Cognitive effects attributed to angiotensin II may result from its conversion to angiotensin IV

Acute Angiotensin II Receptor Blockade Facilitates Parahippocampal Processing During Memory Encoding in High-Trait-Anxious Individuals

Background

Angiotensin II receptor blockers (ARBs) have been associated with preventing posttraumatic stress disorder symptom development and improving memory. However, the underlying neural mechanisms are poorly understood. This study investigated ARB effects on memory encoding and hippocampal functioning that have previously been implicated in posttraumatic stress disorder development.

Methods

In a double-blind randomized design, 40 high-trait-anxious participants (33 women) received the ARB losartan (50 mg) or placebo. At drug peak level, participants encoded images of animals and landscapes before undergoing functional magnetic resonance imaging, where they viewed the encoded familiar images and unseen novel images to be memorized and classified as animals/landscapes. Memory recognition was assessed 1 hour after functional magnetic resonance imaging. To analyze neural effects, whole-brain analysis, hippocampus region-of-interest analysis, and exploratory multivariate pattern similarity analysis were employed.

Results

ARBs facilitated parahippocampal processing. In the whole-brain analysis, losartan enhanced brain activity for familiar images in the parahippocampal gyrus (PHC), anterior cingulate cortex, and caudate. For novel images, losartan enhanced brain activity in the PHC only. Pattern similarity analysis showed that losartan increased neural stability in the PHC when processing novel and familiar images. However, there were no drug effects on memory recognition or hippocampal activation.

Conclusions

Given that the hippocampus receives major input from the PHC, our findings suggest that ARBs may modulate higher-order visual processing through parahippocampal involvement, potentially preserving intact memory input. Future research needs to directly investigate whether this effect may underlie the preventive effects of ARBs in the development of posttraumatic stress disorder.

Angiotensin receptor blocker use is associated with upregulation of the memory-protective angiotensin type 4 receptor (AT4R) in the postmortem brains of individuals without cognitive impairment

The reported primary dementia-protective benefits of angiotensin II type 1 receptor (AT1R) blockers (ARB) are believed, at least in part, to arise from systemic effects on blood pressure. However, there is a specific and independently regulated brain renin-angiotensin system (RAS). Brain RAS acts mainly through three receptor subtypes; AT1R, AT2R, and AT4R. The AT1R promotes inflammation and mitochondrial reactive oxygen species generation. AT2R increases nitric oxide. AT4R is essential for dopamine and acetylcholine release. It is unknown whether ARB use is associated with changes in the brain RAS. Here, we compared the impact of treatment with ARB on not cognitively impaired individuals and individuals with Alzheimer's dementia using postmortem frontal-cortex samples of age- and sex-matched participants (70-90 years old, n = 30 in each group). We show that ARB use is associated with higher brain AT4R, lower oxidative stress, and amyloid-β burden in NCI participants. In AD, ARB use was associated with lower brain AT1R but had no impact on inflammation, oxidative stress, or amyloid-β burden. Our results may suggest a potential role for AT4R in the salutary effects for ARB on the brains of not cognitively impaired older adults.

Association of New Use of Antihypertensives That Stimulate vs Inhibit Type 2 and 4 Angiotensin II Receptors With Dementia Among Medicare Beneficiaries

Importance

Prevalent use of antihypertensive medications that stimulate type 2 and 4 angiotensin II receptors, compared with those that do not stimulate these receptors, has been associated with a lower risk of dementia. However, previous studies were limited by inclusion of individuals with prevalent hypertension and a history of antihypertensive use prior to the start of the study, which can introduce bias.

Objective

To examine the association of new use of antihypertensive medication regimens that stimulate vs inhibit type 2 and 4 angiotensin II receptors with Alzheimer disease and related dementias (ADRD) among Medicare beneficiaries.

Design, Setting, and Participants

This cohort study was conducted among 57 773 Medicare fee-for-service beneficiaries (January 1, 2006, through December 31, 2018) aged 65 years or older with incident hypertension. Data analysis was conducted from January 1 through June 30, 2022.

Exposures

Initiation of antihypertensive medication regimens that stimulate or inhibit type 2 and 4 angiotensin II receptors, or mixed regimens (both stimulating and inhibiting), with the time-dependent measure being each 30-day interval.

Main Outcomes and Measures

The primary outcome was time to first occurrence of ADRD (Centers for Medicare & Medicaid Services Chronic Conditions Data Warehouse definition). Cox proportional hazards regression modeling with time-dependent variables was performed to estimate the association between time-dependent treatment groups and time to ADRD, after adjusting for sociodemographic and clinical characteristics.

Results

The sample included 57 773 Medicare beneficiaries (36 348 women [62.9%]; mean [SD] age, 73.8 [6.3] years; 2954 [5.1%] Black, 1545 [2.7%] Hispanic; 50 184 [86.9%] White, and 3090 [5.4%] Other individuals [the Other category included individuals of American Indian, Asian, other, or unknown race and ethnicity]). During a median of 6.9 years (IQR, 4.7-9.3 years) of follow-up, the unadjusted incidence density rate of ADRD was 2.2 cases per 100 person-years (95% CI, 2.1-2.4 cases per 100 person-years) for the group receiving regimens that stimulate type 2 and 4 angiotensin II receptors compared with 3.1 cases per 100 person-years (95% CI, 3.0-3.2 cases per 100 person-years) for the group receiving regimens that inhibit type 2 and 4 angiotensin II receptors and 2.7 cases per 100 person-years (95% CI, 2.6-2.9 cases per 100 person-years) for the group receiving mixed treatment regimens. In adjusted Cox proportional hazards regression modeling, stimulating treatment was associated with a statistically significant 16% reduction in the hazard of ADRD compared with inhibiting treatment (hazard ratio, 0.84; 95% CI, 0.79-0.90). Mixed regimen use was also associated with reduced hazards of ADRD compared with the inhibiting group (hazard ratio, 0.90; 95% CI, 0.84-0.96).

Conclusions and Relevance

This cohort study of Medicare beneficiaries suggests that use of antihypertensive medications that stimulate type 2 and 4 angiotensin II receptors was associated with lower risk of ADRD compared with antihypertensive medications that inhibit these receptors. Confirmation is needed in a randomized trial.

