Amygdala kindling alters N-methyl-D-aspartate receptor subunit messenger RNA expression in the rat supraoptic nucleus

The effect of hippocampal NR2B-containing NMDA receptors on chronic cognitive dysfunction in rats with temporal lobe epilepsy

Background

We have previously reported that hippocampal long-term potentiation (LTP) was suppressed in temporal lobe epilepsy (TLE) rats. The N-methyl-D-aspartic acid receptors containing 2B subunit (NR2B-NMDARs) are indispensable to induce the LTP of hippocampus. However, it is still unknown whether the NR2B-NMDARs are implied with the cognitive dysfunction in TLE rats.

Methods

The TLE model was lithium chloride-pilocarpine (li-pilo) model. Morris water maze test was used to evaluate the cognitive function of epileptic rats. Expression of hippocampal NR2B-NMDAs was evaluated by western blotting. Stereotactic injection of NMDA, an agonist of NR2B-NMDARs, into the hippocampus of TLE rats was used to investigate the role of NR2B-NMDARs on cognitive dysfunction.

Results

Cognitive function TLE rats was significantly reduced compared with controls in the Morris water maze test (P < 0.05). Western blotting data showed the down-expression of hippocampal NR2B-NMDARs and p-NR2B in TLE rats compared with the control (P < 0.05). In addition, hippocampal stereolocalization of NMDA injection improved partially the learning and memory in TLE rats (P < 0.05).

Conclusions

Down-expression and low activity of hippocampal NR2B-NMDARs may be implied with chronic cognitive dysfunction in TLE rats.

Neuroendocrine changes in the hypothalamic-neurohypophysial system in the Wistar audiogenic rat (WAR) strain submitted to audiogenic kindling

The Wistar audiogenic rat (WAR) strain is used as an animal model of epilepsy, which when submitted to acute acoustic stimulus presents tonic-clonic seizures, mainly dependent on brainstem (mesencephalic) structures. However, when WARs are exposed to chronic acoustic stimuli (audiogenic kindling-AK), they usually present tonic-clonic seizures, followed by limbic seizures, after recruitment of forebrain structures such as the cortex, hippocampus and amygdala. Although some studies have reported that hypothalamic-hypophysis function is also altered in WAR through modulating vasopressin (AVP) and oxytocin (OXT) secretion, the role of these neuropeptides in epilepsy still is controversial. We analyzed the impact of AK and consequent activation of mesencephalic neurocircuits and the recruitment of forebrain limbic (LiR) sites on the hypothalamic-neurohypophysial system and expression of Avpr1a and Oxtr in these structures. At the end of the AK protocol, nine out of 18 WARs presented LiR. Increases in both plasma vasopressin and oxytocin levels were observed in WAR when compared to Wistar rats. These results were correlated with an increase in the expressions of heteronuclear (hn) and messenger (m) RNA for Oxt in the paraventricular nucleus (PVN) in WARs submitted to AK that presented LiR. In the paraventricular nucleus, the hnAvp and mAvp expressions increased in WARs with and without LiR, respectively. There were no significant differences in Avp and Oxt expression in supraoptic nuclei (SON). Also, there was a reduction in the Avpr1a expression in the central nucleus of the amygdala and frontal lobe in the WAR strain. In the inferior colliculus, Avpr1a expression was lower in WARs after AK, especially those without LiR. Our results indicate that both AK and LiR in WARs lead to changes in the hypothalamic-neurohypophysial system and its receptors, providing a new molecular basis to better understaind epilepsy.

Audiogenic kindling activates expression of vasopressin in the hypothalamus of Krushinsky-Molodkina rats genetically prone to reflex epilepsy

The present study analysed the effects of audiogenic kindling on the functional state of the vasopressinergic system of Krushinsky-Molodkina (KM) rats. KM rats represent a genetic model of audiogenic reflex epilepsy. Multiple audiogenic seizures in KM rats lead to the involvement of the limbic structures and neocortex in the epileptic network. The phenomenon of epileptic activity that overspreads from the brain stem to the forebrain is called audiogenic kindling and represents a model of limbic epilepsy. In the present study, audiogenic kindling was induced by 25 repetitive audiogenic seizures (AGS) with 1 AGS per day. A proportion of KM rats did not express AGS to sound stimuli, and these rats were characterised as the AGS-resistant group. The data demonstrated that audiogenic kindling did not change activity of extracellular signal-regulated kinase 1/2 or cAMP response element-binding protein, although it led to an increase in vasopressin (VP) expression in the supraoptic nucleus (SON) and in the magnocellular division of the paraventricular nucleus (PVN). Additionally, we observed a decrease in GABAergic innervation of the hypothalamic neuroendocrine neurones after audiogenic kindling, whereas glutamatergic innervation of the SON and PVN was not altered. By contrast, analysis of AGS-resistant KM rats did not reveal any changes in the activity of the VP-ergic system, confirming that the activation of VP expression was caused by repetitive AGS expression, rather than by repetitive acoustic stress. Thus, we suggest that overspread of epileptiform activity in the brain is the main factor that affects VP expression in the hypothalamic magnocellular neurones.

