Treatment with valproate after status epilepticus: Effect on neuronal damage, epileptogenesis, and behavioral alterations in rats

Abstract

Epileptogenesis, i.e. the process leading to epilepsy with spontaneous recurrent seizures, can be initiated by a number of brain damaging insults, including traumatic brain injury, status epilepticus (SE), and stroke. Such acquired epilepsy is often associated with memory impairment and behavioral problems. There has been a growing interest in the use of antiepileptic drugs (AEDs) for neuroprotection and prevention or modification of epileptogenesis induced by such brain insults. One promising candidate in this respect is valproic acid (VPA), a widely used AED that has been reported to exert neuroprotective activity in a number of in vitro and in vivo models. The present study investigated whether VPA reduces brain damage and improves functional outcome in a rat model of post-SE epilepsy. A self-sustaining SE was induced by prolonged electrical stimulation of the basal amygdala via a depth electrode. SE was terminated after 4 h by diazepam, immediately followed by onset of treatment with VPA. VPA was injected i.p. at a bolus dose of 400 mg/kg, followed by three times daily administration of 200 mg/kg for 4 weeks. A control group received vehicle instead of VPA after SE. Spontaneous seizures were recorded in all rats of both groups following termination of treatment, without significant inter-group difference in seizure frequency or severity. However, treatment with VPA after SE prevented the hyperexcitability and locomotor hyperactivity observed in vehicle-treated epileptic rats. Furthermore, VPA completely counteracted the neuronal damage in the hippocampal formation, including the dentate hilus. The data demonstrate that, although VPA does not prevent the occurrence of spontaneous seizures after SE, it exerts powerful neuroprotective effects and prevents part of the behavioral alterations, demonstrating that administration of VPA immediately after SE exerts a favorable effect on long-term functional outcome.

Introduction

Epilepsy is a common neurological disorder characterized by recurrent, unprovoked seizures. Patients with epilepsy are prone to cognitive and neurobehavioral deficits (Devinsky, 2004). For example, temporal lobe epilepsy (TLE), the most common type of epilepsy in adults, can be associated with memory impairment and behavioral problems, including depression, anxiety and psychoses (Gaitatzis et al., 2004)). Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (DeLorenzo et al., 2005). Acquired epilepsy develops in three phases: (1) the initial brain insult, (2) a latency period, during which epileptogenic processes take place, which in turn lead to (3) spontaneous recurrent seizures (SRS), i.e., the chronic epileptic phase. Status epilepticus (SE), stroke, and traumatic brain injury are three major examples of common brain injuries that can lead to the development of acquired epilepsy (DeLorenzo et al., 2005). The current treatment of epilepsy focuses exclusively on the prevention or suppression of seizures, i.e., the end result of the disease process. Recent findings regarding the sequence of neurobiological changes leading to epilepsy and its molecular basis have raised the question of whether the disease process of epileptogenesis can be prevented, or at least modified in such a way that the epilepsy that develops is milder, easier to treat, non-progressive and without cognitive decline and drug-resistance (Löscher and Schmidt, 2004, Pitkänen, 2004).

Prior clinical attempts to prevent posttraumatic epileptogenesis used various antiepileptic drugs (AEDs), usually given many hours after injury (Temkin, 2001, Temkin et al., 2001). Generally these studies showed that these agents suppressed acute posttraumatic seizures in the first week after trauma, but did not seem to alter the risk of developing posttraumatic epilepsy a year or two later (Temkin, 2001, Temkin et al., 2001). However, the timing of treatment with putative prophylactic drugs may be important, because experimental data suggest that a limited time domain exists for AEDs such as valproate (VPA) to intervene in the epileptogenic process, requiring the earliest possible intervention (Benardo, 2003).

A variety of AEDs have been tested in rat models for their potential to prevent epilepsy, neurodegeneration and behavioral or cognitive defects developing after a brain damaging insult such as a SE (Löscher, 2002a, Pitkänen, 2002a, Pitkänen, 2002b, Pitkänen, 2004). Most of these studies failed to identify any drug which is capable of preventing or attenuating epileptogenesis when prophylactic treatment is started after a SE of sufficient length to induce epileptogenesis (Löscher, 2002a, Pitkänen, 2002a, Pitkänen, 2002b, Pitkänen, 2004). However, AEDs attenuating the severity of the initial insult by being administered during an SE improved the outcome by reducing epileptogenesis, resulting in a milder disease (Pitkänen and Kubova, 2004). Such initial insult modification should be clearly differentiated from drugs being capable of improving the long-term consequences of a brain insult when being administered after the insult. A study by Bolanos et al. (1998) indicated that VPA may constitute such a drug.

In the study of Bolanos et al. (1998), VPA was daily injected for 40 days, 24 h after the onset of a kainate-induced SE in young rats (kainate was injected on postnatal day 35). Compared to rats treated with vehicle or phenobarbital after the SE, the VPA-treated group did not exhibit SRS during treatment, did not develop deficits in learning in the Morris water maze test, and had fewer histologic lesions in the hippocampus, demonstrating that VPA treatment prevents many of the neurologic sequelae typically seen after kainate-induced SE. However, because rats were not monitored for SRS in the absence of VPA treatment, it is not known whether VPA prevented the development of epilepsy.

This prompted us to perform the present study in which prophylactic treatment with VPA was started 4 h after onset of an electrically-induced SE in adult rats. This duration of SE has been shown to induce development of SRS and histological damage in the hippocampal formation, particularly in the dentate hilus (Brandt et al., 2003a). Daily treatment with VPA was continued for 4 weeks after SE, then this treatment was terminated, and 4 weeks later the occurrence of SRS was monitored by EEG/video recordings. In addition to recording SRS, the rats were tested in a battery of behavioral tests, including the elevated-plus maze and the Morris water maze. Finally, the brains of the animals were analyzed for histologic lesions in the hippocampal formation.