Memory impairment in spontaneously hypertensive rats is associated with hippocampal hypoperfusion and hippocampal vascular dysfunction

Impaired memory in Sprague-Dawley rats exposed to complex groundwater mixtures of contaminants is associated with reduced cranial blood flow and hippocampal neurotoxicity

Exposure to industrial contaminants has been implicated in neurobehavioral toxicity in humans. To explore this potential risk, we investigated the neurotoxic effects of oral exposure to a complex groundwater mixture containing petroleum hydrocarbons, pesticides, heavy metals, and unknown parent and breakdown products using male and female Sprague Dawley rats. Rats were randomly divided into six groups and orally exposed daily via drinking water to: (i) tap water, (ii) 10 % v/v low impact groundwater, and (iii) 0.01 %, 0.1 %, 1 %, and 10 % high-impact groundwater for 60 days. Medium- and long-term memory (measured using the novel object recognition task) were impaired. However, no gross motor or coordination deficits were observed by the end of the study period (rotarod test). Doppler ultrasound of the middle cerebral and common carotid arteries was performed to examine the hemodynamic changes. The common carotid blood flow decreased in the groundwater-exposed rats compared to that in the control. However, no significant differences in cerebral blood velocity were observed between the exposed and control groups. A significant reduction in hippocampal serotonin levels was observed in groundwater-exposed rats relative to that in the control group. Collectively, these results indicate that impaired recognition memory in rats exposed to groundwater is accompanied by reduced cranial blood flow and hippocampal neurotoxicity, characterized by altered serotonergic signalling. The levels of detected contaminants known to cause neural or vascular damage were of magnitudes lower than the concentrations of contaminants found in the groundwater mixture, meaning the culprit chemical identity remains unknown. This study emphasizes the need to use whole mixture in exposures when dealing with complex contaminated sites rather than the use of individual compounds.

Age-Related Effects of AT1 Receptor Antagonist Losartan on Cognitive Decline in Spontaneously Hypertensive Rats

Both hypertension and aging are known to increase the vulnerability of the brain to neurovascular damage, resulting in cognitive impairment. The present study investigated the efficacy of the antihypertensive drug losartan on age- and hypertension-associated cognitive decline and the possible mechanism underlying its effect in spontaneously hypertensive rats (SHRs). Losartan was administered (10 mg/kg, i.p. for 19 days) to 3- and 14-month-old SHRs. Age-matched Wistar rats were used as controls. Working memory, short-term object recognition, and spatial memory were assessed using the Y-maze, object recognition test (ORT) and radial arm maze (RAM) test. The expression of markers associated with aging, oxidative stress, and memory-related signaling was assessed in the frontal cortex (FC) and hippocampus. Motor activity measured over 24 h was not different between groups. Middle-aged vehicle-treated SHRs showed poorer performance in spontaneous alternation behavior (SAB) and activity in the first Y-maze test than their younger counterparts, suggesting age-related reduced "decision making" and reactivity in a novel environment. Losartan improved the age- and hypertension-induced decline in short-term recognition and spatial memory measured in the ORT and the second Y-maze test, particularly in the middle-aged rats, but was ineffective in the young adult rats. Changes in memory and age-related markers such as cAMP response element-binding protein (CREB) and amyloid-β1-42 (Aβ1-42) and increased oxidative stress were observed in the hippocampus but not in the FC between young adult and middle-aged vehicle-treated SHRs. Losartan increased CREB expression while reducing Aβ1-42 levels and concomitant oxidative stress in middle-aged SHRs compared with vehicle-treated SHRs. In conclusion, our study highlights the complex interplay between hypertension, aging, and cognitive impairment. It suggests that there is a critical time window for therapeutic intervention with angiotensin II type 1 receptor blockers.

Differential Effects of LOX-1 Inhibition on Aortic Structure and Posterior Cerebral Artery Structure and Function in an Experimental Model of Preeclampsia

