Whey Protein and Its Components Lactalbumin and Lactoferrin Affect Energy Balance and Protect against Stroke Onset and Renal Damage in Salt-Loaded, High-Fat Fed Male Spontaneously Hypertensive Stroke-Prone Rats

BACKGROUND

Whey protein (WH)-enriched diets are reported to aid in weight loss and to improve cardiovascular health. However, the bioactive components in whey responsible for causing such effects remain unidentified.

OBJECTIVE

We determined the effects of whey and its components [α-lactalbumin (LA) and lactoferrin (LF)] on energy balance, glucose tolerance, gut hormones, renal damage, and stroke onset in rats.

METHODS

Male spontaneously hypertensive stroke-prone (SHRSP) rats (age 8 wk) were fed isocaloric high-fat (40% kcal) and high-salt (4% wt/wt) diets (n = 8-10/group) and randomized for 8 wk to diets enriched as follows: control (CO): 15% kcal from egg albumin, 45% kcal from carbohydrate; WH: 20%kcal WH isolate + 15% kcal egg albumin; LA: 20% kcal LA  + 15% kcal egg albumin; or LF: 20% kcal lactoferrin + 15% kcal egg albumin. Measurements included energy balance (food intake, energy expenditure, and body composition), stroke-related behaviors, brain imaging, glucose tolerance, metabolic hormones, and tissue markers of renal damage. Data were analyzed by linear mixed models with repeated measures or 1-way ANOVA.

RESULTS

Diets enriched with WH, LA, or LF increased survival, with 25% of rats fed these diets exhibiting stroke-associated morbidity, whereas 90% of CO rats were morbid by 8 wk (P < 0.05). The nephritis scores of rats fed WH-, LA-, or LF-enriched diets were 80%, 92%, and 122% lower than those of COs (P = 0.001). The mRNA abundances of renin and osteopontin were 100-600% lower in rats fed WH-, LA-, or LF-enriched diets than in COs (P < 0.05). Urine albumin concentrations and albumin-to-creatinine ratios were 200% lower in rats fed LF-enriched diets than in COs (P < 0.05). Compared with COs, rats fed LF-enriched diets for 2-3 wk had food intake decreased by 29%, body weight decreased by 13-19%, lean mass decreased by 12-19%, and fat mass decreased by 20% (P < 0.001). Relative to COs, rats fed WH and LA had food intake decreased by 10% (P < 0.1), but COs had 12-45% lower weight than rats fed LA- and WH-enriched diets by 3 wk (P < 0.01). Compared with COs, rats fed WH-enriched diets increased energy expenditure by 7%, whereas, rats fed LA-enriched diets had energy expenditure acutely decreased by 7% during the first 4 d, and rats fed LF-enriched diets had energy expenditure decreased by 7-17% throughout the first week ( P < 0.001). Rats fed LA- and LF-enriched diets had blood glucose decreased by 14-19% (P < 0.05) and WH by 9% (P = 0.1), relative to COs. Compared with COs, rats fed LF had GIP decreased by 90% and PYY by 87% (P < 0.05).

CONCLUSION

Together, these findings indicate that whey and its components α-lactalbumin and lactoferrin improved energy balance and glycemic control, and protected against the onset of neurological deficits associated with stroke and renal damage in male SHRSP rats.

Intercellular Conduction Optimizes Arterial Network Function and Conserves Blood Flow Homeostasis During Cerebrovascular Challenges

OBJECTIVE

Cerebral arterial networks match blood flow delivery with neural activity. Neurovascular response begins with a stimulus and a focal change in vessel diameter, which by themselves is inconsequential to blood flow magnitude, until they spread and alter the contractile status of neighboring arterial segments. We sought to define the mechanisms underlying integrated vascular behavior and considered the role of intercellular electrical signaling in this phenomenon. Approach and Results: Electron microscopic and histochemical analysis revealed the structural coupling of cerebrovascular cells and the expression of gap junctional subunits at the cell interfaces, enabling intercellular signaling among vascular cells. Indeed, robust vasomotor conduction was detected in human and mice cerebral arteries after focal vessel stimulation: a response attributed to endothelial gap junctional communication, as its genetic alteration attenuated this behavior. Conducted responses were observed to ascend from the penetrating arterioles, influencing the contractile status of cortical surface vessels, in a simulated model of cerebral arterial network. Ascending responses recognized in vivo after whisker stimulation were significantly attenuated in mice with altered endothelial gap junctional signaling confirming that gap junctional communication drives integrated vessel responses. The diminishment in vascular communication also impaired the critical ability of the cerebral vasculature to maintain blood flow homeostasis and hence tissue viability after stroke.

CONCLUSIONS

Our findings highlight the integral role of intercellular electrical signaling in transcribing focal stimuli into coordinated changes in cerebrovascular contractile activity and expose, a hitherto unknown mechanism for flow regulation after stroke.

Magnetic resonance imaging of ischemic injury produced by varying severities of photothrombosis differs in neonatal and adult brain

Stroke is a major cause of disability in adults and children. Recently, we have developed an adult rat model of minor stroke containing a peri-infarct region with a modest T₂ increase and mild ischemic damage. We hypothesized that a neonatal minor stroke with mild peri-ischemic changes could also be produced, but with potential ontogenic differences. Using our minor photothrombosis method, we produced a range of severities of ischemic lesions (mini, minor, moderate and severe) within magnetic resonance imaging (MRI) slices of adult and neonatal rats. In both age groups, the lesion region showed a marked increase in T₂ and diffusion-weighted intensity and decrease in apparent diffusion coefficient (ADC), corresponding to a cortical infarct detected using fluorojade and hematoxylin and eosin staining. Perilesional regions showed modest increases in T₂ and ADC in adults, but not neonates, and this corresponded to scattered cell death, but not necessarily extravasation of plasma protein, i.e. blood-brain barrier disruption. Mini and minor insults in neonates generally showed homogeneous and rather modest changes in T₂ and ADC. MR perfusion maps demonstrated a penumbral area of greater hypoperfusion in adults compared with neonates. Together, the results indicate that, in neonatal cortex, a similar severity of photothrombosis occurs throughout the area of photoactivation, whereas, in adult brain, spontaneous clot lysis and/or partial thrombosis occurs adjacent to permanently occluded vessels. Thus, by comparing differing severities of photothrombotic ischemia in neonates and adults, ontogenic differences were detectable using MRI, with mature brain having a greater penumbral region. Mild ischemic injury and scattered cell death in both neonates and adults could be identified by a modest increase in T₂ and decrease in ADC. A better understanding of the effects of development on ischemic responses and associated MRI changes will provide a basis for the improved diagnosis of mild or minor ischemic insults relevant to pediatric and adult stroke.

Combined damage produced by multiple mild cerebral insults assessed using MRI in neonatal rats

PURPOSE

To determine whether damage to neonatal brain is exacerbated with multiple mild cerebral insults as detected with MRI and corroborated using histology.

MATERIALS AND METHODS

The combined brain injury produced by multiple procedures was compared in neonatal rats having: Sham surgery at P5, Sham surgery at P5 plus a diffuse mild transient unilateral cerebral hypoxia ischemia (HI) at P7, HI alone, and a minor photothrombotic (PT) stroke at P5 followed by HI. MRI after the ischemic insults was followed by final histology.

RESULTS

PT produced lesions with increased T₂ and decreased apparent diffusion coefficient for water (ADC) but no significant effects of a second HI. However, near the PT lesion/parietal cortex there were patchy areas of enhanced T₂ and decreased ADC in 6/9, 3/8 and 0/8 animals in the PT+HI, Sham+HI and HI groups, respectively (P<0.05). Patches corresponded histologically to increased vacuolation and cell death and were more pronounced in the PT+HI and Sham+PT groups than the HI group.

CONCLUSION

The extent of damage produced by a minor neonatal stroke followed by a diffuse HI two days later results in heterogeneous enhancement of T₂, ADC and histological injury near the lesion. Surgical procedures including mechanical head manipulation followed by HI also produced some enhanced heterogeneity of hypoxic-ischemic injury affirming the need for sham controls.

