Enhanced Cholinergic Activity Improves Cerebral Blood Flow during Orthostatic Stress

Chronic inflammation with microglia senescence at basal forebrain: impact on cholinergic deficit in Alzheimer's brain haemodynamics

Cholinergic innervation in the brain is involved in modulating neurovascular function including cerebral blood flow haemodynamics in response to neuronal activity. Cholinergic deficit is associated with pathophysiology in Alzheimer's disease, albeit the aetiology remains to be clarified. In the current study, neocortex cerebral blood flow response to acetylcholine was evaluated by Laser-Doppler Flowmetry (LDF) in 3xTgAD Alzheimer's disease model) and wild-type mice of two age groups. The peak of cerebral blood flow to acetylcholine (i.v.) from baseline levels (% ΔrCBF) was higher in young 3xTgAD versus in wild-type mice (48.35; 95% CI:27.03-69.67 versus 22.70; CI:15.5-29.91, P < 0.05); this was reversed in old 3xTgAD mice (21.44; CI:2.52-40.35 versus 23.25; CI:23.25-39). Choline acetyltransferase protein was reduced in neocortex, while cerebrovascular reactivity to acetylcholine was preserved in young 3×TgAD mice. This suggests endogenous acetylcholine deficit and possible cholinergic denervation from selected cholinergic nuclei within the basal forebrain. The early deposition of tauopathy moieties (mutant hTau and pTau181) and its coincidence in cholinergic cell clusters (occasionaly), were observed at the basal forebrain of 3xTgAD mice including substantia innominate, nucleus Basalis of Meynert and nucleus of horizontal limb diagonal band of Broca. A prominent feature was microglia interacting tauopathy and demonstrated a variety of morphology changes particularly when located in proximity to tauopathy. The microglia ramified phenotype was reduced as evaluated by the ramification index and Fractal analysis. Increased microglia senescence, identified as SASP (senescence-associated secretory phenotype), was colocalization with p16Ink4ɑ, a marker of irreversible cell-cycle arrest in old 3xTgAD versus wild-type mice (P = 0.001). The p16Ink4ɑ was also observed in neuronal cells bearing tauopathy within the basal forebrain of 3xTgAD mice. TNF-ɑ, the pro-inflammatory cytokine elevated persistently in microglia (Pearson's correlation coefficient = 0.62) and the loss of cholinergic cells in vulnerable basal forebrain environment, was indicated by image analysis in 3xTgAD mice, which linked to the cholinergic deficits in neocortex rCBF haemodynamics. Our study revealed the early change of CBF haemodynamics to acetylcholine in 3xTgAD model. As a major effector of brain innate immune activation, microglia SASP with age-related disease progression is indicative of immune cell senescence, which contributes to chronic inflammation and cholinergic deficits at the basal forebrain. Targeting neuroinflammation and senescence may mitigate cholinergic pathophysiology in Alzheimer's disease.

Neurobiological Mechanisms of Cognitive Decline Correlated with Brain Aging

Cognitive decline has emerged as one of the greatest health threats of old age. Meanwhile, aging is the primary risk factor for Alzheimer's disease (AD) and other prevalent neurodegenerative disorders. Developing therapeutic interventions for such conditions demands a greater understanding of the processes underlying normal and pathological brain aging. Despite playing an important role in the pathogenesis and incidence of disease, brain aging has not been well understood at a molecular level. Recent advances in the biology of aging in model organisms, together with molecular- and systems-level studies of the brain, are beginning to shed light on these mechanisms and their potential roles in cognitive decline. This chapter seeks to integrate the knowledge about the neurological mechanisms of age-related cognitive changes that underlie aging.

Correlation between heart rate variability and cerebral autoregulation in septic patients

BACKGROUND

Heart rate variability (HRV) may provide an estimation of the autonomous nervous system (ANS) integrity in critically ill patients. Disturbances of cerebral autoregulation (CAR) may share common pathways of ANS dysfunction.

AIM

To explore whether changes in HRV and CAR index correlate in critically ill septic patients.

METHODS

Prospectively collected data on septic adult (> 18 years) patients admitted into a mixed Intensive Care between February 2016 and August 2019 with a recorded transcranial doppler CAR assessment. CAR was assessed calculating the Pearson's correlation coefficient (i.e. mean flow index, Mxa) between the left middle cerebral artery flow velocity (FV), insonated with a 2-MHz probe, and invasive blood pressure (BP) signal, both recorded simultaneously through a Doppler Box (DWL, Germany). MATLAB software was used for CAR assessment using a validated script; a Mxa >0.3 was considered as impaired CAR. HRV was assessed during the same time period using a specific software (Kubios HRV 3.2.0) and analyzed in both time-domain and frequency domain methods. Correlation between HRV-derived variables and Mxa were assessed using the Spearman's coefficient.

