Altered brain fluid management in a rat model of arterial hypertension

In Vivo Measurement of Rat Brain Water Content at 9.4 T MR Using Super-Resolution Reconstruction: Validation With Ex Vivo Experiments

BACKGROUND

Given that changes in brain water content are often correlated with disease, investigating water content non-invasively and in vivo could lead to a better understanding of the pathogenesis of several neurologic diseases.

PURPOSE

To adapt a super-resolution-based technique, previously developed for humans, to the rat brain and report in vivo high-resolution (HR) water content maps in comparison with ex vivo wet/dry methods.

STUDY TYPE

Prospective.

ANIMAL MODEL

Eight healthy male Wistar rats.

FIELD STRENGTH/SEQUENCE

9.4-T, multi-echo gradient-echo (mGRE) sequence.

ASSESSMENT

Using super-resolution reconstruction (SRR), a HR mGRE image (200 μm isotropic) was reconstructed from three low-resolution (LR) orthogonal whole-brain images in each animal, which was followed by water content mapping in vivo. The animals were subsequently sacrificed, the brains excised and divided into five regions (front left, front right, middle left, middle right, and cerebellum-brainstem regions), and the water content was measured ex vivo using wet/dry measurements as the reference standard. The water content values of the in vivo and ex vivo methods were then compared for the whole brain and also for the different regions separately.

STATISTICAL TESTS

Friedman's non-parametric test was used to test difference between the five regions, and Pearson's correlation coefficient was used for correlation between in vivo and ex vivo measurements. A P-value <0.05 was considered statistically significant.

RESULTS

Water content values derived from in vivo MR measurements showed strong correlations with water content measured ex vivo at a regional level (r = 0.902). Different brain regions showed significantly different water content values. Water content values were highest in the frontal brain, followed by the midbrain, and lowest in the cerebellum and brainstem regions.

DATA CONCLUSION

An in vivo technique to achieve HR isotropic water content maps in the rat brain using SRR was adopted in this study. The MRI-derived water content values obtained using the technique showed strong correlations with water content values obtained using ex vivo wet/dry methods.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY: Stage 1.

Altered Brain Glymphatic Function at Diffusion-Tensor MRI in Pre-cirrhotic Metabolic Dysfunction-Associated Fatty Liver Disease

RATIONALE AND OBJECTIVES

To evaluate glymphatic function changes and their relationships with clinical features in patients with metabolic dysfunction-associated fatty liver disease (MAFLD), thereby facilitating early intervention before this disease progresses to cirrhosis.

MATERIALS AND METHODS

A cross-sectional cohort of 46 pre-cirrhotic MAFLD patients and 30 age-, sex-, and education-matched controls was enrolled, with diffusion-tensor imaging (DTI) data, laboratory and neurocognitive scores collected. The DTI analysis along the perivascular space (DTI-ALPS) index was computed for qualifying glymphatic function. Generalized linear model and partial correlation analyses were applied to evaluate relationships between the ALPS index and clinical variables.

RESULTS

MAFLD group exhibited a decreased ALPS index and increased diffusivity along the y-axis in the projection fiber compared to the controls. The altered ALPS index was associated with clock drawing test (CDT) score (3.931 [0.914, 6.947], P = 0.011) and was correlated with diastolic pressure level (r = -0.315, P = 0.033) in MAFLD group. The relationships of ALPS index with CDT score (6.263 [2.069, 10.458], P = 0.003) and diastolic pressure level (r = -0.518, P = 0.014) remained in the MAFLD with metabolic syndrome (MetS) group. Furthermore, the ALPS index was even associated with Auditory Verbal Learning Test-Immediate recall score (-23.853 [-45.417, -2.289], P = 0.030) in MAFLD with MetS group.

CONCLUSION

MAFLD patients may have a glymphatic dysfunction prior to cirrhosis, and this alteration may be related to cognition and diastolic pressure. Glymphatic dysfunction has a more severe impact on cognition when MAFLD patient is accompanied by MetS.

Relationship of Perivascular Space Markers With Incident Dementia in Cerebral Small Vessel Disease

BACKGROUND

Recent studies, using diffusion tensor image analysis along the perivascular space (DTI-ALPS), suggest impaired perivascular space (PVS) function in cerebral small vessel disease, but they were cross-sectional, making inferences on causality difficult. We determined associations between impaired PVS, measured using DTI-ALPS and PVS volume, and cognition and incident dementia.

