Does angiotensin II cross the blood-brain barrier?

Chronic administration of pharmacological doses of angiotensin 1-7 and iodoangiotensin 1-7 has minimal effects on blood pressure, heart rate, and cognitive function of spontaneously hypertensive rats

Cardiovascular diseases are the principal cause of death worldwide, with hypertension being the most common cardiovascular disease risk factor. High blood pressure (BP) is also associated with an increased risk of poor cognitive performance and dementia including Alzheimer's disease. Angiotensin 1–7 (Ang 1‐7), a product of the renin‐angiotensin system (RAS), exhibits central and peripheral actions to reduce BP. Recent data from our lab reveals that the addition of a non‐radioactive iodine molecule to the tyrosine in position 4 of Ang 1‐7 (iodoAng 1‐7) makes it ~1000‐fold more potent than Ang 1‐7 in competing for the ¹²⁵I‐Ang 1‐7 binding site (Stoyell‐Conti et al., 2020). Moreover, the addition of the non‐radioactive iodine molecule increases (~4‐fold) iodoAng 1‐7’s ability to bind to the AT1 receptor (AT1R), the primary receptor for Ang II. Preliminary data indicates that iodoAng 1‐7 can also compete for the ¹²⁵I‐Ang IV binding site with a low micromolar IC50. Thus, our aims were to compare the effects of chronic treatment of the Spontaneously Hypertensive Rat (SHR) with iodoAng 1‐7 (non‐radioactive iodine isotope) and Ang 1‐7 on arterial pressure, heart rate, and cognitive function. For this study, male SHRs were divided into three groups and treated with Saline, Ang 1‐7, or iodoAng 1‐7 administrated subcutaneously using a 28‐day osmotic mini pump. Systolic BP was measured non‐invasively by the tail‐cuff technique. Cognitive function was assessed by Y‐Maze test and novel object recognition (NOR) test. We have demonstrated in SHRs that subcutaneous administration of high doses of iodoAng 1‐7 prevented the increase in heart rate with age, while Ang 1‐7 showed a trend toward preventing the increase in heart rate, possibly by improving baroreflex control of the heart. Conversely, neither Ang 1‐7 nor iodoAng 1‐7 administered subcutaneously affected BP nor cognitive function.

Chronically infused angiotensin II induces depressive-like behavior via microglia activation

Brain inflammation is one of hypotheses explaining complex pathomechanisms of depression. Angiotensin II (ANGII), which is associated with hypertension, also induces brain inflammation. However, there is no animal study showing the direct relationship between ANGII and depression. To address this issue, ANGII-containing osmotic pumps were implanted into adult male C57BL/6 mice subcutaneously for subacute (7 days) and chronic (at least 21 days) periods and behavioral and molecular analyses were conducted. Chronic infusion of ANGII into mice induced depressive-like behaviors, including the tail suspension test and forced swimming test, which were reversed by imipramine. Chronic infusion of ANGII also induced microglial activation in the hippocampus with increase of Il-1β mRNA and decrease of Arg1 mRNA. In addition, chronic ANGII infusion activated the hypothalamic–pituitary–adrenal axis (HPA axis) and resulted in decreased hippocampal glucocorticoid receptor level. However, subacute ANGII infusion did not induce significant molecular and behavioral changes in mice compared to that of control. The molecular and behavioral changes by chronic ANGII infusion were reversed by co-treatment of minocycline or telmisartan. In addition, ANGII treatment also induced the pro-inflammatory changes in BV-2 microglial cells. Our results indicate that ANGII can induce depressive-like behaviors via microglial activation in the hippocampus and HPA axis hyperactivation in mice. These might suggest possible mechanism on depressive symptom in chronic hypertensive state.

Neuroinflammation in heart failure: new insights for an old disease

Heart failure (HF) is a complex clinical syndrome affecting roughly 26 million people worldwide. Increased sympathetic drive is a hallmark of HF and is associated with disease progression and higher mortality risk. Several mechanisms contribute to enhanced sympathetic activity in HF, but these pathways are still incompletely understood. Previous work suggests that inflammation and activation of the renin-angiotensin system (RAS) increases sympathetic drive. Importantly, chronic inflammation in several brain regions is commonly observed in aged populations, and a growing body of evidence suggests neuroinflammation plays a crucial role in HF. In animal models of HF, central inhibition of RAS and pro-inflammatory cytokines normalizes sympathetic drive and improves cardiac function. The precise molecular and cellular mechanisms that lead to neuroinflammation and its effect on HF progression remain undetermined. This review summarizes the most recent advances in the field of neuroinflammation and autonomic control in HF. In addition, it focuses on cellular and molecular mediators of neuroinflammation in HF and in particular on brain regions involved in sympathetic control. Finally, we will comment on what is known about neuroinflammation in the context of preserved vs. reduced ejection fraction HF.

