Aging-related changes in the nigral angiotensin system enhances proinflammatory and pro-oxidative markers and 6-OHDA-induced dopaminergic degeneration

The potential role of brain renin-angiotensin system in the neuropathology of Parkinson disease: Friend, foe or turncoat?

Parkinson disease (PD) is one of the most common neurodegenerative diseases of the brain. Of note, brain renin-angiotensin system (RAS) is intricate in the PD neuropathology through modulation of oxidative stress, mitochondrial dysfunction and neuroinflammation. Therefore, modulation of brain RAS by angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEIs) may be effective in reducing the risk and PD neuropathology. It has been shown that all components including the peptides and enzymes of the RAS are present in the different brain areas. Brain RAS plays a critical role in the regulation of memory and cognitive function, and in the controlling of central blood pressure. However, exaggerated brain RAS is implicated in the pathogenesis of different neurodegenerative diseases including PD. Two well-known pathways of brain RAS are recognized including; the classical pathway which is mainly mediated by AngII/AT1R has detrimental effects. Conversely, the non-classical pathway which is mostly mediated by ACE2/Ang1-7/MASR and AngII/AT2R has beneficial effects against PD neuropathology. Exaggerated brain RAS affects the viability of dopaminergic neurons. However, the fundamental mechanism of brain RAS in PD neuropathology was not fully elucidated. Consequently, the purpose of this review is to disclose the mechanistic role of RAS in in the pathogenesis of PD. In addition, we try to revise how the ACEIs and ARBs can be developed for therapeutics in PD.

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

The AT1/AT2 Receptor Equilibrium Is a Cornerstone of the Regulation of the Renin Angiotensin System beyond the Cardiovascular System

The AT₁ receptor has mainly been associated with the pathological effects of the renin-angiotensin system (RAS) (e.g., hypertension, heart and kidney diseases), and constitutes a major therapeutic target. In contrast, the AT₂ receptor is presented as the protective arm of this RAS, and its targeting via specific agonists is mainly used to counteract the effects of the AT₁ receptor. The discovery of a local RAS has highlighted the importance of the balance between AT₁/AT₂ receptors at the tissue level. Disruption of this balance is suggested to be detrimental. The fine tuning of this balance is not limited to the regulation of the level of expression of these two receptors. Other mechanisms still largely unexplored, such as S-nitrosation of the AT₁ receptor, homo- and heterodimerization, and the use of AT₁ receptor-biased agonists, may significantly contribute to and/or interfere with the settings of this AT₁/AT₂ equilibrium. This review will detail, through several examples (the brain, wound healing, and the cellular cycle), the importance of the functional balance between AT₁ and AT₂ receptors, and how new molecular pharmacological approaches may act on its regulation to open up new therapeutic perspectives.

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.

Angiotensin II increases the firing activity of pallidal neurons and participates in motor control in rats

The globus pallidus has emerged as a crucial node in the basal ganglia motor control circuit under both healthy and parkinsonian states. Previous studies have shown that angiotensin II (Ang II) and angiotensin subtype 1 receptor (AT1R) are closely related to Parkinson's disease (PD). Recent morphological study revealed the expression of AT1R in the globus pallidus of mice. To investigate the functions of Ang II/AT1R on the globus pallidus neurons of both normal and parkinsonian rats, electrophysiological recordings and behavioral tests were performed in the present study. Electrophysiological recordings showed that exogenous and endogenous Ang II mainly excited the globus pallidus neurons through AT1R. Behavioral tests further demonstrated that unilateral microinjection of Ang II into the globus pallidus induced significantly contralateral-biased swing in elevated body swing test (EBST), and bilateral microinjection of Ang II into the globus pallidus alleviated catalepsy and akinesia caused by haloperidol. AT1R was involved in Ang II-induced behavioral effects. Immunostaining showed that AT1R was expressed in the globus pallidus of rats. On the basis of the present findings, we concluded that pallidal Ang II/AT1R alleviated parkinsonian motor deficits through activating globus pallidus neurons, which will provide a rationale for further investigations into the potential of Ang II in the treatment of motor disorders originating from the basal ganglia.

Role of neuroinflammation mediated potential alterations in adult neurogenesis as a factor for neuropsychiatric symptoms in Post-Acute COVID-19 syndrome-A narrative review

Persistence of symptoms beyond the initial 3 to 4 weeks after infection is defined as post-acute COVID-19 syndrome (PACS). A wide range of neuropsychiatric symptoms like anxiety, depression, post-traumatic stress disorder, sleep disorders and cognitive disturbances have been observed in PACS. The review was conducted based on PRISMA-S guidelines for literature search strategy for systematic reviews. A cytokine storm in COVID-19 may cause a breach in the blood brain barrier leading to cytokine and SARS-CoV-2 entry into the brain. This triggers an immune response in the brain by activating microglia, astrocytes, and other immune cells leading to neuroinflammation. Various inflammatory biomarkers like inflammatory cytokines, chemokines, acute phase proteins and adhesion molecules have been implicated in psychiatric disorders and play a major role in the precipitation of neuropsychiatric symptoms. Impaired adult neurogenesis has been linked with a variety of disorders like depression, anxiety, cognitive decline, and dementia. Persistence of neuroinflammation was observed in COVID-19 survivors 3 months after recovery. Chronic neuroinflammation alters adult neurogenesis with pro-inflammatory cytokines supressing anti-inflammatory cytokines and chemokines favouring adult neurogenesis. Based on the prevalence of neuropsychiatric symptoms/disorders in PACS, there is more possibility for a potential impairment in adult neurogenesis in COVID-19 survivors. This narrative review aims to discuss the various neuroinflammatory processes during PACS and its effect on adult neurogenesis.