Targeting brain Renin-Angiotensin System for the prevention and treatment of Alzheimer's disease: Past, present and future

Alzheimer's disease (AD) is a well-known neurodegenerative disease characterized by the presence of two main hallmarks - Tau hyperphosphorylation and Aβ deposits. Notwithstanding, in the last few years the scientific evidence about the drivers of AD have been changing and nowadays age-related vascular alterations and several cardiovascular risk factors have been shown to trigger the development of AD. In this context, drugs targeting the Renin Angiotensin System (RAS), commonly used for the treatment of hypertension, are evidencing a high potential to delay AD development due to their action on brain RAS. Indeed, the ACE 1/Ang II/AT1R axis is believed to be upregulated in AD and to be responsible for deleterious effects such as increased oxidative stress, neuroinflammation, blood-brain barrier (BBB) hyperpermeability, astrocytes dysfunction and a decrease in cerebral blood flow. In contrast, the alternative axis - ACE 1/Ang II/AT2R; ACE 2/Ang (1-7)/MasR; Ang IV/ AT4R(IRAP) - seems to counterbalance the deleterious effects of the principal axis and to exert beneficial effects on memory and cognition. Accordingly, retrospective studies demonstrate a reduced risk of developing AD among people taking RAS medication as well as several in vitro and in vivo pre-clinical studies as it is herein critically reviewed. In this review, we first revise, at a glance, the pathophysiology of AD focused on its classic hallmarks. Secondly, an overview about the impact of the RAS on the pathophysiology of AD is also provided, focused on their four essential axes ACE 1/Ang II/AT2R; ACE 2/Ang (1-7)/MasR; Ang IV/ AT4R(IRAP) and ACE 1/Ang II/AT1R. Finally, the therapeutic potential of available drugs targeting RAS on AD, namely angiotensin II receptor blockers (ARBs) and angiotensin converting enzyme inhibitors (ACEIs), is highlighted and data supporting this hope will be presented, from in vitro and in vivo pre-clinical to clinical studies.

Multiple Aspects of Inappropriate Action of Renin-Angiotensin, Vasopressin, and Oxytocin Systems in Neuropsychiatric and Neurodegenerative Diseases

The cardiovascular system and the central nervous system (CNS) closely cooperate in the regulation of primary vital functions. The autonomic nervous system and several compounds known as cardiovascular factors, especially those targeting the renin–angiotensin system (RAS), the vasopressin system (VPS), and the oxytocin system (OTS), are also efficient modulators of several other processes in the CNS. The components of the RAS, VPS, and OTS, regulating pain, emotions, learning, memory, and other cognitive processes, are present in the neurons, glial cells, and blood vessels of the CNS. Increasing evidence shows that the combined function of the RAS, VPS, and OTS is altered in neuropsychiatric/neurodegenerative diseases, and in particular in patients with depression, Alzheimer’s disease, Parkinson’s disease, autism, and schizophrenia. The altered function of the RAS may also contribute to CNS disorders in COVID-19. In this review, we present evidence that there are multiple causes for altered combined function of the RAS, VPS, and OTS in psychiatric and neurodegenerative disorders, such as genetic predispositions and the engagement of the RAS, VAS, and OTS in the processes underlying emotions, memory, and cognition. The neuroactive pharmaceuticals interfering with the synthesis or the action of angiotensins, vasopressin, and oxytocin can improve or worsen the effectiveness of treatment for neuropsychiatric/neurodegenerative diseases. Better knowledge of the multiple actions of the RAS, VPS, and OTS may facilitate programming the most efficient treatment for patients suffering from the comorbidity of neuropsychiatric/neurodegenerative and cardiovascular diseases.

Association of Antihypertensives That Stimulate vs Inhibit Types 2 and 4 Angiotensin II Receptors With Cognitive Impairment

Importance

Use of antihypertensive medications that stimulate type 2 and 4 angiotensin II receptors, compared with those that do not stimulate these receptors, has been associated with a lower risk of dementia. However, this association with cognitive outcomes in hypertension trials, with blood pressure levels in the range of current guidelines, has not been evaluated.

Objective

To examine the association between use of exclusively antihypertensive medication regimens that stimulate vs inhibit type 2 and 4 angiotensin II receptors on mild cognitive impairment (MCI) or dementia.

Design, Setting, and Participants

This cohort study is a secondary analysis (April 2011 to July 2018) of participants in the randomized Systolic Blood Pressure Intervention Trial (SPRINT), which recruited individuals 50 years or older with hypertension and increased cardiovascular risk but without a history of diabetes, stroke, or dementia. Data analysis was conducted from March 16 to July 6, 2021.

Exposures

Prevalent use of angiotensin II receptor type 2 and 4-stimulating or -inhibiting antihypertensive medication regimens at the 6-month study visit.

Main Outcomes and Measures

The primary outcome was a composite of adjudicated amnestic MCI or probable dementia.

Results

Of the 8685 SPRINT participants who were prevalent users of antihypertensive medication regimens at the 6-month study visit (mean [SD] age, 67.7 [11.2] years; 5586 [64.3%] male; and 935 [10.8%] Hispanic, 2605 [30.0%] non-Hispanic Black, 4983 [57.4%] non-Hispanic White, and 162 [1.9%] who responded as other race or ethnicity), 2644 (30.4%) were users of exclusively stimulating, 1536 (17.7%) inhibiting, and 4505 (51.9%) mixed antihypertensive medication regimens. During a median of 4.8 years of follow-up (95% CI, 4.7-4.8 years), there were 45 vs 59 cases per 1000 person-years of amnestic MCI or probable dementia among prevalent users of regimens that contained exclusively stimulating vs inhibiting antihypertensive medications (hazard ratio [HR], 0.76; 95% CI, 0.66-0.87). When comparing stimulating-only vs inhibiting-only users, amnestic MCI occurred at rates of 40 vs 54 cases per 1000 person-years (HR, 0.74; 95% CI, 0.64-0.87) and probable dementia at rates of 8 vs 10 cases per 1000 person-years (HR, 0.80; 95% CI, 0.57-1.14). Negative control outcome analyses suggested the presence of residual confounding.

Conclusions and Relevance

In this secondary analysis of SPRINT, prevalent users of regimens that contain exclusively antihypertensive medications that stimulate vs inhibit type 2 and 4 angiotensin II receptors had lower rates of incident cognitive impairment. Residual confounding cannot be ruled out. If these results are replicated in randomized clinical trials, certain antihypertensive medications could be prioritized to prevent cognitive decline.

Association of Antihypertensives and Cognitive Impairment in Long-Term Care Residents

BACKGROUND

Certain classes of antihypertensive medication may have different associations with cognitive impairment.