RESULTS

A total of 141 septic patients was studied; median Mxa was 0.35 [0.13-0.60], with 77 (54.6 %) patients having an impaired CAR. Mxa had a significant although weak correlation with HRV time domain (SDNN, r = 0.17, p = 0.04; RMSSD, r = 0.18, p = 0.03; NN50, r = 0.23, p = 0.006; pNN50, r = 0.23, p = 0.007), frequency domain (FFT-HF, r = 0.21; p = 0.01; AR-HF, r = 0.19; p = 0.02), and non-linear domain (SD1, r = 0.18, p = 0.03) parameters. Impaired CAR patients had also all of these HRV-derived parameters higher than those with intact CAR.

CONCLUSIONS

In this exploratory study, a potential association of ANS dysfunction and impaired CAR during sepsis was observed.

A high salt meal does not impair cerebrovascular reactivity in healthy young adults

A high sodium (Na⁺) meal impairs peripheral vascular function. In rodents, chronic high dietary Na⁺ impairs cerebral vascular function, and in humans, habitual high dietary Na⁺ is associated with increased stroke risk. However, the effects of acute high dietary Na⁺ on the cerebral vasculature in humans are unknown. The purpose of this study was to determine if acute high dietary Na⁺ impairs cerebrovascular reactivity in healthy adults. Thirty‐seven participants (20F/17M; 25 ± 5 years; blood pressure [BP]: 107 ± 9/61 ± 6 mm Hg) participated in this randomized, cross‐over study. Participants were given a low Na⁺ meal (LSM; 138 mg Na⁺) and a high Na⁺ meal (HSM; 1,495 mg Na⁺) separated by ≥ one week. Serum Na⁺, beat‐to‐beat BP, middle cerebral artery velocity (transcranial Doppler), and end‐tidal carbon dioxide (P_ETCO₂) were measured pre‐ (baseline) and 60 min post‐prandial. Cerebrovascular reactivity was assessed by determining the percent change in middle cerebral artery velocity to hypercapnia (via 8% CO₂, 21% oxygen, balance nitrogen) and hypocapnia (via mild hyperventilation). Peripheral vascular function was measured using brachial artery flow‐mediated dilation (FMD). Changes in serum Na⁺ were greater following the HSM (HSM: Δ1.6 ± 1.2 mmol/L vs. LSM: Δ0.7 ± 1.2 mmol/L, p < .01). Cerebrovascular reactivity to hypercapnia (meal effect: p = .41) and to hypocapnia (meal effect: p = .65) were not affected by the HSM. Contrary with previous findings, FMD was not reduced following the HSM (meal effect: p = .74). These data suggest that a single high Na⁺ meal does not acutely impair cerebrovascular reactivity, and suggests that despite prior findings, a single high Na⁺ meal does not impair peripheral vascular function in healthy adults.

Ten days of high dietary sodium does not impair cerebral blood flow regulation in healthy adults

High dietary sodium impairs cerebral blood flow regulation in rodents and is associated with increased stroke risk in humans. However, the effects of multiple days of high dietary sodium on cerebral blood flow regulation in humans is unknown. Therefore, the purpose of this study was to determine whether ten days of high dietary sodium impairs cerebral blood flow regulation. Ten participants (3F/7M; age: 30 ± 10 years; blood pressure (BP): 113 ± 8/62 ± 9 mmHg) participated in this randomized, cross-over design study. Participants were placed on 10-day diets that included either low- (1000 mg/d), medium- (2300 mg/d) or high- (7000 mg/d) sodium separated by ≥four weeks. Urinary sodium excretion, beat-to-beat BP (finger photoplethysmography), middle cerebral artery velocity (transcranial Doppler), and end-tidal carbon dioxide (capnography) was measured. Dynamic cerebral autoregulation during a ten-minute baseline was calculated and cerebrovascular reactivity assessed by determining the percent change in middle cerebral artery blood flow velocity to hypercapnia (8% CO₂, 21% oxygen, balance nitrogen) and hypocapnia (via mild hyperventilation). Urinary sodium excretion increased in a stepwise manner (ANOVA P = 0.001) from the low, to medium, to high condition. There were no differences in dynamic cerebral autoregulation between conditions. While there was a trend for a difference during cerebrovascular reactivity to hypercapnia (ANOVA P = 0.06), this trend was abolished when calculating cerebrovascular conductance (ANOVA: P = 0.28). There were no differences in cerebrovascular reactivity (ANOVA P = 0.57) or conductance (ANOVA: P = 0.73) during hypocapnia. These data suggest that ten days of a high sodium diet does not impair cerebral blood flow regulation in healthy adults.

