Pain threshold, learning and formation of brain edema in mice lacking the angiotensin II type 2 receptor

Evolving views on the first two ligands of the angiotensin II type 2 receptor. From putative antagonists to potential agonists?

The renin-angiotensin system is one of the most complex regulatory systems that controls multiple organ functions. One of its key components, angiotensin II (Ang II), stimulates two G-protein coupled class A receptors: the Ang II type 1 (AT1) receptor and the Ang II type 2 (AT2) receptor. While stimulation of the AT1 receptor causes G-protein-dependent signaling and arrestin recruitment, the AT2 receptor seems to have a constitutively active-like conformation and appears to act via G-protein-dependent and -independent pathways. Overstimulation of the AT1 receptor may lead to unwanted effects like inflammation and fibrosis. In contrast, stimulation of the AT2 receptor leads to opposite effects thus restoring the balance. However, the role of the AT2 receptor has become controversial due to beneficial effects of putative AT2 receptor antagonists. The two first synthetic AT2 receptor-selective ligands, peptide CGP42112 and small molecule PD123319, were initially both considered antagonists. CGP42112 was subsequently considered a partial agonist and it was recently demonstrated to be a full agonist. Based on the search-term PD123319 in Pubmed, 1652 studies have investigated putative AT2 receptor antagonist PD123319. Here, we put forward literature that shows beneficial effects of PD123319 alone, even at doses too low for antagonist efficacy. These beneficial effects appear compatible with agonist-like activity via the AT2 receptor. Taken together, a more consistent image of a therapeutic role of stimulated AT2 receptor emerges which may clarify current controversies.

Epigenetic regulation of beta-endorphin synthesis in hypothalamic arcuate nucleus neurons modulates neuropathic pain in a rodent pain model

Although beta-endorphinergic neurons in the hypothalamic arcuate nucleus (ARC) synthesize beta-endorphin (β-EP) to alleviate nociceptive behaviors, the underlying regulatory mechanisms remain unknown. Here, we elucidated an epigenetic pathway driven by microRNA regulation of β-EP synthesis in ARC neurons to control neuropathic pain. In pain-injured rats miR-203a-3p was the most highly upregulated miRNA in the ARC. A similar increase was identified in the cerebrospinal fluid of trigeminal neuralgia patients. Mechanistically, we found histone deacetylase 9 was downregulated following nerve injury, which decreased deacetylation of histone H3 lysine-18, facilitating the binding of NR4A2 transcription factor to the miR-203a-3p gene promoter, thereby upregulating miR-203a-3p expression. Further, increased miR-203a-3p was found to maintain neuropathic pain by targeting proprotein convertase 1, an endopeptidase necessary for the cleavage of proopiomelanocortin, the precursor of β-EP. The identified mechanism may provide an avenue for the development of new therapeutic targets for neuropathic pain treatment.

Angiotensin-Related Peptides and Their Role in Pain Regulation

Angiotensin (Ang)-generating system has been confirmed to play an important role in the regulation of fluid balance and blood pressure and is essential for the maintenance of biological functions. Ang-related peptides and their receptors are found throughout the body and exhibit diverse physiological effects. Accordingly, elucidating novel physiological roles of Ang-generating system has attracted considerable research attention worldwide. Ang-generating system consists of the classical Ang-converting enzyme (ACE)/Ang II/AT1 or AT2 receptor axis and the ACE2/Ang (1-7)/MAS1 receptor axis, which negatively regulates AT1 receptor-mediated responses. These Ang system components are expressed in various tissues and organs, forming a local Ang-generating system. Recent findings indicate that changes in the expression of Ang system components under pathological conditions are involved in the development of neuropathy, inflammation, and their associated pain. Here, we summarized the effects of changes in the Ang system on pain transmission in various organs and tissues involved in pain development process.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Angiotensin receptors and neuropathic pain

