NK1 receptor antagonist blocks angiotensin II responses in renin transgenic rat medulla oblongata

The functional role of PI3K in maintenance of blood pressure and baroreflex suppression in (mRen2)27 and mRen2.Lewis rat

The phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathway in brain of spontaneously hypertensive rats, but not Wistar-Kyoto (WKY) rats, contributes to elevated mean arterial pressure (MAP). The role of PI3K in the regulation of blood pressure or autonomic function in the nucleus tractus solitarii (NTS) is yet to be established in other Ang II-dependent models of hypertension. Thus, we microinjected PI3K inhibitors, wortmannin or LY294002, into the NTS, and measured MAP, baroreflex sensitivity (BRS) for heart rate (HR) control, and HR variability (HRV) in mRen2.Lewis congenic and (mRen2)27 transgenic rats. Bilateral NTS microinjections of wortmannin (100 nmol/L; 50 nL) reduced MAP in (mRen2)27 and mRen2.Lewis rats (33 ± 5 mm Hg, n = 7, and 32 ± 6 mm Hg, n = 9, respectively) for approximately 90 minutes. Spectral and sequence analysis showed improvements in spontaneous BRS and HRV (50%-100%) after treatment in both hypertensive strains. Injections of wortmannin into NTS of Hannover Sprague-Dawley or Lewis control rats failed to alter MAP, BRS, or HRV. In mRen2.Lewis, but not in control Lewis rats, LY294002 (50 μmole/L) reduced MAP and increased BRS and HRV similar to wortmannin. Thus, the pharmacologic blockade of the PI3K signaling pathway in NTS reveals an important contribution to resting MAP and BRS in rats with overexpression of the Ren2 gene.

Global disturbances in autonomic function yield cardiovascular instability and hypertension in the chromogranin a null mouse

We reported previously that chromogranin A (Chga) knockout (KO) mice are hypertensive and hyperadrenergic. Here we sought to determine the basis of such alterations by probing physiological, biochemical, and pharmacological responses to perturbations of the autonomic nervous system. In the conscious state, KO mice had substantially elevated basal high blood pressure (BP) and heart rate (HR); immobilization stress caused increments in systolic BP and HR in both wild-type (WT) and KO mice, with higher maxima but blunted increments in the KO state. Catestatin (CST; CHGA(352-372)) selectively diminished stress-induced increments in BP and HR in KO mice, implicating CST as an antihypertensive peptide, even in stressful conditions. Heightened plasma catecholamines in KO mice returned to WT level after CST. Stress caused further increments in catecholamines in WT mice but no change in KO mice. KO mice displayed diminished baroreflex sensitivity in response to either phenylephrine or sodium nitroprusside, accounting for exaggerated pressor and depressor responses to these compounds; baroreceptor function was normalized by CST. To probe the relative roles of endogenous/basal sympathetic vs. parasympathetic tone in control of BP and HR, we used the muscarinic-cholinergic antagonist atropine or the beta-adrenergic antagonist propranolol; HR and BP responses to each antagonist were exaggerated in KO animals. We conclude that ablation of Chga expression results in global disturbances in autonomic function, both sympathetic and parasympathetic, that can be abrogated (or rescued), at least in part, by replacement of CST. The results point to mechanisms whereby CHGA and its CST fragment act to control cardiovascular homeostasis.

IMPORTANCE OF THE ANGIOTENSIN TYPE 1 RECEPTOR IN ANGIOTENSIN II–INDUCED BRONCHOCONSTRICTION AND BRONCHIAL HYPERRESPONSIVENESS IN THE GUINEA PIG

