Oswaldo U. Lopes

Role of the Medulla Oblongata in Hypertension

Brain pathways controlling arterial pressure are distributed throughout the neuraxis and are organized in topographically selective networks. In this brief review, we will focus on the medulla oblongata. The nucleus tractus solitarius (NTS) is the primary site of cardiorespiratory reflex integration. It is well accepted that lesions or other perturbations in the NTS can result in elevations of arterial pressure (AP), with many of the associated features so commonly found in humans. However, recent studies have shown 2 distinct subpopulations of neurons within the NTS that can influence AP in opposite ways. Commissural NTS neurons located on the midline may contribute to maintenance of hypertension in spontaneously hypertensive rats (SHR), because small lesions in this area result in a very significant reduction in AP. Also involved in this blood pressure regulation network are 2 distinct regions of the ventrolateral medulla: caudal (CVLM) and rostral (RVLM). Neurons in CVLM are thought to receive baroreceptor input and to relay rostrally to control the activity of the RVLM. Projections from CVLM to RVLM are inhibitory, and a lack of their activity may contribute to development of hypertension. The RVLM is critical to the tonic and reflexive regulation of AP. In different experimental models of hypertension, RVLM neurons receive significantly more excitatory inputs. This results in enhanced sympathetic neuronal activity, which is essential for the development and maintenance of the hypertension.

Role of the Rostral Ventrolateral Medulla in Maintenance of Blood Pressure in Rats With Goldblatt Hypertension

The aim of the present study was to examine the participation of the rostral ventrolateral medulla (RVLM) in the maintenance of hypertension in rats submitted to the renovascular Goldblatt (two-kidney, one clip) procedure. We inhibited or stimulated this area with the use of drugs such as glycine, L-glutamate, or kynurenic acid. (1) Bilateral microinjection of glycine (100 nmol, 200 nL, n = 13) into the RVLM of hypertensive rats produced a decrease in mean arterial blood pressure (MAP) from 177.2 +/- 29.3 to 102.3 +/- 20.9 mm Hg (P < .05), which was similar to the decrease produced by intravenous administration of hexamethonium. The inhibition of RVLM with glycine in normotensive rats produced a decrease in MAP from 106 +/- 17.1 to 59.7 +/- 7.3 mm Hg (P < .05, n = 9). (2) An impressive increase in MAP from 153.3 +/- 16.3 to 228 +/- 34.9 mm Hg (P < .05) occurred in hypertensive rats after microinjection of L-glutamate (50 nmol, 200 nL, n = 6) into the RVLM. The same procedure caused a significant but less intense increase in MAP from 105 +/- 13.8 to 148.3 +/- 24.9 mm Hg in normotensive rats (P < .05, n = 6). (3) A decrease in MAP from 151.6 +/- 25.3 to 96.8 +/- 22.5 mm Hg occurred in hypertensive rats after microinjection of the broad-spectrum glutamate antagonist kynurenic acid (4 nmol, 200 nL, n = 6) into the RVLM, whereas the same procedure did not change MAP in normotensive animals (n = 6). Heart rate was not significantly affected in any group.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory Effects of Nitric Oxide Within the Rostral Ventrolateral Medulla of Freely Moving Rats

The aim of the present study was to examine the participation of NO in the rostral ventrolateral medulla (RVLM) of freely moving rats. We utilized NO donors and L-arginine, which were microinjected into the RVLM. Unilateral microinjection (100 nL) of 2.5 nmol sodium nitroprusside produced a biphasic response consisting of an initial, rapid increase in arterial pressure (AP) from 125+/-5 to 161+/-8 mm Hg (P<.01) and a second, long-lasting response with a progressive increase in AP (maximum delta peak, 34+/-9 mm Hg; P<.01). Another NO donor, S-nitroso-N-acetylpenicillamine (SNAP; 2.5 nmol), also produced immediate hypertension from 118+/-5 mm Hg to 168+/-7 mm Hg (P<.01) but without the second, long-lasting response. L-Arginine (5, 24, and 140 nmol) produced a gradual increase in AP. L-Glutamate (5 nmol) microinjected into the RVLM produced an increase in AP from 122+/-9 mm Hg to 171+/-8 mm Hg (P<.01) and bradycardia from 342+/-10 to 315+/-8 beats/min. This AP response was significantly attenuated, from 115+/-7 to 128+/-9 mm Hg (P<.05), after microinjection of methylene blue (3 nmol) without alterations in heart rate. These results indicate that NO may have an excitatory effect on the RVLM of freely moving rats, probably in association with glutamatergic synapses via cGMP mechanisms.