Growing evidence implicates the renin-angiotensin system (RAS) in multiple facets of neuropathic pain (NP). This narrative review focuses primarily on the major bioactive RAS peptide, Angiotensin II (Ang II), and its receptors, namely type 1 (AT1R) and type 2 (AT2R). Both receptors are involved in the development of NP and represent potential therapeutic targets. We first discuss the potential role of Ang II receptors in modulation of NP in the central nervous system. Ang II receptor expression is widespread in circuits associated with the perception and modulation of pain, but more studies are required to fully characterize receptor distribution, downstream signaling, and therapeutic potential of targeting the central nervous system RAS in NP. We then describe the peripheral neuronal and nonneuronal distribution of the RAS, and its contribution to NP. Other RAS modulators (such as Ang (1-7)) are briefly reviewed as well. AT1R antagonists are analgesic across different pain models, including NP. Several studies show neuronal protection and outgrowth downstream of AT2R activation, which may lead to the use of AT2R agonists in NP. However, blockade of AT2R results in analgesia. Furthermore, expression of the RAS in the immune system and a growing appreciation of neuroimmune crosstalk in NP add another layer of complexity and therapeutic potential of targeting this pathway. A growing number of human studies also hint at the analgesic potential of targeting Ang II signaling. Altogether, Ang II receptor signaling represents a promising, far-reaching, and novel strategy to treat NP.

Angiotensin Type 2 Receptors: Painful, or Not?

Pain in response to various types of acute injury can be a protective stimulus to prevent the organism from using the injured part and allow tissue repair and healing. On the other hand, neuropathic pain, defined as ‘pain caused by a lesion or disease of the somatosensory nervous system’, is a debilitating pathology. The TRPA1 neurons in the Dorsal Root Ganglion (DRG) respond to reactive oxygen species (ROS) and induce pain. In acute nerve injury and inflammation, macrophages infiltrating the site of injury undergo an oxidative burst, and generate ROS that promote tissue repair and induce pain via TRPA1. The latter discourages using the injured limb, with a lack of movement helping wound healing. In chronic inflammation caused by diabetes, cancer etc., ROS levels increase systemically and modulate TRPA1 neuronal functions and cause debilitating neuropathic pain. It is important to distinguish between drug targets that elicit protective vs. debilitating pain when developing effective drugs for neuropathic pain. In this context, the connection of the Angiotensin type 2 receptor (AT₂R) to neuropathic pain presents an interesting dilemma. Several lines of evidence show that AT₂R activation promotes anti-inflammatory and anti-nociceptive signaling, tissue repair, and suppresses ROS in chronic inflammatory models. Conversely, some studies suggest that AT₂R antagonists are anti-nociceptive and therefore AT₂R is a drug target for neuropathic pain. However, AT₂R expression in nociceptive neurons is lacking, indicating that neuronal AT₂R is not involved in neuropathic pain. It is also important to consider that Novartis terminated their phase II clinical trial (EMPHENE) to validate that AT₂R antagonist EMA401 mitigates post-herpetic neuralgia. This trial, conducted in Australia, United Kingdom, and a number of European and Asian countries in 2019, was discontinued due to pre-clinical drug toxicity data. Moreover, early data from the trial did not show statistically significant positive outcomes. These facts suggest that may AT₂R not be the proper drug target for neuropathic pain in humans and its inhibition can be harmful.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Propofol inhibits expression of angiotensin II receptor type 2 in dorsal root ganglion neurons