Although angiotensin II (Ang II) causes bronchoconstriction and bronchial hyperresponsiveness to methacholine in mildly asthmatic patients, the responsible mechanisms for these reactions are unclear. The authors examined the effect of intravenous infusion of Ang II on airway constriction in guinea pigs. Furthermore, the effects of subthreshold concentrations of Ang II on bronchial responsiveness to methacholine were investigated. Airway opening pressure (Pao), an index of bronchoconstriction, increased dose dependently after intravenous infusion of 3 and 10 nmol/kg Ang II (72.2 and 236.5 increase above the baseline value, respectively). In another set of experiments, animals received a methacholine inhalation challenge under a constant intravenous infusion of a subthreshold dose of Ang II (2 nmol/kg/min). The Ang II infusion elicited bronchial hyperresponsiveness to methacholine. The provocative concentration of methacholine, which produced a 200% increase above the baseline Pao (PC200), decreased from 306.9 to 156.1 micrograms/mL upon Ang II infusion. Pretreatment with TCV-116, a type 1 Ang II (AT1) receptor antagonist, but not PD123319, a type 2 Ang II (AT2) receptor antagonist, dose dependently prevented both the Ang II-induced bronchoconstriction and bronchial hyperresponsiveness to methacholine. The authors conclude that Ang II caused bronchoconstriction and induced bronchial hyperresponsiveness to methacholine via the AT1 receptors and that this effect did not involve the release of other bronchoactive mediators.

Angiotensin-(1-7) and baroreflex function in nucleus tractus solitarii of (mRen2)27 transgenic rats

BACKGROUND

Endogenous angiotensin (Ang)-(1-7) enhances, while Ang II attenuates, baroreceptor sensitivity (BRS) for reflex control of heart rate (HR) in Sprague-Dawley (SD) rats. In (mRen2)27 renin transgenic rats [(mRen2)], there is overexpression of the mouse Ren2 gene in brain, leading to elevated Ang II and reduced Ang-(1-7) in brain medullary, and associated with hypertension and impaired BRS.

METHODS

We therefore tested the contribution of endogenous Ang-(1-7) to BRS for control of HR and responses to cardiac vagal chemosensitive afferent fiber activation (CVA) with phenylbiguanide (PBG) in anesthetized SD and (mRen2) 27 rats before and after bilateral nucleus of the solitary tract (nTS) injection of the Ang-(1-7) receptor antagonist (D-Ala7)-Ang-(1-7).

RESULTS

(mRen2) 27 rats exhibited a approximately 50% impairment in BRS as compared with SD (P < 0.05). (D-Ala7)-Ang-(1-7) attenuated BRS by approximately 50% in SD rats, but was without effect in (mRen2) 27 rats. (D-Ala7)-Ang-(1-7) did not alter the responses to CVA by PBG (iv bolus) in either strain. There were no differences in the depressor effects of Ang-(1-7) injected into the nTS, nor were levels of mRNA different for angiotensin-converting enzyme, angiotensin-converting enzyme 2, neprilysin, or the mas receptor in medullary tissue from SD versus (mRen2)27 rats.

CONCLUSION

Endogenous Ang-(1-7) does not provide tonic input in the nTS to modulate BRS for control of HR in (mRen2)27 rats, which may contribute to impairment of BRS in these animals.

Injections of angiotensin-converting enzyme 2 inhibitor MLN4760 into nucleus tractus solitarii reduce baroreceptor reflex sensitivity for heart rate control in rats

Injections of the angiotensin(1-7) [Ang(1-7)] antagonist [d-Ala7]-Ang(1-7) into the nucleus of the solitary tract (NTS) of Sprague-Dawley rats reduce baroreceptor reflex sensitivity (BRS) for control of heart rate by approximately 40%, whereas injections of the angiotensin II (Ang II) type 1 receptor antagonist candesartan increase BRS by 40% when reflex bradycardia is assessed. The enzyme angiotensin-converting enzyme 2 (ACE2) is known to convert Ang II to Ang(1-7). We report that ACE2 activity, as well as ACE and neprilysin activities, are present in plasma membrane fractions of the dorsomedial medulla of Sprague-Dawley rats. Moreover, we show that BRS for reflex bradycardia is attenuated (1.16 +/- 0.29 ms mmHg-1 before versus 0.33 +/- 0.11 ms mmHg-1 after; P < 0.05; n = 8) 30-60 min following injection of the selective ACE2 inhibitor MLN4760 (12 pmol in 120 nl) into the NTS. These findings support the concept that within the NTS, local synthesis of Ang(1-7) from Ang II is required for normal sensitivity for the baroreflex control of heart rate in response to increases in arterial pressure.