Impact of aging on animal models of Parkinson's disease

Aging is the biggest risk factor for developing Parkinson's disease (PD), the second most common neurodegenerative disorder. Several animal models have been developed to explore the pathophysiology underlying neurodegeneration and the initiation and spread of alpha-synuclein-related PD pathology, and to investigate biomarkers and therapeutic strategies. However, bench-to-bedside translation of preclinical findings remains suboptimal and successful disease-modifying treatments remain to be discovered. Despite aging being the main risk factor for developing idiopathic PD, most studies employ young animals in their experimental set-up, hereby ignoring age-related cellular and molecular mechanisms at play. Consequently, studies in young animals may not be an accurate reflection of human PD, limiting translational outcomes. Recently, it has been shown that aged animals in PD research demonstrate a higher susceptibility to developing pathology and neurodegeneration, and present with a more disseminated and accelerated disease course, compared to young animals. Here we review recent advances in the investigation of the role of aging in preclinical PD research, including challenges related to aged animal models that are limiting widespread use. Overall, current findings indicate that the use of aged animals may be required to account for age-related interactions in PD pathophysiology. Thus, although the use of older animals has disadvantages, a model that better represents clinical disease within the elderly would be more beneficial in the long run, as it will increase translational value and minimize the risk of therapies failing during clinical studies. Furthermore, we provide recommendations to manage the challenges related to aged animal models.

Higher Angiotensin II Type 1 Receptor Levels and Activity in the Postmortem Brains of Older Persons with Alzheimer's Dementia

Aging is a key risk factor in Alzheimer's dementia (AD) development and progression. The primary dementia-protective benefits of angiotensin II subtype 1 receptor (AT1R) blockers are believed to arise from systemic effects on blood pressure. However, a brain-specific renin-angiotensin system (b-RAS) exists, which can be altered by AT1R blockers. Brain RAS acts mainly through 3 angiotensin receptors: AT1R, AT2R, and AT4R. Changes in these brain angiotensin receptors may accelerate the progression of AD. Using postmortem frontal cortex brain samples of age- and sex-matched cognitively normal individuals (n = 30) and AD patients (n = 30), we sought to dissect the b-RAS changes associated with AD and assess how these changes correlate with brain markers of oxidative stress, inflammation, and mitochondrial dysfunction as well as amyloid-β and paired helical filament tau pathologies. Our results show higher protein levels of the pro-inflammatory AT1R and phospho-ERK (pERK) in the brains of AD participants. Brain AT1R levels and pERK correlated with higher oxidative stress, lower cognitive performance, and higher tangle and amyloid-β scores. This study identifies molecular changes in b-RAS and offers insight into the role of b-RAS in AD-related brain pathology.

Angiotensin Type-1 Receptor Inhibition Reduces NLRP3 Inflammasome Upregulation Induced by Aging and Neurodegeneration in the Substantia Nigra of Male Rodents and Primary Mesencephalic Cultures

The tissue renin–angiotensin system (RAS) has been shown to be involved in prooxidative and proinflammatory changes observed in aging and aging-related diseases such as dopaminergic degeneration in Parkinson’s disease (PD). We studied the activation of the NLRP3 inflammasome in the substantia nigra with aging and early stages of dopaminergic degeneration in PD models and, particularly, if the brain RAS, via its prooxidative proinflammatory angiotensin II (AngII) type 1 (AT1) receptors, mediates the inflammasome activation. Nigras from aged rats and mice and 6-hydroxydopamine PD models showed upregulation in transcription of inflammasome-related components (NLRP3, pro-IL1β and pro-IL18) and IL1β and IL18 protein levels, which was inhibited by the AT1 receptor antagonist candesartan. The role of the AngII/AT1 axis in inflammasome activation was further confirmed in rats intraventricularly injected with AngII, and in primary mesencephalic cultures treated with 6-hydroxydopamine, which showed inflammasome activation that was blocked by candesartan. Observations in the nigra of young and aged AT1 and AT2 knockout mice confirmed the major role of AT1 receptors in nigral inflammasome activation. In conclusion, the inflammasome is upregulated by aging and dopaminergic degeneration in the substantia nigra, possibly related with a decrease in dopamine levels, and it is mediated by the AngII/AT1 axis.

Age-related biochemical dysfunction in 6-OHDA model rats subject to induced- endurance exercise

Exercise can alleviate the disorders considered as the normal consequences of aging. Whether or not the treadmill endurance training affects the biochemical markers in the Parkinson's disease model rats after the 6-hydroxydopamine (6-OHDA) injection is assessed in this article. The experimental groups of N=8 rats consist of 1) Saline and Young sedentary (S-Young); 2) Saline and Old sedentary (S-Old); 3) Young and 6-OHDA without exercise (Y); 4) Young and 6-OHDA with exercise (YE); 5) Old and 6-OHDA without exercise (O); and 6) Old and 6-OHDA with exercise (OE). An 8 μg of 6-OHDA is injected into the right MFB. The rotation due to apomorphine, weight variation, and some biochemical expression are measured in the rats' striatum. Exposure to 6-OHDA: increase weight loss by (%8) and rotation by (%90), reduce the protein levels of Bdnf by (30%), Th by (43%), and Tfam by (24%), in aging rats (P<0.05). The P53 level rose after the injection compared with the same Saline group (Old rats: 27% and Young rats: 14%), the highest in the O group. The findings indicate that endurance exercise amends the mitochondrial parameters and the apomorphine-induced rotation impairments in the presence of 6-OHDA injection. These positive effects of treadmill running in unilateral 6-OHDA lesioned rat model are age-dependent and are more significant in younger rats.

An ACE2/Mas-related receptor MrgE axis in dopaminergic neuron mitochondria

ACE2 plays a pivotal role in the balance between the pro-oxidative pro-inflammatory and the anti-oxidative anti-inflammatory arms of the renin-angiotensin system. Furthermore, ACE2 is the entry receptor for SARS-CoV-2. Clarification of ACE2-related mechanisms is crucial for the understanding of COVID-19 and other oxidative stress and inflammation-related processes. In rat and monkey brain, we discovered that the intracellular ACE2 and its products Ang 1–7 and alamandine are highly concentrated in the mitochondria and bind to a new mitochondrial Mas-related receptor MrgE (MrgE) to produce nitric oxide. We found MrgE expressed in neurons and glia of rodents and primates in the substantia nigra and different brain regions. In the mitochondria, ACE2 and MrgE expressions decreased and NOX4 increased with aging. This new ACE2/MrgE/NO axis may play a major role in mitochondrial regulation of oxidative stress in neurons, and possibly other cells. Therefore, dysregulation of the mitochondrial ACE2/MrgE/NO axis may play a major role in neurodegenerative processes of dopaminergic neurons, where mitochondrial dysfunction and oxidative stress play a crucial role. Since ACE2 binds SARS-CoV-2 spike protein, the mitochondrial ACE2/MrgE/NO axis may also play a role in SARS-CoV-2 cellular effects.