The renin-angiotensin system (RAS) is involved in nociception and has functions in the cardiovascular system. The primary role of the RAS is to mediate the effect of angiotensin II (Ang II) through Ang II receptor type 2 (AT2). Due to this, AT2 has become a novel therapeutic target for the relief of peripheral neuropathic pain in humans. As it is one of the most popular induction agents of general anesthesia, propofol also exerts peripheral antinociceptive effects. The present study assessed the effect of propofol on the expression of AT2 in cultured dorsal root ganglion (DRG) neurons. The results indicate that propofol decreases AT2 mRNA expression in a statistically significant dose- and time-dependent manner (P<0.05). This resulted in a marked decrease in AT2 protein expression and the density of Ang II-binding AT2 on the cell membrane of DRG neurons. The effect of propofol was reversed by LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor. Although propofol exhibited no significant effect on AT2 gene promoter activity, it significantly decreased the stability of AT2 mRNA (P<0.05). However, this effect was reversed by LY294002. In addition, propofol increased PI3K activity in a concentration-dependent manner in DRG neurons. In conclusion, to the best of our knowledge, the current study provides the first evidence suggesting that propofol inhibits the expression of AT2 in DRG neurons by decreasing the stability of AT2 mRNA through a PI3K-dependent mechanism. The present study provides novel insights into the mechanisms of the peripheral antinociceptive action of propofol and suggests a potential means of regulating Ang II/AT2 signaling in the peripheral nervous system.

Chronic nerve injury-induced Mas receptor expression in dorsal root ganglion neurons alleviates neuropathic pain

Neuropathic pain, which is characterized by hyperalgesia, allodynia and spontaneous pain, is one of the most painful symptoms that can be experienced in the clinic. It often occurs as a result of injury to the peripheral nerves, dorsal root ganglion (DRG), spinal cord or brain. The renin-angiotensin system (RAS) plays an important role in nociception. As an essential component of the RAS, the angiotensin (Ang)-(1-7)/Mas axis may be involved in antinociception. The aim of the present study was to explore the expression pattern of Mas in DRG neurons following chronic nerve injury and examine the effects of Mas inhibition and activation on neuropathic pain in a chronic constriction injury (CCI) rat model. The results showed, that compared with the sham group, CCI caused a time-dependent induction of Mas expression at both the mRNA and the protein levels in DRG neurons. Consistent with the results, isolated DRG neurons showed a time-dependent increase in Ang-(1-7) binding on the cell membrane following the CCI surgery, but not the sham surgery. Compared with the sham control groups, CCI significantly decreased the paw withdrawal latency and threshold, and this was markedly improved and aggravated by intrathecal injection of the selective Mas agonist Ang-(1-7) and the selective Mas inhibitor D-Pro7-Ang-(1-7), respectively. In conclusion, this study has provided the first evidence, to the best of our knowledge, that the Mas expression in DRG neurons is time-dependently induced by chronic nerve injury and that the intrathecal activation and inhibition of Mas can improve and aggravate CCI-induced neuropathic pain, respectively. This study has provided novel insights into the pathophysiological process of neuropathic pain and suggests that the Ang-(1-7)/Mas axis could be an effective therapeutic target for neuropathic pain, warranting further study.

Angiotensin-(1-7) through Mas receptor activation induces peripheral antinociception by interaction with adrenoreceptors

Angiotensin-(1-7) [Ang-(1-7)] develops its functions interacting with Mas receptor. Mas receptor was recently identified in the DRG and its activation by Ang-(1-7) resulted in peripheral antinociception against PGE2 hyperalgesia in an opioid-independent pathway. Nevertheless, the mechanism by which Ang-(1-7) induce peripheral antinociception was not yet elucidated. Considering that endogenous noradrenaline could induce antinociceptive effects by activation of the adrenoceptors the aim of this study was verify if the Ang-(1-7) is able to induce peripheral antinociception by interacting with the endogenous noradrenergic system. Hyperalgesia was induced by intraplantar injection of prostaglandin E2 (2μg). Ang-(1-7) was administered locally into the right hindpaw alone and after either agents, α2-adrenoceptor antagonist, yohimbine (5, 10 and 20 μg/paw), α2C-adrenoceptor antagonist rauwolscine (10, 15 and 20 μg/paw), α1-adrenoceptor antagonist prazosin (0.5, 1 and 2 μg/paw), β-adrenoceptor antagonist propranolol (150, 300 and 600 ng/paw). Noradrenaline (NA) reuptake inhibitor reboxetine (30 μg/paw) was administered prior to Ang-(1-7) low dose (20 ng) and guanetidine 3 days prior to experiment (30 mg/kg/animal, once a day), depleting NA storage. Intraplantar Ang-(1-7) induced peripheral antinociception against hyperalgesia induced by PGE2. This effect was reversed, in dose dependent manner, by intraplantar injection of yohimbine, rauwolscine, prazosin and propranolol. Reboxetine intensified the antinociceptive effects of low-dose of Ang-(1-7) and guanethidine, which depletes peripheral sympathomimetic amines, reversed almost 70% the Ang-(1-7)-induced peripheral antinociception. Then, this study provides evidence that Ang-(1-7) induce peripheral antinociception stimulating an endogenous noradrenaline release that activates peripheral adrenoceptors inducing antinociception.

