GENDER DIFFERENCES IN SYMPATHETIC NERVOUS SYSTEM REGULATION

By Carmen Hinojosa-Laborde, Irene Chapa, Darrell Lange, and Joseph R. Haywood

Departments of Physiology and Pharmacology, University of Texas Health Science Center, San Antonio, Texas, USA

Journal of Clinical and Experimental Pharmacology and Physiology

The risk for hypertension is lower in premenopausal women than men. However, at the onset of menopause, this difference disappears. It is hypothesized that female hormones serve to protect against the development of hypertension and other cardiovascular diseases. It is suggested that the hormones act to reduce the sympathetic nervous system (SNS) to protect women against hypertension. This study researched the many possible sites within the sympatho-adrenal nervous system that the circulating hormones could act to protect females against hypertension.

Studies have suggested that the pathways that control the sympatho-adrenal function in the central nervous system are different in females compared to males. Upon the severing of baroreceptor afferents, both females and males have similar rises in arterial pressure, heart rate, and plasma noradrenaline (NA). However, females have a lower rise in adrenaline when compared to males. Females in dioestrus and estrus had reduced changes in blood pressure and heart rate when their NMDA receptors were stimulated. However, the proestrus females responded similarly to males. Lastly, clonidine, an alpha2-adrenoceptor agonist, caused a greater decrease in plasma NA in females compared to males.

There are also possible gender differences in the regulation of the SNS through the baroreceptor reflex. Many human studies have shown that women have a lower reflex bradycardia in response to blood pressure increases. However, these results have not been found in rats. Many of the rat studies focused primarily on the sympathetic nervous system. Thus, it is suggested that both the sympathetic and parasympathetic nervous systems are modulated by female hormones and lead to sex differences seen in the baroreflex control of heart rate.

Evaluation of the lumbar sympathetic nerve activity and its role in the regulation of heart rate were also studied. No sex differences were measured in the baroreflex control of the lumbar sympathetic nerve activity, but nothing is said on whether or not the stage of the estrous cycle was taken into consideration. When grouped into their respective cycle stages, those in the pro-estrous stage had a greater baroreflex gain and maximal increase in the renal sympathetic nerve activity when compared to males. Those in the estrus or dioestrus phase have both a reduced baroreflex gain and renal sympathetic nerve activity. The renal sympathetic nerve results suggest that females could have a greater baroreflex gain when compared to males. Similar studies should be done on the lumbar sympathetic nerve in order to better understand the effects of the different estrus stages on the activity and baroreflex control there.

The sex-differences of the cardiopulmonary baroreflex was also investigated. It controls blood pressure based on the shifts of blood volume when individuals go from standing to sitting to supine positions. May studies were completed, but there was no consistent pattern determined. For example, in one study women had a greater increase in heart rate and total peripheral resistance (TPR) when standing from a sitting position. However, when an orthostatic-like stress was simulated with lower body negative pressure, the increase in TPR was either less than or similar to men.

Nevertheless, the cardiopulmonary receptors seem to be important in women when they are sensing an increase in blood volume due to increased sodium or water. Studies have shown that the receptors in females may be more efficient at lowering SNA under these conditions than men. Female rats showed a greater increase in renal sodium excretion with a lower SNA, thus suggesting that there is a difference between how efficient the kidneys are at responding to the neurological regulation of the blood volume. The efficiency measured here could be another method utilized by women to protect themselves from hypertension. However, there is nothing stated about how female sex hormones affect this system.

