Angiotensin and Alzheimer´s disease: therapeutic prospects

Angiotensin-converting enzyme inhibitors: a therapeutic option for controlling blood pressure associated with delayed cognitive processing speed

Antihypertensive treatment (AT) is essential for preventing hypertension-related cognitive decline. The goals of this observational study were to compare cognitive performance (CP) between non-hypertensive (NH) volunteers and hypertensive patients and to evaluate the correlation between CP and antihypertensive drugs (AHD). Three groups were constituted: NH (n = 30) [group 1], hypertensive with systolic blood pressure (SBP) < 140 mmHg and diastolic blood pressure (DBP) < 90 mmHg (n = 54) [group 2] and hypertensive with SBP ≥ 140 or DBP ≥ 90 (n = 31) [group 3]. To analyze the cognitive domains, a neuropsychological battery was applied and the raw performance values in these tests were transformed into z-scores. The domain was considered impaired if it presented a z-score below -1.5 SD. Compared to group 1, both groups of hypertensive were older (51 [ ± 12] years) and showed a worse CP in episodic memory (p = 0.014), language (p = 0.003) and processing speed (PS) [p = 0.05]. Angiotensin-converting enzyme inhibitors (ACEi) and angiotensin receptor blockers (ARB) were the most used AHD (46.3%, p = 0.01 [group 2] and 64.5%, p = 0.005 [group 3]) and showed correlations with PS. Linear regression models revealed a negative association of PS with the use of ACEi (β = -0.230, p = 0.004), but not with the use of ARB (β = 0.208, p = 0.008). The effect of AT on cognition appears to go beyond the search for lower blood pressure targets and also includes the mechanism of action of AHD on the brain, so that additional benefits may possibly be achieved with simple adaptations in the treatment regimen, particularly in patients without clinically manifest cognitive impairment.

Environmental Nanoparticles, SARS-CoV-2 Brain Involvement, and Potential Acceleration of Alzheimer's and Parkinson's Diseases in Young Urbanites Exposed to Air Pollution

Alzheimer's and Parkinson's diseases (AD, PD) have a pediatric and young adult onset in Metropolitan Mexico City (MMC). The SARS-CoV-2 neurotropic RNA virus is triggering neurological complications and deep concern regarding acceleration of neuroinflammatory and neurodegenerative processes already in progress. This review, based on our MMC experience, will discuss two major issues: 1) why residents chronically exposed to air pollution are likely to be more susceptible to SARS-CoV-2 systemic and brain effects and 2) why young people with AD and PD already in progress will accelerate neurodegenerative processes. Secondary mental consequences of social distancing and isolation, fear, financial insecurity, violence, poor health support, and lack of understanding of the complex crisis are expected in MMC residents infected or free of SARS-CoV-2. MMC residents with pre-SARS-CoV-2 accumulation of misfolded proteins diagnostic of AD and PD and metal-rich, magnetic nanoparticles damaging key neural organelles are an ideal host for neurotropic SARS-CoV-2 RNA virus invading the body through the same portals damaged by nanoparticles: nasal olfactory epithelium, the gastrointestinal tract, and the alveolar-capillary portal. We urgently need MMC multicenter retrospective-prospective neurological and psychiatric population follow-up and intervention strategies in place in case of acceleration of neurodegenerative processes, increased risk of suicide, and mental disease worsening. Identification of vulnerable populations and continuous effort to lower air pollution ought to be critical steps.

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.

