Cerebrovascular permeability and cognition in the aging rat

HUCBC Treatment Improves Cognitive Outcome in Rats With Vascular Dementia

Background and purpose: Vascular dementia (VaD) is the second common cause of dementia after Alzheimer's disease in older people. Yet, there are no FDA approved drugs specifically for VaD. In this study, we have investigated the therapeutic effects of human umbilical cord blood cells (HUCBC) treatment on the cognitive outcome, white matter (WM) integrity, and glymphatic system function in rats subject to a multiple microinfarction (MMI) model of VaD. Methods: Male, retired breeder rats were subjected to the MMI model (800 ± 100 cholesterol crystals/300 μl injected into the internal carotid artery), and 3 days later were treated with phosphate-buffered saline (PBS) or HUCBC (5 × 10⁶, i.v.). Sham rats were included as naïve control. Following a battery of cognitive tests, rats were sacrificed at 28 days after MMI and brains extracted for immunohistochemical evaluation and Western blot analysis. To evaluate the glymphatic function, fluorescent tracers (Texas Red dextran, MW: 3 kD and FITC-dextran, MW: 500 kD) was injected into the cisterna magna over 30 min at 14 days after MMI. Rats (3-4/group/time point) were sacrificed at 30 min, 3 h, and 6 h, and the tracer movement analyzed using laser scanning confocal microscopy. Results: Compared to control MMI rats, HUCBC treated MMI rats exhibit significantly improved short-term memory and long-term memory exhibited by increased discrimination index in novel object recognition task with retention delay of 4 h and improved novel odor recognition task with retention delay of 24 h, respectively. HUCBC treatment also improves spatial learning and memory as measured using the Morris water maze test compared to control MMI rats. HUCBC treatment significantly increases axon and myelin density increases oligodendrocyte and oligodendrocyte progenitor cell number and increases Synaptophysin expression in the brain compared to control MMI rats. HUCBC treatment of MMI in rats significantly improves glymphatic function by reversing MMI induced delay in the penetration of cerebrospinal fluid (CSF) into the brain parenchyma via glymphatic pathways and reversing delayed clearance from the brain. HUCBC treatment significantly increases miR-126 expression in serum, aquaporin-4 (AQP4) expression around cerebral vessels, and decreases transforming growth factor-β (TGF-β) protein expression in the brain which may contribute to HUCBC induced improved glymphatic function. Conclusions: HUCBC treatment of an MMI rat model of VaD promotes WM remodeling and improves glymphatic function which together may aid in the improvement of cognitive function and memory. Thus, HUCBC treatment warrants further investigation as a potential therapy for VaD.

Sigma-1 Agonist Binding in the Aging Rat Brain: a MicroPET Study with [(11)C]SA4503

PURPOSE

Sigma-1 receptor ligands modulate the release of several neurotransmitters and intracellular calcium signaling. We examined the binding of a radiolabeled sigma-1 agonist in the aging rat brain with positron emission tomography (PET).

PROCEDURES

Time-dependent uptake of [(11)C]SA4503 was measured in the brain of young (1.5 to 3 months) and aged (18 to 32 months) Wistar Hannover rats, and tracer-kinetic models were fitted to this data, using metabolite-corrected plasma radioactivity as input function.

RESULTS

In aged animals, the injected probe was less rapidly metabolized and cleared. Logan graphical analysis and a 2-tissue compartment model (2-TCM) fit indicated changes of total distribution volume (V T) and binding potential (BP ND) of the tracer. BP ND was reduced particularly in the (hypo)thalamus, pons, and medulla.

CONCLUSIONS

Some areas showed reductions of ligand binding with aging whereas binding in other areas (cortex) was not significantly affected.

Models and mechanisms of vascular dementia

Vascular dementia (VaD) is the second leading form of dementia after Alzheimer's disease (AD) plaguing the elderly population. VaD is a progressive disease caused by reduced blood flow to the brain, and it affects cognitive abilities especially executive functioning. VaD is poorly understood and lacks suitable animal models, which constrain the progress on understanding the basis of the disease and developing treatments. This review article discusses VaD, its risk factors, induced cognitive disability, various animal (rodent) models of VaD, pathology, and mechanisms of VaD and treatment options.