MK-801, but not naloxone, attenuates high-dose dextromethorphan-induced convulsive behavior: Possible involvement of the GluN2B receptor

Dextromethorphan (DM) is a dextrorotatory isomer of levorphanol, a typical morphine-like opioid. When administered at supra-antitussive doses, DM produces psychotoxic and neurotoxic effects in humans. Although DM abuse has been well-documented, few studies have examined the effects of high-dose DM. The present study aimed to explore the effects of a single high dose of DM on mortality and seizure occurrence. After intraperitoneal administration with a high dose of DM (80mg/kg), Sprague-Dawley rats showed increased seizure occurrence and intensity. Hippocampal expression levels of N-methyl-d-aspartate (NMDA) receptor subunits (GluN1<GluN2A<GluN2B), c-Fos and pro-apoptotic factors (Bax and cleaved caspase-3) were upregulated by DM treatment; while levels of anti-apoptotic factors (Bcl-2 and Bcl-xL) were downregulated. Consistently, DM also induced ultrastructural degeneration in the hippocampus. A non-competitive NMDA receptor antagonist, MK-801, attenuated these effects of high-dose DM, whereas an opioid antagonist, naloxone, did not affect DM-induced neurotoxicity. Moreover, pretreatment with a highly specific GluN2B subunit inhibitor, traxoprodil, was selectively effective in preventing DM-induced c-Fos expression and apoptotic changes. These results suggest that high-dose DM produces convulsive behaviors by activating GluN2B/NMDA signaling that leads to pro-apoptotic changes.

Increased NRG1-ErbB4 signaling in human symptomatic epilepsy

Previous studies have shown that the neuregulin 1 (NRG1)-ErbB4 signaling pathway may regulate the excitability of fast-spiking neurons in the frontal cortex and participate in primary epilepsy pathogenesis. However, the exact roles and mechanism for NRG1/ErbB4 in human symptomatic epilepsy are still unclear. Using fresh human symptomatic epilepsy tissues, we found that the protein levels of NRG1 and ErbB4 were significantly increased in the temporal cortex. In addition, NRG1-ErbB4 signaling suppressed phosphorylation of GluN2B at position 1472 by Src kinase, and decreased levels of phosphorylation level of GluN2B and Src were detected in human symptomatic epilepsy tissues. Our study revealed a critical role of the NRG1-ErbB4 signaling pathway in symptomatic epilepsy, which is different from that in primary epilepsy, and we propose that the NRG1-ErbB4 signaling may act as a homeostasis modulator that protects the brain from aggravation of epileptiform activity.

N-methyl-D-aspartate receptor NR2B subunit involved in depression-like behaviours in lithium chloride-pilocarpine chronic rat epilepsy model

Depression is a common comorbidity in patients with epilepsy with unclear mechanisms. This study is to explore the role of glutamate N-methyl-D-aspartate (NMDA) receptor NR1, NR2A and NR2B subunits in epilepsy-associated depression. Lithium chloride (Licl)-pilocarpine chronic rat epilepsy model was established and rats were divided into epilepsy with depression (EWD) and epilepsy without depression (EWND) subgroups based on forced swim test. Expression of NMDA receptor NR1, NR2A and NR2B subunits was measured by western blot and immunofluorescence methods. The immobility time (IMT) was significantly greater in Licl-pilocarpine model group than in Control group, which was also greater in EWD group than in EWND group. No differences of spontaneous recurrent seizure (SRS) counts over two weeks and latency were found between EWD and EWND groups. The number of NeuN positive cells was significantly less in Licl-pilocarpine model group than in Control group, but had no difference between EWD and EWND groups. The ratios of phosphorylated NR1 (p-NR1)/NR1 and p-NR2B/NR2B were significantly greater in the hippocampus in EWD group than in EWND group. Moreover, the expression of p-NR1 and p-NR2B in the CA1 subfield of hippocampus were both greater in Licl-pilocarpine model group than Control group. Selective blockage of NR2B subunit with ifenprodil could alleviate depression-like behaviours of Licl-pilocarpine rat epilepsy model. In conclusion, glutamate NMDA receptor NR2B subunit was involved in promoting depression-like behaviours in the Licl-pilocarpine chronic rat epilepsy model and might be a target for treating epilepsy-associated depression.

