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Journal of Cerebral Blood Flow & Metabolism logoLink to Journal of Cerebral Blood Flow & Metabolism
. 2010 Nov 17;31(4):1119–1132. doi: 10.1038/jcbfm.2010.202

Chronic hyperperfusion and angiogenesis follow subacute hypoperfusion in the thalamus of rats with focal cerebral ischemia

Nick MEA Hayward 1,4, Pavel Yanev 1,2,4, Annakaisa Haapasalo 2, Riitta Miettinen 2,5, Mikko Hiltunen 2, Olli Gröhn 1, Jukka Jolkkonen 2,3,*
PMCID: PMC3070972  PMID: 21081957

Abstract

Cerebral blood flow (CBF) is disrupted after focal ischemia in rats. We examined long-term hemodynamic and cerebrovascular changes in the rat thalamus after focal cerebral ischemia. Cerebral blood flow quantified by arterial spin labeling magnetic resonance imaging was decreased in the ipsilateral and contralateral thalamus 2 days after cerebral ischemia. Partial thalamic CBF recovery occurred by day 7, then the ipsilateral thalamus was chronically hyperperfused at 30 days and 3 months compared with its contralateral side. This contrasted with permanent hypoperfusion in the ipsilateral cortex. Angiogenesis was indicated by endothelial cell (RECA-1) immunohistochemistry that showed increased blood vessel branching in the ipsilateral thalamus at the end of the 3-month follow-up. Only transient thalamic IgG extravasation was observed, indicating that the blood–brain barrier was intact after day 2. Angiogenesis was preceded by transiently altered expression levels of cadherin family adhesion molecules, cadherin-7, protocadherin-1, and protocadherin-17. In conclusion, thalamic pathology after focal cerebral ischemia involved long-term hemodynamic changes and angiogenesis preceded by altered expression of vascular adhesion factors. Postischemic angiogenesis in the thalamus represents a novel type of remote plasticity, which may support removal of necrotic brain tissue and aid functional recovery.

Keywords: angiogenesis, behavior, cerebral blood flow, focal cerebral ischemia, magnetic resonance imaging, thalamus

Introduction

The thalamus houses the somatosensory relay nuclei that process information flow between the spinal cord and somatosensory cortex. Because of its synaptic connections to the cortex, the thalamus is affected by focal cerebral ischemia (Nakane et al, 2002). In addition to delayed retrograde degeneration of thalamocortical neurons (Ross and Ebner, 1990), thalamic pathology involves activation of inflammatory processes (Block et al, 2005), impairment of calcium homeostasis (Watanabe et al, 1998; Mäkinen et al, 2008), and complex alterations in amyloid precursor protein processing leading eventually to amyloid-β deposition (van Groen et al, 2005; Hiltunen et al, 2009). The reliability of this system is essential for sensorimotor recovery after stroke (Staines et al, 2002), yet its hemodynamic response after focal cerebral ischemia in rats has, to our knowledge, not been studied.

Magnetic resonance imaging (MRI) is valuable for long-term hemodynamic studies of focal cerebral ischemia. Magnetic resonance imaging is noninvasive, thus the cerebral blood flow (CBF) changes in animals or patients can be quantified successively over extended periods. Dynamic susceptibility contrast MRI provides quantitative multislice mapping of relative cerebral blood volume and relative CBF. Arterial spin labeling (ASL) MRI provides absolute quantification of regional CBF commonly in a single slice of interest. Both techniques have been validated for the study of focal ischemia in rodents (Bratane et al, 2010). Arterial spin labeling (Jiang et al, 1998) and dynamic susceptibility contrast MRI (Rudin et al, 2001; Lin et al, 2008) have shown prolonged cortical and striatal hypoperfusion after permanent middle cerebral artery occlusion in rats, which is consistent with studies using autoradiography or laser Doppler flowmetry (Bolander et al, 1989; Borlongan et al, 2004; Li et al, 2007; Eve et al, 2009). Relatively few studies have looked beyond the first few days after injury and at regions outside the primary infarct, although knowledge of how the cerebral blood supply adapts long after ischemic injury may aid our understanding of brain recovery processes.

Here, we hypothesized that long-term hemodynamic and cerebrovascular changes occur in the thalamus after transient focal cerebral ischemia of rats in association with continuous neurodegenerative processes. We used ASL MRI to quantify thalamic CBF at baseline, at 2, 7, and 30 days, and 3 months after focal cerebral ischemia in rats. T2-weighted MRI described the final lesion size and location. Sensorimotor functions were examined and correlated with thalamic blood flow during the follow-up. Thalamic angiogenesis was studied by RECA-1 immunohistochemistry 3 months after cerebral ischemia, as regional CBF is known to correlate with microvessel density (Gross et al, 1986) and hemodynamic changes coincide with angiogenesis after ischemia in rats (Lin et al, 2002). In addition, we supposed that the expression of certain vascular growth factors and adhesion molecules is altered in the thalamus after cerebral ischemia, potentially contributing to angiogenesis. Thus, the expression levels of angiogenesis-related adhesion molecules of the cadherin family was studied over the same time period in another cohort of ischemic rats (Krishna and Redies, 2009).

Materials and methods

Animals

Seventy-eight male Wistar rats (3 to 4 months, 279 to 353 g) were used in the study. Rats were divided into three cohorts. The first (n=35) was used to study CBF, behavioral performance, lesion volume, and angiogenesis by immunohistochemistry. The second (n=24) was used to study blood–brain barrier (BBB) permeability. The third (n=29) was used to study the expression of genes related to thalamic angiogenesis. The animals had free access to food and water and were housed in individual cages in a temperature-controlled environment (20°C±1°C) with lights on from 0700 to 1900 hours. All animal procedures were approved by the Animal Ethics Committee (Hämeenlinna, Finland) and conducted in accordance with the guidelines set by the European Community Council Directives 86/609/EEC. All efforts were made to minimize the number of animals used and to ensure their welfare throughout.