Pharmacological treatment of pain: future trends and novel insights

The pharmacological treatment of chronic pain is generally hampered by a limited clinical outcome. Hence, there is a strong need for new therapeutic concepts considering the identification of novel targets and related drugs, but also optimization of established therapeutic regimes through individualization. In this issue, focused on "Pain," we discuss some of the recent new concepts in pain treatment, understanding of pain heterogeneity, and subsequent optimization of analgesic treatment, but also novel insights into interactions of nonopioids.

Role of angiotensin modulation in primary headaches

The renin-angiotensin system (RAS) is a major regulatory system controlling many different homeostatic mechanisms both within the brain and in the periphery. While it is primarily associated with blood pressure and salt/water regulation, increasing evidence points to the involvement of the RAS in both headache disorders specifically and pain regulation in general. Several publications have indicated that drugs blocking various elements of the renin-angiotensin system lead to a reduction in migraine. Additionally, interventions on different angiotensin peptides or their receptors have been shown to both reduce and increase pain in animal models. As such, modulation of the renin-angiotensin system is a promising approach to the treatment of headaches and other pain conditions.

Participation of AT1 and Mas receptors in the modulation of inflammatory pain

We investigated the mechanisms underlying the endogenous control of nociception at the peripheral level during inflammation. We hypothesized that angiotensin receptors could modulate pain at the peripheral level via endogenous processes because angiotensin receptors are present in peripheral nerve terminals. We evaluated the role of the angiotensin receptors system (RAS) in the modulation of inflammatory and neuropathic pain states. Mas receptor KO mice exhibited major inflammatory pain compared to wild-type mice. Similar results were observed when rats were injected with the Mas receptor antagonist A779 or the AT1 receptor antagonist, losartan after inflammatory stimulation by carrageenan. However, these antagonists were not effective in animals with neuropathic-induced pain (e.g., sciatic nerve constriction). Therefore, RAS seems to play an important role in inflammatory but not neuropathic pain.

Angiotensin II type 2 receptor (AT2 R) localization and antagonist-mediated inhibition of capsaicin responses and neurite outgrowth in human and rat sensory neurons

BACKGROUND

The angiotensin II (AngII) receptor subtype 2 (AT2 R) is expressed in sensory neurons and may play a role in nociception and neuronal regeneration.

METHODS

We used immunostaining with characterized antibodies to study the localization of AT2 R in cultured human and rat dorsal root ganglion (DRG) neurons and a range of human tissues. The effects of AngII and AT2 R antagonist EMA401 on capsaicin responses in cultured human and rat (DRG) neurons were measured with calcium imaging, on neurite length and density with Gap43 immunostaining, and on cyclic adenosine monophosphate (cAMP) expression using immunofluorescence.