METHODS

In patients with lacunar stroke and confluent white matter hyperintensities, without dementia at baseline, recruited prospectively in a single center, magnetic resonance imaging was performed annually for 3 years, and cognitive assessments, including global, memory, executive function, and processing speed, were performed annually for 5 years. We determined associations between DTI-ALPS and PVS volume with cerebral small vessel disease imaging markers (white matter hyperintensity volume, lacunes, and microbleeds) at baseline and with changes in imaging markers. We determined whether DTI-ALPS and PVS volume at baseline and change over 3 years predicted incident dementia. Analyses were controlled for conventional diffusion tensor image metrics using 2 markers (median mean diffusivity [MD] and peak width of skeletonized MD) and adjusted for age, sex, and vascular risk factors.

RESULTS

A total of 120 patients, mean age 70.0 years and 65.0% male, were included. DTI-ALPS declined over 3 years, while no change in PVS volume was found. Neither DTI-ALPS nor PVS volume was associated with cerebral small vessel disease imaging marker progression. Baseline DTI-ALPS was associated with changes in global cognition (β=0.142, P=0.032), executive function (β=0.287, P=0.027), and long-term memory (β=0.228, P=0.027). Higher DTI-ALPS at baseline predicted a lower risk of dementia (hazard ratio, 0.328 [0.183-0.588]; P<0.001), and this remained significant after including median MD as a covariate (hazard ratio, 0.290 [0.139-0.602]; P<0.001). Change in DTI-ALPS predicted dementia conversion (hazard ratio, 0.630 [0.428-0.964]; P=0.048), but when peak width of skeletonized MD and median MD were entered as covariates, the association was not significant. There was no association between baseline PVS volume, or PVS change over 3 years, and conversion to dementia.

CONCLUSIONS

DTI-ALPS predicts future dementia risk in patients with lacunar strokes and confluent white matter hyperintensities. However, the weakening of the association between change in DTI-ALPS and incident dementia after controlling for peak width of skeletonized MD and median MD suggests part of the signal may represent conventional diffusion tensor image metrics. PVS volume is not a predictor of future dementia risk.

The relationship between amyloid pathology, cerebral small vessel disease, glymphatic dysfunction, and cognition: a study based on Alzheimer's disease continuum participants

BACKGROUND

Glymphatic dysfunction is a crucial pathway for dementia. Alzheimer's disease (AD) pathologies co-existing with cerebral small vessel disease (CSVD) is the most common pathogenesis for dementia. We hypothesize that AD pathologies and CSVD could be associated with glymphatic dysfunction, contributing to cognitive impairment.

METHOD

Participants completed with amyloid PET, diffusion tensor imaging (DTI), and T2 fluid-attenuated inversion-recovery (FLAIR) sequences were included from the Alzheimer's Disease Neuroimaging Initiative (ADNI). White matter hyperintensities (WMH), the most common CSVD marker, was evaluated from T2FLAIR images and represented the burden of CSVD. Amyloid PET was used to assess Aβ aggregation in the brain. We used diffusion tensor image analysis along the perivascular space (DTI-ALPS) index, the burden of enlarged perivascular spaces (PVS), and choroid plexus volume to reflect glymphatic function. The relationships between WMH burden/Aβ aggregation and these glymphatic markers as well as the correlations between glymphatic markers and cognitive function were investigated. Furthermore, we conducted mediation analyses to explore the potential mediating effects of glymphatic markers in the relationship between WMH burden/Aβ aggregation and cognition.

RESULTS

One hundred and thirty-three participants along the AD continuum were included, consisting of 40 CN - , 48 CN + , 26 MCI + , and 19 AD + participants. Our findings revealed that there were negative associations between whole-brain Aβ aggregation (r =  - 0.249, p = 0.022) and WMH burden (r =  - 0.458, p < 0.001) with DTI-ALPS. Additionally, Aβ aggregation (r = 0.223, p = 0.041) and WMH burden (r = 0.294, p = 0.006) were both positively associated with choroid plexus volume. However, we did not observe significant correlations with PVS enlargement severity. DTI-ALPS was positively associated with memory (r = 0.470, FDR-p < 0.001), executive function (r = 0.358, FDR-p = 0.001), visual-spatial (r = 0.223, FDR-p < 0.040), and language (r = 0.419, FDR-p < 0.001). Conversely, choroid plexus volume showed negative correlations with memory (r =  - 0.315, FDR-p = 0.007), executive function (r =  - 0.321, FDR-p = 0.007), visual-spatial (r =  - 0.233, FDR-p = 0.031), and language (r =  - 0.261, FDR-p = 0.021). There were no significant correlations between PVS enlargement severity and cognitive performance. In the mediation analysis, we found that DTI-ALPS acted as a mediator in the relationship between WMH burden/Aβ accumulation and memory and language performances.

CONCLUSION

Our study provided evidence that both AD pathology (Aβ) and CSVD were associated with glymphatic dysfunction, which is further related to cognitive impairment. These results may provide a theoretical basis for new targets for treating AD.

