Atrial natriuretic peptide regulation of noradrenaline release in the anterior hypothalamic area of spontaneously hypertensive rats

C-type natriuretic peptide (CNP) in the paraventricular nucleus-mediated renal sympatho-inhibition

Volume reflex produces sympatho-inhibition that is mediated by the hypothalamic paraventricular nucleus (PVN). However, the mechanisms for the sympatho-inhibitory role of the PVN and the neurochemical factors involved remain to be identified. In this study, we proposed C-type natriuretic peptide (CNP) as a potential mediator of this sympatho-inhibition within the PVN. Microinjection of CNP (1.0 μg) into the PVN significantly decreased renal sympathetic nerve activity (RSNA) (−25.8% ± 1.8% vs. −3.6% ± 1.5%), mean arterial pressure (−15.0 ± 1.9 vs. −0.1 ± 0.9 mmHg) and heart rate (−23.6 ± 3.5 vs. −0.3 ± 0.9 beats/min) compared with microinjection of vehicle. Picoinjection of CNP significantly decreased the basal discharge of extracellular single-unit recordings in 5/6 (83%) rostral ventrolateral medulla (RVLM)-projecting PVN neurons and in 6/13 (46%) of the neurons that were not antidromically activated from the RVLM. We also observed that natriuretic peptide receptor type C (NPR-C) was present on the RVLM projecting PVN neurons detected by dual-labeling with retrograde tracer. Prior NPR-C siRNA microinjection into the PVN significantly blunted the decrease in RSNA to CNP microinjections into the PVN. Volume expansion-mediated reduction in RSNA was significantly blunted by prior administration of NPR-C siRNA into the PVN. These results suggest a potential role for CNP within the PVN in regulating RSNA, specifically under physiological conditions of alterations in fluid balance.

Cardiovascular Autonomic Dysfunction in Chronic Kidney Disease: a Comprehensive Review

Cardiovascular autonomic dysfunction is a major complication of chronic kidney disease (CKD), likely contributing to the high incidence of cardiovascular mortality in this patient population. In addition to adrenergic overdrive in affected individuals, clinical and experimental evidence now strongly indicates the presence of impaired reflex control of both sympathetic and parasympathetic outflow to the heart and vasculature. Although the principal underlying mechanisms are not completely understood, potential involvements of altered baroreceptor, cardiopulmonary, and chemoreceptor reflex function, along with factors including but not limited to increased renin-angiotensin-aldosterone system activity, activation of the renal afferents and cardiovascular structural remodeling have been suggested. This review therefore analyzes potential mechanisms underpinning autonomic imbalance in CKD, covers results accumulated thus far on cardiovascular autonomic function studies in clinical and experimental renal failure, discusses the role of current interventional and therapeutic strategies in ameliorating autonomic deficits associated with chronic renal dysfunction, and identifies gaps in our knowledge of neural mechanisms driving cardiovascular disease in CKD.

Ligand-dependent activation of ER{beta} lowers blood pressure and attenuates cardiac hypertrophy in ovariectomized spontaneously hypertensive rats

AIMS

The biological effects of oestrogens are mediated by two different oestrogen receptor (ER) subtypes, ERalpha and ERbeta, which might play different, redundant, or opposing roles in cardiovascular disease. Previously, we have shown that the selective ERalpha agonist 16alpha-LE2 improves vascular relaxation, attenuates cardiac hypertrophy, and increases cardiac output without lowering elevated blood pressure in spontaneously hypertensive rats (SHR). Because ERbeta-deficient mice exhibit elevated blood pressure and since the ERbeta agonist 8beta-VE2 attenuated hypertension in aldosterone-salt-treated rats, we have now tested the hypothesis that the isotype-selective ERbeta agonist 8beta-VE2 might be capable of lowering elevated blood pressure in ovariectomized SHR.