RESULTS

AT2 R expression was localized in small-/medium-sized cultured neurons of human and rat DRG. Treatment with the AT2 R antagonist EMA401 resulted in dose-related functional inhibition of capsaicin responses (IC50  = 10 nmol/L), which was reversed by 8-bromo-cAMP, and reduced neurite length and density; AngII treatment significantly enhanced capsaicin responses, cAMP levels and neurite outgrowth. The AT1 R antagonist losartan had no effect on capsaicin responses. AT2 R was localized in sensory neurons of human DRG, and nerve fibres in peripheral nerves, skin, urinary bladder and bowel. A majority sub-population (60%) of small-/medium-diameter neuronal cells were immunopositive in both control post-mortem and avulsion-injured human DRG; some very small neurons appeared to be intensely immunoreactive, with TRPV1 co-localization. While AT2 R levels were reduced in human limb peripheral nerve segments proximal to injury, they were preserved in painful neuromas.

CONCLUSIONS

AT2 R antagonists could be particularly useful in the treatment of chronic pain and hypersensitivity associated with abnormal nerve sprouting.

Effect of subpressor dose of angiotensin II on pain-related behavior in relation with neuronal injury and activation of satellite glial cells in the rat dorsal root ganglia

To clarify the role of angiotensin II (Ang II) in the regulation of sensory signaling, we studied the effect of subpressor dose (150 ng/kg/min) of Ang II on pain-related behavior in relation with neuronal injury and activation of satellite glial cells (SGCs) in the dorsal root ganglia (DRGs) after chronic constriction injury (CCI). Systemic continuous delivery of Ang II induced the tactile, heat and cold hyperlagesia, when measured at 7 days ofpost-injury. Blockade of the AT1 receptor with losartan (2.5 mg/kg/day) prevented tactile hyperalgesia and attenuated cold hyperalgesia, but did not affect the response to noxious heat stimulus. A marked increase of large-sized injured primary afferent neurons, detected by ATF3 immunolabeling, was seen in lower lumbar DRGs on ipsilateral side after Ang II treatment. Subpressor dose of Ang II induced an increase of activated SGCs (detected by GFAP immunolabeling) enveloping large-diameter neurons. Our results suggested that Ang II through the AT1 receptor activation is an important regulatory factor in neuropathic pain perception and plays an important role in the injury of large-sized primary afferent neurons and activation of SGCs elicited by the CCI.

Angiotensin II receptor type 2 activation is required for cutaneous sensory hyperinnervation and hypersensitivity in a rat hind paw model of inflammatory pain

Many pain syndromes are associated with abnormal proliferation of peripheral sensory fibers. We showed previously that angiotensin II, acting through its type 2 receptor (AT2), stimulates axon outgrowth by cultured dorsal root ganglion neurons. In this study, we assessed whether AT2 mediates nociceptor hyperinnervation in the rodent hind paw model of inflammatory pain. Plantar injection of complete Freund's adjuvant (CFA), but not saline, produced marked thermal and mechanical hypersensitivity through 7 days. This was accompanied by proliferation of dermal and epidermal PGP9.5-immunoreactive (ir) and calcitonin gene-related peptide-immunoreactive (CGRP-ir) axons, and dermal axons immunoreactive for GFRα2 but not tyrosine hydroxylase or neurofilament H. Continuous infusion of the AT2 antagonist PD123319 beginning with CFA injection completely prevented hyperinnervation as well as hypersensitivity over a 7-day period. A single PD123319 injection 7 days after CFA also reversed thermal hypersensitivity and partially reversed mechanical hypersensitivity 3 hours later, without affecting cutaneous innervation. Angiotensin II-synthesizing proteins renin and angiotensinogen were largely absent after saline but abundant in T cells and macrophages in CFA-injected paws with or without PD123319. Thus, emigrant cells at the site of inflammation apparently establish a renin-angiotensin system, and AT2 activation elicits nociceptor sprouting and heightened thermal and mechanical sensitivity.