OBJECTIVE

To examine the association between prevalent use of antihypertensive medications that stimulate (thiazides, dihydropyridine calcium channel blockers, angiotensin type I receptor blockers) versus inhibit (angiotensin-converting enzyme inhibitors, beta-blockers, non-dihydropyridine calcium channel blockers) type 2 and 4 angiotensin II receptors on cognitive impairment among older adults residing in Veterans Affairs (VA) nursing homes for long-term care.

METHODS

Retrospective cohort study. Long-term care residents aged 65 + years admitted to a VA nursing home from 2012 to 2019 using blood pressure medication and without cognitive impairment at admission. Main exposure was prevalent use of angiotensin II receptor type 2 and 4-'stimulating' (N = 589), 'inhibiting' (N = 3,219), or 'mixed' (N = 1,715) antihypertensive medication regimens at admission. Primary outcome was any cognitive impairment (Cognitive Function Scale).

RESULTS

Over an average of 5.4 months of follow-up, prevalent use of regimens containing exclusively 'stimulating' antihypertensives was associated with a lower risk of any incident cognitive impairment as compared to prevalent use of regimens containing exclusively 'inhibiting' antihypertensives (HR 0.83, 95% CI 0.74-0.93). Results for the comparison between 'mixed' versus 'inhibiting' regimens were in the same direction but not statistically significant (HR 0.96, 95% CI 0.88-1.06).

CONCLUSION

For residents without cognitive impairment at baseline, prevalent users of regimens containing exclusively antihypertensives that stimulate type 2 and 4 angiotensin II receptors had lower rates of cognitive impairment as compared to prevalent users of regimens containing exclusively antihypertensives that inhibit these receptors. Residual confounding cannot be ruled out.

Tackling Alzheimer's Disease with Existing Drugs: A Promising Strategy for Bypassing Obstacles

The unmet need for the development of effective drugs to treat Alzheimer 's disease has been steadily growing, representing a major challenge in drug discovery. In this context, drug repurposing, namely the identification of novel therapeutic indications for approved or investigational compounds, can be seen as an attractive attempt to obtain new medications reducing both the time and the economic burden usually required for research and development programs. In the last years, several classes of drugs have evidenced promising beneficial effects in neurodegenerative diseases, and for some of them, preliminary clinical trials have been started. This review aims to illustrate some of the most recent examples of drugs reprofiled for Alzheimer's disease, considering not only the finding of new uses for existing drugs but also the new hypotheses on disease pathogenesis that could promote previously unconsidered therapeutic regimens. Moreover, some examples of structural modifications performed on existing drugs in order to obtain multifunctional compounds will also be described.

Brain angiotensin II and angiotensin IV receptors as potential Alzheimer's disease therapeutic targets

Alzheimer's disease (AD) is a neurodegenerative disorder that is multifactorial in nature. Yet, despite being the most common form of dementia in the elderly, AD's primary cause remains unknown. As such, there is currently little to offer AD patients as the vast majority of recently tested therapies have either failed in well-controlled clinical trials or inadequately treat AD. Recently, emerging preclinical and clinical evidence has associated the brain renin angiotensin system (RAS) to AD pathology. Accordingly, various components of the brain RAS were shown to be altered in AD patients and mouse models, including the angiotensin II type 1 (AT1R), angiotensin IV receptor (AT4R), and Mas receptors. Collectively, the changes observed within the RAS have been proposed to contribute to many of the neuropathological hallmarks of AD, including the neuronal, cognitive, and vascular dysfunctions. Accumulating evidence has additionally identified antihypertensive medications targeting the RAS, particularly angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEIs), to delay AD onset and progression. In this review, we will discuss the emergence of the RAS's involvement in AD and highlight putative mechanisms of action underlying ARB's beneficial effects that may explain their ability to modify the risk of developing AD or AD progression. The RAS may provide novel molecular targets for recovering memory pathways, cerebrovascular function, and other pathological landmarks of AD.

Human Extinction Learning Is Accelerated by an Angiotensin Antagonist via Ventromedial Prefrontal Cortex and Its Connections With Basolateral Amygdala

BACKGROUND

Deficient extinction learning and threat adaptation in the ventromedial prefrontal cortex (vmPFC)-amygdala circuitry strongly impede the efficacy of exposure-based interventions in anxiety disorders. Recent animal models suggest a regulatory role of the renin-angiotensin system in both these processes. Against this background, the present randomized placebo-controlled pharmacologic functional magnetic resonance imaging experiment aimed at determining the extinction enhancing potential of the angiotensin II type 1 receptor antagonist losartan (LT) in humans.

METHODS

Seventy healthy male subjects underwent Pavlovian threat conditioning and received single-dose LT (50 mg) or placebo administration before extinction. Psychophysiological threat reactivity (skin conductance response) and neural activity during extinction served as primary outcomes. Psychophysiological interaction, voxelwise mediation, and novel multivariate pattern classification analyses were used to determine the underlying neural mechanisms.

RESULTS

LT significantly accelerated the decline of the psychophysiological threat response during within-session extinction learning. On the neural level, the acceleration was accompanied and critically mediated by threat-specific enhancement of vmPFC activation. Furthermore, LT enhanced vmPFC-basolateral amygdala coupling and attenuated the neural threat expression, particularly in the vmPFC, during early extinction.

CONCLUSIONS

Overall the results indicate that LT facilitates within-session threat memory extinction by augmenting threat-specific encoding in the vmPFC and its regulatory control over the amygdala. The findings document a pivotal role of angiotensin regulation of extinction learning in humans and suggest that adjunct LT administration has the potential to facilitate the efficacy of exposure-based interventions in anxiety disorders.

The angiotensin II type I receptor contributes to impaired cerebral blood flow autoregulation caused by placental ischemia in pregnant rats

BACKGROUND

Placental ischemia and hypertension, characteristic features of preeclampsia, are associated with impaired cerebral blood flow (CBF) autoregulation and cerebral edema. However, the factors that contribute to these cerebral abnormalities are not clear. Several lines of evidence suggest that angiotensin II can impact cerebrovascular function; however, the role of the renin angiotensin system in cerebrovascular function during placental ischemia has not been examined. We tested whether the angiotensin type 1 (AT1) receptor contributes to impaired CBF autoregulation in pregnant rats with placental ischemia caused by surgically reducing uterine perfusion pressure.