METHODS AND RESULTS

Treatment of ovariectomized SHR with 8beta-VE2 for 12 weeks conferred no uterotrophic effects but lowered elevated systolic blood pressure (-38 +/- 5 mmHg, n = 31, P < 0.001 vs. placebo) as well as peripheral vascular resistance (-31.3 +/- 4.6%, P < 0.001 vs. placebo). 8beta-VE2 enhanced aortic ERbeta expression (+75.7 +/- 7.1%, P < 0.01 vs. placebo), improved NO-dependent vasorelaxation, augmented phosphorylation of the vasodilator-stimulated phosphoprotein in isolated aortic rings (P < 0.05 vs. placebo), increased cardiac output (+20.4 +/- 2.5%, P < 0.01 vs. placebo), and attenuated cardiac hypertrophy (-22.2 +/- 3.2%, p < 0.01 vs. placebo). 8beta-VE2, in contrast to oestradiol, did not enhance cardiac alpha-myosin heavy chain expression.

CONCLUSION

Ligand-dependent activation of ERbeta confers blood pressure lowering effects in SHR that are superior to those of 17beta-estradiol or the ERalpha agonist 16alpha-LE2 and attenuates cardiac hypertrophy primarily by a reduction of cardiac afterload without promoting uterine growth.

C-type natriuretic peptide inhibits L-type Ca2+ current in rat magnocellular neurosecretory cells by activating the NPR-C receptor

Magnocellular neurosecretory cells (MNCs), of the paraventricular and supraoptic nuclei of the hypothalamus, secrete the hormones vasopressin and oxytocin. As a result, they have an essential role in fundamental physiological responses including regulation of blood volume and fluid homeostasis. C-type natriuretic peptide (CNP) is present at high levels in the hypothalamus. Although CNP is known to decrease hormone secretion from MNCs, no studies have examined the role of the natriuretic peptide C receptor (NPR-C) in these neurons. In this study, whole cell recordings from acutely isolated MNCs, and MNCs in a coronal slice preparation, show that CNP (2 x 10(-8) M) and the selective NPR-C agonist, cANF (2 x 10(-8) M), significantly inhibit L-type Ca2+ current (I(Ca(L))) by approximately 50%. This effect on I(Ca(L)) is mimicked by dialyzing a G(i)-activator peptide (10(-7) M) into these cells, implicating a role for the inhibitory G protein, G(i). These NPR-C-mediated effects were specific to I(Ca(L)). T-type Ca2+ channels were unaffected by CNP. Current-clamp experiments revealed the ability of CNP, acting via the NPR-C receptor, to decrease (approximately 25%) the number of action potentials elicited during a 500 ms depolarizing stimulus. Analysis of action potential duration revealed that CNP and cANF significantly decreased 50% repolarization time (APD50) in MNCs. In summary, our findings show that CNP has a potent and selective inhibitory effect on I(Ca(L)) and on excitability in MNCs that is mediated by the NPR-C receptor. These data represent the first electrophysiological evidence of a functional role for the NPR-C receptor in the mammalian hypothalamus.

Postnatal ontogeny of natriuretic peptide systems in the rat hypothalamus

Our study has attempted to clarify the developmental profile of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) along with the expression of their receptors in the rat hypothalamus. Radioimmunoassay (RIA) of dissected hypothalamic tissue revealed that ANP rose from 167 +/- 50 pg/mg protein immediately after birth to 516 +/- 78 pg/mg protein in the next 24 h and to 928 +/- 100 pg/mg protein by postnatal day (PD) 5. A second increment of ANP in the hypothalamus was noted between PD 10 and PD 20 (from 780 +/- 110 to 2,650 +/- 136 pg/mg protein). These changes were not gender-related and consistent with a rise of ANP mRNA. Diethylstilbestrol treatment of immature rats increased hypothalamic ANP concentration from 2.11 +/- 0.24 to 2.97 +/- 0.44 ng/mg protein (P<0.001), but equine chorionic gonadotropin had no effect, indicating that estrogen is a potential stimulus of ANP only at supra-physiological concentrations. CNP, the most abundant natriuretic peptide in the brain, gradually increased in the developing hypothalamus, but did not plateau at PD 20. Reverse transcription-polymerase chain reaction analysis of ANP receptor mRNA demonstrated higher guanylyl cyclase (GC) A, no changes in GC-B, and lower C-receptor levels in adult compared to newborn rats. In conclusion, we have shown that hypothalamic ANP undergoes a dramatic rise after birth, and progresses further until the 3rd postnatal week. ANP and CNP changes in the developing hypothalamus can influence brain maturation.