Baroreceptor reflex regulation in anesthetized transgenic rats with low glia-derived angiotensinogen

Endogenous angiotensin (ANG) II and ANG-(1-7) act at the nucleus tractus solitarius (NTS) to differentially modulate neural control of the circulation. The role of these peptides endogenous to NTS on cardiovascular reflex function was investigated in transgenic rats with low brain angiotensinogen (Aogen) due to glial overexpression of an antisense to Aogen (ASrAOGEN) and in Sprague-Dawley (SD) rats. Arterial baroreceptor reflex sensitivity (BRS) for control of heart rate (HR) in response to increases in mean arterial pressure (MAP) was tested before and after bilateral microinjection of the angiotensin type 1 (AT(1)) receptor blocker candesartan or the ANG-(1-7) receptor blocker (d-Ala(7))-ANG-(1-7) into the NTS of urethane-chloralose-anesthetized ASrAOGEN and SD rats. Baseline MAP was higher in ASrAOGEN than in SD rats under anesthesia (P < 0.01). Injection of candesartan or (d-Ala(7))-ANG-(1-7) decreased MAP (P < 0.01) and HR (P < 0.05) in ASrAOGEN, but not SD, rats. The BRS at baseline was similar in ASrAOGEN and SD rats. Candesartan increased BRS by 41% in SD rats (P < 0.01) but was without effect in ASrAOGEN rats. In contrast, the reduction in BRS after (d-Ala(7))-ANG-(1-7) administration was comparable in SD (31%) and ASrAOGEN rats (34%). These findings indicate that the absence of glia-derived Aogen is associated with 1) an increase in MAP under anesthesia mediated via AT(1) and ANG-(1-7) receptors within the NTS, 2) the absence of an endogenous ANG II contribution to tonic inhibition of BRS, and 3) a continued contribution of endogenous ANG-(1-7) to tonic enhancement of BRS.

Characteristics of substance P transport across the blood-brain barrier

PURPOSE

Substance P (SP; NH3(+)-Arg(+)-Pro-Lys(+)-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) belongs to a group of neurokinins that are widely distributed in the central nervous system and peripheral nervous system. The biological effects mediated by SP in the central nervous system include regulation of affective behavior, emesis, and nociception. Many of these actions are believed to be the result of the binding of SP to the neurokinin-1 (NK-1) receptor and subsequent transport across the blood-brain barrier (BBB). The objective of the study was to investigate the involvement of the NK-1 receptor in the permeation of SP across the BBB.

METHODS

Transport of 3H SP (1-13 nM) was investigated using BBMEC monolayers grown on polycarbonate membranes mounted on a Side-bi-Side diffusion apparatus. 3H SP samples were analyzed by scintillation spectrometry. Liquid chromatography-tandem mass spectrometry was used to monitor the transport at higher concentrations (micromolar).

RESULTS

SP transport across BBMEC monolayers was found to be saturable (Km = 8.57 +/- 1.59 nM, Vmax = 0.017 +/- 0.005 pmol min(-1) mg(-1) protein) in the concentration range of 0-13 nM. Significant (p < 0.05) decline in 3H SP permeation was observed in the presence of unlabeled SP and at 4 degrees C, indicating that the transport process is carrier-mediated. High-performance liquid chromatography analysis showed no significant metabolism of 3H SP in either the donor or receiver chambers. 3H SP transport was inhibited by 2-11 SP (p < 0.05) but not by any other fragments, indicating that both the C- and N-terminal regions are essential for molecular recognition by the receptor. Endocytic inhibitors (chloroquine, phenylarsine oxide, monensin, and brefeldin) did not inhibit SP transport, suggesting the involvement of a nonendocytic mechanism in SP permeation. Pro(9) SP, a high-affinity substrate for the NK-1 major subtype receptor, significantly (p < 0.05) inhibited the transport of SP. However, Sar(9)Met(O2)(11) SP, a high-affinity substrate for the NK-1 minor subtype receptor, septide, and neurokinin A, inhibitors of NK-1 and neurokinin-2 (NK-2) receptors, respectively, did not produce any inhibition of SP transport. Western blot analysis confirmed the presence of the NK-1 receptor in BBMEC monolayers.

CONCLUSIONS

The above results provide functional and molecular evidence for the existence of a carrier-mediated mechanism in the transport of SP across the BBB. The effects of specific inhibitors and the results of Western blot analyses demonstrate the involvement of the NK-1 receptor in the transport of SP across the BBB.

