Diuretic and natriuretic action of adrenomedullin administered intracerebroventricularly in conscious rats

Effect of Valsartan on Cerebellar Adrenomedullin System Dysregulation During Hypertension

Adrenomedullin (AM) and its receptors components, calcitonin-receptor-like receptor (CRLR), and receptor activity-modifying protein (RAMP1, RAMP2, and RAMP3) are expressed in cerebellum. Cerebellar AM, AM binding sites and receptor components are altered during hypertension, suggesting a role for cerebellar AM in blood pressure regulation. Thus, we assessed the effect of valsartan, on AM and its receptor components expression in the cerebellar vermis of Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Additionally, we evaluated AM action on superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activity, and thiobarbituric acid reactive substances (TBARS) production in cerebellar vermis. Animals were treated with valsartan or vehicle for 11 days. Rats were sacrificed by decapitation; cerebellar vermis was dissected; and AM, CRLR, RAMP1, RAMP2, and RAMP3 expression was quantified by Western blot analysis. CAT, SOD, and GPx activity was determined spectrophotometrically and blood pressure by non-invasive plethysmography. We demonstrate that AM and RAMP2 expression was lower in cerebellum of SHR rats, while CRLR, RAMP1, and RAMP3 expression was higher than those of WKY rats. AM reduced cerebellar CAT, SOD, GPx activities, and TBARS production in WKY rats, but not in SHR rats. Valsartan reduced blood pressure and reversed the altered expression of AM and its receptors components, as well the loss of AM capacity to reduce antioxidant enzyme activity and TBARS production in SHR rats. These findings demonstrate that valsartan is able to reverse the dysregulation of cerebellar adrenomedullinergic system; and they suggest that altered AM system in the cerebellum could represent the primary abnormality leading to hypertension.

Dysregulation of Cerebellar Adrenomedullin Signaling During Hypertension

Adrenomedullin (AM) is a peptide involved in blood pressure regulation. AM activates three different receptors, the AM type 1 (AM1), type 2 (AM2), and calcitonin gene-related peptide 1 (CGRP1) receptors. AM triggers several signaling pathways such as adenylyl cyclase (AC), guanylyl cyclase (GC), and extracellular signal-regulated kinases (ERK) and modulates reactive oxygen species (ROS) metabolism. Cerebellar AM, AM-binding sites, and its receptor components are altered during hypertension, although it is unknown if these alterations are associated with changes in AM signaling. Thus, we assessed AM signaling pathways in cerebellar vermis of 16-week-old Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). Animals were sacrificed by decapitation, and cerebellar vermis was microdissected under stereomicroscopic control. Tissue was stimulated in vitro with AM. Then the production of cyclic guanosine monophosphate (cGMP), nitric oxide (NO) and cyclic adenosine monophosphate (cAMP) were assessed along with ERK1/2 activation and three antioxidant enzymes' activity: glutathione peroxidase (GPx), catalase (CAT), and superoxide dismutase (SOD). Our findings demonstrate that in the cerebellar vermis of normotensive rats, AM increases cGMP, NO, cAMP production, and ERK1/2 phosphorylation, while decreases basal antioxidant enzyme activity. In addition, AM antagonizes angiotensin II (ANG II)-induced increment of antioxidant enzyme activity. Hypertension blunts AM-induced cGMP and NO production and AM-induced decrease of antioxidant enzyme activity. Meanwhile, AM-induced effects on cAMP production, ERK1/2 activation, and AM-ANG II antagonism were not altered in SHR rats. Our results support a dysregulation of several AM signaling pathways during hypertension in cerebellar vermis.