Focal Cerebral Ischemia

Focal cerebral ischemia was induced using the intraluminal filament technique (Longa et al, 1989). Anesthesia was induced in a chamber using 5% isoflurane in 30% O2/70% N2O. A surgical depth of anesthesia was maintained throughout the operation with 0.5% to 1% isoflurane delivered through a nose mask. The right common carotid artery was exposed through a midline ventral cervical incision under a surgical microscope and carefully separated from the adjacent sympathetic nerves. The right common carotid artery and the internal carotid artery were then clamped with microvascular clips to prevent bleeding during insertion of the filament. The external carotid artery was ligated distally with a nylon suture, cut with microscissors, and electrocoagulated. The heparinized intraluminal filament (∅ 0.25 mm, rounded tip) was inserted into the stump of the external carotid artery and advanced 1.9 to 2.1 cm into the internal carotid artery until resistance was felt. The filament was held in place by tightening a suture around the internal carotid artery and placing a microvascular clip around the artery. Body temperature was monitored and maintained at 37°C using a heating pad connected to a rectal probe (Harvard Homeothermic Blanket System, Harvard Apparatus, Holliston, MA, USA). After 90 minutes of occlusion, the filament was removed and the external carotid artery was permanently closed by electrocoagulation. To relieve postoperative pain, rats were treated with 0.03 mg/kg of buprenorfin (intraperitoneally). In addition, postoperative care of ischemic rats included supplemental 0.9% NaCl (intraperitoneally) and softened food pellets to prevent weight loss. All rats had a severe sensorimotor impairment and corticostriatal damage based on limb-placing test on postoperative day 2.

Cerebral Blood Flow Measurements by Magnetic Resonance Imaging

All MRI were performed using a 4.7 T scanner interfaced with a Varian Inova console (Varian, Palo Alto, CA, USA) with an actively decoupled linear volume transmission coil (length=80 mm) and quadrature surface receiver coil pair (Rapid Biomedical, Rimpar, Germany), which provided imaging coverage of the whole brain. Imaging was performed at baseline, 2, 7, and 30 days, and 3 months after focal cerebral ischemia or sham operation. Regions of interest included the ipsilateral perilesional cortex, contralateral cortex, and ipsilateral and contralateral thalamus. Outlines of the ipsilateral regions of interests are shown in Figure 1.

Figure 1.

Figure 1

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T2-weighted coronal slices (A–E) and corresponding cerebral blood flow (CBF) maps (F–J) from a representative rat acquired at baseline (A, F), 2 days (B, G), 7 days (C, H), 30 days (D, I), and 3 months (E, J) after focal cerebral ischemia. Abbreviations for ipsilateral regions of interest: C, cortex; T, thalamus. The scale bar applies to all CBF maps.

Rats were anesthetized under isoflurane in a carrier gas mixture of 70% N2O, 30% O2. Rats were secured in a holder (Rapid Biomedical) using ear bars and a bite bar, while anesthesia was delivered through a nose cone. Breath rate was continually measured throughout imaging via a pressure probe between the rat and the holder. The breathing rate was kept between 62 and 68 breaths/min by adjusting the anesthesia concentration to 0.7% to 1.7%. The holder was electronically heated to sustain a temperature of 37°C throughout imaging.

Anatomical T2-weighted images were acquired using a spin-echo sequence whereby the time to echo=70 milliseconds, repetition time=2,500 milliseconds, and field of view=4 × 4 cm2 covered with 128 × 256 points. Data sets comprised of 15 stacked slices (thickness=1 mm each). The center of the stack was positioned at −3.2 mm from the bregma with the help of axial pilot images and with the aid of a rat brain atlas (Paxinos and Watson, 1998). The estimated slice positioning error between repeated scans was <50% of the slice thickness. The total anatomical imaging time was 12 minutes.

Absolute CBF was quantified using continuous ASL (Williams et al, 1992), with a fast spin-echo read out with a field of view=4 × 4 cm2, 128 × 128 points, slice thickness=2 mm, repetition time=6 seconds, echo spacing=7 milliseconds, and number of echoes=16. The duration and amplitude of the square labeling pulse causing flow driven adiabatic inversion were 3 seconds and 0.1 G, respectively, with a postlabeling delay of 800 milliseconds. The labeling pulse was positioned on the neck 2 cm from the imaging slice and the control image was acquired with an identical radiofrequency pulse positioned symmetrically opposite the imaging slice. Subtraction images from six pairs of label and control images were averaged, and used to provide a coronal CBF map from one thalamic slice, which was equivalent to the center of the T2-weighted image stack. As CBF measured by ASL is influenced by T1 variations in tissue, T1 was mapped in the same coronal slice as CBF using an inversion recovery fast spin-echo sequence (repetition time=4 seconds, echo spacing=13 milliseconds, 4 to 8 echoes/excitation, field of view=4 × 4 cm2 with 64 × 64 points, slice thickness=2 mm, incremented inversion times=5, 300, 600, 1,000, and 1,500 milliseconds). The total ASL imaging time was 25 minutes.

Behavioral Outcome Measures in Rats After Focal Cerebral Ischemia

On postoperative day 2, behavioral deficit of rats was assessed by using a modified limb-placing test. Only animals with a score <10 and thus severe corticostriatal damage (Puurunen et al, 2001) were included in the study. The test has seven limb-placing tasks, which assess forelimb and hindlimb responses to tactile and proprioceptive stimulation. Tactile stimulation was elicited by contacting the limb being tested with a table surface and proprioceptive stimulation elicited by pulling down the limb being tested (i.e., limb position in space).