Growth, metabolism, and blood pressure disturbances during aging in transgenic rats with altered brain renin-angiotensin systems

Transgenic rats with targeted decreased glial expression of angiotensinogen (ASrAogen rats) did not show an increase in systolic pressure compared with Sprague-Dawley (SD) rats during aging (15-69 wk of age). ASrAogen animals had lower body weights throughout the study, similar to reports for animals with systemic knockout of angiotensinogen or treated long term with renin-angiotensin system (RAS) blockers. Further characterization of indexes of growth and metabolism in ASrAogen rats compared with (mRen2)27 and SD rats, which express elevated versus normal brain and tissue angiotensin II levels, respectively, revealed that serum leptin was 100-200% higher in SD and (mRen2)27 rats at 46 wk and 69 wk of age. Consistent with low serum leptin, ASrAogen rats had higher food intake (73%) compared with SD or (mRen2)27 rats. (mRen2)27 rats had higher resting insulin levels than ASrAogen rats at all ages. Insulin levels were constant during aging in ASrAogen rats, whereas an increase occurred in SD rats, leading to higher insulin levels at 46 and 69 wk of age compared with ASrAogen rats. IGF-1 was comparable among strains at all ages, but (mRen2)27 rats had longer and ASrAogen rats had shorter tail lengths versus SD rats at 15 wk of age. In conclusion, reduced expression of glial angiotensinogen blunts the age-dependent rise in insulin levels and weight gain, findings that mimic the effects of long-term systemic blockade of the RAS or systemic knockout of angiotensinogen. These data implicate glial angiotensinogen in the regulation of body metabolism as well as hormonal mechanisms regulating blood pressure.

Intracisternal galanin/angiotensin II interactions in central cardiovascular control

The aim of this work was to investigate the interactions between angiotensin II (Ang II) and galanin(1-29) [GAL(1-29)] or its N-terminal fragment galanin(1-15) [GAL(1-15)] on central cardiovascular control. The involvement of angiotensin type1 (AT1) receptor subtype was analyzed by the AT1 antagonist, DuP 753. Anesthesized male Sprague-Dawley rats received intracisternal microinjections of Ang II (3 nmol) with GAL(1-29) (3 nmol) or GAL(1-15) (0.1 nmol) alone or in combination. The changes in mean arterial pressure (MAP) and heart rate (HR) recorded from the femoral artery were analyzed. The injection of Ang II and GAL(1-15) alone did not produce any change in MAP. However, coinjections of both Ang II and GAL(1-15) elicited a significant vasopressor response. This response was blocked by DuP 753. Ang II and GAL(1-15) alone produced an increase in HR. The coinjections of Ang II with GAL(1-15) induced an increase in HR not significantly different from the tachycardia produced by each peptide. The presence of DuP 753 counteracted this response. GAL(1-29) alone elicited a transient vasopressor response that disappeared in the presence of Ang II. The coinjections of Ang II with GAL(1-29) and with DuP 753 restored the transient vasopressor effect produced by GAL(1-29). GAL(1-29) produced a slight but significant tachycardic effect that was not modified in the presence of Ang II. The presence of DuP 753 did not modify the tachycardic response produced by Ang II and GAL(1-29). These results give indications for the existence of a differential modulatory effect of Ang II with GAL(1-15) and GAL(1-29) on central blood pressure response that might be dependent on the activity of the angiotensin AT1 receptor subtype.

Angiotensin II modulates the cardiovascular responses to microinjection of NPY Y1 and NPY Y2 receptor agonists into the nucleus tractus solitarii of the rat

