Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis.

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

Glutathione pegylated liposomal methylprednisolone administration after the early phase of status epilepticus did not modify epileptogenesis in the rat.

It has been reported that glucocorticoids (GCs) can effectively control seizures in pediatric epilepsy syndromes, possibly by inhibition of inflammation. Since inflammation is supposed to be involved in epileptogenesis, we hypothesized that treatment with GCs would reduce brain inflammation and thereby modify epileptogenesis in a rat model for temporal lobe epilepsy, in which epilepsy gradually develops after electrically induced status epilepticus (SE). To prevent the severe adverse effects that are inevitable with long-term GC treatment, we used liposome nanotechnology (G-Technology(®)) to enhance the sustained delivery to the brain. Starting 4h after onset of SE, rats were treated with glutathione pegylated liposomal methylprednisolone (GSH-PEG liposomal MP) according to a treatment protocol (1× per week; 10mg/kg) that is effective in other models of neuroinflammation. Continuous electro-encephalogram (EEG) recordings revealed that SE duration and onset of spontaneous seizures were not affected by GSH-PEG liposomal MP treatment. The number and duration of spontaneous seizures were also not different between vehicle and GSH-PEG liposomal MP-treated animals. Six weeks after SE, brain inflammation, as assessed by quantification of microglia activation, was not reduced by GSH-PEG liposomal MP-treatment. Also, neuronal cell loss and mossy fiber sprouting were not affected. Our study shows that the selected GSH-PEG liposomal MP treatment regimen that was administered beyond the acute SE phase does not reduce brain inflammation and development of temporal lobe epilepsy.

Blood plasma inflammation markers during epileptogenesis in post-status epilepticus rat model for temporal lobe epilepsy.

PURPOSE: Brain inflammation occurs during epileptogenesis and may contribute to the development and progression of temporal lobe epilepsy. Recently, several studies have indicated that seizures may also increase specific blood plasma cytokine levels in animal models as well as in human patients with epilepsy, suggesting that peripheral inflammation may serve as a biomarker for epilepsy. Moreover, studies in epilepsy animal models have shown that peripheral inflammation may play either a pathogenic or neuroprotective role. METHODS: We evaluated the inflammatory response in blood plasma after electrically induced status epilepticus (SE) in a rat model for temporal lobe epilepsy. We measured blood plasma levels of the inflammation markers interleukin 1ß (IL-1ß), interleukin 6 (IL-6), by enzyme-linked immunosorbent assays (ELISAs) and C-reactive protein (CRP) by immunoturbidimetry, at 1 day after SE (acute period), at 1 week (during the latent period), and at 2 months after SE, which is the chronic epileptic phase when spontaneous seizures occur. Plasma levels were also measured during pilocarpine-induced SE. These were compared with plasma levels after lipopolysaccharide injection, which causes sepsis. KEY FINDINGS: Although sepsis induced a huge surge in IL-1ß and IL-6 levels, we did not detect a change in IL-1ß, IL-6, or CRP plasma levels at any time point after electrically induced SE compared to control animals. SE induced by pilocarpine produced a rise in IL-6 and CRP but not IL-1ß levels. SIGNIFICANCE: These findings suggest that plasma levels of these inflammatory proteins cannot be used as biomarkers for temporal lobe epileptogenesis.

Inhibition of mammalian target of rapamycin reduces epileptogenesis and blood-brain barrier leakage but not microglia activation.

PURPOSE: Previous studies have shown that inhibition of the mammalian target of rapamycin (mTOR) pathway with rapamycin prevents epileptogenesis after pharmacologically induced status epilepticus (SE) in rat models of temporal lobe epilepsy. Because rapamycin is also known for its immunosuppressant properties we hypothesized that one of the mechanisms by which it exerts this effect could be via suppression of brain inflammation, a process that has been suggested to play a major role in the development and progression of epilepsy. METHODS: Rats were treated with rapamycin or vehicle once daily for 7 days (6 mg/kg/day, i.p.) starting 4 h after the induction of SE, which was evoked by electrical stimulation of the angular bundle. Hereafter rapamycin was administered every other day until rats were sacrificed, 6 weeks after SE. Video-electroencephalography was used to monitor the occurrence of seizures. Neuronal death, synaptic reorganization, and microglia and astrocyte activation were assessed by immunohistologic staining. Fluorescein was administered to quantify blood-brain barrier leakage. KEY FINDINGS: Rapamycin treatment did not alter SE severity and duration compared to vehicle treatment rats. Rapamycin-treated rats developed hardly (n = 9) or no (n = 3) seizures during the 6-week treatment, whereas vehicle-treated rats showed a progressive increase of seizures starting 1 week after SE (mean 8 ± 2 seizures per day during the sixth week). Cell loss and sprouting that normally occur after SE were prominent but on average significantly less in rapamycin-treated rats versus vehicle-treated rats. Nevertheless, various inflammation markers (CD11b/c and CD68) were dramatically upregulated and not significantly different between post-SE groups. Of interest, blood-brain barrier leakage was barely detected in the rapamycin-treated group, whereas it was prominent in the vehicle-treated group. SIGNIFICANCE: mTOR inhibition led to strong reduction of seizure development despite the presence of microglia activation, suggesting that effects of rapamycin on seizure development are not due to a control of inflammation. Whether the effects on blood-brain barrier leakage in rapamycin-treated rats are a consequence of seizure suppressing properties of the drug, or contribute to a real antiepileptogenic effect still needs to be determined.

