Angiotensin II inhibits long-term potentiation within the lateral nucleus of the amygdala through AT1 receptors

Potential molecular pathways of angiotensin receptor blockers in the brain toward cognitive improvement in dementia

The alarming rise of cognitive impairment and memory decline and limited effective solutions present a worldwide concern for dementia patients. The multivariant role of the renin-angiotensin system (RAS) in the brain offers strong evidence of a role for angiotensin receptor blockers (ARBs) in the management of memory impairment by modifying glutamate excitotoxicity, downregulating inflammatory cytokines such as interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)α, inhibiting kynurenine aminotransferase (KAT)-II, nucleotide-binding domain, leucine-rich-containing family and pyrin-domain-containing-3 (NLRP3) inflammasomes, boosting cholinergic activity, activating peroxisome proliferator-activated receptor (PPAR)-γ, countering cyclooxygenase (COX) and mitigating the hypoxic condition. The present work focuses on the intricate molecular mechanisms involved in brain-RAS, highlighting the role of ARBs, connecting links between evidence-based unexplored pathways and investigating probable biomarkers involved in dementia through supported preclinical and clinical literature.

From cardiovascular system to brain, the potential protective role of Mas Receptors in COVID-19 infection

Coronavirus disease 2019 (COVID-19) has been declared a new pandemic in March 2020. Although most patients are asymptomatic, those with underlying cardiovascular comorbidities may develop a more severe systemic infection which is often associated with fatal pneumonia. Nonetheless, neurological and cardiovascular manifestations could be present even without respiratory symptoms. To date, no COVID-19-specific drugs are able for preventing or treating the infection and generally, the symptoms are relieved with general anti-inflammatory drugs. Angiotensin-converting-enzyme 2 (ACE2) may function as the receptor for virus entry within the cells favoring the progression of infection in the organism. On the other hand, ACE2 is a relevant enzyme in renin angiotensin system (RAS) cascade fostering Ang1-7/Mas receptor activation which promotes protective effects in neurological and cardiovascular systems. It is known that RAS is composed by two functional countervailing axes the ACE/AngII/AT1 receptor and the ACE/AngII/AT2 receptor which counteracts the actions mediated by AngII/AT1 receptor by inducing anti-inflammatory, antioxidant and anti-growth functions. Subsequently an "alternative" ACE2/Ang1-7/Mas receptor axis has been described with functions similar to the latter protective arm. Here, we discuss the neurological and cardiovascular effects of COVID-19 highlighting the role of the stimulation of the RAS "alternative" protective arm in attenuating pulmonary, cerebral and cardiovascular damages. In conclusion, only two clinical trials are running for Mas receptor agonists but few other molecules are in preclinical phase and if successful these drugs might represent a successful strategy for the treatment of the acute phase of COVID-19 infection.

The Renin Angiotensin System as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a major public health problem, with limited pharmacological options available beyond symptomatic relief. The renin angiotensin system (RAS) is primarily known as a systemic endocrine regulatory system, with major roles controlling blood pressure and fluid homeostasis. Drugs that target the RAS are used to treat hypertension, heart failure and kidney disorders. They have now been used chronically by millions of people and have a favorable safety profile. In addition to the systemic RAS, it is now appreciated that many different organ systems, including the brain, have their own local RAS. The major ligand of the classic RAS, Angiotensin II (Ang II) acts predominantly through the Ang II Type 1 receptor (AT1R), leading to vasoconstriction, inflammation, and heightened oxidative stress. These processes can exacerbate brain injuries. Ang II receptor blockers (ARBs) are AT1R antagonists. They have been shown in several preclinical studies to enhance recovery from TBI in rodents through improvements in molecular, cellular and behavioral correlates of injury. ARBs are now under consideration for clinical trials in TBI. Several different RAS peptides that signal through receptors distinct from the AT1R, are also potential therapeutic targets for TBI. The counter regulatory RAS pathway has actions that oppose those stimulated by AT1R signaling. This alternative pathway has many beneficial effects on cells in the central nervous system, bringing about vasodilation, and having anti-inflammatory and anti-oxidative stress actions. Stimulation of this pathway also has potential therapeutic value for the treatment of TBI. This comprehensive review will provide an overview of the various components of the RAS, with a focus on their direct relevance to TBI pathology. It will explore different therapeutic agents that modulate this system and assess their potential efficacy in treating TBI patients.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3 , APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ. We found that compared to E3FAD mice, E4FAD mice had lower basal synaptic activity, but higher levels of paired pulse facilitation (PPF) and Long-Term Potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aβ are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype and angiotensin II in AD.

AT1 Receptors: Their Actions from Hypertension to Cognitive Impairment

Hypertension is one of the most prevalent cardiovascular disorders worldwide, affecting 1.13 billion people, or 14% of the global population. Hypertension is the single biggest risk factor for cerebrovascular dysfunction. According to the American Heart Association, high blood pressure (BP), especially in middle-aged individuals (~ 40 to 60 years old), is associated with an increased risk of dementia, later in life. Alzheimer’s disease and cerebrovascular disease are the two leading causes of dementia, accounting for around 80% of the total cases and usually combining mixed pathologies from both. Little is known regarding how hypertension affects cognitive function, so the impact of its treatment on cognitive impairment has been difficult to assess. The brain renin-angiotensin system (RAS) is essential for BP regulation and overactivity of this system has been established to precede the development and maintenance of hypertension. Angiotensin II (Ang-II), the main peptide within this system, induces vasoconstriction and impairs neuro-vascular coupling by acting on brain Ang-II type 1 receptors (AT₁R). In this review, we systemically analyzed the association between RAS and biological mechanisms of cognitive impairment, from the perspective of AT₁R located in the central nervous system. Additionally, the possible contribution of brain AT₁R to global cognition decline in COVID-19 cases will be discussed as well.