Recurrent mild cerebral ischemia: enhanced brain injury following acute compared to subacute recurrence in the rat

BACKGROUND

In the current study, a transient cerebral ischemia producing selective cell death was designated a mild ischemic insult. A comparable insult in humans is a transient ischemic attack (TIA) that is associated with functional recovery but can have imaging evidence of minor ischemic damage including cerebral atrophy. A TIA also predicts a high risk for early recurrence of a stroke or TIA and thus multiple ischemic insults are not uncommon. Not well understood is what the effect of differing recovery times between mild ischemic insults has on their pathophysiology. We investigated whether cumulative brain damage would differ if recurrence of a mild ischemic insult occurred at 1 or 3 days after a first insult.

RESULTS

A transient episode of middle cerebral artery occlusion via microclip was produced to elicit mild ischemic changes-predominantly scattered necrosis. This was followed 1 or 3 days later by a repeat of the same insult. Brain damage assessed histologically 7 days later was substantially greater in the 1 day recurrent group than the 3 days recurrent group, with areas of damage consisting predominantly of regions of incomplete infarction and pannecrosis in the 1 day group but predominantly regions of selective necrosis and smaller areas of incomplete infarction in the 3 days group (P < 0.05). Enhanced injury was reflected by greater number of cells staining for macrophages/microglia with ED1 and greater alterations in GFAP staining of reactive astrocytes in the 1 day than 3 days recurrent groups. The differential susceptibility to injury did not correspond to higher levels of injurious factors present at the time of the second insult such as BBB disruption or increased cytokines (tumor necrosis factor). Microglial activation, with potential for some beneficial effects, appeared greater at 3 days than 1 day. Also blood analysis demonstrated changes that included an acute increase in granulocytes and decrease in platelets at 1 day compared to 3 days post transient ischemia.

CONCLUSIONS

Dynamic changes in multiple inflammatory responses likely contribute to the time dependence of the extent of damage produced by recurrent mild ischemic insults. The time of mild stroke recurrence is crucial with early recurrence producing greater damage than subacute recurrence and this supports urgency for determining and implementing optimal stroke management directly after a TIA.

Very Mild Hypothermia (35°C) Postischemia Reduces Infarct Volume and Blood/Brain Barrier Breakdown Following tPA Treatment in the Mouse

Reperfusion therapies for stroke diminish in effectiveness and safety as time to treatment increases. Hypothermia neuroprotection for stroke is established, but its clinical translation has been hampered by uncertainties regarding optimal temperature and complications associated with moderate hypothermia. Also, hypothermia targeting temperatures of 32-33°C is associated with clinical and logistical problems related to induction and adverse side effects. We hypothesized that ischemic damage and tPA-exacerbated blood/brain barrier (BBB) breakdown produced following 30 minutes of middle cerebral artery occlusion and either 1 hour of saline or tPA infusion would be reduced by treatment with very mild cooling of 1.5°C for 48 hours followed by 24 hours of gradual rewarming. Infarct volume was reduced by 29.6% (p<0.001) and 41.9% (p<0.001) in hypothermic-tPA (Hypo_tPA)-treated and hypothermic-saline (Hypo_Sal)-treated animals compared to normothermic-tPA (Norm_tPA) and saline (Norm_Sal)-treated animals, respectively. Hypothermia also reduced IgG extravasation in tPA-treated, but not saline-treated groups compared to their normothermic controls (p<0.001). The ipsilateral-contralateral changes in optical density for IgG extravasation were 18.4% greater in the Norm_tPA than Norm_Sal (p<0.001) group. The ipsilateral-contralateral changes in optical density for IgG extravasation were reduced by 17.8% (p<0.001) in the Hypo_tPA compared to Norm_tPA group. No significant mean difference in IgG extravasation was seen between Hypo_tPA and Hypo_Sal groups (p>0.05). Very modest hypothermia to reduce the BBB breakdown could improve the availability and safety of reperfusion treatments for stroke.

Cellular correlates of longitudinal diffusion tensor imaging of axonal degeneration following hypoxic-ischemic cerebral infarction in neonatal rats

Ischemically damaged brain can be accompanied by secondary degeneration of associated axonal connections e.g. Wallerian degeneration. Diffusion tensor imaging (DTI) is widely used to investigate axonal injury but the cellular correlates of many of the degenerative changes remain speculative. We investigated the relationship of DTI of directly damaged cerebral cortex and secondary axonal degeneration in the cerebral peduncle with cellular alterations in pan-axonal neurofilament staining, myelination, reactive astrocytes, activation of microglia/macrophages and neuronal cell death. DTI measures (axial, radial and mean diffusivity, and fractional anisotropy (FA)) were acquired at hyperacute (3 h), acute (1 and 2 d) and chronic (1 and 4 week) times after transient cerebral hypoxia with unilateral ischemia in neonatal rats. The tissue pathology underlying ischemic and degenerative responses had a complex relationship with DTI parameters. DTI changes at hyperacute and subacute times were smaller in magnitude and tended to be transient and/or delayed in cerebral peduncle compared to cerebral cortex. In cerebral peduncle by 1 d post-insult, there were reductions in neurofilament staining corresponding with decreases in parallel diffusivity which were more sensitive than mean diffusivity in detecting axonal changes. Ipsilesional reductions in FA within cerebral peduncle were robust in detecting both early and chronic degenerative responses. At one or four weeks post-insult, radial diffusivity was increased ipsilaterally in the cerebral peduncle corresponding to pathological evidence of a lack of ontogenic myelination in this region. The detailed differences in progression and magnitude of DTI and histological changes reported provide a reference for identifying the potential contribution of various cellular responses to FA, and, parallel, radial, and mean diffusivity.

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Diffusion tensor imaging (DTI) widely used; cellular correlates often speculative

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Studied longitudinal DTI and histological changes following hypoxia–ischemia

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Compared neonatal cortex changes to those in degenerating cerebral peduncle

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DTI and cellular changes were often transient or delayed in cerebral peduncle.

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This provides a reference for potential cellular contributions to DTI changes.

Magnetization transfer and diffusion imaging of acute axonal damage in the cerebral peduncle following hypoxia-ischemia in neonatal rats

BACKGROUND

Magnetic resonance imaging (MRI) of axonal degenerative changes in the cerebral peduncle of the corticospinal tract following cerebral hypoxic-ischemic damage might distinguish infants most appropriate for receiving prompt treatment. The optimal MRI sequence for very early diagnosis of axonal degenerative changes is unknown. We hypothesized that magnetization transfer ratio (MTR) imaging would be more sensitive than traditional MRI, e.g., T(2) or diffusion weighted imaging.

METHODS

Transient unilateral cerebral hypoxia-ischemia was produced in the neonatal rat followed by MRI of changes in T(2), the apparent diffusion coefficient (ADC) of water, and MTR, with a focus on the parietal cortex (an ischemic damaged region) and the cerebral peduncle (remote within the corticospinal tract). Rats were imaged at 2 h, 1 d, or 1 wk postinsult.

RESULTS

In the cerebral peduncle, MTR and T(2) responded similarly, with alterations occurring ipsilaterally at 1 d postinsult. ADC was most sensitive for detecting changes as early as 2 h postinsult, and this corresponded to a reduced staining of axonal filaments ipsilaterally.

CONCLUSION

MTR and T(2) imaging have comparable sensitivity for distinguishing early axonal damage in the cerebral peduncle. ADC imaging is highly sensitive for detecting early disruption of corticospinal axons, supporting its potential hyperacute diagnostic use clinically.