Alpha2A-adrenergic receptors mediate sympathoinhibitory responses to atrial natriuretic peptide in the mouse anterior hypothalamic nucleus

In the rat, activation of alpha2-adrenergic receptors in the anterior hypothalamic nucleus inhibits sympathetic nervous system activity. Furthermore, local release of atrial natriuretic peptide inhibits norepinephrine release in this nucleus, blocking local activation of alpha2-adrenergic receptors, and thereby contributes to NaCl-sensitive hypertension in spontaneously hypertensive rats. To further test the specificity of this mechanism, either alpha2-adrenergic receptor agonists or atrial natriuretic peptide was microinjected into anterior hypothalamic nucleus of conscious C57BL/6 mice in which the alpha2-adrenergic receptor was functionally deleted by a single point mutation (n=10 per group). In control mice, microinjection of either clonidine or guanabenz (10-3 to 10-7 mol/L) caused a rapid fall in mean arterial pressure that lasted for several minutes. In the knockout mice there was no response to the injection of either dose of either agonist. Microinjection of atrial natriuretic peptide (10-6 to 10-7 mol/L) caused a rapid increase in mean arterial pressure (8.2+/-1.3 and 6.55+/-1.2 mm Hg, respectively) in the control mice that was similar to the responses previously observed in Wistar-Kyoto rats. In contrast, the microinjections did not significantly alter mean arterial pressure in the knockout mice. These experiments demonstrate that in the anterior hypothalamic nucleus of the mouse (and probably in the rat) alpha2A-adrenergic receptors mediate both sympathoinhibitory responses to alpha2-adrenergic receptor agonists and the action of atrial natriuretic peptide.

A-type natriuretic peptide receptor in the spontaneously hypertensive rat kidney

Renal NPR-A binding characteristics was examined in SHR. Renal ANP binding sites of NPR-A showed a lower maximal binding capacity and higher affinity in SHR than in WKY at all intrarenal sites. Despite the lower B(max) in SHR, both ANP(1-28) and ANP(5-25) stimulate similar or greater cGMP production in isolated glomeruli. Studies on guanylate cyclase from glomerular and papillary membranes have reported an increased basal and stimulated guanylate cyclase activity in SHR. The present study provides further evidences for altered NPR-A receptors in SHR kidney, which might act as a negative feedback in response to hypertension.

Localization of cGMP-dependent protein kinase type II in rat brain

In brain, signaling pathways initiated by atrial natriuretic peptide, or transmitters which stimulate nitric oxide synthesis, increase cGMP as their second messenger. One important class of target molecules for cGMP is cGMP-dependent protein kinases, and in the present study, biochemical and immunocytochemical analyses demonstrate the widespread distribution of type II cGMP-dependent protein kinase in rat brain, from the cerebral cortex to the brainstem and cerebellum. Also, colocalization of cGMP-dependent protein kinase type II with its activator, cGMP, was found in several brain regions examined after in vitro stimulation of brain slices with sodium nitroprusside. In western blots, cGMP-dependent protein kinase type II was observed in all brain regions examined, although cerebellar cortex and pituitary contained comparatively less of the kinase. Immunocytochemistry revealed cGMP-dependent protein kinase type II in certain neurons, and occasionally in putative oligodendrocytes and astrocytes, however, its most striking and predominant localization was in neuropil. Electron microscopy examination of neuropil in the medial habenula showed localization of the kinase in both axon terminals and dendrites. As a membrane-associated protein, cGMP-dependent protein kinase type II often appeared to be transported to cell processes to a greater extent than being retained in the cell body. Thus, immunocytochemical labeling of cGMP-dependent protein kinase type II often did not coincide with the localization of kinase mRNA previously observed by others using in situ hybridization. We conclude that in contrast to cGMP-dependent protein kinase type I, which has a very restricted localization to cerebellar Purkinje cells and a few other sites, cGMP-dependent protein kinase type II is a very ubiquitous brain protein kinase and thus a more likely candidate for relaying myriad cGMP effects in brain requiring protein phosphorylation.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