The aim of this work was to investigate the modulation of the cardiovascular effects of neuropeptide Y (NPY) in the nucleus tractus solitarii (NTS) by angiotensin II (Ang II) and to determine the NPY receptor subtype involved in this modulation. Anesthesized Sprague-Dawley rats received microinjections in the NTS of Ang II (threshold and ED(50) doses) with NPY Y(1) agonist Leu(31)Pro(34)NPY and NPY Y(2) agonist NPY(13-36) (threshold and ED(50) doses). The changes in mean arterial pressure (MAP) and heart rate (HR) recorded in the femoral artery were analyzed during 60 min after the microinjections. The injection of threshold doses of Ang II, Y(1) agonist or Y(2) agonist alone did not produce any change in cardiovascular parameters. However, the co-injections into the NTS of threshold doses of both Ang II and the Y(1) agonist elicited significant increases of MAP and HR of about 12 and 10%, respectively. The co-administration of threshold doses of Ang II with the Y(2) agonist also induced a significant vasopressor response. The vasodepressor and bradycardiac effect of an ED(50) dose of the Y(1) agonist was significantly counteracted (P<0.01) by a threshold dose of Ang II. The vasopressor effect elicited by an ED(50) dose of the Y(2) agonist was significantly enhanced by a threshold dose of Ang II (P<0.01). No significant change of cardiovascular responses elicited by an ED(50) dose of Ang II was observed in the presence of threshold doses of the Y(1) agonist or of the Y(2) agonist. The present study gives functional evidences for a differential modulatory activity of Ang II on the cardiovascular responses mediated by Y(1) and Y(2) receptor subtypes, which may be of relevance for central cardiovascular regulation in the NTS.

Drinking behavior elicited by central injection of angiotensin II: roles for protein kinase C and Ca2+/calmodulin-dependent protein kinase II

Prior studies utilizing neurons cultured from the hypothalamus and brain stem of newborn rats have demonstrated that ANG II-induced modulation of neuronal firing involves activation of both protein kinase C (PKC) and Ca2+/calmodulin-dependent protein kinase II (CaMKII). The present studies were performed to determine whether these signaling molecules are also involved in physiological responses elicited by ANG II in the brain in vivo. Central injection of ANG II (10 ng/2 microl) into the lateral cerebroventricle (icv) of Sprague-Dawley rats increased water intake in a time-dependent manner. This ANG II-mediated dipsogenic response was attenuated by central injection of the PKC inhibitors chelerythrine chloride (0.5-50 microM, 2 microl) and Go-6976 (2.3 nM, 2 microl) and by the CaMKII inhibitor KN-93 (10 microM, 2 microl). Conversely, icv injection of chelerythrine chloride (50 microM, 2 microl) and KN-93 (10 microM, 2 microl) had no effect on the dipsogenic response elicited by central injection of carbachol (200 ng/2 microl). Furthermore, injection of ANG II (10 ng/2 microl) icv increases the activity of both PKC-alpha and CaMKII in rat septum and hypothalamus. These data suggest that signaling molecules involved in ANG II-induced responses in vitro are also relevant in physiological responses elicited by ANG II in the whole animal model.

Pharmacological aspects of anticancer drug-induced emesis with emphasis on serotonin release and vagal nerve activity

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somatoautonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs such as cisplatin is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestine and the brainstem. It is proposed that cytotoxic drugs evoke 5-HT release from the enterochromaffin (EC) cells in the intestinal mucosa and that the released 5-HT stimulates the 5-HT receptors on the adjacent vagal afferent nerves. The depolarization of the vagal afferent nerves stimulates the vomiting center in the brainstem and eventually induces a vomiting reflex. 5-HT released from EC cells seems to mediate the cisplatin-induced emesis sensitive to 5-HT(3) receptor antagonists. The release of 5-HT from the EC cells, however, is regulated by polymodal mechanisms on autoreceptors or heteroreceptors. The precise role of 5-HT on the occurrence of vomiting has not been fully elucidated. The present review aims to describe the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagus nerve activity. Various methods for predicting emesis are also evaluated.

Angiotensin potentiates excitatory sensory synaptic transmission to medial solitary tract nucleus neurons

Femtomole doses of angiotensin (ANG) II microinjected into nucleus tractus solitarii (nTS) decrease blood pressure and heart rate, mimicking activation of the baroreflex, whereas higher doses depress this reflex. ANG II might generate cardioinhibitory responses by augmenting cardiovascular afferent synaptic transmission onto nTS neurons. Intracellular recordings were obtained from 99 dorsal medial nTS region neurons in rat medulla horizontal slices to investigate whether ANG II modulated short-latency excitatory postsynaptic potentials (EPSPs) evoked by solitary tract (TS) stimulation. ANG II (200 fmol) increased TS-evoked EPSP amplitudes 20-200% with minimal membrane depolarization in 12 neurons excited by ANG II and glutamate, but not substance P (group A). Blockade of non-N-methyl-d-aspartate receptors eliminated TS-evoked EPSPs and responses to ANG II. ANG II did not alter TS-evoked EPSPs in 14 other neurons depolarized substantially by ANG II and substance P (group B). ANG II appeared to selectively augment presynaptic sensory transmission in one class of nTS neurons but had only postsynaptic effects on another group of cells. Thus ANG II is likely to modulate cardiovascular function by more than one nTS neuronal pathway.