Protection against recurrent stroke with resveratrol: endothelial protection

Despite increased risk of a recurrent stroke following a minor stroke, information is minimal regarding the interaction between injurious mild cerebral ischemic episodes and the possible treatments which might be effective. The aim of the current study was to investigate recurrent ischemic stroke and whether resveratrol, a nutritive polyphenol with promising cardio- and neuro- protective properties, could ameliorate the associated brain damage. Experiments in adult rats demonstrated that a mild ischemic stroke followed by a second mild cerebral ischemia exacerbated brain damage, and, daily oral resveratrol treatment after the first ischemic insult reduced ischemic cell death with the recurrent insult (P<0.002). Further investigation demonstrated reduction of both inflammatory changes and markers of oxidative stress in resveratrol treated animals. The protection observed with resveratrol treatment could not be explained by systemic effects of resveratrol treatment including effects either on blood pressure or body temperature measured telemetrically. Investigation of resveratrol effects on the blood-brain barrier in vivo demonstrated that resveratrol treatment reduced blood-brain barrier disruption and edema following recurrent stroke without affecting regional cerebral blood flow. Investigation of the mechanism in primary cell culture studies demonstrated that resveratrol treatment significantly protected endothelial cells against an in vitro ‘ischemia’ resulting in improved viability against oxygen and glucose deprivation (39.6±6.6% and 81.3±9.5% in vehicle and resveratrol treated cells, respectively). An inhibition of nitric oxide synthesis did not prevent the improved cell viability following oxygen glucose deprivation but SIRT-1 inhibition with sirtinol partially blocked the protection (P<0.001) suggesting endothelial protection is to some extent SIRT-1 dependent. Collectively, the results support that oral resveratrol treatment provides a low risk strategy to protect the brain from enhanced damage produced by recurrent stroke which is mediated in part by a protective effect of resveratrol on the endothelium of the cerebrovasculature.

Infrared optical imaging of matrix metalloproteinases (MMPs) up regulation following ischemia reperfusion is ameliorated by hypothermia

Background

We investigated the use of a new MMP activatable probe MMPSense^™ 750 FAST (MMPSense750) for in-vivo visualization of early MMP activity in ischemic stroke. Following middle cerebral artery occlusion (MCAO) optical imaging was performed. Near-infrared (NIR) fluorescent images of MMPSense activation were acquired using an Olympus fluorescent microscope, 1.25x objective, a CCD camera and an appropriate filter cube for detecting the activated probe with peak excitation and emission at 749 and 775 nm, respectively. Images were acquired starting at 2 or 24 hours after reperfusion over the ipsilateral and contralateral cortex before and for 3 hours after, MMPSense750 was injected.

Results

Increased intensities ipsilaterally were observed following MMPSense750 injection with ischemic injury but not in sham animals. There were significant ipsilateral and contralateral differences at 15 minutes (P <0.05) in early ischemic reperfusion and at time 0 in 24 hours post ischemia (P <0.05) which persisted at 180 minutes in both these groups (P <0.01), but not following sham surgery. The increase in ipsilateral signal intensity was attenuated by hypothermia. These observations corresponded with a significant increase in the total MMP-9 protein levels, 5 and 24 hours following ischemia reperfusion (P <0.05) and their reduction by hypothermia.

Conclusions

Matrix-metalloproteinase upregulation in ischemia reperfusion can be imaged acutely in-vivo with NIRF using MMPSense750. Hypothermia attenuated both the optical increase in intensity after MMPSense750 and the increase in MMP-9 protein expression supporting the proof of concept that NIRF imaging using MMPSense can be used to assess potential therapeutic strategies for stroke treatment.

Persistent enhancement of functional MRI responsiveness to sensory stimulation following repeated seizures

PURPOSE

Neural reorganization and interictal behavioral anomalies have been documented in people with epilepsy and in animal seizure models. Alterations in behavior could be due to somatosensory dysfunction. This study was designed to determine whether seizures can lead to changes in somatosensory representations and whether those changes are persistent.

METHODS

Twice-daily seizures were elicited by delivering 1 s of electrical stimulation through carbon fiber electrodes implanted in both the corpus callosum and sensorimotor neocortex of young adult male Long-Evans rats until a total of 20 seizures were elicited. Either 1-3 days or 3-5 weeks following the last seizure, functional magnetic resonance imaging (MRI) was used to image the brain during electrical stimulation of each forepaw independently.

KEY FINDINGS

Forepaw stimulation in control rats resulted in a focused and contralateral fMRI signal in the somatosensory neocortex. Rats that had repeated seizures had a 151% increase in the number of voxels activated in the contralateral hemisphere 1-3 days after the last seizure and a 166% increase at 3-5 weeks after the last seizure. The number of voxels activated in response to forepaw stimulation was positively correlated with the duration of the longest seizure experienced by each rat. The intensity of the activated voxels was not significantly increased at either time interval from the last seizure.

SIGNIFICANCE

The increased area of activation in somatosensory cortex, which is persistent at 3-5 weeks, is consistent with previous observations of larger motor maps following seizures. Seizure-induced changes in the functioning of sensory cortex may also contribute to interictal behavioral anomalies.

Changes in differential functional magnetic resonance signals in the rodent brain elicited by mixed-nutrient or protein-enriched meals

BACKGROUND & AIMS

The hypothalamus and brain stem have important roles in regulating food intake; the roles of other nonhomeostatic centers in detecting nutrient content of ingested food have been poorly characterized. We used blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) to map brain regions that are responsive to intragastric infusion of isocaloric amounts of a mixed nutrient or protein, and assessed the role of blood glucose in the observed BOLD signal changes.

METHODS

Brain images were acquired, using a 9.4 T MRI system, from anesthetized rats during intragastric infusion of saline (n = 7), or 12 kcal of a mixed nutrient (n = 13) or protein (n = 6). Nutrient-induced changes in blood parameters and the effects of intravenous infusion of saline or glucose (n = 5/treatment) on BOLD fMRI signal changes were also evaluated. Intragastric nutrient infusion reduced the BOLD fMRI signal intensity in homeostatic (hypothalamus, nucleus tractus solitarius) and nonhomeostatic (thalamus, hippocampus, caudate putamen, cerebral cortex, cerebellum) centers; these effects were mimicked qualitatively by intravenous glucose. In contrast to a mixed meal, protein load reduced the BOLD fMRI signal in the amygdala. BOLD fMRI signal changes were inversely correlated with circulating concentrations of amylin, insulin, peptide YY, and glucagon-like peptide-1.

CONCLUSIONS

The caloric content of a meal is signaled from the gut to the brain and affects activity in homeostatic and non-homeostatic centers; blood glucose concentrations have an important role. The satiety effects of protein are associated with activity changes specifically in the amygdala.

Small unilamellar vesicles: a platform technology for molecular imaging of brain tumors

Molecular imaging enables the non-invasive investigation of cellular and molecular processes. Although there are challenges to overcome, the development of targeted contrast agents to increase the sensitivity of molecular imaging techniques is essential for their clinical translation. In this study, spontaneously forming, small unilamellar vesicles (sULVs) (30 nm diameter) were used as a platform to build a bimodal (i.e., optical and magnetic resonance imaging (MRI)) targeted contrast agent for the molecular imaging of brain tumors. sULVs were loaded with a gadolinium (Gd) chelated lipid (Gd-DPTA-BOA), functionalized with targeting antibodies (anti-EGFR monoclonal and anti-IGFBP7 single domain), and incorporated a near infrared dye (Cy5.5). The resultant sULVs were characterized in vitro using small angle neutron scattering (SANS), phantom MRI and dynamic light scattering (DLS). Antibody targeted and nontargeted Gd loaded sULVs labeled with Cy5.5 were assessed in vivo in a brain tumor model in mice using time domain optical imaging and MRI. The results demonstrated that a spontaneously forming, nanosized ULVs loaded with a high payload of Gd can selectively target and image, using MR and optical imaging, brain tumor vessels when functionalized with anti-IGFBP7 single domain antibodies. The unique features of these targeted sULVs make them promising molecular MRI contrast agents.

Differential impact of diabetes and hypertension in the brain: adverse effects in white matter

Humans subjected to diabetes mellitus (DM) and/or hypertension (HTN) develop cognitive decline, cerebral atrophy and white matter abnormalities, but the relative effects of DM and HTN upon myelin and axonal integrity is unknown. We studied models of Type 1 (streptozotocin-induced) and Type 2 DM (ZDF) ± HTN (ZSF-1, SHR) in adult rats using magnetic resonance imaging (MRI) and structural and molecular techniques. Type 1 or 2 DM independently led to loss of myelin associated with changes with MRI T2 and magnetization tensor ratios throughout white matter regions. HTN's effect on myelin loss was minimal. Loss of oligodendroglia and myelin proteins was only identified in either Type 1 or Type 2 DM. Activation of the signal transduction pathways initiated by the receptor for advanced glycation end products (AGEs), RAGE, including upregulation of the signal transducer nuclear factor (NF) κB only occurred with DM. Diabetes is a greater contributor to white matter loss than hypertension in the rat brain, while hypertension only plays a mild additive effect upon neurodegeneration in the presence of diabetes.

