Salusin-β in rostral ventrolateral medulla increases sympathetic outflow and blood pressure via superoxide anions in hypertensive rats

Within this article, the laboratory is trying to find out the role of salsulin-b within the RVLM in sympathetic nerve activity. They used Sprague dawley rats with induced renovascular hypertension. The two test groups were 2K1C and sham. The 2K1C rats had a clip on the right renal artery while the sham rats did not.  Salusin-b was injected into the RVLM and throughout the course of 4 weeks, an increase in sympathetic nerve activity was found in 2K1C rats and not in sham rats (the two rat models utilized). This finding shows significance due to the role it can play in understanding possible RVLM regulation of sympathetic nerve activity and ultimately blood pressure.

-Hashbrown

Different types of barosensory synaptic inputs to rostral ventrolateral medulla neurons of the rat

By: Antonio R. Granata & Morton I. Cohen

Baroreceptor inputs modulate the bulbospinal neurons in the RVLM. To study these inputs, they used firing patterns to visualize them. Inputs that are started in low/high pressure baroreceptors express inhibition of the sympathoexcitatory neurons in the RVLM. This is done by either direct inhibition (IPSPs) or by disfacilitation. The purpose of the study was to investigate the barosensory properties of RVLM neurons using quantitative analysis between electrophys of the neurons and the barosensory activity in the AP. Time and frequency domain analysis were used to analyze the data. Frequency domain analysis is useful to look at signals containing multiple frequencies and it measures the coherence, which is a measure of the strength of coupling between two signals. The responses to electrical stimulation of the aortic nerve were examined in order to investigate the barosensory properties of the RVLM neurons. It was found that the potentials produced by aortic baroreceptor afferents were IPSPs. Also, a cardiac related rhythm was observed. Concluding the data showed that the neurons in the RVLM have baroreceptor input profiles that are different suc has excitation, excitation/inhibition, and inhibition. 

By: Tsetse Fly 

Spontaneously Hypertensive Rats Are Highly Vulnerable to AMPA-Induced Brain Lesions

In this study, the experimenters were curious if there was a difference in the brain tissue in spontaneously hypertensive rats (SHRs) and normotensive rats (WKYs) that formed hypertension(R-WKYs). They were looking specifically if the SHRs or WKYs contained damaging/killing neurons from the over activation of glutamate receptors. The 2 glutamate receptors they looked at were NMDA or AMPA. These receptors were compared to see if the vulnerability of the brain tissue between the SHRs and WYKs might contribute to increased brain damage after ischemia (loss of blood to brain/heart). They induced ischemia by the intraluminal technique and a laser Doppler flowmetry probe to locate the exact area of ischemia and occlusion (blocking of an artery).  After an ischemic event, an increased release of glutamate caused over activation of NMDA and AMPA triggering neuronal death. They found that the SHRs had significant increase in dead tissue due to inadequate blood flow and higher mortality rate (63% SHRs to 17% WKY) in comparable aged WKY rats. This provides evidence that SHRs have sensitivity to AMPA, and not the NMDA. But they proved that it is not fully dependent on the level of arterial hypertension, it is also caused by the increased phosphorylation of GluR1 leading to a high vulnerability to AMPA-induced brain damage in the SHRs.

Differential Activation of Adrenal, Renal, and Lumbar Sympathetic Nerves Following Stimulation of the Rostral Ventrolateral Medulla of the Rat

By: Patrick J. Mueller, Nicholas A. Mischel, and Tadeusz J. Scislo

The RVLM is involved in resting and reflex control of sympathetic activity and blood pressure. The RVLM employs a variety of mechanisms of control, both direct and indirect, to carry out its functions: for example, blood pressure and sympathetic may be controlled directly by vasoconstriction initiated by the RVLM. However, they may also be controlled indirectly through activation of the adrenal and renal nerves. Activating the adrenal nerve stimulates the release of epinephrine, resulting in an increase in cardiac rate and contractility, as well as skeletal muscle vasodilation. Activation of the renal nerve influences renal absorption, renin release, and mediates renal vasoconstriction, thereby controlling blood pressure through manipulation of blood volume. This raises the question: does activation of the RVLM result in differing patterns of nerve activity in the adrenal, renal, and lumbar nerves?