METHODS

Placental ischemic or sham operated rats were treated with vehicle or losartan from gestational day (GD) 14 to 19 in the drinking water. On GD 19, we assessed CBF autoregulation in anesthetized rats using laser Doppler flowmetry.

RESULTS

Placental ischemic rats had impaired CBF autoregulation that was attenuated by treatment with losartan. In addition, we examined whether an agonistic autoantibody to the AT1 receptor (AT1-AA), reported to be present in preeclamptic women, contributes to impaired CBF autoregulation. Purified rat AT1-AA or vehicle was infused into pregnant rats from GD 12 to 19 via mini-osmotic pumps after which CBF autoregulation was assessed. AT1-AA infusion impaired CBF autoregulation but did not affect brain water content.

CONCLUSIONS

These results suggest that the impaired CBF autoregulation associated with placental ischemia is due, at least in part, to activation of the AT1 receptor and that the RAS may interact with other placental factors to promote cerebrovascular changes common to preeclampsia.

Memory and cerebrovascular deficits recovered following angiotensin IV intervention in a mouse model of Alzheimer's disease

Angiotensin II type 1 receptor antagonists like losartan have been found to lower the incidence and progression to Alzheimer's disease (AD), as well as rescue cognitive and cerebrovascular deficits in AD mouse models. We previously found that co-administration of an angiotensin IV (AngIV) receptor (AT4R) antagonist prevented losartan's benefits, identifying AT4Rs as a possible target to counter AD pathogenesis. Therein, we investigated whether directly targeting AT4Rs could counter AD pathogenesis in a well-characterized mouse model of AD. Wild-type and human amyloid precursor protein (APP) transgenic (J20 line) mice (4.5 months old) received vehicle or AngIV (~1.3 nmol/day, 1 month) intracerebroventricularly via osmotic minipumps. AngIV restored short-term memory, spatial learning and memory in APP mice. AngIV normalized hippocampal AT4R levels, increased hippocampal subgranular zone cellular proliferation and dendritic arborization, and reduced oxidative stress. AngIV rescued whisker-evoked neurovascular coupling, endothelial- and smooth muscle cell-mediated cerebral vasodilatory responses, and cerebrovascular nitric oxide bioavailability. AngIV did not alter blood pressure, neuroinflammation or amyloid-β (Aβ) pathology. These preclinical findings identify AT4R as a promising target to counter Aβ-related cognitive and cerebrovascular deficits in AD.

Recruitment of central angiotensin II type 1 receptor associated neurocircuits in carbon dioxide associated fear

Individuals with fear-associated conditions such as panic disorder (PD) and posttraumatic stress disorder (PTSD) display increased emotional responses to interoceptive triggers, such as CO₂ inhalation, that signal a threat to physiological homeostasis. Currently, effector systems and mechanisms underlying homeostatic modulation of fear memory are not well understood. In this regard, the renin angiotensin system (RAS), particularly the angiotensin receptor type 1 (AT1R), a primary homeostatic regulatory target, has gained attention. RAS polymorphisms have been reported in PD and PTSD, and recent studies report AT1R-mediated modulation of fear extinction. However, contribution of AT1Rs in fear evoked by the interoceptive threat of CO₂ has not been investigated. Using pharmacological, behavioral, and AT1R/ACE gene transcription analyses, we assessed central AT1R recruitment in CO₂-associated fear. CO₂ inhalation led to significant AT1R and ACE mRNA upregulation in homeostatic regulatory regions, subfornical organ (SFO) and paraventricular nucleus (PVN), in a temporal manner. Intracerebroventricular infusion of selective AT1R antagonist, losartan, significantly attenuated freezing during CO₂ inhalation, and during re-exposure to CO₂ context, suggestive of AT1R modulation of contextual fear. Regional Fos mapping in losartan-treated mice post-behavior revealed significantly attenuated labeling in areas regulating defensive behavior, contextual fear, and threat responding; such as, the bed nucleus of stria terminalis, dorsal periaqueductal gray, hypothalamic nuclei, hippocampus, and prefrontal areas such as the prelimbic, infralimbic, and anterior cingulate cortices. Sub-regions of the amygdala did not show CO₂-associated AT1R regulation or altered Fos labeling. Collectively, our data suggests central AT1R recruitment in modulation of fear behaviors associated with CO₂ inhalation via engagement of neurocircuits regulating homeostasis and defensive behaviors. Our data provides mechanistic insights into the interoceptive regulation of fear, relevant to fear related disorders such as PD and PTSD.

Fiend and friend in the renin angiotensin system: An insight on acute kidney injury

Besides assisting the maintenance of blood pressure and sodium homeostasis, the renin-angiotensin system (RAS) plays a pivotal role in pathogenesis of acute kidney injury (AKI). The RAS is equipped with two arms i) the pressor arm composed of Angiotensin II (Ang II)/Angiotensin converting enzyme (ACE)/Angiotensin II type 1 receptor (AT1R) also called conventional RAS, and ii) the depressor arm consisting of Angiotensin (1-7) (Ang 1-7)/Angiotensin converting enzyme 2 (ACE2)/MasR known as non-conventional RAS. Activation of conventional RAS triggers oxidative stress, inflammatory, hypertrophic, apoptotic, and pro-fibrotic signaling cascades which promote AKI. The preclinical and clinical studies have reported beneficial as well as deleterious effects of RAS blockage either by angiotensin receptor blocker or ACE inhibitor in AKI. On the contrary, the depressor arm opposes the conventional RAS, has beneficial effects on the kidney but has been less explored in pathogenesis of AKI. This review focuses on significance of RAS in pathogenesis of AKI and provides better understanding of novel and possible therapeutic approaches to combat AKI.

Cognitive benefits of angiotensin IV and angiotensin-(1-7): A systematic review of experimental studies

OBJECTIVES

To explore effects of the brain renin-angiotensin system (RAS) on cognition.

DESIGN

Systematic review of experimental (non-human) studies assessing cognitive effects of RAS peptides angiotensin-(3-8) [Ang IV] and angiotensin-(1-7) [Ang-(1-7)] and their receptors, the Ang IV receptor (AT4R) and the Mas receptor.

RESULTS

Of 450 articles identified, 32 met inclusion criteria. Seven of 11 studies of normal animals found Ang IV had beneficial effects on tests of passive or conditioned avoidance and object recognition. In models of cognitive deficit, eight of nine studies found Ang IV and its analogs (Nle¹-Ang IV, dihexa, LVV-hemorphin-7) improved performance on spatial working memory and passive avoidance tasks. Two of three studies examining Ang-(1-7) found it benefited memory. Mas receptor removal was associated with reduced fear memory in one study.