PERSPECTIVE

Short-term AT2 activation is a potent contributor to thermal hypersensitivity, whereas long-term effects (such as hyperinnervation) also contribute to mechanical hypersensitivity. Pharmacologic blockade of AT2 signaling represents a potential therapeutic strategy aimed at biologic mechanisms underlying chronic inflammatory pain.

The role of the angiotensin AT2 receptor on the diurnal variations of nociception and motor coordination in rats

Phasic pain demonstrates significant diurnal variation in rats. Angiotensin II modulates pain transmission and the diurnal variation in nociception in several rodent pain models. The participation of AT2 receptors in the diurnal regulation of nociception is not yet elucidated. In the present study we investigated the effects of selective peptide AT2 agonist CGP 42112A and the nonpeptide AT2 receptor antagonist PD 123319 on the nociception, motor coordination and arterial blood pressure. Male Wistar 12 weeks old rats were used. CGP 42112A was injected at single doses of 1 and 5 μg/rat intracerebroventricularly (ICV) and infused chronically ICV at a dose of 12 μg/rat/day during 14 days by osmotic minipumps. PD123319 was injected at single doses of 1 and 5 μg/rat, ICV and chronically subcutaneously at a dose of 10 mg/kg/day/14 days. Nociception was assessed by an analgesimeter, arterial blood pressure (ABP) was measured by tail cuff method, and motor coordination by Rota-rod method. Single doses of CGP 42112A (1 and 5 μg/rat) provoked a short lasting antinociception. Unlike acute injection, chronic CGP 42112A infusion increased nociception at the beginning and the end of light phase thus attenuating the diurnal variations observed in the controls. Moreover, it produced an increase of ABP and improved motor coordination. Both acute (1 μg/rat) and chronic PD 123319 treatment resulted in a decrease of pain threshold and chronic treatment attenuated its diurnal fluctuation. Our data support a role for Ang II type 2 receptors in the control of diurnal variations of nociception in rats.

Increased angiotensin II AT1 receptor mRNA and binding in spleen and lung of AT2 receptor gene disrupted mice

To clarify the relationship between Angiotensin II AT(1) and AT(2) receptors, we studied AT(1) receptor mRNA and binding expression in tissues from AT(2) receptor gene disrupted (AT(2)(-/-)) female mice, where AT(2) receptors are not expressed in vivo, using in situ hybridization and quantitative autoradiography. Wild type mice expressed AT(1A) receptor mRNA and AT(1) receptor binding in lung parenchyma, the spleen, predominantly in the red pulp, and in liver parenchyma. In wild type mice, lung AT(2) receptors were expressed in lung bronchial epithelium and smooth muscle, and were not present in the lung parenchyma, the spleen or the liver. This indicates that AT(1) and AT(2) receptors were not expressed in the same cells. In AT(2)(-/-) mice, we found higher AT(1A) receptor mRNA and AT(1) receptor binding in lung parenchyma and in the red pulp of the spleen, but not in the liver, when compared to littermate wild type controls. Our results suggest that impaired AT(2) receptor function upregulates AT(1) receptor transcription and expression in a tissue-specific manner and in cells not expressing AT(2) receptors. AT(1) upregulation explains the increased sensitivity to Angiotensin II characteristic of the AT(2)(-/-) phenotype, consistent with enhanced AT(1) receptor activation in a number of tissues.

Candidate Agtr2 influenced genes and pathways identified by expression profiling in the developing brain of Agtr2(-/y) mice