The tapered/ledged beam and cylinder tests selected for the study are sensitive to detect long-term impairment in sensorimotor functions. The animals were tested before operation and on postoperative days 7, 14, 21, 30, and at 3 months. All behavioral analyses were performed in a blinded manner.

Sensorimotor functions of hindlimbs were tested using a tapered/ledged beam (Zhao et al, 2005). The rats were pretrained for 3 days to traverse the beam before ischemia induction. The beam-walking apparatus consists of a tapered beam with underhanging ledges on each side to permit foot faults without falling. The end of the beam is connected to a black box (20.5 × 25 × 25 cm3) with a platform at the starting point. A bright light is placed above the start point to motivate the rats to traverse the beam. Each rat's performance was videotaped and later analyzed by calculating the slip ratio for the impaired (contralateral to lesion) forelimb and hindlimb. The more slips indicate a greater degree of impairment. Steps onto the ledge were scored as a full slip, and a half slip was given if the limb touched the side of the beam. Slip ratio was calculated as: ((number of full slips+½ × number of half slips)/(number of total steps)) × 100%. The mean of three trials was used for the statistical analysis.

The cylinder test was used to assess imbalance between the impaired and nonimpaired forelimb use (Karhunen et al, 2003). For the test, the rat was placed in a transparent cylinder (∅ 20 cm) and videotaped during the light part of the light/dark cycle. A mirror was placed at 45° angle beneath the cylinder so that behavior could be filmed from below the cylinder. Exploratory activity for 1 to 3 minutes was analyzed by using a video recorder with slow motion capabilities. The number of contacts by both forelimbs and by either impaired or unimpaired forelimb was counted. A cylinder score for impaired forelimb was calculated as: ((contralateral contacts+½ bilateral contacts)/(total contacts)) × 100%.

RECA-1 Immunohistochemistry and Quantification of Angiogenesis by Stereological Analysis

Rats were perfused transcardially at 3 months after surgery with 0.9% NaCl followed by 4% paraformaldehyde in 0.1 mol/L phosphate buffer, pH 7.4. The brains were removed from the skulls, postfixed, and cryoprotected. Frozen sections (35 μm) were cut with a microtome and stored in a cryoprotectant tissue-collection solution at −20°C.

Double immunostaining for endothelial cell antibody (RECA-1) and neuron-specific nuclear protein (NeuN) was applied as a marker for blood vessels and to visualize loss of neurons in the thalamus. The sections were washed in phosphate buffer three times and blocked in 5% normal goat serum after which the sections were transferred to a solution containing the primary antibody (NeuN at 1:4,000, Millipore, Temecula, CA, USA) and Tris-buffered saline (TBS) with 5% normal goat serum and 0.5% Triton X-100 (TBS-T). Following 18 hours of incubation in this solution on a shaker table at room temperature (20°C) in the dark, the sections were rinsed three times with TBS-T and transferred to a solution containing the secondary antibody (goat anti-mouse*biotin at 1:1,000, Vector Laboratories, Burlingame, CA, USA). After 2 hours, the sections were rinsed three times with TBS-T and transferred to a solution containing mouse ExtrAvidin (Sigma, St Louis, MO, USA) for 1 hour and then rinsed in TBS-T and incubated for ∼4 to 6 minutes with Ni-enhanced diaminobenzidine (blue). Then, the staining protocol was repeated with antibody for RECA-1 (1:2,000, AbD Serotec, Oxford, UK). Immunostaining for RECA-1 was visualized with diaminobenzidine (brown).

A point-counting method was implemented to estimate the density of blood vessels in the RECA-1 and NeuN double-stained sections. Sections were systematically sampled with 150 μm interval from 1-in-5 series using a fixed starting point. The part of the thalamus, which corresponds to the MRI region of interest was included in the analysis. Brain sections were examined under a light microscope (BX50, Olympus, Tokyo, Japan) fitted with the stereological image analysis system, equipped with a motorized stage controller and a camera (HVC20A, Hitachi, Kokusai Electric, Japan). The analysis was performed with the aid of the Stereo Investigator software (Version 2006, MicroBrightField, Williston, VT, USA). The instrumentation was calibrated before each series of measurements.

Briefly, the area of the ipsilateral and contralateral thalamus was outlined under a low magnification ( × 2.5), and the outlined region was measured with a systematic random design of dissector counting frames. A sampling grid of 400 × 400 μm2 was laid on the section. A three-dimensional probe consisting of the counting frame of 65 × 65 μm2 (an ‘optical dissector') with height (z axis) of 10 μm was focused through a known depth of the section to estimate the total number of vascular branch hit points. Counting was performed throughout the section depth, according to point-counting rules as described previously (West et al, 1991). A × 40 PlanApo oil immersion objective having a 1.4 numerical aperture was used in the analyses. The total number of branching points was estimated using the formula: Ntot=∑Q × 1/ssf × 1/asf × 1/tsf, where section sampling fraction (ssf) is 1/5, area sampling fraction (asf; the area of counting frame, 4225.00 divided by the area of sampling grid, 160,000) is 0.03, and tissue sampling fraction (tsf) is 1.76. For statistical analysis, the branching point counts per ipsilateral and contralateral thalamus from all sections from each subject were combined with the help of Stereo Investigator system (Version 2006, MicroBrightField). In order to make an estimate of the branching point density in the thalamus, we used the formula NV=Ntot/VSN. Here, NV is the numerical density and VSN is the volume of the analyzed tissue. The variability within groups was assessed via the coefficient of error. Shrinkage in the ipsilateral thalamus was assessed by comparing the area of ipsilateral thalamus and contralateral thalamus from three sections.

