Dopaminergic degeneration is enhanced by chronic brain hypoperfusion and inhibited by angiotensin receptor blockage

The role of the brain renin-angiotensin system in Parkinson´s disease

The renin-angiotensin system (RAS) was classically considered a circulating hormonal system that regulates blood pressure. However, different tissues and organs, including the brain, have a local paracrine RAS. Mutual regulation between the dopaminergic system and RAS has been observed in several tissues. Dysregulation of these interactions leads to renal and cardiovascular diseases, as well as progression of dopaminergic neuron degeneration in a major brain center of dopamine/angiotensin interaction such as the nigrostriatal system. A decrease in the dopaminergic function induces upregulation of the angiotensin type-1 (AT1) receptor activity, leading to recovery of dopamine levels. However, AT1 receptor overactivity in dopaminergic neurons and microglial cells upregulates the cellular NADPH-oxidase-superoxide axis and Ca2+ release, which mediate several key events in oxidative stress, neuroinflammation, and α-synuclein aggregation, involved in Parkinson's disease (PD) pathogenesis. An intraneuronal antioxidative/anti-inflammatory RAS counteracts the effects of the pro-oxidative AT1 receptor overactivity. Consistent with this, an imbalance in RAS activity towards the pro-oxidative/pro-inflammatory AT1 receptor axis has been observed in the substantia nigra and striatum of several animal models of high vulnerability to dopaminergic degeneration. Interestingly, autoantibodies against angiotensin-converting enzyme 2 and AT1 receptors are increased in PD models and PD patients and contribute to blood-brain barrier (BBB) dysregulation and nigrostriatal pro-inflammatory RAS upregulation. Therapeutic strategies addressed to the modulation of brain RAS, by AT1 receptor blockers (ARBs) and/or activation of the antioxidative axis (AT2, Mas receptors), may be neuroprotective for individuals with a high risk of developing PD or in prodromal stages of PD to reduce progression of the disease.

AT 1 inhibition mediated neuroprotection after experimental traumatic brain injury is dependent on neutrophils in male mice

After traumatic brain injury (TBI) cerebral inflammation with invasion of neutrophils and lymphocytes is a crucial factor in the process of secondary brain damage. In TBI the intrinsic renin-angiotensin system is an important mediator of cerebral inflammation, as inhibition of the angiotensin II receptor type 1 (AT1) reduces secondary brain damage and the invasion of neutrophil granulocytes into injured cerebral tissue. The current study explored the involvement of immune cells in neuroprotection mediated by AT1 inhibition following experimental TBI. Four different cohorts of male mice were examined, investigating the effects of neutropenia (anti-Ly6G antibody mediated neutrophil depletion; C57BL/6), lymphopenia (RAG1 deficiency, RAG1^-/-), and their combination with candesartan-mediated AT1 inhibition. The present results showed that reduction of neutrophils and lymphocytes, as well as AT1 inhibition in wild type and RAG1^-/- mice, reduced brain damage and neuroinflammation after TBI. However, in neutropenic mice, candesartan did not have an effect. Interestingly, AT1 inhibition was found to be neuroprotective in RAG1^-/- mice but not in neutropenic mice. The findings suggest that AT1 inhibition may exert neuroprotection by reducing the inflammation caused by neutrophils, ultimately leading to a decrease in their invasion into cerebral tissue.

Abnormal circadian blood pressure regulation and cognitive impairment in α-synucleinopathies

Circadian blood pressure (BP) rhythm is important for the maintenance of healthy daily life, and its disruption is associated with poor outcomes. Cardiovascular autonomic failure is often observed in older populations but has a greater impact on neurodegenerative disorders such as α-synucleinopathies. These BP abnormalities include orthostatic hypotension (OH), supine hypertension (SH), and a loss of nocturnal BP fall. OH not only causes falls or syncope but is also related to cognitive impairment in α-synucleinopathies. For example, OH doubles or triples the risk for the development of cognitive impairment in Parkinson's disease (PD). The diffuse central and peripheral neuropathology of α-synuclein may contribute to both OH and cognitive impairment. Moreover, repeated cerebral hypoperfusion in OH is thought to be related to cerebrovascular and neuronal damage, which may cause cognitive impairment. SH, which often coexists with OH, is also associated with cognitive impairment through cerebrovascular damage, such as white matter lesions and cerebral microbleeds. The reverse-dipping (riser) pattern on ambulatory BP monitoring is commonly observed in PD (∼56%), regardless of disease duration and severity. It is also related to cognitive impairment and more pronounced when coexisting with OH. These abnormal circadian BP profiles may be synergistically associated with cognitive impairment and poor outcomes in α-synucleinopathies. Although evidence for aggressive control of BP dysregulation improving cognitive impairment and outcomes is limited, regular BP monitoring appears to be important for total management of α-synucleinopathies.