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ACE2 products Ang1-7 and alamandine (Ala) highly concentrate in brain mitochondria.

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Ang1-7 and Ala bind to mitochondrial Mas-related receptor MrgE producing nitric oxide.

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ACE2/MrgE may play a major role in mitochondrial function and oxidative stress.

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Clarification of ACE2-related mechanisms is also crucial for understanding COVID-19.

Aβ-accelerated neurodegeneration caused by Alzheimer's-associated ACE variant R1279Q is rescued by angiotensin system inhibition in mice

Recent genome-wide association studies identified the angiotensin-converting enzyme gene (ACE) as an Alzheimer's disease (AD) risk locus. However, the pathogenic mechanism by which ACE causes AD is unknown. Using whole-genome sequencing, we identified rare ACE coding variants in AD families and investigated one, ACE1 R1279Q, in knockin (KI) mice. Similar to AD, ACE1 was increased in neurons, but not microglia or astrocytes, of KI brains, which became elevated further with age. Angiotensin II (angII) and angII receptor AT1R signaling were also increased in KI brains. Autosomal dominant neurodegeneration and neuroinflammation occurred with aging in KI hippocampus, which were absent in the cortex and cerebellum. Female KI mice exhibited greater hippocampal electroencephalograph disruption and memory impairment compared to males. ACE variant effects were more pronounced in female KI mice, suggesting a mechanism for higher AD risk in women. Hippocampal neurodegeneration was completely rescued by treatment with brain-penetrant drugs that inhibit ACE1 and AT1R. Although ACE variant-induced neurodegeneration did not depend on β-amyloid (Aβ) pathology, amyloidosis in 5XFAD mice crossed to KI mice accelerated neurodegeneration and neuroinflammation, whereas Aβ deposition was unchanged. KI mice had normal blood pressure and cerebrovascular functions. Our findings strongly suggest that increased ACE1/angII signaling causes aging-dependent, Aβ-accelerated selective hippocampal neuron vulnerability and female susceptibility, hallmarks of AD that have hitherto been enigmatic. We conclude that repurposed brain-penetrant ACE inhibitors and AT1R blockers may protect against AD.

Novel Interactions Involving the Mas Receptor Show Potential of the Renin-Angiotensin system in the Regulation of Microglia Activation: Altered Expression in Parkinsonism and Dyskinesia

The renin-angiotensin system (RAS) not only plays an important role in controlling blood pressure but also participates in almost every process to maintain homeostasis in mammals. Interest has recently increased because SARS viruses use one RAS component (ACE2) as a target-cell receptor. The occurrence of RAS in the basal ganglia suggests that the system may be targeted to improve the therapy of neurodegenerative diseases. RAS-related data led to the hypothesis that RAS receptors may interact with each other. The aim of this paper was to find heteromers formed by Mas and angiotensin receptors and to address their functionality in neurons and microglia. Novel interactions were discovered by using resonance energy transfer techniques. The functionality of individual and interacting receptors was assayed by measuring levels of the second messengers cAMP and Ca2+ in transfected human embryonic kidney cells (HEK-293T) and primary cultures of striatal cells. Receptor complex expression was assayed by in situ proximity ligation assay. Functionality and expression were assayed in parallel in primary cultures of microglia treated or not with lipopolysaccharide and interferon-γ (IFN-γ). The proximity ligation assay was used to assess heteromer expression in parkinsonian and dyskinetic conditions. Complexes formed by Mas and the angiotensin AT1 or AT2 receptors were identified in both a heterologous expression system and in neural primary cultures. In the heterologous system, we showed that the three receptors-MasR, AT1R, and AT2R-can interact to form heterotrimers. The expression of receptor dimers (AT1R-MasR or AT2R-MasR) was higher in microglia than in neurons and was differentially affected upon microglial activation with lipopolysaccharide and IFN-γ. In all cases, agonist-induced signaling was reduced upon coactivation, and in some cases just by coexpression. Also, the blockade of signaling of two receptors in a complex by the action of a given (selective) receptor antagonist (cross-antagonism) was often observed. Differential expression of the complexes was observed in the striatum under parkinsonian conditions and especially in animals rendered dyskinetic by levodopa treatment. The negative modulation of calcium mobilization (mediated by AT1R activation), the multiplicity of possibilities on RAS affecting the MAPK pathway, and the disbalanced expression of heteromers in dyskinesia yield new insight into the operation of the RAS system, how it becomes unbalanced, and how a disbalanced RAS can be rebalanced. Furthermore, RAS components in activated microglia warrant attention in drug-development approaches to address neurodegeneration.

The Tissue Renin-Angiotensin System and Its Role in the Pathogenesis of Major Human Diseases: Quo Vadis?

Evidence has arisen in recent years suggesting that a tissue renin-angiotensin system (tRAS) is involved in the progression of various human diseases. This system contains two regulatory pathways: a pathological pro-inflammatory pathway containing the Angiotensin Converting Enzyme (ACE)/Angiotensin II (AngII)/Angiotensin II receptor type 1 (AGTR1) axis and a protective anti-inflammatory pathway involving the Angiotensin II receptor type 2 (AGTR2)/ACE2/Ang1–7/MasReceptor axis. Numerous studies reported the positive effects of pathologic tRAS pathway inhibition and protective tRAS pathway stimulation on the treatment of cardiovascular, inflammatory, and autoimmune disease and the progression of neuropathic pain. Cell senescence and aging are known to be related to RAS pathways. Further, this system directly interacts with SARS-CoV 2 and seems to be an important target of interest in the COVID-19 pandemic. This review focuses on the involvement of tRAS in the progression of the mentioned diseases from an interdisciplinary clinical perspective and highlights therapeutic strategies that might be of major clinical importance in the future.