Gastric distention induced functional magnetic resonance signal changes in the rodent brain

Investigating the localization of gastric sensation within the brain is important for understanding the neural correlates of satiety. Previous rodent studies have identified the brain-stem and hypothalamus as key mediators of gastric distention-induced satiation. Although, recent blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) studies in humans have identified a role for higher cortico-limbic structures in mediating the satiation effects of gastric distention, the role of these regions in rodents remains to be characterized. We determined the effects of gastric distention on global spatio-temporal BOLD fMRI signal changes in the rodent brain. Brain images were acquired with a high resolution 9.4 T magnet during gastric distention with continuous monitoring of blood pressure in adult male Sprague Dawley rats (n=8-10). Distention of the stomach with an intragastric balloon, at rates which mimicked the rate of consumption and emptying of a mixed nutrient liquid meal, resulted in robust reduction in food intake and increase in blood pressure. Gastric distention increased BOLD fMRI activity within homeostatic regions such as the hypothalamus and nucleus tractus solitarius, as well as non homeostatic regions including the hippocampus, amygdala, thalamus, cerebellum and the cortex (cingulate, insular, motor and sensory cortices). Further, the increase in BOLD fMRI activity following distention was strongly correlated to an increase in blood pressure. These results indicate that gastric distention, mimicking the rate of intake and emptying of a liquid meal, increases BOLD fMRI activity in both homeostatic and non homeostatic brain circuits which regulate food intake, and that these BOLD fMRI signal changes may in part be attributable to transient increases in blood pressure.

Magnetic resonance molecular imaging of post-stroke neuroinflammation with a P-selectin targeted iron oxide nanoparticle

We have developed a magnetic resonance molecular imaging method using a novel iron-oxide contrast agent targeted towards P-selectin - MNP-PBP (magnetic nanoparticle-P-selectin binding peptide) - to image endothelial activation following cerebral ischemia/reperfusion. MNP-PBP consists of approximately 1000 PBP ligands (primary sequence: GSIQPRPQIHNDGDFEEIPEEYLQ GGSSLVSVLDLEPLDAAWL) conjugated to a 50 nm diameter aminated dextran iron oxide particle. In vitro P- and E-selectin binding was assessed by competition ELISA. Transient focal cerebral ischemia was induced in male C57/BL 6 mice followed by contrast injection (MNP-PBP; MNP-NH2; Feridex; MNP-PBP-FITC) at 24 h after reperfusion and T(2) magnetic resonance imaging at 9.4 T was performed. Infarction and microvasculature accumulation of contrast agent was assessed in coronal brain sections. MNP-PBP attenuated antibody binding to P-selectin by 34.8 +/- 1.7%. P-selectin was preferentially increased in the infarct hemisphere and MNP-PBP-FITC accumulation in the infarct hemisphere microvasculature was observed. Compared with the nontargeted iron oxide agents MNP-NH2 and Feridex, MNP-PBP showed a significantly greater T(2) effect within the infarction. MR imaging of P-selectin expression with a targeted iron oxide nanoparticle contrast agent may reveal early endothelial activation in stroke and other neuroinflammatory states.

Reduced blood brain barrier breakdown in P-selectin deficient mice following transient ischemic stroke: a future therapeutic target for treatment of stroke

BACKGROUND

The link between early blood- brain barrier (BBB) breakdown and endothelial cell activation in acute stroke remain poorly defined. We hypothesized that P-selectin, a mediator of the early phase of leukocyte recruitment in acute ischemia is also a major contributor to early BBB dysfunction following stroke. This was investigated by examining the relationship between BBB alterations following transient ischemic stroke and expression of cellular adhesion molecule P-selectin using a combination of magnetic resonance molecular imaging (MRMI), intravital microscopy and immunohistochemistry. MRMI was performed using the contrast, gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) conjugated to Sialyl Lewis X (Slex) where the latter is known to bind to activated endothelium via E- or P selectins. Middle cerebral artery occlusion was induced in male C57/BL 6 wild-type (WT) mice and P-selectin-knockout (KO) mice. At 24 hours following middle cerebral artery occlusion, T1 maps were acquired prior to and following contrast injection. In addition to measuring P- and E-selectin expression in brain homogenates, alterations in BBB function were determined immunohistochemically by assessing the extravasation of immunoglobulin G (IgG) or staining for polymorphonuclear (PMN) leukocytes. In vivo assessment of BBB dysfunction was also investigated optically using intravital microscopy of the pial circulation following the injection of Fluorescein Isothiocyanate (FITC)-dextran (MW 2000 kDa).

RESULTS

MRI confirmed similar infarct sizes and T1 values at 24 hours following stroke for both WT and KO animals. However, the blood to brain transfer constant for Gd DTPA (Kgd) demonstrated greater tissue extravasation of Gd DTPA in WT animals than KO mice (P < 0.03). In the P selectin KO mice, Delta T1 stroke -Delta T1 contralateral control cortex, decreased significantly in the Gd-DTPA(sLeX) group compared to Gd-DTPA, indicative of sLeX mediated accumulation of the targeted contrast agent. Regarding BBB function, in the P-selectin KO mice compared to WT control mice, there was an attenuation in the extravasation of IgG (P < 0.001), a trend for decreased FITC extravasation and less infiltration of PMN leukocytes (P < 0.001) thereby supporting the observed increase in Kgd permeability in stroke brain of WT compared to KO mice.

CONCLUSION

P-selectin expression contributes to enhanced BBB dysfunction at 24 hours after transient focal cerebral ischemia.

Quantified T1 as an adjunct to apparent diffusion coefficient for early infarct detection: a high-field magnetic resonance study in a rat stroke model

BACKGROUND

Thrombolytic treatment for acute stroke has focused attention on accurate identification of injured vs. salvageable brain tissue, particularly if reperfusion occurs. However, our knowledge of differences in acute magnetic resonance imaging changes between transient and permanent ischemia and how they reflect permanently damaged tissue remain incomplete.

AIMS AND/OR HYPOTHESIS

Magnetic resonance imaging characteristics vary widely following ischemia and, at acute times, T1, T2 or apparent diffusion coefficient quantification may differentiate viable tissue from that destined to infarct.

METHODS

High-resolution magnetic resonance imaging was performed at 9.4 T following permanent or transient (90 min) middle cerebral artery occlusion in spontaneously hypertensive male rats or Wistar rats. Within 30 min, quantified maps of the apparent diffusion coefficient, T1, and T2 were performed and measures determined for sequences in the infarct and compared with that in the contralateral region. Lesion area for each magnetic resonance imaging sequence (T1, T2, apparent diffusion coefficient, and perfusion maps) was delineated for different time points using quantitative threshold measures and compared with final histological damage.

RESULTS

Early extensive changes in T1 following both transient and permanent middle cerebral artery occlusion provided a sensitive early indicator of the final infarct area. Following reperfusion, small but measurable early T2 changes indicative of early development of vasogenic edema occurred in the transient but not permanent groups. In transient middle cerebral artery occlusion, at 70 min apparent diffusion coefficient decreased (P<0.001) and then pseudonormalized at 150 min. In permanent middle cerebral artery occlusion, apparent diffusion coefficient declined over time. Lesion area detected using T1 maps exceeded that with T2 and apparent diffusion coefficient at 70 and 150 min in both groups (P<0.001).

CONCLUSIONS

The results indicate that, independent of reperfusion, quantified T1 is superior for detecting early ischemic changes that are not necessarily detected with T2 or apparent diffusion coefficient.