To test this, microinjections of glutamate were performed in the RVLM of the rats. The rats were anesthetized, and nerve activity was recorded simultaneously in all three nerves: renal, adrenal, and lumbar. It was assumed that nerve activity was at its baseline prior to activation; under this assumption, nerve activity was compared as a percent increase in activity in each nerve. Similar studies in cats have previously shown differential and selective nerve activation patterns after activation of the RVLM. This study is the first to observe such patterns in rats.  The authors contribute this to the proportionally smaller size of the rat as compared to the cat and the minute changes in volume and location that produce an effect. Specifically, changes in nerve activity were observed only when the position was shifted by 100–200 μm or when volume was increased by 15 nl. 

Four factors were tested, each leading to results that demonstrate differential and selective activation patterns: concentration of glutamate, volume of glutamate, decreasing mean arterial pressure, and blocking GABAA receptors. 

Concentration:
All nerve activity increased as concentration of glutamate increased.
Adrenal nerves (Pre- ASNA) were the most sensitive to changes in glutamate concentration; therefore, adrenal nerve activation increased by the greatest percentage in comparison to both renal and lumbar nerves.
Renal nerve (RSNA) activation also increased significantly in comparison to Lumbar nerve (LSNA) activation.

Volume:
Pre-ASNA and RSNA increased when volume increased.
LSNA did not change in response to volume. 
Pre-ASNA and RSNA also increased by a greater percentage than LSNA, just in response to the presence of glutamate. 

Decreasing Mean Arterial Pressure (MAP):
Sodium nitroprusside was used to decrease MAP
Pre-ASNA increased significantly compared to LSNA
No significant increase in RSNA compared to pre-ASNA or LSNA

Blocking GABAA Receptors:
Receptors were blocked with bicuculline 
Pre-ASNA and RSNA both increased significantly compared to LSNA
Bicuculline (blocking GABAA Receptors) is 3 to 4 times more effective at increasing nerve activity in all three nerves as sodium nitroprusside (decreasing MAP)

This study is important because determining differential and selective patterns of nerve activity can help "define the contribution of different sympathetic nerves and CNS sites to a given physiological or pathophysiological process."

Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation

By: C. Michael Foley and Patrick J. Mueller

American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, July 2005

Being exposed to periods of microgravity or bed rest can result in physiological adaptations, such as an increased susceptibility to orthostatic intolerance. Being returned to a normal 1 G environment or an upright position after exposure to microgravity or bed rest seems to cause more orthostatic intolerance in females compared to males. One cause of orthostatic intolerance may be decreased arterial baroreflex compensation. The purpose of this experiment was to test the hypothesis that female rats would exhibit a greater reduction in arterial baroreflex function after hindlimb unloading (HU) compared to female rats. 

To test this hypothesis, male and female rats were randomly assigned to the HU group or the cage control (CC) group. Female rats were examined, and vaginal swabs were taken daily to track the stages of their estrous cycles. All rats underwent surgery to implant the suspension apparatus through the 5th and 7th sacral intervertebral space. The animals were allowed to recover and went though an acclimation period before being subject to 13-15 days of suspension (HU group) or cage time (CC group). After 13-15 days of these conditions the HU rats were taken down from the suspension apparatus, and all rats were placed in a horizontal position. Then, arterial blood pressure, mean arterial pressure (MAP), heart rate (HR) and nerve activity were recorded for each of the rats. For the nerve activity, the absolute renal sympathetic nerve activity (RSNA) was determined by the recorded nerve activity present after the background noise was removed. After the experiment was over, all rats were euthanized so their soleus and plantaris muscles and adrenal glands could be removed to be weighed and checked for atrophy. 

There were many results produced from this study. First, there was evidence of atrophy of the soleus and plantaris muscles in the male and female HU groups compared to the male and female CC groups. Second, resting MAP was lower in the females compared to the males, and higher in the HU groups compared to the CC groups. Third, resting HR was higher in the females compared to the males, but the baroreflex control of HR was similar among all groups. Fourth, both gender and HU affected the baroreflex control of the RSNA. The female HU group had the least baroreflex sympathetic ability to increase RSNA in response to returning to the horizontal position. Overall, the baroreflex control of HR was similar in males and females, but there was an upward shift in the response in the HU groups of both genders. This produces the major finding that: both female gender and HU result in blunted baroreflex-mediated sympathoexcitation. This is consistent with other studies involving humans that found that females are more affected by orthostatic intolerance after return from microgravity or bed rest conditions. 

