doi:10.1093/brain/awl318Brain (2007), 130, 521–534
Blood–brain barrier leakage may lead to
progression of temporal lobe epilepsy
E. A. van Vliet,1,2S. da Costa Arau ´jo,2S. Redeker,3R. van Schaik,2E. Aronica3
and J. A. Gorter1,2
1Epilepsy Institute of The Netherlands (SEIN), Heemstede,2Swammerdam Institute for Life Sciences, Center for
Neuroscience and3Academic Medical Center, Department of (Neuro)Pathology, University of Amsterdam,
Amsterdam, The Netherlands
Correspondence to: Dr J. A. Gorter, Swammerdam Institute for Life Sciences, Center for Neuroscience, University of
Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The Netherlands
Leakage of the blood–brain barrier (BBB) is associated with various neurological disorders, including
temporal lobe epilepsy (TLE). However, it is not known whether alterations of the BBB occur during
epileptogenesis and whether this can affect progression of epilepsy. We used both human and rat epileptic
brain tissue and determined BBB permeability using various tracers and albumin immunocytochemistry.
In addition, we studied the possible consequences of BBB opening in the rat for the subsequent progression
of TLE. Albumin extravasation in human was prominent after status epilepticus (SE) in astrocytes and
neurons, and also in hippocampus of TLE patients. Similarly, albumin and tracers were found in microglia,
astrocytes and neurons of the rat. The BBB was permeable in rat limbic brain regions shortly after SE, but
also in the latent and chronic epileptic phase. BBB permeability was positively correlated to seizure frequency
in chronic epileptic rats. Artificial opening of the BBB by mannitol in the chronic epileptic phase induced a
persistent increase in the number of seizures in the majority of rats. These findings indicate that BBB leakage
occurs during epileptogenesis and the chronic epileptic phase and suggest that this can contribute to the
progression of epilepsy.
Keywords: albumin; seizure; fluorescein; Evans Blue; mannitol; status epilepticus
Abbreviations: BBB ¼ blood–brain barrier; FJB ¼ fluoro-jade B; TLE ¼ temporal lobe epilepsy; SE ¼ status epilepticus
Received June 23, 2006. Revised September 26, 2006. Accepted October 9, 2006. Advance Access publication November 22, 2006.
Due to its unique structure, the blood–brain barrier (BBB)
is capable of limiting the penetration of a variety of
substances from the blood into the brain. The BBB plays an
important role in the homeostasis and is generally seen as a
defence mechanism that protects the brain against various
molecules that may enter the BBB. The BBB is composed of
endothelial cells which form a diffusion barrier, due to
the presence of tight junctions that firmly connect
endothelial cells (Kettenmann and Ransom, 2005). In
addition to this, efflux transporters of the ATP binding
resistance-associated proteins), located at the luminal side
of endothelial cells, may restrict further entry of substances
into the brain. To provide the brain with essential nutrients
and remove excreted substances, endothelial cells also
contain numerous membrane transporters (for review see
Lee et al., 2001) involved in the influx/efflux of essential
substrates such as glucose, amino acids, electrolytes and
nucleosides or removal of xenobiotics.
Due to the development of small molecular weight tracers
that enter the damaged BBB, disruption was found to be
associated with various neurological disorders such as
migraine (Dreier et al., 2005), postconcussion syndrome
(Korn et al., 2005), multiple sclerosis (Minagar and
Alexander, 2003) and epilepsy (Roch et al., 2002; Ballabh
et al., 2004; Neuwelt, 2004; Seiffert et al., 2004). BBB
disruption has been shown both in human (Mihaly and
Bozoky, 1984) as well as in animal studies after acute
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seizures (Nitsch and Klatzo, 1983; Zucker et al., 1983;
Lassmann et al., 1984; Ruth, 1984; Saija et al., 1992; Pont
et al., 1995; Ilbay et al., 2003; Leroy et al., 2003; Oztas et al.,
2003) and has been associated with abnormal EEG patterns
(Tomkins et al., 2001; Korn et al., 2005; Pavlovsky et al.,
2005). Moreover, Friedman and colleagues showed that focal
opening of the BBB by direct cortical application of
albumin-containing solution can lead to the generation of
an epileptic focus in rats (Seiffert et al., 2004). However, it is
not known howthe BBB
epileptogenesis and whether alterations in BBB permeability
can contribute to spontaneous seizure progression. To get
more insight into the role of BBB disruption in epilepto-
genesis and progression of epilepsy, we determined BBB
permeability in epileptic rats and humans and studied the
possible consequences of a compromised BBB for the
subsequent seizure progression. Since previous studies have
compromised BBB function in a variety of pathophysiolo-
gical conditions (Cornford and Hyman, 1999; Seiffert et al.,
2004), we used albumin and albumin-binding dyes to
visualize BBB leakage.
Material and methods
Albumin immunoreactivity (IR) in human brain
To determine BBB permeability in human epileptic brain, albumin
extravasation was studied by immunocytochemistry. Brain material
was obtained from the files of the departments of neuropathology
of the Academic Medical Center (University of Amsterdam).