Arterial Blood Pressure Variability and Other Vascular Factors Contribution to the Cognitive Decline in Parkinson's Disease

Dementia is one of the most disabling non-motor symptoms in Parkinson’s disease (PD). Unlike in Alzheimer’s disease, the vascular pathology in PD is less documented. Due to the uncertain role of commonly investigated metabolic or vascular factors, e.g., hypertension or diabetes, other factors corresponding to PD dementia have been proposed. Associated dysautonomia and dopaminergic treatment seem to have an impact on diurnal blood pressure (BP) variability, which may presumably contribute to white matter hyperintensities (WMH) development and cognitive decline. We aim to review possible vascular and metabolic factors: Renin-angiotensin-aldosterone system, vascular endothelial growth factor (VEGF), hyperhomocysteinemia (HHcy), as well as the dopaminergic treatment, in the etiopathogenesis of PD dementia. Additionally, we focus on the role of polymorphisms within the genes for catechol-O-methyltransferase (COMT), apolipoprotein E (APOE), vascular endothelial growth factor (VEGF), and for renin-angiotensin-aldosterone system components, and their contribution to cognitive decline in PD. Determining vascular risk factors and their contribution to the cognitive impairment in PD may result in screening, as well as preventive measures.

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

Angiotensin type 2 receptors: Role in aging and neuroinflammation in the substantia nigra

Overactivity of the angiotensin-type-1 receptor (AT1)/NADPH-oxidase axis enhances aging processes, neuroinflammation and neurodegeneration. The role of AT2 receptors in the above-mentioned AT1-related effects in the aged brain, particularly substantia nigra, was investigated in this study. In the nigra, we observed a progressive decrease in AT2 mRNA expression with aging, and AT2 deletion led to changes in spontaneous motor behavior, dopamine receptors, renin-angiotensin system, and pro-oxidative and pro-inflammatory markers similar to those observed in aged wild type (WT) mice. Both aged WT mice and young AT2 KO mice showed an increased AT1, decreased MAS receptor and increased angiotensinogen mRNA and/or protein expression, as well as upregulation of pro-oxidative and pro-inflammatory markers. In cultures of microglial cells, activation of AT2 receptors inhibited the LPS-induced increase in AT1 mRNA and protein expression and neuroinflammatory markers. Both in AT2 KO microglial cultures and microglia obtained from adult AT2 KO mice, an increase in AT1 mRNA expression was observed. In cultured dopaminergic neurons, AT2 activation down-regulated AT1 mRNA and protein, and dopaminergic neurons from adult AT2 KO mice showed upregulation of AT1 mRNA expression. Both in microglia and dopaminergic neurons the pathway AT2/nitric oxide/cyclic guanosine monophosphate mediates the regulation of the AT1 mRNA and protein expression through downregulation of the Sp1 transcription factor. MAS receptors are also involved in the regulation of AT1 mRNA and protein expression by AT2. The results suggest that an aging-related decrease in AT2 expression plays a major role in the aging-related AT1 overexpression and AT1-related pro-inflammatory pro-oxidative effects.

Impact of Aging on the 6-OHDA-Induced Rat Model of Parkinson's Disease

Parkinson's disease (PD) is the second most common age-related neurodegenerative disorder. The neurodegeneration leading to incapacitating motor abnormalities mainly occurs in the nigrostriatal pathway due to the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Several animal models have been developed not only to better understand the mechanisms underlying neurodegeneration but also to test the potential of emerging disease-modifying therapies. However, despite aging being the main risk factor for developing idiopathic PD, most of the studies do not use aged animals. Therefore, this study aimed at assessing the effect of aging in the unilateral 6-hydroxydopamine (6-OHDA)-induced animal model of PD. For this, female young adult and aged rats received a unilateral injection of 6-OHDA into the medial forebrain bundle. Subsequently, the impact of aging on 6-OHDA-induced effects on animal welfare, motor performance, and nigrostriatal integrity were assessed. The results showed that aging had a negative impact on animal welfare after surgery. Furthermore, 6-OHDA-induced impairments on skilled motor function were significantly higher in aged rats when compared with their younger counterparts. Nigrostriatal histological analysis further revealed an increased 6-OHDA-induced dopaminergic cell loss in the SNpc of aged animals when compared to young animals. Overall, our results demonstrate a higher susceptibility of aged animals to 6-OHDA toxic insult.

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.

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.

Four main therapeutic keys for Parkinson's disease: A mini review

OBJECTIVES

Parkinson's disease (PD) is characterized by motor and cognitive dysfunctions. The progressive degeneration of dopamine-producing neurons that are present in the substantia nigra pars compacta (SNpc) has been the main focus of study and PD therapies since ages.

MATERIALS AND METHODS

In this manuscript, a systematic revision of experimental and clinical evidence of PD-associated cell process was conducted.

RESULTS

Classically, the damage in the dopaminergic neuronal circuits of SNpc is favored by reactive oxidative/nitrosative stress, leading to cell death. Interestingly, the therapy for PD has only focused on avoiding the symptom progression but not in finding a complete reversion of the disease. Recent evidence suggests that the renin-angiotensin system imbalance and neuroinflammation are the main keys in the progression of experimental PD.

CONCLUSION

The progression of neurodegeneration in SNpc is due to the complex interaction of multiple processes. In this review, we analyzed the main contribution of four cellular processes and discussed in the perspective of novel experimental approaches.