Rostral Ventrolateral Medulla: A Source of Sympathetic Activation in Rats Subjected to Long-Term Treatment With L-NAME

The major aim of the present study was to evaluate the role of the rostral ventrolateral medulla (RVLM) in the maintenance of hypertension in rats subjected to long-term treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) (70 mg/kg orally for 1 week). We inhibited or stimulated RVLM neurons with the use of drugs such as glycine, L-glutamate, or kynurenic acid in urethane-anesthetized rats (1.2 to 1.4 g/kg IV). Bilateral microinjection of glycine (50 nmol, 100 nL) into the RVLM of hypertensive rats produced a decrease in mean arterial blood pressure (MAP) from 158+/-4 to 71+/-4 mm Hg (P<0.05), which was similar to the decrease produced by intravenous administration of hexamethonium. In normotensive rats, glycine microinjection reduced MAP from 106+/-4 to 60+/-3 mm Hg (P<0.05). Glutamate microinjection into the RVLM produced a significant increase in MAP in both hypertensive rats (from 157+/-3 to 201+/-6 mm Hg) and normotensive rats (from 105+/-5 to 148+/-9 mm Hg). No change in MAP was observed in response to kynurenic acid microinjection into the RVLM in either group. These results suggest that hypertension in response to long-term L-NAME treatment is dependent on an increase in central sympathetic drive, mediated by RVLM neurons. However, glutamatergic synapses within RVLM are probably not involved in this response.

Attenuated pressor responses to amino acids in the rostral ventrolateral medulla after swimming training in conscious rats.

The cardiovascular effects of microinjection of the amino acids glutamate and glycine within the rostral ventrolateral medulla (RVLM) after swimming training (ST) in unrestrained awake rats were investigated. Unilateral microinjection of l-glutamate (5, 20 and 50 mM, in 100 nl) produced a dose dependent increase in mean arterial pressure (MAP) in control (C) (16+/-5 mm Hg; 29+/-6 mm Hg; 43+/-6 mm Hg) and swim (SW) (1+/-1 mm Hg; 16+/-2 mm Hg; 25+/-3 mm Hg) groups. However, the magnitude of this response was lower in the swim group. Prazosin injection produced hypotension and tachycardia in both groups (C=-43+/-3 mm Hg/98+/-16 bpm; SW=-61+/-5 mm Hg/115+/-32 bpm). In the SW group the hypotension caused by prazosin was greater compared to C group, but the tachycardia was not different between them. After prazosin, glutamate response in RVLM was blocked in both groups as well. When glycine (10 mM or 1 M, in 100 nl) were microinjected into the RVLM of C group we observed two different effects: decrease in MAP with the lower dose and an increase in MAP with the higher dose (10 mM=-13+/-2 mm Hg; 1 M=47+/-6 mm Hg). However, after ST the hypertensive response to glycine was blunted with no alterations in the hypotensive response (10 mM=-14+/-1 mm Hg; 1 M=18+/-4 mm Hg). These findings suggest that RVLM is involved in the modulation of the sympathetic outflow to the cardiovascular system during exercise training.