Spontaneously hypertensive rats can become hydrocephalic despite undisturbed secretion and drainage of cerebrospinal fluid

BACKGROUND

Hydrocephalus constitutes a complex neurological condition of heterogeneous origin characterized by excessive cerebrospinal fluid (CSF) accumulation within the brain ventricles. The condition may dangerously elevate the intracranial pressure (ICP) and cause severe neurological impairments. Pharmacotherapies are currently unavailable and treatment options remain limited to surgical CSF diversion, which follows from our incomplete understanding of the hydrocephalus pathogenesis. Here, we aimed to elucidate the molecular mechanisms underlying development of hydrocephalus in spontaneously hypertensive rats (SHRs), which develop non-obstructive hydrocephalus without the need for surgical induction.

METHODS

Magnetic resonance imaging was employed to delineate brain and CSF volumes in SHRs and control Wistar-Kyoto (WKY) rats. Brain water content was determined from wet and dry brain weights. CSF dynamics related to hydrocephalus formation in SHRs were explored in vivo by quantifying CSF production rates, ICP, and CSF outflow resistance. Associated choroid plexus alterations were elucidated with immunofluorescence, western blotting, and through use of an ex vivo radio-isotope flux assay.

RESULTS

SHRs displayed brain water accumulation and enlarged lateral ventricles, in part compensated for by a smaller brain volume. The SHR choroid plexus demonstrated increased phosphorylation of the Na⁺/K⁺/2Cl^- cotransporter NKCC1, a key contributor to choroid plexus CSF secretion. However, neither CSF production rate, ICP, nor CSF outflow resistance appeared elevated in SHRs when compared to WKY rats.

CONCLUSION

Hydrocephalus development in SHRs does not associate with elevated ICP and does not require increased CSF secretion or inefficient CSF drainage. SHR hydrocephalus thus represents a type of hydrocephalus that is not life threatening and that occurs by unknown disturbances to the CSF dynamics.

Effect of long-term antihypertensive treatment on cerebrovascular structure and function in hypertensive rats

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.

Neuroimaging findings related to glymphatic system alterations in older adults with metabolic syndrome

OBJECTIVE

The glymphatic system is a glial-based perivascular network that promotes brain metabolic waste clearance. Reduced glymphatic flow has been observed in rat models of type 2 diabetes and hypertension, indicating the role of vascular risk factors in the glymphatic system. However, little is known about how vascular risk factors affect the human glymphatic system. The present study aims to assess the relationships between metabolic syndrome (MetS), a cluster of vascular risk factors, and the glymphatic system function using diffusion magnetic resonance imaging (MRI)-based measures of water diffusivity in the glymphatic compartments, including the brain interstitial space and perivascular spaces around the deep medullary vein. We hypothesized that vascular risk factors are associated with glymphatic dysfunction, leading to cognitive impairment in older adults.

METHODS

This cross-sectional study assessed 61 older adults (age range, 65-82 years) who had participated in the Bunkyo Health Study, including 15 healthy controls (mean age, 70.87 ± 4.90 years) and 46 individuals with MetS (mean age, 71.76 ± 4.61 years). Fractional volume of extracellular-free water (FW) and an index of diffusion tensor imaging along the perivascular space (DTI-ALPS) were used as indirect indicators of water diffusivity in the interstitial extracellular and perivenous spaces of white matter, respectively.

RESULTS

After adjusting for age, sex, years of education, total Fazekas scale, Pittsburgh sleep quality index (PSQI) score, and intracranial volume (ICV), a significantly (P = 0.030; Cohen's d = 1.01) higher FW was observed in individuals with MetS than in the healthy controls. Furthermore, individuals with MetS had a significantly (P = 0.031; Cohen's d = 0.86) lower ALPS index than the healthy controls, with age, sex, years of education, total Fazekas scale, PSQI score, ICV, fractional anisotropy, and mean diffusivity included as confounding factors. Higher FW was significantly associated with lower ALPS index (r = -0.37; P = 0.004). Multiple linear regression (MLR) with backward elimination analyses showed that higher diastolic blood pressure (BP; standardized β = 0.33, P = 0.005) was independently associated with higher FW, whereas higher fasting plasma glucose levels (standardized β = -0.63, P = 0.002) or higher Brinkman index of cigarette consumption cumulative amount (standardized β = -0.27, P = 0.022) were associated with lower ALPS index. The lower ALPS index (standardized β, 0.28; P = 0.040) was associated with poorer global cognitive performance, which was determined using the Japanese version of the Montreal Cognitive Assessment (MOCA-J) scores. Finally, partial correlation analyses showed a significant correlation between higher FW and lower MOCA-J scores (r = -0.35; P = 0.025) and between higher FW and higher diastolic BP (r = 0.32, P = 0.044).