Functional brain mapping at 9.4T using a new MRI-compatible electrode chronically implanted in rats

There is a need for acute and chronic stimulation of the brain within the MRI for studies of epilepsy, as well as deep brain stimulation for movement and behavioral disorders. This work describes the production and characteristics of carbon fiber-based electrodes for acute and chronic stimulation in the brain. Increasing MRI field strengths are making it increasingly difficult to introduce foreign objects without a susceptibility artifact. We describe the production of, and the characteristics of carbon fiber-based electrodes. These are biocompatible and can be implanted for chronic studies. We show the use of these electrodes at 9.4T for studying functional activation. Data are presented showing regional connectivity. Activation not only occurs near the electrode, but at sites distant and often contralateral to the electrode. In addition, there were sites showing strong negative activation to stimulation both with direct stimulation and during a kindling-associated seizure.

X-linked inhibitor of apoptosis protein expression after ischemic injury in the human and rat developing brain

X-linked inhibitor of apoptosis protein (XIAP) is a potent suppressor of neuronal death. The aim of this study was to investigate the expression of XIAP after ischemia in the human and rat developing brain. Autopsy specimens from 19 children with neuropathologic diagnosis of focal cerebral ischemic infarct were processed immunohistochemically for XIAP expression. XIAP positive cells were compared in pathologically classified acute (1-4 d), subacute (5-30 d), and chronic (months) strokes vs. age-matched controls with normal brain histology. For the animal studies, ischemia was induced in 1-wk-old rats by unilateral carotid artery occlusion and transient hypoxia. XIAP expression was quantified at four time points after ischemia in the infarct core and peri-infarct area. Neuronal XIAP expression was higher in the penumbra of subacute human infarcts compared with controls (p < 0.05). XIAP expression in the peri-infarct of rat pup was highest at 7 d postischemic injury (p < 0.05). The increase in XIAP expression was associated with a reduction in activated caspase-3 in ischemic neonatal rat brain. Our results demonstrate that XIAP expression postischemic injury is delayed in both species and may continue for several days. Therefore, potentiation of XIAP expression may be neuroprotective in the developing brain.

Cerebral expression of DNA repair protein, Ku70, and its association with cell proliferation following cerebral hypoxia-ischemia in neonatal rats

We hypothesized that increased Ku70 expression could be involved in recovery following cerebral hypoxia-ischemia. We investigated the progression of cerebral alterations in Ku70 expression at different time points (24 h, 72 h, 1 week, 4 weeks and 8 weeks) after hypoxia-ischemia (right carotid artery occlusion plus 1.5h of hypoxia) in neonatal rats. To determine whether in addition to its known role of DNA repair, Ku70 was associated with cell death or cell proliferation we performed double staining for Ku70 and DNA fragmentation or bromodeoxyuridine, respectively. The results show that Ku70 expression was increased in the infarct core and peri-infarct regions at 24h following hypoxia-ischemia. The increased Ku70 expression was transient in the infarct core with a loss of Ku70 positive cells over days. In contrast, in the peri-infarct region the expression of Ku70 remained increased at chronic times 8 weeks following the insult. Cells positive for DNA fragmentation were not co-localized with cells positive for Ku70 after an insult. However, most of the cells positive for bromodeoxyuridine indicative of cell proliferation were positive for Ku70 in the peri-infarct region at 8 weeks after the insult. Considering the roles of Ku70 in DNA repair or inhibiting apoptosis and its co-localization within cells that had undergone proliferation, Ku70 may be considered a potential novel target to enhance recovery following hypoxia-ischemia.

Differential progression of magnetization transfer imaging changes depending on severity of cerebral hypoxic-ischemic injury

We hypothesized that magnetic resonance magnetization transfer (MT) imaging would be sensitive for detecting cerebral ischemic injury in white matter of neonatal brain. We compared the progression of changes in T(2) and the MT ratio (MTR) after cerebral hypoxic-ischemic insults of differing severity in neonatal rats. Magnetization transfer imaging parameters were first optimized, and then MTR and T(2) maps were acquired at various times after a mild (rather selective white matter) or substantial insult produced by unilateral cerebral hypoxia-ischemia. Depending on insult severity, time after insult, and region (e.g., subcortical white matter or cortex), cerebral hypoxia-ischemia produced reductions in MTR and an increase in T(2). The exception was acutely at 1 to 5 h at which time points MTR was reduced ipsilaterally in white matter, whereas T(2) was not affected significantly. Progression of imaging changes differed in rats grouped according to whether gross damage was present after chronic recovery. Behavioral changes were generally associated with chronic reductions in MTR and gross brain damage. Magnetization transfer imaging was capable of early detection of hypoxic-ischemic injury and particularly sensitive for identifying the progression of cerebral injury in white matter. Magnetization transfer ratio has potential for assisting with early diagnosis and treatment assessment for infants affected by perinatal hypoxia-ischemia.

Functional magnetic resonance imaging within the rat spinal cord following peripheral nerve injury

Functional magnetic resonance imaging (fMRI) was used to detect the effects of graded peripheral nerve injury at the spinal level. Graded peripheral nerve injury in rats was accomplished by transection of nerves entering the spinal cord at the L3 and L4 levels of the spinal cord segments. Electrical stimulation of the hindpaw was used to elicit activity within the spinal cord. The stimulation experimental paradigm consisted of 62 functional images, 5 slices each, with a total of 3 rest and 2 stimulation periods. A 9.4 T MRI system and a quadrature volume rf coil covering the lumbar spinal cord were used for the fMRI study. Sets of fast spin echo images were acquired repeatedly following sham preparatory surgery under control conditions and in rats following sham surgery (pre nerve cut), followed by L3 nerve and then L4 nerve section. In rats with sham surgery, there was a significant activation within the dorsal horn of slices corresponding to L3 and L4 spinal cord segments. Following section of the L3 nerve, there was a reduction in the number of active voxels in the L3 and L4 spinal cord segments. The activation was reduced further by sectioning of the L4 nerve. Thus, following an increasing loss of axonal connections to the spinal cord, there was a decreasing number of active voxels within the spinal cord. The results demonstrate that spinal fMRI in the rat has sufficient sensitivity to detect within the spinal cord the effects of a graded reduction in peripheral connectivity.

Transient hypertension concurrent with forepaw stimulation enhances functional MRI responsiveness in infarct and peri-infarct regions

Although functional magnetic resonance imaging (fMRI) is gaining use as a tool to assess cerebral recovery following various insults, the effects of potential confounders such as hypertension are poorly defined. We hypothesized that after stroke, transient hypertension during an fMRI study could produce a detected activation unrelated to neuronal activity within the infarct. Thus, the effect of norepinephrine induced increases in blood pressure (BP) on the fMRI response to forepaw stimulation were investigated in controls or 1 week after transient middle cerebral artery occlusion in rats. Images were smoothed spatially and voxels correlating to either forepaw stimulation or the change in BP time courses were analyzed. Transient hypertension increased the signal intensity and numbers of voxels correlating to the BP time courses within and adjacent to the ischemic infarct and these exceeded the response in the contralateral hemisphere or in controls. With left paw stimulation at normotension, there was a loss of activation in right sensory-motor cortex -- a region with necrosis and disruption of cerebral vessels. As BP increased left paw stimulation also resulted in the detection of activation in the infarcted sensory-motor cortex and peri-infarct regions. Thus, BP changes synchronous with tasks in fMRI studies can result in MR signal changes consistent with a loss of cerebral blood flow (CBF) autoregulation rather than neuronal activation in necrotic brain. After stroke, the use of stressful tasks associated with BP changes in fMRI studies should be limited or the BP change should be considered as a potential source of MR signal changes.

Blood-oxygen-level-dependent magnetic resonance signal and cerebral oxygenation responses to brain activation are enhanced by concurrent transient hypertension in rats

Neuronal activation results in increases in blood-oxygen-level-dependent (BOLD) signal increases in magnetic resonance images, increases in cerebral blood flow (CBF), and changes in tissue oxygenation. We hypothesized that transient hypertension concurrent with neuronal activation would interfere with the normal physiological responses to neuronal activation potentially leading to additive responses. Anesthetized rats were prepared for functional magnetic resonance imaging studies in which increases in BOLD signal were measured in response to: (1) electrical forepaw stimulation, (2) different graded levels of transient hypertension produced with norepinephrine, and both 1 and 2. In other experiments with a similar protocol, changes in CBF and cortical oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) were measured using Laser Doppler Flowmetry and near-infrared (IR) spectroscopy. BOLD signal within the sensory-motor cortex increased during forepaw stimulation. These matched increases in CBF and oxyHb and decreases in deoxyHb. During moderate or severe transient hypertension, there was a blood pressure-dependent increase in BOLD signal, CBF, and oxyHb; and a decrease in deoxyHb. When transient hypertension and forepaw stimulation were combined, the responses of oxyHb, deoxyHb, or BOLD signal were generally a summation of each response. In contrast, the CBF response to forepaw stimulation was relatively unaffected by transient hypertension. We conclude that during stimulation with concurrent hypertension, the normal changes in tissue oxygenation that accompany neuronal activation are enhanced by the increases produced by hypertension despite an excellent autoregulation of CBF. The latter could reflect highly transient decreases in oxygen consumption or likely a redistribution of flow through more nonexchange vessels.