Testing the calcium hypothesis of aging in the rat hippocampus in vivo using manganese-enhanced MRI

In this study, we noninvasively tested the hypothesis that Mn(2+)-enhanced magnetic resonance imaging (MEMRI) is sensitive to age-related changes in Ca(2+) influx occurring in the hippocampal region CA1. Uptake of Mn(2+), an MRI contrast agent and Ca(2+) surrogate with low cellular efflux rates (days to weeks), was measured in longitudinal MEMRI studies involving 2 separate groups of male Long-Evans rats: one group was studied at 2.5 and 7 months of age, whereas the other was studied at 7 and 19 months of age. Separate or combined analysis revealed that the extent of Mn(2+) accumulation in CA1 significantly increased with age (p < 0.05). These results provide first-time in vivo confirmation of the calcium hypothesis of aging and justify future longitudinal studies combining MEMRI with behavioral testing to investigate mechanisms of age-related cognitive decline.

Combined age- and trauma-related proteomic changes in rat neocortex: a basis for brain vulnerability

This proteomic study investigates the widely observed clinical phenomenon, that after comparable brain injuries, geriatric patients fare worse and recover less cognitive and neurologic function than younger victims. Utilizing a rat traumatic brain injury model, sham surgery or a neocortical contusion was induced in 3 age groups. Geriatric (21 months) rats performed worse on behavioral measures than young adults (12-16 weeks) and juveniles (5-6 weeks). Motor coordination and certain cognitive deficits showed age-dependence both before and after injury. Brain proteins were analyzed using silver-stained two-dimensional electrophoresis gels. Spot volume changes (>2-fold change, p<0.01) were identified between age and injury groups using computer-assisted densitometry. Sequences were determined by mass spectrometry of tryptic peptides. The 19 spots identified represented 13 different genes that fell into 4 general age- and injury-dependent expression patterns. Fifteen isoforms changed differentially with respect to both age and injury (p<0.05). Further investigations into the nature and function of these isoforms may yield insights into the vulnerability of older patients and resilience of younger patients in recovery after brain injuries.

Angiogenesis in old-aged subjects after ischemic stroke: a cautionary note for investigators

Angiogenesis represents a form of neovascularisation of exceptional importance in numerous pathological conditions including stroke. In this context it is directly related to neuroregeneration which is seen in close proximity. However, numerous experimental data have been drawn from studies that have ignored the age criterion. This is extremely important as angiogenesis is different in young versus old subjects. Extrapolating data obtained from studies performed in young subjects or "in vitro" to old-age patients could lead to inexact conclusions since the dynamics of angiogenesis is age-dependent.

The current review covers the key features of brain senescence including morphological and functional changes related to the brain parenchyma, its vascular network and blood flow which could possibly influence the process of angiogenesis. This is followed by a description of post-stroke angiogenesis and its relationship to neuroregeneration and its modulation by vascular endothelial growth factor (VEGF) and insulin-like growth factor 1 (IGF 1), the most important factors active in old brain after ischemic injury.

Experimental models of vascular dementia and vascular cognitive impairment: a systematic review

Vascular cognitive impairment (VCI) encompasses vascular dementia and is the second most common cause of dementing illness after Alzheimer's disease. The main causes of VCI are: cerebral small vessel disease; multi-infarct dementia; strategic infarct (i.e. located in a functionally-critical brain area); haemorrhage/microbleed; angiopathy (including cerebral amyloid angiopathy); severe hypoperfusion (e.g. cardiac arrhythmia); and hereditary vasculopathy (e.g. cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, CADASIL). In this systematic analysis, we aimed to relate cognitive and neuropathological features of experimental models to clinical VCI. We extracted data from 107 studies covering 16 models. These included: brief global ischaemic insults (in rats, mice or gerbils); chronic global hypoperfusion (rats, mice, gerbils); chronic hypertension (in primates or stroke-prone, spontaneously-hypertensive rats); multiple ischaemic lesions because of intra-vascular emboli (in rodents, rabbits or primates); strategic ischaemic lesions (in rats or mini-pigs); generalised vasculopathies, because of mutant Notch3, hyperhomocysteinaemia, experimental diabetes mellitus or lack of cerebral vasodilator M(5) receptors (rats or mice). Most cognitive testing showed deficits in working and reference memory. The lesions observed were microinfarcts, diffuse white matter lesions, hippocampal neuronal death, focal ischaemic lesions and micro-haemorrhages. The most-used model was bilateral carotid artery occlusion in rats, leading to chronic hypoperfusion and white matter injury.