Preeclampsia is a hypertensive disorder of pregnancy marked by vascular dysfunction, large artery stiffness, and excess oxidized low-density lipoprotein (oxLDL). oxLDL activates oxidative stress pathways which contribute to arterial stiffness through interaction with the lectin-like oxLDL receptor 1 (LOX-1). Increased vascular stiffness is associated with higher pulse wave velocity and downstream microvasculature damage. Here we evaluated the ability of LOX-1 inhibition (LOX-1i) to prevent large artery structural and microvascular structural and functional changes via assessment of the descending thoracic aorta (DTAo) and posterior cerebral arteries (PCA) in a high cholesterol model of preeclampsia. Adult female Sprague Dawley normal late-pregnant (LP) and experimentally preeclamptic (ePE, high cholesterol diet d7-19) animals underwent intraperitoneal (i.p.) implantation of a mini-osmotic pump at d12 containing LOX-1 neutralizing antibodies (ePE + LOX-1i, n = 7) or goat IgG as vehicle control (LP + IgG, n = 8 and ePE + IgG, n = 8). Animals were studied at d19. DTAos and PCAs were removed for histologic assessment and isolated vessel experiments, respectively. Fetuses and placentas were weighed individually. Plasma was analyzed for markers of oxidative stress. ePE + IgG DTAo elastin content (an indirect metric of stiffness) was not significantly different from the LP + IgG group. Nonetheless, trending elastin break and sinuosity data (higher number of breaks and lower sinuosity in the ePE + IgG group compared to LP + IgG) suggested increased stiffness in this high cholesterol PE model. LOX-1i appeared to prevent a decrease in elastin. PCAs showed no structural changes with ePE or LOX-1i. ePE PCAs had increased reactivity to the nitric oxide donor sodium nitroprusside and decreased tone that was unaffected by LOX-1i. ePE animals had increased plasma oxLDL and 3-nitrotyrosine that was unaffected by LOX-1i. Taken together, LOX-1i may improve large artery stiffness without mitigation of the oxidative stress or cerebral microvascular dysfunction seen in ePE. Understanding these mechanisms is important in abating the long-term risks of preeclampsia.

Hippocampus under Pressure: Molecular Mechanisms of Development of Cognitive Impairments in SHR Rats

Data from clinical trials and animal experiments demonstrate relationship between chronic hypertension and development of cognitive impairments. Here, we review structural and biochemical alterations in the hippocampus of SHR rats with genetic hypertension, which are used as a model of essential hypertension and vascular dementia. In addition to hypertension, dysfunction of the hypothalamic-pituitary-adrenal system observed in SHR rats already at an early age may be a key factor of changes in the hippocampus at the structural and molecular levels. Global changes at the body level, such as hypertension and neurohumoral dysfunction, are associated with the development of vascular pathology and impairment of the blood-brain barrier. Changes in multiple biochemical glucocorticoid-dependent processes in the hippocampus, including dysfunction of steroid hormones receptors, impairments of neurotransmitter systems, BDNF deficiency, oxidative stress, and neuroinflammation are accompanied by the structural alterations, such as cellular signs of neuroinflammation micro- and astrogliosis, impairments of neurogenesis in the subgranular neurogenic zone, and neurodegenerative processes at the level of synapses, axons, and dendrites up to the death of neurons. The consequence of this is dysfunction of hippocampus, a key structure of the limbic system necessary for cognitive functions. Taking into account the available results at various levels starting from the body and brain structure (hippocampus) levels to molecular one, we can confirm translational validity of SHR rats for modeling mechanisms of vascular dementia.

Angiotensin II-mediated hippocampal hypoperfusion and vascular dysfunction contribute to vascular cognitive impairment in aged hypertensive rats

INTRODUCTION

Chronic hypertension increases the risk of vascular cognitive impairment (VCI) by ∼60%; however, how hypertension affects the vasculature of the hippocampus remains unclear but could contribute to VCI.

METHODS

Memory, hippocampal perfusion, and hippocampal arteriole (HA) function were investigated in male Wistar rats or spontaneously hypertensive rats (SHR) in early (4 to 5 months old), mid (8 to 9 months old), or late adulthood (14 to 15 months old). SHR in late adulthood were chronically treated with captopril (angiotensin converting enzyme inhibitor) or apocynin (antioxidant) to investigate the mechanisms by which hypertension contributes to VCI.

RESULTS

Impaired memory in SHR in late adulthood was associated with HA endothelial dysfunction, hyperconstriction, and ∼50% reduction in hippocampal blood flow. Captopril, but not apocynin, improved HA function, restored perfusion, and rescued memory function in aged SHR.

DISCUSSION

Hippocampal vascular dysfunction contributes to hypertension-induced memory decline through angiotensin II signaling, highlighting the therapeutic potential of HAs in protecting neurocognitive health later in life.

HIGHLIGHTS

Vascular dysfunction in the hippocampus contributes to vascular cognitive impairment. Memory declines with age during chronic hypertension. Angiotensin II causes endothelial dysfunction in the hippocampus in hypertension. Angiotensin II-mediated hippocampal arteriole dysfunction reduces blood flow. Vascular dysfunction in the hippocampus impairs perfusion and memory function.

