Development of therapeutics for brain diseases has remained challenging, in particular due to the difficulty of passing the blood-brain barrier.

Midlife hypertension is an important risk factor for cognitive impairment and dementia, including Alzheimer’s disease. We investigated the effects of long-term treatment with two classes of antihypertensive drugs to determine whether diverging mechanisms of blood pressure lowering impact the brain differently. Spontaneously hypertensive rats (SHR) were either left untreated or treated with a calcium channel blocker (amlodipine) or beta blocker (atenolol) until one year of age. The normotensive Wistar Kyoto rat (WKY) was used as a reference group. Both drugs lowered blood pressure equally, while only atenolol decreased heart rate. Cerebrovascular resistance was increased in SHR, which was prevented by amlodipine but not atenolol. SHR showed a larger carotid artery diameter with impaired pulsatility, which was prevented by atenolol. Cerebral arteries demonstrated inward remodelling, stiffening and endothelial dysfunction in SHR. Both treatments similarly improved these parameters. MRI revealed that SHR have smaller brains with enlarged ventricles. In addition, neurofilament light levels were increased in cerebrospinal fluid of SHR. However, neither treatment affected these parameters. In conclusion, amlodipine and atenolol both lower blood pressure, but elicit a different hemodynamic profile. Both medications improve cerebral artery structure and function, but neither drug prevented indices of brain damage in this model of hypertension.

There is a discrepancy between successful recanalization and good clinical outcome after endovascular treatment (EVT) in acute ischemic stroke patients. During removal of a thrombus, a shower of microemboli may release and lodge to the distal circulation. The objective of this study was to determine the extent of damage on brain tissue caused by microemboli. In a rat model of microembolization, a mixture of microsphere (MS) sizes (15, 25 and 50 mu m diameter) was injected via the left internal carotid artery. A 3D image of the left hemisphere was reconstructed and a point-pattern spatial analysis was applied based on G- and K-functions to unravel the spatial correlation between MS and the induced hypoxia or infarction. We show a spatial correlation between MS and hypoxia or infarction spreading up to a distance of 1000-1500 mu m. These results imply that microemboli, which individually may not always be harmful, can interact and result in local areas of hypoxia or even infarction when lodged in large numbers.

Objectives: Endovascular treatment (EVT) has become the standard of care for acute ischemic stroke. Despite successful recanalization, a limited subset of patients benefits from the new treatment. Human MRI studies have shown that during removal of the thrombus, a shower of microclots is released from the initial thrombus, possibly causing new ischemic lesions. The aim of the current study is to quantify tissue damage following microembolism. Materials and methods: In a rat model, microembolism was generated by injection of a mixture of polystyrene fluorescent microspheres (15, 25 and 50 mu m in diameter). The animals were killed at three time-points: day 1, 3 or 7. AMIRA and IMARIS software was used for 3D reconstruction of brain structure and damage, respectively. Conclusions: Microembolism induces ischemia, hypoxia and infarction. Infarcted areas persist, but hypoxic regions recover over time suggesting that repair processes in the brain rescue the regions at risk.

The publisher regrets that in the abstract, the RESULTS section was omitted from the published version. The following information has been added to the abstract: Results Microscopic analysis of 500 μm thick brain volumes revealed multiple ischemic, hypoxic and infarcted regions. Both the number and total volume of hypoxic and ischemic areas declined over time, whereas the infarcts remained. The publisher would like to apologise for any inconvenience caused. Recovery of Hypoxic Regions in a Rat Model of MicroembolismJournal of Stroke and Cerebrovascular DiseasesVol. 30Issue 6PreviewAcute ischemic stroke accounts for about 80% of stroke cases.1 The introduction of endovascular treatment (EVT), where a thrombus is removed by means of a retrieval device, has led to a breakthrough for the therapy of acute ischemic stroke.2 However, although in the large majority of cases successful recanalization of the occluded vessel is attained3,4, only one third of patients is left with a good clinical outcome, defined as modified Rankin Scale from 0 to 2 at 90 days follow-up.5 Full-Text PDF Open Access