Importance of Glycinergic and Glutamatergic Synapses Within the Rostral Ventrolateral Medulla for Blood Pressure Regulation in Conscious Rats

In this study we used a method that permits bilateral or unilateral microinjections of drugs into the rostral ventrolateral medulla (RVLM) of conscious, freely moving rats. There is only limited information about how sympathetic vasomotor tone is maintained by premotor RVLM neurons in conscious animals. It has long been known that glycine microinjection into the RVLM region leads to a decrease in blood pressure (BP) in anesthetized animals. In the present study we show that both unilateral and bilateral microinjection of glycine at the same dose used for anesthetized rats (50 nmol, 50 nL) into the RVLM increases BP in conscious animals. A similar response was also observed when the excitatory amino acid L-glutamate was microinjected into the RVLM. The microinjection of kynurenic acid into the RVLM did not change the basal level of BP but blocked the increase in BP after glycine or glutamate microinjection. A decrease in BP was only observed when low doses of glycine were used (1 to 10 nmol). We conclude that, in conscious animals, the hypertension occurring in response to high doses of glycine into the RVLM is dependent on glutamatergic synapses within the RVLM. A decrease in BP observed when low doses of glycine were used shows that in conscious animals, the RVLM, in association with other premotor neurons, is probably responsible for the maintenance of sympathetic vasomotor tone, because glycine is less effective in decreasing BP under these circumstances than in anesthetized animals.

Sympathetic activation in rats with L-NAME-induced hypertension

We evaluated the hemodynamic pattern and the contribution of the sympathetic nervous system in conscious and anesthetized (1.4 g/kg urethane, iv) Wistar rats with L-NAME-induced hypertension (20 mg/kg daily). The basal hemodynamic profile was similar for hypertensive animals, conscious (N = 12) or anesthetized (N = 12) treated with L-NAME for 2 or 7 days: increase of total peripheral resistance associated with a decrease of cardiac output (CO) compared to normotensive animals, conscious (N = 14) or anesthetized (N = 14). Sympathetic blockade with hexamethonium essentially caused a decrease in total peripheral resistance in hypertensive animals (conscious, 2 days: from (means +/- SEM) 2.47 +/- 0.08 to 2.14 +/- 0.07; conscious, 7 days: from 2.85 +/- 0.13 to 2.07 +/- 0.33; anesthetized, 2 days: from 3.00 +/- 0.09 to 1.83 +/- 0.25 and anesthetized, 7 days: from 3.56 +/- 0.11 to 1.53 +/- 0.10 mmHg mL-1 min-1) with no change in CO in either group. However, in the normotensive group a fall in CO (conscious: from 125 +/- 4.5 to 96 +/- 4; anesthetized: from 118 +/- 1.5 to 104 +/- 5.5 mL/min) was observed. The responses after hexamethonium were more prominent in the hypertensive anesthetized group. However, no difference was observed between conscious and anesthetized normotensive rats in response to sympathetic blockade. The present study shows that the vasoconstriction in response to L-NAME was mediated by the sympathetic drive. The sympathetic tone plays an important role in the initiation and maintenance of hypertension.

Role of endogenous angiotensin II on glutamatergic actions in the rostral ventrolateral medulla in Goldblatt hypertensive rats.

Abstract—In this study, we investigated the cardiovascular responses mediated by rostral ventrolateral medulla neurons (RVLM) in the Goldblatt hypertension model (2K-1C) treated or not treated with captopril. The actions of glutamate into the RVLM were tested, injecting glutamate (0.1 mol/L, 100 nL) and its antagonist kynurenic acid (0.02 mol/L, 100 nL). Glycine (0.5 mol/L, 100 nL) was also microinjected. Experiments were performed in male Wistar rats (weight, 250 to 300 g); 5 groups were studied: (1) 2K-1C nontreated (H, n=6); (2) 2K-1C treated with captopril, 10 mg/kg per day (Ht10, n=10); (3) 2K-1C treated with captopril, 50 mg/kg per day (Ht50, n=7); (4) control normotensive rats (N, n=7); and (5) normotensive rats treated with captopril, 50 mg/kg per day (Nt50, n=8). All experiments in 2K-1C were performed 6 weeks after renal surgery; captopril treatment lasted for the last 2 weeks. In urethane-anesthetized rats (1.2 g/kg IV), bilateral microinjection of glycine into the RVLM caused a depressor response; there was no difference between groups in relation to the change of variation (N: 54±2; H: 46±12; Ht10: 50±3, and Ht50: 42±7 mm Hg). Only in the H group, kynurenic acid microinjection into the RVLM caused a depressor response (H: 158±8 to 132±8 mm Hg). Glutamate response was larger in hypertensive than in normotensive rats (N: 38±2.6 and H: 55±6); no difference was observed between hypertensive groups. The data suggest that glutamate acts tonically to drive the RVLM in 2K-1C rats, and this action is modulated by endogenous angiotensin II. The increase in the glutamate actions within the RVLM may contribute to the pathogenesis of renovascular hypertension.