Mitochondria-Endoplasmic Reticulum Crosstalk in Parkinson's Disease: The Role of Brain Renin Angiotensin System Components

The past few decades have seen an increased emphasis on the involvement of the mitochondrial-associated membrane (MAM) in various neurodegenerative diseases, particularly in Parkinson’s disease (PD) and Alzheimer’s disease (AD). In PD, alterations in mitochondria, endoplasmic reticulum (ER), and MAM functions affect the secretion and metabolism of proteins, causing an imbalance in calcium homeostasis and oxidative stress. These changes lead to alterations in the translocation of the MAM components, such as IP3R, VDAC, and MFN1 and 2, and consequently disrupt calcium homeostasis and cause misfolded proteins with impaired autophagy, distorted mitochondrial dynamics, and cell death. Various reports indicate the detrimental involvement of the brain renin–angiotensin system (RAS) in oxidative stress, neuroinflammation, and apoptosis in various neurodegenerative diseases. In this review, we attempted to update the reports (using various search engines, such as PubMed, SCOPUS, Elsevier, and Springer Nature) demonstrating the pathogenic interactions between the various proteins present in mitochondria, ER, and MAM with respect to Parkinson’s disease. We also made an attempt to speculate the possible involvement of RAS and its components, i.e., AT1 and AT2 receptors, angiotensinogen, in this crosstalk and PD pathology. The review also collates and provides updated information on the role of MAM in calcium signaling, oxidative stress, neuroinflammation, and apoptosis in PD.

Captopril and losartan attenuate behavioural sensitization in mice chronically exposed to toluene

Inhalants are consumed worldwide for recreational purposes. The main component found in many inhalants is toluene. One of the most deleterious behavioural effects caused by chronic exposure to inhalants is addiction. This response has been associated with activation of the mesolimbic dopaminergic pathway, and it is known that the renin angiotensin system plays a role in the modulation of this dopaminergic system. In the present work, we hypothesize that blockade of the RAS with angiotensin converting enzyme inhibitors or angiotensin II type 1 receptor blockers is able to attenuate the addictive response induced by toluene. We exposed mice to toluene for four weeks to induce locomotor sensitization. In the second phase of the work, captopril or losartan were administered for 20 days. Subsequently, the expression of behavioural sensitization was evaluated with a toluene challenge. To exclude false associations between the observed responses and treatments, motor coordination and blood pressure were analysed in animals treated with captopril or losartan. At the end of the behavioural studies, animal brains were harvested and Ang II/Ang-(1-7) and Ang-(1-7)/Ang II ratios were analysed in the nucleus accumbens (NAc) and prefrontal cortex (PFCx). The results showed that toluene induced behavioural sensitization, while captopril or losartan treatment attenuated the expression of this response. No significant differences were observed in motor coordination or blood pressure. Repeated toluene administration decreased Ang-(1-7)/Ang II ratio in the PFCx. On the other hand, treatment with captopril or losartan decreased the Ang II/Ang-(1-7) ratio and enhanced the Ang-(1-7)/Ang II ratio in the NAc. This work suggests that blockade of RAS attenuates the toluene-induced behavioural sensitization.

White matter hyperintensities in patients with Parkinson's disease: A systematic review and meta-analysis

INTRODUCTION

Mechanisms driving neurodegeneration in Parkinson's disease (PD) are unclear and neurovascular dysfunction may be a contributing factor. White matter hyperintensities (WMH) are commonly found on brain MRI in patients with PD. It is controversial if they are more prevalent or more severe in PD compared with controls. This systematic review aims to answer this question.

METHODS

A systematic search of electronic databases was conducted for studies of WMH in patients with PD. A qualitative synthesis was done for studies reporting WMH prevalence or WMH scores on a visual rating scale (VRS). In studies reporting total WMH volume, the difference between patients with PD and controls was pooled using random effects meta-analysis.

RESULTS

Among 3860 subjects from 24 studies, 2360 were cases and 1500 controls. Fifteen studies reported WMH scores and four studies reported the prevalence of WMH. On VRS, five studies reported no difference in WMH scores, three found higher WMH scores in PD compared to controls, three reported increased WMH scores either in periventricular or deep white matter, and four reported higher scores only in PD with dementia. In studies reporting WMH volume, there was no difference between patients with PD and controls (pooled standardized mean difference = 0.1, 95%CI: -0.1-0.4, I² = 81%).

CONCLUSION

WMH are not more prevalent or severe in patients with PD than in age-matched controls. PD dementia may have more severe WMH compared to controls and PD with normal cognition. Prospective studies using standardized methods of WMH assessment are needed.

White Matter Hyperintensities, Dopamine Loss, and Motor Deficits in De Novo Parkinson's Disease

BACKGROUND

White matter hyperintensities, prevalent in patients with Parkinson's disease (PD), significantly affect parkinsonian motor symptoms. The objective of this study was to investigate the relationship between white matter hyperintensities and nigrostriatal dopamine depletion and their interaction or mediating effects on motor symptoms in patients with drug-naive early-stage PD.

METHODS

This cross-sectional study enrolled 501 patients with de novo PD who initially underwent [¹⁸ F] N-(3-fluoropropyl)-2β-carbonethoxy-3β-(4-iodophenyl) nortropane positron emission tomography and brain magnetic resonance imaging scans between April 2009 and September 2015 in a tertiary-care university hospital. We quantified dopamine transporter availability in each striatal subregion and assessed the severity of periventricular and lobar white matter hyperintensities using the Scheltens scale. The relationship between white matter hyperintensities, dopamine transporter availability in the posterior putamen, and Unified Parkinson's Disease Rating Scale (UPDRS) motor scores was assessed using multivariate linear regression and mediation analyses.