Dynamic Changes in the Renin-Angiotensin-Aldosterone System and the Beneficial Effects of Renin-Angiotensin-Aldosterone Inhibitors on Spatial Learning and Memory in a Rat Model of Chronic Cerebral Ischemia

Renin-angiotensin-aldosterone system (RAAS) plays an important role in the regulation of blood pressure and brain function. Therefore, we studied the dynamic changes in the RAAS in the blood, cerebral cortex, and hippocampus and the effects of RAAS inhibitors on spatial learning and memory and hippocampal apoptosis in a rat model of chronic cerebral ischemia (CCI) established by bilateral ligation of the common carotid arteries of rats. The levels of renin, angiotensin II (Ang II), and aldosterone (ALD) in the plasma, and the homogenates of the left side of cerebral cortex and whole hippocampus of rats were detected on day 1, 3, 7, 14, 21, and 30 by radioimmunoassay. Spatial learning and memory and hippocampal apoptosis were evaluated on day 30 by Morris water maze test (navigation and space exploration tests) and terminal dexynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) assay, respectively, after rats were orally administered with distilled water (DW), renin inhibitor aliskiren (30 mg/kg), Ang converting enzyme inhibitor enalapril (4 mg/kg), or Ang II receptor antagonist candesartan (2 mg/kg) daily for 30 days. The results showed that the levels of renin and Ang II were significantly higher but ALD fluctuated in the blood, cerebral cortex, and hippocampus in CCI rats compared to normal rats. However, aliskiren and enalapril could significantly decrease (p < 0.05) the levels of renin, Ang II and ALD in the blood, cerebral cortex, and hippocampus compared to DW treatment; while candesartan had similar effect on renin and ALD but no effect on Ang II in CCI rats. Furthermore, spatial learning and memory were significantly decreased but apoptosis in the hippocampus was obviously increased in CCI rats compared to normal rats (p < 0.05). However, aliskiren, enalapril, and candesartan were equally effective to improve spatial learning and memory and decrease apoptosis in the hippocampus. Therefore, RAAS plays an important role in the development of cerebral ischemia and RAAS inhibitors aliskiren, enalapril, and candesartan improve spatial learning and memory and protect brain injury by inhibiting hippocampal apoptosis in CCI rats.

Slowing the progression of Alzheimer's disease; what works?

Alzheimer's disease (AD) is an age-related progressive dementia, which is increasing in prevalence world-wide. Typically affecting short-term memory at onset, this devastating illness advances to impair all aspects of cognition, as well as non-cognitive domains. Although much effort has been made in recent years to develop disease-modifying treatments, medications which provided promising results in pre-clinical research have so far faltered in human clinical trials. Attention has recently shifted into trying to identify preventative measures that may delay the onset of the illness. Preventative factors include physical activity, proper diet, cognitive stimulation and the management of conditions such as hypertension, diabetes and obesity. However, it remains imperative to identify approaches that may help patients already diagnosed with the illness. Alongside pharmacological research, much work has been done on uncovering strategies which may slow down the progression of AD. This review aims to summarize evidence supporting or refuting methods impacting on the progression of the disease. AD remains a chronic and serious condition, therefore any intervention delaying the onset of moderate/severe symptoms will have a significant impact on patients and their families.

Antihypertensives, angiotensin, glucose and Alzheimer's disease

Evidence supporting a link between vascular disorders such as hypertension and Alzheimer's disease (AD) is increasing. Population studies have suggested an association between hypertension and an increased risk of developing AD. A potential role for antihypertensive medications in the management of cognitive disorders has also been suggested, although findings are mixed. However, it is of interest that evidence is now leaning towards the possibility that some of these antihypertensive medications may improve cognition independent of their blood pressure lowering effects. Many of these drugs cross the blood-brain barrier and may influence neurotransmitters involved in cognition. Increasing knowledge of the actions of antihypertensives in the brain and the vascular system could lead to better treatment and/or prevention options for AD.

Inhibition of central angiotensin II enhances memory function and reduces oxidative stress status in rat hippocampus