Thiol oxidation and altered NR2B/NMDA receptor functions in in vitro and in vivo pilocarpine models: implications for epileptogenesis

Hippocampal sclerosis, the main pathological sign of chronic temporal lobe epilepsy (TLE), is associated with oxidative injury, altered N-methyl d-aspartate receptor (NMDAR) stoichiometry, and loss of hippocampal neurons. However, the mechanisms that drive the chronic progression of TLE remain elusive. Our previous studies have shown that NADPH oxidase activation and ERK 1/2 phosphorylation are required for the up-regulation of the predominantly pre-synaptic NR2B subunit auto-receptor in both in vitro and in vivo pilocarpine (PILO) models of TLE. To provide further understanding of the cellular responses during the early-stages of hyper excitability, we investigated the role of oxidative damage and altered NR2B functions. In rat primary hippocampal cultures, we found that N-acetylcysteine (NAC) prevented PILO-mediated thiol oxidation, apoptosis, cell death and NR2B subunit over-expression. Interestingly, NAC did not block thiol oxidation when added to the neurons 6h after the PILO exposure, suggesting that disulfide formation could rapidly become an irreversible phenomenon. Moreover, NAC pre-treatment did not prevent PILO-induced NR2A subunit over-expression, a critical event in hippocampal sclerosis. Pre-treatment with the highly specific NR2B subunit inhibitor, ifenprodil, partially decreased PILO-mediated thiol oxidation and was not effective in preventing apoptosis and cell death. However, if acutely administered 48h after PILO exposure, ifenprodil blocked glutamate-induced aberrant calcium influx, suggesting the crucial role of NR2B over-expression in triggering neuronal hyper-excitability. Furthermore, ifenprodil treatment was able to prevent NR2A subunit over-expression by means of ERK1/2 phosphorylation. Our findings indicate oxidative stress and NR2B/NMDA signaling as promising therapeutic targets for co-treatments aimed to prevent chronic epilepsy following the seizure onset.

Propofol effectively inhibits lithium-pilocarpine- induced status epilepticus in rats via downregulation of N-methyl-D-aspartate receptor 2B subunit expression

Status epilepticus was induced via intraperitoneal injection of lithium-pilocarpine. The inhibitory effects of propofol on status epilepticus in rats were judged based on observation of behavior, electroencephalography and 24-hour survival rate. Propofol (12.5-100 mg/kg) improved status epilepticus in a dose-dependent manner, and significantly reduced the number of deaths within 24 hours of lithium-pilocarpine injection. Western blot results showed that, 24 hours after induction of status epilepticus, the levels of N-methyl-D-aspartate receptor 2A and 2B subunits were significantly increased in rat cerebral cortex and hippocampus. Propofol at 50 mg/kg significantly suppressed the increase in N-methyl-D-aspartate receptor 2B subunit levels, but not the increase in N-methyl-D-aspartate receptor 2A subunit levels. The results suggest that propofol can effectively inhibit status epilepticus induced by lithium-pilocarpine. This effect may be associated with downregulation of N-methyl-D-aspartate receptor 2B subunit expression after seizures.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

NMDA receptor subunit expression in the supraoptic nucleus of adult rats: dominance of NR2B and NR2D

