Blockade of pro-cognitive effects of angiotensin IV and physostigmine in mice by oxytocin antagonism

Low doses of oxytocin enhance learning and memory in animal models. Angiotensin IV inhibits cysteine aminopeptidase, also known as insulin-regulated aminopeptidase and oxytocinase, and enhances memory in animals. The mechanism of this effect of angiotensin IV is unknown. This study explored the role of oxytocin in the cognitive effects of angiotensin IV with physostigmine as a control and used isolated smooth muscle to assess the pharmacological selectivity of the observed antagonism. Using novel object recognition in male mice, the effects of angiotensin IV (4.7 μg/kg), oxytocin (0.1 ng/kg) or physostigmine (200 μg/kg) administered subcutaneously immediately after the second training trial, were assessed in the presence and absence of 10 μg/kg β-mercapto-β-β-cyclopenta-methylenepropionyl; O-Me-Tyr2, Orn?-oxytocin, an oxytocin antagonist; n=8 in all cases. The effects of the antagonist on angiotensin IV, oxytocin and acetylcholine-induced contractions of rat isolated uterus were also determined. Oxytocin, angiotensin IV and physostigmine significantly enhanced consolidation of learning (P=0.04, 0.004 and 0.008 respectively), and there were no significant effects on locomotor activity. The oxytocin antagonist similarly not only significantly improved novel object recognition (P=0.03) but also significantly increased locomotor activity (P=0.04). In the learning paradigm the oxytocin antagonist prevented the effects of oxytocin, angiotensin IV and physostigmine but in the uterus, contractions induced by angiotensin IV and acetylcholine were unaffected whilst effects of oxytocin were significantly reduced. These results suggest that the pro-cognitive effects of angiotensin IV may be mediated by accumulation of endogenous oxytocin although the mechanisms underlying the observed interaction between the oxytocin antagonist and physostigmine are unclear.     

Comparative effects of angiotensin II and its degradation products angiotensin III and angiotensin IV in rat aorta.

1. In the present study, the contractile effects of angiotensin III (AIII) and angiotensin IV (AIV) compared with those of angiotensin II (AII) were determined in rat aortic ring preparations. 2. All three peptides caused concentration-dependent contractions with similar maximal responses. AIII proved approximately 4 times less potent than AII, whereas AIV was about 1000 times less active than AII. 3. The selective AT1-receptor antagonist, losartan (10-300 nM) caused parallel rightward shifts of the concentration-response curves (CRC) for all three peptides. The Schild plot slopes for the effect of losartan on AIII curves were significantly lower than unity (P < 0.05). The selective AT2-receptor antagonist, PD123177 did not influence the CRCs for AII and AIV. However, the AIII curves were moderately shifted leftward in the presence of PD123177 (0.1 and 1 microM). 4. Destruction of the endothelium or incubation with the NO-synthesis inhibitor NG-monomethyl-L-arginine acetate (L-NMMA) (0.1 mM) significantly enhanced the contractile responses to all three peptides. 5. Tachyphylaxis was investigated by constructing a second CRC for all three peptides, after an interval of 1 h. The presence of endothelium significantly enhanced the development of tachyphylaxis to all three peptides. However, in endothelium-denuded preparations, the Emax value of the second curve elicited by AII was about 50%, compared with the first one, whereas for AIII and AIV Emax values were as high as 90% and 100%, respectively. 6. Our results indicate that both AIII and AIV are less potent but similarly efficacious vasoconstrictor agents compared with AII. Their contractile effects are also mediated by AT1-receptors and probably modulated by endothelium. Tachyphylaxis induced by AIII and AIV proved weaker than that for AII. Tachyphylaxis appears to be enhanced by the presence of an intact endothelium.     

