L. James Willmore

Vigabatrin: 2008 update.

Vigabatrin (VGB) is a structural analogue of γ‐aminobutyric acid (GABA) that irreversibly inhibits GABA‐transaminase (GABA‐T), increasing brain levels of GABA. VGB is under assessment for treatment of infantile spasms (IS) and refractory complex partial seizures (CPS). Response can be rapid with spasm cessation following approximately 2 weeks of therapy. Patients with symptomatic tuberous sclerosis (TS) and other patients have achieved spasm cessation. Comparison with ACTH has been performed. Patients with refractory CPS have responded as well. Adverse effects and structural findings on imaging occur with VGB treatment. T2 hyperintensities within brain have been observed. Psychotic disorders or hallucinations have occurred rarely. A specific adverse effects is associated VGB, with a peripheral visual field defect (VFD) detected in some patients. Prevalence and incidence of the VGB‐induced peripheral VFD varied depending on the age of the patient and the extent of exposure to VGB, with 25% to 50% prevalence in adults; the prevalence in children was 15% and retinal defect in infants ranged from 15% to 31%. A bilateral nasal defect may be the first clinical indication and may progress to a concentric, bilateral field defect observed in many affected patients; central visual acuity is almost always preserved. The earliest finding of the first abnormal field examination in adults was after 9 months of treatment; with a mean duration of VGB exposure of 4.8 years. In children, the earliest onset of a first abnormal field examination was after 11 months, with a mean time to onset of 5.5 years. The earliest sustained onset of the VGB‐induced retinal defect in infants was 3.1 months.

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.

The role of iron-induced hippocampal peroxidation in acute epileptogenesis

Intracortical injection of iron salts causes lipid peroxidation, focal edema, necrosis, gliosis, and the development of behavioral and electrographic seizures. Tocopherol pretreatment prevents the histopathologic perturbations associated with iron injection, and appears to accelerate the resolution of focal accumulation of peroxidation products. In this experiment, rats were pretreated with 500 mg/kg DL-alpha-tocopherol acetate prior to the injection of 3 microliter of 100 mM FeCl2 into the dorsal hippocampus, or induction of convulsive seizures by s.c. injection of 0.8 mg/100 g bicucullin. Tocopherol pretreatment prevented the occurrence of convulsive seizures in a significant number of iron-salts injected animals. Lipid peroxidation measured in the dissected hippocampus was significantly increased in untreated rats developing iron-induced seizures and in rats treated with tocopherol, but developing convulsive seizures. Tocopherol failed to prevent bicucullin-induced seizures. Further, convulsive seizures induced by bicucullin failed to alter hippocampal fluorescence levels. Hence, we concluded that the epileptogenic effects of hippocampal injection of iron salts appear to be related to the induction of peroxidation of neural lipids within the injection site.

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.

Antiepileptic drugs and neuroprotection: Current status and future roles

There has been a growing interest in the use of antiepileptic drugs (AEDs) for neuroprotection, and in the possible role of AEDs in disease modification (i.e., antiepileptogenesis). Increased understanding of the mechanisms underlying brain injury has led to advances in the study of neuroprotection. However, defining the clinical paradigm and selecting appropriate outcomes to detect neuroprotective effects present challenges to clinicians studying the neuroprotective properties of drugs. Established AEDs, such as phenytoin, phenobarbital, and carbamazepine, have shown neuroprotective activity in an ischemic/hypoxic model of neuronal injury. Animal model studies also have suggested that newer AEDs, such as levetiracetam, topiramate, and zonisamide, may have neuroprotective or antiepileptogenic properties. However, the prevention of epileptogenesis by an AED has yet to be demonstrated in clinical trials. The future of neuroprotection may involve established and newer AEDs, as well as other compounds, such as immunophilins, caspase inhibitors, endocannabinoids, and antioxidants.

Iron-induced lipid peroxidation and brain injury responses

Head trauma with cerebral contusion causes extravasation of red blood cells, followed by hemolysis and deposition of iron‐containing blood products within the neuropil. Liberation of heme compounds is associated with deposition of hemosiderin, and with gliosis, neuronal loss and occasionally the development of seizures. In this experiment we injected components of red blood cell contents into rat amygdala, and then measured the rate of appearance of products of lipid peroxidation. Injection of microliter volumes of hemin and hemoglobin, with hematoprotoporphyrin and rodent plasma injection and contralateral uninjected tissue as controls, showed that the presence of the iron moiety within the protoporphyrin ring was required to initiate and propagate peroxidation. Free radical reactions initiated by iron or heme deposited within the neuropil may be a fundamental reaction associated with brain injury responses, and possibly with posttraumatic epileptogenesis.