Intellectual disability (ID) is a common developmental disability observed in 1 to 3% of the human population. A possible role for the Angiotensin II type 2 receptor (AGTR2) in brain function, affecting learning, memory, and behavior, has been suggested in humans and rodents. Mice lacking the Agtr2 gene (Agtr2(-/y)) showed significant impairment in their spatial memory and exhibited abnormal dendritic spine morphology. To identify Agtr2 influenced genes and pathways, we performed whole genome microarray analysis on RNA isolated from brains of Agtr2(-/y) and control male mice at embryonic day 15 (E15) and postnatal day one (P1). The gene expression profiles of the Agtr2(-/y) brain samples were significantly different when compared to profiles of the age-matched control brains. We identified 62 differently expressed genes (p< or =0.005) at E15 and in P1 brains of the Agtr2(-/y) mice. We verified the differential expression of several of these genes in brain samples using quantitative RT-PCR. Differentially expressed genes encode molecules involved in multiple cellular processes including microtubule functions associated with dendritic spine morphology. This study provides insight into Agtr2 influenced candidate genes and suggests that expression dysregulation of these genes may modulate Agtr2 actions in the brain that influences learning and memory.

Expression and transport of Angiotensin II AT1 receptors in spinal cord, dorsal root ganglia and sciatic nerve of the rat

To clarify the role of Angiotensin II in the regulation of peripheral sensory and motor systems, we initiated a study of the expression, localization and transport of Angiotensin II receptor types in the rat sciatic nerve pathway, including L(4)-L(5) spinal cord segments, the corresponding dorsal root ganglia (DRGs) and the sciatic nerve. We used quantitative autoradiography for AT(1) and AT(2) receptors, and in situ hybridization to detect AT(1A), AT(1B) and AT(2) mRNAs. We found substantial expression and discrete localization of Angiotensin II AT(1) receptors, with much higher numbers in the grey than in the white matter. A very high AT(1) receptor expression was detected in the superficial dorsal horns and in neuronal clusters of the DRGs. Expression of AT(1A) mRNA was significantly higher than that of AT(1B). AT(1) receptor binding and AT(1A) and AT(1B) mRNAs were especially prominent in ventral horn motor neurons, and in the DRG neuronal cells. Unilateral dorsal rhizotomy significantly reduced AT(1) receptor binding in the ipsilateral side of the superficial dorsal horn, indicating that a substantial number of dorsal horn AT(1) receptors have their origin in the DRGs. After ligation of the sciatic nerve, there was a high accumulation of AT(1) receptors proximal to the ligature, a demonstration of anterograde receptor transport. We found inconsistent levels of AT(2) receptor binding and mRNA. Our results suggest multiple roles of Angiotensin II AT(1) receptors in the regulation of sensory and motor functions.

Impaired spatial memory and altered dendritic spine morphology in angiotensin II type 2 receptor-deficient mice

Mental retardation is the most frequent cause of serious handicap in children and young adults. Mutations in the human angiotensin II type 2 receptor (AT2) have been implicated in X-linked forms of mental retardation. We here demonstrate that mice lacking the AT2 receptor gene are significantly impaired in their performance in a spatial memory task and in a one-way active avoidance task. As no difference was observed between the genotypes in fear conditioning, the detected deficit in spatial memory may not relate to fear. Notably, receptor knockout mice showed increased motility in an activity meter and elevated plus maze. Importantly, these mice are characterized by abnormal dendritic spine morphology and length, both features also found to be associated with some cases of mental retardation. These findings suggest a crucial role of AT2 in normal brain function and that dysfunction of the receptor has impact on brain development and ultrastructural morphology with distinct consequences on learning and memory.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

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.

Chronic ethanol intake modifies renin-angiotensin system-regulating aminopeptidase activities in mouse cerebellum

In developing cerebellum, where critical periods of vulnerability have been established for several basic substances, it has been extensively studied the wide array of abnormalities induced by exposure to ethanol (EtOH). However, little is known about the effects of EtOH consumption on cerebellar functions in adult individuals. Several studies show participation in cognitive activities to be concentrated in the lateral cerebellum (hemispheres), whereas basic motor functions such as balance and coordination are represented in the medial parts of the cerebellum (vermis and paravermis). In addition to the circulating renin angiotensin system (RAS), a local system has been postulated in brain. The effector peptides of the RAS are formed via the activity of several aminopeptidases (AP). The present work analyses the effect of chronic EtOH intake on the RAS-regulating AP activities in the soluble and membrane-bound fractions of two cerebellar locations: the hemispheres and the vermis. We hypothesize that cerebellar RAS is involved in basic motor functions rather than in cognitive activities.