The supraoptic nucleus (SON) of the hypothalamus contains magnocellular neurosecretory neurons (MNC) which synthesize and release the peptide hormones vasopressin and oxytocin. Glutamate is a prominent excitatory neurotransmitter in the SON and regulates MNC excitability. NMDA receptors (NMDAR), a type of ionotropic glutamate receptor, mediate synaptic plasticity of MNCs and are necessary for characteristic burst firing patterns which serve to maximize hormone release. NMDARs are di- or tri-heteromeric complexes of NR1 and NR2 subunits. Receptor properties depend on NR2 subunit composition and variable splicing of NR1. We investigated the expression profile of NR1 and NR2 subunits in the SON at the mRNA and protein levels plus protein expression of NR1 splice variants in control and salt-loaded adult rats. There was robust mRNA expression of all subunits, with NR2D levels being the highest. At the protein level, NR1, NR2B, and NR2D were robustly expressed, while NR2A was weakly expressed. NR2C protein was not detected with either of the two antibodies tested. All four NR1 splice variant cassettes (N1, C1, C2, C2') were detected in the SON, although NR1 N1 expression was too low for accurate analysis. Three days of salt-loading did not alter mRNA, protein, or splice variant expression of NMDAR subunits in the SON. Robust NR2D protein expression has not been previously shown in MNCs and is uncommon in the adult brain. Although the functional significance of this unusual expression profile is unknown, it may contribute to important physiological characteristics of SON neurons, such as burst firing and resistance to excitotoxicity.

Tonic facilitation of glutamate release by presynaptic NR2B-containing NMDA receptors is increased in the entorhinal cortex of chronically epileptic rats

We have shown previously that when postsynaptic NMDA receptors are blocked, the frequency, but not amplitude, of spontaneous EPSCs (sEPSCs) at synapses in the entorhinal cortex is reduced by NMDA receptor antagonists, demonstrating that glutamate release is tonically facilitated by presynaptic NMDA autoreceptors. In the present study, we recorded sEPSCs using whole-cell voltage clamp in neurons in layer V in slices of the rat entorhinal cortex. Using specific antagonists for NR2A [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid] and NR2B [(alphaR, betaS)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidinepropanol hydrochloride (Ro 25-6981)] subunit-containing receptors, we confirmed that in slices from juvenile rats (4-6 weeks of age), the autoreceptor is predominantly of the NR1-NR2B subtype. In older (4-6 months of age) control animals, the effect of the NR2B antagonist was less marked, suggesting a decline in autoreceptor function with development. In slices from rats (aged 4-6 months) exhibiting spontaneous recurrent seizures induced with a lithium-pilocarpine protocol, Ro 25-6981 again robustly reduced sEPSC frequency. The effect was equal to or greater than that seen in the juvenile slices and much more pronounced than that seen in the age-matched control animals. In all three groups, the NR2A antagonist was without effect on sEPSCs. These results suggest that there is a developmental decrease in NMDA autoreceptor function, which is reversed in a chronic epileptic condition. The enhanced autoreceptor function may contribute to seizure susceptibility and epileptogenesis in temporal lobe structures.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Novel anticonvulsant medications in development

Epilepsy is currently the most prevalent neurological disorder worldwide. Pharmacological therapy remains the cornerstone of epilepsy treatment, however, refractory epilepsy is still a significant clinical problem despite the release of the second generation of anticonvulsants. Anticonvulsant treatment failures may result from lack of efficacy and presence of significant side effects. One rationale for incomplete effectiveness of the currently available anticonvulsants is that they were identified using the same classical models and therefore work largely by the same actions. These mechanisms fail to consider variations in the pathophysiological process that results in epilepsy, nor have they been shown to prevent the process of developing epilepsy (epileptogenesis). The next generation of anticonvulsants has taken into account the shortcomings of existing agents and attempted to improve on the currently available treatments using rationale drug design. This group of investigational anticonvulsants may be broadly classified as possessing one or more of the following: 1) increased tolerability through improvement in drug chemical structure or better delivery to the site of action, 2) new mechanisms (or combinations of mechanisms) of action, 3) improved pharmacokinetic properties. This article will discuss the next generation of anticonvulsants (carabersat, CGX-1007, fluorofelbamate, harkoseride, losigamone, pregabalin, retigabine, safinamide, SPD-421, talampanel, valrocemide) and the possible populations in which they would be clinically useful.

Role of excitatory amino acids in developmental epilepsies

Altered excitatory amino acid (EAA) neurotransmission, mediated primarily by glutamate, is a major cause of the imbalance of excitation and inhibition which characterizes both early development and epileptogenesis. Glutamate's actions are mediated by three classes of receptors: NMDA, non-NMDA (AMPA and kainate), and metabotropic. Several features of normal EAA development contribute to hyperexcitability in the immature brain, making it more prone to development of seizures. These features include increased density of NMDA receptors, differences in NMDA receptor subunit composition and activation kinetics, which result in reduced voltage-dependent Mg(2+) blockade and longer receptor openings in early development. Also, the unique subunit composition of AMPA receptors present at synapses during early development results in increased Ca(2+) influx. These and other differences in EAA signaling, in combination with developmental alterations in inhibitory neurotransmission, contribute to the increased seizure susceptibility seen in young animals and children. In turn, seizures themselves may alter EAA neurotransmission in an age-dependent manner. Age related changes in excitatory neurotransmission may, therefore, lead to differences in basic mechanisms of epileptogenesis between the immature and mature brain, and may also alter the activity and efficacy of antiepileptic drugs in the pediatric age group.