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.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Aging-related dysregulation in enteric dopamine and angiotensin system interactions: implications for gastrointestinal dysfunction in the elderly

Gastrointestinal dysfunction is a common problem in the elderly. Aging-related changes in interactions between local dopaminergic and renin-angiotensin systems (RAS) have been observed in the brain, renal and vascular tissues. However, it is not known if these interactions also occur in the gut, and are dysregulated with aging. We showed a mutual regulation between the colonic dopaminergic system and RAS using young and aged mice deficient for major angiotensin and dopamine receptors. Aged rats showed a marked decrease in colonic dopamine D2 receptor expression, together with an increase in angiotensin type 1 (AT1) receptor expression, a decrease in angiotensin type 2 (AT2) receptor expression (i.e. an increase in the RAS pro-inflammatory arm activity), and increased levels of inflammatory and oxidative markers. Aged rats also showed increased levels of colonic dopamine and noradrenalin, and a marked decrease in acetylcholine and serotonin levels. The present observations contribute to explain an aging-related pro-inflammatory state and dysregulation in gastrointestinal function, which may be counteracted by treatment of aged animals with the AT1 receptor blocker candesartan.

Crosstalk between insulin-like growth factor-1 and angiotensin-II in dopaminergic neurons and glial cells: role in neuroinflammation and aging

The local renin-angiotensin system (RAS) and insulin-like growth factor 1 (IGF-1) have been involved in longevity, neurodegeneration and aging-related dopaminergic degeneration. However, it is not known whether IGF-1 and angiotensin-II (AII) activate each other. In the present study, AII, via type 1 (AT1) receptors, exacerbated neuroinflammation and dopaminergic cell death. AII, via AT1 receptors, also increased the levels of IGF-1 and IGF-1 receptors in microglial cells. IGF-1 inhibited RAS activity in dopaminergic neurons and glial cells, and also inhibited the AII-induced increase in markers of the M1 microglial phenotype. Consistent with this, IGF-1 decreased dopaminergic neuron death induced by the neurotoxin MPP⁺ both in the presence and in the absence of glia. Intraventricular administration of AII to young rats induced a significant increase in IGF-1 expression in the nigral region. However, aged rats showed decreased levels of IGF-1 relative to young controls, even though RAS activity is known to be enhanced in aged animals. The study findings show that IGF-1 and the local RAS interact to inhibit or activate neuroinflammation (i.e. transition from the M1 to the M2 phenotype), oxidative stress and dopaminergic degeneration. The findings also show that this mechanism is impaired in aged animals.

NADPH oxidases in Parkinson's disease: a systematic review

Parkinson's disease (PD) is a progressive movement neurodegenerative disease associated with a loss of dopaminergic neurons in the substantia nigra of the brain. Oxidative stress, a condition that occurs due to imbalance in oxidant and antioxidant status, is thought to play an important role in dopaminergic neurotoxicity. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases are multi-subunit enzymatic complexes that generate reactive oxygen species as their primary function. Increased immunoreactivities for the NADPH oxidases catalytic subunits Nox1, Nox2 and Nox4 have been reported in the brain of PD patients. Furthermore, knockout or genetic inactivation of NADPH oxidases exert a neuroprotective effect and reduce detrimental aspects of pathology in experimental models of the disease. However, the connections between NADPH oxidases and the biological processes believed to contribute to neuronal death are not well known. This review provides a comprehensive summary of our current understanding about expression and physiological function of NADPH oxidases in neurons, microglia and astrocytes and their pathophysiological roles in PD. It summarizes the findings supporting the role of both microglial and neuronal NADPH oxidases in cellular disturbances associated with PD such as neuroinflammation, alpha-synuclein accumulation, mitochondrial and synaptic dysfunction or disruption of the autophagy-lysosome system. Furthermore, this review highlights different steps that are essential for NADPH oxidases enzymatic activity and pinpoints major obstacles to overcome for the development of effective NADPH oxidases inhibitors for PD.

Insulin-Like Growth Factor-1 and Neuroinflammation

Insulin-like growth factor-1 (IGF-1) effects on aging and neurodegeneration is still controversial. However, it is widely admitted that IGF-1 is involved in the neuroinflammatory response. In peripheral tissues, several studies showed that IGF-1 inhibited the expression of inflammatory markers, although other studies concluded that IGF-1 has proinflammatory functions. Furthermore, proinflammatory cytokines such as TNF-α impaired IGF-1 signaling. In the brain, there are controversial results on effects of IGF-1 in neuroinflammation. In addition to direct protective effects on neurons, several studies revealed anti-inflammatory effects of IGF-1 acting on astrocytes and microglia, and that IGF-1 may also inhibit blood brain barrier permeability. Altogether suggests that the aging-related decrease in IGF-1 levels may contribute to the aging-related pro-inflammatory state. IGF-1 inhibits the astrocytic response to inflammatory stimuli, and modulates microglial phenotype (IGF-1 promotes the microglial M2 and inhibits of M1 phenotype). Furthermore, IGF-1 is mitogenic for microglia. IGF-1 and estrogen interact to modulate the neuroinflammatory response and microglial and astrocytic phenotypes. Brain renin-angiotensin and IGF-1 systems also interact to modulate neuroinflammation. Induction of microglial IGF-1 by angiotensin, and possibly by other pro-inflammatory inducers, plays a major role in the repression of the M1 microglial neurotoxic phenotype and the enhancement of the transition to an M2 microglial repair/regenerative phenotype. This mechanism is impaired in aged brains. Aging-related decrease in IGF-1 may contribute to the loss of capacity of microglia to undergo M2 activation. Fine tuning of IGF-1 levels may be critical for regulating the neuroinflammatory response, and IGF-1 may be involved in inflammation in a context-dependent mode.