While it is now well established that the independent brain renin-angiotensin system (RAS) has some important central functions besides the vascular ones, the relevance of its main bioactive peptide angiotensin II (Ang II) on the memory processes, as well as on oxidative stress status is not completely understood. The purpose of the present work was to evaluate the effects of central Ang II administration, as well as the effects of Ang II inhibition with either AT1 and AT 2 receptor specific blockers (losartan and PD-123177, respectively) or an angiotensin-converting enzyme (ACE) inhibitor (captopril). These effects were studied on the short-term memory (assessed through Y-maze) or long-term memory (as determined in passive avoidance) and on the oxidative stress status of the hippocampus. Our results demonstrate memory deficits induced by the administration of Ang II, as showed by the significant decrease of the spontaneous alternation in Y-maze (p=0.015) and latency-time in passive avoidance task (p=0.001) when compared to saline. On the other side, the administration of all the aforementioned Ang II blockers significantly improved the spontaneous alternation in Y-maze task, while losartan also increased the latency time as compared to saline in step-through passive avoidance (p=0.042). Also, increased oxidative stress status was induced in the hippocampus by the administration of Ang II, as demonstrated by increased levels of lipid peroxidation markers (malondialdehyde-MDA concentration) (p<0.0001) and a decrease in both antioxidant enzymes determined: superoxide dismutase-SOD (p<0.0001) and glutathione peroxidase-GPX (p=0.01), as compared to saline. Additionally, the administration of captopril resulted in an increase of both antioxidant enzymes and decreased levels of lipid peroxidation (p=0.001), while PD-123177 significantly decreased MDA concentration (p>0.0001) vs. saline. Moreover, significant correlations were found between all of the memory related behavioral parameters and the main oxidative stress markers from the hippocampus, which is known for its implication in the processes of memory and also where RAS components are well expressed. This could be relevant for the complex interactions between Ang II, behavioral processes and neuronal oxidative stress, and could generate important therapeutic approaches.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Effects of lisinopril on NMDA receptor subunits 2A and 2B levels in the hippocampus of rats with L-NAME-induced hypertension

Hypertension is major risk factor leading to cerebrovascular pathologies. N-methyl D-aspartate receptors (NMDARs) and renin-angiotensin system are involved in neuronal plasticity, as well as cognitive functions in the hippocampus. In this study, we examined the effects of lisinopril, an ACE inhibitor, on the levels of hippocampal NMDAR subunits; NR2A and NR2B in L-NAME (N(ε)-nitro-L-arginine Methyl Ester)-induced hypertensive rats. In addition, malondialdehyde (MDA) levels were measured as a marker for lipid peroxidation. Compared with the control group, the MDA level was significantly increased after 8 weeks in the L-NAME-treated group. Rats treated with lisinopril and L-NAME plus lisinopril were found to have significantly decreased hippocampal MDA levels. Regarding the hippocampal concentrations of NR2A and NR2B, there were no statistically significant differences between groups. We demonstrated that lisinopril treatment has no direct regulatory effect on the levels of NR2A and NR2B in the rat hippocampus. Our results showed that Lisinopril could act as an antioxidant agent against hypertension-induced oxidative stress in rat hippocampus. The findings support that the use of lisinopril may offer a good alternative in the treatment of hypertension by reducing not only blood pressure but also prevent hypertensive complications in the brain.

Chronic inhibition of central Angiotensin-converting enzyme ameliorates colchicine-induced memory impairment in mice

Preclinical and clinical studies indicated involvement of the central renin-angiotensin system (RAS) in memory functions. However, the role of central angiotensin-converting enzyme (ACE) in memory function is still unclear. The present study investigated the involvement of central ACE in colchicine-induced memory impairment in the context of cholinergic function and oxidative stress. Memory impairment was induced by intracerebral colchicine administration in mice. The ACE inhibitor, perindopril (0.05 and 0.1 mg/kg/day), was administered orally for 14 days. Memory function was evaluated by the Morris water maze (MWM) test from the 14^th day on after colchicine injection. Donepezil was used as a standard. Parameters of oxidative stress and cholinergic function, ACE activity in serum and the brain were estimated after the completion of behavioral studies. Colchicine caused memory impairment as revealed by no significant change in latency to reach a hidden platform in the MWM test. Furthermore, there was a significant increase in MDA, ROS, and nitrite levels with a reduction in GSH level and acetylcholinesterase (AChE) activity in the brain of colchicine-treated mice. Colchicine significantly increased brain ACE activity without affecting serum ACE. Donepezil prevented colchicine-induced memory impairment in mice. The antidementic effect of perindopril may be attributed to reduced oxidative stress and improvement in cholinergic function. Moreover, the elevated brain ACE activity was also inhibited by perindopril. The study showed that central ACE plays an important role in colchicine-induced memory deficit, corroborating a number of studies that show that treatment with ACE inhibitors could be neuroprotective.