Patients underwent resection of the hippocampus (n = 6) for
medically intractable epilepsy. To reduce metabolic injury, the
fissural blood supply was kept intact until removal of the hippo-
campus. The surgical material is directly fixed after dissection and
therefore in optimal condition. In addition, autopsy material was
used of two epilepsy patients that died during an acute status
epilepticus (SE). These patients had a long history of epilepsy
(Table 1) and died before pharmacological treatment was started to
stop the SE. Pathological examination excluded encephalitis or
meningitis. This material was compared to normal-appearing
hippocampi of five autopsy specimens from patients without
history of seizures or other neurological diseases. Tissue was
obtained and used in a manner compliant with the Declaration of
Helsinki. Table 1 summarizes the clinical features of all patients.
Brain tissue was fixed in 10% buffered formalin, paraffin
embedded, sectioned at 6 mm and mounted on organosilane-
coated slides (Sigma, St Louis, MO, USA). Two hippocampal
sections of each patient were processed for immunocytochemistry.
Sections were deparaffinated in xylene, rinsed in ethanol (100, 95
and 70%) and incubated for 20 min in 0.3% hydrogen peroxide
diluted in methanol. Slides were then washed with phosphate-
buffered saline (PBS; 10 mM, pH 7.4) and incubated overnight in
anti-albumin (rabbit anti-human albumin; 1 : 20000; DakoCyto-
mation, Glostrup, Denmark) at 4?C. Hereafter, sections were
washed in PBS and stained with a polymer based peroxidase
immunocytochemistry detection kit (PowerVision Peroxidase
system, ImmunoVision, Brisbane, CA, USA). After washing,
sections were stained with 3,30-diaminobenzidine tetrahydrochloride
(50 mg DAB, Sigma-Aldrich, Zwijndrecht, The Netherlands) and 5
ml 30% hydrogen peroxide in a 10 ml solution of Tris–HCl. Sections
were counterstained with haematoxylin, dehydrated in alcohol and
xylene and coverslipped. Sections incubated without anti-albumin
or with preimmune serum were essentially blank.
To evaluate whether changes of BBB permeability occurred during
epileptogenesis, the SE rat model for temporal lobe epilepsy (TLE)
was used (Gorter et al., 2001).
Adult male Sprague–Dawley rats (Harlan CPB Laboratories,
Zeist, The Netherlands) weighing 400–550 g were housed
individually in a controlled environment (21 6 1?C; humidity
60%; lights on from 08:00 a.m. to 8:00 p.m.; food and water
available ad libitum). The study was approved by the University
Animal Welfare committee.
Rats were anaesthetized with ketamine (57 mg/kg; Alfasan,
Woerden, The Netherlands) and xylazine (9 mg/kg; Bayer AG,
Table 1 Summary of the clinical and neuropathological data of the patients with epilepsy
AEDs = antiepileptic drugs; CBZ = carbamazepine; CPS = complex partial seizures; HMEG = hemimegalencephaly; HS = hippocampal
sclerosis; LEV = levetiracetam; ND = pathology not defined at autopsy; PB = phenobarbital; PHT = phenytoin; SE = status epilepticus;
SGS = secondary generalized seizures; TLE = temporal lobe epilepsy; VPA = valproate; W = Wyler grading system.
522 Brain (2007), 130, 521–534E. A. van Vliet et al.
Leverkusen, Germany) and placed in a stereotactic frame. In order
to record hippocampal EEG, a pair of insulated stainless steel
electrodes (70 mm wire diameter, tips were 0.8 mm apart) were
implanted into the left dentate gyrus under electrophysiological
control as previously described (Gorter et al., 2001). A pair of
stimulation electrodes was implanted in the angular bundle.
Two weeks after electrode implantation, each rat was transferred to
a recording cage (40 · 40 · 80 cm3) and connected to a recording
and stimulation system (NeuroData Digital Stimulator, Cygnus
Technology Inc., Delaware Water Gap, NJ, USA) with a shielded
multi-strand cable and electrical swivel (Air Precision, Le Plessis
Robinson, France). A week after habituation to the new condition,
rats underwent tetanic stimulation (50 Hz) of the hippocampus in
the form of a succession of trains of pulses every 13 s. Each train
had a duration of 10 s and consisted of biphasic pulses
(pulse duration 0.5 ms, maximal intensity 500 mA). Stimulation
was stopped when the rats displayed sustained forelimb clonus
and salivation for minutes, which usually occurred within 1 h.
However, stimulation never lasted >90 min. Behaviour was
continuously monitored during electrical stimulation and several
hours thereafter. Immediately after termination of the stimulation,
periodic epileptiform discharges (PEDs) occurred at a frequency of
1–2 Hz and were accompanied by behavioural generalized seizures
and EEG seizures SE. The total PED duration was considered as the
total SE duration. In 34 rats a SE was electrically evoked, which
lasted from at least 3 h, up to 13 h. Electrode implanted control rats
were handled and recorded identically, but did not receive electrical
Differential EEG signals were amplified (10·) via a FET transistor
that connected the headset of the rat to a differential amplifier
(20·; CyberAmp, Axon Instruments, Burlingame, CA, USA),
filtered (1–60 Hz), and digitized by a computer. A seizure detection
program (Harmonie, Stellate Systems, Montreal, Canada) sampled
the incoming signal at a frequency of 200 Hz per channel. All EEG
recordings were visually screened and seizures were confirmed by
trained human observers. All rats were monitored continuously
from the SE onwards, until the first spontaneous seizure appeared.