Estrogen regulates myogenic tone in hippocampal arterioles by enhanced basal release of nitric oxide and endothelial SK Ca channel activity

Arteries and arterioles exhibit myogenic tone, a partially constricted state that allows further constriction or dilation in response to moment-to-moment fluctuations in blood pressure. The vascular endothelium that lines the internal surface of all blood vessels controls a wide variety of essential functions, including the contractility of the adjacent smooth muscle cells by providing a tonic vasodilatory influence. Studies conducted on large (pial) arteries on the surface of the brain have shown that estrogen lowers myogenic tone in female mice by enhancing nitric oxide (NO) release from the endothelium, however, whether this difference extends to the intracerebral microcirculation remains ambiguous. The existing incomplete picture of sex differences in cerebrovascular physiology combined with a deficiency in treatments that fully restore cognitive function after cerebrovascular accidents places heavy emphasis on the necessity to investigate myogenic tone regulation in the microcirculation from both male and female mice. We hypothesized that sex-linked hormone regulation of myogenic tone extends its influence on the microcirculation level, and sought to characterize it in isolated arterioles from the hippocampus, a major cognitive brain area. Using diameter measurements both in vivo (acute cranial window vascular diameter) and ex vivo (pressure myography experiments), we measured lower myogenic tone responses in hippocampal arterioles from female than male mice. By using a combined surgical and pharmacological approach, we found myogenic tone in ovariectomized (OVX) female mice matches that of males, as well as in endothelium-denuded arterioles. Interestingly, eNOS inhibition induced a larger constriction in female arterioles but only partially abolished the difference in tone. We identified that the remnant difference was mediated by a higher activity and expression of the small-conductance Ca 2+ -sensitive K + (SK) channels. Collectively, these data indicate that eNOS and SK channels exert greater vasodilatory influence over myogenic tone in female mice at physiological pressures.

Spontaneously hypertensive rats vs. Wistar Kyoto and Wistar rats: an assessment of anxiety, motor activity, memory performance, and seizure susceptibility

Spontaneously hypertensive rats (SHRs) are widely accepted for modeling essential hypertension and Attention deficit hyperactivity disorder (ADHD). However, data concerning central nervous system changes associated with behavioral responses of this strain and usage of Wistar Kyoto (WKY) rats as controls are confounding. The objective of the present study was to assess the impact of anxiety and motor activity on the cognitive responses of SHRs compared to Wistar and WKY rats. In addition, the role of brain-derived neurotrophic factor (BDNF) in the hippocampus on cognitive behavior and seizure susceptibility in the three strains was evaluated. In Experiment#1, SHR demonstrated impulsive responses in the novelty suppression feeding test accompanied by impaired spatial working and associative memory in the Y maze and object recognition test compared with the Wistar rat but not WKY rats. In addition, the WKY rats exhibited diminished activity compared to Wistar rats in an actimeter. In Experiment#2, the seizure susceptibility was assessed by 3-min electroencephalographic (EEG) recording after two consecutive injections of pentylenetetrazol (PTZ) (20+40 mg/kg). The WKY rats were more vulnerable to rhythmic metrazol activity (RMA) than the Wistar rats. In contrast, Wistar rats were more prone to generalized tonic-clonic seizures (GTCS) than WKY rats and SHRs. Control SHR had lower BDNF expression in the hippocampus compared to Wistar rats. However, while the BDNF levels were elevated in the Wistar and WKY rats after PTZ injection, no change in this signaling molecule was observed in the SHR in the seizure condition. The results suggest Wistar rats as a more appropriate control of SHR than WKY rats for studying memory responses mediated by BDNF in the hippocampus. The higher vulnerability to seizures in Wistar and WKY rats compared to SHR might be linked to PTZ-induced decreased expression of BDNF in the hippocampus.

Neurovascular adaptations modulating cognition, mood, and stress responses

The neurovascular unit (NVU) is a dynamic center for substance exchange between the blood and the brain, making it an essential gatekeeper for central nervous system (CNS) homeostasis. Recent evidence supports a role for the NVU in modulating brain function and cognition. In addition, alterations in NVU processes are observed in response to stress, although the mechanisms via which they can affect mood and cognitive functions remain elusive. Here, we summarize recent studies of neurovascular regulation of emotional processes and cognitive function, including under stressful conditions. We also highlight relevant RNA-sequencing (RNA-seq) databases aiming to profile the NVU along with innovative tools to study and manipulate NVU function that can be exploited in the context of cognition and stress research throughout development, aging, or brain disorders.

Hippocampal Vascular Supply and Its Role in Vascular Cognitive Impairment

The incidence of age-related dementia is increasing as the world population ages and due to lack of effective treatments for dementia. Vascular contributions to cognitive impairment and dementia are increasing as the prevalence of pathologies associated with cerebrovascular disease rise, including chronic hypertension, diabetes, and ischemic stroke. The hippocampus is a bilateral deep brain structure that is central to learning, memory, and cognitive function and highly susceptible to hypoxic/ischemic injury. Compared with cortical brain regions such as the somatosensory cortex, less is known about the function of the hippocampal vasculature that is critical in maintaining neurocognitive health. This review focuses on the hippocampal vascular supply, presenting what is known about hippocampal hemodynamics and blood-brain barrier function during health and disease, and discusses evidence that supports its contribution to vascular cognitive impairment and dementia. Understanding vascular-mediated hippocampal injury that contributes to memory dysfunction during healthy aging and cerebrovascular disease is essential to develop effective treatments to slow cognitive decline. The hippocampus and its vasculature may represent one such therapeutic target to mitigate the dementia epidemic.