Lastly, the researchers considered the protective effects of noradrenergic neurotransimission in females. Male rats were more sensitive to adrenergic nerve stimulation in rat tail arteries than females, thus leading to an lower vasoconstriction in females. There was no difference in NA. In ovariectomized females, the difference compared to male rats was lost. The study suggests that the results seen with the ovariectomized rats could not be due to an alteration in tyrosine hydroxylase activity—there is no observed sex-difference in this enzyme and thus would not be impacted by changing levels of hormones. However, they do suggest that presynaptic alpha2-adrenoceptors may be elevated in female rats when compared to male rats. Females had an increase in NA release when the receptors were blocked, and this difference was abolished when females were given ovariectomies. Additionally, the study offers another pathway that could lead to the sex-differences in adrenergic nerve activity. The corelease of neuropeptide Y from sympathetic nerve terminals can enhance the NA-induced vasoconstriction. The effects are greater in males than in females in many different models, again leading to a lower vasoconstriction in females than in males. The researchers suggest that further studies should be completed on the clearance of released NA in order to understand all of the sex-differences that exist in the adrenergic neurotransmission system.  

The adrenal medulla was studied, simply due to the idea that responses to stress are sex-dependent. The adrenal medulla has an important role in stress responses, released catecholamines into the plasma to alter different functions appropriately. Females have a lower adrenal activation, which is used as support for the claim that males have greater increases in blood pressure, heart rate, and release of catecholamines during stress. Studies have shown that the release may be lower in females because their content of NA is lower than males. Furthermore, the amount is estrous stage-dependent. Other studies have shown that with an increase in estradiol, there is a decrease in NA content and release from the medulla. Lastly, the activity of catecholamine degrading enzymes like monoamine oxidase and catechol-O-methyl transferase changes with the estrous cycle. Ovariectomies are linked to a lower activity in the enzymes and estrogen replacement therapy establishes the activity again.

Overall, the study helped to show that there are sex-dependent differences in the sympatho-adrenal nervous system. Many of the components are altered by female sex hormones and may offer possible hypertensive-protective mechanisms in females.

-LivIn la Vida

GABA in the female brain--Oestrous cycle-related changes in GABAergic function in the periaqueductal grey matter

by T.A. Lovick
Department of Physiology, University of Birmingham B15 2TT, UK
Journal of Pharmacology, Biochemistry, and Behavior

The late luteal (premenstrual) phase of the estrous cycle coincides with a decrease in progesterone blood and brain levels. The reduction in hormone levels has been linked to an increase in anxiety symptoms in those suffering from anxiolytic disorders. Furthermore, research has suggested that the environment influences the severity of the symptoms, although the mechanism of which this occurs is unknown. The periaqueductal grey (PAG) in the midbrain is associated with the regulation of anxiolytic responses and is the center of this study.

Neuroactive progesterone metabolite allopregnanolone (ALLO) has been shown to act as a positive modulator of GABA on GABA_A receptors. Progesterone withdrawals have demonstrated an upregulation of GABA_A alpha4, beta1, and delta subunit proteins, along with changes in neuronal responsiveness to GABA within the midbrain periaqueductal grey matter. Thus, the purpose of this study was to investigate if the upregulation of the GABA_A subunits will lead to an increase in excitability in the PAG, which in turn could increase the anxiolytic symptoms seen in women suffering from PMS. The researchers believed that since the delta subunit is usually located extrasynaptically, it would be upregulated on the interneurons of the PAG circuits. The interneurons would be suppressed and the PAG output would increase. Additionally, the influence of the environment on the PAG response throughout the estrous cycle was investigated.

Animals were housed in either a “standard,” noisy environment with other animals or in an “isolated,” quiet environment with no other animals. Daily smears were taken with each group to track the estrous cycle of each rat. Two rats from each cohort were used for immunohistochemical processing. The study used immunoreactivity methods to locate PAG neurons that expressed GABA_A subunits or glutamic acid decarboxylase (GAD), a GABA synthesizing enzyme. The immunoreactive cells within the PAG were individually counted to determine the density of the cells at different points of the estrous cycle.

Extracellular recordings of individual neurons were taken while iontophoretically applying drugs within the dorsal half of the PAG. Male rats were used to study the effects of progesterone on PAG GABAergic systems by directly applying GABA, the ALLO analog ORG20599, and bicuculline (BIC), a GABA antagonist. Female rats were used to study the effects of the estrous cycle on the GABAergic tone in the PAG. BIC responses were measured at different stages of the estrous cycle. Additionally, PAG circuitry responsiveness was investigated through the use of panicogenic CCK₂ receptor agonist pentagastrin at different cycle stages.