Quantifying Blood–Brain Barrier Leakage

IgG leakage from serum into the brain was assessed as a marker of the BBB integrity. Rats were perfused transcardially at 2 days to 3 months after surgery with 0.9% NaCl followed by 4% paraformaldehyde in phosphate buffer. The brains were removed from the skulls, postfixed, and cryoprotected. Frozen sections (35 μm) were cut with a microtome and stored in a cryoprotectant tissue-collection solution at −20°C.

The sections were washed in phosphate buffer three times after which they were incubated in 1% hydrogen peroxide for 15 minutes. Then the sections were blocked in 2% normal goat serum. This was followed by incubation with biotinylated sheep anti-rat IgG (1:200, AbD Serotec, Oxford, UK) for 48 hours at 4°C. The sections were washed with TBS-T and transferred to a solution containing mouse ExtrAvidin (1:1,000, Sigma, St Louis, MO, USA) for 1 hour and then incubated for ∼4 to 6 minutes with diaminobenzidine.

Images ( × 1.25 magnification) in the ipsilateral and contalateral thalamus were acquired using an Olympus BX40 microscope and Olympus digital camera DP50-CU (Japan) plus image acquisition software Viewfinder (Pixera Corporation, San Jose, CA, USA). ImageJ (NIH) software was used for image analysis. All images were converted to gray scale and then the entire thalamus was outlined to provide a mean gray scale value for statistical analysis.

Expression of Adhesion Molecules and Vascular Endothelial Growth Factor in the Thalamus After Focal Cerebral Ischemia

The ipsilateral and contralateral thalamus was dissected and frozen immediately on dry ice and stored in −70°C. Samples were mechanically homogenized in cold phosphate-buffered saline (Lonza, Basel, Switzerland). Total protein contents were extracted in TPER tissue extraction buffer (Pierce, Rockford, IL, USA) containing protease and phosphatase inhibitors (Pierce) and the protein concentrations were determined by using a BCA protein assay kit (Pierce). A measure of 30 μg of proteins were subjected to 4% to 12% Bis-Tris polyacrylamide gel electrophoresis (Invitrogen, Carlsbad, CA, USA) under reducing conditions and blotted on polyvinylidene fluoride membranes (Hybond-P; GE Healthcare, Uppsala, Sweden). The membranes were blocked in 5% nonfat dry milk in TBS, pH 7.4, containing 0.1% Tween-20 (TBS-T) for 1 hour and incubated overnight at 4°C with the following antibodies: rabbit anti-cadherin-7 (1:200; Santa Cruz, Santa Cruz, CA, USA); mouse anti-protocadherin-1 (PCDH1) (1:1,000; Santa Cruz); rabbit anti-PCDH17 (1:200; Santa Cruz); and rabbit anti-vascular endothelial growth factor (anti-VEGF) (1:1,000; Abcam, Cambridge, MA, USA). Appropriate horse-radish peroxidase-conjugated secondary antibodies (GE Healthcare) and enhanced chemiluminescence were used to visualize the proteins using ImageQuant RT ECL imager (GE Healthcare).

Western blot images were quantified using Quantity One software (Bio-Rad, Hercules, CA, USA). The levels of each protein were normalized to the levels of glyceraldehyde 3-phosphate dehydrogenase in the same samples. The data are shown as expression levels of each protein in the ipsilateral thalamus as percentage of the levels in contralateral thalamus.

Statistical Analyses

SPSS software (Version 14.0) was used for all statistical analyses. Cerebral blood flow, beam walking, and cylinder data were analyzed for the overall group effect using analysis of variance (ANOVA) for repeated measures. Comparisons between groups or ipsilateral and contralateral regions were then made using unpaired or paired Student's t-tests, respectively. The number of blood vessel branching points were compared between ipsilateral and contralateral sides of the thalamus using a paired t-test. For BBB breakdown studies by IgG immunoreactivity, one-way ANOVA followed by post hoc multiple comparisons (Bonferroni) were used to find differences over time. For gene expression data, one-way ANOVA was used to find differences between groups followed by post hoc multiple comparisons (Newman–Keuls' test) for differences between the ipsilateral and contralateral thalamus. All correlations are presented as Pearson's two-way correlations. The level of statistical significance was P<0.05 throughout this study.

Results

Mortality and General Pathology After Focal Cerebral Ischemia

For the rats used to study CBF, behavior, and angiogenesis, 8/26 ischemic rats did not survive within 72 hours. This corresponds to a mortality rate of 31%. Only rats surviving the entire follow-up period were included in the analyses. The size of the lesion was 122.3±19.2 mm3 (mean±s.e.m.) as measured 3 months after ischemia by T2-weighted lesion volumetry MRI. A typical infarct comprised extensive cortical and striatal damage and also variable partial damage to the hypothalamus and amygdala outside the vascular territory of the middle cerebral artery. The thalamus was spared from acute ischemic damage because of blood supply through the posterior cerebral artery. However, mediodorsal, submedial, ventrolateral, and ventromedial thalamic nuclei showed delayed secondary damage depending on the size and location of cortical infarct. The thalamic reticular nucleus was also affected due to loss of GABAergic input from the globus pallidus (Dihné et al, 2002). These together resulted in severe shrinkage of the ipsilateral thalamus in ischemic rats (31%, P<0.05).