From attention-deficit hyperactivity disorder to sporadic Alzheimer's disease-Wnt/mTOR pathways hypothesis

Alzheimer's disease (AD) is the most common neurodegenerative disorder with the majority of patients classified as sporadic AD (sAD), in which etiopathogenesis remains unresolved. Though sAD is argued to be a polygenic disorder, apolipoprotein E (APOE) ε4, was found three decades ago to pose the strongest genetic risk for sAD. Currently, the only clinically approved disease-modifying drugs for AD are aducanumab (Aduhelm) and lecanemab (Leqembi). All other AD treatment options are purely symptomatic with modest benefits. Similarly, attention-deficit hyperactivity disorder (ADHD), is one of the most common neurodevelopmental mental disorders in children and adolescents, acknowledged to persist in adulthood in over 60% of the patients. Moreover, for ADHD whose etiopathogenesis is not completely understood, a large proportion of patients respond well to treatment (first-line psychostimulants, e.g., methylphenidate/MPH), however, no disease-modifying therapy exists. Interestingly, cognitive impairments, executive, and memory deficits seem to be common in ADHD, but also in early stages of mild cognitive impairment (MCI), and dementia, including sAD. Therefore, one of many hypotheses is that ADHD and sAD might have similar origins or that they intercalate with one another, as shown recently that ADHD may be considered a risk factor for sAD. Intriguingly, several overlaps have been shown between the two disorders, e.g., inflammatory activation, oxidative stress, glucose and insulin pathways, wingless-INT/mammalian target of rapamycin (Wnt/mTOR) signaling, and altered lipid metabolism. Indeed, Wnt/mTOR activities were found to be modified by MPH in several ADHD studies. Wnt/mTOR was also found to play a role in sAD and in animal models of the disorder. Moreover, MPH treatment in the MCI phase was shown to be successful for apathy including some improvement in cognition, according to a recent meta-analysis. In several AD animal models, ADHD-like behavioral phenotypes have been observed indicating a possible interconnection between ADHD and AD. In this concept paper, we will discuss the various evidence in human and animal models supporting the hypothesis in which ADHD might increase the risk for sAD, with common involvement of the Wnt/mTOR-pathway leading to lifespan alteration at the neuronal levels.

Electroacupuncture and manual acupuncture at LR3 and ST36 have attenuating effects on hypertension and subsequent cognitive dysfunction in spontaneously hypertensive rats: A preliminary resting-state functional magnetic resonance imaging study

Introduction

Chronic hypertension may have a contributory role toward cognitive impairment. Acupuncture exerts protective effects on cognitive functions while controlling the blood pressure. However, the neural mechanism underlying the dual attenuating effect of acupuncture remains unclear. In this study, we investigated the effects of electroacupuncture (EA) and manual acupuncture (MA) on the functional activity of the brain regions of spontaneously hypertensive rats (SHRs) by through resting-state functional magnetic resonance imaging (rs-fMRI). We also evaluated the differences in these functional activities between the EA and MA groups.

Methods

We randomly assigned 30 SHRs into the EA, MA, and model (SHR) groups. Wistar Kyoto rats (n = 10) were used as normal control (WKY). The interventions were administered once every alternate day for 12 weeks. The systolic blood pressure of all rats was recorded every 2 weeks until the end of the intervention. After the intervention, rs-fMRI scanning was performed to access the whole brain data of rats randomly selected from each group evenly. The amplitude of low frequency fluctuation (ALFF) analysis, regional homogeneity (ReHo) analysis, and functional connectivity (FC) analysis were also conducted. The Morris water maze (MWM) test was conducted to evaluate the learning and memory of the rats. Hematoxylin-eosin staining and Nissl staining were performed to observe histopathological changes in the key brain regions.

Results

We demonstrated that, when compared with the SHR group, the EA and MA groups had significantly lower blood pressure and better performance for behavioral test indices, and that the effect of EA was better than that of MA. ALFF and ReHo analyses revealed enhancement of the neuronal activity of some functionally impaired brain areas in the EA and MA groups. The main callback brain regions included the hypothalamus, entorhinal cortex, brain stem, prelimbic cortex, cingulate cortex, corpus callosum, and cerebellum. The FC analysis demonstrated that EA and MA enhanced the functional connectivity between the seeds and brain regions such as the brain stem, entorhinal cortex, hippocampus, prelimbic cortex, and cerebellum. The pathological test of the entorhinal cortex also verified the protective effect of acupuncture on the neuronal functional activity.