CONCLUSION

Studies of cognitive impairment show salutary effects of acute administration of Ang IV and its analogs, as well as AT4R activation. Brain RAS peptides appear most effective administered intracerebroventricularly, close to the time of learning acquisition or retention testing. Ang-(1-7) shows anti-dementia qualities.

Within the Brain: The Renin Angiotensin System

For many years, modulators of the renin angiotensin system (RAS) have been trusted by clinicians for the control of essential hypertension. It was recently demonstrated that these modulators have other pleiotropic properties independent of their hypotensive effects, such as enhancement of cognition. Within the brain, different components of the RAS have been extensively studied in the context of neuroprotection and cognition. Interestingly, a crosstalk between the RAS and other systems such as cholinergic, dopaminergic and adrenergic systems have been demonstrated. In this review, the preclinical and clinical evidence for the impact of RAS modulators on cognitive impairment of multiple etiologies will be discussed. In addition, the expression and function of different receptor subtypes within the RAS such as: Angiotensin II type I receptor (AT1R), Angiotensin II type II receptor (AT2R), Angiotensin IV receptor (AT4R), Mas receptor (MasR), and Mas-related-G protein-coupled receptor (MrgD), on different cell types within the brain will be presented. We aim to direct the attention of the scientific community to the plethora of evidence on the importance of the RAS on cognition and to the different disease conditions in which these agents can be beneficial.

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.

Angiotensin IV Receptors Mediate the Cognitive and Cerebrovascular Benefits of Losartan in a Mouse Model of Alzheimer's Disease

The use of angiotensin receptor blockers (ARBs) correlates with reduced onset and progression of Alzheimer's disease (AD). The mechanism depicting how ARBs such as losartan restore cerebrovascular and cognitive deficits in AD is unknown. Here, we propose a mechanism underlying losartan's benefits by selectively blocking the effects of angiotensin IV (AngIV) at its receptor (AT4R) with divalinal in mice overexpressing the AD-related Swedish and Indiana mutations of the human amyloid precursor protein (APP mice) and WT mice. Young (3-month-old) mice were treated with losartan (∼10 mg/kg/d, 4 months), followed by intracerebroventricular administration of vehicle or divalinal in the final month of treatment. Spatial learning and memory were assessed using Morris water mazes at 3 and 4 months of losartan treatment. Cerebrovascular reactivity and whisker-evoked neurovascular coupling responses were measured at end point (∼7 months of age), together with biomarkers related to neuronal and vascular oxidative stress (superoxide dismutase-2), neuroinflammation (astroglial and microglial activation), neurogenesis (BrdU-labeled newborn cells), and amyloidosis [soluble amyloid-β (Aβ) species and Aβ plaque load]. Divalinal countered losartan's capacity to rescue spatial learning and memory and blocked losartan's benefits on dilatory function and baseline nitric oxide bioavailability. Divalinal reverted losartan's anti-inflammatory effects, but failed to modify losartan-mediated reductions in oxidative stress. Neither losartan nor divalinal affected arterial blood pressure or significantly altered the amyloid pathology in APP mice. Our findings identify activation of the AngIV/AT4R cascade as the underlying mechanism in losartan's benefits and a target that could restore Aβ-related cognitive and cerebrovascular deficits in AD.SIGNIFICANCE STATEMENT Antihypertensive medications that target the renin angiotensin system, such as angiotensin receptor blockers (ARBs), have been associated with lower incidence and progression of Alzheimer's disease (AD) in cohort studies. However, the manner by which ARBs mediate their beneficial effects is unknown. Here, the angiotensin IV receptor (AT4R) was identified as mediating the cognitive and cerebrovascular rescue of losartan, a commonly prescribed ARB, in a mouse model of AD. The AT4R was further implicated in mediating anti-inflammatory benefits. AT4R-mediated effects were independent from changes in blood pressure, amyloidosis, and oxidative stress. Overall, our results implicate the angiotensin IV/AT4R cascade as a promising candidate for AD intervention.

The development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's (PD) diseases are neurodegenerative diseases presently without effective drug treatments. AD is characterized by general cognitive impairment, difficulties with memory consolidation and retrieval, and with advanced stages episodes of agitation and anger. AD is increasing in frequency as life expectancy increases. Present FDA approved medications do little to slow disease progression and none address the underlying progressive loss of synaptic connections and neurons. New drug design approaches are needed beyond cholinesterase inhibitors and N-methyl-d-aspartate receptor antagonists. Patients with PD experience the symptomatic triad of bradykinesis, tremor-at-rest, and rigidity with the possibility of additional non-motor symptoms including sleep disturbances, depression, dementia, and autonomic nervous system failure. This review summarizes available information regarding the role of the brain renin-angiotensin system (RAS) in learning and memory and motor functions, with particular emphasis on research results suggesting a link between angiotensin IV (AngIV) interacting with the AT4 receptor subtype. Currently there is controversy over the identity of this AT4 receptor protein. Albiston and colleagues have offered convincing evidence that it is the insulin-regulated aminopeptidase (IRAP). Recently members of our laboratory have presented evidence that the brain AngIV/AT4 receptor system coincides with the brain hepatocyte growth factor/c-Met receptor system. In an effort to resolve this issue we have synthesized a number of small molecule AngIV-based compounds that are metabolically stable, penetrate the blood-brain barrier, and facilitate compromised memory and motor systems. These research efforts are described along with details concerning a recently synthesized molecule, Dihexa that shows promise in overcoming memory and motor dysfunctions by augmenting synaptic connectivity via the formation of new functional synapses.