The Renin-Angiotensin System in Huntington's Disease: Villain or Hero?

Huntington’s Disease (HD) is an autosomal dominant, progressive neurodegenerative disorder characterized by severe symptoms, including motor impairment, cognitive decline, and psychiatric alterations. Several systems, molecules, and mediators have been associated with the pathophysiology of HD. Among these, there is the Renin-Angiotensin System (RAS), a peptide hormone system that has been associated with the pathology of neuropsychiatric and neurodegenerative disorders. Important alterations in this system have been demonstrated in HD. However, the role of RAS components in HD is still unclear and needs further investigation. Nonetheless, modulation of the RAS components may represent a potential therapeutic strategy for the treatment of HD.

How Does SARS-CoV-2 Affect the Central Nervous System? A Working Hypothesis

Interstitial pneumonia was the first manifestation to be recognized as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, in just a few weeks, it became clear that the coronavirus disease-2019 (COVID-19) overrun tissues and more body organs than just the lungs, so much so that it could be considered a systemic pathology. Several studies reported the involvement of the conjunctiva, the gut, the heart and its pace, and vascular injuries such as thromboembolic complications and Kawasaki disease in children and toddlers were also described. More recently, it was reported that in a sample of 214 SARS-CoV-2 positive patients, 36.4% complained of neurological symptoms ranging from non-specific manifestations (dizziness, headache, and seizures), to more specific symptoms such hyposmia or hypogeusia, and stroke. Older individuals, especially males with comorbidities, appear to be at the highest risk of developing such severe complications related to the Central Nervous System (CNS) involvement. Neuropsychiatric manifestations in COVID-19 appear to develop in patients with and without pre-existing neurological disorders. Growing evidence suggests that SARS-CoV-2 binds to the human Angiotensin-Converting Enzyme 2 (ACE2) for the attachment and entrance inside host cells. By describing ACE2 and the whole Renin Angiotensin Aldosterone System (RAAS) we may better understand whether specific cell types may be affected by SARS-CoV-2 and whether their functioning can be disrupted in case of an infection. Since clear evidences of neurological interest have already been shown, by clarifying the topographical distribution and density of ACE2, we will be able to speculate how SARS-CoV-2 may affect the CNS and what is the pathogenetic mechanism by which it contributes to the specific clinical manifestations of the disease. Based on such evidences, we finally hypothesize the process of SARS-CoV-2 invasion of the CNS and provide a possible explanation for the onset or the exacerbation of some common neuropsychiatric disorders in the elderly including cognitive impairment and Alzheimer disease.

Angiotensin II Type 2 Receptor-Expressing Neurons in the Central Amygdala Influence Fear-Related Behavior

BACKGROUND

The renin-angiotensin system has been implicated in posttraumatic stress disorder; however, the mechanisms responsible for this connection and the therapeutic potential of targeting the renin-angiotensin system in posttraumatic stress disorder remain unknown. Using an angiotensin receptor bacterial artificial chromosome (BAC) and enhanced green fluorescent protein (eGFP) reporter mouse, combined with neuroanatomical, pharmacological, and behavioral approaches, we examined the role of angiotensin II type 2 receptor (AT₂R) in fear-related behavior.

METHODS

Dual immunohistochemistry with retrograde labeling was used to characterize AT₂R-eGFP⁺ cells in the amygdala of the AT₂R-eGFP-BAC reporter mouse. Pavlovian fear conditioning and behavioral pharmacological analyses were used to demonstrate the effects of AT₂R activation on fear memory in male C57BL/6 mice.

RESULTS

AT₂R-eGFP⁺ neurons in the amygdala were predominantly expressed in the medial amygdala and the medial division of the central amygdala (CeM), with little AT₂R-eGFP expression in the basolateral amygdala or lateral division of the central amygdala. Characterization of AT₂R-eGFP⁺ neurons in the CeM demonstrated distinct localization to gamma-aminobutyric acidergic projection neurons. Mice receiving acute intra-central amygdala injections of the selective AT₂R agonist compound 21 prior to tests for cued or contextual fear expression displayed less freezing. Retrograde labeling of AT₂R-eGFP⁺ neurons projecting to the periaqueductal gray revealed AT₂R-eGFP⁺ neuronal projections from the CeM to the periaqueductal gray, a key brain structure mediating fear-related freezing.

CONCLUSIONS

These findings suggest that CeM AT₂R-expressing neurons can modulate central amygdala outputs that play a role in fear expression, providing new evidence for a novel angiotensinergic circuit in the regulation of fear.

Roles and functions of Atp6ap2 in the brain

The classical renin-angiotensin system (RAS) in the body has been studied intensively in the last decades, since it is known that this system is involved in the regulation of blood pressure. Since nearly all members of the classical RAS have also been identified within the brain in the last decades and due to the existence of the blood-brain barrier, a RAS within the brain (bRAS) that is largely independent from the peripheral RAS has been postulated. All members of the angiotensin family as e.g., angiotensin II, angiotensin IV and angiotensin II (1-7) along with the respective receptors (e.g., angiotensin II receptor type 1 (AT1), angiotensin II receptor type 2 (AT2), angiotensin IV receptor (AT4), angiotensin II (1-7) receptor (Mas)) have been identified within the brain. Moreover, a receptor capable of binding renin and the renin precursor prorenin with high affinity has also been detected within the brain. This protein functions as a membrane receptor for (pro)renin and also represents a V-ATPase subunit and is therefore termed (P)RR or Atp6ap2, respectively. In this review we shed light on the (known as well as putative) roles and functions of Atp6ap2 in the brain under physiological and pathophysiological conditions.

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.