Modelling physiology of haemodynamic adaptation in short-term microgravity exposure and orthostatic stress on Earth

Cardiovascular haemodynamics alters during posture changes and exposure to microgravity. Vascular auto-remodelling observed in subjects living in space environment causes them orthostatic intolerance when they return on Earth. In this study we modelled the human haemodynamics with focus on head and neck exposed to different hydrostatic pressures in supine, upright (head-up tilt), head-down tilt position, and microgravity environment by using a well-developed 1D-0D haemodynamic model. The model consists of two parts that simulates the arterial (1D) and brain-venous (0D) vascular tree. The cardiovascular system is built as a network of hydraulic resistances and capacitances to properly model physiological parameters like total peripheral resistance, and to calculate vascular pressure and the related flow rate at any branch of the tree. The model calculated 30.0 mmHg (30%), 7.1 mmHg (78%), 1.7 mmHg (38%) reduction in mean blood pressure, intracranial pressure and central venous pressure after posture change from supine to upright, respectively. The modelled brain drainage outflow percentage from internal jugular veins is 67% and 26% for supine and upright posture, while for head-down tilt and microgravity is 65% and 72%, respectively. The model confirmed the role of peripheral veins in regional blood redistribution during posture change from supine to upright and microgravity environment as hypothesized in literature. The model is able to reproduce the known haemodynamic effects of hydraulic pressure change and weightlessness. It also provides a virtual laboratory to examine the consequence of a wide range of orthostatic stresses on human haemodynamics.

Increased One-Year Recurrent Ischemic Stroke after First-Ever Ischemic Stroke in Males with Benign Prostatic Hyperplasia

(1) Background: Patients with benign prostatic hyperplasia (BPH) were questioned about quality of life and sleep. Most BPH patients were treated with alpha-1 adrenergic receptor antagonists, which could improve cerebral blood flow for 1–2 months. Patients with ischemic stroke (IS) could experience cerebral autoregulation impairment for six months. The relationship between BPH and recurrent IS remains unclear. The aim of this study was to determine the risk of one-year recurrent IS conferred by BPH. (2) Methods: We used data from the Taiwanese National Health Insurance Database to identify newly diagnosed IS cases entered from 1 January 2008 to 31 December 2008. Patients were followed until the recurrent IS event or 365 days after the first hospitalization. The risk factors associated with one-year recurrent IS were assessed using Cox proportional hazards regression. (3) Results: Patients with BPH had a higher risk of recurrent IS (12.11% versus 8.15%) (adjusted hazard ratio (HR): 1.352; 95% confidence interval (CI): 1.028–1.78, p = 0.031). Other risk factors included hyperlipidemia (adjusted HR: 1.338; 95% CI: 1.022–1.751, p = 0.034), coronary artery disease (adjusted HR: 1.487; 95% CI: 1.128–1.961, p = 0.005), chronic obstructive pulmonary disease (adjusted HR: 1.499; 95% CI: 1.075–2.091, p = 0.017), and chronic kidney disease (adjusted HR: 1.523; 95% CI: 1.033–2.244, p = 0.033). (4) Conclusion: Patients with BPH who had these risk factors had an increased risk of one-year recurrent IS. The modification of risk factors may prevent recurrent IS.

Comparison of diurnal variation, anatomical location, and biological sex within spontaneous and driven dynamic cerebral autoregulation measures

Presently, the literature describing the influence of diurnal variation on dynamic cerebral autoregulation (dCA) metrics is sparse. Additionally, there is little data with respect to dCA comparisons between anterior/posterior circulation beds and biological sexes using squat-stand maneuvers. Eight male and eight female participants (n = 16) performed 5 min of spontaneous upright rest and squat-stand maneuvers at 0.05 and 0.10 Hz across seven time points throughout the day. All testing sessions commenced at 8:00 a.m. each day and dCA parameters were quantified across the cardiac cycle (diastole, mean, and systole) using transcranial Doppler ultrasound to insonate cerebral blood velocity within the middle and posterior cerebral arteries (MCA, PCA). No cardiac cycle alternations were seen spontaneous (all p > .207) while a trend was noted in some driven (all p > .051) dCA metrics. Driven dCA produced much lower coefficient of variances (all <21%) compared with spontaneous (all <58%). Moreover, no sex differences were found within driven metrics (all p > .096). Between vessels, PCA absolute gain was reduced within all spontaneous and driven measures (all p < .014) whereas coherence, phase, and normalized gain were unchanged (all p > .099). There appears to be little influence of diurnal variation on dCA measures across the day (8:00 a.m. to 6:00 p.m.). Absolute gain was blunted in the PCA relative to the MCA and consistent with previous literature, driven methods demonstrated vastly improved reproducibility metrics compared to spontaneous methods. Finally, no dCA differences were found between biological sexes, demonstrating that males and females regulate in a harmonious manner, when females are tested within the early follicular phase of the menstrual cycle.

