Cardiovascular homeostasis is regulated in large part by the rostral ventrolateral medulla (RVLM) in mammals. Projections from the RVLM to the intermediolateral column of the thoracolumbar spinal cord innervate preganglionic neurons of the sympathetic nervous system causing elevation of blood pressure and heart rate. A large proportion, but not all, of the neurons in the RVLM contain the enzymes necessary for the production of epinephrine and are identified as the C1 cell group. Angiotensin II (Ang II) activates the RVLM acting upon AT1 receptors. To assess the proportion of AT1 receptors that are located on C1 neurons in the rat RVLM this study employed an antibody to dopamine-beta-hydroxylase conjugated to saporin, to selectively destroy C1 neurons in the RVLM. Expression of tyrosine hydroxylase immunoreactive neurons in the RVLM was reduced by 57 % in the toxin injected RVLM compared to the contralateral RVLM. In contrast, densitometric analysis of autoradiographic images of (125)I-sarcosine(1), isoleucine(8) Ang II binding to AT1 receptors of the injected side RVLM revealed a small (10 %) reduction in AT1-receptor expression compared to the contralateral RVLM. These results suggest that the majority of AT1 receptors in the rat RVLM are located on non-C1 neurons or glia.

The rostral ventrolateral medulla (RVLM) is critical to the maintenance of blood pressure. It has been proposed that blood-borne Ang II can influence the RVLM via a neural connection between the circumventricular organs and paraventricular nucleus of the hypothalamus (PVH) and that a component of this pathway is angiotensinergic. A period of water deprivation leads to increased ability of angiotensin type 1 (AT(1)) receptor antagonists to reduce blood pressure when administered into the RVLM and PVH. We studied the differences in AT(1) receptor and angiotensin-converting enzyme (ACE) expression in these and other brain regions involved in blood pressure regulation and water intake following dehydration. AT(1) receptor and ACE expression in brains of rats deprived of water for 48 h were compared to that of water-replete rats by quantitative receptor autoradiography. AT(1) receptor expression was increased in the subfornical organ and periventricular nucleus of the hypothalamus, but not in other brain regions measured. ACE expression was increased in the RVLM, PVH, choroid plexus, median preoptic nucleus, and organosum vasculosum of the lamina terminalis. These findings suggest that increased Ang II production but not increased receptor expression in the PVH and RVLM is the mechanism by which Ang II in the brain helps to sustain systemic blood pressure during periods of water deprivation.

Spontaneously hypertensive rats (SHR) have an activated brain angiotensin system that contributes to the elevation of blood pressure in this animal model. Physiological and pharmacological studies suggest that hyperactivation of brain AT1 angiotensin receptors is a major pathophysiological factor. Consistent with these observations, radioligand binding studies indicate widespread up-regulation of brain angiotensin receptors in SHR. One key brainstem site in which AT1 receptor stimulation appears to contribute to the elevated blood pressure in SHR is the rostral ventrolateral medulla (RVLM). However, no quantitative comparison of AT1 receptor binding in the RVLM has been made in SHR versus normotensive rats. A novel, non-AT1, non-AT2 binding site, specific for angiotensins II and III, has recently been discovered in the brain. To determine if radioligand binding to either AT1 receptors or this novel angiotensin binding site is altered in the RVLM and other caudal brainstem regions of SHR, a quantitative densitometric autoradiographic comparison of radioligand binding in SHR versus normotensive Wistar–Kyoto rats was made. In both the RVLM and caudal ventrolateral medulla (CVLM) as well as dorsomedial medulla (DMM), there was increased expression of AT1 receptor binding in SHR (13%, 9%, and 23%, respectively). Conversely, expression of the novel, non-AT1, non-AT2, angiotensin II and III binding site was decreased in the RVLM and DMM of SHR (37% and 13%, respectively). This increased AT1 receptor binding in the RVLM may contribute to the hypertension of SHR. Reduced radioligand binding to the novel, non-AT1, non-AT2, angiotensin binding site in the RVLM of SHR may indicate a role for this binding site to reduce blood pressure via its interactions with angiotensins II and III.

Angiotensin II acts on Ang II type I (AT(1)) receptors in areas of the caudal brainstem involved in cardiovascular regulation. In particular, activation of AT(1) receptors in the rostral ventrolateral medulla (RVLM) has been suggested to contribute to hypertension. However, the characteristics of AT(1) receptors in the RVLM of rat, the species in which the most experimental work has been done, are not well documented. This study evaluated AT(1) receptor binding along a 2.7-mm length of rat medulla, which included the full extent of the RVLM and the caudal ventrolateral medulla (CVLM). Sections of medulla from female rats cut on a cryostat were incubated with five concentrations of I-125-sarcosine(1), isoleucine(8) angiotensin II to assess the density (B-max) and dissociation constant (K-D) of the receptors for the radioligand. The dorsomedial medulla (DMM) displayed a high density of AT(1) binding (1207 +/- 100 fmol/g), which peaked at 0.4 mm rostral to the calamus scriptorius (approximately 14 mm caudal to Bregma). The RVLM and CVLM displayed significantly lower (p < 0.01) densities of AT(1) binding, 278 +/- 38 and 379 +/- 64 fmol/g, respectively. However, the dissociation constants were significantly lower (i.e., higher affinity) in RVLM and CVLM (164 +/- 38 and 178 +/- 27 pM, respectively,) than in DMM (328 +/- 12 pM, p<0.01 and p<0.05, respectively). These results provide an anatomical and pharmacological framework for future studies on the role in cardiovascular regulation of AT(1) receptors in the caudal brainstem. (C) 2009 Elsevier B.V. All rights reserved.