Cerebellar Adrenomedullinergic System. Role in Cardiovascular Regulation

Adrenomedullin (AM) is a multifunctional peptide which exerts numerous biological activities through the activation of AM1 (CRLR + RAMP2) and AM2 (CRLR + RAMP3) receptors. AM immunoreactivity, AM binding sites and CRLR, RAMP1, RAMP2 and RAMP3 are expressed in rat cerebellar vermis. AM binding sites are discretely and differentially distributed in the rat cerebellar cortex with higher levels detected in SHR when compared with WKY rats. In addition, there is an up-regulation of cerebellar CGRP1 (CRLR + RAMP1) and AM2 (CRLR + RAMP3) receptors and a down-regulation of AM1 (CRLR + RAMP2) receptor during hypertension associated with a decreased AM expression. These changes may constitute a mechanism which contributes to the development of hypertension, and supports the notion that cerebellar AM is involved in the regulation of blood pressure. Cerebellar AM activates ERK, increases cAMP, cGMP and nitric oxide, and decreases antioxidant enzyme activity. These effects are mediated through AM₁ receptor since they are blunted by AM(22-52). AM-stimulated cAMP production is mediated through AM2 and CGRP receptors. In vivo administration of AM into the cerebellar vermis caused a profound, specific and dose-dependent hypotensive effect in SHR, but not in normotensive WKY rats. This effect was mediated through AM1 receptor since it was abolished by AM(22-52). In addition, AM injected into the cerebellar vermis reduced vasopressor response to footshock stress. These findings demonstrate dysregulation of cerebellar AM system during hypertension, and suggest that cerebellar AM plays an important role in the regulation of blood pressure. Likewise, they constitute a novel mechanism of blood pressure control which has not been described so far.

Role of cerebellar adrenomedullin in blood pressure regulation

Adrenomedullin (AM) and their receptor components, calcitonin-receptor-like receptor (CRLR) and receptor activity-modifying protein (RAMP1, RMP2 and RAMP3) are widely expressed in the central nervous system, including cerebellum. We have shown that AM binding sites are altered in cerebellum during hypertension, suggesting a role for cerebellar adrenomedullinergic system in blood pressure regulation. To further evaluate the role of AM in cerebellum, we assessed the expression of AM, RAMP1, RAMP2, RAMP3 and CRLR in the cerebellar vermis of 8 and 16week old spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. In addition, the effect of microinjection of AM into rat cerebellar vermis on arterial blood pressure (BP) was determined. Animals were sacrificed by decapitation and cerebellar vermis was dissected for quantification of AM, CRLR, RAMP1, RAMP2 and RAMP3 expression using western blot analysis. Another group of male, 16week old SHR and WKY rats was anesthetized, and a cannula was implanted in the cerebellar vermis. Following recovery AM (0.02 to 200pmol/5μL) or vehicle was injected into cerebellar vermis. BP was determined, before and after treatments, by non-invasive plethysmography. In addition, to establish the receptor subtype involved in AM action in vivo, animals received microinjections of AM22-52 (200pmol/5μL), an AM1 receptor antagonist, or the CGRP1 receptor antagonist, CGRP8-37 (200pmol/5μL) into the cerebellar vermis, administered simultaneously with AM or vehicle microinjection. Cannulation was verified post mortem with the in situ injection of a dye solution. Our findings demonstrated that the expression of CRLR, RAMP1 and RAMP3 was higher in cerebellum of SHR rats, while AM and RAMP2 expression was lower than those of WKY rats, both in 8 and 16week old rats. In vivo microinjection of AM into the cerebellar vermis caused a profound, dose dependent, hypotensive effect in SHR but not in normotensive WKY rats. Coinjections of a putative AM receptor antagonist, AM22-52 abolished the decreases in mean arterial pressure (MAP) evoked by AM, showing that AM acts through its AM1 receptor in the vermis to reduce MAP. These findings demonstrate a dysregulation of cerebellar AM-system during hypertension, and suggest that cerebellar AM plays an important role in the regulation of BP. Likewise; they constitute a novel mechanism of BP control which has not been described so far.

Potent osmoregulatory actions of homologous adrenomedullins administered peripherally and centrally in eels