Microvascular changes in estrogen-α sensitive brainstem structures of aging female hamsters

Structural neuronal plasticity is present in the nucleus para-retroambiguus (NPRA) and the commissural nucleus of the solitary tract/A2 group (NTScom/A2) in female hamsters. Both brainstem nuclei play a role in estrous cycle related autonomic adaptations. We investigated how aging affects the capillary condition in these adaptive brainstem regions. Senescent female hamsters (+/-95 weeks) were tested weekly for their 4-day estrous cycle. Subsequently morphological changes of NPRA and NTScom/A2 were compared with those of young (+/-20 weeks) females in an ultrastructural study. The medial tegmental field served as control area. In 841 capillaries (n=319 capillaries, young females (N=3); n=522 capillaries, aged females (N=4)) vascular aberrations were classified into 3 categories: endothelial and tight junction, basement membrane and pericyte aberrations. In old animals, capillaries showed marked endothelial changes, disrupted tight junctions, and thickening and splitting of basement membranes. Aberrations were found in 40-60% of all capillaries. About 70% of the pericytes contained degenerative inclusions. Despite this generalized vascular degeneration, the reproductive cycle of female hamsters was unaffected by vascular senescence. Perivascular fibrosis as reported in aging rats was never observed, which suggests the existence of species differences.

Elevation of CSF albumin in old sheep: relations to CSF turnover and albumin extraction at blood-CSF barrier

Albumin is the most abundant protein in both CSF and plasma, and albumin quotient is often used to assess the functions of brain barriers especially that of the blood-CSF barrier [i.e. the choroid plexus (CP) which also secretes CSF]. In this study, we took albumin as a model molecule to investigate ageing-related alterations in the CSF-CP system in sheep. We found significant ageing-related increases in the weight of lateral CP [122.4 +/- 14.0 mg in the young, 198.6 +/- 35.4 mg in the middle aged, 286.1 +/- 25.1 mg in the old (p < 0.05)], in the CSF albumin as well as the albumin quotient. Albumin protein spots in old CSF displayed wider on 2D western immunoblotting images, and had higher densities on images of 2D large gels stained with Pro-Q Emerald 488 compared to the young samples, suggesting ageing-related post-translational modification in the albumin. CSF secretion was reduced with age: 0.148 +/- 0.013 mL/min/g in the young, 0.092 +/- 0.02 mL/min/g in the middle aged, 0.070 +/- 0.013 mL/min/g in the old (p < 0.05). The (125)I-BSA extraction was not different among the sheep groups, nor was altered by temperature reduction, monensin, nocodazole, anti-transforming growth factor beta receptor II antibody, as well as unlabelled albumins. In conclusion, elevation of albumin in old CSF is associated with reduced CSF secretion by the CP, which size increases with age. (125)I-BSA extract, reflecting the extracellular space rather than the active albumin uptake in the CP, is not different between ages. These early changes in health ageing may result in the accumulation and modifications of CSF proteins leading to neurotoxicity.

Cognitive performance and cerebrohemodynamics associated with the Persian Gulf Syndrome

The Persian Gulf Syndrome generally manifests as a set of nonspecific complaints with emphasis on central nervous system impairment. The purpose of this study was to determine if cognitive performance and middle cerebral artery blood flow velocity (MCABFV) were altered in symptomatic Gulf War veterans (sGWVs) and asymptomatic Gulf War veterans (aGWVs) by exposure to low levels of acetone. MCABFV was assessed in male aGWVs (n = 8) and sGWVs (n = 8) during cognitive challenges while breathing 1) clean air, 2) a clean air placebo, and 3) a mixture of air and 40 parts per million (ppm) acetone. Pulmonary function was also evaluated. Pulmonary function tests showed no statistical differences between aGWVs and sGWVs while breathing clean air or 40 ppm acetone in air. Cognitive performance was similar during the clean air, placebo, and acetone test conditions for sGWVs and aGWVs. Data pooled across test conditions for each group indicated a statistically significant (P < 0.05) poorer performance primarily in memory and executive function tasks by sGWVs. sGWVs had a 34.2% higher baseline MCABFV than aGWVs (P < 0.05). Increases in MCABFV for aGWVs (averaged over all cognitive tasks for each test condition) ranged between 7.8% and 8.8%, and were not statistically significant. Increases for sGWVs ranged between 0.3% and 4.8%, averaged over all cognitive tasks for each test condition. No significant differences were noted between the clean air and placebo test conditions but both were significantly different to the acetone condition. Differences in MCABFV increases for each of the test conditions between aGWVs and sGWVs were also statistically significant. sGWV did not appear to demonstrate pulmonary dysfunction following exposure to acetone. They did, however, appear to have generally lower cognitive function as compared to aGWVs. sGWVs appeared to have a significant degree of autoregulatory disruption in cerebral perfusion, resulting in reduced cognitive reserve capacity and potentially impaired ability to handle complex cognitive tasks.