Paracrine and Intracrine Angiotensin 1-7/Mas Receptor Axis in the Substantia Nigra of Rodents, Monkeys, and Humans

In addition to the classical hormonal (tissue-to-tissue) renin-angiotensin system (RAS), there are a paracrine (cell-to-cell) and an intracrine (intracellular/nuclear) RAS. A local paracrine brain RAS has been associated with several brain disorders, including Parkinson’s disease (PD). Classically, angiotensin II (Ang II) is the main RAS effector peptide and acts through two major receptors: Ang II type 1 and 2 (AT1 and AT2) receptors. It has been shown that enhanced activation of the Ang II/AT1 axis exacerbates dopaminergic cell death. Several new components of the RAS have more recently been discovered. However, the role of new Ang 1-7/Mas receptor RAS component was not investigated in the brain and particularly in the dopaminergic system. In the present study, we observed Mas receptor labeling in dopaminergic neurons and glial cells in rat mesencephalic primary cultures; substantia nigra of rats, monkeys, and humans; and human induced pluripotent stem (iPS) cells derived from healthy controls and sporadic PD patients. The present data support a neuroprotective role of the Ang 1-7/Mas receptor axis in the dopaminergic system. We observed that this axis is downregulated with aging, which may contribute to the aging-related vulnerability to neurodegeneration. We have also identified an intracellular Ang 1-7/Mas axis that modulates mitochondrial and nuclear levels of superoxide. The present data suggest that nuclear RAS receptors regulate the adequate balance between the detrimental and the protective arms of the cell RAS. The results further support that the brain RAS should be taken into account for the design of new therapeutic strategies for PD.

The renin angiotensin system and the brain: New developments

The traditional renin-angiotensin system (RAS) is indispensable system in adjusting sodium homeostasis, body fluid volume, and controlling arterial blood pressure. The key elements are renin splitting inactive angiotensinogen to yield angiotensin (Ang-I). Ang-1 is then changed by angiotensin-1 converting enzyme (ACE) into angiotensin II (Ang-II). Using PubMed, Google Scholar, and other means, we searched the peer-reviewed literature from 1990 to 2013 for articles on newly discovered findings related to the RAS, especially focusing on how the system influences the central nervous system (CNS). The classical RAS is now considered to be only part of the picture; the discovery of additional RAS pathways in the brain and elsewhere has yielded a vastly improved understanding of how the RAS influences the CNS. Newly discovered effects of the RAS on brain tissue include neuroprotection, cognition, and cerebral vasodilation. A number of brain biochemical pathways are influenced by the brain RAS. Within various pathways, there are potential opportunities for classical pharmacologic interventions as well as the possibility of controlling gene expression.

The intracellular angiotensin system buffers deleterious effects of the extracellular paracrine system

The 'classical' renin-angiotensin system (RAS) is a circulating system that controls blood pressure. Local/paracrine RAS, identified in a variety of tissues, including the brain, is involved in different functions and diseases, and RAS blockers are commonly used in clinical practice. A third type of RAS (intracellular/intracrine RAS) has been observed in some types of cells, including neurons. However, its role is still unknown. The present results indicate that in brain cells the intracellular RAS counteracts the intracellular superoxide/H₂O₂ and oxidative stress induced by the extracellular/paracrine angiotensin II acting on plasma membrane receptors. Activation of nuclear receptors by intracellular or internalized angiotensin triggers a number of mechanisms that protect the cell, such as an increase in the levels of protective angiotensin type 2 receptors, intracellular angiotensin, PGC-1α and IGF-1/SIRT1. Interestingly, this protective mechanism is altered in isolated nuclei from brains of aged animals. The present results indicate that at least in the brain, AT1 receptor blockers acting only on the extracellular or paracrine RAS may offer better protection of cells.

The renin-angiotensin system: a possible new target for depression

Depression remains a debilitating condition with an uncertain aetiology. Recently, attention has been given to the renin-angiotensin system. In the central nervous system, angiotensin II may be important in multiple pathways related to neurodevelopment and regulation of the stress response. Studies of drugs targeting the renin-angiotensin system have yielded promising results. Here, we review the potential beneficial effects of angiotensin blockers in depression and their mechanisms of action. Drugs blocking the angiotensin system have efficacy in several animal models of depression. While no randomised clinical trials were found, case reports and observational studies showed that angiotensin-converting enzyme inhibitors or angiotensin receptor blockers had positive effects on depression, whereas other antihypertensive agents did not. Drugs targeting the renin-angiotensin system act on inflammatory pathways implicated in depression. Both preclinical and clinical data suggest that these drugs possess antidepressant properties. In light of these results, angiotensin system-blocking agents offer new horizons in mood disorder treatment.

Candesartan, angiotensin II type 1 receptor blocker is able to relieve age-related cognitive impairment

BACKGROUND

Candesartan is one of the standard antihypertensive drug belonging to AT1R angiotensin receptor blockers (ARBs) group. Beneficial effects of this drug in the treatment of hypertension are well recognized. In this study we tested a hypothesis that candesartan could alleviate age-related memory decline.

METHODS

Aged and young rats have been treated with candesartan (0.1mg kg^-1) for 21days and then underwent a battery of behavioral tests: for assessment of long-term memory (Passive avoidance test - PA), recognition memory (Object recognition test - OR), locomotor functions (Open field - OF) and anxiety behavior (Elevated plus maze - EPM).

RESULTS

Aged rats (2-years-old) displayed clear declining tendency in the retrieval of passive avoidance behavior showing thus increased forgetting. Prolonged administration of candesartan significantly (p<0.01) reversed this phenomenon causing recall measured as the avoidance latency, and surprisingly also showed the tendency to recall deterioration observed in the young rats. More optimistic results were achieved in the OR, where candesartan significantly improved recognition memory (p<0.001) of aged rats who performed even better than the young ones (p<0.05).

CONCLUSIONS

It appears that candesartan potently abolishes some kinds of aging-induced memory impairments and cognitive declines in aged rats, but in some circumstances it may even could increase the damage of memory. It seems that the use of sartans in the treatment of hypertension for patients with associated cognitive impairment, or for people in risk groups for such disorders can be an interesting alternative.