The hippocampus is susceptible to protein aggregation in neurodegenerative diseases such as Alzheimer's disease. This protein accumulation is partially attributed to an impaired clearance; however, the removal pathways for fluids and waste products are not fully understood. The aim of this study was therefore to map the clearance pathways from the mouse brain. A mixture of two fluorescently labeled tracers with different molecular weights was infused into the hippocampus. A small subset of mice (n = 3) was sacrificed directly after an infusion period of 10 min to determine dispersion of the tracer due to the infusion, while another group was sacrificed after spreading of the tracers for an additional 80 min (n = 7). Upon sacrifice, mice were frozen and sectioned as a whole by the use of a custom-built automated imaging cryomicrotome. Detailed 3D reconstructions were created to map the tracer spreading. We observed that tracers distributed over the hippocampus and entered adjacent brain structures, such as the cortex and cerebroventricular system. An important clearance pathway was found along the ventral part of the hippocampus and its bordering interpeduncular cistern. From there, tracers left the brain via the subarachnoid spaces in the directions of both the nose and the spinal cord. Although both tracers followed the same route, the small tracer distributed further, implying a major role for diffusion in addition to convection. Taken together, these results reveal an important clearance pathway of solutes from the hippocampus.

BackgroundMany processes contributing to the functional and structural regulation of the coronary circulation have been identified. A proper understanding of the complex interplay of these processes requires a quantitative systems approach that includes the complexity of the coronary network. The purpose of this study was to provide a detailed quantification of the branching characteristics and local hemodynamics of the human coronary circulation.MethodsThe coronary arteries of a human heart were filled post-mortem with fluorescent replica material. The frozen heart was alternately cut and block-face imaged using a high-resolution imaging cryomicrotome. From the resulting 3D reconstruction of the left coronary circulation, topological (node and loop characteristics), topographic (diameters and length of segments), and geometric (position) properties were analyzed, along with predictions of local hemodynamics (pressure and flow).ResultsThe reconstructed left coronary tree consisted of 202,184 segments with diameters ranging from 30 μm to 4 mm. Most segments were between 100 μm and 1 mm long. The median segment length was similar for diameters ranging between 75 and 200 μm. 91% of the nodes were bifurcations. These bifurcations were more symmetric and less variable in smaller vessels. Most of the pressure drop occurred in vessels between 200 μm and 1 mm in diameter. Downstream conductance variability affected neither local pressure nor median local flow and added limited extra variation of local flow. The left coronary circulation perfused 358 cm3 of myocardium. Median perfused volume at a truncation level of 100 to 200 μm was 20 mm3 with a median perfusion of 5.6 ml/min/g and a high local heterogeneity.ConclusionThis study provides the branching characteristics and hemodynamic analysis of the left coronary arterial circulation of a human heart. The resulting model can be deployed for further hemodynamic studies at the whole organ and local level.

In sickle cell disease (SCD), oxygen delivery is impaired due to anemia, especially during times of increased metabolic demand, and cerebral blood flow (CBF) must increase to meet changing physiologic needs. But hyperemia limits cerebrovascular reserve (CVR) and ischemic risk prevails despite elevated CBF. The cerebral metabolic rate of oxygen (CMRO2 ) directly reflects oxygen supply and consumption and may therefore be more insightful than flow-based CVR measures for ischemic risk in SCD. We hypothesized that adults with SCD have impaired CMRO2 at rest and that a vasodilatory challenge with acetazolamide would improve CMRO2 . CMRO2 was calculated from CBF and oxygen extraction fraction (OEF), measured with arterial spin labeling and T2 -prepared tissue relaxation with inversion recovery (T2 -TRIR) MRI. We studied 36 adults with SCD without a clinical history of overt stroke, and nine healthy controls. As expected, CBF was higher in patients with SCD versus controls (mean ± SD: 74 ± 16 versus 46 ± 5 mL/100 g/min, P < .001), resulting in similar oxygen delivery (SCD: 377 ± 67 versus controls: 368 ± 42 μmol O2 /100g/min, P = .69). OEF was lower in patients versus controls (27 ± 4 versus 35 ± 4%, P < .001), resulting in lower CMRO2 in patients versus controls (102 ± 24 versus 127 ± 20 μmol O2 /100g/min, P = .002). After acetazolamide, CMRO2 declined further in patients (P < .01) and did not decline significantly in controls (P = .78), indicating that forcing higher CBF worsened oxygen utilization in SCD patients. This lower CMRO2 could reflect variation between healthy and unhealthy vascular beds in terms of dilatory capacity and resistance whereby dysfunctional vessels become more oxygen-deprived, hence increasing the risk of localized ischemia.

