prevented later-life increases in seizure-induced neuronal injury as assessed by in situ DNA nick end-labeling (ISEL).

DISCUSSION

We have previously demonstrated efficacy of other AMPAR antagonists such as NBQX and topiramate in this model. The present finding shows that the novel agent talampanel, under evaluation as an antiepileptic drug in children and adults, may have clinical potential in the treatment of neonatal seizures, particularly those occurring in the context of hypoxic encephalopathy.

Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia

Disruption to brain development at an early stage can potentially alter chemically coded neural networks and can affect behavior in later life. During early brain development antagonism of glutamate NMDA receptors, which play an important role in neuronal outgrowth and survival, leads to neuronal damage in several brain regions and causes behavioral alterations in rodents that mimic schizophrenia symptoms and endophenotypes. There are several lines of evidence implicating involvement of a dysfunctional glutamate system in schizophrenia. In normal subjects, NMDA receptor antagonists produce behavioral and neurochemical changes that mimic schizophrenia symptoms better than any other psychotomimetic drug. Moreover, these drugs worsen symptoms in schizophrenia patients and can trigger a recrudescence of the acute psychotic state in patients with stable chronic schizophrenia. In addition, genes consistently reported as being altered in schizophrenia play roles in development, neuroplasticity and glutamate/GABAergic neurotransmission. Perinatal NMDA receptor antagonist treatment is a useful model for studying the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia because neurochemical and behavioral changes, reminiscent of those seen in schizophrenia, are present long after cessation of drug administration, which suggests that a permanent change in brain structure and organization has occurred during brain development.

Plasticity of both excitatory and inhibitory synapses is associated with seizures induced by removal of chronic blockade of activity in cultured hippocampus

One factor common to many neurological insults that can lead to acquired epilepsy is a loss of afferent neuronal input. Neuronal activity is one cellular mechanism implicated in transducing deafferentation into epileptogenesis. Therefore the effects of chronic activity blockade on seizure susceptibility and its underlying mechanisms were examined in organotypic hippocampal slice cultures treated chronically with the sodium channel blocker, tetrodotoxin (TTX), or the N-methyl-D-aspartate receptor (NMDAR) antagonist, D-2-amino-5-phosphonovaleric acid (D-APV). Granule cell field potential recordings in physiological buffer revealed spontaneous electrographic seizures in 83% of TTX-, 9% of D-APV-, but 0% of vehicle-treated cultures. TTX-induced seizures were not associated with membrane property alterations that would elicit granule cell hyperexcitability. Seizures were blocked by glutamate receptor antagonists, suggesting that plasticity in excitatory synaptic circuits contributed to seizures. The morphology of granule cells and their mossy fiber axons remained largely unchanged, and the number of synapses onto granule cells measured immunohistochemically was not increased in TTX- or D-APV-treated cultures. However, voltage-clamp recordings revealed that miniature excitatory postsynaptic current frequency and kinetics were increased and miniature inhibitory postsynaptic current kinetics were decreased in D-APV- and TTX-treated cultures compared with vehicle. Changes were more profound and qualitatively different in TTX- compared with D-APV-treated cultures, consistent with the dramatic effects of TTX treatment on seizure expression. We propose that chronic blockade of action potentials by TTX induces homeostatic responses including plasticity of both excitatory and inhibitory synapses. Removal of TTX unmasks the impact of these synaptic plasticities on local circuit excitability, resulting in spontaneous seizures.

Potent protection of ferulic acid against excitotoxic effects of maternal intragastric administration of monosodium glutamate at a late stage of pregnancy on developing mouse fetal brain