Temporal Profile of Cerebral Blood Flow Changes in the Rat Thalamus After Focal Cerebral Ischemia

We quantified absolute CBF in the thalamus by ASL-MRI (Figures 1 and 2A). The flow data were analyzed by ANOVA for repeated measures and there was a significant overall group effect (P<0.05). In addition, there was a significant group × time × hemispheric interaction (P<0.001), indicating that CBF was different over time between groups and also between ipsilateral and contralateral sides of the thalamus. A more detailed analysis (Figure 2A) showed that CBF was decreased compared with respective values in sham-operated rats in the ipsilateral (73% of sham mean, P<0.001) and contralateral (64% of sham mean, P<0.001) thalamus 2 days after ischemia. In addition, the ipsilateral CBF in ischemic rats was greater than that contralaterally (120% of contralateral mean, P<0.01). Partial recovery of thalamic CBF occurred by day 7 both ipsilaterally and contralaterally. Still, perfusion in ischemic rats at day 7 was not comparable to sham-operated measures and the thalamus remained hypoperfused in this regard ipsilaterally (88% of sham mean, P<0.05) and contralaterally (77% of sham mean, P<0.01). Greater ipsilateral perfusion compared with contralateral perfusion in ischemic rats still prevailed at day 7 (116% of contralateral mean, P<0.001).

Figure 2.

Figure 2

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Absolute cerebral blood flow (CBF) in the (A) thalamus and (B) cortex at baseline and at 2, 7, and 30 days and 3 months after focal cerebral ischemia (Ischemic, n=18) or sham operation (Sham, n=9). Cerebral blood flow was quantified by arterial spin labeling (ASL). Values are presented as mean±standard error of the mean (s.e.m.). Ipsilateral versus contralateral thalamus in ischemic rats (###P<0.001, ##P<0.01, #P<0.05) and ipsilateral thalamus in ischemic rats versus sham (***P<0.001, **P<0.01, *P<0.05) are the comparisons presented for each time point.

At chronic time points, day 30 and 3 months, the ipsilateral thalamus was hyperperfused compared with its contralateral side in ischemic animals at day 30 (117% of contralateral mean, P<0.001) and at 3 months (118% of contralateral mean, P<0.001).

Cortical flow data differed significantly over time (P<0.05, ANOVA) and there was a significant group × time × hemispheric interaction (P<0.01), indicating that CBF was different over time between groups and also between the ipsilateral and contralateral cortex (Figure 2B). Specifically, bilateral hypoperfusion was observed at 2 days after ischemia, as compared with shams (73% to 80% of sham mean, P<0.01). The ipsilateral cortex, which was variably affected between ischemic rats, on average remained hypoperfused with respect to sham control measures and to the contralateral side for the study duration. Contralateral cortical CBF recovered by day 7 and subsequently remained comparable to control measures.

In addition, there were perfusion changes in the hippocampus and amygdala (data not shown). The hippocampus was hypoperfused at day 2 both ipsilaterally and contralaterally (∼55% of the sham value, P<0.001). This bilateral hypoperfusion recovered partially over the next 3 months but remained at the end of the study (∼74% to 86% of the sham value, P<0.001) and was more pronounced on the ipsilateral side (P<0.05). The amygdala was hypoperfused at day 2 and day 7 (between 67% and 79% of the sham value, P<0.001) both ipsilaterally and contralaterally. Contralaterally, CBF had recovered to sham measures by day 30. Ipsilaterally, CBF remained decreased chronically at 3 months (40% of the sham value, P<0.001). The ipsilateral amygdala was part of the lesion in the filament model.

Angiogenesis in the Thalamus After Focal Cerebral Ischemia

Angiogenesis in the ipsilateral thalamus was studied at the end of the study period (3 months) by RECA-1 immunohistochemistry and counting the vessel branching points (Figure 3). The density of vessels was increased in the ipsilateral thalamus of ischemic rats throughout the mediodorsal, submedial, ventrolateral, ventromedial, and very often the paracentral and oval paracentral thalamic nuclei (Figures 3E and 3G). Quantitative analysis showed that the number of vessel branching points in ischemic rats was significantly higher in the ipsilateral thalamus compared with the contralateral thalamus (141% of contralateral mean, P<0.001; Figure 3B). There was no such difference in sham-operated animals.

Figure 3.

Figure 3

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(A) RECA-1 and NeuN double-labeled coronal section from a rat subjected to focal cerebral ischemia. (B) Ratio±standard error of the mean (s.e.m.) of small blood vessel branching points between the ipsilateral and contralateral thalamus in ischemic rats (Ischemic, n=13) and sham-operated rats (Sham, n=6). Statistical significance between the ipsilateral and contralateral thalamus; ***P<0.001. (C) Three-dimensional representation of the actual hits (blood vessel branching points) counted with the fractionators probe. (D–G) Digital photomicrographs demonstrating the distribution and density of blood vessels in the contralateral (D, F) and the ipsilateral (E, G) thalamus 3 months after cerebral ischemia. Please note the parallel increases in the number of vessels and in neuronal loss in the ipsilateral side. The high background staining in the ipsilateral thalamus is due to degenerative processes. Scale bar=100 μm.

The number of vessel branching points in the ipsilateral thalamus of ischemic rats did not correlate with infarct size assessed at the end of the follow-up (r=−0.128) or ipsilateral CBF in the thalamus (r=0.427) assessed 2 days after ischemia. There were only occasional correlations between blood vessel branching points and CBF during the 3-month follow-up.

Transient Thalamic Blood–Brain Barrier Damage After Focal Cerebral Ischemia

To study whether BBB damage had occurred and could affect other results, IgG immunoreactivity was stained from additional ischemic animals (Figure 4). Semiquantitative analysis showed a diffuse pattern of IgG immunoreactivity in the ipsilateral thalamus at day 2 after cerebral ischemia. This may be related to reperfusion damage to existing vasculature and is in line with the BBB leakage in this model (Belayev et al, 1996).