Discussion

Our findings suggested that EA and MA exhibited attenuating effects on hypertension and cognitive dysfunction by enhancing the functional activities in the corresponding brain regions. Moreover, EA activated more callback brain regions and functional connectivity than MA, which may explain why the effect of EA was better than that of MA.

Mineralocorticoid receptor antagonism improves transient receptor potential vanilloid 4-dependent dilation of cerebral parenchymal arterioles and cognition in a genetic model of hypertension

OBJECTIVE

In a model of secondary hypertension, mineralocorticoid receptor (MR) antagonism during the development of hypertension prevents the impairment of transient receptor potential vanilloid 4 (TRPV4) activation in parenchymal arterioles (PAs) and cognitive impairment. However, it is unknown whether MR antagonism can improve these impairments when treatment begins after the onset of essential hypertension. We tested the hypothesis that MR activation in stroke-prone spontaneously hypertensive rats (SHRSP) leads to impaired TRPV4-mediated dilation in PAs that is associated with cognitive dysfunction and neuroinflammation.

METHODS

20-22-week-old male SHRSP ± eplerenone (EPL; 100 mg/kg daily for 4 weeks) were compared to normotensive Sprague-Dawley (SD) rats. Pressure myography was used to assess PA function. Cognition was tested using Y-maze. Neuroinflammation was assessed using immunofluorescence and qRT-PCR.

RESULTS

Carbachol-mediated endothelium-dependent dilation was impaired in SHRSP, and MR antagonism improved this without affecting myogenic tone. Dilation to TRPV4 agonist GSK1016790A was impaired in SHRSP, and ELP treatment restored this. Intermediate conductance potassium channel (IKCa)/small conductance potassium channel (SKCa)-mediated dilation was impaired by hypertension and unaffected by EPL treatment. TRPV4 and IKCa/SKCa channel mRNA expression were reduced in PAs from hypertensive rats, and EPL did not improve this. Impairments in PA dilation in SHRSP were associated with cognitive decline, microglial activation, reactive astrogliosis, and neuroinflammation; cognitive and inflammatory changes were improved with MR blockade.

CONCLUSIONS

These data advance our understanding of the effects of hypertension on cerebral arterioles using a clinically relevant model and treatment paradigm. Our studies suggest TRPV4 and the MR are potential therapeutic targets to improve cerebrovascular function and cognition during hypertension.

Treatment with apocynin selectively restores hippocampal arteriole function and seizure-induced hyperemia in a model of preeclampsia

Preeclampsia (PE) is a hypertensive disorder of pregnancy associated with neurovascular dysfunction, cognitive impairment and increased seizure susceptibility. Here, we sought to determine if treatment of experimental PE (ePE) rats with apocynin could prevent hippocampal arteriolar (HA) dysfunction and impaired seizure-induced hyperemia within the hippocampus, a brain region central to cognition and seizure generation. Isolated and pressurized HAs from Sprague Dawley rats that were normal pregnant (Preg; n = 8), ePE (n = 8) or ePE treated with apocynin for 2 weeks of gestation (ePE + apo; n = 8) were compared. Hippocampal blood flow (n = 6/group) was measured using hydrogen clearance before and during seizure. Aorta elastin was quantified using histochemistry. ePE was associated with HA dysfunction including reduced contraction to endothelin-1 and diminished dilation to the endothelium-dependent vasodilator NS309 that was prevented by apocynin. However, apocynin had no effect on ePE-induced impairment of dilation to the nitric oxide donor sodium nitroprusside, but increased myogenic tone and substantially increased HA distensibility. Seizure-induced hyperemia was impaired in ePE rats that was restored by apocynin. Aorta from ePE rats had reduced elastin content, suggesting large artery stiffness, that was unaffected by apocynin. Thus, while apocynin partially prevented HA dysfunction, its restoration of functional hyperemia may be protective of seizure-induced injury during eclampsia.

HIV Promotes Neurocognitive Impairment by Damaging the Hippocampal Microvessels

Current evidence suggests that mild cerebrovascular changes could induce neurodegeneration and contribute to HIV-associated neurocognitive disease (HAND) in HIV patients. We investigated both the quantitative and qualitative impact of HIV infection on brain microvessels, especially on hippocampal microvessels, which are crucial for optimal O2 supply, and thus for maintaining memory and cognitive abilities. The results obtained using cultured human brain microvascular endothelial cells (HBMEC) were reproduced using a suitable mouse model and autopsied human HIV hippocampus. In HBMEC, we found significantly higher oxidative stress-dependent apoptotic cell loss following 5 h of treatment of GST-Tat (1 µg/ml) compared to GST (1 µg/ml) control. We noticed complete recovery of HBMEC cells after 24 h of GST-Tat treatment, due to temporal degradation or inactivation of GST-Tat. Interestingly, we found a sustained increase in mitochondrial oxidative DNA damage marker 8-OHdG, as well as an increase in hypoxia-inducible factor hypoxia-inducible factor-1α (HIF-1α). In our mouse studies, upon short-term injection of GST-Tat, we found the loss of small microvessels (mostly capillaries) and vascular endothelial growth factor (VEGF), but not large microvessels (arterioles and venules) in the hippocampus. In addition to capillary loss, in the post-mortem HIV-infected human hippocampus, we observed large microvessels with increased wall cells and perivascular tissue degeneration. Together, our data show a crucial role of Tat in inducing HIF-1α-dependent inhibition of mitochondrial transcriptional factor A (TFAM) and dilated perivascular space. Thus, our results further define the underlying molecular mechanism promoting mild cerebrovascular disease, neuropathy, and HAND pathogenesis in HIV patients.