In the male rats, GABA produced a dose-response in individual neurons within the PAG. BIC blocked all GABA response in the same cells. ALLO and ORG20599 decreased the firing rate at the same neurons in male rats. However, the firing rate decreased more slowly and was sustained over time. BIC also decreased the effect of ALLO and ORG20599, similarly to the GABA studies, and increased the firing rate by 15-29%. The results suggested that there is a GABAergic tone that is altered by progesterone metabolites.

The female rats showed similar responses to GABA and ALLO/ORG20599 injections. However, the firing rates that increased with BIC were the highest in estrous and late diestrous phases on the cycle. Thus, the excitability is increased during these phases. The females also underwent exposure to pentagastrin, the CCK₂ agonist. The firing rate of the neurons in the PAG increased the most during the estrous and late dioestrus phases of the cycle, similarly to the BIC responses.

According to the immunohistochemical studies, progesterone withdrawal in the female rats led to the increase in the GABA_A receptor subunits in neurons in the PAG. Spontaneously-cycling rats saw similar elevations in the subunits during the late dioestrus phase. GAD-positive cells were present throughout the PAG, and their levels remained constant during all phases of the cycle. However, it appeared that the neurons within elevated-expression of the subunits co-localized near the GAD neurons during the late-dioestrus phase.

Two cohorts of female animals from each environment were used to determine the influence the environment has on GABA_A receptor subunit expression. The standard-housed animals showed the expected increase in subunit expression. However, the isolated animals did not show an increase. They remained at similar levels to the proestrous phase, when subunit expression is expected to be low. All animals were cycling normally.

The study showed that progesterone, and more specifically its neuroactive metabolite ALLO, can inhibit the GABA_A neuronal firing directly through receptor-interaction. The male studies determined that the hormone can rapidly modulate the GABAergic tone within the PAG. The female studies demonstrated that the decreased levels of progesterone during the late dioestrus phase are correlated with an upregulation of subunits in the PAG. Thus, it is suggested that the cycling of hormones modulates the expression of GABA_A receptor subunits on neurons. Importantly, the delta subunit, which is located extrasynaptically, was also upregulated. Because of the increased excitability of PAG, along with the upregulated GABA_A receptor delta subunit, it is suggested that the interneurons are receiving an increased inhibition from GABA. The output neurons are receiving a reduce basal tone, which increased the “intrinsic excitability” of the PAG as a whole.

These results also explain why there is the increased sensitivity to pentagastrin during the late dioestrus phases. With a decreased GABAergic tone after the progesterone withdrawal of this phase, the CCK₂ receptors can continue to fire. The CCK₂ receptor is anxiolytic-regulating, suggesting that there is a correlation between the reduced GABA-tone from the progesterone withdrawal and increased anxiety.

The studies supported the hypothesis that the environment can influence the effects of progesterone on the GABA_A receptor subunit expression. The researchers suggest that the isolated animals underwent social-isolation stress. The influence of the stress on the animals prevented the upregulation of the subunits during late dioestrus, despite cycling normally. These results are supported by male rat studies, as well. Nevertheless, more studies need to be done to determine how the housing environment can influence the expression of the subunits.

-LivIn la Vida

Spike detection in human muscle sympathetic nerve activity using a matched wavelet approach

Aryan Salmanpour, Lyndon J. Brown, J. Kevin Shoemaker
Journal of Neuroscience Methods (2010)

Microneurography is often used to visualize nerve activity. Although these recordings can show changes in nerve activity, it is difficult to differentiate between background noise and actual nerve activity. This study discusses a new method for detecting action potentials in nerve activity from human muscle. The method attempts to find the location of action potentials in high levels of noise by improving wavelet techniques.