Hereafter some rats were disconnected from the set-up. All rats
were connected again 4 months later and continuous EEG
recordings (24 h/day) were started to determine seizure frequency
and duration. As previously described (Gorter et al., 2001; van Vliet
et al., 2004), a stable baseline of seizure frequency is normally
reached in chronic epileptic rats at this time-point, and no seizure
clusters occur. Rats were monitored for at least 1 week and
experiments were not started before a stable baseline was reached.
Albumin IR in rat brain
Albumin extravasation in the rat was studied by fluorescent
albumin immunocytochemistry. A subset of free-floating sections
that was used in Evans Blue (EB) tracer experiments (see below)
were washed (2 · 10 min) in 0.05 M PBS and then incubated with
Glostrup, Denmark). After 24 h, the sections were washed in PBS
(3 · 10 min) and incubated for 1.5 h in anti-rabbit Alexa Fluor 488
(1 : 200, Molecular Probes). Following three additional washes in
PBS, sections were mounted on slides (Superfrost Plus, Menzel,
Braunschweig, Germany) and coverslipped with mounting medium
for fluorescence, containing 40,6-diamidino-2-phenylindole, which
labels cell nuclei (Vectashield with DAPI, Vector Laboratories,
Burlingame, CA, USA). Images were acquired using a confocal-laser
scanning microscope and Adobe Photoshop.
Quantification of BBB permeability
Since the detection of extravasated albumin by immunocytochem-
istry is not an accurate measurement to determine whether the BBB
was permeable at a specific time-point, additional experiments were
performed in rats. In order to quantify BBB permeability during
epileptogenesis and relate changes in permeability directly to
seizure activity, rats were injected with two different fluorescent
tracers that do not enter the brain under normal circumstances
(except for the circumventricular organs). In addition, these tracers
bind to albumin, so that a comparison could be made with
immunocytochemical data. Fluorescein (FSC) was used to quantify
BBB permeability microscopically, while EB was used in a limited
number of rats to detect BBB permeability macroscopically and to
confirm the distribution of BBB permeability microscopically as
detected by FSC. These tracers were intravenously (i.v.) adminis-
tered via the tail vein (EB, 50 mg/kg i.v., Sigma-Aldrich, Steinheim,
Germany; FSC; 100 mg/kg i.v., Merck, Darmstadt, Germany) under
isoflurane anaesthesia (4 vol%). EEG recordings were discontinued
during anaesthesia, which never lasted longer than several minutes.
Rats were injected in the acute seizure period (1 day after SE
induction; EB n = 2; FSC n = 5; and 2 days after SE; FSC n = 4), in
the latent period (1 week after SE; EB n = 1; FSC n = 5) and in the
chronic seizure period (4 months after SE; EB n = 2; FSC n = 7),
when rats display spontaneous seizures (average seizure frequency
0.3 seizures/h). In addition, electrode implanted control rats that
were not stimulated were included as well (EB n = 2; FSC n = 5).
Rats were disconnected from the EEG recording set-up 2 h after
tracer injection and deeply anaesthetized with pentobarbital
(Nembutal, intraperitoneally (i.p.), 60 mg/kg). The animals were
perfused through the ascending aorta with 100 ml of physiological
salt solution, followed by 300 ml 4% paraformaldehyde/0.2%
glutaraldehyde in 0.1 M phosphate buffer, pH 7.4. The brains were
post-fixed in situ overnight at 4?C, dissected and cryoprotected in
30% phosphate-buffered sucrose solution, pH 7.4. After overnight
incubation at 4?C, the brains were frozen in isopentane (?30?C)
and stored at ?80?C until sectioning. Sagittal sections (40 mm)
were cut using a sliding microtome. Sections were collected in 0.1 M
phosphate buffer and processed for immunocytochemistry.
Detection of EB and FSC
To detect extravasation of the fluorescent albumin-binding dyes EB
and FSC, sagittal sections were mounted on slides (Superfrost Plus,
Menzel, Braunschweig, Germany) and coverslipped with mounting
Burlingame, CA, USA). Tracers were detected using a confocal-
laser scanning microscope (Zeiss LSM510) with appropriate filter
settings (EB excitation 546 nm, emission 611 nm; FSC excitation
488 nm, emission 520 nm). Images were made using Zeiss software
(Zeiss LSM Image browser) and Adobe Photoshop. A quantifica-
tion of FSC sections was made for each rat using three different
sections: 2.4, 3.4 and 4.6 mm lateral to bregma (Paxinos and
Watson, 1998). Tracers were analysed in limbic brain regions that
BBB leakage during progression of epilepsyBrain (2007), 130, 521–534 523
are thought to be involved in the generation and/or spread of
seizure activity, and also in the cerebellum. The following brain
regions were analysed: hippocampus (granule cell layer), entorhinal
cortex (layer II/III), anterior piriform cortex (layer II/III), amygdala
(basolateral amygdala nucleus), thalamus (ventral postero-medial/
lateral nucleus) and cerebellum. The confocal grid (271 · 271 mm2,
15 · 15 squares) was placed on the selected brain region and the
number of squares that contained a FSC signal was counted. The
intensity of the FSC signal was evaluated using the histogram
function in Adobe Photoshop. The average signal intensity
measured in control rats (which was close to zero in all analysed
regions), was used for the background correction. We constructed a
‘permeability index’ (number of squares that contained a FSC
signal · FSC intensity) and all data were expressed as mean 6 SEM.
