By: Gang Wang
American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 2008
Previous studies have shown that females develop hypertension later in life (and it is usually less severe) than males do. This indicates that sex differences can play a role in determining risk factors for development of high blood pressure. One of the things involved in tonic regulation of arterial pressure is the presence of bulbospinal neurons in the RVLM. When Angiotensin II (ANG II) is injected into the RVLM, an increase in sympathetic nerve activity and blood pressure is observed due to being mediated by angiotensin type I (AT1) receptors. As part of the experiment, the research team analyzed RVLM bulbospinal neurons to determine if there was a sex difference in the following: 1. Immunolabeling for AT1 receptors and the key NADPH-oxidase subunit p47 using dual-label immunoelectron microscopy and 2. Effects of ANG II on ROS production and Ca2+ currents hydroethidine fluoromicroscopy and patch-clamping. The purpose of these analyses were to test the hypothesis that an increase in AT1 receptors in female RVLM neurons is counterbalanced by reduction in p47 levels, so that ANG II-induced production does not differ between males and females.
To test the hypothesis, 4 month and 12 month old male and female Sprague-Dawley rats were used. All the young adult and middle-aged female rats underwent isoflurane anesthesia to receive bilateral ovariectomies. This was done to determine more directly the role of estrogen, since the females were given an estrogen replacement following the surgery. To analyze the data, the following methods were used: retrograde labeling of RVLM neurons, immunolabeling of isolated RVLM neurons, patch-clamp recordings of isolated RVLM neurons, and detection of intracellular reactive oxygen species (ROS) using dihydroethidium as an indicator.
The results showed that female tyrosine hydroxylase (TH) RVLM neurons displayed significantly more AT1 and less p47 than males. In addition, in the population of TH-labeled RVLM neurons, females had a higher level of AT1 receptor-ImG labeling than males. In terms of the L-type Ca2+ channels, currents were used to assess their function. ANG II increased L-type Ca2+ currents of female RVLM bulbospinal neurons through the AT1 receptors and NADPH oxidase. This lead to the discovery that ANG II-induced L-type Ca2+ currents, but not the ROS production, was greater in females than in males.
From the results above, three things can be concluded: 1. Female P23 rats display more AT1 receptor-ImG labeling and less NADPH oxidase subunit p47-ImG labeling in the TH-labeled RVLM neurons compared to males, 2. The elevated levels of AT1 receptor-ImG labeling in females persist into adulthood and midlife and 3. AT1 receptor-ImG labeling was higher in estradiol than in vehicle-treated OVX females.
In summary, female rats contained more AT1-receptors in TH-containing neurons of the RVLM than male rats, and this increase was associated with the reduction of NADPH oxidase subunit p47 in female rats. Overall these findings revealed previously unknown sex differences in ANG II receptor expression and signaling in autonomic neurons, which are critically involved in controlling the cardiovascular system and blood pressure.
Tuesday, July 3, 2018
Fos expression in brain stem nuclei of pregnant rats after hydralazine-induced hypotension
Kathleen S. Curtis, J. Thomas Cunningham, and Cheryl M. Heesch
Am. J. Physiol. 277 (1999)
Am. J. Physiol. 277 (1999)
Pregnancy in humans and other
animals may cause changes in baroreflex function. Specifically, the ability to
increase sympathetic nerve activity (SNA) in response to a hypotensive
challenge appears to be decreased during pregnancy. This could be due to the
fact that the control of sympathetic outflow is altered by hormonal changes
associated with pregnancy. 3a-OH-DHP (reproductive
hormone) has been shown to increase the activity of inhibitory GABAergic pathways
that play an important role in the baroreflex. When 3a-OH-DHP was injected into the RVLM of non-pregnant rats, they
showed inability to increase SNA during hypotensive challenges, similar to what
is seen in pregnant rats. This attenuated increase in SNA in response to a
hypotensive challenge during pregnancy may indicate that there are differences
in the central processing of information related to control of sympathetic outflow.
This study examined Fos immunoreactivity
in pregnant rats after acute decreases in blood pressure to determine whether
the activation of brain stem areas known to be involved in the central control
of SNA is altered by pregnancy. Immunolabeling for dopamine b-hydroxylase (DBH) was also performed to
determine whether pregnancy differentially affected the activity of
catecholamine-containing neurons in response to acute decreases in blood pressure.
To test these points pregnant and virgin rats were injected with either hydralazine
(HDZ; induces hypotension) or an isotonic saline vehicle (VEH; control) and
compared.
Among other things, this study
found that Fos expression in the RVLM of HDZ-treated pregnant rats was less
than that of HDZ-treated virgin female rats, and the difference was specific to
noncatecholaminergic neurons (DBH negative). Additional experiments are
necessary to determine the phenotype and projections of the neurons affected by
pregnancy. The attenuated SNA in HDZ-treated pregnant rats could be attributed
to reproductive hormones augmenting GABAergic inhibitory pathways, or attenuating
ANG II-mediated excitatory pathways.
- Ben Huber
- Ben Huber
Thursday, June 28, 2018
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."
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."
Tuesday, June 26, 2018
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.
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.
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