Muscle sympathetic nerve activity (MSNA) data was recorded from seven healthy individuals between the ages of 23 and 30. There were two males and five females. The peroneal nerve was used to retrieve the recordings because it contains nerves with skin and muscle blood vessels as their destination. The main purpose of wavelet analysis is to take the signal that has all the action potentials that are buried in noise and decompose it using continuous wavelet transformation (CWT) to filter the MSNA signal into a matched wavelet called a mother wavelet. To create the mean action potential template, the ten largest sympathetic bursts from each patient were used. The non-overlapping action potentials in ten bursts for each patients were then averaged to create the mean action potentials. The participant’s mean action potentials were then averaged to create one mean action potential template called the mother wavelet template. The mother wavelet helps create a resemblance index which can give you wavelet coefficients between the signal of interest and the mother wavelet. The wavelet coefficients are large in the presence of action potentials, which means there is a strong resemblance, and very low in segments with only noise. Thresholds were then created to determine the location of individual action potentials. The action potentials are then separated from the original filtered MSNA.

This new method has advanced the ability to look at spikes in sympathetic nerve activity in humans and helps to further understand just how the nervous system sends signals. Although this process was designed for human sympathetic action potentials, it could possibly be used for spike detection in other nerve recordings. The algorithm should work fine, just as long as there is an action potential template for recordings being studied. The optimized threshold and mother wavelet used in this study does not apply to all other tests. It was created specifically for this study. Future experiments could attempt to find a threshold that can become standard for all tests. Also, this method finds the locations of action potentials in high levels of noise, but cannot distinguish action potentials from different axons. 

Paul M

Effects of intrinsic aerobic capacity and ovariectomy on voluntary wheel running and nucleus accumbens dopamine receptor expression

Young-Min Park et. al

Physiology and Behavior

           

As mentioned in the last blog paper, there is a high incidence of cardiovascular disease in post-menopausal women. The current paper states that this is partially due to an increase in physical inactivity in post-menopausal women. Many studies have shown that increasing physical activity leads to a decreased risk of many diseases and so this decrease in physical activity in post-menopausal women is particularly interesting. What is the cause of the physical activity? Is it due to in inability to exercise as much, or maybe a decreased motivation for voluntary exercise? Rats can be selectively bred for high (HCR) and low (LCR) capacity running and these two different breeds show differing amount of voluntary wheel running. The mesolimbic dopamine system in the brain has been implicated in this motivation.

In the current paper rats were selectively bred for HCR and LCR which reflects their ability to exercise. Rats were kept on a 12:12 light dark cycle and then were randomized into four groups: HCR sham, HCR OVX, LCR sham, and LCR OVX (OVX is ovariectomy, sham is sham surgery). At 27 weeks surgeries were performed, either sham or ovariectomy, and then all rats were given access to a cage with a running wheel and running distance was monitored for 11 weeks. This was measured weekly (so variations due to the hormones were not seen on a daily basis as we do in our lab). After this period of 11 weeks the rats were sacrificed, and brain tissue was harvested for analysis. mRNA was extracted from the nucleus accumbens tissue that was harvested and levels of D1 like receptors, D2 like receptor and the dopamine transporter (DAT) were all measured.

As expected the high capacity group ran more throughout the entire 11 week period as compared to the low capacity group. In both ovariectomy groups running was reduced at all 11 weeks compared to the correlated sham group. It was also found that the high capacity group had a greater ovariectomy induced reduction in the amount of weekly running than the low capacity group. As for the general trend of running within each group, all groups did increase their running throughout the experiment with the exception of the high capacity ovariectomy group which actually decreased their running throughout 11 weeks of the experiment. In gene expression analysis it was found that the HCR group had greater excitatory dopamine expression than the LCR group and that inhibitory dopamine gene expression was actually greater in the LCR group than the HCR group. There were interesting finding in expression when looking at the different in ovariectomy groups. OVX increased the amount of inhibitory dopamine expression in the HCR group, but in the LCR group, OVX, decreased the amount of dopamine expression. In this paper the estimation of dopamine activation was the ratio of excitatory dopamine expression vs. inhibitory dopamine expression. When this was assessed it was found that this ratio was greater in HCR compared to LCR and that OVX only affected this ratio in the HCR group. This ratio was also positively correlated with weekly running distances across all of the groups.