CONCLUSION

The present study shows the changes in diffusion MRI-based measures reflected by the higher FW and lower ALPS index in older adults with MetS, possibly due to the adverse effect of vascular risk factors on the glymphatic system. Our findings also indicate the associations between the diffusion MRI-based measures and elevated diastolic BP, hyperglycemia, smoking habit, and poorer cognitive performance. However, owing to the limitations of this study, the results should be cautiously interpreted.

Determination of brain water content by dry/wet weight measurement for the detection of experimental brain edema

Brain edema is a fatal pathological state in which brain volume increases as a result of abnormal accumulation of fluid within the brain parenchyma. A key attribute of experimentally induced brain edema - increased brain water content (BWC) - needs to be verified. Various methods are used for this purpose: specific gravimetric technique, electron microscopic examination, magnetic resonance imaging (MRI) and dry/wet weight measurement. In this study, the cohort of 40 rats was divided into one control group (CG) and four experimental groups with 8 rats in each group. The procedure for determining BWC using dry/wet weight measurement was initiated 24 h after the completion of edema induction by the water intoxication method (WI group); after the intraperitoneal administration of Methylprednisolone (MP) together with distilled water during edema induction (WI+MP group); 30 min after osmotic blood brain barrier disruption (BBBd group); after injection of MP via the internal carotid artery immediately after BBBd (BBBd + MP group). While induction of brain edema (WI, BBBd) resulted in significantly higher BWC, there was no increase in BWC in the MP groups (WI+MP, BBBd+MP), suggesting a neuroprotective effect of MP in the development of brain edema.

Altered Expression of AQP1 and AQP4 in Brain Barriers and Cerebrospinal Fluid May Affect Cerebral Water Balance during Chronic Hypertension

Hypertension is the leading cause of cardiovascular affection and premature death worldwide. The spontaneously hypertensive rat (SHR) is the most common animal model of hypertension, which is characterized by secondary ventricular dilation and hydrocephalus. Aquaporin (AQP) 1 and 4 are the main water channels responsible for the brain’s water balance. The present study focuses on defining the expression of AQPs through the time course of the development of spontaneous chronic hypertension. We performed immunofluorescence and ELISA to examine brain AQPs from 10 SHR, and 10 Wistar−Kyoto (WKY) rats studied at 6 and 12 months old. There was a significant decrease in AQP1 in the choroid plexus of the SHR-12-months group compared with the age-matched control (p < 0.05). In the ependyma, AQP4 was significantly decreased only in the SHR-12-months group compared with the control or SHR-6-months groups (p < 0.05). Per contra, AQP4 increased in astrocytes end-feet of 6 months and 12 months SHR rats (p < 0.05). CSF AQP detection was higher in the SHR-12-months group than in the age-matched control group. CSF findings were confirmed by Western blot. In SHR, ependymal and choroidal AQPs decreased over time, while CSF AQPs levels increased. In turn, astrocytes AQP4 increased in SHR rats. These AQP alterations may underlie hypertensive-dependent ventriculomegaly.

Evolution of Brain Morphology in Spontaneously Hypertensive and Wistar-Kyoto Rats From Early Adulthood to Aging: A Longitudinal Magnetic Resonance Imaging Study

The influence of hypertension and aging alone on brain structure has been described extensively. Our understanding of the interaction of hypertension with aging to brain morphology is still limited. We aimed to detect the synergistic effects of hypertension and aging on brain morphology and to describe the evolution patterns of cerebral atrophy from spatial and temporal perspectives. In 8 spontaneously hypertensive rats (SHRs) and 5 Wistar-Kyoto rats, high-resolution magnetic resonance imaging scans were longitudinally acquired at 10, 24, 52, and 80 weeks. We analyzed the tissue volumes of gray matter, white matter, cerebral spinal fluid, and total intracranial volume (TIV), and then evaluated gray matter volume in detail using voxel-based morphometry (VBM) and region of interest-based methods. There were interactive effects on hypertension and aging in tissue volumes of gray matter, white matter, and TIV, of which gray matter atrophy was most pronounced, especially in elderly SHRs. We identified the vulnerable gray matter volume with combined effects of hypertension and aging in the septal region, bilateral caudate putamen, hippocampus, primary somatosensory cortex, cerebellum, periaqueductal gray, right accumbens nucleus, and thalamus. We automatically extracted the septal region, anterior cingulate cortex, primary somatosensory cortex, caudate putamen, hippocampus, and accumbens nucleus and revealed an inverted-U trajectory of volume change in SHRs, with volume increase at the early phase and decline at the late phase. Hypertension interacts with aging to affect brain volume changes such as severe atrophy in elderly SHRs.