Alzheimer's disease and age-related memory decline (preclinical)

An unfortunate result of the rapid rise in geriatric populations worldwide is the increasing prevalence of age-related cognitive disorders such as Alzheimer's disease (AD). AD is a devastating neurodegenerative illness that is characterized by a profound impairment of cognitive function, marked physical disability, and an enormous economic burden on the afflicted individual, caregivers, and society in general. The rise in elderly populations is also resulting in an increase in individuals with related (potentially treatable) conditions such as "Mild Cognitive Impairment" (MCI) which is characterized by a less severe (but abnormal) level of cognitive impairment and a high-risk for developing dementia. Even in the absence of a diagnosable disorder of cognition (e.g., AD and MCI), the perception of increased forgetfulness and declining mental function is a clear source of apprehension in the elderly. This is a valid concern given that even a modest impairment of cognitive function is likely to be associated with significant disability in a rapidly evolving, technology-based society. Unfortunately, the currently available therapies designed to improve cognition (i.e., for AD and other forms of dementia) are limited by modest efficacy and adverse side effects, and their effects on cognitive function are not sustained over time. Accordingly, it is incumbent on the scientific community to develop safer and more effective therapies that improve and/or sustain cognitive function in the elderly allowing them to remain mentally active and productive for as long as possible. As diagnostic criteria for memory disorders evolve, the demand for pro-cognitive therapeutic agents is likely to surpass AD and dementia to include MCI and potentially even less severe forms of memory decline. The purpose of this review is to provide an overview of the contemporary therapeutic targets and preclinical pharmacologic approaches (with representative drug examples) designed to enhance memory function.

Is heart rate variability related to memory performance in middle-aged men?

OBJECTIVE

Heart rate variability (HRV), a measure of autonomic function, has been associated with cognitive function, but studies are conflicting. Previous studies have also not controlled for familial and genetic influences.

METHODS

We performed power spectral analysis on 24-hour ambulatory ECGs in 416 middle-aged male twins from the Vietnam Era Twin Registry. Memory and learning were measured by verbal and visual Selective Reminding Tests (SRTs). Mixed-effect regression models were used to calculate associations between and within twin pairs, while adjusting for covariates.

RESULTS

The mean age (standard deviation) was 55 (2.9) years. A statistically significant positive association was found between measures of HRV and verbal, but not visual, SRT scores. The most statistically significant unadjusted association was found between very low frequency HRV and verbal total recall SRT, such that each logarithm of increase in very low frequency was associated with an increased verbal SRT score of 4.85 points (p = .002). The association persisted despite the adjustment for demographic and cardiovascular risk factors, and after accounting for familial and genetic factors by comparing twins within pairs. A significant interaction was found between posttraumatic stress disorder (PTSD) and HRV, such that total power and ultra low frequency were associated with SRT in twins (n = 362) without PTSD, but not in those with PTSD.

CONCLUSIONS

Lower frequency spectra of HRV are associated with verbal, but not visual, learning and memory, particularly in subjects without PTSD. This association may indicate that autonomic nervous system dysregulation plays a role in cognitive decline.

Non-adherence to antihypertensive medication and impaired cognition: which comes first?

OBJECTIVE

Antihypertensive medications are important in the prevention of serious consequences of hypertension, such as stroke and heart failure. Up to one-third of elderly hypertensive patients, however, do not adhere to their medication. Adherence to medication decreases with increasing age, and with decreasing cognitive ability, thus elderly, cognitively-impaired patients have poorer control of blood pressure. Good control of blood pressure is associated with decreased prevalence of dementia and Alzheimer's disease. This study assessed the evidence that antihypertensive medications have effects on the prevalence or severity of mild cognitive impairment, dementia or Alzheimer's disease.