The findings of this paper are very interesting. It is interesting to see that the high capacity group did not protect against the decrease in voluntary wheel running in the OVX group. I think that It would be extremely interesting to get at the mechanisms that the female hormones work by in the brain that cause these behavioral changes in the rats. This could result in possible therapeutic interventions in post-menopausal women that could help decrease the incidence of cardiovascular disease in these women.

- BeN RoShAk

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla (Second try)

By: Sean D. Stocker, Susan M. Lang, Sarah S. Simmonds, Megan M. Wenner, William B. Farquhar (Hypertension, 2015)

Hypertension due to increased salt levels have been shown to be associated with increased levels of Na in cerebrospinal fluid (CSF) and increased sympathetic nerve activity (SNA). Those who are salt sensitive have seen an increased level of Na in the CSF. This study focuses more so on the effects of hypernatremia in CSF and how the pathway is relayed. The rostral ventrolateral medulla (RVLM) is involved in the signaling mechanism, but it is unclear how it is related to and increased concentration of Na. 

CSF with varying levels of Na concentrations were infused in male rats. During these infusions, recordings of the ABP and SNA were active. 

Various experiments were performed in order to test the different aspects of the study.

Experiment 1: Different concentrations of CSF NaCl were infused to observe effects on various end organs. Lumbar SNA, adrenal SNA, heart rate, and mean ABP were increased as concentration increased. Renal SNA decreased and splanchnic SNA was unchanged. 

Experiment 2: The Na levels was observed in the fourth ventricle of the brain. When CSF NaCl was infused, the concentration in the fourth ventricle was increased.

Experiment 3: To assess contribution of SNA to NaCl induced responses, infusions were performed after ganglionic blockade. Chlorisondamine was injected and decreased baseline SNA and mean ABP. It also abolished the SNA, tachycardiac, and pressor response to the NaCl injection. 

Experiment 5: To test whether RVLM neurons mediate a response to CSF NaCl, infusions occurred after the inhibition of the RVLM. Bilateral injections of GABA to RVLM reduced SNA activity in lumbar, renal, adrenal, splanchnic regions. Also, lumbar sympathoexcitatory and pressor response to the infusion was abolished. 

Experiment 6: To identify the neurotransmitter in the RVLM that mediates the responses in experiment 5, Na infusions were performed after blockade of ionotropic glutamate or angiotensin type 1 receptors in RVLM. Bilateral injection of kynurenic acid in RVLM partially reduced lumbar SNA but didn't change renal adrenal or splanchnic SNA, as well as mean ABP and heart rate. Blockade of glutamate receptors significantly weakened the increase in lumbar and adrenal SNA, as well as heart rate and ABP. It also reduced renal sympathoinhibitory response. Bilateral injection of angiotensin type 1 did not alter any of the responses. 

Experiment 7: Last experiment was to establish that CSF hypernatremia altered activity of barosensitive spinally projecting RVLM neurons. After infusion of NaCl in type 1 neurons occurred, cell discharge activity increased within 2 minutes and remained elevated. Decreased in Type 2 neurons. Type 3 neurons were unchanged. Identified C1 neurons displayed an increased discharge response to infusion of NaCl. 

The results led to the conclusions that acute infusions of different concentrations of NaCl increased lumbar SNA and adrenal SNA while decreasing renal SNA and not changing splanchnic SNA. In relation to the RVLM, sympathetic and ABP responses were reduced when the RVLM neurons were inhibited with a GABA agonist or glutamate receptor blockers. When the RVLM neurons were inhibited, the sympathetic and pressor responses to the NaCl was reduced, explaining the role of the RVLM in ABP and SNA control. We look at the splanchnic SNA and when RVLM neurons were inhibited, the response to increased NaCl was inhibited! The RVLM plays a role in how the SNA responses to increased levels of NaCl. 

