#rvlm

Kay L.H.  Wu, Yung-Mei Chao, Shiow-Jen Tsay, Chen Hsiu Chen, Samuel H.H. Chan, Ima  Dovinova, Julie Y.H. Chan

Metabolic  syndrome (MetS) is known to be associated with hypertension and recently with  oxidative stress. Of note is that oxidative stress in the rostral ventrolateral  medulla (RVLM), where sympathetic premotor neurons reside, contributes to sympathoexcitation  and hypertension.

 The aim of  the present study is to identify the source of tissue oxidative stress in  RVLM and their roles in neural mechanism of hypertension associated with  MetS.

 Adult normotensive  rats subjected to a high-fructose diet for 8 weeks developed metabolic traits  of MetS, alongside increases in sympathetic vasomotor activity and blood  pressure.

 In RVLM of  these MetS rats, the tissue level of reactive oxygen species was increased,  nitric oxide (NO) was decreased, and mitochondrial electron transport  capacity was reduced. Whereas the protein expression of neuronal NO synthase (nNOS)  or protein inhibitor of nNOS was increased, the ratio of nNOS dimer/monomer  was significantly decreased.

 Oral intake  of pioglitazone or intracisternal infusion of tempol or coenzyme Q10 significantly  nullified all those molecular events in high-fructose diet–fed rats and increased  sympathoexcitation and hypertension.

 Gene  silencing using lentivirus carrying small hairpin RNA of protein inhibitor of  nNOS mRNA in RVLM inhibited protein inhibitor of nNOS expression, increased the  ratio of nNOS dimer/monomer, restored NO content, and alleviated oxidative  stress in RVLM of high-fructose diet–fed rats.

 This also significantly  reduced sympathoexcitation and hypertension. These results suggest that  redoxsensitive and protein inhibitor of nNOS–mediated nNOS uncoupling is responsible  for sustained production of reactive oxygen species in RVLM, resulting in sympathoexcitation  and hypertension associated with MetS.

Mahesh Kumar Sivasubramanian

Sean D.  Stocker, Susan M. Lang, Sarah S. Simmonds, Megan M. Wenner, William B.  Farquhar

Elevated  NaCl concentrations of the cerebrospinal fluid increase sympathetic nerve  activity (SNA) in saltsensitive hypertension. RVLM neurons play an important  role in regulation of SNA. The aim of the present study is to investigate the  contribution of RVLM neurons to the SNA and pressor response to cerebrospinal  fluid hypernatremia.

 For this  purpose male Sprague dawley rats were used. Intracerebro ventricular infusion  of 0.15 mol/L, 0.6 mol/L, and 1.0 mol/L NaCl produced concentration-dependent  increases in lumbar SNA, adrenal SNA, heart rate, and mean ABP (Figure 1).  However, the increase in adrenal SNA and heart rate was delayed and increased  ≈8 to 10 minutes after start of the intracerebroventricular  infusion. Intracerebroventricular infusion of 1.0 mol/L NaCl decreased renal  SNA but did not alter splanchnic SNA.

 Intravenous  injection of chlorisondamine promptly decreased baseline SNA and mean ABP and  abolished the SNA, tachycardic, and pressor response to 0.6 mol/L and 1.0  mol/L NaCl. Intracerebroventricular infusions after acute electrolytic lesion  of the AV3V significantly attenuated the increase in lumbar SNA, adrenal SNA,  mean ABP, and heart rate to infusion of 0.6 mol/L and 1.0 mol/L NaCl.

 Bilateral injection  of muscimol abolished the lumbar sympathoexcitatory and pressor  response to intracerebroventricular infusion of 1.0 mol/L  NaCl. The adrenal sympathoexcitatory response was partially attenuated. Blockade  of RVLM glutamate receptors with kynurenic acid significantly  attenuated the increase in lumbar SNA, adrenal SNA, heart  rate, mean ABP and also reduced the renal sympathoinhibitory response to  intracerebroventricular infusion of 1.0 mol/L NaCl.

 Acute  intracerebroventricular infusion of hypertonic NaCl differentially affected  the discharge frequency of spinally projecting RVLM neurons. All these  findings suggest that the sympathoexcitatory response to CSF hypernatremia depends  on AV3V neurons to increase glutamatergic drive onto a selective population  of RVLM neurons. 