METHODS

The ISI Web of Knowledge database was searched; including replicates, the nine searches identified 14400 publications since 1952, of which 9.9% had been published in 2009. This review considers the 18 studies meeting the set criteria published in 2009 or later.

KEY FINDINGS

Not all antihypertensive medications are equivalent in their positive cognitive effects, with brain-penetrating angiotensin-converting-enzyme inhibitors and possibly angiotensin receptor antagonists being the most effective.

CONCLUSIONS

Based on evidence of blood-pressure control and cost, UK National Institute for Health and Clinical Excellence guidelines recommend calcium-channel blockers or thiazide-type diuretics for the treatment of hypertension in patients over 55 years. These guidelines take no account of the potential cognitive effects of the antihypertensive therapies, consideration of which might lead to a review. There may be benefit in stressing that adherence to antihypertensive medication not only decreases the risk of cardiovascular disease and death, but may also decrease the risk or severity of mild cognitive impairment, dementia and Alzheimer's disease.

Emerging cognitive enhancing drugs

BACKGROUND

Advances in health sciences during the last century have increased the average age in industrialized nations. Despite this progress, neurodegenerative diseases that affect higher order thinking and memory continue to increase in prevalence as they take a devastating toll on human productivity in the later years. There is an acute need for new drugs and therapeutic approaches for treating these severe diseases, and also for improving the quality of cognitive function associated with normal aging and in many other disorders and syndromes that present with cognitive dysfunction.

OBJECTIVE

The purpose of this review is to ascertain the pharmacological approaches being exploited to improve cognition and memory and to determine the most relevant and effective directions taken for new drug discovery. Limitations and difficulties encountered in this effort also are discussed.

METHODS

This review focuses primarily on compounds already undergoing clinical trials for improving cognition and memory with some discussion of rising new drug targets.

RESULTS/CONCLUSION

Compounds that act on allosteric sites on neurotransmitter receptors are expected to lead the field with new levels of specificity and reduced side effects. New multi-functional compounds can be designed that can both improve cognition and slow the process of disease.

Angiotensins in Alzheimer's disease - friend or foe?

The renin-angiotensin system (RAS) is an important regulator of blood pressure. Observational and experimental studies suggest that alterations in blood pressure and components of the brain RAS contribute to the development and progression of Alzheimer's disease (AD), resulting in changes that can lead or contribute to cognitive decline. The complexity of the RAS and diversity of its interactions with neurological processes have recently become apparent but large gaps in our understanding still remain. Modulation of activity of components of the brain RAS offers substantial opportunities for the treatment and prevention of dementia, including AD. This paper reviews molecular, genetic, experimental and clinical data as well as the therapeutic opportunities that relate to the involvement of the RAS in AD.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Angiotensin-converting enzyme converts amyloid beta-protein 1-42 (Abeta(1-42)) to Abeta(1-40), and its inhibition enhances brain Abeta deposition

The abnormal deposition of the amyloid beta-protein (Abeta) in the brain appears crucial to the pathogenesis of Alzheimer's disease (AD). Recent studies have suggested that highly amyloidogenic Abeta(1-42) is a cause of neuronal damage leading to AD pathogenesis and that monomeric Abeta(1-40) has less neurotoxicity than Abeta(1-42). We found that mouse and human brain homogenates exhibit an enzyme activity converting Abeta(1-42) to Abeta(1-40) and that the major part of this converting activity is mediated by the angiotensin-converting enzyme (ACE). Purified human ACE converts Abeta(1-42) to Abeta(1-40) as well as decreases Abeta(1-42)/Abeta(1-40) ratio and degrades Abeta(1-42) and Abeta(1-40). Importantly, the treatment of Tg2576 mice with an ACE inhibitor, captopril, promotes predominant Abeta(1-42) deposition in the brain, suggesting that ACE regulates Abeta(1-42)/Abeta(1-40) ratio in vivo by converting secreted Abeta(1-42) to Abeta(1-40) and degrading Abetas. The upregulation of ACE activity can be a novel therapeutic strategy for AD.