Small vessel disease to subcortical dementia: a dynamic model, which interfaces aging, cholinergic dysregulation and the neurovascular unit

Background

Small vessels have the pivotal role for the brain’s autoregulation. The arteriosclerosis-dependent alteration of the brain perfusion is one of the major determinants in small vessel disease. Endothelium distress can potentiate the flow dysregulation and lead to subcortical vascular dementia (sVAD). sVAD increases morbidity and disability. Epidemiological studies have shown that sVAD shares with cerebrovascular disease most of the common risk factors. The molecular basis of this pathology remains controversial.

Purpose

To detect the possible mechanisms between small vessel disease and sVAD, giving a broad vision on the topic, including pathological aspects, clinical and laboratory findings, metabolic process and cholinergic dysfunction.

Methods

We searched MEDLINE using different search terms (“vascular dementia”, “subcortical vascular dementia”, “small vessel disease”, “cholinergic afferents”, etc). Publications were selected from the past 20 years. Searches were extended to Embase, Cochrane Library, and LILIACS databases. All searches were done from January 1, 1998 up to January 31, 2018.

Results

A total of 560 studies showed up, and appropriate studies were included. Associations between traditional vascular risk factors have been isolated. We remarked that SVD and white matter abnormalities are seen frequently with aging and also that vascular and endothelium changes are related with age; the changes can be accelerated by different vascular risk factors. Vascular function changes can be heavily influenced by genetic and epigenetic factors.

Conclusion

Small vessel disease and the related dementia are two pathologies that deserve attention for their relevance and impact in clinical practice. Hypertension might be a historical problem for SVD and SVAD, but low pressure might be even more dangerous; CBF regional selective decrease seems to be a critical factor for small vessel disease-related dementia. In those patients, endothelium damage is a super-imposed condition. Several issues are still debatable, and more research is needed.

The Neuroprotective Effects of Exercise: Maintaining a Healthy Brain Throughout Aging

Physical activity plays an essential role in maintaining a healthy body, yet it also provides unique benefits for the vascular and cellular systems that sustain a healthy brain. While the benefit of exercise has been observed in humans of all ages, the availability of preclinical models has permitted systematic investigations into the mechanisms by which exercise supports and protects the brain. Over the past twenty-five years, rodent models have shown that increased physical activity elevates neurotrophic factors in the hippocampal and cortical areas, facilitating neurotransmission throughout the brain. Increased physical activity (such as by the voluntary use of a running wheel or regular, timed sessions on a treadmill) also promotes proliferation, maturation and survival of cells in the dentate gyrus, contributing to the process of adult hippocampal neurogenesis. In this way, rodent studies have tremendous value as they demonstrate that an 'active lifestyle' has the capacity to ameliorate a number of age-related changes in the brain, including the decline in adult neurogenesis. Moreover, these studies have shown that greater physical activity may protect the brain health into advanced age through a number of complimentary mechanisms: in addition to upregulating factors in pro-survival neurotrophic pathways and enhancing synaptic plasticity, increased physical activity promotes brain health by supporting the cerebrovasculature, sustaining the integrity of the blood-brain barrier, increasing glymphatic clearance and proteolytic degradation of amyloid beta species, and regulating microglia activation. Collectively, preclinical studies demonstrate that exercise initiates diverse and powerful neuroprotective pathways that may converge to promote continued brain health into old age. This review will draw on both seminal and current literature that highlights mechanisms by which exercise supports the functioning of the brain, and aids in its protection.

Dynamic cerebral autoregulation is impaired in Veterans with Gulf War Illness: A case-control study

Neurological dysfunction has been reported in Gulf War Illness (GWI), including abnormal cerebral blood flow (CBF) responses to physostigmine challenge. However, it is unclear whether the CBF response to normal physiological challenges and regulation is similarly dysfunctional. The goal of the present study was to evaluate the CBF velocity response to orthostatic stress (i.e., sit-to-stand maneuver) and increased fractional concentration of carbon dioxide. 23 cases of GWI (GWI+) and 9 controls (GWI) volunteered for this study. Primary variables of interest included an index of dynamic autoregulation and cerebrovascular reactivity. Dynamic autoregulation was significantly lower in GWI+ than GWI- both for autoregulatory index (2.99±1.5 vs 4.50±1.5, p = 0.017). In addition, we observed greater decreases in CBF velocity both at the nadir after standing (-18.5±6.0 vs -9.8±4.9%, p = 0.001) and during steady state standing (-5.7±7.1 vs -1.8±3.2%, p = 0.042). In contrast, cerebrovascular reactivity was not different between groups. In our sample of Veterans with GWI, dynamic autoregulation was impaired and consistent with greater cerebral hypoperfusion when standing. This reduced CBF may contribute to cognitive difficulties in these Veterans when upright.