Memory beyond expression

The idea that memories are not invariable after the consolidation process has led to new perspectives about several mnemonic processes. In this framework, we review our studies on the modulation of memory expression during reconsolidation. We propose that during both memory consolidation and reconsolidation, neuromodulators can determine the probability of the memory trace to guide behavior, i.e. they can either increase or decrease its behavioral expressibility without affecting the potential of persistent memories to be activated and become labile. Our hypothesis is based on the findings that positive modulation of memory expression during reconsolidation occurs even if memories are behaviorally unexpressed. This review discusses the original approach taken in the studies of the crab Neohelice (Chasmagnathus) granulata, which was then successfully applied to test the hypothesis in rodent fear memory. Data presented offers a new way of thinking about both weak trainings and experimental amnesia: memory retrieval can be dissociated from memory expression. Furthermore, the strategy presented here allowed us to show in human declarative memory that the periods in which long-term memory can be activated and become labile during reconsolidation exceeds the periods in which that memory is expressed, providing direct evidence that conscious access to memory is not needed for reconsolidation. Specific controls based on the constraints of reminders to trigger reconsolidation allow us to distinguish between obliterated and unexpressed but activated long-term memories after amnesic treatments, weak trainings and forgetting. In the hypothesis discussed, memory expressibility--the outcome of experience-dependent changes in the potential to behave--is considered as a flexible and modulable attribute of long-term memories. Expression seems to be just one of the possible fates of re-activated memories.

Angiotensin II type 1 receptor blocker losartan prevents and rescues cerebrovascular, neuropathological and cognitive deficits in an Alzheimer's disease model

Angiotensin II (AngII) receptor blockers that bind selectively AngII type 1 (AT1) receptors may protect from Alzheimer's disease (AD). We studied the ability of the AT1 receptor antagonist losartan to cure or prevent AD hallmarks in aged (~18months at endpoint, 3months treatment) or adult (~12months at endpoint, 10months treatment) human amyloid precursor protein (APP) transgenic mice. We tested learning and memory with the Morris water maze, and evaluated neurometabolic and neurovascular coupling using [(18)F]fluoro-2-deoxy-D-glucose-PET and laser Doppler flowmetry responses to whisker stimulation. Cerebrovascular reactivity was assessed with on-line videomicroscopy. We measured protein levels of oxidative stress enzymes (superoxide dismutases SOD1, SOD2 and NADPH oxidase subunit p67phox), and quantified soluble and deposited amyloid-β (Aβ) peptide, glial fibrillary acidic protein (GFAP), AngII receptors AT1 and AT2, angiotensin IV receptor AT4, and cortical cholinergic innervation. In aged APP mice, losartan did not improve learning but it consolidated memory acquisition and recall, and rescued neurovascular and neurometabolic coupling and cerebrovascular dilatory capacity. Losartan normalized cerebrovascular p67phox and SOD2 protein levels and up-regulated those of SOD1. Losartan attenuated astrogliosis, normalized AT1 and AT4 receptor levels, but failed to rescue the cholinergic deficit and the Aβ pathology. Given preventively, losartan protected cognitive function, cerebrovascular reactivity, and AT4 receptor levels. Like in aged APP mice, these benefits occurred without a decrease in soluble Aβ species or plaque load. We conclude that losartan exerts potent preventive and restorative effects on AD hallmarks, possibly by mitigating AT1-initiated oxidative stress and normalizing memory-related AT4 receptors.

Non-canonical signalling and roles of the vasoactive peptides angiotensins and kinins

GPCRs (G-protein-coupled receptors) are among the most important targets for drug discovery due to their ubiquitous expression and participation in cellular events under both healthy and disease conditions. These receptors can be activated by a plethora of ligands, such as ions, odorants, small ligands and peptides, including angiotensins and kinins, which are vasoactive peptides that are classically involved in the pathophysiology of cardiovascular events. These peptides and their corresponding GPCRs have been reported to play roles in other systems and under pathophysiological conditions, such as cancer, central nervous system disorders, metabolic dysfunction and bone resorption. More recently, new mechanisms have been described for the functional regulation of GPCRs, including the transactivation of other signal transduction receptors and the activation of G-protein-independent pathways. The existence of such alternative mechanisms for signal transduction and the discovery of agonists that can preferentially trigger one signalling pathway over other pathways (called biased agonists) have opened new perspectives for the discovery and development of drugs with a higher specificity of action and, therefore, fewer side effects. The present review summarizes the current knowledge on the non-canonical signalling and roles of angiotensins and kinins.

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.

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.

Plasma renin-angiotensin system-regulating aminopeptidase activities are modified in early stage Alzheimer's disease and show gender differences but are not related to apolipoprotein E genotype

Alterations in blood pressure and components of the renin-angiotensin system (RAS) contribute to the development and progression of Alzheimer's disease (AD), resulting in changes that can lead or contribute to cognitive decline. Aspartyl aminopeptidase (ASAP), aminopeptidase A (APA), aminopeptidase N (APN) and aminopeptidase B (APB) catabolise circulating angiotensins, whereas insulin-regulated aminopeptidase (IRAP) has been described as the AT4 receptor. We have found in AD patients a significant decrease of APA activity in men but not in women, and of APN, APB and IRAP in both genders, when compared with control subjects. No changes were found in ASAP activity. Also, APN, APB and IRAP but not APA correlated with the Mini-Mental test, but no relationship with APOE genotype was found. We conclude that several components of the RAS are modified in AD patients, with gender differences. Furthermore, ROC analysis indicates that APN, APB and IRAP activities could be useful non-invasive biomarkers of AD from the earliest stages.

Importance of the brain Angiotensin system in Parkinson's disease

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

Assessment of biological activity of novel peptide analogues of angiotensin IV

Objectives

Angiotensin IV (Ang IV) is a metabolite of angiotensin II which acts on specific AT4 receptors identified as the enzyme insulin regulated aminopeptidase (IRAP). The transduction process of these receptors is unresolved, but Ang IV inhibits the aminopeptidase activity. Ang IV improves cognition in animal models thus there is a desire to develop metabolically stable analogues for further development.

Methods

Peptide analogues of Ang IV were obtained commercially or synthesised. Each peptide was tested in vitro for its ability to inhibit the aminopeptidase activity (IRAP) of mouse brain homogenates and for its effects on isolated rat uterine smooth muscle.

Key findings

[Des-Val1]-Ang IV, acetylated-Ang IV-amide, Ang IV-amide and [des-His4]-Ang IV all inhibited IRAP. [Sar1, Ile8]-Angiotensin II (10 µm) had an effect greater than that of Ang IV or any of the other analogues studied. In isolated uterine smooth muscle, angiotensins II and IV induced contractions, which could be antagonised by an AT1-receptor antagonist. None of the novel peptides induced uterine smooth muscle contractions, but [Sar1, des Arg2-Gly8]-angiotensin II showed significant antagonism of the contractile effects of angiotensin II and carboxyamide-terminated Ang IV-NH2 showed antagonism of Ang IV-induced contractions.

