Jun Tokumaru

Collapse of extracellular glutamate regulation during epileptogenesis: down-regulation and functional failure of glutamate transporter function in rats with chronic seizures induced by kainic acid

We used northern and western blotting to measure the quantity of glutamate and GABA transporters mRNA and their proteins within the hippocampal tissue of rats with epileptogenesis. Chronic seizures were induced by amygdalar injection of kainic acid 60 days before death. We found that expression of the mRNA and protein of the glial glutamate transporters GLAST and GLT‐1 were down‐regulated in the kainic acid‐administered group. In contrast, EAAC‐1 and GAT‐3 mRNA and their proteins were increased, while GAT‐1 mRNA and protein were not changed. We performed in vivo microdialysis in the freely moving state. During the interictal state, the extracellular glutamate concentration was increased, whereas the GABA level was decreased in the kainic acid group. Following potassium‐induced depolarization, glutamate overflow was higher and the recovery time to the basal release was prolonged in the kainic acid group relative to controls. Our data suggest that epileptogenesis in rats with kainic acid‐induced chronic seizures is associated with the collapse of extracellular glutamate regulation caused by both molecular down‐regulation and functional failure of glutamate transport.

Effect of zonisamide on molecular regulation of glutamate and GABA transporter proteins during epileptogenesis in rats with hippocampal seizures.

Epileptiform discharges and behavioral seizures may be the consequences of excess excitation associated with the neurotransmitter glutamate, or from inadequate inhibitory effects associated with gamma-aminobutyric acid (GABA). Synaptic effects of these neurotransmitters are terminated by the action of transporter proteins that remove amino acids from the synaptic cleft. Excitation initiated by the synaptic release of glutamate is attenuated by the action of glial transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1), and the neuronal transporter excitatory amino-acid carrier-1 (EAAC-1). GABA is removed from synaptic regions by the action of the transporters proteins GABA transporter-1 (GAT-1) and GABA transporter-3 (GAT-3). In this experiment, albino rats with chronic, spontaneous recurrent seizures induced by the amygdalar injection of FeCl3 were treated for 14 days with zonisamide (ZNS) (40 mg/kg, i.p.). Control animals underwent saline injection into the same amygdalar regions. Treatment control for both groups of intracerebrally injected animals was i.p. injection of equal volumes of saline. Western blotting was used to measure the quantity of glutamate and GABA transporters in hippocampus and frontal cortex. ZNS caused increase in the quantity of EAAC-1 protein in hippocampus and cortex and down regulation of the GABA transporter GAT-1. These changes occurred in both experimental and ZNS treated control animals. These data show that the molecular effect of ZNS, with up-regulation of EAAC-1 and decreased production of GABA transporters, should result in increased tissue and synaptic concentrations of GABA. Although many antiepileptic drugs have effects on ion channels when measured in vitro our study suggests that additional mechanisms of action may be operant. Molecular effects on regulation of transporter proteins may aid in understanding epileptogenesis and inform investigators about future design and development of drugs to treat epilepsy.

Glutamate excess and free radical formation during and following kainic acid-induced status epilepticus

Abstract. Kainic acid (KA) induces seizures and degeneration in CA1 of the ventral hippocampus, though its mechanism of action is unknown. We used KA to induce seizures in freely moving rats prepared for in vivo microdialysis with probe placement, and then measured extracellular glutamate with an online fluorometric detector. Generation of free radicals was monitored by electron paramagnetic resonance (EPR) spectroscopy coupled with perfusion of the spin-trapping agent, α-(4-pyridyl-N-oxide)-N-tert-butylnitrone (POBN). Regional antioxidant efficacy was measured by observing the eliminating ratio of nitroxide radicals, using 3-carbamoyl-2, 2, 5, 5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) applied exogenously from the probe. Increased levels of extracellular glutamate observed at the initiation of KA-induced seizures appear to be associated with generation of lipid free radicals and with a decrease in residual antioxidant effects. These data suggest that collapse of the redox state in the hippocampus, the region most vulnerable to injury from seizure activity, may be critical in the regional injury induced by seizures. Further, we propose that the functional failure of glutamate transporters due to oxidative stress results in high levels of extracellular glutamate during sustained generalized seizures induced with KA.