Figure 4.

Figure 4

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Digital photomicrographs of IgG-stained sections used to assess the blood–brain barrier (BBB) damage in (A) a sham-operated rat and (B) a rat 2 days after focal cerebral ischemia. (C) Semiquantitative analysis of IgG immunoreactivity in the ipsilateral thalamus after cerebral ischemia. Data are expressed as mean±standard error of the mean (s.e.m.), n=4 to 6. Statistical significances; **P<0.01 (compared with sham group), #P<0.05 (compared with 3 months ischemic group). Scale bar=1 mm.

Behavioral Performance and Its Correlations with Cerebral Blood Flow, Angiogenesis, and Lesion Size After Focal Cerebral Ischemia

Sham-operated rats and ischemic rats performed similarly in the cylinder test before operation (Figure 5A). When the data were analyzed by ANOVA for repeated measures, there was a significant overall group effect (P<0.001). That is, ischemic rats were more impaired than sham-operated rats. There was also a significant group by day interaction (P<0.01), showing that the initial severe deficit recovered in ischemic rats. Indeed, forelimb impairment was more severe in ischemic rats compared with shams on day 7 (P<0.01) but not significantly different from the sham-operated mean from day 14 onwards.

Figure 5.

Figure 5

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Sensorimotor recovery after focal cerebral ischemia or sham operation in rats. (A) Contralateral forelimb use is measured in the limb asymmetry (cylinder) test at baseline and at 7, 14, 21, and 30 days and 3 months after cerebral ischemia (Ischemic, n=18) or sham-operation (Sham, n=9). (B) Hindlimb performance is measured by the tapered/ledged beam-walking test at baseline and at 7, 14, 21, and 30 days and 3 months after cerebral ischemia (n=18) or sham-operation (n=9). Mean values±standard error of the mean (s.e.m.) are presented. Statistical significance between ischemic and sham-operated rats; ***P<0.001, **P<0.01.

When beam-walking data were analyzed by ANOVA for repeated measures, there was a significant overall group effect (P<0.001) and a significant group by day interaction (P<0.001; Figure 5B). Behavioral performance of ischemic rats was different from sham-operated rats on all postoperative days (P<0.001), which means that no spontaneous recovery occurred.

Beam-walking performance in ischemic rats correlated with lesion size in all postoperative days (Table 1); therefore, the lesion size is the major determinant of impairment. Correlations between the use of the impaired forelimb in the cylinder test and lesion size were less pronounced and possibly various restorative mechanisms contribute to spontaneous recovery. Only occasional correlations were found between behavioral impairment and the number of vessel branching points in the ipsilateral thalamus or CBF in ischemic rats (data not shown). However, there was a temporal association between cylinder data and CBF in the ipsilateral thalamus, that is, initial hypoperfusion and severe sensorimotor impairment was followed by hyperperfusion and recovery of forelimb function.

Table 1. Correlations between lesion size and behavioral scores after focal cerebral ischemia.

  Time after ischemia
  7 days 14 days 21 days 30 days 3 months
Beam-walking test 0.532* 0.671** 0.603** 0.750** 0.534*
Cylinder test −0.434 −0.574* −0.225 −0.387 −0.492*
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Pearson correlation (n=18): *P<0.05; **P<0.01.

Expression of Adhesion Molecules and Vascular Endothelial Growth Factor in the Thalamus After Focal Cerebral Ischemia

It has been previously shown that expression of several cadherin superfamily members peaks during vascular development (Krishna and Redies, 2009). Thus, expression levels of cadherin-7, PCDH1, PCDH17, and VEGF were examined in the ipsilateral and contralateral thalamus of ischemic and sham-operated rats at 2, 7, and 30 days after cerebral ischemia by Western blotting. Cadherin-7, PCDH1, PCDH17, and VEGF were found to be expressed in both the ipsilateral and contralateral thalamus after cerebral ischemia (Figures 6A–6D). These proteins were also expressed in the thalamus of the sham-operated control animals. No major differences were observed in the levels of the proteins between the ipsilateral and contralateral thalamus (ipsilateral levels ≈100% of contralateral levels) at any of the time points in the sham-operated animals (Figures 6A–6D). In contrast, in the ischemic animals, the levels of cadherin-7 were significantly increased by ∼60% at 2 days after cerebral ischemia in the ipsilateral thalamus as compared with the contralateral thalamus. The levels in the ipsilateral thalamus remained elevated at 7 days after cerebral ischemia and returned to similar levels to those in the contralateral thalamus by 30 days after cerebral ischemia (Figures 6A and 6E). PCDH1 levels were upregulated by ∼20% between 7 days and 30 days after cerebral ischemia in the ipsilateral thalamus as compared with the contralateral side (Figures 6B and 6E). In contrast to cadherin-7 or PCDH1, the levels of PCDH17 decreased at 7- and 30-day time points in the ipsilateral thalamus when compared with the contralateral thalamus (Figures 6C and 6E). Furthermore, while there were no differences in the levels of VEGF between the ipsilateral and contralateral thalamus at 2 days after cerebral ischemia, VEGF levels were significantly diminished by ∼70% in the ipsilateral thalamus at 7 days. The levels remained decreased also at the 30-day time point by ∼40% (Figures 6D and 6E). There were no longer differences in the expression levels of any of the studied proteins between the ipsilateral and contralateral thalamus at the end of the 3-month follow-up (data not shown), suggesting that these expressional changes are mostly related to events taking place shortly after the ischemic insult.

Figure 6.

