Pathophysiological aspects of blood-brain barrier permeability in epileptic seizures

Peri-ictal magnetic resonance imaging characteristics in dogs with suspected idiopathic epilepsy

Background

The pathophysiology of changes in magnetic resonance imaging (MRI) detected after a seizure is not fully understood.

Objective

To characterize and describe seizure‐induced changes detected by MRI.

Animals

Eighty‐one client‐owned dogs diagnosed with idiopathic epilepsy.

Methods

Data collected retrospectively from medical records and included anatomical areas affected, T1‐, T2‐weighted and T2‐FLAIR (fluid‐attenuated inversion recovery) appearance, whether changes were unilateral or bilateral, symmetry, contrast enhancement, mass effect, and, gray and white matter distribution. Diffusion‐ and perfusion weighted maps were evaluated, if available.

Results

Seizure‐induced changes were T2‐hyperintense with no suppression of signal on FLAIR. Lesions were T1‐isointense (55/81) or hypointense (26/81), local mass effect (23/81) and contrast enhancement (12/81). The majority of changes were bilateral (71/81) and symmetrical (69/71). The most common areas affected were the hippocampus (39/81) cingulate gyrus (33/81), hippocampus and piriform lobes (32/81). Distribution analysis suggested concurrence between cingulate gyrus and pulvinar thalamic nuclei, the cingulate gyrus and parahippocampal gyrus, hippocampus and piriform lobe, and, hippocampus and parahippocampal gyrus. Diffusion (DWI) characteristics were a mixed‐pattern of restricted, facilitated, and normal diffusion. Perfusion (PWI) showed either hypoperfusion (6/9) or hyperperfusion (3/9).

Conclusions and Clinical Importance

More areas, than previously reported, have been identified that could incur seizure‐induced changes. Similar to human literature, DWI and PWI changes have been identified that could reflect the underlying metabolic and vascular changes.

An exploration of the spectrum of peri-ictal MRI change; a comprehensive literature review

PURPOSE

The aim of this review was to identify published studies in the literature relating to ictal induced MRI change and to identify certain common themes, practical points for clinicians and areas for future research.

METHODS

We identified 96 articles that satisfied our inclusion criteria yielding 575 cases. All articles were analysed; number of subjects, spectrum of MRI and EEG change, aetiology, and follow-up (both clinical and imaging) were noted.

RESULTS

The most frequent imaging changes were restricted diffusion, T2-hyperintensity and reduced ADC values. The mesial temporal structures and neocortex were most commonly affected locations though subcortical structures like the thalamus and pulvinar were also described. Practical clinical points included; the development of PLEDS concordant with ictal imaging change was associated with worse clinical prognosis, patients with seizures due to symptomatic aetiology may be more likely to develop ictal related imaging change and follow up is vitally important to ensure that ictal related oedema is not misidentified as a mass lesion or conversely that a mass lesion is not misidentified as ictal related change.

CONCLUSION

Qualitative MRI studies have provided clinicians with useful in-vivo insights into the dynamic ictal neuronal environment. Changes are not only localised to the ictal focus but can be remote and irreversible. Small patient numbers varying study design and high numbers of symptomatic seizures makes comparison between studies problematic. Also there is possible microstructural quantitative MRI changes that are missed on qualitative MRI. There is a need for prospective quantitative MRI studies in patients with epilepsy peri-icatlly with a uniform period of follow up and comparison to control data.

Fit for purpose application of currently existing animal models in the discovery of novel epilepsy therapies