Statistical analysis on the permeability index was performed using
ANOVA, followed by the Student’s t-test. Differences with P < 0.05
were considered significant. A correlation between two ordinal
variables was calculated using a Spearman’s rank correlation test
(P < 0.05).
To confirm that EB and FSC bind to albumin and to determine
whether EB and FSC colocalized with specific cell types, double
labelling was performed on a subset of sections (at least two
sections/rat, 2.4, 3.4 and 4.6 mm lateral to bregma) with anti-
albumin (rabbit anti-albumin, 1:100, DakoCytomation, Glostrup,
Denmark), the microglial marker anti-OX-42 [mouse anti-rat
CD11b/c (OX-42), 1:100, PharMingen, CA, USA], the astrocytic
marker anti-glial fibrillary acidic protein (mouse anti-GFAP,
1:1000, DakoCytomation, Glostrup, Denmark) and the neuronal
marker anti-NeuN (mouse anti-NeuN, 1:1000, Chemicon, UK).
Free-floating sections were washed (2 · 10 min) in 0.05 M PBS,
followed by washing (1 · 60 min) in PBS + 0.4% bovine serum
albumin (BSA). BSA was omitted in all solutions for albumin
staining. Sections were then incubated in primary antibodies. After
24 h of incubation with the primary antibody, the sections were
washed in PBS (3 · 10 min) and incubated for 1.5 h in Alexa Fluor
568 (FSC sections; goat anti-mouse IgG, 1:200, Molecular Probes)
or Alexa Fluor 488 (EB sections; goat anti-mouse IgG Alexa, 1:200,
Molecular Probes). Following three additional washes in PBS,
sections were mounted on slides (Superfrost Plus, Menzel,
Braunschweig, Germany) and coverslipped with mounting medium
for fluorescence (Vectashield, Vector Laboratories, Burlingame, CA,
USA). Images were acquired using a confocal-laser scanning
microscope and Adobe Photoshop.
Fluoro-Jade B staining
To evaluate whether tracer/albumin containing cells were dege-
nerating cells, a Fluoro-Jade B (FJB) staining was performed as
described previously (Schmued and Hopkins, 2000) on a subset of
sections (at least two sections/rat, 2.4, 3.4 and 4.6 mm lateral to
bregma) of rats that were injected with EB. Sections were mounted
on coated slides (Superfrost Plus, Menzel, Braunschweig, Germany)
and dried overnight at room temperature. They were immersed in
absolute alcohol for 3 min. followed by 70% ethanol for 1 min. and
distilled water for 1 min. The slides were transferred to 0.06%
potassium permanganate for 15 min. After rinsing with distilled
water (1 min), the slides were transferred to a 0.001% polyanionic
FSC derivative solution (FJB, Histo-Chem Inc., Jefferson, AR, USA)
made in 0.1% acetic acid. Slides were rinsed in water, dried,
immersed in xylene and coverslipped with mountant for histology
(DPX, Sigma-Aldrich, Zwijndrecht, The Netherlands). Images were
acquired using a confocal-laser scanning microscope and Adobe
Artificial opening of the BBB
To investigate whether alterations in BBB permeability could
influence seizure activity, the BBB was opened with mannitol
(1.5 g/kg i.v., 25% solution, once daily for 3 consecutive days)
under isoflurane anaesthesia (4 vol%) in both control rats (n = 5)
and in chronic epileptic rats with a stable seizure frequency (n = 8).
EEG recordings were discontinued during anaesthesia, which never
lasted longer than several minutes. We confirmed that this protocol
resulted in BBB extravasation of FSC (data not shown) and that
short isoflurane anaesthesia combined with physiological salt
administration, does not influence daily seizure activity (van Vliet
et al., 2006). Rats were under continuous EEG monitoring and the
number of seizures and the seizure duration were evaluated
before, during and after mannitol treatment. Statistical analysis
was performed using the paired Student’s t-test. Differences with
P < 0.05 were considered significant.
Albumin IR in human and rat brain
Alterations in BBB permeability, resulting in albumin
extravasation, were detected using immunocytochemistry.
In the human hippocampus of autopsy controls (n = 5),
no albumin extravasation was observed (Fig. 1A and B).
In contrast, in resected hippocampi of patients with TLE
(n = 6) strong albumin IR was present in parenchyma
throughout the hippocampus, next to blood vessels (Fig. 1E).