Lesions of the Commissural Nucleus of the Solitary Tract Reduce Arterial Pressure in Spontaneously Hypertensive Rats

It has been suggested that increased sympathetic activity and arterial chemoreceptors are important for the high blood pressure in spontaneously hypertensive rats (SHR). Electrolytic lesions of the commissural nucleus of the solitary tract (commNTS) abolish (1) the cardiovascular responses to chemoreflex activation with potassium cyanide (KCN) in normotensive rats and (2) the hypertension that follows acute aortic baroreceptor denervation in rats. Therefore, in this study we investigated the effects of electrolytic lesions of the commNTS on basal mean arterial pressure (MAP), baroreflex, and chemoreflex in SHR and in normotensive control Wistar-Kyoto (WKY) and Wistar rats. CommNTS lesions elicited a dramatic fall in MAP to normal levels during the period of study (from the first to fourth day following lesions) in SHR and almost no changes in WKY and Wistar rats. The pressor responses to chemoreflex activation with KCN tested in the days 1 and 4 after commNTS lesions were abolished in SHR and in normotensive strains. The reflex tachycardia induced by sodium nitroprusside was also attenuated in days 1 and 4 after commNTS lesions in SHR, WKY, and Wistar rats. The data suggest that the integrity of commNTS is important for the maintenance of high blood pressure in SHR and for the reflex responses dependent on sympathetic activation either in SHR or in normotensive strains.

Nitric Oxide–Dependent Guanylyl Cyclase Participates in the Glutamatergic Neurotransmission Within the Rostral Ventrolateral Medulla of Awake Rats

A well-known action of nitric oxide (NO) is to stimulate the soluble form of guanylyl cyclase, evoking an accumulation of cyclic GMP in target cells. The aim of the present study was to examine the effects of inhibition of guanylyl cyclase dependent on NO during cardiovascular responses induced by L-glutamate and S-nitrosoglutathione (SNOG) microinjected into the rostral ventrolateral medulla (RVLM) of awake rats. Three days before the experiments, adult male Wistar rats (280 to 320 g) were anesthetized for implantation of guide cannulas to the desired stereotaxic position (AP=-2.5 mm, L=1.8 mm) in relation to lambda. The cannulas were fixed to the skull with acrylic cement. Twenty-four hours before the experiments, a femoral artery and vein were cannulated for recording arterial pressure (AP) and heart rate (HR) and injection of anesthetic. Unilateral microinjections (100 nL) of L-glutamate (5 nmol/L) and SNOG (2.5 nmol/L) were made into the histologically confirmed RVLM. The cardiovascular responses to these drugs were evaluated before and after microinjection (3 nmol/L, 200 nL) of either methylene blue or oxodiazoloquinoxaline (ODQ). The hypertensive effect of L-glutamate was attenuated by 74% after methylene blue (DeltaAP=49+/-8 to 13+/-4 mm Hg) and by 80.5% after ODQ (DeltaAP=30+/-2 to 6+/-2 mm Hg). The increase in AP produced by SNOG was fully blocked by ODQ (DeltaAP=39+/-8 to 1+/-2 mm Hg). These data indicate that cyclic GMP mechanisms have a key role in glutamatergic neurotransmission in the RVLM of awake rats, and it is most probable that NO participates in this response.