Animal seizure and epilepsy models continue to play an important role in the early discovery of new therapies for the symptomatic treatment of epilepsy. Since 1937, with the discovery of phenytoin, almost all anti-seizure drugs (ASDs) have been identified by their effects in animal models, and millions of patients world-wide have benefited from the successful translation of animal data into the clinic. However, several unmet clinical needs remain, including resistance to ASDs in about 30% of patients with epilepsy, adverse effects of ASDs that can reduce quality of life, and the lack of treatments that can prevent development of epilepsy in patients at risk following brain injury. The aim of this review is to critically discuss the translational value of currently used animal models of seizures and epilepsy, particularly what animal models can tell us about epilepsy therapies in patients and which limitations exist. Principles of translational medicine will be used for this discussion. An essential requirement for translational medicine to improve success in drug development is the availability of animal models with high predictive validity for a therapeutic drug response. For this requirement, the model, by definition, does not need to be a perfect replication of the clinical condition, but it is important that the validation provided for a given model is fit for purpose. The present review should guide researchers in both academia and industry what can and cannot be expected from animal models in preclinical development of epilepsy therapies, which models are best suited for which purpose, and for which aspects suitable models are as yet not available. Overall further development is needed to improve and validate animal models for the diverse areas in epilepsy research where suitable fit for purpose models are urgently needed in the search for more effective treatments.

Claudin-1, -2 and -3 Are Selectively Expressed in the Epithelia of the Choroid Plexus of the Mouse from Early Development and into Adulthood While Claudin-5 is Restricted to Endothelial Cells

A primary function of epithelial and endothelial monolayers is the formation of barriers that separate tissues into functional compartments. Tight junctions (TJs) seal the intercellular space between the single cells of a monolayer. TJs thus contribute importantly to the homeostasis of the cerebrospinal fluid as they help in maintaining the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (CSF). The composition of TJs differs by its localization as well as the stage of development according to its respective function. Claudin-3 is typically present in the epithelia and has been claimed to be a constituent of the BBB. It is, however, notoriously difficult to demonstrate its expression in endothelial cells of the brain vasculature at the morphological level by means of immunohistochemical techniques. Using an improved fixation strategy (4% paraformaldehyde at pH 11, in the presence of EDTA) and the sensitive alkaline phosphatase as a detection system, we show that claudin-3 is present in mouse epithelia from embryonic day 14 onwards. In brain, it is restricted to the anlage of choroid plexus in the ventricles, together with claudin-1 and -2. In adult mice, it is clearly delineating the epithelium of the choroid plexus in the lateral and fourth ventricles. In contrast, in cerebral blood vessels claudin-3 as well as claudin-1 and -2 are absent in cerebral blood vessels during all developmental stages up to adulthood. Rather, the BBB is characterized by the presence of claudin-5, ZO-1 and occludin. Thus, in mice claudin-3 is an important constituent of TJ in the embryonic and in the adult choroid plexus.

Transient postictal MRI changes in patients with brain tumors may mimic disease progression

BACKGROUND

Transient postictal imaging abnormalities in patients with non-tumor-related seizures are well documented and include fluid-attenuated inversion recovery/T2 hyperintensity and parenchymal and meningeal contrast enhancement. In contrast, transient postictal imaging abnormalities in patients with tumor-related seizures have been poorly described. Fifty percent of patients with brain tumors have a seizure during the course of their illness and are often imaged after a seizure or after a change in seizure character or frequency. Interval changes on repeat imaging can mimic disease progression or other pathologic processes.

METHODS

We describe 3 patients with brain tumors and transient postictal MRI changes that mimicked disease progression and infection.

RESULTS

Our patients demonstrated fluid-attenuated inversion recovery/T2 hyperintensity and gadolinium enhancement on MRI studies performed shortly after ictal events. These changes were suspicious for tumor progression in 2 cases and for recurrent infection in the third. Control of seizure activity resulted in resolution of these changes on scans obtained 10 to 21 days later.

CONCLUSIONS

Imaging shortly after an ictal event can potentially mislead the clinician to interpret changes as tumor or pathologic progression. Unnecessary intervention in these patients with new and suspicious imaging findings should be avoided. We recommend repeat imaging be performed in patients with brain tumors and seizures several weeks after seizure control if clinically feasible. Further research is needed to delineate the time course of seizure-induced MRI changes.