Long-term intracerebroventricular infusion of angiotensin II after kainate-induced status epilepticus: Effects on epileptogenesis, brain damage, and diurnal behavioral changes

Our previous studies revealed that Angiotensin (Ang) II has anticonvulsant effects in acute seizure models. However, data on its role in experimental models of epilepsy are missing. In the present study, we tested whether posttreatment with Ang II after kainate (KA)-induced status epilepticus (SE) can affect epileptogenesis, concomitant behavioral changes, and brain damage. The Wistar rats were intracerebroventricularly infused via osmotic mini-pumps with Ang II (1.52μg/μl/day for 28days) after SE. Spontaneous motor seizures (SMS) were video-recorded for up to three months. Locomotor activity, anxiety, and depression-like behavior were evaluated during the last week of drug infusion, while spatial memory was assessed during the 3rd month after SE. Angiotensin II decreased the latency for onset of the first SMS and increased the frequency of SMS two months after SE. The continuous peptide infusion exacerbated the KA-induced hyperactivity and caused depression-like behavior. The reduced anxiety of KA-treated rats was alleviated by Ang II exposure. The KA-induced deficit in the hippocampal-dependent spatial memory was not influenced by Ang II. However, Ang II partially prevented the neuronal damage in the hippocampus, specifically in the CA1 area. The role of AT1 and AT2 receptor activation in the effects of the octapeptide is discussed.

Angiotensin type 1a receptors on corticotropin-releasing factor neurons contribute to the expression of conditioned fear

Although generally associated with cardiovascular regulation, angiotensin II receptor type 1a (AT1a R) blockade in mouse models and humans has also been associated with enhanced fear extinction and decreased post-traumatic stress disorder (PTSD) symptom severity, respectively. The mechanisms mediating these effects remain unknown, but may involve alterations in the activities of corticotropin-releasing factor (CRF)-expressing cells, which are known to be involved in fear regulation. To test the hypothesis that AT1a R signaling in CRFergic neurons is involved in conditioned fear expression, we generated and characterized a conditional knockout mouse strain with a deletion of the AT1a R gene from its CRF-releasing cells (CRF-AT1a R((-/-)) ). These mice exhibit normal baseline heart rate, blood pressure, anxiety and locomotion, and freeze at normal levels during acquisition of auditory fear conditioning. However, CRF-AT1a R((-/-)) mice exhibit less freezing than wild-type mice during tests of conditioned fear expression-an effect that may be caused by a decrease in the consolidation of fear memory. These results suggest that central AT1a R activity in CRF-expressing cells plays a role in the expression of conditioned fear, and identify CRFergic cells as a population on which AT1 R antagonists may act to modulate fear extinction.

Effects of AT1 receptor antagonism on kainate-induced seizures and concomitant changes in hippocampal extracellular noradrenaline, serotonin, and dopamine levels in Wistar-Kyoto and spontaneously hypertensive rats

In the management of epilepsy, AT1 receptor antagonists have been suggested as an additional treatment strategy. A hyperactive brain angiotensin (Ang) II system and upregulated AT1 receptors are implicated in the cerebrovascular alterations in a genetic form of hypertension. Uncontrolled hypertension could also, in turn, be a risk factor for a seizure threshold decrease and development of epileptogenesis. The present study aimed to assess the effects of the selective AT1 receptor antagonist ZD7155 on kainic acid (KA)-induced status epilepticus (SE) development and accompanying changes in the hippocampal extracellular (EC) neurotransmitter levels of noradrenaline (NAD), serotonin (5-HT), and dopamine (DA) in spontaneously hypertensive rats (SHRs) and their parent strain Wistar-Kyoto (WKY) rats, since monoamines are well-known neurotransmitters involved in mechanisms of both epilepsy and hypertension. Status epilepticus was evoked in freely moving rats by a repetitive intraperitoneal (i.p.) administration of KA in subconvulsant doses. In the treatment group, ZD7155 (5mg/kg i.p.) was coadministered with the first KA injection. Spontaneously hypertensive rats exhibited higher susceptibility to SE than WKY rats, but the AT1 receptor antagonist did not alter the development of SE in SHRs or in WKY rats. In vivo microdialysis demonstrated significant KA-induced increases of the hippocampal NAD and DA levels in SHRs and of NAD, 5-HT, and DA in WKY rats. Although SHRs developed more severe seizures while receiving a lower dose of KA compared to WKY rats, AT1 receptor antagonism completely prevented all KA-induced increases of hippocampal monoamine levels in both rat strains without affecting seizure development per se. These results suggest a lack of direct relationship between KA-induced seizure susceptibility and adaptive changes of hippocampal NAD, 5-HT, and DA levels in the effects of ZD7155 in WKY rats and SHRs.

Angiotensin and Alzheimer´s disease: therapeutic prospects

This review describes the features of Alzheimer's disease and discusses the evidence that antihypertensive therapies may be beneficial in its treatment and prevention. Drugs acting via the renin-angiotensin system are considered and it is suggested that these drugs may produce their effects via mechanisms other than by their antihypertensive actions. Using evidence from animal studies, the role of angiotensin as a neurotransmitter and its involvement in the control of normal cognitive function is described. Studies of angiotensin and human cognition are reviewed along with studies investigating the possible link between disorders of the system, either inherited or acquired, and the symptoms of Alzheimer's disease. The therapeutic potential of screening for markers of renin-angiotensin abnormality for the prediction of Alzheimer's disease is considered, as is the potential use of agents known to influence the renin-angiotensin system in the treatment or prevention of the disease.

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.