Interactions between sodium and angiotensin

1. Increased sodium intake causes decreased formation of angiotensin (Ang) II and increased AngII causes increased Na+ retention. 2. Increased sodium intake and increased AngII causes cardiac hypertrophy, but decreased sodium intake regresses cardiac hypertrophy despite high AngII levels. Likewise, decreased Na+ and blockers of the renin-angiotensin system (RAS) in neonatal rats have similar effects on subsequent blood pressure development. 3. Cardiac hypertrophy due to renal hypertension does not regress when the RAS is blocked and rats are on a high salt intake. Likewise, sodium restriction alone does not cause regression; combination of reduced NaCl intake and RAS blockade is required. 4. High doses of perindopril and losartan in combination cause a syndrome in rats on 0.2% NaCl that leads to profound hypotension, polyuria, renal impairment and involution of the heart and death. This is reversed or prevented by a high (4%) NaCl intake, which also prevents the plasma angiotensinogen depletion that occurs with combined blockade on 0.2% NaCl intake. 5. Intake of NaCl and AngII interact at many levels. It is postulated that there is an important interaction at the cellular level that can explain the above events.     

Interactions between apelin and angiotensin II on rat portal vein.

Apelin (AP), an endogenous ligand of the APJ receptor, is involved in the regulation of cardiovascular homeostasis. Regardless the multiple similarities between AP and angiotensin II (Ang II), their roles seem to be different. We studied both the interactions between Apelin 13 (AP13) and Ang II and to what extent, if any, nitric oxide (NO) is involved. The experiments were performed in endothelium-denuded or endothelium-intact rat portal vein in the presence of 10 microM N(G)-nitro L-arginine methyl ester or 10 microM aminoguanidine. AP13 did not modify the isolated rat portal vein tone by itself, but inhibited the Ang II-induced contractions acting mainly by a NO-dependent mechanism. Our results sustain the hypothesis that AP13 could increase the activity of both constitutive and inducible NO synthase on either endothelium intact or endothelium denuded rat portal vein rings.     

Des-aspartate-angiotensin-I and angiotensin IV improve glucose tolerance and insulin signalling in diet-induced hyperglycaemic mice

Although clinical studies suggested that blockade of the renin-angiotensin system may prevent diabetes, the mechanism is uncertain. As a follow-up to an earlier study, we investigated how des-aspartate-angiotensin-1 (DAA-1) and its metabolite, angiotensin IV (Ang-IV) improved glucose tolerance in diet-induced hyperglycaemic mice. Male C57BL/6J mice were fed a high-fat-high-sucrose (HFD) or normal (ND) diet for 52 weeks. HFD animals were orally administered either DAA-I (600 nmol/kg/day), Ang-IV (400 nmol/kg/day) or distilled water. Body weight, blood glucose and insulin were measured fortnightly. Inflammatory and insulin signalling transducers that are implicated in hyperglycaemia were analyzed in skeletal muscles at 52 weeks. HFD animals developed hyperglycemia, hyperinsulinemia and obesity. DAA-I and Ang-IV improved glucose tolerance but had no effect on hyperinsulinemia and obesity. Skeletal muscles of HFD animals showed increased level of ROS, gp91 of NADPH oxidase, pJNK and AT₁R-JAK-2-IRS-1 complex. Both DAA-I and Ang-IV attenuated these increases. Insulin-induced activation of IR, IRS-1, IRS-1-PI3K coupling, phosphorylation of Akt, and GLUT4 translocation were attenuated in skeletal muscles of HFD animals. The attenuation was significantly ameliorated in DAA-I-treated HFD animals. In corresponding Ang-IV treated animals, insulin induced IRAP and PI3K interaction, activation of pAkt and GLUT4 translocation, but no corresponding activation of IR, IRS-1 and IRS-1-PI3 K coupling were observed. DAA-I and Ang-IV improved glucose tolerance, insulin signalling, and para-inflammatory processes linked to hyperglycaemia. DAA-I acts via the angiotensin AT₁ receptor and activates the insulin pathway. Ang-IV acts via IRAP, which couples PI3K and activates the later part of the insulin pathway.     