(In)activity-dependent alterations in resting and reflex control of splanchnic sympathetic nervous activity

By Nicholas A. Mischel and Patrick J. Mueller

Journal of Applied Physiology, October 2011

Sympathetic motor neurons stemming from the RVLM are directly regulated by excitatory (glutamate) and inhibitory (GABA) neurotransmitters. Based on previous studies from the Mischel/Mueller lab, it has become clear that is possible that chronic physical inactivity affects the excitability of RVLM neurons directly and contributes to sympathetic over activity and increased risk of cardiovascular disease in sedentary individuals. The purpose of this study is to test the hypothesis that chronic physical inactivity leads to augmented resting and reflex-mediated splanchnic sympathetic nerve activity (SSNA), resting blood pressure, and vascular reactivity.

To address this hypothesis, rats were housed in the lab for 10-13 weeks under experimental conditions. During this time they were randomly split into two groups: active (running wheel in cage, n=16) and inactive (no running wheel, n=17). After the 10-13 week period, the rats were anesthetized and mean arterial pressure (MAP) and SSNA were recorded from the rats. Next, the rats' responses to RVLM microinjections of glutamate, baroreceptor unloading (sodium nitroprusside), and vascular reactivity (preformed under autonomic blockade with Phenylephrine) were measured. In order to record this data, catheters were inserted into the femoral artery and vein of the rats. The catheters were used for recordings of MAP, as well as administration of drugs. Along with the catheters, an electrode was placed on the splanchnic sympathetic nerve to record SSNA in response to the various experimental conditions.

Sedentary animals exhibited increased resting SSNA and MAP, increases in SSNA in response to RVLM activation and baroreceptor unloading, and increased vascular reactivity to alpha 1-mediated vasoconstriction. It is thought that the augmented response of the sedentary RVLM to glutamate, whether through direct activation or baroreceptor unloading, is one of the mechanisms through which sedentary conditions lead to increased sympathoexcitation. In addition, these data showed that the vasculature of sedentary animals is more apt to vasoconstrict in response to stimulation.  These results suggest that a sedentary lifestyle increases the risk of cardiovascular disease by augmenting resting and reflex-mediated sympathetic output to the splanchnic circulation and also by increasing vascular sensitivity to adrenergic stimulation.

- Ben Huber

Immunoreactivity for the NMDA NR1 subunit in bulbospinal catecholamine and serotonin neurons of rat ventral medulla

Ida Llewellyn-Smith and Patrick J. Mueller
Autonomic Neuroscience: Basic and Clinical (2013)

Two of the major autonomic cell groups in the ventral medulla are the bulbospinal catecholamine- and serotonin-containing neurons. The number and distribution of glutamate receptors located on these two types of neurons is likely to shape their function and how they contribute to various physiological and pathophysiological processes. NMDA receptors are a type of glutamate receptor that are made up of a five subunits (pentameric), one of which, that will serve as a marker for NMDA receptors in this study, is the NR1 subunit. The purpose of this study was to determine the number of bulbospinal catecholamine- and serotonin-containing neurons that contained the NR1 subunit, and thus NMDA receptors. 

To test this hypothesis, a group of 10 rats were housed in the lab with unlimited food and water. The rat spinal cords were injected with a retrograde tracer called cholera toxin B (CTB) before perfusion and subsequent sacrifice. The rat brains were sectioned and triple immunofluorescently labeled for NR1 (NMDA marker), CTB (bulbospinal indicator), and either TH or TpTH (catecholamine and serotonin enzyme, respectively) on 12 sections in the RVLM. The reason this study focused on TH and TpTH neurons is because they are known to be important for sympathetic control of the cardiovascular system.  The stained sections were examined under a microscope and the NR1-positive neurons were counted.

The results showed that virtually all bulbospinal TH and non-TH neurons in the RVLM, and all bulbospinal TpTH and non-TpTH in raphe pallidus and parapyramidal region, express NR1. However, even though the NR1 subunit is considered obligatory for functional NMDA channels, it is possible that some of the neurons express the NR1 subunit but do not have functional NMDA channels. It is thought that NMDA-mediated neurotransmission in the RVLM may contribute to neuroplasticity.

- Ben Huber