Neurons and astrocytes located around these vessels were
also albumin positive (Fig. 1F). Most albumin extravasation
was observed in autopsy material of patients that had
died during SE (n = 2). Very strong albumin IR was seen
around all blood vessels within the hippocampus and
cortex (Fig. 1C). In addition, many neurons and astrocytes
were also highly immunoreactive (Fig. 1D). No albumin
extravasation was observed in the cerebellum of these
In control rats no albumin could be detected in limbic
brain regions (e.g. hippocampus, Fig. 2A). However, in the
acute (1–2 days after SE) and latent phase (1 week after SE)
albumin extravasation was evident in the hippocampus
(Fig. 2B and C), entorhinal cortex, piriform cortex, thalamus,
amygdala and olfactory bulb. In chronic epileptic rats
albumin was present, but not as widespread as acutely after
SE. Albumin was detected especially in the piriform cortex,
but the hippocampus (Fig. 2D), entorhinal cortex, thalamus
and amygdala were also immunoreactive for albumin.
BBB permeability during epileptogenesis
To assess leakage of the BBB, rats were sacrificed in the acute
seizure period (1–2 days after SE), in the latent period when
524Brain (2007), 130, 521–534E. A. van Vliet et al.
Fig. 1 Albumin immunocytochemistry in the human hippocampus. In autopsy control tissue (A and B) no albumin staining is present.
Most albumin extravasation was observed in autopsy material of patients that died during SE. Very strong albumin IR was seen around
all blood vessels within the hippocampus and cortex (C and D). In addition, many neurons (arrowheads in D) and astrocytes (arrows in D)
were also highly immunoreactive. In resected hippocampi from temporal lobe epilepsy patients strong albumin IR was present in parenchyma
throughout the hippocampus, next to blood vessels (arrows in E). Neurons (arrowheads in F) and astrocytes (black arrows in F)
were also albumin positive. The white arrow in F shows a blood vessel with albumin extravasation. Scale bar A, C and E = 800 mm, B, D
and F = 75 mm.
BBB leakage during progression of epilepsy Brain (2007), 130, 521–534 525
no seizures were observed (1 week after SE) and in the
chronic epileptic period (4 months after SE), which is
characterized by recurrent spontaneous seizures. The tracers
EB or FSC were injected at these different time points.
In control rats, no EB staining was observed macroscopically
(Fig. 3A, D and G) or microscopically (Fig. 4A) in the
analysed brain regions. The average FSC signal intensity was
close to zero in all analysed regions (Fig. 6F). Since the BBB
is not permeable to FSC in control rats these data were used
for the background correction.
In the acute seizure phase (1 day after SE), brains were
oedematous and EB extravasation was observed macro-
scopically in the hippocampus, entorhinal cortex, piriform
cortex, thalamus, septum and olfactory bulb (Fig. 3B, E and
H). In these brain regions EB was abundantly present
(Fig. 4B) in neurons, as shown by colocalization with the
Fig. 2 Albumin immunocytochemistry in the rat hippocampus. In control rats (A) albumin could not be detected in the dentate gyrus
of the hippocampus. Sections were counterstained with DAPI (blue) to visualize cells. In the acute (B) and latent phase (C), albumin
extravasation was evident throughout the dentate gyrus (green). In chronic epileptic rats (D) albumin was present in the dentate gyrus
(arrowheads), but not as widespread as acutely after SE. Inset in D shows high magnification of the hilus, containing albumin particles (green).
Scale bar = 100 mm, gcl = granule cell layer.
526 Brain (2007), 130, 521–534 E. A. van Vliet et al.
neuronal marker NeuN (Fig. 4K). In addition, EB colocalized
with the microglial marker OX-42 (Fig. 4E) and astrocytic
marker GFAP (Fig. 4H), although these cells were less often
observed compared with neurons that contained EB. No EB
staining was observed in the cerebellum. Immunostainings
with anti-albumin confirmed that EB containing cells
also contained albumin (Fig. 5A–C). A large number of
these cells colocalized with FJB (Fig. 5D–F), indicating
degeneration. However, some neurons contained EB, but no
FJB (arrowhead in Fig. 5F), which suggests that not all
albumin-containing cells die.
A very large increase of FSC staining was detected in most
limbic brain regions (Fig. 6G). One day after SE, the FSC
permeability index increased significantly compared to
controls in the parenchyma of the hippocampus, entorhinal
cortex, piriform cortex, thalamus and amygdala (Fig. 6A–E).
The BBB was still permeable, 2 days after SE, however, when
compared with 1 day SE, the FSC permeability index
decreased significantly 1.5–2 times in most analysed regions
(Fig. 6). The cerebellum did not contain FSC.
In the latent phase (1 week after SE), EB extravasation was
observed in the same brain regions as described at 1 day after
SE, however to a smaller extent (data not shown). Similarly,
the FSC permeability index decreased further (Fig. 6) and
was 3–4 times less compared with 2 days after SE in most
regions, except for the entorhinal cortex in which the most
intense FSC was observed (Fig. 6B). Compared to control
rats the FSC permeability index was still increased.