NR2B-containing NMDA autoreceptors at synapses on entorhinal cortical neurons

We have previously shown that presynaptic N-methyl-D-aspartate receptors (NMDARs) can facilitate glutamate release onto principal neurons in the entorhinal cortex (EC). In the present study, we have investigated the subunit composition of these presynaptic NMDARs. We recorded miniature alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs), from visually identified neurons in layers II and V of the EC in vitro. In both layers, bath application of the NR2A/B subunit-selective agonist, homoquinolinic acid (HQA), resulted in a marked facilitation of mEPSC frequency. Blockade of presynaptic Ca(2+) entry through either NMDARs or voltage-gated Ca(2+) channels with Co(2+) prevented the effects of HQA, confirming that Ca(2+) entry to the terminal was required for facilitation. When the NR2B-selective antagonist, ifenprodil, was applied prior to HQA, the increase in mEPSC frequency was greatly reduced. In addition, we found that an NMDAR antagonist blocked frequency-dependent facilitation of evoked release and reduced mEPSC frequency in layer V. Thus we have demonstrated that NMDA autoreceptors in layer V of the EC bear the NR2B subunit, and that NMDARs are also present at terminals onto superficial neurons.

Inhibition of hippocampal kindling by metabotropic glutamate receptor antisense oligonucleotides

Recent work has shown that metabotropic glutamate receptors (mGluRs) increase in response to seizure activity and can contribute significantly to the expression and progression of partial seizures. Using the kindling model of temporal lobe seizures, we evaluated the ability of local hippocampal injections of mGluR1 antisense or mGluR3 antisense oligonucleotides to suppress receptor expression and alter hippocampal kindling. Daily antisense injections in the hippocampus resulted in a significant decrease in mGluR1 or mGluR2/3 immunoreactivity. Rats injected with mGluR3 antisense showed a brief suppression of afterdischarge duration when compared to matched rats injected with a nonsense-oligonucleotide. Rats injected with a mGluR1 antisense oligonucleotide had a dramatic suppression of the rate of seizure progression with no significant effect on afterdischarge duration. Suppression of mGluR1 synthesis by local antisense inhibition may provide a new therapeutic approach for the control of epileptogenesis.

7-nitroindazole differentially affects the anticonvulsant activity of antiepileptic drugs against amygdala-kindled seizures in rats

PURPOSE

The objective of this study was to evaluate the interaction of the preferential brain nitric oxide synthase (NOS) inhibitor, 7-nitroindazole (7-NI), with conventional antiepileptic drugs (AEDs) against amygdala-kindled seizures in rats.

METHODS

Experiments were performed on fully kindled rats. Adverse effects were evaluated with the rotorod test, which assesses motor coordination, and the passive-avoidance task, which assesses memory. Plasma levels of AEDs were measured by immunofluorescence.

RESULTS

7-NI (up to 100 mg/kg) failed to modify seizure parameters. However, it reduced the severity and duration of kindled seizures when coadministered with otherwise ineffective doses of carbamazepine (CBZ) (10-20 mg/kg) or phenobarbital (PB) (20 mg/kg). Combinations of 7-NI with valproate (VPA), diphenylhydantoin (DPH), or clonazepam (CLO) were not protective. L-Arginine (500 mg/kg) did not reverse the seizure-suppressing interactions between 7-NI and the conventional AEDs. The combinations of 7-NI and CBZ or PB did not impair performance in the rotorod test. Coadministration of 7-NI with CBZ did not affect long-term memory, and 7-NI given with PB didn't affect the mnemonic effect of PB. Finally, 7-NI did not affect the free plasma levels of CBZ or PB.

CONCLUSIONS

Pharmacokinetic interactions do not seem to account for the anticonvulsant effects of 7-NI combined with CBZ or PB. Central nitric oxide (NO) is possibly not involved in the synergism between 7-NI and these AEDs.