Figure 6

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Expression levels of cadherin family adhesion molecules and vascular endothelial growth factor (VEGF) in the thalamus after focal cerebral ischemia. Western blots showing the expression levels of cadherin-7 (A), protocadherin-1 (PCDH1; B), protocadherin-17 (PCDH17; C), and VEGF (D) at 2, 7, and 30 days (d) after cerebral ischemia in the ipsilateral (Ipsi) and contralateral (Contra) thalamus of ischemic (Ischemic) or sham-operated (Sham) rats. The levels of these proteins did not significantly differ between the ipsilateral and contralateral thalamus in the sham-operated control animals at any of the time points. Molecular weights are shown as kilodaltons (kDa) on the right. (E) Quantification of the expression levels of cadherin family members and VEGF in the thalamus after cerebral ischemia. The data are shown as expression levels of each protein (normalized to glyceraldehyde 3-phosphate dehydrogenase (GAPDH) levels) in the ipsilateral thalamus as percentage of the levels in the contralateral thalamus±standard error of the mean (s.e.m.), n=3 to 7. Statistical significance between ipsilateral and contralateral thalamus; **P<0.01, *P<0.05.

Discussion

Most rodent studies of hemodynamics after cerebral ischemia have focused on the acute period and primarily the periinfarct regions. To our knowledge, we report for the first time initial bilateral hypoperfusion in the thalamus after focal cerebral ischemia followed by long-term ipsilateral hyperperfusion. An increase in blood vessel branching points showed thalamic angiogenesis, which could contribute to chronic hyperperfusion. Enhanced expression of vascular adhesion factors, cadherin-7, and PCDH1 at 2 to 7 days after ischemia preceded angiogenesis in the thalamus.

Hypoperfusion at the Acute Phase in the Cortex and Thalamus After Focal Cerebral Ischemia

We observed bilateral hypoperfusion in the cortex and thalamus up to day 7 after cerebral ischemia. It is noteworthy that early hypoperfusion did recover except in the ipsilateral cortex. Consistent with the present data, a reduction in CBF in the striatum and periinfarct cortex has been shown by autoradiography at 7 days after permanent middle cerebral artery occlusion in rats (Bolander et al, 1989). This hypoperfusion resolved by 28 days after ischemia, which may be due to the establishment of collateral blood supply. More recently, laser Doppler flowmetry has shown that periinfarct hypoperfusion prevails for several weeks after permanent (Eve et al, 2009) and transient (Borlongan et al, 2004) middle cerebral artery occlusion in rats. Dynamic susceptibility contrast MRI has shown that hypoperfusion prevails for at least 3 days in the ipsilateral striatum (Rudin et al, 2001) and up to 14 days in the ipsilateral cortex (Lin et al, 2002). After transient focal cerebral ischemia in rats, ASL showed that acute hypoperfusion normalized by 3 hours (Jiang et al, 1998), which contrasts with our findings perhaps due to a milder ischemia severity.

The precise mechanisms for subacute hypoperfusion in our study remain uncertain. Previous studies in focal cerebral ischemia models have shown that CBF is not affected in the thalamus during occlusion (Tsuchidate et al, 1997). Thus, cytotoxic and vasogenic edema (Nordborg et al, 1994), and BBB breakdown (Belayev et al, 1996) that occur after transient focal cerebral ischemia in rats may contribute to hypoperfusion (Lin et al, 2002). We also observed slight BBB breakdown in the ipsilateral thalamus at 2 days after cerebral ischemia, which together with extensive edema (Nordborg et al, 1994) elevates intracranial pressure and reduces cerebral perfusion pressure. As described before (Nordborg et al, 1994), we also observed midline shift in ischemic rats by anatomical MRI at day 2, which resolved by day 7 and could explain the extensive subacute contralateral hypoperfusion we observed. Also, remote metabolic depression could be responsible for hypoperfusion and this has been shown within 1 week after focal ischemia (Watanabe et al, 1998; Barbelivien et al, 2002). In stroke patients, thalamic hypometabolism is thought to result from depressed synaptic activity (diaschisis) due to acute structural damage in the infarcted cortex (Binkofski et al, 1996).

Delayed Hyperperfusion Coincides with Angiogenesis in the Thalamus After Focal Cerebral Ischemia

Early hypoperfusion in the thalamus after focal cerebral ischemia was followed by recovery in the ipsilateral thalamus at 30 days and 3 months. Interestingly, we also observed angiogenesis in the ipsilateral thalamus at 3 months after ischemia. This agrees with a recent report of ipsilateral thalamic angiogenesis at 14 days after permanent middle cerebral artery occlusion in stroke-prone hypertensive rats (Ling et al, 2009).

Regional CBF has long been known to correlate with microvessel density (Gross et al, 1986), suggesting that increased thalamic blood flow is at least partly due to increased blood vessel density. This has been proposed for the cortex during 2 weeks after permanent ischemia in rats (Lin et al, 2002). Also, it is likely that the prolonged hypoperfusion in the thalamus at day 2 and day 7 after cerebral ischemia leads to hypoxia that promotes or induces angiogenesis (Hermann and Zechariah, 2009).

The chronic vascular changes we observed may be related to functional improvement. New vasculature created by angiogenesis at the ischemic border becomes evident at 2 to 4 days after focal ischemia in rats (Hermann and Zechariah, 2009) and correlates with survival in ischemic stroke patients (Krupinski et al, 1994), suggesting that it is a natural recovery process in periinfarct regions. Chronically, hyperperfusion occurs when vasoactive agents such as nitric oxide or adenosine arise from the endothelium or glia (Iadecola and Nedergaard, 2007) and promote CBF increase to serve cellular processes with high energy demands. These may include removal of neuronal debris by macrophages after neurodegeneration (Manoonkitiwongsa et al, 2001).