The rostral ventrolateral medulla (RVLM) is a brainstem region critical to the maintenance of blood pressure. A period of water deprivation leads to increased sensitivity of the RVLM and paraventricular nucleus of the hypothalamus (PVH) to angiotensin II (Ang II). This Ang II helps sustain blood pressure. It has been proposed that peripheral Ang II can influence the RVLM via an angiotensinergic neural connection involving circumventricular organs and the PVH. We studied the differences in angiotensin type 1 (AT1) receptor and angiotensin converting enzyme (ACE) expression in these and other brain regions involved in blood pressure regulation and water intake following a 48 hr period of water‐deprivation. Brains were sectioned and used for quantitative AT1 receptor and ACE autoradiography. AT1 receptor expression was increased in the subfornical organ and periventricular nucleus of the hypothalamus, but not in other brain regions measured. ACE expression was increased in the RVLM, PVH, choroid plexus, median preoptic nucleus, and organosum vasculosum of the lamina terminalis. These findings suggest that increased brain‐derived Ang II production, but not increased receptor expression or sensitivity is the mechanism by which Ang II in the brain helps to sustain systemic blood pressure during water‐deprivation. Supported by the Peptide Radioiodination Service Center of the University of Mississippi.

The RVLM, a brainstem site critical in cardiovascular regulation, contains angiotensin Type 1 receptors (AT1R) and stimulation of these receptors increases arterial pressure. Elevated stimulation of these receptors may contribute to sympathoexcitation seen in some rat models of hypertension, as injection of AT1R antagonists into RVLM decreases arterial pressure in these hypertensive models. Within the RVLM, there are two groups of spinally‐projecting sympathoexcitatory neurons based on neurochemical phenotype: those that contain catecholamine biosynthetic enzymes, termed C1 cells, and those that do not. Although evidence supports the hypothesis that the C1 and the AT1R‐expressing populations of the RVLM overlap, this hypothesis has not been tested. Local injection of saporin toxin conjugated to a dopamine‐beta‐hydroxylase antibody(antiDBH‐Sap) selectively destroys C1 cells. Using this lesioning approach, we examined the effect of C1 cell depletion on density of AT1R binding sites in the RVLM. Autoradiographic localization of (125)‐I‐sarcosine(1), isoleucine(8) Ang II revealed high affinity AT1R binding in the RVLM. Selective unilateral lesions of C1 neurons in the RVLM reduced AT1R binding in the RVLM compared to the unlesioned side and the RVLM of rats injected with a control toxin. These data suggest that a large proportion of AT1R in the RVLM are located on C1 neurons. (NIH HL‐55687, HL‐76083)

The solitary tract nucleus (NTS), caudal ventrolateral medulla (CVLM) and rostral ventrolateral medulla (RVLM) are brainstem structures that have been identified as central sites at which blood pressure is controlled. Each are reported to be responsive to angiotensin II (Ang II) and to contain type 1 Ang II receptors (AT1R). This study used quantitative densitometric saturation autoradiography to determine the density of AT1R in the RVLM and CVLM of the female rat. Results showed that specific binding of 125I‐Sarcosine1, Isoleucine8 Ang II in the NTS had high density (1207 ± 99.8 fmol/g wet weight) and high affinity (KD of 328 ± 12.3 pM). Specific binding was much lower in the CVLM and RVLM. The CVLM (−14.24 to −13.76 mm relative to Bregma) contained 379 ± 63.9 fmol/g wet weight. The KD (178 ± 27.2 pM) was significantly lower than NTS (p<0.01). The density of AT1R in the RVLM (−13.64 to −12.08 mm relative to Bregma) was 275 ± 38.1 fmol/g wet weight with KD=164 ± 20.4 pM (p<0.01 versus NTS). These results indicate that there is a low density, but high affinity population of AT1Rs in the CVLM and RVLM that are the likely mediators of the effects of locally acting and exogenously applied Ang II on blood pressure in rats. This study was supported by the Peptide Radioiodination Service Center, University of Mississippi.

The rostral ventrolateral medulla (RVLM) and the presympathetic bulbospinal neurons in this region play a critical role in cardiovascular regulation. However, there is ambiguity regarding the precise anatomical coordinates of the RVLM and much still needs to be learned regarding the regulation and neurochemistry of this region. This brief review discusses some of these issues and focuses on the role of angiotensin-mediated signaling in the RVLM in blood pressure regulation.

Reduced uterine perfusion initiated in late gestation in the rat results in intrauterine growth restriction (IUGR) and development of hypertension by 4 wk of age. We hypothesize that the renin angiotensin system (RAS), a regulatory system important in the long-term control of blood pressure, may be programmed by placental insufficiency and may contribute to the etiology of IUGR hypertension. We previously reported that RAS blockade abolished hypertension in adult IUGR offspring; however, the mechanisms responsible for the early phase of hypertension are unresolved. Therefore, the purpose of this study was to examine RAS involvement in early programmed hypertension and to determine whether temporal changes in RAS expression are observed in IUGR offspring. Renal renin and angiotensinogen mRNA expression were significantly decreased at birth (80 and 60%, respectively); plasma and renal RAS did not differ in conjunction with hypertension (mean increase of 14 mmHg) in young IUGR offspring; however, hypertension (mean increase of 22 mmHg) in adult IUGR offspring was associated with marked increases in renal angiotensin-converting enzyme (ACE) activity (122%) and renal renin and angiotensinogen mRNA (7-fold and 7.4-fold, respectively), but no change in renal ANG II or angiotensin type 1 receptor. ACE inhibition (enalapril, 10 mg x kg(-1) x day(-1), administered from 2 to 4 wk of age) abolished hypertension in IUGR at 4 wk of age (decrease of 15 mmHg, respectively) with no significant depressor effect in control offspring. Therefore, temporal alterations in renal RAS are observed in IUGR offspring and may play a key role in the etiology of IUGR hypertension.