Renin Angiotensin System in Cognitive Function and Dementia

Angiotensin II represents a key molecule in hypertension and cerebrovascular pathology. By promoting inflammation and oxidative stress, enhanced Ang II levels accelerate the onset and progression of cell senescence. Sustained activation of RAS promotes end-stage organ injury associated with aging and results in cognitive impairment and dementia. The discovery of the angiotensin-converting enzyme ACE2-angiotensin (1–7)-Mas receptor axis that exerts vasodilator, antiproliferative, and antifibrotic actions opposed to those of the ACE-Ang II-AT1 receptor axis has led to the hypothesis that a decrease in the expression or activity of angiotensin (1–7) renders the systems more susceptible to the pathological actions of Ang II. Given the successful demonstration of beneficial effects of increased expression of ACE2/formation of Ang1–7/Mas receptor binding and modulation of Mas expression in animal models in containing cerebrovascular pathology in hypertensive conditions and aging, one could reasonably hope for analogous effects regarding the prevention of cognitive decline by protecting against hypertension and cerebral microvascular damage. Upregulation of ACE2 and increased balance of Ang 1–7/Ang II, along with positive modulation of Ang II signaling through AT2 receptors and Ang 1–7 signaling through Mas receptors, may be an appropriate strategy for improving cognitive function and treating dementia.

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.

Angiotensin-(1-7) central administration induces anxiolytic-like effects in elevated plus maze and decreased oxidative stress in the amygdala

There is increasing evidence that besides the well-known angiotensin (Ang) II, other renin-angiotensin system (RAS) peptides, including Ang-(1-7), could have important effects at the central level. However, very few things are known about the central actions of Ang-(1-7), while the effects of its administration alone on anxiety have not been tested to date, to the best of our knowledge. In this way, we were interested in studying the effects of Ang-(1-7) intracerebroventricular administration on anxiety levels, as studied through some main behavioral parameters in the elevated plus maze, as well as the importance of Ang-(1-7) in the oxidative stress status from the amygdala, which is one of the key brain regions involved in mediating anxiety. We report here a possible anxiolytic-like effect of Ang-(1-7) administration, as demonstrated by the increased percentage of time spent and frequency of entries in the open arms of the elevated plus maze, as well as increased head-dipping behavior in the open arms and decreased stretching in closed arms. Also some antioxidant effects of Ang-(1-7) are suggested since a significant increase of GPX specific activity and a decrease of the main peroxidation marker MDA were observed in the amygdala. Moreover, we found a significant correlation between most of the behavioral parameters in the elevated plus maze and the levels of the oxidative stress markers. However, further studies are necessary in order to elucidate the effects of Ang-(1-7) administration on anxiety and oxidative stress status and also on the possible correlation that might exists between these aspects.

Cortical response to somatosensory stimulation in medication overuse headache patients is influenced by angiotensin converting enzyme (ACE) I/D genetic polymorphism

BACKGROUND

Medication overuse headache (MOH) is a disabling health problem. Convincing evidence attributes a pathophysiologic role to central sensitization. By recording somatosensory evoked potentials (SSEPs) in patients with MOH, we observed increased sensitization and deficient habituation to repetitive sensory stimuli consistent with drug overuse. The renin-angiotensin system in the brain seems to play a relevant role in neural plasticity and dependence behavior. We therefore sought differences in SSEP sensitization and habituation in patients with MOH who underwent angiotensin converting enzyme (ACE) I/D polymorphism analysis.

METHODS

We recorded median-nerve SSEPs (two blocks of 100 sweeps) in 43 patients with MOH. We measured N20-P25 amplitudes, and assessed sensitization using the first block amplitudes, and habituation using amplitude changes between the two sequential blocks. According to their genotype, subjects were divided into three groups: "D/D", "D/I" and "I/I" carriers.

RESULTS

The habituation slope of the two SSEP block amplitudes was significantly increased in the D/D subgroup (n = 16) with respect to that of the I/I subgroup (n = 6), with the D/I subgroup (n = 21) falling in between. In D/D carriers, the habituation slope correlated positively with the duration of the overuse headache, and the first SSEP block amplitudes, a measure of sensitization, increased in strict relationship with the type of overused medication in the MOH patients overall and in the D/D subgroup; this was not so in the D/I and I/I subgroups.

CONCLUSION

In patients with MOH, the homozygote D/D ACE polymorphism influences habituation and sensitization to repeated sensory stimuli in strict relationship with medication overuse. We suggest that angiotensin peptides influence neuronal mechanisms of plasticity by interacting with central monoaminergic synaptic transmission.

Distribution of angiotensin type 1a receptor-containing cells in the brains of bacterial artificial chromosome transgenic mice

In the central nervous system, angiotensin II (AngII) binds to angiotensin type 1 receptors (AT(1)Rs) to affect autonomic and endocrine functions as well as learning and memory. However, understanding the function of cells containing AT(1)Rs has been restricted by limited availability of specific antisera, difficulties discriminating AT(1)R-immunoreactive cells in many brain regions and, the identification of AT(1)R-containing neurons for physiological and molecular studies. Here, we demonstrate that an Agtr1a bacterial artificial chromosome (BAC) transgenic mouse line that expresses type A AT(1)Rs (AT1aRs) identified by enhanced green fluorescent protein (EGFP) overcomes these shortcomings. Throughout the brain, AT1aR-EGFP was detected in the nuclei and cytoplasm of cells, most of which were neurons. EGFP often extended into dendritic processes and could be identified either natively or with immunolabeling of GFP. The distribution of AT1aR-EGFP cells in brain closely corresponded to that reported for AngII binding and AT1aR protein and mRNA. In particular, AT1aR-EGFP cells were in autonomic regions (e.g., hypothalamic paraventricular nucleus, central nucleus of the amygdala, parabrachial nucleus, nuclei of the solitary tract and rostral ventrolateral medulla) and in regions involved in electrolyte and fluid balance (i.e., subfornical organ) and learning and memory (i.e., cerebral cortex and hippocampus). Additionally, dual label electron microscopic studies in select brain areas demonstrate that cells containing AT1aR-EGFP colocalize with AT(1)R-immunoreactivity. Assessment of AngII-induced free radical production in isolated EGFP cells demonstrated feasibility of studies investigating AT1aR signaling ex vivo. These findings support the utility of Agtr1a BAC transgenic reporter mice for future studies understanding the role of AT(1)R-containing cells in brain function.