Diabetes, leukoencephalopathy and rage

Longstanding diabetes mellitus damages kidney, retina, peripheral nerve and blood vessels, but brain is not usually considered a primary target. We describe direct involvement of the brain, particularly white matter, in long-term (9 months) experimental diabetes of mice, not previously modeled, correlating magnetic resonance (MR) imaging with quantitative histological assessment. Leukoencephalopathy and cerebral atrophy, resembling that encountered in diabetic humans, developed in diabetic mice and was accompanied by time-related development of cognitive changes in behavioural testing. Increased RAGE (receptor for advanced glycation end products) expression, a mediator of widespread diabetic complications, increased dramatically at sites of white matter damage in regions of myelination. RAGE expression was also elevated within neurons, astrocytes and microglia in grey matter and within oligodendrocytes in white matter. RAGE null diabetic mice had significantly less neurodegenerative changes when compared to wild-type diabetic mice. Our findings identify a robust and novel model of cerebral, particularly white matter, involvement with diabetes associated with abnormal RAGE signaling.

Transient blood pressure changes affect the functional magnetic resonance imaging detection of cerebral activation

Functional magnetic resonance imaging (fMRI) provides an indirect measure of cerebral activation that could be altered by factors directly affecting cerebral blood flow independent of changes in neuronal activation. Presently, we investigate how changes in blood pressure (BP) affect the activation detected with fMRI. fMRI scans were acquired in 33 rats under control conditions and following transient BP increases (norepinephrine, IV) or decreases (arfonad, IV) with and without electrical stimulation of the forepaw. Voxels correlating to either the stimulation or the change in BP time courses were identified. During transient hypertension, irrespective of forepaw stimulation, BP increases (i.e., >10 mm Hg) produced a transient increase in the blood oxygen level-dependent (BOLD) intensity resulting in a significant numbers of voxels correlating to the BP time courses (P < 0.05), and the number of these voxels increased as BP increased, becoming substantial at BP > 30 mm Hg. The activation patterns with BP increases and stimulation overlapped spatially resulting in an enhanced cerebral activation to simultaneous forepaw stimulation (P < 0.05). BP decreases (>10 mm Hg) produced corresponding decreases in BOLD intensity, causing significant numbers of voxels correlating to the BP decreases (P < 0.005), and these numbers increased as BP decreased (P < 0.001). The BP decreases and stimulation time courses and responses were distinct, and hypotension did not affect the detection of the activation response to forepaw stimulation. The results indicate that substantial hypertension accompanying a stimulation paradigm produces a BOLD response that enhances the cerebral activation detected, whereas hypotension does not affect the detection of neuronal activation but does produce responses that could be interpreted as a 'deactivation'.

Evolution of magnetic resonance imaging changes associated with cerebral hypoxia-ischemia and a relatively selective white matter injury in neonatal rats

We hypothesized that a combination of quantitative magnetic resonance imaging (MRI) sequences would detect a differential evolution of hypoxic-ischemic changes in white matter compared with gray matter in a recently developed model of unilateral mild cerebral hypoxia-ischemia in the 7-d-old rat. Using this model, which involved unilateral carotid artery occlusion and exposure to hypoxia for 45-50 min, maps of apparent diffusion coefficients of water (ADC), T1, T2, and cerebral blood flow (CBF) were acquired either before hypoxia-ischemia or at 1, 24, or 48 h and at 7 d post-hypoxia-ischemia followed by brain processing for histology. At 1 h post-hypoxia-ischemia, MRI changes in white matter ipsilateral to the hypoxia-ischemia were not as pronounced as those in gray matter. However, increases in T1, T2 and ADC and decreases in CBF within white matter enhanced over time, with changes being maximal at 48 h post-hypoxia-ischemia, whereas changes in the cortical gray matter normalized over this time. By 7 d post-hypoxia-ischemia, there were no differences in ADC, T1, T2, or CBF between hemispheres despite there being histologic changes in white matter within the hypoxic-ischemic hemisphere including increased glial proliferation and reactivity, reduced myelin basic protein, and increased cell death. The results demonstrate that increases in ADC and T2 observed subacutely in the days following hypoxia-ischemia are associated with rather selective white matter damage and suggest that diffuse white matter hyperintensities and increased ADC reported in infants are transient MRI changes post- hypoxia-ischemia.

Simultaneous functional magnetic resonance imaging in the rat spinal cord and brain

Functional magnetic resonance imaging (fMRI) method was developed to investigate the pattern and temporal relationship in neuronal pathways of brain and spinal cord. Signal intensity changes correlating with stimulation patterns were observed simultaneously in the rat spinal cord and brain using fMRI at 9.4 T. Electrical stimulation of the forepaw was used to elicit activity. A quadrature volume RF coil covering both brain and the cervical spinal cord was used. Sets of fast spin echo (FSE) images were acquire simultaneously for both brain and spinal cord fMRI. Experiments were repeated in single animal and across animals. Activities within the dorsal horn of the spinal cord and within the somatosensory cortex were observed consistently within each animal as well as across animals.

White matter damage precedes that in gray matter despite similar magnetic resonance imaging changes following cerebral hypoxia-ischemia in neonatal rats

We hypothesized that the cerebral injury produced by hypoxia-ischemia (HI) in neonatal rats would differ in white compared with gray matter as detected histologically or with magnetic resonance (MR) imaging methods. Maps of T2 and the apparent diffusion coefficient (ADC) of water were acquired in 1-week-old rats at times prior to cerebral HI (right carotid artery occlusion plus 1.5 h of hypoxia), within the last 5-10 min of HI, and 1 h or 24 h after HI. Near the end of HI, ADC decreased and T2 increased in both cortical gray and subcortical white matter within the cingulum of the HI hemisphere. One hour after HI, ADC partially recovered, but T2 remained increased and then increased further by 24 h post-HI. In contrast to the similar MR responses in white and gray matter, histological evidence for irreversible cell damage occurred in white matter earlier than in gray matter within the HI hemisphere. At 1 h post-HI, rarefied or disrupted nerve fibers and an increase in TUNEL-positive cells were observed within white matter in the cingulum, whereas neurons within the cortical gray matter appeared normal. By 24 h post-HI, damage was apparent in both white and gray matter. Thus, MR imaging detected acute tissue edema following cerebral HI in both gray and white matter but did not distinguish between the early irreversible tissue injury detected histologically in white but not gray matter in this rather severe model of neonatal encephalopathy.

Cerebral blood flow response to a hypoxic-ischemic insult differs in neonatal and juvenile rats

To compare the cerebral blood flow (CBF) response to a transient episode of hypoxia-ischemia producing damage in neonatal and juvenile rats. One- and four-week-old rats were subjected to unilateral carotid artery occlusion plus hypoxia (8% oxygen). Perfusion MR images were acquired either in sham controls or in hypoxic-ischemic rats before, during, 1 h and 24 h after hypoxia-ischemia. At 24 h post hypoxia-ischemia, T2 maps and histology were used to assess damage. In sham controls, CBF increased twofold between the age of one and four weeks. Reductions in CBF ipsilateral to the occlusion occurred during hypoxia-ischemia followed by a substantial recovery at 1 h post in both age groups. However, contralaterally, hyperemia occurred during hypoxia-ischemia in four-week but not one-week-old rats. Similarly, hyperemia occurred ipsilaterally at 24 h post hypoxia-ischemia in four-week but not one-week-olds, corresponding to the distribution of elevations in T2. Despite CBF differences, extensive cell death occurred ipsilaterally in both age groups. The CBF responses to hypoxia-ischemia and reperfusion differ depending on postnatal age, with hyperemia occurring in juvenile but not neonatal rats. The results suggest a greater CBF responsiveness and differential relationship between post-ischemic vascular perfusion and tissue injury in older compared with immature animals.