Chronic epileptic phase
In chronic epileptic rats (4 months after SE), EB staining was
macroscopically similar to control rats (Fig. 3C, F and I).
However, microscopic examination
increased EB staining, although to a much smaller extent
than in the acute seizure period. Immunostainings with anti-
albumin showed that EB colocalized with albumin in most
cases (Fig. 5G–I). However, some EB particles did not
colocalize with albumin (arrowheads in Fig. 5I). EB particles
were most frequently seen in layer III of the piriform cortex
(Fig. 4C), although the hippocampus, entorhinal cortex,
thalamus, and amygdala also contained some EB particles.
EB was mainly present in reactive microglial cells (Fig. 4F)
and in astrocytes that surrounded blood vessels (Fig. 4I).
Sparse labelling was found in neurons (Fig. 4L). No
colocalization was found with FJB (Fig. 5J–L). The
cerebellum did not contain EB.
The FSC permeability index was significantly increased
compared to control rats in the piriform cortex (Fig. 6C),
hippocampus, entorhinal cortex, thalamus and amygdala
(Fig. 6A, B, D and E). FSC was found as ‘green particles’
Fig. 3 Evans Blue (EB) extravasation during epileptogenesis. In control rats, no EB staining was observed macroscopically lateral view (A);
midsagittal cut (D); ventral view right half of the brain (G). In the acute seizure period, brains were oedematous and EB extravasation was
observed in the cerebellum. In chronic epileptic rats (C, F and I) EB staining was similar to control rats. However, an increased EB staining was
observed microscopically (Fig. 4). E=entorhinal cortex, O=olfactory bulb, S=septum, H=hippocampus, T=thalamus, P=piriform cortex.
BBB leakage during progression of epilepsyBrain (2007), 130, 521–534527
Fig. 4 Evans Blue (EB) extravasation during epileptogenesis. In control rats, no EB staining (red) was observed microscopically (A, D, G and
J). In the acute seizure period EB (red) was abundantly present (B). EB colocalized with the microglial marker OX-42 (E, arrowhead shows
activated microglia migrating towards EB particle) and astrocytic marker GFAP (H) and the neuronal marker NeuN (K). In chronic epileptic
rats increased EB staining was observed microscopically, mainly in layer III of the piriform cortex (C). Inset shows high power magnification.
EB colocalized with reactive microglial cells (F) and astrocytes that surrounded blood vessels (I). Sparse labeling was found in neurons (L).
Scale bar A–C = 100 mm, inset in C = 18 mm, D–F = 20 mm, G–I = 25 mm, J–L = 20 mm.
528Brain (2007), 130, 521–534 E. A. van Vliet et al.
Fig. 5 Colocalization of Albumin, Fluoro-Jade B and Evans Blue. Immunostainings with anti-albumin confirmed that EB containing cells also
contained albumin, both in the acute period (1 day after SE; A–C) as well as in chronic epileptic rats (4 months after SE; arrows in I).
However, not all EB particles colocalized with albumin in the chronic period (arrowheads in I). A large number of EB positive cells
colocalized with Fluoro-Jade B in the acute period (arrows in F), indicating degeneration. However, some neurons contained EB, but no
Fluoro-Jade B (arrowhead in F), which suggests that not all albumin containing cells die. In chronic epileptic rats most EB were found
in the piriform cortex (arrowheads in L). These particles did not colocalize with Fluoro-Jade B (arrows in L). Scale bar = 20 mm.
BBB leakage during progression of epilepsyBrain (2007), 130, 521–534529
with high fluorescence intensity close to cell nuclei. These
particles were most evident in rats with a high seizure
frequency. Similarly as has been shown for EB in Fig. 4A,
FSC was mainly present in reactive microglial cells, located
in layer III of the piriform cortex (data not shown). FSC
was also present in astrocytes and neurons in this
region, although less frequently compared with microglial
Seizure activity and changes of BBB
FSC presence in the piriform cortex (which had the highest
FSC permeability index), was related with the seizure
frequency (average number of seizures/hour of every chronic
epileptic rat during the week before sacrifice). Figure 7A
shows that a higher seizure frequency was related to a more
permeable BBB (Spearman’s rank, two tailed, P < 0.01,
r value = 0.92). We further investigated the permeability of
BBB in relation to the occurrence of the last seizure. For
each rat, the time between the last seizure and sacrifice was
calculated and compared with the permeability to FSC in the
piriform cortex. Figure 7B shows that the time span between
the last seizure and sacrifice was negatively correlated with
BBB permeability (Spearman’s rank, one tailed, P < 0.05,
r value = ?0.70). Out of the seven rats, three experienced a
seizure during the presence of the tracer (shaded area in
Fig. 7B). The other four rats experienced a seizure prior to
tracer injection. In two of these rats FSC was also present
and in the other two, in which the last seizure occurred at
least 1 day before sacrifice, no FSC signal was detected.
To determine whether changes in BBB permeability can
affect seizure activity, additional experiments were per-
formed. In order to artificially open the BBB, both control
and chronic epileptic rats were infused with mannitol.