Capsaicin-induced changes in LTP in the lateral amygdala are mediated by TRPV1

The transient receptor potential vanilloid type 1 (TRPV1) channel is a well recognized polymodal signal detector that is activated by painful stimuli such as capsaicin. Here, we show that TRPV1 is expressed in the lateral nucleus of the amygdala (LA). Despite the fact that the central amygdala displays the highest neuronal density, the highest density of TRPV1 labeled neurons was found within the nuclei of the basolateral complex of the amygdala. Capsaicin specifically changed the magnitude of long-term potentiation (LTP) in the LA in brain slices of mice depending on the anesthetic (ether, isoflurane) used before euthanasia. After ether anesthesia, capsaicin had a suppressive effect on LA-LTP both in patch clamp and in extracellular recordings. The capsaicin-induced reduction of LTP was completely blocked by the nitric oxide synthase (NOS) inhibitor L-NAME and was absent in neuronal NOS as well as in TRPV1 deficient mice. The specific antagonist of cannabinoid receptor type 1 (CB1), AM 251, was also able to reduce the inhibitory effect of capsaicin on LA-LTP, suggesting that stimulation of TRPV1 provokes the generation of anandamide in the brain which seems to inhibit NO synthesis. After isoflurane anesthesia before euthanasia capsaicin caused a TRPV1-mediated increase in the magnitude of LA-LTP. Therefore, our results also indicate that the appropriate choice of the anesthetics used is an important consideration when brain plasticity and the action of endovanilloids will be evaluated. In summary, our results demonstrate that TRPV1 may be involved in the amygdala control of learning mechanisms.

Different isoforms of nitric oxide synthase are involved in angiotensin-(1-7)-mediated plasticity changes in the amygdala in a gender-dependent manner

BACKGROUND

The amygdala receives afferent sensory input and processes information related to hydromineral balance. Angiotensin acts on and through the amygdala to stimulate thirst and sodium appetite. In addition, different angiotensins seem to play a role in cognition and learning mechanisms by acting on and through the amygdala. Recently, we showed that angiotensin-(1-7) (Ang-(1-7)) enhances the magnitude of long-term potentiation (LTP) in the lateral nucleus of the amygdala (LA) via the Mas receptor.

METHODS

Extracellular field potentials were measured in the LA.

RESULTS

LA-LTP induced by stimulation of the external capsule was nitric oxide (NO)-dependent because the NO synthase (NOS) inhibitor L-NAME reduced LA-LTP. The LA-LTP was also reduced in both male and female nNOS and eNOS knockout mice. In male eNOS(-/-) mice, Ang-(1-7) enhanced LA-LTP, whereas the LTP-enhancing effect of Ang-(1-7) was missing in female eNOS(-/-) mice. Therefore, the LTP-enhancing effect of Ang-(1-7) was mediated by eNOS in females. In contrast, Ang-(1-7) strongly enhanced the LTP in nNOS(-/-) females, whereas the effect of Ang-(1-7) was missing in nNOS(-/-) males. Thus, Ang-(1-7) induced an increase in the magnitude of LTP via the involvement of nNOS in males.

CONCLUSION

Our data support not only the hypothesis that NO contributes to plasticity changes in the lateral amygdala, but also show for the first time a gender-dependent involvement of different isoforms of NOS in the mediation of Ang-(1-7) on LTP in the amygdala.

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.

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.

Evidence for a role of AT(2) receptors at the CVLM in the cardiovascular changes induced by low-intensity physical activity in renovascular hypertensive rats

In the present study, we evaluated the involvement of the rennin-angiotensin system (RAS) in the control of the blood pressure (BP), baroreceptor-mediated bradycardia and the reactivity of caudal ventrolateral medulla (CVLM) neurons to Ang II and to AT(2) receptor antagonist in sedentary or trained renovascular hypertensive rats. Physical activity did not significantly change the baseline mean arterial pressure (MAP), heart rate (HR) or the sensitivity of the baroreflex bradycardia in normotensive Sham rats. However, in 2K1C hypertensive rats, physical activity induced a significant fall in baseline MAP and HR and produced an improvement of the baroreflex function (bradycardic component). The microinjections of Ang II into the CVLM produced similar decreases in MAP in all groups, Sham and 2K1C, sedentary and trained rats. The hypotensive effect of Ang II at the CVLM was blocked by previous microinjection of the AT(2) receptors antagonist, PD123319, in all groups of rats. Unexpectedly, microinjection of PD123319 at the CVLM produced a depressor effect in 2K1C sedentary that was attenuated in 2K1C trained rats. No significant changes in MAP were observed after PD123319 in Sham rats, sedentary or trained. These data showed that low-intensity physical activity is effective in lowering blood pressure and restoring the sensitivity of the baroreflex bradycardia, however these cardiovascular effects are not accompanied by changes in the responsiveness to Ang II at CVLM in normotensive or hypertensive, 2K1C rats. In addition, the blood pressure changes observed after AT(2) blockade in 2K1C rats suggest that hypertension may trigger an imbalance of AT(1)/AT(2) receptors at the CVLM that may be restored, at least in part, by low-intensity physical activity.