Magnetic resonance imaging of differential gray versus white matter injury following a mild or moderate hypoxic-ischemic insult in neonatal rats

Selective white matter injury in the pre-mature infants suggests it has a greater susceptibility to hypoxia-ischemia. To investigate whether white matter injury would predominate following a mild hypoxic-ischemic insult, 7-day-old rats underwent either mild or moderate hypoxia-ischemia and magnetic resonance imaging 24 h later. Mild and moderate hypoxia-ischemia were produced by unilateral carotid artery occlusion plus exposure to hypoxia for either 45-50 or 90 min at ambient temperatures of 34.5 or 35.5 degrees C, respectively. Following mild hypoxia-ischemia, there was a significant increase in T(1) and T(2) within periventricular white matter (e.g. corpus callosum) in the hemisphere ipsilateral to the occlusion compared to that contralaterally and less of an increase within gray matter (e.g. cortex and striatum). This corresponded to relatively selective white matter injury detected histologically. Following a moderate hypoxia-ischemia, both gray and white matter was severely injured with marked increases in T(1) and T(2) occurring in both white and gray matter regions ipsilateral to the hypoxia-ischemia. We conclude that a mild insult, consisting of a short duration of hypoxia-ischemia at a slightly lower body temperature than a moderate hypoxic-ischemic insult, produces enhanced injury in white matter and a relative sparing of gray matter.

Correspondence of AQP4 expression and hypoxic-ischaemic brain oedema monitored by magnetic resonance imaging in the immature and juvenile rat

Whether the water channel protein AQP4 is involved in the very early cell swelling and brain oedema observed with cerebral hypoxia-ischaemia (HI) and whether this response depends on the maturity of brain were investigated by comparing regional changes in AQP4 protein expression and signal intensity on magnetic resonance (MR) images in immature and juvenile brains. Maps of T2 and the apparent diffusion coefficient (ADC) of water were acquired in 1- and 4-week-old rats at times prior to HI, within the last 5 min of HI and 1 h or 24 h afterwards. AQP4 expression assessed with Western blotting was not significantly reduced until 24 h post-HI irrespective of age. However, AQP4 immunostaining was decreased at the end of HI and at 1 h or 24 h after HI in the hemisphere ipsilateral to the occlusion with changes being similar in both age groups and coinciding well with regional reductions in ADC. IgG immunostaining to assess blood-brain barrier integrity and T2 were unchanged at early time points in 4-week old rats despite decreases in AQP4 immunostaining. Thus, at early time points there were decreases in AQP4 detected with immunostaining but not Western blotting methods. However, the good correlation between alterations in ADC and AQP4 immunostaining suggests that changes in the AQP4 are involved in some of the early changes in brain water distribution observed in hypoxia-ischemia, and supports the speculation that AQP4 is involved in the transport of water across the perivascular membranes into the vascular lumen.

MR molecular imaging of early endothelial activation in focal ischemia

Focal ischemia followed by reperfusion initiates a harmful P- and E-selectin-mediated recruitment of leukocytes in brain microvasculature. In this study, we tested whether a novel magnetic resonance (MR) contrast agent (Gd-DTPA-sLe(x) A), which is designed to bind to activated endothelium could be detected by MR imaging (MRI) in a focal stroke mouse model. MRIs (9.4T) of the brain were acquired 24 hours after transient middle cerebral artery occlusion. T1 maps were acquired repeatedly before and up to 1.5 hours after the intravenous injection of either Gd-DTPA or Gd-DTPA-sLe(x) A. Analysis of images included a pixel-by-pixel subtraction of T1 maps from the precontrast T1 maps and quantification of T1 within the ischemic area. After injection of Gd-DTPA-sLe(x) A, T1 decreased compared with precontrast levels, and an interhemispheric difference between the pre-post contrast T1 developed within the stroke lesion at a mean time of 52 minutes after injection (p < 0.05). Animals injected with Gd-DTPA did not exhibit changes in T1 signal intensity between regions of the ipsilateral and contralateral hemispheres, indicating that the reductions in T1 observed with Gd-DTPA-sLe(x) A were unrelated to blood-brain barrier breakdown. Fluorescent-labeled sLe(x) A administered intravenously was observed to bind to the endothelium of injured but not control brain. The study suggests that the contrast agent Gd-DTPA-sLe(x) A can be used to visualize early endothelial activation after transient focal ischemia in vivo with MRI.

Development of acute edema following cerebral hypoxia-ischemia in neonatal compared with juvenile rats using magnetic resonance imaging

We hypothesized that the evolution of cerebral edema accompanying cerebral hypoxia-ischemia is dependent on age and that such differences would be detectable using magnetic resonance imaging methods. Thus we examined in immature and juvenile rats the relationship between hypoxic-ischemic changes in T1 and T2 and the alterations in brain water content, as assessed by differences in tissue wet-dry weights. One- and 4-wk-old rats were anesthetized and subjected to unilateral carotid artery occlusion and subsequent exposure to hypoxia (8% oxygen). T1 and T2 maps were acquired at 9.4 T, and then brain water content was measured in sham controls or in hypoxic-ischemic animals before, during, and 1 or 24 h after hypoxia-ischemia. In sham controls, T1, T2, and proton density decreased with increasing age, corresponding to an ontogenic decrease in water content. In 1-wk-old rats, increases in T1 and T2 were observed during and at 1 and 24 h after hypoxia-ischemia, corresponding to elevations in water content. In 4-wk-old rats, T1 and water content increased during and at 1 and 24 h after hypoxia-ischemia whereas T2 was not increased until 24 h after hypoxia-ischemia. Regression analysis showed that T1 correlated better with total water content than T2. In both immature and older brain, an increase in total brain water develops acutely and persists after an episode of cerebral hypoxia-ischemia, and T1 imaging detects this change better than T2. Hypoxic-ischemic changes in T2 are age dependent, reflecting other physicochemical changes of water in the tissue than water content alone.

Functional magnetic resonance imaging of tonic pain and vasopressor effects in rats

In functional magnetic resonance imaging (fMRI) studies, an elevation in blood pressure (BP) in individuals with a poor autoregulatory response may increase cerebral blood flow, potentially enhancing the blood oxygenation level dependent response. To investigate the role of BP changes, the cerebral activation to either tonic pain or the infusion of the vasopressor norepinephrine was correlated with the accompanying BP changes in alpha-chloralose anesthetized rats. Immediately after formalin (2%) injection into the forepaw, fMRI detected an activation that was correlated with the BP increase and additional activations that were independent of blood pressure changes 5-40 minutes later. The activation detected with the administration of the vasopressor norepinephrine, which does not cross the blood-brain barrier was correlated to both the amount and rate of increase in BP. The response ranged from being sparse, localized within cortex or widespread during modest, moderate or severe elevations in BP, respectively. The cerebral circulatory effects of hypertension should be considered as contributing to changes in cerebral blood oxygenation in fMRI studies involving increases in BP.

Transient hypoxia-ischemia in rats: changes in diffusion-sensitive MR imaging findings, extracellular space, and Na+-K+ -adenosine triphosphatase and cytochrome oxidase activity

PURPOSE

To investigate the correlation between diffusion-weighted (DW) magnetic resonance (MR) image changes with alterations in extracellular volume and changes in cytochrome oxidase and Na(+)-K(+)-adenosine triphosphatase (ATPase) activity at various times during and after cerebral hypoxia-ischemia in neonatal and juvenile rats.

MATERIALS AND METHODS

One- and 4-week-old rats were randomly assigned to control or transient cerebral hypoxia-ischemia (ie, right carotid artery occlusion plus exposure to 8% oxygen) groups. Hypoxic-ischemic changes compared with normal ipsilateral brain tissue on DW images and the apparent diffusion coefficient of water were measured during and at 1 and 24 hours after hypoxia-ischemia ended. Hypoxic-ischemic changes in extracellular space and ipsilateral versus contralateral differences in Na(+)-K(+)-ATPase and cytochrome oxidase activity were measured.