The effects of sarmesin, an Angiotensin II analogue on seizure susceptibility, memory retention and nociception

The present research studies the effects of sarmesin [Sar(1)Tyr(OMe)(4)] Angiotensin II (ANG II), an analogue of ANG II, on the seizure susceptibility, memory activity and nociception. It was found that this octapeptide, administered i.c.v., dose-dependently decreased the seizure intensity (pentylenetetrazol (PTZ) generalized seizure model and PTZ kindling) and augmented PTZ seizure threshold in mice. Sarmesin impaired the memory upon re-testing of rats 24 h later in the passive avoidance test. It decreased the pain threshold in a paw pressure nociceptive assay in rats. ANG II exerted pronociceptive effect as well. Taken together, these results reveal sarmesin as a behaviorally active peptide in the studied experimental animal models.

Autonomic control of blood pressure in mice: basic physiology and effects of genetic modification

Control of blood pressure and of blood flow is essential for maintenance of homeostasis. The hemodynamic state is adjusted by intrinsic, neural, and hormonal mechanisms to optimize adaptation to internal and environmental challenges. In the last decade, many studies showed that modification of the mouse genome may alter the capacity of cardiovascular control systems to respond to homeostatic challenges or even bring about a permanent pathophysiological state. This review discusses the progress that has been made in understanding of autonomic cardiovascular control mechanisms from studies in genetically modified mice. First, from a physiological perspective, we describe how basic hemodynamic function can be measured in conscious conditions in mice. Second, we focus on the integrative role of autonomic nerves in control of blood pressure in the mouse, and finally, we depict the opportunities and insights provided by genetic modification in this area.

Changes in pain perception during treatment with angiotensin converting enzyme-inhibitors and angiotensin II type 1 receptor blockade

OBJECTIVES

Besides the well-known role of the angiotensin system in blood pressure control, an interaction of angiotensin and pain perception has been suggested. This study sought to investigate whether an angiotensin converting enzyme inhibitor, which facilitates bradykinins, algesic peptides, and/or an AT1 receptor antagonist may modify hypertension-related hypoalgesia in humans. The study was approved by the ethical committee of our Department.

METHODS

A total of 22 hypertensive patients were submitted to dental pulp stimulation to obtain the dental pain threshold and tolerance, and to 24 h blood pressure monitoring together with a control group of 55 normotensives. Then the hypertensives were randomized to enalapril or losartan treatment and were re-evaluated (dental pain perception and ambulatory monitoring) after 8 weeks of the first treatment and after an additional 8 weeks of the second treatment.

RESULTS

Untreated hypertensives showed a reduced perception to painful stimuli when compared with normotensives. A significant reduction of both pain threshold and tolerance was observed during the anti-hypertensive treatments (Friedman test: P = 0.007 and P = 0.006, respectively). Pain sensitivity was similar during the two treatments and it did not differ from pain sensitivity values of normotensive controls. ANCOVAs were computed to evaluate the relationship between anti-hypertensive agents and pain sensitivity, after controlling for blood pressure. A 24 h mean pressure served as covariate, removing any effect of blood pressure; a significant difference was observed entering both pain threshold and tolerance as dependent variables (F = 5.28, P = 0.0076; F = 8.16, P = 0.0007, respectively).

CONCLUSIONS

Both the angiotensin converting enzyme inhibitor enalapril and the AT1 receptor blocking agent losartan acted similarly on pain threshold and tolerance, pain sensitivity being increased during the two anti-hypertensive treatments. The blood pressure reduction during drug assumption could not account for the pain sensitivity changes observed. The latter may be due to a specific pharmacodynamic mechanism mediated through angiotensin II AT1 receptors.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.