Angiotensin peptides as neurotransmitters/neuromodulators in the dorsomedial medulla

1. The present review provides an update on evidence of the neurotransmitter pathways and location of receptors within the nucleus tractus solitarii (NTS) mediating the baroreflex and other haemodynamic actions of angiotensin (Ang) II. 2. A series of studies suggests a significant role for substance P in the acute cardiovascular and carotid sinus chemoreceptor facilitatory actions of AngII in the NTS. The use of antisense oligonucleotides to AT1 receptors indicates both pre- and post-synaptic AngII receptors are likely to be involved in these actions. 3. With respect to baroreceptor reflex actions, it is clear that endogenous AngII impairs the gain for operation of the baroreceptor reflex, because AT1 receptor antagonists facilitate reflex function. This effect is either independent of substance P or involves inhibition of release. Moreover, initial data obtained using antisense oligonucleotides to AT1 receptors suggest that, in the NTS, the effect of endogenous AngII on the baroreceptor reflex is mainly due to presynaptic actions on vagal or carotid sinus afferent fibres. In contrast, the level of endogenous AngII within the NTS appears to have variable effects on activation of cardiopulmonary vagal afferent fibres by phenylbiguanide. These results indicate a divergence of effects of AngII on reflexes evoked by these two different types of sensory input. 4. Use of transgenic rats with alterations in brain angiotensin peptides allowed us to assess the effect of long-term alterations in brain Ang peptides on reflex function. We studied (mRen2)27 transgenic rats (TGR(mRen2)) with high brain medulla AngII levels and transgenic rats with angiotensinogen (Aogen) antisense linked to glial fibrillary acidic protein promoter (TGR(ASrAogen)) with greatly reduced brain Aogen. The reflex evoked by activation of cardiac vagal chemosensitive afferent fibres was enhanced in TGR(ASrAogen), whereas the baroreceptor reflex control of heart rate was attenuated in TGR(mRen2), further confirming a divergence of effects of AngII on these two sensory modalities. 5. The overall results are consistent with a sustained inhibitory effect of AngII on the baroreceptor reflexes, with dose-dependent or activation-dependent effects on cardiac vagal afferent fibre activation. Moreover, alterations in substance P pathways may contribute to the actions of AngII on reflex function.

Angiotensin AT1 receptor signalling pathways in neurons

1. The aim of the present article is to review the intracellular signal transduction pathways that are influenced by the peptide angiotensin (Ang) II, acting via its type 1 (AT1) receptor, in neurons. 2. The AT1 receptors couple to a wide variety of signalling pathways in peripheral tissues, such as kidney, heart and vascular smooth muscle. A similar diversity of signalling mechanisms exists for AT1 receptors in neurons. 3. We outline the known neuronal AT1 receptor signalling pathways as they relate to function. Pathways that couple activation of AT1 receptors to short-term changes in neuronal membrane ionic currents and firing rate will be reviewed. These are different from the pathways that elicit longer-term changes in enzyme activity and gene expression and, ultimately, increases in noradrenaline synthesis. 4. Novel AT1 receptor signalling pathways discovered through gene expression profiling and their potential functional significance have been discussed.

Substance P: a new era, a new role

Substance P has been extensively studied and is considered the prototypic neuropeptide of the more than 50 known neuroactive molecules. The understanding of substance P has evolved beyond the original concept as the pain transmitter of the dorsal horn. Animal and genetic research, recent developments of nonpeptide substance P antagonists, and important changes in the understanding of neurotransmission have each contributed to the current understanding of substance P After 7 decades, the physiologic role of substance P is known as a modulator of nociception, involved in signaling the intensity of noxious or aversive stimuli. Genetic studies in mice and development of substance P antagonists provide more recent results that support the redefinition of the central role of substance P Evidence suggests that this neuropeptide is an integral part of central nervous system pathways involved in psychologic stress.