Enalapril Alone or Co-Administered with Losartan Rescues Cerebrovascular Dysfunction, but not Mnemonic Deficits or Amyloidosis in a Mouse Model of Alzheimer's Disease

The co-administration of angiotensin converting enzyme inhibitors (ACEi) and angiotensin II (AngII) receptor blockers (ARB) that bind angiotensin type 1 receptors (AT1R) may protect from Alzheimer's disease (AD) better than each treatment taken alone. We tested the curative potential of the non brain-penetrant ACEi enalapril (3 mg/kg/day) administered for 3 months either alone or in combination with the brain penetrant ARB losartan (10 mg/kg/day) in aged (∼15 months) transgenic mice overexpressing a mutated form of the human amyloid-β protein precursor (AβPP, thereafter APP mice). We studied cerebrovascular function, protein levels of oxidative stress markers (superoxide dismutases SOD1, SOD2 and the NADPH oxidase subunit p67phox), amyloid-β (Aβ) pathology, astrogliosis, cholinergic innervation, AT1R and angiotensin IV receptor (AT4R) levels, together with cognitive performance. Both treatments normalized cerebrovascular reactivity and p67phox protein levels, but they did not reduce the cerebrovascular levels of SOD1. Combined treatment normalized cerebrovascular SOD2 levels, significantly attenuated astrogliosis, but did not reduce the increased levels of cerebrovascular AT1R. Yet, combined therapy enhanced thioflavin-S labeled Aβ plaque burden, a tendency not significant when Aβ1 - 42 plaque load was considered. None of the treatments rescued cognitive deficits, cortical AT4R or cholinergic innervation. We conclude that both treatments normalized cerebrovascular function by inhibiting the AngII-induced oxidative stress cascade, and that the positive effects of the combined therapy on astrogliosis were likely due to the ability of losartan to enter brain parenchyma. However, enalapril did not potentiate, and may even dampen, the reported cognitive benefits of losartan, raising caution when selecting the most appropriate antihypertensive therapy in AD patients.

How the Body Talks to the Brain; Peripheral Mediators of Physical Activity-Induced Proliferation in the Adult Hippocampus

In the hippocampal dentate gyrus, stem cells maintain the capacity to produce new neurons into adulthood. These adult-generated neurons become fully functional and are incorporated into the existing hippocampal circuit. The process of adult neurogenesis contributes to hippocampal functioning and is influenced by various environmental, hormonal and disease-related factors. One of the most potent stimuli of neurogenesis is physical activity (PA). While the bodily and peripheral changes of PA are well known, e.g. in relation to diet or cardiovascular conditions, little is known about which of these also exert central effects on the brain. Here, we discuss PA-induced changes in peripheral mediators that can modify hippocampal proliferation, and address changes with age, sex or PA duration/intensity. Of the many peripheral factors known to be triggered by PA, serotonin, FGF-2, IGF-1, VEGF, β-endorphin and adiponectin are best known for their stimulatory effects on hippocampal proliferation. Interestingly, while age negatively affects hippocampal proliferation per se, also the PA-induced response to most of these peripheral mediators is reduced and particularly the response to IGF-1 and NPY strongly declines with age. Sex differences per se have generally little effects on PA-induced neurogenesis. Compared to short term exercise, long term PA may negatively affect proliferation, due to a parallel decline in FGF-2 and the β-endorphin receptor, and an activation of the stress system particularly during conditions of prolonged exercise but this depends on other variables as well and remains a matter of discussion. Taken together, of many possible mediators, serotonin, FGF-2, IGF-1, VEGF, β-endorphin and adiponectin are the ones that most strongly contribute to the central effects of PA on the hippocampus. For a subgroup of these factors, brain sensitivity and responsivity is reduced with age.