RESULTS

Hyperintensities on DW images obtained during hypoxia-ischemia correlated well (P <.05) with extracellular space reductions, which occurred 15 minutes earlier in the brains of 4-week-old rats than in the brains of 1-week-old rats. Similarly, within 1 hour after hypoxia-ischemia ended, DW image and extracellular space changes normalized. In contrast, Na(+)-K(+)-ATPase and cytochrome oxidase activity decreased in some regions during hypoxia-ischemia and remained reduced 1 hour after the end of hypoxia-ischemia. Twenty-four hours after signal intensity normalization, hyperintense areas reappeared on DW images, and Na(+)-K(+)-ATPase and cytochrome oxidase activity remained decreased.

CONCLUSION

Signal intensity alterations with diffusion-sensitive MR imaging during and after transient hypoxia-ischemia are closely associated with a corresponding shrinkage and reexpansion of the extracellular space, irrespective of age. Mechanisms other than Na(+)-K(+)-ATPase changes may induce the early cell volume changes detected with diffusion-sensitive MR imaging.

Correlation of cerebral hypoxic-ischemic T2 changes with tissue alterations in water content and protein extravasation

BACKGROUND AND PURPOSE

Age-dependent changes in T2-weighted MR images have been reported in cerebral hypoxia-ischemia. However, the biophysical mechanisms responsible for the image changes remain poorly defined. We investigated whether cerebral hypoxia-ischemia-induced T2 changes correlate with alterations in either water content or protein extravasation.

METHODS

One- and 4-week-old rats were subjected to unilateral carotid artery occlusion plus hypoxia in 8% oxygen. T2 images were acquired before, during, and 1 or 24 hours after hypoxia-ischemia. Blood-brain barrier disruption and brain edema were evaluated by immunohistological detection of IgG extravasation and measurement of water content by dry-wet weight and specific gravity methods.

RESULTS

In 1-week-old rats, T2 values, areas of hyperintensity on T2-weighted images, and water content in the ipsilateral hemisphere increased during hypoxia-ischemia, recovered at 1 hour after hypoxia-ischemia, and increased again at 24 hours after hypoxia-ischemia. Extravasation of IgG occurred during hypoxia-ischemia and remained detectable 24 hours after hypoxia-ischemia. In 4-week-old rats, an increase in T2 or extravasation of IgG did not occur until 24 hours after hypoxia-ischemia despite a comparable elevation in water content during and soon after hypoxia-ischemia.

CONCLUSIONS

T2 imaging appears reliable for detecting edema associated with disruption of the blood-brain barrier but not necessarily an increase in cerebral water or plasma proteins alone. The different hypoxic-ischemic changes in T2 in immature and older brain are associated with differences in alterations in water content plus extravasation of protein, consistent with age-dependent differences in hypoxic-ischemic alterations in vascular permeability.

Long-term deficits following cerebral hypoxia-ischemia in four-week-old rats: correspondence between behavioral, histological, and magnetic resonance imaging assessments

We examined whether following a hypoxic-ischemic insult in young animals there are long-lasting functional deficits that correlate either to histological tissue damage or to potential compensatory plasticity changes. Four-week-old rats were subjected to an episode of cerebral hypoxia-ischemia (right carotid artery occlusion + 30 min of hypoxia) or a sham operation. In hypoxic-ischemic animals there were gross neurological deficits 1, 24, and 48 h postinsult with recovery by 1 week. Behavioral deficits were observed in both the acquisition and the performance of a response duration differentiation test and a fine motor control test (staircase test) 3 months after the hypoxia-ischemia. Functional magnetic resonance imaging studies demonstrated less activation in the sensory-motor cortex of hypoxic-ischemic animals in response to left vs right forepaw stimulation 4 months postinsult. Histological assessment delineated striatal, cortical, and hippocampal damage in the hypoxic-ischemic hemisphere and a reduction in cortical thickness, bilaterally. GFAP immunoreactivity was increased in injured striatum and cortex. Neurofilament heavy chain (NF200) immunoreactivity was normally most intense in white matter and decreased in areas of ipsilateral cortical damage. Synaptophysin immunoreactivity was reduced around areas of infarction and somewhat increased in adjacent undamaged striatum and in layer IV of parietal cortex. The histological damage or chronic degenerative changes could account for much of the variance in functional outcome detected with sensitive behavioral tests so that overall the compensatory or plasticity changes evident within the juvenile brain are rather modest.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

We examined whether cerebral activation to two different intense and painful stimuli could be detected using functional magnetic resonance imaging (fMRI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain. A set of gradient echo images were acquired and analyzed using our software based on fuzzy cluster analysis (EvIdent). Following the injection of 50 microl of formalin (5%) into the forepaw we observed a regional increase in signal intensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated frequently and often bilaterally. Surprisingly, activation appeared sequentially, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-treatment (1 mg/kg, i. v.) delayed and/or reduced the intensity of the activation resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0. 3 ms, 3 Hz) of the forepaw, activation was observed consistently in the sensory-motor cortex contralateral to the stimulation. Activation also occurred frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were markedly reduced during nitrous oxide inhalation. Treatment with morphine resulted in an inhibition of the activation response to electrical stimulation in most regions except for sensory-motor cortex. Thus, electrical and chemical noxious stimuli activated regions that are known to be involved in the central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provide an effective new tool with which to study novel analgesics and their effects on the CNS processing of pain in animal models.

Effect of long-term vigabatrin administration on the immature rat brain

PURPOSE

To determine whether the neuropathologic changes produced by vigabatrin (VGB; gamma-vinyl GABA) administration in the developing rat brain are reversible.

METHODS

We injected rats daily with VGB (25-40 mg/kg/day, s.c.) from age 12 days for 2 weeks followed by 2 weeks of a drug-free period. Behavioral testing, magnetic resonance (MR) imaging, biochemical assays, and histologic technique were used to assess the adverse effect of VGB in developing brain and its reversibility.

RESULTS

At the end of 2 weeks' VGB administration: (a) there was a hyperactivity and a shortened latency to escape out of cool water; (b) white matter appeared hyperintense in T2 and diffusion-weighted MR images with 4-15% increases in T2; (c) microvacuolation, TUNEL-positive nuclei, and swollen axons were observed in the corpus callosum; (d) myelin staining indicated a reduction in myelination, as did the reduction in activities of myelin and oligodendrocyte-associated enzymes and the decrease in myelin basic protein on Western blots. Two weeks after stopping VGB administration: (a) MR images were normal, and microvacuolation was no longer in the white matter; (b) reduction in myelination reversed partially; (c) the T2 relaxation time remained elevated in the hypothalamus; and (d) the behavioral response remained abnormal.

CONCLUSIONS

Long-term VGB administration to young rats causes brain injury, which recovers partially on its cessation. The observed cell death, disrupted myelination, and alterations in behavior indicate a need for further safety assessment in infants and children.

Prevention of both T2- and diffusion-weighted increases in image intensity during cerebral hypoxia-ischemia in infant rats pretreated with dexamethasone

The present study examines the effect of dexamethasone treatment on the intensity of changes in T2-weighted and diffusion-weighted (DW) magnetic resonance images occurring in infant rats during and after cerebral hypoxia-ischemia. The right carotid artery was occluded under isoflurane anesthesia in 7-day-old rats and images were acquired in sedated animals using a Bruker 9.4 T magnetic resonance (MR) system. Imaging changes were markedly different in rats pretreated with dexamethasone phosphate (0.1 mg/kg, i.p.) 24 h before hypoxia than in controls. In control animals, areas of hyperintensity ipsilateral to the occlusion occurred during hypoxia-ischemia in both the DW- and T2-weighted images with some recovery of the changes in early posthypoxia. In contrast, in dexamethasone-treated animals, areas of increased hyperintensity in the MR images did not occur. Thus, dexamethasone treatment prevents MR imaging changes during ischemia, suggesting that the cytotoxic edema associated with energy depletion and/or ionic disturbances during ischemia are also prevented by dexamethasone treatment.