RESULTS

Periventricular and frontal white matter hyperintensities were generally associated with dopamine transporter availability in striatal subregions after adjusting for age at symptom onset, sex, disease duration, and vascular risk factors. There was an interaction effect between periventricular white matter hyperintensities and dopamine transporter availability in the posterior putamen for the axial motor score. The effect of white matter hyperintensities on UPDRS total score and bradykinesia subscore was indirectly mediated by dopamine transporter availability in the posterior putamen, whereas the axial sub-score was directly affected by white matter hyperintensities.

CONCLUSIONS

This study suggests that the detrimental effect of white matter hyperintensities on parkinsonian motor symptoms is more relevant and independent for axial motor impairments in the status of mildly decreased striatal dopamine transporter availability. © 2021 International Parkinson and Movement Disorder Society.

Emerging Concepts of Motor Reserve in Parkinson's Disease

The concept of cognitive reserve (CR) in Alzheimer's disease (AD) explains the differences between individuals in their susceptibility to AD-related pathologies. An enhanced CR may lead to less cognitive deficits despite severe pathological lesions. Parkinson's disease (PD) is also a common neurodegenerative disease and is mainly characterized by motor dysfunction related to striatal dopaminergic depletion. The degree of motor deficits in PD is closely correlated to the degree of dopamine depletion; however, significant individual variations still exist. Therefore, we hypothesized that the presence of motor reserve (MR) in PD explains the individual differences in motor deficits despite similar levels of striatal dopamine depletion. Since 2015, we have performed a series of studies investigating MR in de novo patients with PD using the data of initial clinical presentation and dopamine transporter PET scan. In this review, we summarized the results of these published studies. In particular, some premorbid experiences (i.e., physical activity and education) and modifiable factors (i.e., body mass index and white matter hyperintensity on brain image studies) could modulate an individual's capacity to tolerate PD pathology, which can be maintained throughout disease progression.

Candesartan Neuroprotection in Rat Primary Neurons Negatively Correlates with Aging and Senescence: a Transcriptomic Analysis

Preclinical experiments and clinical trials demonstrated that angiotensin II AT₁ receptor overactivity associates with aging and cellular senescence and that AT₁ receptor blockers (ARBs) protect from age-related brain disorders. In a primary neuronal culture submitted to glutamate excitotoxicity, gene set enrichment analysis (GSEA) revealed expression of several hundred genes altered by glutamate and normalized by candesartan correlated with changes in expression in Alzheimer's patient's hippocampus. To further establish whether our data correlated with gene expression alterations associated with aging and senescence, we compared our global transcriptional data with additional published datasets, including alterations in gene expression in the neocortex and cerebellum of old mice, human frontal cortex after age of 40, gene alterations in the Werner syndrome, rodent caloric restriction, Ras and oncogene-induced senescence in fibroblasts, and to tissues besides the brain such as the muscle and kidney. The most significant and enriched pathways associated with aging and senescence were positively correlated with alterations in gene expression in glutamate-injured neurons and, conversely, negatively correlated when the injured neurons were treated with candesartan. Our results involve multiple genes and pathways, including CAV1, CCND1, CDKN1A, CHEK1, ICAM1, IL-1B, IL-6, MAPK14, PTGS2, SERPINE1, and TP53, encoding proteins associated with aging and senescence hallmarks, such as inflammation, oxidative stress, cell cycle and mitochondrial function alterations, insulin resistance, genomic instability including telomere shortening and DNA damage, and the senescent-associated secretory phenotype. Our results demonstrate that AT₁ receptor blockade ameliorates central mechanisms of aging and senescence. Using ARBs for prevention and treatment of age-related disorders has important translational value.

Interaction between Angiotensin Type 1, Type 2, and Mas Receptors to Regulate Adult Neurogenesis in the Brain Ventricular-Subventricular Zone

The renin-angiotensin system (RAS), and particularly its angiotensin type-2 receptors (AT2), have been classically involved in processes of cell proliferation and maturation during development. However, the potential role of RAS in adult neurogenesis in the ventricular-subventricular zone (V-SVZ) and its aging-related alterations have not been investigated. In the present study, we analyzed the role of major RAS receptors on neurogenesis in the V-SVZ of adult mice and rats. In mice, we showed that the increase in proliferation of cells in this neurogenic niche was induced by activation of AT2 receptors but depended partially on the AT2-dependent antagonism of AT1 receptor expression, which restricted proliferation. Furthermore, we observed a functional dependence of AT2 receptor actions on Mas receptors. In rats, where the levels of the AT1 relative to those of AT2 receptor are much lower, pharmacological inhibition of the AT1 receptor alone was sufficient in increasing AT2 receptor levels and proliferation in the V-SVZ. Our data revealed that interactions between RAS receptors play a major role in the regulation of V-SVZ neurogenesis, particularly in proliferation, generation of neuroblasts, and migration to the olfactory bulb, both in young and aged brains, and suggest potential beneficial effects of RAS modulators on neurogenesis.