Neurochemistry and neuropharmacology of emesis - the role of serotonin

Emesis is an instinctive defense reaction caused by the somato-autonomic nerve reflex which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs and radiation is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestinal mucosa and in the brainstem. 5-HT released from enterochromaffin (EC) cells, which synthesize and secrete 5-HT, stimulates the 5-HT(3) receptors on the adjacent vagal afferent nerves. This vagal afferent nerve depolarization may evoke the vomiting reflex. This review describes the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release from EC cells and afferent vagus nerve activity.

Mode of action of ANG II on ion transport in guinea pig distal colon

The effect of ANG II on mucosal ion transport and localization of ANG type 1 receptor (AT(1)R) in the guinea pig distal colon was investigated. Submucosal/mucosal segments were mounted in Ussing flux chambers, and short-circuit current (I(sc)) was measured as an index of ion transport. Serosal addition of ANG II produced a concentration-dependent (10(-9)-10(-5) M) increase in I(sc). The maximal response was observed at 10(-6) M; the increase in I(sc) was 164.4 +/- 11.8 microA/cm(2). The ANG II (10(-6) M)-evoked response was mainly due to Cl(-) secretion. Tetrodotoxin, atropine, the neurokinin type 1 receptor antagonist FK-888, and piroxicam significantly reduced the ANG II (10(-6) M)-evoked response to 28, 45, 58, and 16% of control, respectively. Pretreatment with prostaglandin E(2) (10(-5) M) resulted in a threefold increase in the ANG II-evoked response. The AT(1)R antagonist FR-130739 completely blocked ANG II (10(-6) M)-evoked responses, whereas the ANG type 2 receptor antagonist PD-123319 had no effect. Localization of AT(1)R was determined by immunohistochemistry. In the immunohistochemical study, AT(1)R-immunopositive cells were distributed clearly in enteric nerves and moderately in surface epithelial cells. These results suggest that ANG II-evoked electrogenic Cl(-) secretion may involve submucosal cholinergic and tachykinergic neurons and prostanoid synthesis pathways through AT(1)R on the submucosal plexus and surface epithelial cells in guinea pig distal colon.

Angiotensin peptides and baroreflex control of sympathetic outflow: pathways and mechanisms of the medulla oblongata

The baroreceptor reflex is a relatively high gain control system that maintains arterial pressure within normal limits. To a large extent, this is accomplished through central neural pathways responsible for autonomic outflow residing in the medulla oblongata. The circulating renin-angiotensin system also contributes to the regulation of blood pressure, predominantly through its effects on the control of hydromineral balance and fluid volume. All the components of the renin-angiotensin system are also found in the brain. One of the principal products of the renin-angiotensin system cascade (brain or blood), angiotensin II, modulates the baroreceptor reflex by diminishing the sensitivity of the reflex and shifting the operating point for regulation of sympathetic outflow to higher blood pressures. This paper reviews our current knowledge about the neuronal pathways in the medulla oblongata through which angiotensin peptides alter the baroreceptor reflex control of sympathetic nerve activity. Emphasis is placed on the probable components and neural mechanisms of the medullary baroreflex arc that account for the ability of angiotensin peptides to change the sensitivity of the baroreceptor reflex and to shift the baroreceptor reflex control of sympathetic outflow to higher blood pressures in a pressure-independent manner.

Differential effects of angiotensin II in the nucleus tractus solitarii of the rat--plausible neuronal mechanism