The teleost adrenomedullin (AM) family consists of three groups, AM1/AM4, AM2/AM3, and AM5. In the present study, we examined the effects of homologous AM1, AM2, and AM5 on drinking and renal function after peripheral or central administration in conscious freshwater eels. AM2 and AM5, but not AM1, exhibited dose-dependent (0.01-1 nmol/kg) dipsogenic and antidiuretic effects after intra-arterial bolus injection. The antidiuretic effect was significantly correlated with the degree of associated hypotension. To avoid the potential indirect osmoregulatory effects of AM-induced hypotension, infusion of AMs was also performed at nondepressor doses. Drinking was enhanced dose-dependently at 0.1-3 pmol.kg(-1).min(-1) of AM2 and AM5, matching the potency and efficacy of angiotensin II (ANG II), the most potent dipsogenic hormone known thus far. AM2 and AM5 infusion also induced mild antidiuresis, while AM1 caused antinatriuresis. Additionally, AMs were injected into the third and fourth ventricles of conscious eels to assess their site of dipsogenic action. However, none of the AMs at 0.05-0.5 nmol induced drinking, while ANG II was highly dipsogenic. AM2 and ANG II injected into the third ventricle increased arterial pressure while AM5 decreased it in a dose-dependent manner, and both AM2 and AM5 decreased blood pressure when injected into the fourth ventricle. These data suggest that circulating AM2 and AM5 act on a target site in the brain that lacks the blood-brain barrier. Collectively, the present study showed that AM2 and AM5 are potent osmoregulatory hormones in the eel, and their actions imply involvement in seawater adaptation of this euryhaline species.

Adrenomedullin facilitates reinnervation of phenol-injured perivascular nerves in the rat mesenteric resistance artery

Our previous report showed that innervation of calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-containing nerves in rat mesenteric resistance arteries was markedly reduced by topical application of phenol, and that nerve growth factor (NGF) facilitates the reinnervation of both nerves. We also demonstrated that a CGRP superfamily peptide, adrenomedullin, is distributed in perivascular nerves of rat mesenteric resistance arteries. In the present study, we investigated the influence of adrenomedullin on the reinnervation of mesenteric perivascular nerves following topical phenol treatment. Under pentobarbital-Na anesthesia, 8-week-old Wistar rats underwent in vivo topical application of phenol (10% phenol in 90% ethanol) to the superior mesenteric artery proximal to the bifurcation of the abdominal aorta. After the treatment, the animals were subjected to immunohistochemistry of the third branch of small arteries proximal to the intestine and to vascular responsiveness testing on day 7. Topical phenol treatment caused marked reduction of the density of NPY-like immunoreactive (LI)- and CGRP-LI nerve fibers in the arteries. Adrenomedullin (360 or 1000 ng/h) or NGF (250 ng/h), which was administered intraperitoneally for 7 days using an osmotic mini-pump immediately after topical phenol treatment, significantly increased the density of CGRP-LI- and NPY-LI nerve fibers compared with saline. Treatment with adrenomedullin (1000 ng/h) or NGF restored adrenergic nerve-mediated vasoconstriction and CGRP nerve-mediated vasodilation in the perfused mesenteric artery treated topically with phenol. These results suggest that adrenomedullin, like NGF, has a facilitatory effect on the reinnervation of perivascular nerves.

Evolutionary history of the calcitonin gene-related peptide family in vertebrates revealed by comparative genomic analyses

The calcitonin gene-related peptide (CGRP) family is composed of CGRP, amylin and adrenomedullin (AM) in mammals. In teleost fish, AM forms an independent subfamily of five members (AM1-5), which inspired us to trace the evolutionary history of the CGRP family throughout vertebrates by comparative genomic approach. Linkage mapping and synteny analyses of the CGRP family genes in medaka, Oryzias latipes, revealed that AM1/CGRP, AM2/amylin, and AM5 genes were located on respective proto-chromosomes before the divergence of teleost lineage. In teleost fish, additional whole genome duplication generated AM1/4, CGRP1/2, AM2/3, but one of the duplicated amylin and AM5 genes was silenced. In mammals, the amylin or AM2 gene was translocated to different chromosomes, while the CGRP gene was multiplied in tandem to generate CGRP-alpha,beta, and recently identified calcitonin receptor-stimulating peptide genes. Based on these data, we identified a novel AM5 gene in several mammalian species as we previously did for AM2.

Possible involvement of adrenomedullin in lipopolysaccharide-induced small-intestinal motility changes in conscious rats

BACKGROUND

Adrenomedullin is a vasodilator peptide that displays a variety of effects, such as hypotension and vasodilatation. The aim of this study was to test the effect of intravenous adrenomedullin on the motility pattern of the small intestine, and the functional involvement of adrenomedullin in endotoxin-induced small-intestinal motility disturbance.

METHODS

Jejunal motility was recorded in fasted conscious rats, using miniature strain-gauge force transducers sutured to the serosal surface of the small-intestinal wall.