The present study was conducted to investigate a possible protection of ferulic acid against excitotoxic effects of maternal intragastric (ig) administration of monosodium glutamate (MSG) at a late stage of pregnancy on developing mouse fetal brain. [(3)H]-labeled glutamate was used as radiotracer to study the effect of ferulic acid on distribution of MSG in mouse fetal brain. MSG dissolved in distilled water (2.0 g/kg body weight, 640 kBq of [(3)H]glutamate/mouse, ig) or/and sodium ferulate (SF) (20, 40, 80 mg/kg body weight, ip), was given to pregnant mice at 17-19 days; the distribution of [(3)H] glutamate in the mouse fetal brains was measured at 30, 60, 90, 120 min after administration of MSG or/and SF. Maternal mice were given MSG (1.0, 2.0, 4.0 g/kg body weight, ig) or/and SF (20, 40, 80 mg/kg body weight, ip) simultaneously at 17-19 days of pregnancy, and then behavioral tests and histopathological observations were used to analyze glutamate-induced functional and morphological changes of the brains of their offspring, and Western blot analysis was performed for examining expressions of bcl-2 and caspase-3. The results showed that SF obviously inhibited the uptake of labeled glutamate in fetal brain. In addition, SF countered the effects of MSG on behavior, histopathology, genetic toxicity, and expression of apoptosis-related gene. The results suggest that ferulic acid is a novel competitive N-methyl-D-aspartate (NMDA) receptor antagonist and neuroprotector. In conclusion, maternal administration of ferulic acid has potent protective effects against glutamate-induced neurotoxicity in their filial mice.

Antiepileptic effects of botulinum neurotoxin E

Experimental studies suggest that the delivery of antiepileptic agents into the seizure focus might be of potential utility for the treatment of focal-onset epilepsies. Botulinum neurotoxin E (BoNT/E) causes a prolonged inhibition of neurotransmitter release after its specific cleavage of the synaptic protein synaptosomal-associated protein of 25 kDa (SNAP-25). Here, we show that BoNT/E injected into the rat hippocampus inhibits glutamate release and blocks spike activity of pyramidal neurons. BoNT/E effects persist for at least 3 weeks, as determined by immunodetection of cleaved SNAP-25 and loss of intact SNAP-25. The delivery of BoNT/E to the rat hippocampus dramatically reduces both focal and generalized kainic acid-induced seizures as documented by behavioral and electrographic analysis. BoNT/E treatment also prevents neuronal loss and long-term cognitive deficits associated with kainic acid seizures. Moreover, BoNT/E-injected rats require 50% more electrical stimulations to reach stage 5 of kindling, thus indicating a delayed epileptogenesis. We conclude that BoNT/E delivery to the hippocampus is both antiictal and antiepileptogenic in experimental models of epilepsy.

Neonatal seizures: to treat or not to treat?

The immature brain is intrinsically hyperexcitable, a feature that, despite being crucial for learning, synaptogenesis and neuronal plasticity, predisposes the neonate to seizures. Seizures represent the most common neurologic manifestation of impaired brain function in this age group. Importantly, although seizure-induced neuronal injury is minimal in the "healthy" neonatal brain, the "metabolically-compromised" brain appears more vulnerable. Even in the "healthy" brain, however, seizures result in impaired learning, enhanced susceptibility to further seizures, and increased risk of brain injury with seizures later in life, as a result of altered hippocampal circuitry. Given these findings, an aggressive approach to neonatal seizures appears warranted. However, our current conventional therapies (including phenobarbital, phenytoin, and benzodiazepines), even when used in combination, are often ineffective in controlling seizures. Lidocaine may yield better efficacy but requires more study. Recent animal data suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) antagonists such as topiramate may have a neuroprotective role. However, further work is needed to confirm the safety of excitatory amino acid antagonists in neonates because there remains a prevailing concern that such agents may impair normal neurodevelopmental processes.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

NMDA but not AMPA receptor antagonists impair the delay-interposed radial maze performance of rats

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonists CGS19755 and MK801 and the 2-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor antagonist YM90K on spatial working memory were investigated by using a delay-interposed radial-arm maze (RAM) task in rats. CGS19755 and MK801, at the largest dose that had no effect on the performance in the nondelayed RAM task, significantly decreased the initial correct response after the 5-min delay in the delay-interposed RAM task. In contrast, YM90K had no effect on the initial correct response and arm reentries in both the delay-interposed and nondelayed RAM task. CGS19755, MK801 and YM90K, at all doses tested, did not alter the running time in either the delayed or the nondelayed RAM tasks. These results suggest that spatial working memory can be impaired by a blockade of NMDA receptor function and that such impairment is particularly sensitive to delay interposition. The lack of effect of the AMPA receptor antagonist provides additional evidence of the importance of the NMDA subtype of the glutamate receptors in cognitive processes.