Angiotensin II modulates amphetamine-induced glial and brain vascular responses, and attention deficit via angiotensin type 1 receptor: Evidence from brain regional sensitivity to amphetamine

Amphetamine-induced neuroadaptations involve vascular damage, neuroinflammation, a hypo-functioning prefrontal cortex (PFC), and cognitive alterations. Brain angiotensin II, through angiotensin type 1 receptor (AT₁ -R), mediates oxidative/inflammatory responses, promoting endothelial dysfunction, neuronal oxidative damage and glial reactivity. The present work aims to unmask the role of AT₁ -R in the development of amphetamine-induced changes over glial and vascular components within PFC and hippocampus. Attention deficit was evaluated as a behavioral neuroadaptation induced by amphetamine. Brain microvessels were isolated to further evaluate vascular alterations after amphetamine exposure. Male Wistar rats were administered with AT₁ -R antagonist, candesartan, followed by repeated amphetamine. After one week drug-off period, animals received a saline or amphetamine challenge and were evaluated in behavioral tests. Afterward, their brains were processed for cresyl violet staining, CD11b (microglia marker), GFAP (astrocyte marker) or von Willebrand factor (vascular marker) immunohistochemistry, and oxidative/cellular stress determinations in brain microvessels. Statistical analysis was performed by using factorial ANOVA followed by Bonferroni or Tukey tests. Repeated amphetamine administration increased astroglial and microglial markers immunoreactivity, increased apoptotic cells, and promoted vascular network rearrangement at the PFC concomitantly with an attention deficit. Although the amphetamine challenge improved the attentional performance, it triggers detrimental effects probably because of the exacerbated malondialdehyde levels and increased heat shock protein 70 expression in microvessels. All observed amphetamine-induced alterations were prevented by the AT₁ -R blockade. Our results support the AT₁ -R involvement in the development of oxidative/inflammatory conditions triggered by amphetamine exposure, affecting cortical areas and increasing vascular susceptibility to future challenges.

Physical Exercise Improves Aging-Related Changes in Angiotensin, IGF-1, SIRT1, SIRT3, and VEGF in the Substantia Nigra

Dysregulation of tissue renin-angiotensin system (RAS) is involved in oxidative and inflammatory processes observed in major aging-related diseases, including neurodegenerative diseases such as Parkinson's disease (PD). Physical exercise has beneficial effects against aging-related changes, dopaminergic neuron vulnerability, and PD progression. The present study indicates that sedentary aged rats have an increase in activity of the nigral angiotensin (Ang) II/Ang type 1 receptor (AT1) axis (ie, the pro-oxidative pro-inflammatory arm), and a decrease in the activity of the RAS protective arm (ie, Ang II/AT2 and Ang 1-7/Mas receptor axis) in comparison with young rats. In addition, sedentary aged rats showed a decrease in levels of nigral IGF-1, SIRT1, SIRT3, and VEGF. Treadmill running induced a significant increase in levels of IGF-1, SIRT1, SIRT3, and VEGF, as well as an increase in expression of the protective Ang 1-7/Mas axis and inhibition of the Ang II/AT1 axis. The exercise-induced increase in IGF-1 and sirtuins may mediate the effects of exercise on the nigral RAS. However, exercise may induce the increase in VEGF and modulation of RAS activity by different pathways. Exercise, via RAS, contributes to inhibition of the pro-oxidative and proinflammatory state that increase dopaminergic neuron vulnerability and risk of PD with aging.

White matter hyperintensities as a predictor of freezing of gait in Parkinson's disease

INTRODUCTION

To investigate the effect of white matter hyperintensities (WMH) on long-term motor outcomes in Parkinson's disease (PD).

METHODS

We retrospectively reviewed medical records of 268 patients with de novo PD (follow-up > 3 years). According to the Clinical Research Center for Dementia of South Korea (CREDOS) WMH visual rating scale scores, the patients were divided into two groups: a PD group with minimal WMH (PD-WMH-; n = 198) and a PD group with moderate to severe WMH (PD-WMH+; n = 70). We compared longitudinal increases in doses of dopaminergic medications between the two groups using a mixed model. We also assessed the effects of WMH on the development of freezing of gait (FOG).

RESULTS

Patients in the PD-WMH + group were older than those in the PD-WMH- group, and had more severe motor deficits and more severely decreased striatal dopamine transporter availability. The PD-WMH + group required higher doses of dopaminergic medications for symptom control, compared to the PD-WMH- group, over the follow-up period. After adjusting for age, sex, striatal dopamine transporter availability, and levodopa-equivalent dose, the PD-WMH + group showed a higher risk of developing FOG (HR, 3.29; 95% CI, 1.79-6.05; p < 0.001) than the PD-WMH- group.