Epileptic rats that had a relatively stable daily seizure
Fig. 6 Permeability index of fluorescein (FSC) during epileptogenesis in the hippocampus (A), entorhinal cortex (B), piriform cortex (C),
thalamus (D) and amygdala (E). F and G show typical examples of FSC staining (green) in the hippocampus of a control and a rat sacrificed in
the acute period. Sections were counterstained with DAPI (blue) to visualize cells. While no FSC is present in the control, abundant
parenchymal FSC staining is present 1 day after SE. The permeability index is expressed as mean 6 SEM. In the acute seizure period, a
tremendous increase of the FSC permeability index was detected in all limbic brain regions. In the latent period, the FSC permeability index
decreased and was three and four times less compared with 2 days after SE in most regions, except for the entorhinal cortex in which the
most intense FSC was observed (B). However, the FSC permeability index was still significantly increased compared to control rats. In
chronic epileptic rats the FSC permeability index was significantly increased in the piriform cortex (C), but also in the hippocampus,
entorhinal cortex, thalamus and amygdala (A, B, D and E). ‘*’ Indicates significant difference compared to control values, ANOVA followed
by the Student’s t-test (P < 0.05). ‘+’ Indicates significant difference compared with previous time-point (Student’s t-test, P < 0.05).
530Brain (2007), 130, 521–534E. A. van Vliet et al.
frequency (on average 4.2 6 0.6 seizures/day, n = 8) were
selected and the effect on seizure frequency was measured
during and after mannitol treatment. Mannitol did not
induce seizures in control rats. In contrast, the seizure
frequency was significantly enhanced during the 3-day
mannitol treatment in all chronic epileptic rats (compared
with pre-treatment values, paired Student’s t-test, P < 0.01;
Fig. 8). The seizure frequency was still significantly increased
in all rats during 3 days after mannitol treatment was
stopped. After the first 3 days, two groups of rats could be
distinguished on the basis of their seizure frequency. In the
majority of rats (n = 5) the seizure frequency increased
progressively over time (progressive; Fig. 8). In 3 out of 8
rats the seizure frequency returned to pre-treatment values
(non-progressive). Mannitol treatment did not affect seizure
duration (pre-treatment period, 60 6 5 s; mannitol
treatment, 61 6 4 s; after mannitol treatment was stopped,
58 6 3 s).
The main new findings of our study are: (i) opening of the
BBB is likely to occur after a single seizure in the chronic
epileptic phase; and (ii) transient opening of the BBB by
hyperosmotic treatment could aggravate epileptic seizures,
often in a persistent manner.
The BBB was disrupted in epileptic rats and humans, both
shortly after SE as well as in the chronic epileptic phase. In
rats, we showed that the BBB is not only open during a
seizure but remains open for at least 1 h thereafter.
Fig. 7 Relationship between seizure frequency and the FSC permeability index in the piriform cortex (A). A higher seizure frequency was
related to a more permeable BBB (Spearman’s rank, two tailed, P < 0.01, r value = 0.92). B shows the relationship between the occurrence
of the last seizure and the FSC permeability index in the piriform cortex. The time span between the last seizure and sacrifice was negatively
correlated with BBB permeability (Spearman’s rank, one tailed, P < 0.05, r value = ?0.70). The shaded area shows the presence of the tracer,
which was injected 2 h before sacrifice.
BBB leakage during progression of epilepsyBrain (2007), 130, 521–534 531
Nevertheless, rats that just experienced a seizure had a more
permeable BBB, indicating that the BBB is even more
open during a spontaneous seizure. Changes in BBB
integrity may be caused by increased local blood pressure
that occurs during seizures (Nitsch and Klatzo, 1983; Oztas
and Kaya, 1991), free radical formation (Oztas et al., 2001),
inflammatory responses such as leucocyte recruitment,
cytokine and interleukin production (Patel, 2004; Vezzani
and Granata, 2005; Gorter et al., 2006) and/or loss of tight
junction molecules (Ballabh et al., 2004).
One of the consequences of increased BBB permeability
is the accumulation of serum proteins that enter the brain,
which may contribute to increased excitability. It has been
shown recently that epileptiform activity can be induced by
direct cortical application of albumin-containing solution in
rats, suggesting that serum proteins play a role in the
pathogenesis of focal epilepsies (Seiffert et al., 2004).
Interestingly, albumin was found specifically in regions
with more EEG spiking activity in human (Cornford et al.,
1998) and induced calcium waves in rat cortical astrocytes
(Nadal et al., 1997). We showed a positive correlation
between BBB permeability and the occurrence of sponta-
neous seizures in chronic epileptic rats. The increased BBB
permeability was mainly observed in the piriform cortex,
which is considered to be highly epileptogenic (Loscher and
Ebert, 1996; Demir et al., 1999; Ebert et al., 2000). However,
several other limbic regions also showed BBB leakage,
although to lesser extent. This positive correlation and the
finding that artificial opening of the BBB by mannitol
increased seizure frequency in chronic epileptic rats add up
to the increasing evidence that BBB leakage could contribute
to the increased excitability or maintenance of the epileptic
condition. The fact that the rats do not experience seizures
during the latent period when the BBB leakage is many times
higher that during the chronic epileptic phase might be due
to other factors such as transient decreased expression of
genes related to synaptic transmission which has been
observed during the latent period (Gorter et al., 2006).