The neuroendocrine stress response impairs hippocampal vascular function and memory in male and female rats

Chronic psychological stress affects brain regions involved in memory such as the hippocampus and accelerates age-related cognitive decline, including in Alzheimer's disease and vascular dementia. However, little is known about how chronic stress impacts hippocampal vascular function that is critically involved in maintaining neurocognitive health that could contribute to stress-related memory dysfunction. Here, we used a novel experimental rat model that mimics the neuroendocrine and cardiovascular aspects of chronic stress to determine how the neuroendocrine components of the stress response affect hippocampal function. We studied both male and female rats to determine potential sex differences in the susceptibility of the hippocampus and its vasculature to neuroendocrine stress-induced dysfunction. We show that activation of neuroendocrine stress pathways impaired the vasoreactivity of hippocampal arterioles to mediators involved in coupling neuronal activity with local blood flow that was associated with impaired memory function. Interestingly, we found more hippocampal arteriolar dysfunction and scarcer hippocampal microvasculature in male compared to female rats that was associated with greater memory impairment, suggesting the male sex may be at increased risk of neuroendocrine-derived hippocampal dysfunction during chronic stress. Overall, this study revealed the therapeutic potential of targeting hippocampal arterioles to prevent or slow memory decline in the setting of prolonged and/or unavoidable stress.

Blood pressure variability and structural brain changes: a systematic review

BACKGROUND

Blood pressure variability (BPV) has been linked with cognitive impairment and dementia. However, the pathophysiological mechanisms by which BPV affects cognition are unclear. This systematic review aims to assess the links between different BPV measures and white and grey matter structures.

METHODS AND RESULTS

The following databases were searched from inception through to January 2021; EMBASE, MEDLINE, EMCARE and SCOPUS. Studies that reported on the relationship between within-individual BPV (short, medium or long-term variability) or a circadian blood pressure (BP) measurement and MRI assessed brain structures were included. Overall, 20 studies met the criteria and were included, of which 11 studies looked at short-term BPV, eight articles investigated visit-to-visit BPV and one study looked at a compositional BPV measurement. Due to heterogeneity in study samples, meta-analysis was not possible. Across the included studies, associations between MRI indices and BP dipping patterns were mixed; higher long-term BPV and higher sleep systolic BPV was found to be associated with lower whole brain volume and hippocampal volume.

CONCLUSION

Increased BPV, in particular systolic long-term and systolic night-time BPV, appears to be associated with lower brain volume and hippocampal volume. This highlights the adverse effect that increased BPV has upon the brain, potentially contributing to cognitive decline, including dementia, in late-life.

Experimental Preeclampsia Causes Long-Lasting Hippocampal Vascular Dysfunction and Memory Impairment

Preeclampsia (PE) is a hypertensive disorder of pregnancy that is associated with memory impairment, cognitive decline and brain atrophy later in life in women at ages as young as early-to-mid 40 s. PE increases the risk of vascular dementia three-fold, however, long-lasting effects of PE on the vasculature of vulnerable brain regions involved in memory and cognition, such as the hippocampus, remain unknown. Here, we used a rat model of experimental PE (ePE) induced by maintaining rats on a 2% cholesterol diet beginning on day 7 of gestation to investigate hippocampal function later in life. Hippocampal-dependent memory and hippocampal arteriole (HA) function were determined in Sprague Dawley rats 5 months after either a healthy pregnancy or ePE (n = 8/group). Rats that had ePE were hypertensive and had impaired vasoreactivity of HAs to mediators involved in matching neuronal activity with local blood flow (i.e., neurovascular coupling). ePE rats also had impaired long-term memory, but not spatial memory. Thus, this model of ePE mimics some of the long-lasting cardiovascular and cognitive consequences that occur in women who previously had PE. These findings suggest endothelial and vascular smooth muscle dysfunction of HAs were present months after PE that could impair hippocampal neurovascular coupling. This represents a novel vascular mechanism by which PE causes early-onset dementia.

Hippocampus is more susceptible to hypoxic injury: has the Rosetta Stone of regional variation in neurovascular coupling been deciphered?