Effect of levetiracetam on molecular regulation of hippocampal glutamate and GABA transporters in rats with chronic seizures induced by amygdalar FeCl3 injection

Enhancement of the glutamatergic excitatory synaptic transmission efficacy in the FeCl3 induced epilepsy model is associated with changes in the levels of glutamate and GABA transporter proteins. This study examined the effect of levetiracetam (LEV) on glutamate overflow and glutamate/GABA transporters expression in rats with epileptogenesis induced by the amygdalar injection of 1.0 microl of 100 mM FeCl3 (epileptic rat) and in control rats receiving amygdalar acidic saline injection (non-epileptic rat). In amygdalar acidic saline injected rats, 40 mM KCl-evoked glutamate overflow was significantly suppressed by both 32 and 100 microM LEV co-perfusion. In unilateral amygdalar FeCl3 injected rats, 32 microM LEV was ineffective, but the 100 microM LEV statistically suppressed glutamate overflow. Western blotting was employed to determine the hippocampal expression of glutamate/GABA transporters in epileptic or non-epileptic rats. The rats were treated for 14 days with 54 mg/kg LEV or vehicle intraperitoneally injection. Following 14 days of treatment, the ipsilateral hippocampus was removed for a Western blot analysis. In non-epileptic rats, the expression increased for all of the glutamate and GABA transporters (GLAST, GLT-1, EAAC-1, GAT-1 and GAT-3) while the glutamate transporter regulating protein (GTRAP3-18) decreased in comparison to those of normal rats that were treated with the vehicle. In epileptic rats receiving LEV, the EAAC-1 and GAT-3 levels increased while GTRAP3-18 (89%) decreased in comparison to those of the epileptic rats treated with the vehicle. GTRAP3-18 inhibitor regulates glutamate-binding affinity to EAAC-1. The anti-epileptic action of LEV may be partially due to a reduction of glutamate-induced excitotoxicity and an enhancement of the GABAergic inhibition as observed with the inhibitory effect on the 40 mM KCl-evoked glutamate overflow. These conclusions are supported by the increase in the expression of glial glutamate transporters (GLAST and GLT-1), and the increase in the expression of EAAC-1 and GAT-3 associated with a decrease in GTRAP3-18. The increased expression of EAAC-1 and the decreased expression of GTRAP3-18 in association with the up-regulation of GAT-3 due to such continual LEV administration was thus found to enhance GABA synthesis and reverse the transport of GABA both in non-epileptic and epileptic rats. The suppression of glutamate excitation and the enhancement of GABA inhibition in the rats with continual LEV administration is a result of the up-regulation of glutamate and GABA transporters with the down-regulation of GTRAP3-18. These observations together demonstrated the critical molecular mechanism of the anti-epileptic activity of LEV.

Kindling phenomena induced by the repeated short-term high potassium stimuli in the ventral hippocampus of rats: on-line monitoring of extracellular glutamate overflow

We observed in this study that transient periodic stimuli in response to high potassium (40 mM, 5 min at 40-min intervals, 13–15 stimuli) perfusion in the ventral hippocampus of rats led to the appearance of a kindling-like phenomenon. In this kindling-like phenomenon, we confirmed the augmentation of glutamate release and the prolongation of spike discharge. Changes in the extracellular glutamate levels before and after the stimuli were monitored by the application of in vivo microdialysis combined with on-line enzyme fluorometric detection of glutamate. This kindling-like phenomenon was not observed when microdialysis was carried out using a Ca++-free medium. The augmentation of glutamate release and the prolongation of spike discharge with epileptic convulsions are completely Ca++ dependent. These data show that repeated short-term increases in extracellular glutamate levels results in the enhancement of excitatory neuronal systems, causing an excessive propagation of seizure activity and culminating in secondary generalized seizures.

Sequential changes in glutamate transporter mRNA levels during Fe3+-induced epileptogenesis

Severe head injury in humans can cause recurrent seizures; this form of epilepsy appears to correlate with the occurrence of parenchymal hemorrhage. The injection of ferric cations, one component of hemoglobin, into rat amygdala, causes lipid peroxidation, and recurrent spontaneous seizures. We wondered whether the regulation of glutamate might be perturbed as a result of severe head injury, which might then act as a mechanism of chronic epileptogenesis. Levels of glutamate transporter glutamate-aspartate transporter (GLAST), glutamate transporter-1 (GLT-1), and excitatory amino-acid carrier (EAAC-1) mRNA were measured in ipsilateral and contralateral hippocampi and cerebral cortex removed from rats at 60 min, 24 h, and 5, 15 and 30 days after FeCl(3) injection into the amygdaloid body. While the neuronal transporter EAAC-1 mRNA was elevated bilaterally for up to 30 days following the microinjection that initiated seizures, GLT-1 mRNA, derived from glial cells, returned to basal levels. At 15 and 30 days after injection, however, when the experimental animals were experiencing spontaneous limbic behavioral seizures, GLAST mRNA was down-regulated. Epileptogenesis may correlate with the impairment of glial glutamate transport, leading to an excitation and imbalance of transmitter influences within the hippocampi and cerebral cortex.