Mahesh Kumar Sivasubramanian

Eloise  Streeter, Mohammad Al-Magableh, Joanne Louise Hart and Emilio Badoer

Hydrogen sulphide  (H2S) is now recognized as an important signalling molecule and has been shown  to have vasodilator and cardio protectant effects. More recently it has been suggested  that H2S may also act within the brain to reduce blood pressure (BP). Male  Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats were used.

In the present  study we have demonstrated the presence of the H2S-producing enzyme, cystathionine-β-synthase  (CBS) in the rostral ventrolateral medulla (RVLM), and the hypothalamic paraventricular  nucleus (PVN), brain regions with key cardiovascular regulatory functions.

 In the present  study using western blot we found that the enzyme CBS but not CSE was present  in the pressor region of the RVLM and in the PVN, suggesting H2S may be endogenously  produced in these brain regions.

 Microinjection  of the H2S donor, NaHS, directly into these regions, however, did not significantly  alter MAP, HR, or LSNA in WKY and SHR rats.

 In WKY rats,  inhibition of the production of H2S, using inhibitors of CBS, in those brain regions  also had no marked or consistent effects on the cardiovascular variables.

 The results  suggest that hydrogen sulfide in the RVLM and PVN does not have a significant  role in cardiovascular regulation.

Mahesh Kumar Sivasubramanian

Mei-Fang  Zhong, Yu-Hong Zhao, Hua Xu, Xing Tan, Yang-Kai Wang and Wei-Zhong Wang    

Homocysteine  (HCY) causes overactivity of sympathetic tone thereby acting as a significant  risk factor for hypertension. Excessive oxidative stress in the rostral  ventrolateral medulla (RVLM), a key region for control of sympathetic  outflow, contributes to sympathetic hyperactivity in hypertension. Therefore,  the goal of the present study is to determine the effect of systemic HCY on  production of reactive oxygen species (ROS) in the RVLM.

 For this  purpose male Sprague Dawley rats of 12 weeks old are used. Animals were  divided into two groups. HCY group was fed with L-methionine (1 g/kg body  weight per day) in tap water for a period of 8 weeks. The control group was  fed with equal quantity of water.

The diet  induced hyperhomocysteinemia in HCY group was identified by the assessment of  plasma HCY concentration 8 weeks after L-methionine treatment. In HCY group  there was constant elevation of Mean arterial pressure (MAP) when compared  with vehicle group. The HCY group also had increase in body when compared  with Control group.

In HCY  group, MAP and basal renal sympathetic nerve activity was higher when  compared with control. There was no significant difference between heart rate  (HR) in both HCY and control group.

In HCY group  there was significant increase in ROS production in the RVLM when compared  with control group as determined by fluorescent labelling with  Dihydroethidium (DHE).

 In HCY group  protein expression level of NOX4 was significantly increased whereas SOD1  level was significantly reduced. All these results suggest that the diet-induced  hyperhomocysteinemia leads to accelerated oxidative stress in the RVLM, which  is associated with high levels in BP and sympathetic overactivity.

Mahesh Kumar Sivasubramanian

Maria J.Barnes  and David H. McDougal    

Leptin is a hormone  released from adipose tissue. While this hormone normally acts to reduce feeding  behaviour and increase energy expenditure, in obesity, resistance to these effects  occurs eventhough the hormone is released in large amounts. Although leptin no  longer works to suppress feeding in the obese, leptin retains its potent effects  on other autonomic functions such as blood pressure regulation. Leptin has been  associated with hypertension and increased sympathetic autonomic activity. Therefore,  leptin is emerging as a major contributor to the hypertensive state observed in  obesity.

 Our present study  using pseudo-rabies multi-synaptic retrograde tract tracing and immune histochemical  methods revealed that neurons within the RVLM that send sympathetic projections  to the kidney express leptin receptors. For this purpose Male long Evan rats  of 8-10 weeks old were used.

 RVLM neurons  were identified by nano-injection of glutamate which elicited a minimum increase  of 15mmHg in blood pressure within 15s and an increase in RSNA which preceded  the change in blood pressure.

 Leptin (1  and 3µg) microinjection into the RVLM increased the Mean Arterial Pressure (MAP)  and renal sympathetic nerve activity (RSNA) when compared with leptin (0.3µg)  and saline.

 Pretreatment  of the RVLM with the rat superactive leptin antagonist, SLAN-4 prior to  leptin treatment reduced the MAP and RSNA compared with saline.

 All these  results suggest that the neurons within the RVLM express functional leptin  receptors and can respond to leptin by increasing RSNA and mean arterial  blood pressure. Taken together, our results suggest that leptin may regulate  renal sympathetic tone and ultimately blood pressure by modulating the  activity of neurons within the RVLM.