Angiotensin-(1-7)-induced plasticity changes in the lateral amygdala are mediated by COX-2 and NO

It is known from studies outside the brain that upon binding to its receptor, angiotensin-(1-7) elicits the release of prostanoids and nitric oxide (NO). Cyclooxygenase (COX) is a key enzyme that converts arachidonic acid to prostaglandins. Since there are no data available so far on the role of COX-2 in the amygdala, in a first step we demonstrated that the selective COX-2 inhibitor NS-398 significantly reduced the probability of long-term potentiation (LTP) induction in the lateral nucleus of the amygdala. Similarly, in COX-2(-/-) mice, LTP induced by external capsule (EC) stimulation was impaired. Second, we evaluated the action of angiotensin-(1-7) in the amygdala. In wild-type mice, angiotensin-(1-7) increased LTP. This LTP-enhancing effect of Ang-(1-7) was not observed in COX-2(+/-) mice. However, in COX-2(-/-) mice, Ang-(1-7) caused an enhancement of LTP similar to that in wild-type mice. The NO synthetase inhibitor L-NAME blocked this angiotensin-(1-7)-induced increase in LTP in COX-2(-/-) mice. Low-frequency stimulation of external capsule fibers did not cause long-term depression (LTD) in drug-free and angiotensin-(1-7)-treated brain slices in wild-type mice. In contrast, in COX-2(-/-) mice, angiotensin-(1-7) caused stable LTD. Increasing NO concentration by the NO-donor SNAP also caused LTD in wild-type mice. Our study shows for the first time that LTP in the amygdala is dependent on COX-2 activity. Moreover, COX-2 is involved in the mediation of angiotensin-(1-7) effects on LTP. Finally, it is recognized that there is a molecular cross-talk between COX-2 and NO that may regulate synaptic plasticity.

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

This study tests the hypothesis that the facilitation of learning and improvement of memory observed after an intracerebroventricular (i.c.v.) injection of angiotensin II (Ang II) is, in fact, caused by its derivative angiotensin IV (Ang IV). We ran two memory tests as well as an auxiliary test assessing motor performance in rats injected (i.c.v., 1 nmol in 2 microl saline) with Ang II or Ang IV. There were separate groups receiving peptide or saline five, 10 and 15 minutes before testing. Ang IV significantly increased step-through latencies in a passive avoidance paradigm as well as improved discrimination between familiar and unfamiliar objects in an object recognition test in all groups showing better retrieval of memory of aversive as well as appetitive stimuli in the peptide-treated groups regardless of the time of its injection. In contrast, rats treated with Ang II demonstrated significant improvement of memory of aversive and appetitive stimuli in the same tests only 15 minutes after its i.c.v. injection, with no effect in the groups injected five minutes before testing and slight efficacy in those injected 10 minutes before the test. Numbers of crossings, rearings and bar approaches in an open field were similar both in the peptide-treated and control groups making it unlikely that changes in motor performance affected the memory tests. In line with the present views on the intracellular metabolism of Ang II, these results suggest degradation to Ang IV by aminopeptidases A and N is necessary before the cognitive effects can occur.

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.

Angiotensin II disrupts inhibitory avoidance memory retrieval

The brain renin-angiotensin system (RAS) is involved in learning and memory, but the actual role of angiotensin II (A(II)) and its metabolites in this process has been difficult to comprehend. This has been so mainly due to procedural issues, especially the use of multi-trial learning paradigms and the utilization of pre-training intracerebroventricular infusion of RAS-acting compounds. Here, we specifically analyzed the action of A(II) in aversive memory retrieval using a hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs. Rats bilaterally implanted with infusion cannulae aimed to the CA1 region of the dorsal hippocampus were trained in IA and tested for memory retention 24 h later. We found that when given into CA1 15 min before IA memory retention test, A(II), but not angiotensin IV or angiotensin(1-7) induced a dose-dependent and reversible amnesia without altering locomotor activity, exploratory behavior or anxiety state. The effect of A(II) was blocked in a dose-dependent manner by the A(II)-type 2 receptor (AT(2)) antagonist PD123319 but not by the A(II)-type 1 receptor (AT(1)) antagonist losartan. By themselves, neither PD123319 nor losartan had any effect on memory expression. Our data indicate that intra-CA1 A(II) hinders retrieval of avoidance memory through a process that involves activation of AT(2) receptors.

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.

Repeated ethanol exposure and withdrawal impairs human fear conditioning and depresses long-term potentiation in rat amygdala and hippocampus

BACKGROUND

In rats, repeated episodes of alcohol consumption and withdrawal (RWD) impair fear conditioning to discrete cues.

METHODS

Fear conditioning was measured in human binge drinkers as the increased startle response in the presence of a CS+ conditioned to aversive white noise. Secondly, the ability of tone CSs, paired with footshock, to induce c-fos expression, a marker of neuronal activity, in limbic structures subserving emotion was studied in rats. Additionally, consequences of RWD on subsequent induction of long term potentiation (LTP) in external capsule/lateral amygdala and Schaffer collateral/hippocampus CA1 pathways were studied in rat brain slices.

RESULTS

Fear conditioning was impaired in young human binge drinkers. The ability of fear-conditioned CSs to increase c-fos expression in limbic brain areas was reduced following RWD, as was LTP induction. Rats conditioned prior to RWD, following RWD showed generalization of conditioned fear from the tone CS+ to a neutral control stimulus, and a novel tone.

CONCLUSIONS

Binge-like drinking impairs fear conditioning, reduces LTP, and results in inappropriate generalization of learned fear responses. We propose a mechanism whereby RWD-induced synaptic plasticity reduces capacity for future learning, while allowing unconditioned stimuli access to neuronal pathways underlying conditioned fear.

Angiotensin-(1–7) enhances LTP in the hippocampus through the G-protein-coupled receptor Mas

The renin-angiotensin system not only plays a critical role in blood pressure control but is also involved in learning and memory mechanisms. In addition to angiotensin (Ang) II, Ang-(1-7) may also have important biological activities in the brain. Here, we show for the first time that Ang-(1-7) enhances long-term potentiation (LTP) in the CA1 region of the hippocampus. Our studies with AT1 receptor antagonists and selective Ang-(1-7) receptor antagonists demonstrate the existence of a distinct Ang-(1-7) receptor in the brain, the G-protein-coupled receptor Mas, encoded by the Mas protooncogene. We also show that the genetic deletion of this receptor abolishes the Ang-(1-7)-induced enhancement of LTP. Thus, we firstly demonstrate that Ang-(1-7) influences the induction of LTP in limbic structures implicating its distinct function in learning and memory mechanisms; secondly, we have identified Mas as a functional receptor for Ang-(1-7) in the brain.