Prevention of iron-induced epileptiform discharges in rats by treatment with antiperoxidants

Abstract Head trauma and hemorrhagic cortical infarction in humans are associated with the development of late epilepsy. Hemosiderin deposition within brain is a frequent histopathologic accompaniment of chronic seizures induced by trauma. In this study an aqueous solution of iron salts, the principal metallic ion found in whole blood, was injected subpially into rat isocortex. Ninety-four percent of untreated animals developed epileptiform discharges on serial EEG recording. Pretreatment of rats with the antioxidant α-tocopherol, and with selenium, prevented the development of iron-induced epileptiform activity in 72% of the animals. Iron is known to cause formation of superoxide radicals with resulting peroxidation of membranous components of tissue. The use of antioxidants in preventing the development of iron-induced epileptiform discharges suggests that peroxidative injury may be an important mechanism in the development of experimental post-traumatic epilepsy as induced by isocortical injection of ferrous chloride.

Histopathology of the ferric-induced chronic epileptic focus in cat: A Golgi study

Abstract Five cats were rendered chronically epileptic via subpial injection of saturated FeCl 3 solution. Six weeks postinjection, electrocorticographic recording demonstrated focal epileptiform spiking in the region of the injection. This finding was not observed in saline-injected controls. Histopathological analysis of the epileptic focus using Nissl and Golgi-Cox techniques revealed (i) depopulation of Golgi-impregnated neurons, (ii) astocytic gliosis, (iii) loss of dendritic spines, (iv) decreased dendritic branching, and (v) dendritic varicosities. These are similar to the pathological findings which have been described for human epileptogenic foci. These results, in combination with the frequent observation of hemosiderosis in regions of human epileptogenic foci, implicate the release of iron from extravasated blood elements as a possible etiologic mechanism. Therefore, we believe the FeCl 3 experimental epileptogenic focus accurately models the human clinical entity (posttraumatic epilepsy) with respect to both electrophysiology and histopathology.

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

Severe head injury in humans causes recurrent seizures; this form of epilepsy appears to correlate with occurrence of parenchymal hemorrhage. Injection of ferric cations, one component of hemoglobin, into rat amygdala, causes lipid peroxidation, and recurrent spontaneous seizures. We wondered whether regulation of extracellular glutamate might be perturbed as a mechanism of chronic epileptogenesis, therefore levels of glutamate transporter proteins GLT-1, GLAST and EAAC-1 were measured in ipsilateral and contralateral hippocampi removed from rats having spontaneous iron-induced limbic seizures. The neuronal transporter EAAC-1 was elevated bilaterally up to 30 days following the microinjection that initiated seizures. The neuronal transporter EAAC-1 was elevated bilaterally up to 30 days following the microinjection that initiated seizures. The glial transporter GLT-1 increased 5 and 15 days after iron injection on the side contralateral to the injection then returned to basal levels 30 days after the lesion. GLAST also showed an initial increase but at 15 and 30 days after injection, when experimental animals were experiencing spontaneous limbic behavioral seizures, this protein was down-regulated. The results suggest that iron-induced epileptogenesis involves alteration in glial glutamate transport that may lead to enhanced excitation within the hippocampus.

Amygdalar Injection of FeCl3Causes Spontaneous Recurrent Seizures

Rats were microinjected with a 100 mM aqueous solution of ferric chloride into the left amygdaloid body. Behavior was observed and depth electroencephalograms were recorded over the 30 days following injection. All of the FeCl3-injected rats developed isolated epileptiform discharges from the ipsilateral amygdala soon after injection. Within 5 days epileptiform discharges were arising as well from the contralateral amygdala and behavioral seizures were observed. These spontaneous seizures occurred in a pattern associated with stage 4 kindling, with rearing and bilateral forelimb clonus. Seizures persisted during the 30 days of the experiment. Recording from chronically implanted depth electrodes showed development of spike discharges, with recurrent seizures arising from amygdalar regions with propagation into both hippocampi. Aqueous iron is known to initiate lipid peroxidation by free radical mechanisms. Our observations suggest that epileptogenesis followed by chronic, spontaneous seizures could be initiated by deposition of iron-containing compounds into limbic structures of the rat.