Evidence for coupling between glucose metabolism and glutamate cycling using FDG PET and 1H magnetic resonance spectroscopy in patients with epilepsy

The purpose of this study was to examine the relation between glucose metabolism and glutamate concentration in the human brain, in both the normal and diseased state. Regional values of glucose metabolism measured with 2-deoxy-2[F-18]fluoro-D-glucose positron emission tomography (FDG PET) studies and single-voxel proton magnetic resonance spectroscopy (1H MRS) measurements of the glutamate/ glutamine/gamma-aminobutyric acid (Glx) tissue concentration were determined in multiple brain regions in 11 patients (5 girls and 6 boys, mean age 7.5 years) with medically intractable partial epilepsy. FDG PET and 1H MRS studies were performed in the interictal state in seven patients and in the ictal/periictal state in four patients. Regions of interest were identified in epileptic cortex (determined by intracranial and/or scalp electroencephalography) and in contralateral normal brain regions. Lower glucose metabolism and lower Glx concentrations were found in the epileptic focus than in the contralateral normal cortex in all seven patients examined in the interictal state, whereas higher glucose metabolism and higher Glx concentrations were observed in the epileptic focus in the four patients who had ictal/periictal studies. Significant correlations were found between the values of cerebral glucose utilization and Glx concentration in epileptic brain region, in nonepileptic brain regions, and in epileptic and nonepileptic regions combined. These results demonstrate a significant relation between glucose metabolism and glutamate/glutamine concentration in normal and epileptic cerebral cortex. This relation is maintained in both the interictal and ictal states.

Glutamate receptors and transporters in genetic and acquired models of epilepsy

Glutamate, the principal excitatory neurotransmitter in the brain, acts on three families of ionotropic receptor--AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid), kainate and NMDA (N-methyl-D-aspartate) receptors and three families of metabotropic receptor (Group I: mGlu1 and mGlu5; Group II: mGlu2 and mGlu3; Group III: mGlu4, mGlu6, mGlu7 and mGlu8). Glutamate is removed from the synaptic cleft and the extracellular space by Na+-dependent transporters (GLAST/EAAT1, GLT/EAAT2, EAAC/EAAT3, EAAT4, EAAT5). In rodents, genetic manipulations relating to the expression or function of glutamate receptor proteins can induce epilepsy syndromes or raise seizure threshold. Decreased expression of glutamate transporters (EAAC knockdown, GLT knockout) can lead to seizures. In acquired epilepsy syndromes, a wide variety of changes in receptors and transporters have been described. Electrically-induced kindling in the rat is associated with functional potentiation of NMDA receptor-mediated responses at various limbic sites. Group I metabotropic responses are enhanced in the amygdala. To date, no genetic epilepsy in man has been identified in which the primary genetic defect involves glutamate receptors or transporters. Changes are found in some acquired syndromes, including enhanced NMDA receptor responses in dentate granule cells in patients with hippocampal sclerosis.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Kindled seizures increase metabotropic glutamate receptor expression and function in the rat supraoptic nucleus

The spread of experimentally kindled seizures in rats results in sustained increases in plasma vasopressin (VP) and VP mRNA in the supraoptic nucleus (SON). These increases provide an excellent example of the pathological plasticity that can develop in normal cells exposed to recurrent seizure activity. To test whether this plasticity might be due in part to changes in metabotropic glutamate receptors (mGluRs), we examined mGluR mRNA expression in the SON 1 month after stage 5 amygdala kindling. Three mGluR subtypes were detected by in situ hybridization in the SON in the following relative levels: mGluR3 > mGluR1 > mGluR7. Both mGluR1 and mGluR3 mRNAs were significantly increased in the SON (+28-61%) and cortex (+27-42%) after kindling. Immunoreactivity for mGluR1 but not mGluR2/3 was significantly increased in vivo in the SON. Receptor protein expression and intracellular calcium accumulation in response to the mGluR agonist, 1S,3R ACPD, were evaluated after in vitro "kindling" of neuroendocrine cells by Mg2+ deprivation. Increased immunoreactivity for mGluR1 and mGluR2/3 was seen in all cultures 3 days after a brief exposure to Mg2+-free medium. 1S,3R 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) induced rapid peak responses and gradual accumulations of intracellular Ca2+ in neurons. Both responses were increased in the "kindled" cells. Increases in the expression of functional mGluR1 and perhaps mGluR3 receptors may contribute to the development of long-lasting plastic changes associated with seizure activity.