Angiotensin II blocks memory consolidation through an AT2 receptor-dependent mechanism

RATIONALE AND OBJECTIVES

Several studies suggest that the brain renin-angiotensin system is involved in memory consolidation. However, the participation of angiotensin II (AII) in this process is controversial. This is probably due to the fact that many of the studies carried out to elucidate this matter employed multitrial learning paradigms together with pretraining intracerebroventricular infusions, and therefore were unable to distinguish between consolidation and retrieval related events and lacked anatomical specificity. To circumvent this problem, we analyzed the role played in memory consolidation by AII using the hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs.

METHODS AND RESULTS

Rats bilaterally implanted with infusion cannulae into the CA1 region of the dorsal hippocampus (CA1) were trained in IA and tested for memory retention 24 h later. We found that when infused into CA1 immediately or 30 min after training but not later, AII produced a dose-dependent amnesic effect without altering locomotor activity, exploratory behavior or anxiety state. The amnesic effect of AII was not mimicked by angiotensin IV (AIV) and was totally blocked by the AII-type 2 receptor (AT2) antagonist, PD123319, but not by the AII-type 1 receptor (AT1) antagonist, losartan. Importantly, when infused alone, neither PD123319 nor losartan produced any effect on memory retention.

CONCLUSIONS

Our data indicate that, when given into CA1, AII blocks memory formation through a mechanism involving activation of AT2 receptors; however, endogenous AII does not seem to participate in the consolidation of IA long-term memory.

Alterations in Sympathetic Ganglionic Transmission in Response to Angiotensin II in (mRen2)27 Transgenic Rats

Hypertension in (mRen2)27 transgenic rats is partly dependent on activation of the sympathetic nervous system, but the role of ganglionic transmission is unknown. We assessed indices of synaptic plasticity (post-tetanic short-term potentiation [PTP] and long-term potentiation [LTP]) and sympathetic ganglionic transmission without tetany in superior cervical ganglia (SCG) of Hannover Sprague-Dawley rats (HnSD) versus (mRen2)27 rats. There were no differences in decay time constants [PTP=9 minutes; LTP=120 to 150 minutes in both (mRen2)27 and HnSD]. However, angiotensin (Ang) II increased PTP and LTP in SCG isolated from (mRen2)27 rats to a greater extent than HnSD. Candesartan (an AT 1 antagonist) blocked the potentiation in both groups. Without a preceding tetanic pulse, 16-nM Ang II induced similar significant increases in ganglionic transmission of ≈14% in both strains. Assessment of Ang II receptors by 125 I-[Sar 1 Thr 8 ]-Ang II binding showed that the AT 1 -receptor subtype predominates in the ganglia. The density of receptors in the SCG was comparable in (mRen2)27 and HnSD rats, whether measured in tissue from ganglia removed and frozen versus ganglia used in the transmission testing, suggesting that upregulation of receptors in vitro after removal of SCG did not occur. The divergence of effects of Ang II on LTP and PTP [greater in (mRen2)27 than HnSD] and nontetany ganglionic transmission (similar in both strains) may reflect different locations of receptors (pre- versus postsynaptic) or different signaling mechanisms involved in the two responses. We suggest that functional Ang II receptors in SCG mediate physiological actions of Ang II on ganglionic transmission and may play a pivotal role in hypertension.

Long-term depression in horizontal slices of the rat lateral amygdala

Long-term depression (LTD) is an enduring decrease in synaptic efficacy and is thought to underlie memory. In contrast to investigations of plasticity mechanisms in the amygdala in rat coronal slices, this study was done in horizontal slices. Field excitatory postsynaptic potentials (fEPSPs) and EPSPs, respectively, were recorded extracellularly and intracellularly from the lateral nucleus of the amygdala (LA). We show that low-frequency stimulation (LFS) induces LTD in the LA, when stimulation electrodes were located in the LA. No significant differences were found between females and males. In dependence of strain variations, a reduction of GABAergic inhibition either reduced the magnitude of LTD or was a prerequisite for the induction of extracellularly recorded LA-LTD. Theta pulse stimulation (TPS) of afferents within the LA caused a weaker LTD than LFS. Theta burst stimulation (TBS) given 20 min after the end of LFS reversed LTD, whereas high-frequency stimulation (HFS) resulted in long-term potentiation (LTP) that was significantly stronger than that obtained in naive slices. Therefore, primed induction of LTD facilitates high-frequency-induced LTP in the rat lateral amygdala. NMDARs as well as group II mGluRs were involved in the mediation of LA-LTD. In contrast to data obtained by stimulation of afferents running within the LA, LFS of the external capsule fibers induced a weak LA-LTD, and TPS was not able to induce LTD. This study showed for the first time that LTD can be induced in the LA by standard LFS (900 pulses at 1 Hz) and that LTP stimuli reversed LTD. The results also provide further evidence for the broad sensitivity of synaptic plasticity mechanisms to the history of prior activity.

8-OH-DPAT suppresses the induction of LTP in brain slices of the rat lateral amygdala

The effect of 8-OH-DPAT on the induction of long-term potentiation (LTP) in the lateral nucleus of the amygdala was investigated using rat horizontal brain slice preparations. Bath-applied 8-OH-DPAT decreased the field potential amplitudes in a dose-dependent manner. In the lateral amygdala synapses, 8-OH-DPAT significantly suppressed the induction of LTP evoked by a weak theta burst stimulation. This suppression of LTP was also found using a concentration of 8-OH-DPAT, which did not influence the baseline activity significantly. The specific 5-HT(1A) receptor antagonist, WAY 100,635 blocked the inhibitory effect of 8-OH-DPAT on the induction of LTP. The inhibitory effect of 5-HT(1A) receptor stimulation on amygdaloid neuronal plasticity suggests that the amygdala is a site for serotonin to exert its influence on memory of aversive events.