CNS drug delivery: opioid peptides and the blood-brain barrier

Peptides are key regulators in cellular and intercellular physiological responses and possess enormous promise for the treatment of pathological conditions. Opioid peptide activity within the central nervous system (CNS) is of particular interest for the treatment of pain owing to the elevated potency of peptides and the centrally mediated actions of pain processes. Despite this potential, peptides have seen limited use as clinically viable drugs for the treatment of pain. Reasons for the limited use are primarily based in the physiochemical and biochemical nature of peptides. Numerous approaches have been devised in an attempt to improve peptide drug delivery to the brain, with variable results. This review describes different approaches to peptide design/modification and provides examples of the value of these strategies to CNS delivery of peptide drugs. The various modes of modification of therapeutic peptides may be amalgamated, creating more efficacious "hybrid" peptides, with synergistic delivery to the CNS. The ongoing development of these strategies provides promise that peptide drugs may be useful for the treatment of pain and other neurologically-based disease states in the future.

Effects of lipopolysaccharide on blood-brain barrier permeability during pentylenetetrazole-induced epileptic seizures in rats

We investigated the effects of lipopolysachharide (LPS) on functional and structural properties of the blood-brain barrier (BBB) during pentylenetetrazole (PTZ)-induced epileptic seizures in rats. Arterial blood pressure was significantly elevated during epileptic seizures irrespective of LPS pretreatment. Plasma levels of interleukin (IL)-1, interleukin (IL)-6, nitric oxide (NO) and malondialdehyde (MDA) increased while catalase concentrations decreased in animals treated with LPS, PTZ and LPS plus PTZ. The significantly increased BBB permeability to Evans blue (EB) dye in the cerebral cortex, diencephalon and cerebellum regions of rats by PTZ-induced seizures was markedly reduced upon LPS pretreatment. Immunoreactivity for tight junction proteins, zonula occludens-1 and occludin, did not change in brain vessels of animals treated with PTZ and LPS plus PTZ. Glial fibrillary acidic protein immunoreactivity was increased in LPS, but not in PTZ and LPS plus PTZ. These results indicate that LPS pretreatment reduces the passage of EB dye bound to albumin into the brain, at least partly, by increasing plasma NO and IL-6 levels during PTZ-induced epileptic seizures. We suggest that LPS may provide protective effects on the BBB integrity during epileptic seizures through transcellular pathway, since the paracellular route remained unaffected by LPS and LPS plus PTZ.

Limbic seizures induce P-glycoprotein in rodent brain: functional implications for pharmacoresistance

The causes and mechanisms underlying multidrug resistance (MDR) in epilepsy are still elusive and may depend on inadequate drug concentration in crucial brain areas. We studied whether limbic seizures or anticonvulsant drug treatments in rodents enhance the brain expression of the MDR gene (mdr) encoding a permeability glycoprotein (P-gp) involved in MDR to various cancer chemotherapeutic agents. We also investigated whether changes in P-gp levels affect anticonvulsant drug concentrations in the brain. Mdr mRNA measured by RT-PCR increased by 85% on average in the mouse hippocampus 3-24 hr after kainic acid-induced limbic seizures, returning to control levels by 72 hr. Treatment with therapeutic doses of phenytoin or carbamazepine for 7 d did not change mdr mRNA expression in the mouse hippocampus 1-72 hr after the last drug administration. Six hours after seizures, the brain/plasma ratio of phenytoin was reduced by 30% and its extracellular concentration estimated by microdialysis was increased by twofold compared with control mice. Knock-out mice (mdr1a/b -/-) lacking P-gp protein showed a 46% increase in phenytoin concentrations in the hippocampus 1 and 4 hr after injection compared with wild-type mice. A significant 23% increase was found in the cerebellum at 1 hr and in the cortex at 4 hr. Carbamazepine concentrations were measurable in the hippocampus at 3 hr in mdr1a/b -/- mice, whereas they were undetectable at the same time interval in wild-type mice. In rats having spontaneous seizures 3 months after electrically induced status epilepticus, mdr1 mRNA levels were enhanced by 1.8-fold and fivefold on average in the hippocampus and entorhinal cortex, respectively. Thus, changes in P-gp mRNA levels occur in limbic areas after both acute and chronic epileptic activity. P-gp alterations significantly affect antiepileptic drugs concentrations in the brain, suggesting that seizure-induced mdr mRNA expression contributes to MDR in epilepsy.