Alzheimer's disease and Alzheimer's disease-related dementias (AD/ADRD) are associated with cerebral hypoperfusion or reductions in baseline cerebral blood flow (CBF). The neurovascular coupling (NVC) response or functional hyperemia regulates brain perfusion via a retrograde (capillary-to-arteriole) pathway by increasing regional CBF in response to local neuron activation. The hippocampus plays a significant role in spatial and non-spatial memory. Functional MRI (fMRI) has not established a solid positive correlation between hippocampal blood oxygen level-dependent (BOLD) signal and local neuronal activity. The inconsistency of NVC in the hippocampus compared to the neocortex is possibly due to anatomical and methodological difficulties to accurately detect hippocampal blood flow. A recent study reported that NVC and oxygenation are reduced in the hippocampus compared to the cortex using a novel invasive surgical approach by creating a cranial window with and without removing the neocortex. Results from these studies suggest that the hippocampus is more susceptible to hypoxic injury in pathological conditions when NVC is impaired, such as AD/ADRD, stroke, and traumatic brain injury (TBI). The Rosetta Stone of regional variation in the NVC and its significance in AD/ADRD has not been fully deciphered based on these results without addressing remaining concerns; however, we are one step closer, indeed.

A robust bitmap-based real-time position tracking algorithm for rats in radial arm maze tests

This paper aims to develop a position tracking algorithm by which a rat in a radial arm maze can be accurately located in real time. An infrared (IR) night-vision camera was hung above the maze to capture IR images of the rat. The IR images were binarized and then duplicated for subsequent intersection and opening operations. Due to simple operations and a high robustness against the noise spots formed by the droppings of the rat, it took just minutes to process more than 9000 frames, and an accuracy above 99% was reached as well. The maze was intruded by an experimenter to further test the robustness, and the accuracy slightly fell to 98%. For comparison purposes, the same experiments were carried out using a pre-trained YOLO v2 model. The YOLO counterpart gave an accuracy beyond 97% in the absence and in the presence of the intruder. In other words, this work slightly outperformed the YOLO counterpart in terms of the accuracy in both cases, which indicates the robustness of this work. However, it took the YOLO counterpart an hour or so to locate a rat contained in the frames, which highlights the contribution of this work.

Prenatal Exposure to High Cortisol Induces ADHD-like Behaviors with Delay in Spatial Cognitive Functions during the Post-weaning Period in Rats

High levels of cortisol in blood are frequently observed in patients with major depressive disorders and increased cortisol level induces depressivelike symptoms in animal models. However, it is still unclear whether maternal cortisol level during pregnancy is a critical factor resulting in neuropsychiatric disorders in offspring. In this study, we increased cortisol level in rats by repetitively injecting corticosterone subcutaneously (Corti. Mom, 20 mg/kg/day) during pregnancy and evaluated the behavioral patterns of their pups (Corti.Pups) via forced swimming (FS), open field (OF), elevated plus maze (EPM) and Morris water maze (MWM) tests during the immediate post-weaning period (postnatal day 21 to 25). In results, corticosterone significantly increased plasma cortisol levels in both Corti.Moms and Corti.Pups. Unlike depressive animal models, Corti.Pups showed higher hyperactive behaviors in the FS and OF tests than normal pups (Nor.Pups) born from rats (Nor.Moms) treated with saline. Furthermore, Corti.Pups spent more time and traveled longer distance in the open arms of EPM test, exhibiting higher extremity. These patterns were consistent with behavioral symptoms observed in animal models of attention deficit hyperactivity disorder (ADHD), which is characterized by hyperactivity, impulsivity, and inattention. Additionally, Corti.Pups swam longer and farther to escape in MWM test, showing cognitive declines associated with attention deficit. Our findings provide evidence that maternal cortisol level during pregnancy may affect the neuroendocrine regulation and the brain development of offspring, resulting in heterogeneous developmental brain disorders such as ADHD.

HB-EGF depolarizes hippocampal arterioles to restore myogenic tone in a genetic model of small vessel disease

Vascular cognitive impairment, the second most common cause of dementia, profoundly affects hippocampal-dependent functions. However, while the growing literature covers complex neuronal interactions, little is known about the sustaining hippocampal microcirculation. Here we examined vasoconstriction to physiological pressures of hippocampal arterioles, a fundamental feature of small arteries, in a genetic mouse model of CADASIL, an archetypal cerebral small vessel disease. Using diameter and membrane potential recordings on isolated arterioles, we observed both blunted pressure-induced vasoconstriction and smooth muscle cell depolarization in CADASIL. This impairment was abolished in the presence of voltage-gated potassium (KV1) channel blocker 4-aminopyridine, or by application of heparin-binding EGF-like growth factor (HB-EGF), which promotes KV1 channel down-regulations. Interestingly, we observed that HB-EGF induced a depolarization of the myocyte plasma membrane within the arteriolar wall in CADASIL, but not wild-type, arterioles. Collectively, our results indicate that hippocampal arterioles in CADASIL mice display a blunted contractile response to luminal pressure, similar to the defect we previously reported in cortical arterioles and pial arteries, that is rescued by HB-EGF. Hippocampal vascular dysfunction in CADASIL could then contribute to the decreased vascular reserve associated with decreased cognitive performance, and its correction may provide a therapeutic option for treating vascular cognitive impairment.