1. Cellular mechanisms of the actions of angiotensin II (ANGII) within the nucleus of the solitary tract (NTS) were studied using rat brain slices in 78 neurones recorded in the whole-cell configuration. Twenty-nine per cent of cells had an on-going activity and with only one exception these cells responded to tractus solitarii (TS) stimulation with a monophasic excitatory postsynaptic potential (EPSP). In approximately half of the silent cells, TS stimulation evoked an EPSP-inhibitory postsynaptic potential (IPSP) complex. 2. The ANGII (200 or 1000 nM) effect on TS-evoked EPSPs depended on the cell subpopulation. In cells with on-going activity, ANGII (1000 nM) increased evoked EPSP amplitude by +70 +/- 13 % (means +/- s.e.m., n = 5) but reduced it (200 and 1000 nM) in silent cells where both evoked EPSPs and IPSPs were present. ANGII either increased TS-evoked IPSP conductances in cells where they were detectable or revealed an evoked IPSP (200 nM ANGII: IPSP conductance increased from 70 +/- 29 to 241 +/- 34 pS; n = 11). All ANGII effects were prevented by the ANGII type 1 (AT1) receptor blocker losartan. Since 200 nM ANGII did not increase responses to iontophoretically applied GABA, the effect of ANGII on TS-evoked IPSPs may occur presynaptically. 3. The neurokinin type 1 (NK1) receptor antagonist CP-99,994 (5 microM) blocked the ANGII-induced increase in EPSPs but had no effect on TS-evoked IPSP potentiation by ANGII. 4. Thus, ANGII can potentiate both inhibitory and excitatory synaptic transmission within different subpopulations of NTS neurones. Potentiation of evoked EPSPs, but not of IPSPs, involves activation of NK1 receptors. The balance of these actions of ANGII could be reflex specific: for the baroreflex circuitry the inhibitory action might predominate while the peripheral chemoreceptor reflex may be facilitated due to enhanced excitatory transmission.

Blood pressure reduction and diabetes insipidus in transgenic rats deficient in brain angiotensinogen

Angiotensin produced systemically or locally in tissues such as the brain plays an important role in the regulation of blood pressure and in the development of hypertension. We have established transgenic rats [TGR(ASrAOGEN)] expressing an antisense RNA against angiotensinogen mRNA specifically in the brain. In these animals, the brain angiotensinogen level is reduced by more than 90% and the drinking response to intracerebroventricular renin infusions is decreased markedly compared with control rats. Blood pressure of transgenic rats is lowered by 8 mmHg (1 mmHg = 133 Pa) compared with control rats. Crossbreeding of TGR(ASrAOGEN) with a hypertensive transgenic rat strain exhibiting elevated angiotensin II levels in tissues results in a marked attenuation of the hypertensive phenotype. Moreover, TGR(ASrAOGEN) exhibit a diabetes insipidus-like syndrome producing an increased amount of urine with decreased osmolarity. The observed reduction in plasma vasopressin by 35% may mediate these phenotypes of TGR(ASrAOGEN). This new animal model presenting long-term and tissue-specific down-regulation of angiotensinogen corroborates the functional significance of local angiotensin production in the brain for the central regulation of blood pressure and for the pathogenesis of hypertension.

Antiemetic effects of sendide, a peptide tachykinin NK1 receptor antagonist, in the ferret

The antiemetic activity of sendide, a new peptide tachykinin NK1 receptor antagonist, against cisplatin-induced emesis was investigated using ferrets. The frequency of cisplatin (10 mg/kg, i.p.)-induced retching (104.6 +/- 14.3/6 h) and vomiting (19.0 +/- 3.0/6 h) was significantly reduced by pretreatment with sendide (3.0 mg/kg, s.c.) (14.0 +/- 8.1/6 h and 1.8 +/- 1.2/6 h, respectively). Intravenous bolus injection of substance P (1-10 microg/kg) or 5-hydroxytryptamine (5-HT) (10-50 microg/kg) produced a dose-dependent increase in the abdominal afferent vagus nerve activity. The change from pre-injection level in the afferent nerve activity induced by substance P (1 microg/kg, i.v.) (453.7 +/- 51.5%) was significantly reduced by pretreatment with either sendide (100 microg/kg, i.v.) (276.1 +/- 50.1%, P < 0.05) or granisetron, a 5-HT3 receptor antagonist (1 mg/kg, i.v.) (146.3 +/- 14.0%, P < 0.01). The amount of 5-HT released into the solution during a 1-h exposure to 2-methyl-5-HT (10(-6) M), a 5-HT3 receptor agonist, was significantly increased (317.9 +/- 46.7%, P < 0.05) compared with that of the control tissues (160.4 +/- 8.1%). The 2-methyl-5-HT-induced 5-HT release was significantly inhibited by administration of sendide (10(-6) M) (174.0 +/- 21.6%, P < 0.05) or granisetron (10(-6) M) (186.6 +/- 27.3%, P < 0.05). Since sendide does not penetrate the central nervous system, these results suggest that the antiemetic effects of sendide are due to the inhibition of NK1 and 5-HT3 receptors on the emetic peripheral detector sites.