Chronic nicotine pretreatment protects the blood-brain barrier against nicotine-induced seizures in the rat

This study was designed to investigate the possible protective actions of nicotine on cerebrovascular permeability in convulsions during nicotine-induced seizures. We have measured the permeability changes in the blood-brain barrier (BBB) macroscopically and spectrophotometrically by using Evans blue dye. Specific gravity measurements were also performed to assess brain edema which develops after blood-brain barrier opening. The experiments were carried out on Wistar rats. Rats were divided into two groups. They received acutely a convulsive dose of nicotine 3, 5, 8 and 9 mg kg(-1)i.p. or pretreated with a low dose of nicotine (0.8 mg kg(-1)i. p.) for 21 days followed by the procedure mentioned in the first group. Acute nicotine injection induced a significant increase in blood pressure and Evans-blue passage, despite a decline in specific gravity values. Low doses of chronic nicotine administration markedly reduced both the leakage of dye, and brain water content. Chronic treatment with low doses of nicotine (0.8 mg kg(-1)day(-1)s. c.) lessened the intensity of tonic-clonic seizures observed with a single dose of 3, 5, 8 or 9 mg kg(-1)nicotine. The data presented here demonstrate that nicotine pretreatment results in decreased sensitivity to nicotine-induced seizures in rats.

Interleukin-1beta immunoreactivity and microglia are enhanced in the rat hippocampus by focal kainate application: functional evidence for enhancement of electrographic seizures

Using immunocytochemistry and ELISA, we investigated the production of interleukin (IL)-1beta in the rat hippocampus after focal application of kainic acid inducing electroencephalographic (EEG) seizures and CA3 neuronal cell loss. Next, we studied whether EEG seizures per se induced IL-1beta and microglia changes in the hippocampus using bicuculline as a nonexcitotoxic convulsant agent. Finally, to address the functional role of this cytokine, we measured the effect of human recombinant (hr)IL-1beta on seizure activity as one marker of the response to kainate. Three and 24 hr after unilateral intrahippocampal application of 0.19 nmol of kainate, IL-1beta immunoreactivity was enhanced in glia in the injected and the contralateral hippocampi. At 24 hr, IL-1beta concentration increased by 16-fold (p < 0.01) in the injected hippocampus. Reactive microglia was enhanced with a pattern similar to IL-1beta immunoreactivity. Intrahippocampal application of 0.77 nmol of bicuculline methiodide, which induces EEG seizures but not cell loss, enhanced IL-1beta immunoreactivity and microglia, although to a less extent and for a shorter time compared with kainate. One nanogram of (hr)IL-1beta intrahippocampally injected 10 min before kainate enhanced by 226% the time spent in seizures (p < 0.01). This effect was blocked by coinjection of 1 microgram (hr)IL-1beta receptor antagonist or 0.1 ng of 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate, selective antagonists of IL-1beta and NMDA receptors, respectively. Thus, convulsant and/or excitotoxic stimuli increase the production of IL-1beta in microglia-like cells in the hippocampus. In addition, exogenous application of IL-1beta prolongs kainate-induced hippocampal EEG seizures by enhancing glutamatergic neurotransmission.

The presence of autoantibodies to N-terminus domain of GluR1 subunit of AMPA receptor in the blood serum of patients with epilepsy

We have analyzed the serum from patients with refractory epilepsy for the presence of autoreactive antibodies to AMPA glutamate receptor subunits. The presence and the level of autoantibodies were assessed using immunoblot and ELISA with synthetic peptides specific for subregions of AMPA glutamate receptor subunits. Patients with refractory epilepsy exhibited strong immunoreactivity to GluR1 subunit compared to healthy donors and patients with Parkinson's disease and Alzheimer's disease. Weak immunoreactivity to other AMPA glutamate receptor subunits was also detected and the signal was diminished in the raw GluR4>GluR3>GluR2. The occurrence of autoantibodies to specific neurotransmitter subunits in the sera of patients with refractory epilepsy suggest that autoimmune process may underlie this disorder.