An update on non-peptide angiotensin receptor antagonists and related RAAS modulators

The renin-angiotensin-aldosterone-system (RAAS) is an important regulator of blood pressure and fluid-electrolyte homeostasis. RAAS has been implicated in pathogenesis of hypertension, congestive heart failure, and chronic renal failure. Aliskiren is the first non-peptide orally active renin inhibitor approved by FDA. Angiotensin Converting Enzyme (ACE) Inhibitors are associated with frequent side effects such as cough and angio-oedema. Recently, the role of ACE2 and neutral endopeptidase (NEP) in the formation of an important active metabolite/mediator of RAAS, ang 1-7, has initiated attempts towards development of ACE2 inhibitors and combined ACE/NEP inhibitors. Furukawa and colleagues developed a series of low molecular weight nonpeptide imidazole analogues that possess weak but selective, competitive AT1 receptor blocking property. Till date, many compounds have exhibited promising AT1 blocking activity which cause a more complete RAAS blockade than ACE inhibitors. Many have reached the market for alternative treatment of hypertension, heart failure and diabetic nephropathy in ACE inhibitor intolerant patients and still more are waiting in the queue. But, the hallmark of this area of drug research is marked by a progress in understanding molecular interaction of these blockers at the AT1 receptor and unraveling the enigmatic influence of AT2 receptors on growth/anti-growth, differentiation and the regeneration of neuronal tissue. Different modeling strategies are underway to develop tailor made molecules with the best of properties like Dual Action (Angiotensin And Endothelin) Receptor Antagonists (DARA), ACE/NEP inhibitors, triple inhibitors, AT2 agonists, AT1/TxA2 antagonists, balanced AT1/AT2 antagonists, and nonpeptide renin inhibitors. This abstract gives an overview of these various angiotensin receptor antagonists.

Angiotensin II increases evoked release at the frog neuromuscular junction through a receptor sensitive to A779

Receptor mediated presynaptic modulation is a ubiquitous mechanism involved in synaptic plasticity. Here we show that angiotensin II increased quantal content at the frog neuromuscular junction. This presynaptic effect of angiotensin II was insensitive to losartan and PD123319, but was antagonized by a more potent partial agonist of the amphibian angiotensin receptor, L162313. In addition, A779, a blocker of the angiotensin-[1-7] receptor, also abolished the effect of angiotensin II. These results indicate that the effect of angiotensin II on evoked release is mediated through an angiotensin receptor. L162313 alone increased quantal content, and A779 also antagonized this effect of L162313. We conclude that the neuromuscular junction possesses angiotensin receptors involved in presynaptic modulation.

Angiotensin II suppresses long-term depression in the lateral amygdala of mice via L-type calcium channels

Previously we have shown that angiotensin II (Ang II) suppresses long-term potentiation (LTP) in the lateral nucleus of the amygdala (LA) of horizontal slices. This study examines the effect of Ang II on long-term depression (LTD) in the LA. Low frequency stimulation (1 Hz, 15 min; LFS) applied to fibers running within the LA induced a long-lasting reduction of the amplitudes of field potentials in the LA of mice. We have previously shown that this LTD is sensitive to the NMDA receptor blocker D-AP5 and is dependent on group II mGlu receptors. Ang II blocked dose-dependent LTD. Losartan, an AT1 receptor antagonist, blocked the Ang II-induced suppression of LTD, whereas PD 123 319, an AT2 receptor antagonist, had no effect. Co-administration of nifedipine, an L-type calcium channel antagonist, abolished Ang II-induced suppression of LTD. When applied alone nifedipine reduced the magnitude of LA-LTD. According to our previous results, stimulation of external capsule (EC) fibers in rats did not cause LTD in mice. Similarly, Ang II did not induce long-lasting changes of activity when EC stimulation site was used. The results support the view that angiotensins are involved in mechanisms of learning and memory including the plasticity changes in the LA.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.