Hippocampal network dysfunction as a mechanism of early-onset dementia after preeclampsia and eclampsia

Preeclampsia is a hypertensive disorder of pregnancy that can involve dangerous neurological symptoms such as spontaneous seizures (eclampsia). Despite being diseases specific to the pregnant state, preeclampsia and eclampsia have long-lasting neurological consequences later in life, including changes in brain structure and cognitive decline at relatively young ages. However, the effects of preeclampsia on brain regions central to memory and cognition, such as the hippocampus, are unclear. Here, we present a case reporting the progressive and permanent cognitive decline in a woman that had eclamptic seizures in the absence of evidence of brain injury on MRI. We then use rat models of normal pregnancy and preeclampsia to investigate mechanisms by which eclampsia-like seizures may disrupt hippocampal function. We show that experimental preeclampsia causes delayed memory decline in rats and disruption of hippocampal neuroplasticity. Further, seizures in pregnancy and preeclampsia caused acute memory dysfunction and impaired neuroplasticity but did not cause acute neuronal cell death. Importantly, hippocampal dysfunction persisted 5 weeks postpartum, suggesting seizure-induced injury is long lasting and may be permanent. Our data provide the first evidence of a model of preeclampsia that may mimic the cognitive decline of formerly preeclamptic women, and that preeclampsia and eclampsia affect hippocampal network plasticity and impair memory.

Tropisetron But Not Granisetron Ameliorates Spatial Memory Impairment Induced by Chronic Cerebral Hypoperfusion

Tropisetron and Granisetorn are 5-HT3 antagonists with antiemetic effects. Tropisetron also has a partial agonistic effect on alpha-7 nicotinic acetylcholine receptors (α7 nAChRs). On the other hand, chronic cerebral hypoperfusion (CCH) attenuates cerebral blood flow and impairs cognitive functions. The goal of this study was to investigate the effect of Tropisetron and Granisetron on CCH-induced spatial memory impairment in rats. Forty-eight male Wistar rats were used in this study. 2-VO surgery was done to induce CCH and Radial Eight Arm Maz apparatus was used to evaluate spatial memory (working and reference memory). Tropisetron was injected intraperitoneally at the doses of 1 and 5 mg/kg, and Granisetron was injected intraperitoneally at the dose of 3 mg/kg. Dorsal hippocampal (CA1) neurons count, Interleukin 6 (IL-6) serum level, and serotonin-reuptake transporter (SERT) gene expression were also evaluated. The results showed, CCH impaired working and reference memory, increased IL-6 serum level, and decreased CA1 neurons and SERT expression. Tropisetron at the dose of 5 mg/kg restored all the effects of CCH. However, Granisetron did not restore CCH-induced memory impairment. Furthermore, Granisetron had no effect on IL-6. While, it increased SERT expression and CA1 neurons. In conclusion, Tropisetron but not Granisetron, ameliorated spatial memory impairment induced by CCH. We suggested conducting more detailed studies investigating the role of serotonergic system (5-HT3 receptors and serotonin transporters) and also α7 nAChRs in the effects of Tropisetron.

Altered brain fluid management in a rat model of arterial hypertension

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.

Ex Vivo Pressurized Hippocampal Capillary-Parenchymal Arteriole Preparation for Functional Study

From subtle behavioral alterations to late-stage dementia, vascular cognitive impairment typically develops following cerebral ischemia. Stroke and cardiac arrest are remarkably sexually dimorphic diseases, and both induce cerebral ischemia. However, progress in understanding the vascular cognitive impairment, and then developing sex-specific treatments, has been partly limited by challenges in investigating the brain microcirculation from mouse models in functional studies. Here, we present an approach to examine the capillary-to-arteriole signaling in an ex vivo hippocampal capillary-parenchymal arteriole (HiCaPA) preparation from mouse brain. We describe how to isolate, cannulate, and pressurize the microcirculation to measure arteriolar diameter in response to capillary stimulation. We show which appropriate functional controls can be used to validate the HiCaPA preparation integrity and display typical results, including testing potassium as a neurovascular coupling agent and the effect of the recently characterized inhibitor of the Kir2 inward rectifying potassium channel family, ML133. Further, we compare the responses in preparations obtained from male and female mice. While these data reflect functional investigations, our approach can also be used in molecular biology, immunochemistry, and electrophysiology studies.