Participation of angiotensin II in learning and memory. III. Interactions of angiotensin II with GABAergic drugs

The experiments were carried out on male albino rats trained and tested for retention (24 hr later) in a shuttle-box. Angiotensin II (AT II) 0.10 micrograms intracerebroventricularly (i.c.v.), gamma-aminobutyric acid (GABA) 100 micrograms i.c.v., bicuculline 0.5 and 1.0 mg/kg intraperitoneally (i.p.), and picrotoxin 0.5 and 1.0 mg/kg i.p. administered independently or in combinations immediately after training. AT II was found to improve retention. GABA also facilitated retention. Combination of AT II + GABA potentiated the memory-improving effect of AT II. Bicuculline and picrotoxin at a dose of 0.5 mg/kg did not affect retention, while at a dose of 1.0 mg/kg they improved it. Combinations of AT II + bicuculline (0.5 mg/kg) and AT II + picrotoxin (0.5 mg/kg) abolished the retention-improving effect of AT II. Bicuculline (0.5 mg/kg) or picrotoxin (0.5 mg/kg) abolished the retention-facilitating effect of the combination of AT II + GABA as well as the potentiating action of GABA on the memory effect of AT II. These results suggest the participation of GABAergic transmission in the CNS in the mechanisms of the long-term memory-improving effect of AT II.     

Participation of angiotensin II in learning and memory. II. Interactions of angiotensin II with dopaminergic drugs

The effect of angiotensin II (ATII) and of its interactions with dopaminergic drugs injected post-trial on retention in active avoidance tasks in shuttle-box-trained rats were studied. ATII at doses of 0.10 and 0.50 micrograms administered intracerebroventricularly (i.c.v.) immediately after training improved retention. The dopaminergic receptor agonist apomorphine at a dose of 0.10 mg/kg injected intraperitoneally (i.p.) facilitated retention whereas elymoclavine (a dopaminergic agonist) at a dose of 2.5 mg/kg i.p. had no effect. ATII at a dose of 0.10 micrograms i.c.v. administered after apomorphine 0.10 mg/kg or elymoclavine 2.5 mg/kg exerted a stronger retention-facilitating effect. The dopaminergic receptor antagonist haloperidol at a dose of 1 mg/kg i.p. markedly impaired retention. ATII at a dose of 0.50 micrograms administered after haloperidol (1 mg/kg) did not exercise its retention-facilitating effect. It is concluded that the retention facilitating effects of ATII are realized through interactions with brain dopaminergic transmission.     

Centrally administered oxytocin elicits exaggerated grooming in oxytocin null mice

Experiments were conducted to determine if the chronic absence of the neurotransmitter oxytocin (OT) in null mice resulted in alterations in the responsiveness and abundance of central OT receptors. Self-grooming elicited by intracerebroventricularly administered OT was studied as an indicator of the activation of central OT receptors and autoradiography was used to map the distribution and density of OT receptors in OT null and wild type mice. The intracerebroventricular administration of OT, but not vehicle, artificial cerebrospinal fluid (aCSF), produced a robust increase in grooming behavior in both OT null and wild type animals, P<.001. However, OT-induced grooming was significantly greater in OT null than wild type mice, P<.005. The enhanced grooming was selective to OT as indicated by the finding that grooming to intracerebroventricular arginine vasopressin (AVP) was of the same magnitude in both OT null and wild type mice. OT-induced grooming appears to be mediated through the activation of OT receptors because pretreatment of animals with an OT antagonist, Atosiban, abolished OT-induced grooming, but not AVP-induced grooming. OT receptor distribution and binding in brains of OT null and wild type mice were examined by autoradiography and were not significantly different. The results indicate that the chronic absence of OT in null mice leads to an increase in OT receptor responsiveness that contributes to the augmented grooming activity elicited by centrally administered OT.     

Interactions of lithium and drugs that affect signal transduction on behaviour in rats.