CONCLUSION

This study demonstrates that WMH burden negatively affects the longitudinal requirement of dopaminergic medication and the development of FOG. These findings suggest that baseline WMH severity or volume may be a useful prognostic marker of motor outcomes in PD.

miRNAs Identify Shared Pathways in Alzheimer's and Parkinson's Diseases

Despite the identification of several dozens of common genetic variants associated with Alzheimer's disease (AD) and Parkinson's disease (PD), most of the genetic risk remains uncharacterised. Therefore, it is important to understand the role of regulatory elements, such as miRNAs. Dysregulated miRNAs are implicated in AD and PD, with potential value in dissecting the shared pathophysiology between the two disorders. miRNAs relevant to both neurodegenerative diseases are related to axonal guidance, apoptosis, and inflammation, therefore, AD and PD likely arise from similar underlying biological pathway defects. Furthermore, pathways regulated by APP, L1CAM, and genes of the caspase family may represent promising therapeutic miRNA targets in AD and PD since they are targeted by dysregulated miRNAs in both disorders.

Angiotensin II Receptor 1 Blockage Limits Brain Damage and Improves Functional Outcome After Brain Injury in Aged Animals Despite Age-Dependent Reduction in AT1 Expression

Traumatic brain injury (TBI) is a frequent pathology associated with poor neurological outcome in the aged population. We recently observed accelerated cerebral inflammation in aged mice in response to TBI. Candesartan is a potent specific inhibitor of angiotensin II receptor type 1 (AT1) which limits cerebral inflammation and brain damage in juvenile animals after experimental TBI. In the present study, we show significantly lower posttraumatic AT1 mRNA levels in aged (21 months) compared to young (2 months) mice. Despite low cerebral At1 expression, pharmacologic blockade by treatment with candesartan [daily, beginning 30 min after experimental TBI by controlled cortical impact (CCI)] was highly effective in both young and aged animals and reduced histological brain damage by -20% after 5 days. In young mice, neurological improvement was enhanced by AT1 inhibition 5 days after CCI. In older animals, candesartan treatment reduced functional impairment already on day 3 after TBI and post-traumatic body weight (BW) loss was attenuated. Candesartan reduced microglia activation (-40%) in young and aged animals, and neutrophil infiltration (-40% to 50%) in aged mice, whereas T-cell infiltration was not changed in either age group. In young animals, markers of anti-inflammatory microglia M2a polarization [arginase 1 (Arg1), chitinase3-like 3 (Ym1)] were increased by candesartan at days 1 and 5 after insult. In older mice 5 days after insult, expression of Arg1 was significantly higher independently of the treatment, whereas Ym1 gene expression was further enhanced by AT1 inhibition. Despite age-dependent posttraumatic differences in At1 expression levels, inhibition of AT1 was highly effective in a posttreatment paradigm. Targeting inflammation with candesartan is, therefore, a promising therapeutic strategy to limit secondary brain damage independent of the age.

Deciphering variability in the role of interleukin-1β in Parkinson's disease

Although the role of inflammation in neurodegeneration has been well acknowledged, less is known on the issue of each cytokine in specific neurodegenerative diseases. In this review, we will present evidence elucidating that interleukin-1β (IL-1β) has a multi-faceted character in pathogenesis of Parkinson's disease, which is a progressive neurodegenerative disorder. Increased levels of IL-1β were found in PD patients. Besides, PD symptoms were observed in IL-1β wild-type, but not deficient, animals. These lines of evidence suggest that IL-1β may contribute to the initiation or progression of PD. On the other hand, some studies reported decreased levels of IL-1β in PD patients. Also, genetic studies provided evidence suggesting that IL-1β may protect individuals against PD. Presumably, the broad range of IL-1β role is due to its interaction with both upstream and downstream mediators. Differences in IL-1β levels could be because of glia population (i.e. microglia and astrocytes), mitogen-activated protein kinase and nuclear factor κ light-chain-enhancer of activated B cells signaling pathways, and several mediators (including cyclooxygenase, neurotrophic factors, reactive oxygen species, caspases, heme oxygenase-1, and matrix metalloproteinases). Although far from practice at this point, unraveling theoretical therapeutic targets based on the up-down IL-1β neuroweb could facilitate the development of strategies that are likely to be used for pharmaceutical designs of anti-neurodegenerative drugs of the future.

Molecular Mechanisms of Vascular Dementia: What Can Be Learned from Animal Models of Chronic Cerebral Hypoperfusion?

Vascular dementia (VD) is defined as a progressive neurodegenerative disease of cognitive decline, attributable to cerebrovascular factors. Numerous studies have demonstrated that chronic cerebral hypoperfusion (CCH) is associated with the initiation and progression of VD and Alzheimer's disease (AD). Suitable animal models were established to replicate such pathological condition in experimental research, which contributes largely to comprehending causal relationships between CCH and cognitive impairment. The most widely used experimental model of VD and CCH is permanent bilateral common carotid artery occlusion in rats. In CCH models, changes of learning and memory, cerebral blood flow (CBF), energy metabolism, and neuropathology initiated by ischemia were revealed. However, in order to achieve potential therapeutic targets, particular mechanisms in cognitive and neuropathological changes from CCH to dementia should be investigated. Recent studies have shown that hypoperfusion resulted in a chain of disruption of homeostatic interactions, including oxidative stress, neuroinflammation, neurotransmitter system dysfunction, mitochondrial dysfunction, disturbance of lipid metabolism, and alterations of growth factors. Evidence from experimental studies that elucidate the damaging effects of such imbalances suggests their critical roles in the pathogenesis of VD. The present review provides a summary of the achievements in mechanisms made with the CCH models, permits an understanding of the causative role played by CCH in VD, and highlights preventative and therapeutic prospects.