The organum vasculosum of the lamina terminalis regulates noradrenaline release in the anterior hypothalamic nucleus

Changes in either plasma sodium concentration or arterial pressure can differentially affect hypothalamic neurons. For instance, increases in plasma NaCl concentration decrease noradrenaline release from nerve terminals in the anterior hypothalamic nucleus, while increases in arterial pressure unrelated to an elevation in plasma NaCl enhance noradrenaline release in anterior hypothalamic nucleus. The present study tests the hypothesis that in the rat the organum vasculosum of the lamina terminalis (an osmosensitive area of the brain) detects rises in plasma NaCl concentration and conveys this information to anterior hypothalamic nucleus. The axons projecting from the organum vasculosum of the lamina terminalis to the hypothalamus were unilaterally cut immediately caudal to organum vasculosum of the lamina terminalis, and five days later, 3-methoxy-4-hydroxy phenylglycol (the major metabolite of noradrenaline in brain) was continuously monitored in the ipsilateral or contralateral anterior hypothalamic nucleus in response to an intravenous infusion of hypertonic saline. In spontaneously hypertensive rats, the infusion decreased the 3-methoxy-4-hydroxy phenylglycol concentration by 24+/-2% in the anterior hypothalamic nucleus contralateral to the lesion, and in control spontaneously hypertensive rats. In contrast, in the anterior hypothalamic nucleus ipsilateral to the lesion, hypertonic saline infusion caused a 58+/-3% increase in 3-methoxy-4-hydroxy phenylglycol. These data support the hypothesis that the organum vasculosum of the lamina terminalis is part of the circuit that transmits information concerning plasma NaCl concentration to anterior hypothalamic nucleus.

Atrial natriuretic factor inhibits norepinephrine biosynthesis and turnover in the rat hypothalamus

We have previously reported that atrial natriuretic factor (ANF) increased neuronal norepinephrine (NE) uptake and reduced basal and evoked neuronal NE release. Changes in NE uptake and release are generally associated to modifications in the synthesis and/or turnover of the amine. On this basis, the aim of the present work was to study ANF effects in the rat hypothalamus on the following processes: endogenous content, utilization and turn-over of NE; tyrosine hydroxylase (TH) activity; cAMP and cGMP accumulation and phosphatidylinositol hydrolysis. Results showed that centrally applied ANF (100 ng/microl/min) increased the endogenous content of NE (45%) and diminished NE utilization. Ten nM ANF reduced the turnover of NE (53%). In addition, ANF (10 nM) inhibited basal and evoked (with 25 mM KCl) TH activity (30 and 64%, respectively). Cyclic GMP levels were increased by 10 nM ANF (100%). However, neither cAMP accumulation nor phosphatidylinositol breakdown were affected in the presence of 10 nM ANF. The results further support the role of ANF in the regulation of NE metabolism in the rat hypothalamus. ANF is likely to act as a negative putative neuromodulator inhibiting noradrenergic neurotransmission by signaling through the activation of guanylate cyclase. Thus, ANF may be involved in the regulation of several central as well as peripheral physiological processes such as cardiovascular function, electrolyte and fluid homeostasis, endocrine and neuroendocrine synthesis and secretion, behavior, thirst, appetite and anxiety that are mediated by central noradrenergic activity.

The role of the central nervous system in hypertension

Normally, the kidney plays the dominant role in setting long-term arterial pressure, and the nervous system acts primarily as a short-term regulator, adjusting arterial pressure to acute challenges (eg, standing, running, and stress). However, in several animal models and in subsets of hypertensive human patients, the nervous system seems to play a more significant role in the chronic elevation of arterial pressure. Many clinical studies suggest that the peripheral sympathetic nerves are intimately involved in hypertension, and researchers recently characterized abnormalities in the brain that seem to predispose animal models to sympathetic nervous system overactivity and hypertension. Together, the current data strongly suggest that the brain, via the sympathetic nervous system, directly contributes to some forms of hypertension and indirectly contributes to all of them. This review is not intended as an exhaustive examination of all studies on the role of the nervous system in hypertension but rather focuses on several intriguing experiments that provide provocative new insights on this topic.