RESULTS

Intravenous administration of adrenomedullin at doses of 3, 6, and 10 microg/kg per min over 30 min disrupted phase 3 of the migrating motor complex, with the disruption lasting for 61.9 +/- 5.1, 52.2 +/- 10.6, and 74.1 +/- 25.2 min, respectively. The interval from drug administration to the onset of disruption decreased as the dose of adrenomedullin increased to 41.5 +/- 11.0, 11.6 +/- 3.4, and 0 min, respectively (P < 0.05). An increase in the motility index was also dose-dependent (P < 0.05) at these doses of adrenomedullin. Lipopolysaccharide (50 microg/kg) induced disruption of phase 3, which lasted for 138.7 +/- 5.4 min. Previous administration of the putative adrenomedullin-receptor antagonist, AM (22-52), at a 50 microg/kg dose, attenuated the disruption induced by lipopolysaccharide to 74.4 +/- 3.5 min (P < 0.01).

CONCLUSIONS

Our findings (1) suggest that intravenous adrenomedullin causes small-intestinal motility disturbances, and (2) support the hypothesis that adrenomedullin overproduction plays an important role in lipopolysaccharide-induced disruption of the motility pattern in the small intestine in rats.

Sympatho-Inhibitory Action of Endogenous Adrenomedullin Through Inhibition of Oxidative Stress in the Brain

Central sympathetic activation is one of the possible mechanisms underlying hypertension, in which reactive oxygen species may play a role. Thus, we examined whether adrenomedullin, an antioxidant peptide, is involved in the central regulation of arterial pressure through sympatho-modulatory action. Adrenomedullin knockout mice were fed with high-salt diet for 4 weeks to stimulate adrenomedullin production. In the wild-type littermates, brain adrenomedullin content was significantly increased with salt loading, but not in the knockout mice. Intracerebroventricular hyperosmotic saline increased arterial pressure and sympathetic nerve activity in a dose-dependent fashion. With the normal salt diet, the hyperosmotic saline-induced response did not significantly differ between the knockout and wild-type mice; with the high-salt diet, however, the response was significantly greater in the knockout mice than in wild-type littermates (arterial pressure: 35.3±5.7% versus 20.1±2.1%, P <0.05; sympathetic nerve activity: 30.3±4.8% versus 15.9±1.5%, P <0.05; respectively). Moreover, pretreatment with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol), a membrane-permeable superoxide dismutase mimetic, inhibited the augmented response to central hyperosmotic saline in salt-loaded knockout mice. Consistently, the hyperosmotic saline-induced production of reactive oxygen species, measured by the lucigenin chemiluminescence method, was significantly greater in the isolated hypothalamus of salt-loaded knockout mice than in that of salt-loaded wild-type ones. In conclusion, endogenous adrenomedullin in the brain may inhibit sympathetic activation through its antioxidant action.

A possible mechanism for the action of adrenomedullin in brain to stimulate stress hormone secretion

Adrenomedullin (AM) has been reported to have actions at each level of the hypothalamo-pituitary-adrenal (HPA) axis, suggesting that the peptide plays a role in the organization of the neuroendocrine responses to stress. We examined the mechanism by which AM regulates the central nervous system branch of the HPA axis as well as the possible role of AM in the modulation of the releases of two other hormones, prolactin and GH, whose secretions also are altered by stress. Intracerebroventricular administration of AM led to elevated plasma corticosterone levels in unrestrained, conscious male rats. This effect was abrogated by pretreatment with a CRH antagonist, suggesting that AM activates the HPA axis by causing the release of CRH into hypophyseal portal vessels. In addition, AM given intracerebroventricularly stimulated the release of prolactin but did not alter the secretion of GH. We propose that AM produced in the brain may be an important neuromodulator of the hormonal stress response.