-Tsetse Fly

Erythropoietin, a putative neurotransmitter during hypoxia, is produced in RVLM neurons and activates them in neonatal Wistar rats

Naoki Oshima,  Hiroshi Onimaru,  Akira Yamagata,  Seigo Itoh,  Hidehito Matsubara,  Toshihiko Imakiire,  Yasuhiro Nishida,  and Hiroo Kumagai.
Am J Physiol Regul Integr Comp Physiol. 2018 May 1

Hypoxia is a lack of oxygen reaching certain areas of the body. Hypoxia causes increases in the sympathetic response. Erythropoietin (EPO) is a glycoprotein cytokine that has been observed in many areas of the body, especially during hypoxia. EPO has also been shown to be present in the brain. This study looks at the role of EPO in the RVLM during hypoxia and the responses it causes.

To test the effects of EPO on RVLM neurons, medulla preparations were given a solution of EPO. EPO was shown to depolarize the RVLM neurons and increase the action potential frequency. The RVLM neurons were then given a solution of EPO and tetrodotoxin (TTX), which is a neurotoxin. The neurons still showed depolarization which suggests that EPO depolarizes the RVLM neurons.

RVLM neurons project to the mediolateral cell column (IML), where they connect with the sympathetic preganglionic neurons. To test if the activity in the RVLM, which was caused by EPO, was being transmitted to the IML neurons for sympathetic response, the RVLM area were given an EPO solution. Changes in the membrane potential (MP) and IML were then observed. Using a tungsten electrode at the TH-2 level of the spine, the results showed that the EPO solution caused a depolarization of the IML neurons. Latency of the neural signal was then studied by looking at the time from when the EPO solution was given, to the time changes occurred in the membrane potentials of the IML neurons. This time was show to be about 26.8 seconds. When the RVLM was given a solution of soluble erythropoietin receptors (SEPOR), which is an EPO antagonist, the latency was about 25 seconds. This shows that effects on the RVLM neurons are transmitted to IML neurons. 

Lucifer yellow, which is a lithium salt florescent dye, was used to test the depolarization of neurons after given a solution of EPO.  After the medulla preparation was given a solution of EPO, six neurons in the RVLM that showed depolarization were examined for erythropoietin receptor florescence. All six of the neurons were shown to exhibit EPOR activity.

To test if EPO is produced in the RVLM during hypoxia, two groups were created. Two sections of the medulla containing the RVLM were either given a gas high in nitrogen or a gas high in oxygen for 30 minutes. Afterwards they measured the levels of EPO mRNAs in the RVLMs using polymerase chain reaction (PCR). The levels of EPO mRNA was shown to increase in the RVLM areas of the hypoxic group when compared to the group given oxygen, which suggests that EPO production is increased during hypoxic conditions.

In conclusion, this study showed the presence and role of EPO within the RVLM. Hypoxia was shown to increase EPO production, which lead to EPO stimulating the RVLM neurons. This study provides some ideas of the mechanisms behind hypertension induced by hypoxia.

Paul M 

Ovarian Hormone Deprivation Reduces Oxytocin Expression in Paraventricular Nucleus Preautonomic Neurons and Correlates with Baroreflex Impairments

De Melo VU, Saldanha RRM, Dos Santos CR, et al. Ovarian Hormone Deprivation Reduces Oxytocin Expression in Paraventricular Nucleus Preautonomic Neurons and Correlates with Baroreflex Impairment in Rats. Frontiers in Physiology. 2016;7:461. doi:10.3389/fphys.2016.00461.

              Some studies have shown that the incidence of cardiovascular disease in women increases as they go through menopause. The exact mechanisms for this are unclear and need to be further investigated. It has been suggested that it is due to the decrease in estrogen levels in the women’s body. Specifically hypertension is one of the diseases cited that has a dramatic increase after women go through menopause. Some neurons in the paraventricular nucleus of the hypothalamus (PVN) utilize oxytocin as a neurotransmitter and project to the brainstem and are involved in the regulation of the cardiovascular system via sympathetic outflow. These PVN neurons express estrogen receptors. The current study hypothesizes that estrogen binds to these neurons and increases their outflow to the brainstem thus increasing sympathetic outflow, and that if estrogen deprived this system would be dysregulated and could cause hypertension. They also state that the increase in cardiovascular ability is due to the increase in baroreceptor sensitivity and that without estrogen that is impaired.