Cell death, gliosis, and synaptic remodeling in the hippocampus of epileptic rats

Seizures set in motion complex molecular and morphological changes in vulnerable structures, such as the hippocampal complex. A number of these changes are responsible for neuronal death of CA3 and hilar cells, which involves necrotic and apoptotic mechanisms. In surviving dentate granule cells seizures induce an increased expression of tubulin subunits and microtubule-associated proteins, suggesting that an overproduction of tubulin polymers would lead to a remodeling of mossy fibers (the axons of granule cells). In fact, these fibers sprout in the dentate gyrus to innervate granule cell dendrites, creating recurrent excitatory circuits. In contrast, terminal mossy fibers do not sprout in the CA3 field. Navigation of mossy fiber's growth cones may be facilitated by astrocytes, which would exert differential effects by producing and excreting cell adhesion and substrate molecules. In the light of the results discussed here, we suggest that in adult brain activated-resident astrocytes (nonproliferating, tenascin-negative, neuronal cell-adhesion molecule-positive astrocytes) could contribute to the process of axonal outgrowth and synaptogenesis in the dentate gyrus, while proliferating astrocytes, tenascin-positive, could impede any axonal rearrangement in CA3.

Glial reaction after seizure induced hippocampal lesion: immunohistochemical characterization of proliferating glial cells

Kainic acid treatment (a model of temporal lobe epilepsy) induces Ammon's horn sclerosis, which is characterized by degeneration of CA3 pyramidal neurons and reactive gliosis. In the present study we have combined autoradiographic analysis of 3H-thymidine incorporation and immunocytochemistry to investigate this glial scarring phenomenon. The present results demonstrate that in the fields showing neuronal degeneration (i.e. CA3-CA4 fields of Ammon's horn and dentate hilus) the glial reaction consists of a proliferation and hypertrophy of astrocytes and microglia-macrophages. In the regions showing exclusively terminal axonal degeneration (i.e. the molecular layer of kainate-treated rats), glial cells do not proliferate but astrocytes show a transient hypertrophy. These results also demonstrate that oligodendrocytes do not proliferate in the hippocampus of kainate-treated rats. In agreement with our previous report we find that hippocampal astrocytes from kainate-treated rats express A2B5 immunoreactivity, a marker of type-2 astrocytes. A2B5 immunoreactivity was expressed by astrocytes not only in areas showing glial proliferation such as CA3-CA4 fields, but also in the molecular layer, where astrocytes do not proliferate. This suggests that in the CNS, normal resident astrocytes acquire the phenotypic properties of type-2 astrocytes.

Pretreatment with a competitive NMDA antagonist D-CPPene attenuates focal cerebral infarction and brain swelling in awake rats

The purpose of the study was to assess effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D-(E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (D-CPPene) upon focal cerebral infarction and brain oedema in the rat. Focal cerebral ischaemia was produced by permanent occlusion of the middle cerebral artery under halothane anaesthesia. The anaesthetic gas was discontinued immediately after the occlusion and the rats were killed 24 hours later. Cerebral infarction and brain swelling were each assessed on the frozen brain sections at 8 predetermined coronal planes. Pretreatment with D-CPPene (4.5 mg/kg i.v. followed by continuous infusion at 3 mg/kg/h until sacrifice) 15 minutes prior to MCA occlusion, significantly reduced the volume of infarction in the cerebral hemisphere by 29% (p < 0.05). Brain swelling, obtained by subtracting the nonischaemic hemispheric volume from the ischaemic hemispheric volume, was significantly reduced with D-CPPene treatment and the mean reduction in swelling (34% less than the controls: p < 0.001) proportionately similar to the decrease in infarct volume in the same animals. These data indicate that systemic administration of the competitive NMDA receptor antagonist D-CPPene has neuroprotective effects against ischaemic brain damage, and the reduction in brain swelling occurs in parallel with the reduction in ischaemic damage.