The effect of adrenomedullin, amylin fragment 8-37 and calcitonin gene-related peptide on contractile force, heart rate and coronary perfusion pressure in isolated rat hearts

The effect of human adrenomedullin, human amylin fragment 8-37 (amylin 8-37) and rat calcitonin gene-related peptide (CGRP) on contractile force, heart rate and coronary perfusion pressure has been investigated in the isolated perfused rat hearts. Adrenomedullin (2x10(-10), 2x10(-9) and 2x10(-8) M) produced a significant decrease in contractile force and perfusion pressure, but only the peptide caused a decline in heart rate at the highest dose. Amylin (10(-9), 10(-8) and 10(-7) M) significantly increased and then decreased contractile force. Two doses of amylin (10(-8) and 10(-7) M) induced a significant increase in heart rate, however amylin did not change perfusion pressure in all the doses used. Rat alpha CGRP (10(-8), 10(-7) and 10(-6) M) evoked a slight decline in contractile force following a significant increase in contractile force induced by the peptide. CGRP in all the doses raised heart rate and lowered perfusion pressure. Our results suggest that adrenomedullin has negative inotropic, negative chronotropic and coronary vasodilator actions. Amylin produces a biphasic inotropic effect and evokes a positive chronotropy. CGRP causes positive inotropic, positive chronotropic and vasodilatory effects in isolated rat hearts.

Role of brain dopaminergic system in the adrenomedullin-induced diuresis and natriuresis

Intracerebroventricular (IVT) administration of adrenomedullin (AM) to conscious male hydrated rats increases urinary volume and sodium excretion. The possible involvement of brain dopamine (DA) system on the renal action of IVT-AM was investigated. AM-induced diuretic and natriuretic action was prevented following selective central dopaminergic denervation with 6-hydroxydopamine (6OHDA) in combination with desmethylimipramine (DMI). Selective D(2) DA receptor antagonism with haloperidol, sulpiride, and remoxipride; or with the D(1) DA receptor antagonist, SCH 23390, blunted the increase in urinary volume and sodium excretion induced by IVT-AM. The present results suggest that AM acts centrally, at least in part, via an interaction with endogenous DA through the activation of both DA D(1)/D(2) receptor subtype.

Cell and molecular biology of the multifunctional peptide, adrenomedullin

Adrenomedullin (AM) is a recently discovered regulatory peptide involved in many functions including vasodilatation, electrolyte balance, neurotransmission, growth, and hormone secretion regulation, among others. This 52-amino acid peptide is expressed by specific cell types in many organs throughout the body. A complex receptor system has been described for AM; it requires at least the presence of a seven-transmembrane-domain G-protein-coupled receptor, a single-transmembrane-domain receptor activity modifying protein, and a receptor component protein needed to establish the connection with the downstream signal transduction pathway, which usually involves cyclicAMP. In addition, a serum-binding protein regulates the biological actions of AM, frequently by increasing AM functional attributes. Changes in levels of circulating AM correlate with several critical diseases, including cardiovascular and renal disorders, sepsis, cancer, and diabetes. Whether AM is a causal agent, a protective reaction, or just a marker for these diseases is currently under investigation. New technologies seeking to elevate and/or reduce AM levels are being investigated as potential therapeutic avenues.

Autonomic and neuroendocrine actions of adrenomedullin in the brain: mechanisms for homeostasis

In addition to its role as a potent vasodilator, adrenomedullin (ADM) affects an animal's physiological status through its effects in the brain. We have shown that circulating ADM activates neurons, including nitric oxide (NO)-producing neurons, in autonomic centers of the brain such as the hypothalamic paraventricular nucleus (PVN). Systemic ADM gains access to the brain through the area postrema (AP), a brainstem circumventricular organ, and the PVN is a major target of these ADM-sensitive AP neurons. Neurons expressing the preproADM (ppADM) gene are distributed throughout the brain, with high levels in autonomic centers. Lipopolysaccharide (LPS, immune stress), restraint (psychological stress), and 24 h dehydration all down-regulate ppADM gene expression in different subsets of autonomic centers. Receptor-activity-modifying protein (RAMP) 2 and RAMP3, ADM receptor subunits, are expressed in autonomic centers including the PVN and supraoptic nucleus. Intracerebroventricular injections of ADM increase arterial pressure, heart rate, tyrosine hydroxylase mRNA levels in the locus coeruleus, plasma levels of ACTH, and NO production in the hypothalamus. ADM excites putative GABAergic and cholinergic neurons in dissociated cells from a basal forebrain integrative center, the diagonal band of Broca. These results demonstrate that the signalling components necessary for ADM to influence physiological systems are present in the brain and that ADM is an important transmitter of brain autonomic pathways which are involved in regulating homeostatic balance.