Mahesh Kumar Sivasubramanian

Xing Tan, Pei-Lei Jiao, Yang-Kai Wang, Zhao-Tang Wu, Xiao-Rong Zeng, Miao-Ling Li, Wei-Zhong Wang

Neuroinflammation  is associated with cardiovascular dysfunction. The purpose of this study is to  determine the role of the PI3K signaling in neuroinflammation in the RVLM of  hypertension.

For this  purpose 16 weeks old Spontaneously Hypertensive rats (SHR) and Wistar Kyoto  rats (WKY) were used.  Gene silencing  of PI3K in the RVLM attenuated the levels of neuroinflammation and  cardiovascular function in SHR.

Silencing of  PI3K in the RVLM inhibited the cardiovascular excitation and neuroinflammation  induced by central Ang II

ACE2  overexpression in the RVLM attenuated the increased neuroinflammation,  cardiovascular excitation, and upregulation of PI3K/AKT signaling in the  LPS-induced hypertensive rats and SHR.

Taken  together, these findings indicate that the PI3K signaling in the RVLM is involved in the pathogenesis of neuroinflammation in hypertension. It is also suggested that inactivation of the PI3K signaling mediates the antineuroinflammation effect of ACE2 overexpression in the RVLM in hypertensive rats.

All these  results suggest that both neuroinflammation and cardiovascular excitation  induced by central infusion of Ang II are prevented by pretreatment with RVLM  PI3K knockdown.

Mahesh Kumar Sivasubramanian

De-pei  li and Hui-lin pan

The aim of  this study is to determine the afferent pathways underlying the activation of  barosensitive neurons in the RVLM on stimulation of cardiac receptors in  rats.

Epicardial  application of bradykinin (BK) activated the cardiac receptors and thereby  stimulated the barosensitive neurons in the RVLM. To determine the mechanism  underlying this, cervical vagotomy and blockade of stellate ganglia was done.

Epicardial  application of BK after cervical vagotomy did not alter the response of  barosensitive neurons in the RVLM. But the bilateral blockade of stellate ganglia which is the major sympathetic afferent pathway by local infiltration  of lidocaine eliminated the response of RVLM vasomotor neurons to the  epicardial application of BK.

Based on  these results it can be stated that epicardial application of BK stimulated  majority of the barosensitive neurons in the RVLM and for this stimulation of  neurons cardiac sympathetic afferents are essential.  

This study  demonstrates that sympathetic afferent pathway selectively excites the barosensitive neurons in the RVLM on stimulation of cardiac receptors.

Mahesh Kumar Sivasubramanian

Erika E.  Nishi, Cassia T. Bergamaschi, Elizabeth B. Oliveira-Sales, Karin A. Simon and  Ruy R. Campos

The aim of  the present study is to identify whether the angiotensin II type 1 receptor  (AT1R) is involved in the oxidative stress in RVLM and thereby leading to  cardiovascular dysfunction in renovascular hypertension.

For this  purpose male wistar rats were chosen and silver clip was implanted around the  left renal artery of rats (2K-1C). Age matched rats were used as controls.

The mean  arterial pressure (MAP) of 2K-1C rats were higher than the control rats and  this was reduced by losartan treatment. Renal sympathetic nerve activity  (rSNA) was higher in the hypertensive rats (2K-1C) when compared with control  rats. On treatment with losartan the rSNA was normalised only in 2K-1C rats  not in control groups.

Parvicellular  reticular nucleus (PCRt) was chosen as a control area to study the  preferential superoxide production in the RVLM. Superoxide production as  measured by dihydroethidium (DHE) fluorescent intensity showed no significant  difference between the control and 2K-1C rats whereas the DHE fluorescent  intensity in the RVLM of 2K-1C rats was higher than the control rats which on  treatment with losartan significantly reduced the DHE fluorescent intensity  in 2K-1C rats.

Thiobarbituric  acid reactive substances (TBARS) and total glutathione levels (tGSH) were  used as an indicator of systemic oxidative stress. In 2K-1C rats the plasma  TBARS level was increased and tGSH level was decreased when compared with the  control rats. On treatment with losartan, these levels were reversed only in  2K-1C rats (TBARS level decreased and tGSH level increased).

All these  results suggests that the losartan induced AT1R blockade reduced the blood  pressure and rSNA, and oxidative stress was also decreased locally (within  the RVLM) and systemically in the 2K-1C rats.   

Mahesh Kumar Sivasubramanian