Altered Expression of Cadherin Family Adhesion Molecules and Vascular Endothelial Growth Factor Precedes Angiogenesis After Focal Cerebral Ischemia

We monitored the expression levels of different cadherin superfamily members and VEGF in the thalamus after cerebral ischemia. Cadherin-7 and PCDH1 levels peaked at 2 to 7 days after ischemia and returned to baseline in the ipsilateral thalamus by the end of the 3-month follow-up in ischemic rats. The peak in the expression levels of these molecules in the ipsilateral thalamus took place within 1 week after cerebral ischemia and preceded the angiogenesis and hyperperfusion we observed. Furthermore, the decline in the expression levels of PCDH17 also took place within the same time frame, even though these changes persisted until 30 days after ischemia. The notion that alterations in the expression of these molecules occur after focal cerebral ischemia and precede angiogenesis is consistent with the previously suggested role of cadherin family adhesion molecules during brain vascular development (Krishna and Redies, 2009). Interestingly, angiogenesis is mediated by VEGF in periinfarct regions in ischemic rats (Cai et al, 2009), but we could not confirm this in the thalamus. In fact, a decrease in VEGF levels was observed at 7 to 30 days in the ipsilateral thalamus after cerebral ischemia. Clearly, these data suggest that different mechanisms are involved in angiogenesis in the cortex and thalamus after cerebral ischemia in rats. In summary, our data show that altered expression of certain adhesion molecules and VEGF takes place in the ipsilateral thalamus shortly after the ischemic insult and that these alterations precede the observed angiogenesis in the thalamus.

Methodological Considerations

Arterial spin labeling MRI provides noninvasive, absolute quantification of CBF. This allows consecutive imaging of patients or individual animals, thus longitudinal studies of regional hemodynamic changes are feasible. These advantages, combined with better spatial resolution, make ASL-MRI an improvement over Doppler methods for regional CBF quantification.

Repetitive anesthesia is required for follow-up MRI studies of rats. A combination of isoflurane and nitrous oxide, which is a commonly used anesthesia, can increase baseline blood flow (Iltis et al, 2005). Also, 1% isoflurane dosed for 10 minutes invokes transient BBB opening in the thalamus of cats (Tetrault et al, 2008) and this likely occurs in rats. In our study, the dosage of anesthesia was consistently moderated to be minimal by measuring the breathing rate throughout imaging, and all results comparisons were made with sham-operated rats undergoing the same procedures. The measurement of blood-derived physiological variables such as pH, pO2, and pCO2 would provide additional support to a proposal of physiological normality during MRI, yet this has been successfully achieved during longer (90 minutes) imaging sessions in our previous studies under similar conditions (Hayward et al, 2010). Further, catheter insertion and removal would interfere with the behavioral performance tests.

Arterial spin labeling-based CBF quantification has been well validated in rodent models of focal ischemia (Bratane et al, 2010). However, we note that CBF calculation depends on tissue T1 and the blood–brain partition coefficient. Cerebral blood flow values were calculated voxel wise with absolute T1 values mapped in individual animals. It is possible that λ increases from 0.9 to 0.99 in edematous tissue, which creates a maximum 9% uncertainty for CBF (Herscovitch and Raichle, 1985).

Functional and Therapeutic Implications

Sensory, motor, and cognitive pathways are organized and integrated within the thalamus (Briggs and Usrey, 2008). Thus, damage to the thalamus or to its projections has widespread functional consequences. Initial hypoperfusion in the thalamus after focal cerebral ischemia is likely related to systemic metabolic depression (Watanabe et al, 1998), retrograde excitotoxicity (Ross and Ebner, 1990), and extensive edema (Nordborg et al, 1994). These may trigger angiogenesis and chronic hyperperfusion to support the removal of necrotic brain tissue (Manoonkitiwongsa et al, 2001) and aid repair processes. To what extent CBF recovery and angiogenesis promote behavioral improvement after cerebral ischemia is difficult to assess due to the complex pathology in the thalamus (Hiltunen et al, 2009) and multiple mechanisms underlying functional recovery (Witte, 1998). However, there was a temporal association between CBF in the ipsilateral thalamus and cylinder data, that is, initial hypoperfusion and severe sensorimotor impairment was followed by hyperperfusion and recovery of the forelimb function. In addition, the recent data by Zhang et al (2010) showed that attenuation of the thalamic pathology is related to improved sensory functions, again emphasizing the importance of the thalamus in behavioral recovery. Thus, promoting therapeutic angiogenesis may offer a novel treatment target for ischemia-induced secondary pathology and in turn improve functional outcome.

In conclusion, the thalamic response to focal ischemia in rats involves long-term hemodynamic changes and angiogenesis. These changes may be partially mediated by altered expression of different vascular adhesion factors, even though more studies are required to fully understand the molecular mechanisms underlying these events. However, postischemic angiogenesis in the thalamus represents a novel form of remote plasticity, which may support sensorimotor recovery. Noninvasive characterization of such processes may provide recovery biomarkers and assist therapeutic approaches in the future.

Acknowledgments

The authors thank Petra Mäkinen and Nanna Huuskonen for their excellent technical assistance.

The authors declare no conflict of interest.

Footnotes

This study was supported by a Marie Curie Early Stage Trainee Research Fellowship MEST-CT-2005-019217 (NH, OG), the Health Research Council of the Academy of Finland (OG, AH, MH), the Nordic Centre of Excellence in Neurodegeneration (MH), the Finnish Funding Agency for Technology, and Innovation grant 70048/09 (MH).

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