Chronic cerebral hypoperfusion induces vascular plasticity and hemodynamics but also neuronal degeneration and cognitive impairment

Chronic cerebral hypoperfusion (CCH) induces cognitive impairment, but the compensative mechanism of cerebral blood flow (CBF) is not fully understood. The present study mainly investigated dynamic changes in CBF, angiogenesis, and cellular pathology in the cortex, the striatum, and the cerebellum, and also studied cognitive impairment of rats induced by bilateral common carotid artery occlusion (BCCAO). Magnetic resonance imaging (MRI) techniques, immunochemistry, and Morris water maze were employed to the study. The CBF of the cortex, striatum, and cerebellum dramatically decreased after right common carotid artery occlusion (RCCAO), and remained lower level at 2 weeks after BCCAO. It returned to the sham level from 3 to 6 weeks companied by the dilation of vertebral arteries after BCCAO. The number of microvessels declined at 2, 3, and 4 weeks but increased at 6 weeks after BCCAO. Neuronal degeneration occurred in the cortex and striatum from 2 to 6 weeks, but the number of glial cells dramatically increased at 4 weeks after BCCAO. Cognitive impairment of ischemic rats was directly related to ischemic duration. Our results suggest that CCH induces a compensative mechanism attempting to maintain optimal CBF to the brain. However, this limited compensation cannot prevent neuronal loss and cognitive impairment after permanent ischemia.

Loss of angiotensin II receptor expression in dopamine neurons in Parkinson's disease correlates with pathological progression and is accompanied by increases in Nox4- and 8-OH guanosine-related nucleic acid oxidation and caspase-3 activation

Background

In rodent models of Parkinson’s disease (PD), dopamine neuron loss is accompanied by increased expression of angiotensin II (AngII), its type 1 receptor (AT1), and NADPH oxidase (Nox) in the nigral dopamine neurons and microglia. AT1 blockers (ARBs) stymie such oxidative damage and neuron loss. Whether changes in the AngII/AT1/Nox4 axis contribute to Parkinson neuropathogenesis is unknown. Here, we studied the distribution of AT1 and Nox4 in dopamine neurons in two nigral subregions: the less affected calbindin-rich matrix and the first-affected calbindin-poor nigrosome 1 of three patients, who were clinically asymptomatic, but had nigral dopamine cell loss and Braak stages consistent with a neuropathological diagnosis of PD (prePD). For comparison, five clinically- and neuropathologically-confirmed PD patients and seven age-matched control patients (AMC) were examined.

Results

AT1 and Nox4 immunoreactivity was noted in dopamine neurons in both the matrix and the nigrosome 1. The total cellular levels of AT1 in surviving dopamine neurons in the matrix and nigrosome 1 declined from AMC>prePD>PD, suggesting that an AngII/AT1/Nox4 axis orders neurodegenerative progression. In this vein, the loss of dopamine neurons was paralleled by a decline in total AT1 per surviving dopamine neuron. Similarly, AT1 in the nuclei of surviving neurons in the nigral matrix declined with disease progression, i.e., AMC>prePD>PD. In contrast, in nigrosome 1, the expression of nuclear AT1 was unaffected and similar in all groups. The ratio of nuclear AT1 to total AT1 (nuclear + cytoplasmic + membrane) in dopamine neurons increased stepwise from AMC to prePD to PD. The proportional increase in nuclear AT1 in dopamine neurons in nigrosome 1 of prePD and PD patients was accompanied by elevated nuclear expression of Nox4, oxidative damage to DNA, and caspase-3-mediated cell loss.

Conclusions

Our observations are consistent with the idea that AngII/AT1/Nox4 axis-mediated oxidative stress gives rise to the dopamine neuron dysfunction and loss characteristic of the neuropathological and clinical manifestations of PD and suggest that the chance for a neuron to survive increases in association with lower total as well as nuclear AT1 expression. Our results support the need for further evaluation of ARBs as disease-modifying agents in PD.