The therapeutic mechanism of the action of lithium in the treatment of bipolar affective disorder is not known, in spite of a burgeoning number of biochemical studies linking lithium to signal transduction processes. This article reviews a decade of studies examining the behavioural manifestations of manipulating inositol, cyclic adenosine monophosphate (cAMP) and G proteins in rats. Inositol, forskolin, dibutyryl cAMP and pertussis toxin all interacted with lithium when rearing behavior was measured. Lithium potentiated the increase in locomotion induced by injections of cholera toxin into the nucleus accumbens, consistent with the hypothesis that it inactivates inhibitory G proteins. More specific interactions were found between lithium and inositol following cholinergic and serotonergic stimulation. Inositol, but not forskolin, attenuated lithium-pilocarpine seizures and the enhancement of the serotonin syndrome; however, inositol had no effect on lithium-induced attenuation of wet dog shakes following an injection of 5-hydroxytryptophan. Behavioural evidence supports biochemical findings suggesting that lithium's interactions with the phoshphatidyl inositol and cyclic AMP signal transduction systems may be relevant to its therapeutic effects in bipolar disorder. Further research on more specific behaviours may elucidate the relevant pharmacological mechanisms underlying the therapeutic effect of lithium.     

Beta-homo-amino acid scan of angiotensin IV

Angiotensin IV, a metabolite of angiotensin II, inhibits the enzyme insulin regulated aminopeptidase or IRAP and also, although with lower potency, aminopeptidase-N (AP-N). When both beta (2)-homo amino acid- and beta (3)-homo amino acid substitutions were used, allowed the identification of H-( R)beta (2)hVal-Tyr-Ile-His-Pro-beta (3)hPhe-OH as a potent and stable Ang IV analog with high selectivity for IRAP versus AP-N and the AT1 receptor.     

The influence of methionine on rats and mice behaviour.

Methionine given ivc or ip to rats and ic to mice acts only slightly on the central nervous system of these animals; it has either stimulating or inhibiting action, depending on the test applied.     

Alzheimer's, angiotensin IV and an aminopeptidase

The angiotensin AT(4) receptor was originally defined as the specific, high affinity binding site for the hexapeptide angiotensin IV (Ang IV). Subsequently, the peptide LVV-hemorphin 7 was also demonstrated to be a bioactive ligand of the AT(4) receptor. Central administration of Ang IV or LVV-hemorphin 7 (LVV-H7) markedly enhances learning and memory in normal rodents and reverse memory deficits observed in animal models of amnesia. The high affinity binding site has a broad distribution in the brain including areas such as the hippocampus that are involved in memory processing. The high affinity Ang IV binding site (AT(4) receptor) has been identified as the transmembrane enzyme, insulin-regulated membrane aminopeptidase (IRAP). Insulin-regulated aminopeptidase is a type II integral membrane spanning protein belonging to the M1 family of aminopeptidases and in insulin-responsive cells colocalizes with GLUT4 in specific intra-cellular vesicles. Both Ang IV and LVV-H7 are competitive inhibitors of IRAP catalytic activity and are not substrates of the enzyme.     

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.     

Participation of angiotensin receptors in acute hypoxia in mice. I. Effects of angiotensin peptide receptor ligands saralasin and sarmesin.

The effects of angiotensin II (Ang II) and angiotensin receptor ligands sarmesin ([Sar1, Tyr(Me)4] Ang II) and saralasin ([Sar1, Ala8] Ang II) administered intracerebroventricularly (i.c.v.) on acute anoxic hypoxia were studied in mice. The interactions of these ligands and of amastatin (an aminopeptidase A inhibitor) after pretreatment with Ang II and sarmesin, respectively, were also studied. Ang II decreased the latency to hypoxia-induced convulsive seizures and altered survival time (increase or decrease depending on the dose). Sarmesin and saralasin significantly increased the latency to seizures as well as survival time. Pretreatment with saralasin and sarmesin antagonized the Ang II effect on the latency to seizures. Both drugs increased the Ang II effect on the survival time. Amastatin tended to increase the effect of sarmesin on the survival time. Taken together, the results suggest that the antihypoxic effect of sarmesin and saralasin is most likely due to an action on Ang II receptors, with the agents behaving as partial agonists.     