Adrenomedullin and the integrative physiology of fluid and electrolyte balance

Adrenomedullin (AM) is hypothesized to be a physiologically relevant regulator in fluid and electrolyte homeostasis. AM acts within the central nervous system to inhibit both water and salt intake. The peptide has direct actions in the hypothalamus to decrease vasopressin secretion and in the pituitary gland to inhibit ACTH release. AM decreases production and release of aldosterone from the adrenal glands and acts directly in the kidneys to increase renal blood flow and cause diuresis and natriuresis. Whether or not these complementary actions in brain, pituitary, adrenal gland, and kidney reflect coordinated regulatory mechanisms is currently unknown. Development of molecular tools to determine the physiologic role of endogenous AM will greatly enhance our understanding of AM and its regulation of fluid and electrolyte homeostasis.

A review of the biological properties and clinical implications of adrenomedullin and proadrenomedullin N-terminal 20 peptide (PAMP), hypotensive and vasodilating peptides

Adrenomedullin (AM), identified from pheochromocytoma and having 52 amino acids, elicits a long-lasting vasodilatation and diuresis. AM is mainly mediated by the intracellular adenylate cyclase coupled with cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) -cyclic guanosine monophosphate (cGMP) pathway through its specific receptor. The calcitonin receptor-like receptor (CLCR) and receptor-activity modifying protein (RAMP) 2 or RAMP3 models have been proposed as the candidate receptor. AM is produced mainly in cardiovascular tissues in response to stimuli such as shear stress and stretch, hormonal factors and cytokines. Recently established AM knockout mice lines revealed that AM is essential for development of vitelline vessels of embryo. Plasma AM levels elevate in cardiovascular diseases such as heart failure, hypertension and septic shock, where AM may play protective roles through its characteristic biological activities. Human AM gene delivery improves hypertension, renal function, cardiac hypertrophy and nephrosclerosis in the hypertensive rats. AM decreases cardiac preload and afterload and improves cardiac contractility and diuresis in patients with heart failure and hypertension. Advances in gene engineering and receptor studies may contribute to further understandings of biological implication and therapeutic availability of AM.

Adrenomedullin and central cardiovascular regulation

Adrenomedullin gene products have been localized to neurons in brain that innervate sites known to be important in the regulation of cardiovascular function. Those sites also have been demonstrated to possess receptors for the peptide and central administrations of adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) elevate blood pressure and heart rate in both conscious and anesthetized animals. The accumulated evidence points to a role of the sympathetic nervous system in these cardiovascular effects. These sympathostimulatory actions of AM and PAMP have been hypothesized to be cardioprotective in nature and to reflect the central nervous system (CNS) equivalent of the direct cardiostimulatory effects of the peptides in the periphery. This review summarizes the most recent data on the CNS actions of the adrenomedullin gene-derived peptides and suggests future strategies for the elucidation of the physiologic relevance of the already demonstrated, pharmacologic actions of these peptides.

Effect of adrenomedullin receptor and calcitonin gene-related peptide receptor antagonists on centrally mediated adrenomedullin renal action

To determine pharmacological specificity of intracerebroventricular (IVT) administration of adrenomedullin (AM) on water and sodium excretion, studies were performed in rats pretreated with AM (22-52), a putative AM-receptor antagonist or CGRP(8-37), a ligand that preferentially antagonizes the CGRP(1)-receptor subtype. In addition, the effects of IVT injection of calcitonin-gene-related peptide (CGRP) on urinary water and electrolyte excretion was assessed. Intracerebroventricular administration of rat adrenomedullin to conscious hydrated rats resulted in a significant increase in urinary volume and sodium excretion during the 6-h period of urine collection and was most effective at 3 and 6 h. Although less effective than AM, central administration of CGRP induced diuresis and natriuresis. Pretreatment with AM (22-52) or CGRP(8-37) significantly suppressed the diuretic and natriuretic effect of IVT-AM. These data suggest that both CGRP(1) and AM receptors are involved in the centrally mediated diuretic and natriuretic action of the AM. Our results provide evidence supporting the hypothesis that endogenous AM plays a role in the central nervous control of fluid and electrolyte homeostasis.