In this chapter, we briefly summarize the structure and physiology of the cerebral (micro)circulation. Specific features of the brain include the lack of a lymphatic system and the volume restrictions imposed by the skull. Strong autoregulation, collateral vessels, paravascular spaces, and a distribution of resistance over arteries, arterioles, and capillaries are characteristics of the cerebral vasculature. The neurovascular unit, consisting of a close interplay between capillary endothelial cells, pericytes, and astrocytes, allows regulation of blood flow down to the capillary level to meet local demand. At the level of the endothelium, the blood-brain barrier is a distinct feature of the cerebral vasculature. We then review more recent insights, mainly based on experimental research, regarding the pathophysiological changes that occur in the microcirculation associated with hypertension, stroke, microinfarcts, and dementia. These include impaired microvascular perfusion after stroke and novel mechanisms of clot extravasation.

Background Proper neuronal function is directly dependent on the composition, turnover, and amount of interstitial fluid that bathes the cells. Most of the interstitial fluid is likely to be derived from ion and water transport across the brain capillary endothelium, a process that may be altered in hypertension due to vascular pathologies as endothelial dysfunction and arterial remodelling. In the current study, we investigated the effects of hypertension on the brain for differences in the water homeostasis. Methods Magnetic resonance imaging (MRI) was performed on a 7T small animal MRI system on male spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) of 10 months of age. The MRI protocol consisted of T2-weighted scans followed by quantitative apparent diffusion coefficient (ADC) mapping to measure volumes of different anatomical structures and water diffusion respectively. After MRI, we assessed the spatial distribution of aquaporin 4 expression around blood vessels. Results MRI analysis revealed a significant reduction in overall brain volume and remarkably higher cerebroventricular volume in SHR compared to WKY. Whole brain ADC, as well as ADC values of a number of specific anatomical structures, were significantly lower in hypertensive animals. Additionally, SHR exhibited higher brain parenchymal water content. Immunohistochemical analysis showed a profound expression of aquaporin 4 around blood vessels in both groups, with a significantly larger area of influence around arterioles. Evaluation of specific brain regions revealed a decrease in aquaporin 4 expression around capillaries in the corpus callosum of SHR. Conclusion These results indicate a shift in the brain water homeostasis of adult hypertensive rats.

New Findings What is the topic of this review? In this symposium report, we review the glymphatic clearance from the brain. What advances does it highlight? Evaluation of the evidence indicates that cerebrospinal fluid flows along paravascular spaces at the surface of the brain. However, bulk flow along penetrating arteries into the brain, followed by exit along veins, requires further confirmation. Clearance from the brain, based on mixing, might provide an alternative explanation for experimental findings. AbstractThe interstitial fluid of the brain provides the environment for proper neuronal function. Maintenance of the volume and composition of interstitial fluid requires regulation of the influx and removal of water, ions, nutritive and waste products. The recently described glymphatic pathway might contribute to some of these functions. It is proposed that cerebrospinal fluid enters the brain via paravascular spaces along arteries, mixes with interstitial fluid, and leaves the brain via paravascular spaces along veins. In this symposium report, we review the glymphatic concept, its concerns, and alternative views on interstitial fluid–cerebrospinal fluid exchange.