Conclusions

This study provides five novel inhibitors of IRAP worthy of assessment in behavioural models of learning and memory. The analogues are devoid of AT1 receptor agonist properties, and the carboxyamide analogue presents an opportunity to elucidate the mechanism of action of Ang IV as, like Ang IV, it inhibits IRAP, but antagonises the effects of Ang IV on isolated smooth muscle.

(+)-UH 232, a partial agonist of the D3 dopamine receptors, attenuates cognitive effects of angiotensin IV and des-Phe(6)-angiotensin IV in rats

We have recently found that postsynaptic D3 dopamine (DA) receptors appear not to participate in the memory enhancing effects of the angiotensin AT4 receptor agonists angiotensin IV (Ang IV) and des-Phe(6)-Ang IV. In this study we evaluated role of the presynaptic DA D3 receptors in these effects. For that purpose effect of (+)-UH 232, a selective D3 DA receptors partial agonist preferring presynaptic sites, on the pro-cognitive action of intracerebroventricularly (icv) injected Ang IV and des-Phe(6)-Ang IV was examined. Male Wistar rats weighing 180-200 g were used. Both peptides given at the dose of 1 nmol facilitated recall of a passive avoidance (PA) behaviour, improved object recognition (OR), and increased apomorphine-induced stereotype behaviour. In the auxiliary tests performed to control for the unspecific influence of motor (open field, OF) and emotional ('plus' maze, PM) effects of our treatments on the results of the memory tests they had either no (OF) or negligible (PM) effects. Intraperitoneal pre-treatment of the animals with an ineffective on its own dose (1 mg/kg) of (+)-UH 232 abolished or markedly diminished effects of both peptides on PA and OR but did not influence enhancement of stereotypy caused by the peptides.

Influence of up-regulated renin-angiotensin system on the exploration, anxiety-related behavior and object recognition

The renin-angiotensin system (RAS) plays an important role in the development of hypertension and has serious consequences on behaviour. The aim of our study was to investigate the effect of hypertension, induced by up-regulated RAS, on the exploration, anxiety-related behaviour and object recognition in laboratory rats. In the experiment, 12 weeks old normotensive Sprague-Dawley (SD) and hypertensive TGR(mREN2)27 (TGR) male rats with up-regulated RAS were used. In the open-field test, the TGR rats were less active in ambulating, rearing and sniffing and more active in self-grooming and urinating than SD ones. In the elevated plus-maze test, the TGR rats showed lower frequency of total arm entries, closed arm entries and higher frequency of defecation than in controls. In the emergence test, TGR rats did not show significant differences. In the novel object recognition task, the TGR rats spent less time with exploration of both familiar and unfamiliar objects but preferred the novel object over the familiar one and exhibited higher percentage of the total exploring time spent with novel object exploration than SD rats. Our results indicate that the TGR rats are less actively exploring, show some modifications of emotional/anxiety-related behavior and exhibited better recognition abilities.

Involvement of the AT1 receptor subtype in the effects of angiotensin IV and LVV-haemorphin 7 on hippocampal neurotransmitter levels and spatial working memory

Intracerebroventricular (i.c.v.) administration of angiotensin IV (Ang IV) or Leu-Val-Val-haemorphin 7 (LVV-H7) improves memory performance in normal rats and reverses memory deficits in rat models for cognitive impairment. These memory effects were believed to be mediated via the putative 'AT4 receptor'. However, this binding site was identified as insulin-regulated aminopeptidase (IRAP). Correspondingly, Ang IV and LVV-H7 were characterised as IRAP inhibitors. This study investigates whether and how IRAP may be involved in the central effects of Ang IV and LVV-H7. We determined the effects of i.c.v. administration of Ang IV or LVV-H7 on hippocampal neurotransmitter levels using microdialysis in rats. We observed that Ang IV modulates hippocampal acetylcholine levels, whereas LVV-H7 does not. This discrepancy was reflected in the observation that Ang IV binds with micromolar affinity to the AT1 receptor whereas no binding affinity was observed for LVV-H7. Correspondingly, we demonstrated that the AT1 receptor is involved in the effects of Ang IV on hippocampal neurotransmitter levels and on spatial working memory in a plus maze spontaneous alternation task. However, the AT1 receptor was not involved in the spatial memory facilitating effect of LVV-H7. Finally, we demonstrated that Ang IV did not diffuse to the hippocampus following i.c.v. injection, suggesting an extrahippocampal site of action. We propose that AT1 receptors are implicated in the neurochemical and cognitive effects of Ang IV, whereas LVV-H7 may mediate its effects via IRAP.

The brain RAS and Alzheimer's disease

Alzheimer's disease (AD) has become a major world-wide health problem with ever rising costs associated with the treatment and care of afflicted individuals. As life expectancy has increased the occurrence of dementia has also increased. Hypertension during middle adulthood is correlated with a significantly elevated risk of cognitive impairment later in life. Treatment with antihypertensive drugs, particularly angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), has been reported to reduce the likelihood and slow the progression of AD; however, the use of ACE inhibitors may be accompanied by an increase in amyloid beta protein(1-42) accumulation. This review summarizes available information regarding the brain renin-angiotensin system (RAS), and specifically the efficacy of ACE inhibitors as anti-dementia agents, and considers the recently discovered AT(4) receptor and associated agonist drugs as potential new therapeutic targets to treat memory impairments associated with AD. We conclude with a description of recent efforts by members of our laboratory to develop blood-brain barrier penetrant angiotensin IV analogue drugs that facilitate cognition in animal models of AD. These efforts have resulted in a small molecule with desirable hydrophobicity characteristics that shows promise with respect to memory facilitation when peripherally administered.