Moreover, leakage of the BBB early in the process of
epileptogenesis may not directly cause seizure activity as
suggested by the study of Seiffert et al. (2004), in which
increased excitability was not observed until 4–7 days after
In contrast, acute opening of the BBB in chronic epileptic
rats increased the seizure frequency in all rats. In the
majority of rats (62%) this led to a permanent and
progressive increase in the number of seizures later on.
Interestingly, one of the complications of BBB opening by
mannitol in humans to treat brain tumours is the occurrence
of seizures in 4–50% of the patients (Neuwelt et al., 1983,
1986; Roman-Goldstein et al., 1994). In rats, increased BBB
opening did not necessarily lead to a persistent seizure
progression, since in a subset of rats (38%) the number of
spontaneous seizures returned to pre-mannitol treatment
levels. The specific mechanisms that may induce increased
excitability after BBB disruption are not known, but it has
been hypothesized that slow processes regulated at the
transcriptional–translational level (triggered by the increased
concentration of proteins in the brain extracellular fluid),
activation of astrocytes, impaired potassium buffering and
the development of calcium waves in astrocytes may be a few
of the causes (Nadal et al., 1997; Seiffert et al., 2004).
Besides hyperexcitability, serum proteins may be involved
in neurodegeneration. Albumin was mainly found in
neurons in epileptic tissue of human as well as in the rat
shortly after SE, when cell death occurs (Gorter et al., 2003).
Many cells colocalized with FJB, indicating that these cells
were degenerating cells. Similarly, neuronal accumulation of
Fig. 8 Seizure frequency in chronic epileptic rats before, during and after mannitol treatment (mean 6 SEM of consecutive 3 day
periods). The seizure frequency increased significantly in all chronic epileptic rats during mannitol treatment (compared with
pre-treatment values, paired Student’s t-test, P < 0.05). Hereafter, two groups of rats could be distinguished on the basis of their
seizure frequency. In the majority of rats (n = 5) the seizure frequency increased progressively over time (progressive). In 3 out of
8 rats the seizure frequency returned to pre-treatment values (non-progressive). ‘*’ Indicates significant difference compared with
pre-treatment values (paired Student’s t-test P < 0.05).
532 Brain (2007), 130, 521–534 E. A. van Vliet et al.
associated with cytochrome c release, DNA fragmentation,
and cell death (Matz et al., 2001). Neuronal accumulation of
albumin has also been shown after induced seizures (Nitsch
and Klatzo, 1983; Sokrab et al., 1989) and ischemia (Loberg
et al., 1993, 1994).
In addition to the presence of albumin in neurons,
astrocytes also contained albumin, which is in agreement
with previous rat (Lassmann et al., 1984) and human studies
(Mihaly and Bozoky, 1984). The role of albumin-containing
astrocytes is unclear, but it has been hypothesized that
astrocytes may have a protective function by clearing
extravasated albumin from the extracellular space (Mihaly
and Bozoky, 1984).
Albumin deposits were also found in microglial cells in
the rat brain. Shortly after SE, they were abundantly present
in many limbic brain regions, while in the chronic epileptic
phase they were mainly restricted to layer II and III of the
piriform cortex. These cells are thought to be involved in
trapping serum-derived foreign substances that enter the
brain (Xu and Ling, 1994); especially in regions where BBB
integrity is affected. Interestingly, in those limbic regions we
previously reported increased expression of ferritin, an iron-
storage protein that may be induced by local extravasation of
blood and release of iron from haemoglobin-containing
blood cells (Gorter et al., 2005).
In conclusion, this study shows dynamic changes of BBB
increased permeability of the BBB was present in various
limbic regions and correlated to seizure activity suggesting
that it may contribute to increased excitability in the
epileptogenic foci that can lead to progression of epilepsy.
Given that our study suggests that a compromised BBB
could contribute to seizure development and progression of
epilepsy, the BBB may represent an adequate target to
intervene with epileptogenesis. In this respect, agents that
can alter BBB permeability are candidates for strategies in
order to control the progression of epilepsy.
proteinsafter intracerebral haemorrhagewas
We thank Prof. Dr F. H. Lopes da Silva and Prof.
Dr W. J. Wadman for critically reading the manuscript.
Part of this work was supported by the Epilepsy Institute of
The Netherlands. We would like to thank Dr W. G. M. Spliet
and Prof. Dr D. Troost (neuropathologists; Department of
(Neuro)Pathology of University Medical Center Utrecht and
University of Amsterdam) for the collaboration in the
collection of human material. E.A. and J.A.G. are supported
by the National Epilepsy Fund—‘Power of the Small’ and
Hersenstichting Nederland [NEF 02-10 and NEF 05-11
(E.A.) and 03-03 (J.G.)].
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