血管紧张素转化酶抑制药和阿司匹林联合用药的相互作用

血管紧张素转化酶抑制药(ACEI)和阿司匹林(Asp)广泛联合应用于治疗心血管疾病,但有关两药联合应用时的相互作用引起争论,使联合用药的安全性存在疑问,但也有研究反驳了Asp会削弱ACEI有益作用的论点,支持在ACEI治疗心血管疾病中应用Asp的安全性。本文综述了ACEI和Asp在高血压、急性心肌梗死、充血性心力衰竭应用中的相互作用,提示对两药的相互作用应予以特别关注。     

An oxytocin receptor in anococcygeus muscles isolated from male mice.

The nature of the neurohypophyseal peptide receptor in the anococcygeus muscles from male mice was investigated. The rank order of potency of naturally occurring peptides was oxytocin greater than Arg-vasotocin greater than Arg-vasopressin greater than Lys-vasopressin, which is similar to that found in the uterus and mammary gland. Selective agonists on the oxytocin (OT) receptors of the uterus and mammary gland (Thr4-OT; Gly7-OT; Thr4-Gly7-OT) were also potent agonists in the mouse anococcygeus. Competitive antagonists of uterine responses to oxytocin (dP-TyrMe-Thr4-OT; dP-TyrMe-OT; dP-Thr4-OT; dp-Orn8-OT) were also competitive antagonists of oxytocin-induced contractions of the mouse anococcygeus. It is concluded that the neurohypophyseal peptide receptor of the male mouse anococcygeus is of the oxytocin type; antagonist pA2 values suggest that this receptor resembles, but may not be identical to, the uterine oxytocin receptor. Possible physiological and pharmacological implications of these observations are discussed.     

Interactions of ligands at angiotensin II-receptors and imidazoline receptors

Ligands for angiotensin II-(AT)-receptors and imidazoline receptors have structural similarities and influence blood pressure via various mechanisms. The goal of this study was to study the specificity of various ligands by displacement experiments. Antazoline, cimetidine, clonidine, efaroxan, guanabenz, guanethidine, idazoxan, moxonidine and rilmenidine up to a concentration of 100 microM failed to displace the specific binding of [125I]Sar1,Ile8 angiotensin II at the AT1-receptor characterized by losartan (IC50 = 26 +/- 12 nM) in liver homogenate. The same substances up to 100 microM produced no reduction of specific [125I]Sar1,Ile8 angiotensin II binding to the AT2-receptor of phaeochromocytoma cell membranes characterized by PD123319 (IC50 = 20 +/- 5 nM). Displacement experiments at the imidazoline I1-receptors were performed on bovine adrenal medulla membranes using [3H]clonidine after characterization by the I1-ligand clonidine (IC50 = 459 +/- 13 nM) and the I2-ligand idazoxan (IC50 = 3.29 +/- 0.88 microM). The investigated AT-receptor ligands angiotensin II, losartan, EXP 3174 and PD123319 revealed no displacement of [3H]clonidine up to a concentration of 100 microM. The I2-receptor in liver homogenate was characterized by displacement of [3H]idazoxan by cold idazoxan and clonidine (IC50 = 0.37 +/- 0.17 and 68 +/- 31 microM, respectively). The investigated AT-receptor ligands angiotensin II, losartan and PD123319 failed to displace [3H]idazoxan specifically up to 100 microM. Hence, the tested substances showed no cross-reactivity at the corresponding AT- and I-receptors up to 100 microM, a concentration markedly higher than the plasma concentrations achieved after therapeutic application.     

Angiotensin IV elevates oxytocin levels in the rat amygdala and produces anxiolytic-like activity through subsequent oxytocin receptor activation

Introduction  

The effects of angiotensin (Ang) IV result from binding to a constitutively active metallopeptidase known as the AT₄ receptor (or oxytocinase/insulin-regulated membrane aminopeptidase). While in vitro evidence indicates that Ang IV inhibits the peptidase activity of AT₄ receptors, leading to increases in the concentrations of several peptides, including oxytocin, the consequence of inhibiting AT₄ peptidase activity in vivo remains unresolved.