Sequential changes in AMPA and NMDA protein levels during Fe3+-induced epileptogenesis

Seizure susceptibility is related to enhanced glutamatergic excitatory synaptic transmission with alterations in the expressions of ionotropic glutamate receptors. We wondered if levels of AMPA and NMDA receptor subunits changed following epileptogenesis induced by amygdalar FeCl(3) injection. We used Western blots to measure levels of subunits in the ipsilateral and contralateral hippocampus at various times after FeCl(3) injection into the amygdaloid body. With acute seizures, at +5 days after the injection, levels of GluR1, NMDAR1, and NMDAR2 were markedly increased in both hippocampi, with quantities at least 2-4 times baseline. By +15 and +30 days after injection, when chronic spontaneous seizures were occurring, the levels of GluR2 were increased, while GluR1 and NMDAR1&2A/B were decreased. Increased NMDAR1&2A/B levels at +5 days are consistent with the occurrence of upregulation of NMDA receptor production in the early stages of epileptogenesis. Since GluR2 suppresses glutamate receptor-mediated Ca(2+)-influx, increased expression of GluR2 with development of chronic, recurrent seizures may be a compensatory effect during epileptogenesis from neural responses to propagated seizures.

In vivo evaluation of hippocampal anti-oxidant ability of zonisamide in rats.

We evaluated the anti-oxidant property of zonisamide (ZNS) in the rat brain under freely moving conditions by means of in vivo microdialysis of two exogenous nitroxide radicals, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) and 3-methoxy carbonyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (PCAM). Time-dependent changes in the signal intensities of these exogenous nitroxide radicals obtained from the hippocampal perfusates were observed using an X-band ESR spectrometer at 20-min intervals. The ESR signal intensities of nitroxide radicals decreased exponentially in all animals, which indicates that their half-life could be used as a parameter to estimate the decay rate of nitroxide radicals. Nitroxide radicals lose their paramagnetism when exposed to reductants in a biological system. Thus, half-life reflects the in vivo reducing ability. Although the half-life of carbamoyl-PROXYL, which could not pass the blood-brain barrier (BBB), was not changed when compared with the controls, pre-treatment with ZNS significantly shortened the half-life of PCAM, which could pass through the BBB. These findings suggest that the ZNS-induced increase in reducing ability did not occur within the extracellular space, but rather mainly at the neural cell membrane. This study is the first in vivo evaluation of the reducing ability of ZNS in freely moving animals.

Expression of glutamate transporters and ionotropic glutamate receptors in GLAST knockout mice

In order to investigate the molecular mechanism underlying high seizure susceptibility of GLAST knockout mice, we carried out Western blotting for the expression of GLT-1, EAAC-1, and several kinds of glutamate receptors in the hippocampus and the cortex. Although no significant difference was observed between GLAST (+/+) and (-/-) mice in terms of expression of GLT-1 and EAAC-1 in the hippocampus, these proteins were over-expressed in the frontal cortex in GLAST (-/-) mice (GLT-1, about 210% increase; EAAC-1, about 180% increase). Expression of hippocampal Glu-R1 and Glu-R2 in GLAST (-/-) mice was remarkably increased (Glu-R1, about 140% increase; Glu-R2, about 160% increase), while Glu-R3 and NMDA receptors levels (NMDA-R1, 2A and 2B) were equal to those in control. Cortical levels of Glu-R1, -R2 and -R3 receptors in GLAST (-/-) mice were remarkably decreased (Glu-R1, about 60% decrease; Glu-R2, about 60% decrease; Glu-R3, about 70% decrease), while NMDA receptors were remarkably increased in comparison to those in GLAST (+/+) mice (N-R1, about 150% increase; N-R2A, about 150% increase; N-R2B, about 140% increase). These data suggest that the increased susceptibility to seizures in GLAST (-/-) mice might be derived from increased expression of Glu-R1 in the hippocampus coupled with decreased cortical expression of Glu-R2 and increased NMDA-R1 and -2A, -2B expression.

Kinetics of Extracellular Nitroxide Radical and Glutamate Levels in the Hippocampus of Conscious Rats: Cautionary Note to the Application of Nitroxide Radical on Clinical Arena

Recently, novel applications of the nitroxide radicals have been proposed as antioxidant and anticancer agents. In view of the significance of nitroxide radical as a potential pharmaceutical agent for various applications in biological systems, it will be important to investigate further whether nitroxide radicals have a neurotoxicity or not. Blood–brain barrier permeable nitroxide radical, 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (PCAM; five-membered ring nitroxide radical) would provide us more important information to explore the neuronal excitotoxicity of nitroxide radicals on the central nervous system. Every rat injected with PCAM showed limbic seizure with secondary generalization. PCAM administration resulted in neuronal cell loss in CA1 area, which is closely associated with the neurotoxicity of endogenous glutamate and nitroxide itself. More detailed studies on their possible toxicity of nitroxide radicals will be needed before the prospect of moving nitroxide from the experimental to the clinical arena when nitroxide radicals would be used for CNS disease in future.