Striatal vessels receive phosphorylated tyrosine hydroxylase-rich innervation from midbrain dopaminergic neurons

Nowadays it is assumed that besides its roles in neuronal processing, dopamine (DA) is also involved in the regulation of cerebral blood flow. However, studies on the hemodynamic actions of DA have been mainly focused on the cerebral cortex, but the possibility that vessels in deeper brain structures receive dopaminergic axons and the origin of these axons have not been investigated. Bearing in mind the evidence of changes in the blood flow of basal ganglia in Parkinson's disease (PD), and the pivotal role of the dopaminergic mesostriatal pathway in the pathophysiology of this disease, here we studied whether striatal vessels receive inputs from midbrain dopaminergic neurons. The injection of an anterograde neuronal tracer in combination with immunohistochemistry for dopaminergic, vascular and astroglial markers, and dopaminergic lesions, revealed that midbrain dopaminergic axons are in close apposition to striatal vessels and perivascular astrocytes. These axons form dense perivascular plexuses restricted to striatal regions in rats and monkeys. Interestingly, they are intensely immunoreactive for tyrosine hydroxylase (TH) phosphorylated at Ser19 and Ser40 residues. The presence of phosphorylated TH in vessel terminals indicates they are probably the main source of basal TH activity in the striatum, and that after activation of midbrain dopaminergic neurons, DA release onto vessels precedes that onto neurons. Furthermore, the relative weight of this "vascular component" within the mesostriatal pathway suggests that it plays a relevant role in the pathophysiology of PD.

Microglial TNF-α mediates enhancement of dopaminergic degeneration by brain angiotensin

In vitro and in vivo models of Parkinson's disease were used to investigate whether TNF-α plays a major role in the enhancement of the microglial response and dopaminergic degeneration induced by brain angiotensin hyperactivity. Treatment of primary mesencephalic cultures with low doses of the neurotoxin MPP(+) induced a significant loss of dopaminergic neurons, which was enhanced by cotreatment with angiotensin II and inhibited by TNF-α inhibitors. Treatment of primary cultures with angiotensin induced a marked increase in levels of TNF-α, which was inhibited by treatment with angiotensin type-1-receptor antagonists, NADPH-oxidase inhibitors and NFK-β inhibitors. However, TNF-α levels were not significantly affected by treatment with angiotensin in the absence of microglia. The microglial origin of the angiotensin-induced increase in TNF-α levels was confirmed using dopaminergic (MES 23.5) and microglial (N9) cell lines. Inhibition of the microglial Rho-kinase activity also blocked the AII-induced increase in TNF-α levels. Treatment of the dopaminergic cell line with TNF-α revealed that NFK-β activation mediates the deleterious effect of microglial TNF-α on dopaminergic neurons. Treatment of mice with MPTP also induced significant increases in striatal and nigral TNF-α levels, which were inhibited by angiotensin type-1-receptor antagonists or NFK-β inhibitors. The present results show that microglial TNF-α plays a major role in angiotensin-induced dopaminergic cell death and that the microglial release of TNF-α is mediated by activation of angiotensin type-1 receptors, NADPH-oxidase, Rho-kinase and NFK-β.

Brain renin-angiotensin system and dopaminergic cell vulnerability

Although the renin-angiotensin system (RAS) was classically considered as a circulating system that regulates blood pressure, many tissues are now known to have a local RAS. Angiotensin, via type 1 receptors, is a major activator of the NADPH-oxidase complex, which mediates several key events in oxidative stress (OS) and inflammatory processes involved in the pathogenesis of major aging-related diseases. Several studies have demonstrated the presence of RAS components in the basal ganglia, and particularly in the nigrostriatal system. In the nigrostriatal system, RAS hyperactivation, via NADPH-oxidase complex activation, exacerbates OS and the microglial inflammatory response and contributes to progression of dopaminergic degeneration, which is inhibited by angiotensin receptor blockers and angiotensin converting enzyme (ACE) inhibitors. Several factors may induce an increase in RAS activity in the dopaminergic system. A decrease in dopaminergic activity induces compensatory upregulation of local RAS function in both dopaminergic neurons and glia. In addition to its role as an essential neurotransmitter, dopamine may also modulate microglial inflammatory responses and neuronal OS via RAS. Important counterregulatory interactions between angiotensin and dopamine have also been observed in several peripheral tissues. Neurotoxins and proinflammatory factors may also act on astrocytes to induce an increase in RAS activity, either independently of or before the loss of dopamine. Consistent with a major role of RAS in dopaminergic vulnerability, increased RAS activity has been observed in the nigra of animal models of aging, menopause and chronic cerebral hypoperfusion, which also showed higher dopaminergic vulnerability. Manipulation of the brain RAS may constitute an effective neuroprotective strategy against dopaminergic vulnerability and progression of Parkinson's disease.

Brain angiotensin regulates iron homeostasis in dopaminergic neurons and microglial cells

Dysfunction of iron homeostasis has been shown to be involved in ageing, Parkinson's disease and other neurodegenerative diseases. Increased levels of labile iron result in increased reactive oxygen species and oxidative stress. Angiotensin II, via type-1 receptors, exacerbates oxidative stress, the microglial inflammatory response and progression of dopaminergic degeneration. Angiotensin activates the NADPH-oxidase complex, which produces superoxide. However, it is not known whether angiotensin affects iron homeostasis. In the present study, administration of angiotensin to primary mesencephalic cultures, the dopaminergic cell line MES23.5 and to young adult rats, significantly increased levels of transferrin receptors, divalent metal transporter-1 and ferroportin, which suggests an increase in iron uptake and export. In primary neuron-glia cultures and young rats, angiotensin did not induce significant changes in levels of ferritin or labile iron, both of which increased in neurons in the absence of glia (neuron-enriched cultures, dopaminergic cell line) and in the N9 microglial cell line. In aged rats, which are known to display high levels of angiotensin activity, ferritin levels and iron deposits in microglial cells were enhanced. Angiotensin-induced changes were inhibited by angiotensin type-1 receptor antagonists, NADPH-oxidase inhibitors, antioxidants and NF-kB inhibitors. The results demonstrate that angiotensin, via type-1 receptors, modulates iron homeostasis in dopaminergic neurons and microglial cells, and that glial cells play a major role in efficient regulation of iron homeostasis in dopaminergic neurons.