The angiotensin AT4 receptor subtype as a target for the treatment of memory dysfunction associated with Alzheimer's disease

Over recent years antihypertensive drugs, particularly angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), have been reported to have beneficial effects upon cognitive impairment. Such findings suggest that pharmacological manipulation of angiotensin ligands may be of clinical importance in slowing or halting the cognitive deterioration seen in vascular dementia and Alzheimer's disease. The mechanism(s) underlying these improvements in cognitive function remains unclear; however, important leads are emerging. The angiotensin AT4 receptor subtype, discovered by our laboratory in 1992, influences several important behaviours and physiologies, including learning and memory, and may play a role in this cognitive improvement. This review initially describes the therapeutic drugs approved by the Federal Drug Administration and new approaches presently being developed to treat Alzheimer's disease-induced cognitive impairment. Next, the biologically-active angiotensin ligands and their respective receptor subtypes are discussed, followed by the roles of angiotensin II, angiotensin IV, ACE inhibitors and ARBs in cognitive function. We conclude with a working hypothesis concerning the importance of the AT4 receptor subtype as a new potential drug target for the treatment of Alzheimer's disease-associated memory loss.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Dopamine D4 receptor antagonist L745,870 abolishes cognitive effects of intracerebroventricular angiotensin IV and des-Phe(6)-Ang IV in rats

In this study effect of L745,870, a selective D(4) dopamine (DA) receptor blocker, on the pro-cognitive action of intracerebroventricularly (icv) injected angiotensin IV (Ang IV) and des-Phe(6)-Ang IV was examined. Male Wistar rats weighing 180-200 g were used. Both peptides given at the dose of 1 nmol facilitated recall of a passive avoidance (PA) behaviour, improved object recognition (OR) memory, decreased number of errors, increased number of sequential correct entries and shortened time-to-goal in an eight-arm radial maze (RM). In the auxiliary tests performed to control for the participation of unspecific motor (open field, OF) and emotional ('plus' maze, PM) effects of our treatment in the results of memory tests they had either no (OF) or negligible (PM) effects. Intraperitoneal pretreatment of the animals with 1 mg/kg of L745,870 abolished effects of both peptides on PA and OR and slightly diminished those observed in the eight-arm RM.

Cognitive-enhancing effects of angiotensin IV

Angiotensin IV is a derivative of the potent vasoconstrictor angiotensin II and it has been shown to enhance acquisition, consolidation and recall in animal models of learning and memory when administered centrally or peripherally. Whether changes in angiotensin IV activity underlie the cognitive effects of those cardiovascular drugs designed to disrupt the peripheral renin-angiotensin system in humans remains undetermined, but angiotensin IV appears to be a worthy candidate for consideration in drug development programmes. The mechanism of action of angiotensin IV is still debated, although its AT₄ receptor has been convincingly identified as being insulin-regulated amino peptidase, which is also known as oxytocinase and placental leucine aminopeptidase. It is speculated that angiotensin IV may interact with insulin-regulated amino peptidase to enhance neuronal glucose uptake, prevent metabolism of other neuroactive peptides, induce changes in extracellular matrix molecules, or induce release of acetylcholine and/or dopamine. All of these things may be responsible for the beneficial effects on cognition, but none of them are yet proven. Importantly, strain differences in murine responses to angiotensin IV suggest that some individuals may benefit from drugs targeted to the AT₄ receptor whilst others may be refractory. At present it thus appears that those individuals with the poorest baseline cognition may receive greatest benefit, but possible genetic differences in responses to angiotensin IV cannot be ruled-out.

Effect of D(3) dopamine receptors blockade on the cognitive effects of angiotensin IV in rats

Our previous studies showed that D(1) and D(2) dopamine receptors are indispensable for the cognitive effects of angiotensin IV (Ang IV) and its des-Phe(6) derivative des-Phe(6)-Ang IV to occur. As most neuroleptics currently used in the treatment of schizophrenia have variable D(2)/D(3) dopaminolytic selectivity, in this study we searched for the role of the D(3) dopamine receptors in facilitating learning and improving memory actions of Ang IV and des-Phe(6)-Ang IV in rats. For this purpose, we evaluated the recall of the passive avoidance (PA) behaviour, object recognition (OR) memory, and the spatial working memory (WM) in rats treated with the intraperitoneal (i.p.) nafadotride (N[(n-butyl-2-pyrrolidinyl)methyl]-1-methoxy-4-cyanonaphtalene-2-carboxamide), a highly selective D(3) receptor blocker and then by an intracerebroventricular (i.c.v.) Ang IV or des-Phe(6)-Ang IV. Separate groups of rats receiving the same treatments were run to check for the possible participation of unspecific motor (open field) or emotioned (elevated "plus" maze) effects of our treatments in the results of the cognitive tests. The results revealed Ang IV to express its improving recall of PA, OR memory and WM action roughly similarly in all groups showing only minor or null significance of the D(3) receptors blockade. Interestingly, in the nafadotride pretreated rats, des-Phe(6)-Ang IV beneficial effect on the recall of the PA was weaker than that of Ang IV. Improvement of the spatial WM in an eight-arm radial maze, similar after Ang IV and des-Phe(6)-Ang IV, was not significantly affected by nafadotride. There were no motor and only minor anxiogenic effects of Ang IV and des-Phe(6)-Ang IV abolished by nafadotride in the former case. In conclusion, this study points to the minor significance of the D(3) dopamine receptors in the cognitive effects of Ang IV and to the interesting, though unexplained, inhibition by nafadotride of the des-Phe(6)-Ang IV effects.

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.

The effect of angiotensin II and IV on ERK1/2 and CREB signalling in cultured rat astroglial cells

Angiotensin peptides produced by the brain renin-angiotensin system have established roles in cognition, but there is no mechanistic basis of angiotensin effects on memory. Astroglial cells present throughout the whole brain, synthesize all the components of the renin-angiotensin system and express angiotensin receptors; therefore our aim was to assess changes in intracellular signalling pathways related to memory formation, particularly the activation of CREB and ERK1/2 in astroglial cells grown in the presence of angiotensin peptides. Cultured rat astroglial cells were treated for 24 h with 10 microM angiotensin II and/or 10 microM angiotensin IV in the presence or absence of 100 microM losartan (AT1-receptor antagonist) or 100 microM PD123319 (AT2-receptor antagonist). Both angiotensin peptides alone were without effect on culture protein levels and cell viability and did not induce oxidative stress, but both peptides together slightly elevated cell growth rates and increased damaged, apoptotic cell numbers. This effect was most probably mediated by the AT1 receptor. Angiotensin II but not angiotensin IV increased intracellular calcium via activation of AT1 receptor. Angiotensin IV but not angiotensin II increased extracellular-regulated protein kinases 1 and 2 (ERK1/2) by 65% and T202, T204 phosphorylated ERK1/2 levels by 36%; this effect was blocked in part by both losartan and PD123319. Angiotensin II but not angiotensin IV increased cyclic AMP-responsive element binding protein (CREB) expression by almost 100% and elevated Ser 133-phosphorylated CREB levels by 56%. These effects were also inhibited in part by both losartan and PD123319. Our results indicate that CREB activation in cultured rat glial cells is mediated mostly by angiotensin II. Angiotensin IV appears to affect the ERK1/2 pathway.