Cerebral vessels in ageing and Alzheimer's disease

The integrity of the cerebral vasculature is crucial to the maintenance of cognitive functions during ageing. Prevailing evidence suggests that cerebrovascular functions decline during normal ageing, with pronounced effects in Alzheimer's disease (AD). The causes of these changes largely remain unknown. While previous studies recorded ageing-related impairments, such as atherosclerosis and loss of innervation in basal surface arteries of the brain, it only recently has been realized that a number of subtle alterations in both the intracranial resistance vessels and the smaller capillaries is apparent in both ageing animals and humans. The dominant changes include alterations in composition of connective tissues and smooth muscle of large vessel walls, thickening of the vascular basement membrane, thinning of the endothelium in some species, loss of endothelial mitochondria and increased pericytes. Some of these attributes appear more affected in AD. Other abnormalities entail profound irregularities in the course of microvessels, unexplained inclusions in the basement membrane and changes in unique proteins and membrane lipids associated with the blood-brain barrier. Brain imaging and permeability studies show no clear functional evidence to support the structural and biochemical anomalies, but it is plausible that focal and transient breach of the blood-brain barrier in ageing, and more notably in AD, occurs. Thus, circumscribed neuronal populations in certain brain regions could become vulnerable. Furthermore, the characteristic deposition of amyloid in vessels in AD may exacerbate the decline in vascular function and promote chronic hypoperfusion. Although not explicit from current studies, it is likely that the brain vasculature is continually modified by growth and repair mechanisms in attempts to maintain perfusion during ageing and disease.

Shift in the performance of 24-month-old Wistar rats in the Morris water escape task: a comparison across 36 experiments

Spatial discrimination learning in aged rats serves as an animal model of cognitive aging. We assessed the replicability of spatial discrimination performance in the standard Morris water escape task. To this end the learning curves and the performance in a probe trial of 24-month-old outbred Wistar (HsdWin:Wu) control rats from 36 experiments were compared. These experiments had been performed at our laboratory under strictly controlled conditions over a period of 71 weeks. There was a very high variability in the learning curves between experiments. The initial performance level, i.e. the performance during the first session, did not change systematically across the 36 experiments. In contrast, the final performance level, i.e. the level reached in the fifth training session, decreased over the 71 week period, when the platform escape latency and the distance swam to reach the platform, measured as number of line crossings, were considered. In the last experiments of the series, learning curves were no longer seen: the rats did not improve their performance across the acquisition sessions. By contrast, the swimming speed and, in the probe trial, the bias for the quadrant where the platform had been positioned during training, did not change. This indicates that a decrease across experiments occurred predominantly with respect to spatial orientation performance, whereas the motor performance appeared to be unchanged. Explanations for this observation, such as differences in viability between shipments and the possible occurrence of genetic drift, are discussed.

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Potential mechanisms of the age-related changes in the blood-brain barrier

A variety of age-related changes in the blood-brain barrier transport processes have been identified. These include reduced hexose and butyrate transport, reduced choline transport, reduced triiodothyronine transport without a change in the transport of neutral and basic amino acids. The potential mechanisms underlying these age-related changes include hemodynamic alterations in the cerebral circulation of aged rats, notably increased occurrence of arteriovenous shunting. Additional age-related changes in cerebral microvessels include alterations in protein composition, and increased accumulation of lipid peroxidation byproducts, along with changes in membrane fluidity of isolated cerebral microvessels. In addition, neurotransmitter activity notably beta adrenergic neurotransmission, is significantly reduced in cerebral microvessels of aged rats. These alterations taken together may account for some of the age-related changes in the blood-brain barrier.