Laser capture microdissection protocol for gene expression analysis in the brain

Laser capture microdissection (LCM) allows the isolation of specific cell populations from complex tissues that can be then used for gene expression studies. However, there are no reproducible protocols to study RNA in the brain and, particularly, in the substantia nigra. RNA is a very labile biomolecule that is easily degraded during manipulation. LCM studies use low amounts of material and special precautions must be taken to preserve RNA yield and integrity, which are decisive for PCR analysis. The RNA yield and/or integrity can be affected negatively by tissue manipulation, LCM process and RNA extraction. We have optimized these three critical steps using nigral tissue sections, and developed a LCM protocol to obtain high-quality RNA for gene expression analysis. The optimal LCM protocol requires the use of 20 µm-thick tissue sections mounted on glass slides and processed for rapid tyrosine hydroxylase immunofluorescence. Additionally, a total microdissected tissue area of 1 mm² and a column-based RNA extraction method were used to obtain a high RNA yield and integrity. In the rat substantia nigra, we demonstrated the expression of RNA for the angiotensin type 1 and type 2 receptors using this optimized LCM protocol. In conclusion, the LCM protocol reported here can be used to study the expression of both scarcely or abundantly expressed genes in the different brain regions of mammals under both physiological and pathological conditions.

Brain Renin-Angiotensin System and Microglial Polarization: Implications for Aging and Neurodegeneration

Microglia can transform into proinflammatory/classically activated (M1) or anti-inflammatory/alternatively activated (M2) phenotypes following environmental signals related to physiological conditions or brain lesions. An adequate transition from the M1 (proinflammatory) to M2 (immunoregulatory) phenotype is necessary to counteract brain damage. Several factors involved in microglial polarization have already been identified. However, the effects of the brain renin-angiotensin system (RAS) on microglial polarization are less known. It is well known that there is a “classical” circulating RAS; however, a second RAS (local or tissue RAS) has been observed in many tissues, including brain. The locally formed angiotensin is involved in local pathological changes of these tissues and modulates immune cells, which are equipped with all the components of the RAS. There are also recent data showing that brain RAS plays a major role in microglial polarization. Level of microglial NADPH-oxidase (Nox) activation is a major regulator of the shift between M1/proinflammatory and M2/immunoregulatory microglial phenotypes so that Nox activation promotes the proinflammatory and inhibits the immunoregulatory phenotype. Angiotensin II (Ang II), via its type 1 receptor (AT1), is a major activator of the NADPH-oxidase complex, leading to pro-oxidative and pro-inflammatory effects. However, these effects are counteracted by a RAS opposite arm constituted by Angiotensin II/AT2 receptor signaling and Angiotensin 1–7/Mas receptor (MasR) signaling. In addition, activation of prorenin-renin receptors may contribute to activation of the proinflammatory phenotype. Aged brains showed upregulation of AT1 and downregulation of AT2 receptor expression, which may contribute to a pro-oxidative pro-inflammatory state and the increase in neuron vulnerability. Several recent studies have shown interactions between the brain RAS and different factors involved in microglial polarization, such as estrogens, Rho kinase (ROCK), insulin-like growth factor-1 (IGF-1), tumor necrosis factor α (TNF)-α, iron, peroxisome proliferator-activated receptor gamma, and toll-like receptors (TLRs). Metabolic reprogramming has recently been involved in the regulation of the neuroinflammatory response. Interestingly, we have recently observed a mitochondrial RAS, which is altered in aged brains. In conclusion, dysregulation of brain RAS plays a major role in aging-related changes and neurodegeneration by exacerbation of oxidative stress (OS) and neuroinflammation, which may be attenuated by pharmacological manipulation of RAS components.

Mitochondrial angiotensin receptors in dopaminergic neurons. Role in cell protection and aging-related vulnerability to neurodegeneration

The renin–angiotensin system (RAS) was initially considered as a circulating humoral system controlling blood pressure, being kidney the key control organ. In addition to the ‘classical' humoral RAS, a second level in RAS, local or tissular RAS, has been identified in a variety of tissues, in which local RAS play a key role in degenerative and aging-related diseases. The local brain RAS plays a major role in brain function and neurodegeneration. It is normally assumed that the effects are mediated by the cell-surface-specific G-protein-coupled angiotensin type 1 and 2 receptors (AT1 and AT2). A combination of in vivo (rats, wild-type mice and knockout mice) and in vitro (primary mesencephalic cultures, dopaminergic neuron cell line cultures) experimental approaches (confocal microscopy, electron microscopy, laser capture microdissection, transfection of fluorescent-tagged receptors, treatments with fluorescent angiotensin, western blot, polymerase chain reaction, HPLC, mitochondrial respirometry and other functional assays) were used in the present study. We report the discovery of AT1 and AT2 receptors in brain mitochondria, particularly mitochondria of dopaminergic neurons. Activation of AT1 receptors in mitochondria regulates superoxide production, via Nox4, and increases respiration. Mitochondrial AT2 receptors are much more abundant and increase after treatment of cells with oxidative stress inducers, and produce, via nitric oxide, a decrease in mitochondrial respiration. Mitochondria from the nigral region of aged rats displayed altered expression of AT1 and AT2 receptors. AT2-mediated regulation of mitochondrial respiration represents an unrecognized primary line of defence against oxidative stress, which may be particularly important in neurons with increased levels of oxidative stress such as dopaminergic neurons. Altered expression of AT1 and AT2 receptors with aging may induce mitochondrial dysfunction, the main risk factor for neurodegeneration.

Low putamen activity associated with poor reward sensitivity in childhood chronic fatigue syndrome

Motivational signals influence a wide variety of cognitive processes and components of behavioral performance. Cognitive dysfunction in patients with childhood chronic fatigue syndrome (CCFS) may be closely associated with a low motivation to learn induced by impaired neural reward processing. However, the extent to which reward processing is impaired in CCFS patients is unclear. The aim of the present functional magnetic resonance imaging (fMRI) study was to determine whether brain activity in regions related to reward sensitivity is impaired in CCFS patients. fMRI data were collected from 13 CCFS patients (mean age, 13.6 ± 1.0 years) and 13 healthy children and adolescents (HCA) (mean age, 13.7 ± 1.3 years) performing a monetary reward task. Neural activity in high- and low-monetary-reward conditions was compared between CCFS and HCA groups. Severity of fatigue and the reward obtained from learning in daily life were evaluated by questionnaires. Activity of the putamen was lower in the CCFS group than in the HCA group in the low-reward condition, but not in the high-reward condition. Activity of the putamen in the low-reward condition in CCFS patients was negatively and positively correlated with severity of fatigue and the reward from learning in daily life, respectively. We previously revealed that motivation to learn was correlated with striatal activity, particularly the neural activity in the putamen. This suggests that in CCFS patients low putamen activity, associated with altered dopaminergic function, decreases reward sensitivity and lowers motivation to learn.