A Role for the Brain RAS in Alzheimer's and Parkinson's Diseases

The brain renin-angiotensin system (RAS) has available the necessary functional components to produce the active ligands angiotensins II (AngII), angiotensin III, angiotensins (IV), angiotensin (1-7), and angiotensin (3-7). These ligands interact with several receptor proteins including AT1, AT2, AT4, and Mas distributed within the central and peripheral nervous systems as well as local RASs in several organs. This review first describes the enzymatic pathways in place to synthesize these ligands and the binding characteristics of these angiotensin receptor subtypes. We next discuss current hypotheses to explain the disorders of Alzheimer's disease (AD) and Parkinson's disease (PD), as well as research efforts focused on the use of angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), in their treatment. ACE inhibitors and ARBs are showing promise in the treatment of several neurodegenerative pathologies; however, there is a need for the development of analogs capable of penetrating the blood-brain barrier and acting as agonists or antagonists at these receptor sites. AngII and AngIV have been shown to play opposing roles regarding memory acquisition and consolidation in animal models. We discuss the development of efficacious AngIV analogs in the treatment of animal models of AD and PD. These AngIV analogs act via the AT4 receptor subtype which may coincide with the hepatocyte growth factor/c-Met receptor system. Finally, future research directions are described concerning new approaches to the treatment of these two neurological diseases.

Effect of chronic treatment with angiotensin type 1 receptor antagonists on striatal dopamine levels in normal rats and in a rat model of Parkinson's disease treated with L-DOPA

Beneficial effects of angiotensin type-1 receptor (AT1) inhibition have been observed in a number of brain processes mediated by oxidative stress and neuroinflammation, including Parkinson's disease. However, important counterregulatory interactions between dopamine and angiotensin systems have recently been demonstrated in several peripheral tissues, and it is possible that a decrease in dopamine levels due to AT1 inhibition may interfere with neuroprotective strategies. The present experiments involving rats with normal dopaminergic innervation indicate that chronic treatment with the AT1 antagonist candesartan does not significantly affect striatal levels of dopamine, serotonin or metabolites, as does not significantly affect motor behavior, as evaluated by the rotarod test. Interestingly, chronic administration of candesartan to normal rats induced a marked increase in dopamine D1 and a decrease in dopamine D2 receptor expression. In a rat model of Parkinson's disease treated with L-DOPA, no differences in striatal dopamine and serotonin levels were observed between candesartan-treated rats and untreated, which suggests that chronic treatment with candesartan does not significantly affect the process of L-DOPA decarboxylation and dopamine release in Parkinson's disease patients. Candesartan did not induce any differences in the striatal expression of dopamine D1 and D2 and serotonin 5-HT1B receptors in 6ydroxydopamine-lesioned rats treated with L-DOPA. The results suggest that chronic treatment with AT1 antagonists as a neuroprotective strategy does not significantly affect striatal dopamine release or motor behavior. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Brain angiotensin and dopaminergic degeneration: relevance to Parkinson's disease

The pathogenic mechanism of Parkinson's disease (PD) appears to be multifactorial. However, oxidative stress and neuroinflammation, including activation of NADPH-dependent oxidases, play a major role in the progression of dopaminergic cell death. The renin-angiotensin system (RAS) was described as a circulating humoral system that regulates blood pressure and water homeostasis. However, there exist local RAS in many tissues, and locally formed angiotensin activates NADPH-dependent oxidases, which are a major source of superoxide and are upregulated in major aging-related diseases such as hypertension, diabetes and atherosclerosis. Furthermore, an intracellular or intracrine RAS, with still unknown functions, has been identified in several cell types. The brain has an independent local RAS, which has been involved in several brain disorders, including neurodegenerative diseases. It is particularly interesting for PD the important interaction observed between angiotensin and dopamine, which counterregulate each other in renal cells and also in the striatum and substantia nigra. In recent studies, we have observed both a local and an intracellular RAS in the rodent, monkey and human substantia nigra, and that dopamine depletion induced RAS upregulation possibly as a compensatory mechanism. However, RAS hyperactivation also exacerbated oxidative stress and neuroinflammation, which contributed to progression of dopaminergic degeneration. In addition, we observed increased RAS activity in the nigra of animals with higher vulnerability of dopaminergic neurons to degeneration, such as aged males, menopausal females and rats subjected to chronic brain hypoperfusion. RAS activity and dopaminergic vulnerability were significantly reduced by treatment with angiotensin type I receptor antagonists. Manipulation of the brain RAS may constitute an effective neuroprotective strategy against dopaminergic degeneration in PD.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.