Atorvastatin treatment during epileptogenesis in a rat model for temporal lobe epilepsy.

PURPOSE: It has been shown that blood-brain barrier leakage together with inflammation could contribute to epileptogenesis and seizure progression in a rat model for temporal lobe epilepsy. Because statins have been shown to reduce blood-brain barrier permeability and inflammation in neurological diseases, we aimed to restore the integrity of the blood-brain barrier in epileptic rats using atorvastatin. If this drug could restore the blood-brain barrier, a reduction of brain inflammation might be expected, thereby delaying or preventing the development of epilepsy. METHODS: Rats were orally treated with atorvastatin (once daily, 10 mg/kg) or vehicle for 14 days, starting 7 days before the induction of epilepsy (which was evoked by electrical stimulation of the angular bundle until rats developed status epilepticus). Seizure activity was monitored continuously until 6 weeks after status epilepticus using video-EEG (electroencephalography). Fluorescein was administered at this time point to quantify blood-brain barrier leakage. Brain inflammation, neuronal death, and synaptic reorganization were assessed by (immuno)histologic stainings. KEY FINDINGS: Atorvastatin treatment did not affect the duration of status epilepticus or the development of epilepsy. At 6 weeks after status epilepticus, blood-brain barrier leakage was evident both in atorvastatin-treated and vehicle-treated rats in limbic brain regions (hippocampus, entorhinal cortex, piriform cortex). Atorvastatin treatment had not reduced inflammation, neuronal death, or synaptic reorganization. SIGNIFICANCE: The lack of any favorable effect of atorvastatin on the restoration of the blood-brain barrier, cell death, or brain inflammation suggests that atorvastatin is more effective in neurological diseases where the adaptive immune response plays a crucial role and less so in a disease as temporal lobe epilepsy, where the innate immune response is more prominent.

Cox-2 inhibition can lead to adverse effects in a rat model for temporal lobe epilepsy.

PURPOSE: Status epilepticus (SE) leads to upregulation of pro-inflammatory proteins including cyclooxygenase-2 (cox-2) which could be implicated in the epileptogenic process and epileptic seizures. Recent studies show that cox-2 can regulate expression of P-glycoprotein (P-gp) during epileptogenesis and epilepsy. P-gp could cause pharmacoresistance by reducing brain entry of anti-epileptic drugs such as phenytoin (PHT). Here we have investigated the effects of cox-2 inhibition on epileptogenesis, spontaneous seizures and PHT treatment in a rat model for temporal lobe epilepsy (TLE). METHODS: A 3-day treatment with the cox-2 inhibitor SC-58236 (SC) was started 1 day before electrically induced SE. Chronic epileptic rats were treated with SC for 14 days, which was followed by a 7-day period of SC/PHT combination treatment. Seizure activity was monitored continuously using electroencephalography. RESULTS: SC treatment did not affect SE duration, but led to an increased number of rats that died during the first 2 weeks after SE. Cox-2 inhibition during the chronic period led to an increased number of seizures in the 2nd week of treatment in 50% of the rats. SC/PHT treatment reduced seizures significantly for only 2 days. CONCLUSIONS: Both SC treatment that started before SE and the 14-day treatment in chronic epileptic rats led to adverse effects in the TLE rat model. Despite a temporal reduction in seizure frequency with SC/PHT treatment, SC does not seem to be a suitable approach for anti-epileptogenic or anti-epileptic therapy.

COX-2 inhibition controls P-glycoprotein expression and promotes brain delivery of phenytoin in chronic epileptic rats.

Epileptic seizures drive expression of the blood-brain barrier efflux transporter P-glycoprotein via a glutamate/cyclooxygenase-2 mediated signalling pathway. Targeting this pathway may represent an innovative approach to control P-glycoprotein expression in the epileptic brain and to enhance brain delivery of antiepileptic drugs. Therefore, we tested the effect of specific cyclooxygenase-2 inhibition on P-glycoprotein expression in two different status epilepticus models. Moreover, the impact of a cyclooxygenase-2 inhibitor on expression of the efflux transporter and on brain delivery of an antiepileptic drug was evaluated in rats with recurrent spontaneous seizures. The highly selective cyclooxygenase-2 inhibitors SC-58236 and NS-398 both counteracted the status epilepticus-associated increase in P-glycoprotein expression in the parahippocampal cortex and the ventral hippocampus. In line with our working hypothesis, a sub-chronic 2-week treatment with SC-58236 in the chronic epileptic state kept P-glycoprotein expression at control levels. As described previously, enhanced P-glycoprotein expression in chronic epileptic rats was associated with a significant reduction in the brain penetration of the antiepileptic drug phenytoin. Importantly, the brain delivery of phenytoin was significantly enhanced by sub-chronic cyclooxygenase-2 inhibition in rats with recurrent seizures. In conclusion, the data substantiate targeting of cyclooxygenase-2 in the chronic epileptic brain as a promising strategy to control the expression levels of P-glycoprotein despite recurrent seizure activity. Cyclooxygenase-2 inhibition may therefore help to increase concentrations of antiepileptic drugs at the target sites in the epileptic brain. It needs to be further evaluated whether the approach also enhances efficacy.