Background and Purpose- We developed a rat model of silent brain infarcts based on microsphere infusion and investigated their impact on perfusion and tissue damage. Second, we studied the extent and mechanisms of perfusion recovery. Methods- At day 0, 15 µm fluorescent microspheres were injected into the right common carotid artery of F344 rats. At days 1, 7, or 28, the brain was removed, cut in 100-µm cryosections, and processed for immunofluorescent staining and analysis. Results- Injection of microspheres caused mild and transient damage to the treated hemisphere, with a decrease in perfused capillary volume at day 1, as compared with the untreated hemisphere. At day 1 but not at days 7 and 28, we observed IgG staining outside of the vessels, indicating vessel leakage. All microspheres were located inside the lumen of the vessels at day 1, whereas the vast majority (≈80%) of the microspheres were extravascular at day 7, and 100% at day 28. This was accompanied by restoration of perfused capillary volume. Conclusions- Microspheres cause mild and transient damage, and effective extravasation mechanisms exist in the brain to clear microsized emboli from the vessels.

Background:MRI studies have demonstrated a high prevalence of silent cerebral infarcts (SCI) in both children and adults with sickle cell disease (SCD). SCI are associated with cognitive impairment and lesion progression in adults with SCD. Disrupted oxygen transport can contribute to cerebral ischemic lesions, despite the compensatory elevation in cerebral blood flow (CBF) in SCD. Investigating the cerebral metabolic rate of oxygen (CMRO2) may therefore give insight into the hemodynamic etiology of SCI in SCD patients. We hypothesized that CMRO2 is reduced in adult patients with SCD as a result of chronic anemia and that vasodilation can improve CMRO2 by generating an increase in blood and oxygen flow.Aims:The purpose of this study was to measure CBF and OEF to compute CMRO2 and to measure the response to vasodilation with acetazolamide.Methods:We included adults with SCD (HbSS or HbSb0‐thalassaemia), and age‐, sex‐, and race‐matched controls. Exclusion criteria were contraindications to MRI and acetazolamide, and a clinical history of a cerebrovascular event or neurologic disease affecting cerebral autoregulation.The MRI protocol comprised fluid‐attenuated inversion recovery (FLAIR) for anatomy and lesions, T2‐prepared tissue relaxation with inversion recovery (T2‐TRIR) for oxygenation and T1 of blood, and pseudo continuous arterial spin labelling (pCASL) for cerebral blood flow. Paired tests were used to assess statistical significance of changes from baseline to post‐acetazolamide conditions. P < .05 was considered significant. Variables were summarized by means and standard deviations.Results:Hb was lower in patients compared to controls (8.8 ± 1.4 vs 13.7 ± 1.3 g/dL, P < .001). Oxygen delivery was similar in patients compared to controls (371 ± 70 vs 368 ± 39 μmol O2/100 g/min, P = .832), due to compensatory elevated CBF in patients versus controls (73 ± 16 vs 46 ± 4 mL/100 g/min, P < .001). OEF was lower in patients compared to controls (27.1 ± 4.4 vs 35.3 ± 3.6 %, P = .001). SCD patients had lower CMRO2 compared to controls (99.7 ± 24.2 vs 126.6 ± 18.9 μmol O2/100 g/min, P = .003) (Figure 2). Cerebrovascular reserve was lower in patients versus controls (43.1 ± 12.5 vs 67.0 ± 20.0%, P = .003). After acetazolamide, CMRO2 did not change significantly in the healthy controls (P = .844), but declined in SCD patients (P = .036), showing that the ACZ challenge was indeed isometabolic in controls but not in patients. Finally, we found that increased CBF after ACZ was accompanied by a drop in OEF in patients with SCD (P < .001) as well as in controls (P = .003).Summary/Conclusion:Our findings support our hypothesis that SCD patients have reduced CMRO2. This indicates that in this chronic condition, CMRO2, and therefore likely also neuronal function, have adapted to chronic low oxygen availability. Heterogeneous flow distribution could be the reason for the reduced CMRO2 we observed in patients with SCD, whereby oxygen shunted to the venous side without delivering oxygen to the brain tissue, and this was exacerbated by acetazolamide. We conclude that CMRO2 does not adjust quickly to transient changes in oxygen availability at the microvascular level despite the increase in CBFimage