Reciprocal regulation between sirtuin-1 and angiotensin-II in the substantia nigra: implications for aging and neurodegeneration

Local angiotensin II (AII) and sirtuin 1 (SIRT1) play a major role in the modulation of neuroinflammation, oxidative stress and aging-related dopaminergic vulnerability to damage. However, it is not known whether the modulation is related to reciprocal regulation between SIRT1 and AII. In the present study, a single intraventricular injection of AII increased nigral SIRT1 levels in young adult rats. Although AII activity is known to be increased in aged rats, levels of SIRT1 were significantly lower than in young controls. Treatment with the SIRT1-activating compound resveratrol increased nigral SIRT1 levels in aged rats. Levels of SIRT1 were significantly higher in aged wild type mice than in AII type-1 receptor (AT1) deficient mice. In cell culture studies, treatment with AII also induced a transitory increase in levels of SIRT1 in the MES 23.5 dopaminergic neuron and the N9 microglial cell lines. In aged rats, treatment with resveratrol induced a significant decrease in the expression of AT1 receptors and markers of NADPH-oxidase activation (p47phox). In aged transgenic mice over-expressing SIRT1, levels of AT1 and p47 phox were lower than in aged wild type controls. In vitro, the inhibitory effects of resveratrol on AII/AT1/NADPH-oxidase activity were confirmed in primary mesencephalic cultures, the N9 microglial cell line, and the dopaminergic neuron cell line MES 23.5, and they were blocked by the SIRT1 specific inhibitor EX527. The present findings show that SIRT1 and the axis AII/AT1/NADPH-oxidase regulate each other. This is impaired in aged animals and may be mitigated with sirtuin-activating compounds.

Age-related alterations in the expression of genes and synaptic plasticity associated with nitric oxide signaling in the mouse dorsal striatum

Age-related alterations in the expression of genes and corticostriatal synaptic plasticity were studied in the dorsal striatum of mice of four age groups from young (2-3 months old) to old (18-24 months of age) animals. A significant decrease in transcripts encoding neuronal nitric oxide (NO) synthase and receptors involved in its activation (NR1 subunit of the glutamate NMDA receptor and D1 dopamine receptor) was found in the striatum of old mice using gene array and real-time RT-PCR analysis. The old striatum showed also a significantly higher number of GFAP-expressing astrocytes and an increased expression of astroglial, inflammatory, and oxidative stress markers. Field potential recordings from striatal slices revealed age-related alterations in the magnitude and dynamics of electrically induced long-term depression (LTD) and significant enhancement of electrically induced long-term potentiation in the middle-aged striatum (6-7 and 12-13 months of age). Corticostriatal NO-dependent LTD induced by pharmacological activation of group I metabotropic glutamate receptors underwent significant reduction with aging and could be restored by inhibition of cGMP hydrolysis indicating that its age-related deficit is caused by an altered NO-cGMP signaling cascade. It is suggested that age-related alterations in corticostriatal synaptic plasticity may result from functional alterations in receptor-activated signaling cascades associated with increasing neuroinflammation and a prooxidant state.

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.

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.

A survey from 2012 of evidence for the role of neuroinflammation in neurotoxin animal models of Parkinson's disease and potential molecular targets

Parkinson's disease (PD) is a neurodegenerative movement disorder that results from the progressive loss of dopaminergic neurons in the midbrain substantia nigra pars compacta (SNpc). The specific molecular events that cause PD are currently not known; however, progress to better understand PD pathogenesis has been made using various animal models of the disease. In this review, we have highlighted reports from 2012 in which neurochemical/neurotoxins have been used in rodents to specifically address the role of neuroinflammation in the development and/or progression of PD-like pathology and in particular nigral degeneration. A number of studies have been summarized in which plausible pro-inflammatory, anti-inflammatory, or therapeutic agents targeting inflammatory pathways were introduced and/or investigated by various groups for neuroprotective effects. From these studies, it is clear that neuroinflammation acts to exacerbate the toxic outcomes that are set in motion within neurons following exposure to neurotoxins. Additionally, it is noted that future work is still needed to better understand the underlying mechanisms mediating the neuroinflammatory and neurotoxic phenotypes reported in rodent models of PD-like pathology to maximize the translation potential of these interventions to the clinic to prevent and/or delay PD onset and/or progression in humans.

Aging-related dysregulation of dopamine and angiotensin receptor interaction

It is not known whether the aging-related decrease in dopaminergic function leads to the aging-related higher vulnerability of dopaminergic neurons and risk for Parkinson's disease. The renin-angiotensin system (RAS) plays a major role in the inflammatory response, neuronal oxidative stress, and dopaminergic vulnerability via type 1 (AT1) receptors. In the present study, we observed a counterregulatory interaction between dopamine and angiotensin receptors. We observed overexpression of AT1 receptors in the striatum and substantia nigra of young adult dopamine D1 and D2 receptor-deficient mice and young dopamine-depleted rats, together with compensatory overexpression of AT2 receptors or compensatory downregulation of angiotensinogen and/or angiotensin. In aged rats, we observed downregulation of dopamine and dopamine receptors and overexpression of AT1 receptors in aged rats, without compensatory changes observed in young animals. L-Dopa therapy inhibited RAS overactivity in young dopamine-depleted rats, but was ineffective in aged rats. The results suggest that dopamine may play an important role in modulating oxidative stress and inflammation in the substantia nigra and striatum via the RAS, which is impaired by aging.

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.