Intraventricular infusion of N-methyl-D-aspartate. 1. Acute blood-brain barrier consequences

The purpose of this study was to document the early cerebrovascular consequences of excessive N-methyl-D-aspartate (NMDA) receptor activation. Five microliters of NMDA (100 nmol/microliters) or vehicle was infused over a 15-min period into the lateral ventricle of adult rats. The protein tracer horseradish peroxidase (HRP) was injected intravenously for blood-brain barrier (BBB) studies. The intraventricular infusion of vehicle (n = 5) caused no alterations in arterial blood pressure or microvascular damage away from the intraventricular probe tract. In contrast, NMDA infusion (n = 8) led to a gradual increase in arterial blood pressure (mean 36 mm Hg). Multifocal regions of HRP extravasation were observed bilaterally throughout the neuraxis following NMDA infusion. Sites of BBB disruption and hemorrhage included brain regions bordering ventricular spaces. In addition, isolated foci of protein extravasation were commonly detected in the cerebral cortex, thalamus, basal forebrain, septum and cerebellum. Pretreatment with the noncompetitive NMDA antagonist MK-801 (2 mg/kg) substantially reduced the BBB responses to NMDA. However, microvascular abnormalities were seen in NMDA-infused rats where blood pressure elevations were inhibited by blood removal. In addition to neurons, cerebral blood vessels are also acutely affected by NMDA receptor activation. Blockage of NMDA receptor channels following brain injury may potentially provide protection by attenuating BBB breakdown and subsequent brain edema.

Role of neuroexcitation in development of blood-brain barrier and oedematous changes following cerebral ischaemia and traumatic brain injury

Potential involvement of neuroexcitatory mechanisms was studied in: 1) repetitive forebrain ischaemia in gerbils, 2) global cerebral ischaemia in rats and 3) cryogenic injury to the cerebral cortex in rats and gerbils. Uptake of 45Ca was used as a marker of injury, whereas ultrastructural localization of calcium was assessed with an oxalate-pyroantimonate method. The blood-brain barrier was evaluated with immunostaining for serum albumin. Changes in extracellular glutamate were estimated by microdialysis and an enzymatic cycling assay. Changes in water content were assessed by specific gravity measurements. Repetitive ischaemia of 3 x 5 min carotid occlusions produced a cumulative effect with regard to development of oedema and neuronal injury. This was associated with several-fold increments in glutamate release after repeated insults, whereas there was no apparent correlation with energy metabolism disturbances. Other studies revealed in all models a development of secondary foci distant to the primary impact of ischaemia or cold lesions, which were characterized by calcium accumulation in swollen dendrites, chronic neuronal changes and intraneuronal uptake of serum proteins, all of these changes being potentially compatible with involvement of neuroexcitatory mechanisms.

High intensity light exposure increases blood-brain barrier transport in rats

A quantitative technique utilizing 14C-alpha-aminoisobutyric acid as a tracer was used to study the blood-brain barrier permeability modifications induced by environmental stimuli, as the high intensity light exposure. The blood-to-brain transfer constant was significantly increased only in the occipital cortex following the treatment. The possibility that the blood-brain barrier is able to modify its local characteristics in response to cerebral activity changes was discussed.

Distant blood-brain barrier opening in subfields of the rat hippocampus after intrastriatal injections of kainic acid but not ibotenic acid

Blood-brain barrier (BBB) permeability towards proteins was determined in rats 4 h after intrastriatal kainic or ibotenic acid application, using Evans blue as indicator. Whereas, with the exception of the unspecific damage in cortex, after ibotenic acid BBB remained intact in deep brain areas. Evans blue leakage was found ipsilateral to the kainic acid injection, occasionally in striatum, thalamus and amygdala and regularly in hippocampus. There it was confined to the fimbria and the CA3 field. Only rarely a mirror focus-like staining was present in the contralateral hippocampus. The ultrastructural investigation revealed that BBB opening in CA3 is due to increased transendothelial pinocytosis; the tight junctions were intact. Thus, changes in the microenvironment around vessels, elicited by kainic acid and/or seizures, might be responsible for BBB opening.