Sickle cell disease is characterized by chronic hemolytic anemia and vascular inflammation, which can diminish the vasodilatory capacity of the small resistance arteries, making them less adept at regulating cerebral blood flow. Autoregulation maintains adequate oxygen delivery, but when vasodilation is maximized, the low arterial oxygen content can lead to ischemia and silent cerebral infarcts. We used magnetic resonance imaging of cerebral blood flow to quantify whole-brain cerebrovascular reserve in 36 adult patients with sickle cell disease (mean age, 31.9±11.3 years) and 11 healthy controls (mean age, 37.4±15.4 years), and we used high-resolution 3D FLAIR magnetic resonance imaging to determine the prevalence of silent cerebral infarcts. Cerebrovascular reserve was calculated as the percentage change in cerebral blood flow after a hemodynamic challenge with acetazolamide. Co-registered lesion maps were used to demonstrate prevalent locations for silent cerebral infarcts. Cerebral blood flow was elevated in patients with sickle cell disease compared to controls (median [interquartile range]: 82.8 [20.1] vs 51.3 [4.8] mL/100g/min, P<0.001). Cerebral blood flow was inversely associated with age, hemoglobin, and fetal hemoglobin, and correlated positively with bilirubin, and LDH, indicating that cerebral blood flow may reflect surrogates of hemolytic rate. Cerebrovascular reserve in sickle cell disease was decreased by half compared to controls (34.1 [33.4] vs 69.5 [32.4] %, P<0.001) and was associated with hemoglobin and erythrocyte count indicating anemia-induced hemodynamic adaptations. In total, 29/36 patients (81%) and 5/11 controls (45%) had silent cerebral infarcts (median volume of 0.34 vs 0.02 mL, P=0.03). Lesions were preferentially located in the borderzone. In conclusion, patients with sickle cell disease have a globally reduced cerebrovascular reserve as determined by arterial spin labeling with acetazolamide and reflects anemia-induced impaired vascular function in sickle cell disease. This study was registered at clinicaltrials.gov identifier 02824406.

BACKGROUND:Drift is a well-known issue affecting intracoronary pressure measurements. A small pressure offset at the end of the procedure is generally considered acceptable, while repeat assessment is advised for drift exceeding ±2 mmHg. This practice implies that drift assessed after wire pullback equals that at the time of stenosis appraisal, but this assumption has not been systematically investigated. Our aim was to compare intra-and post-procedural pressure sensor drift and assess benefits of correction for intra-procedural drift and its effect on diagnostic classification.METHODS:In 70 patients we compared intra- and post-procedural pressure drift for 120 hemodynamic tracings obtained at baseline and throughout the hyperemic response to intracoronary adenosine. Intra-procedural drift was derived from the intercept of the stenosis pressure gradient-velocity relationship. Diagnostic reclassification after correction for intra-procedural drift was assessed for the mean distal-to-aortic pressure ratio at baseline (Pd/Pa) and hyperemia (fractional flow reserve, FFR), and corresponding stenosis resistances.RESULTS:Post- and intra-procedural drift exceeding the tolerated threshold was observed in 73% and 64% of the hemodynamic tracings, respectively. Discordance in terms of acceptable drift level was present for 42% of the tracings, with avoidable repeat physiological assessment in 25% and unacceptable intra-procedural drift unrecognized at final drift check in 17% of the tracings. Correction for intra-procedural drift caused higher reclassification rates for baseline than hyperemic functional indices.CONCLUSIONS:Post-procedural pressure drift frequently does not match drift during physiological assessment. Tracing-specific correction for intra-procedural drift can potentially lower the risk of inadvertent diagnostic misclassification and prevent unnecessary repeats.

Hypertension is an important risk factor for cerebrovascular disease, including stroke and dementia. Both in humans and animal models of hypertension, neuropathological features such as brain atrophy and oedema have been reported. We hypothesised that cerebrovascular damage resulting from chronic hypertension would manifest itself in a more permeable blood–brain barrier and blood–cerebrospinal fluid barrier. In addition, more leaky barriers could potentially contribute to an enhanced interstitial fluid and cerebrospinal fluid formation, which could, in turn, lead to an elevated intracranial pressure.