Morphological alterations in the amygdala and hippocampus of mice during ageing

Declines in memory function and behavioural dysfunction accompany normal ageing in mammals. However, the cellular and morphological basis of this decline remains largely unknown. It was assumed for a long time that cell losses in the hippocampus accompany ageing. However, recent stereological studies have questioned this finding. In addition, the effect of ageing is largely unknown in another key structure of the memory system, the amygdala. In the present study, we have estimated neuronal density and total neuronal numbers as well as density of fragments of degenerated axons in different hippocampal subfields and amygdaloid nuclei. Comparisons were made among aged (21-26 months old) mice and normal adult littermates (8 months old). No significant volume loss occurs in the hippocampus of aged mice. Small but insignificant reductions in total neuronal numbers were found in the hippocampus and in the amygdaloid nuclei. In contrast to the mild effects of ageing upon neuronal numbers, fragments of degenerated axons were increased in both hippocampus and amygdala of aged mice. These data suggest that ageing does not induce prominent cell loss in the hippocampus or amygdala, but leads to degeneration of axons that innervate these forebrain structures. Thus, mechanisms underlying age-related dysfunction depend on parameters other than neuronal numbers, at least in the hippocampal formation and the amygdala.

The role of angiotensin II in cognition and behaviour

Polymorphisms of the renin-angiotensin system are associated with cardiovascular disorders, possibly as a consequence of increased brain angiotensin II activity. Within the brain, angiotensin controls blood pressure, fluid balance and hormone secretion; it also influences behaviour: reduction of central angiotensin function has both antidepressant-like and axiolytic-like actions. Evidence concerning the role of the renin-angiotensin system in learning and memory is contradictory, although more studies support the proposal that angiotensin reduces cognitive function. Studies of renin-angiotensin system genotype and psychological status have suggested an association between the angiotensin-converting enzyme deletion allele and age related cognitive decline, but a greater prevalence of the insertion allele in Alzheimer's disease. The deletion allele has also been associated with depressive illness, as has the M allele of the angiotensinogen gene although other studies have failed to replicate these findings. The role of the brain renin-angiotensin system in human psychopathology remains to be fully explored.

Interaction between Mas and the angiotensin AT1 receptor in the amygdala

The Mas-protooncogene is a maternally imprinted gene encoding an orphan G protein-coupled receptor expressed mainly in limbic structures of the rodent CNS. Because Mas and the product of the Mas-related gene enhance the effects of angiotensins on cells expressing angiotensin receptors of the AT1 subtype, we first compared the distribution of cells expressing AT1 receptors in different limbic and thalamic brain structures in Mas-knockout mice and in wildtype mice by an immunohistochemical approach. No significant differences could be found between the two strains. The Mas-protooncogene seems to be implicated in the signal transduction of angiotensin receptors and is expressed in the amygdala. Therefore we then analyzed whether field potentials are altered by angiotensin II in brain slices of the basolateral amygdala. An opposite action of angiotensin II was obtained in mice lacking the Mas-protooncogene in comparison to wildtype mice. The use of different angiotensin receptor antagonists provides the first in vitro evidence for a functional interaction between the Mas-protooncogene and the AT1 receptor.

Distribution of angiotensin II binding sites in the mouse thalamus: receptor-binding study with fluorescent coupled peptides and their conversion to a light stable product

Fluorescence-coupled peptides allow a non-radioactive receptor binding study whereby single cells can be examined under a fluorescence microscope. By the combination of such a method with immunohistochemistry, using an HRP-coupled anti-fluorescein antibody, a permanent labeling can be achieved. By using this method the distribution of angiotensin II binding sites has been examined in the mouse thalamus. The results show that a moderate staining was obvious within the thalamus and that the distribution of binding sites in the thalamus is very homogeneous in the mouse brain. In detail, angiotensin II binding sites were found in the anterodorsal nucleus, in the laterodorsal and posterior nucleus of the thalamus, as well as in the lateral geniculate nucleus, the reticular thalamic nucleus and in the zona incerta.

The isolated mammalian brain: an in vivo preparation suitable for pathway tracing

An in vitro preparation of complete brain is presented which allows successful tracing with fast axonal tracers for at least 2 days after in vitro preparation. A simple procedure is used, by which the isolated brain is submerged in oxygenated artificial cerebrospinal fluid, enabling maintenance of adequate integrity without perfusion the vascular system. The use of an isolated brain has many advantages: virtually all brain areas are easily accessible; tracer applications are possible without problems of animal survival; and the tracers are be translocated by blood circulation. The viability of this isolated brain preparation was validated by successful recording of extracellular field potentials 1 day later.

Effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides from 1994 through 1998

Effects of peptides on the central nervous system are presented in two ways so as to provide a cross-listing. In the first table, the peptides are listed alphabetically. In the second table, the central nervous system effects are arranged alphabetically. No longer can there be any doubt that peptides affect the central nervous system, sometimes in several ways.

Mapping of angiotensin AT1 receptors in the rat limbic system

The AT1 receptor is one of the two receptor subtypes able to bind angiotensin II. In the present study, immunohistochemical examination of the distribution of the AT1 receptor in several limbic structures of female rats has been done, revealing new aspects of the distribution of AT1-positive cells. The presence of AT1 receptor expressing cells in the hippocampus and the amygdala is described, but their distribution in these regions has not been examined in a detailed way. We found some notable differences in the distribution of these cells: in female rats, we detected high amounts of labeled cells in the hippocampus, the entorhinal cortex and piriform cortex. In somewhat lower amounts, stained cells could be found in several nuclei of the amygdala (in the basomedial, basolateral, lateral, central and medial nucleus of the amygdala, in the amygdalopiriform transition area and in the amygdalohippocampal transition area as well as in the bed nucleus of the stria terminalis).