              The current study used 33 female Wistar rats that were kept on a 12:12 light dark cycle. These were randomly divided into two groups, one that was ovariectomized (OVX) and one that had a sham surgery. OVX group was used as the group with no circulating estrogen that would be binding in the PVN. Surgeries were performed at 10 weeks of age. After 8 more weeks, rats were implanted with catheter and cardiovascular parameters were measured. At this time spontaneous baroreflex sensitivity was measured as well. Brain slices were obtained from the rats after they were sacrificed and PCR analysis of mRNA expression as well as oxytocin expression was analyzed.

              It was found that the OVX rats weighed more at the end of the study, and that they had a higher resting mean arterial pressure as well as a higher heart rate. In some nuclei of the PVN it was found that oxytocin mRNA and protein expression levels were significantly lower in the OVX rats as compared to the sham surgery rats. In these rats where there was less oxytocin expression, there was a strong correlation with decreased autonomic control as measured by baroreflex sensitivity. This study provides support for the idea that without the estrogen hormone, the baroreceptor reflex is impaired which causes dysregulation of the sympathetic system and possibly increases in blood pressure. It is shown in this study that reduced oxytocin expression in the PVN neurons may be one of the molecular mechanisms behind this increase in blood pressure after hormone deprivation.

              I think the findings from this study are interesting in light of us trying to initiate ovariectomy experiments in rats within our own laboratory. The findings are interesting and give a potential mechanism for the reason that estrogen has a protective cardiovascular effect. This study mainly focuses on the PVN neurons that express estrogen receptors and are oxytogenic in nature, however it is these neurons that modulate activity in the rostral ventrolateral medulla, the focus in this lab.

Ben R - blog alter ego TBD

Correlation of discharges of rostral ventrolateral medullary neurons with the low-frequency sympathetic rhythm in rats

By: Tseng et al

The rhythmicity is an important feature of the sympathetic nervous system. It's what keeps the system functioning at a stable pace. This rhythm has been showed to be controlled by the heart beat, respiratory regulation, and the baroreflex loop. Blood pressure is directly controlled by this loop. The RVLM has been showed to contribute to this rhythm by tonically driving SNA. The purpose of this study was to investigate which types of RVLM neurons contribute to the low frequency (LF) SNA rhythm in rats. To do this they studied the neuronal activities of RVLM neurons, SNA, and BP at the same time. Coherence analysis was used to correlate LF rhythms of SNA with neuronal firing frequency.

Recordings of SNA, BP, and RVLM activity were recorded simultaneously. RVLM neuronal firing and SNA were compared to each other using a cross-correlation computation. Phenylephrine was used to denervate the baroreflex loop. Once infused, BP, SNA, and HR were significantly decreased. 51 neurons were recorded from the RVLM. They were sorted by their responsiveness to raised BP (inhibited, excited, non-responsive)

41% of neurons recorded were correlated with the LF SNA rhythm. When removing baroreceptor input, neurons were still correlated to the LF SNA rhythm which suggests that vasomotor RVLM neurons contribute to the existence of the LF SNA rhythm. When the baroreflex was denervated, SNA was only reduced, but didn't go to 0. This explains that there are other sources for the LF component of SNA that exist. 62% of RVLM neurons that contributed to the LF SNA rhythm that are barosensitive were sympathoexcitatory. The conclusion of the study is that RVLM neurons have the ability to generate the LF SNA rhythm and also the LF BP rhythm.


-Tsetse Fly

Neuroinflammation Contributes to Autophagy Flux Blockage in the Neurons of Rostral Ventrolateral Medulla in Stress-induced Hypertension Rats

D. Dongshu, H. Li, W. Jiaxiang et al. (2017)

There are a lot of interconnected networks that control blood pressure. The Rostral ventrolateral medulla (RVLM) is a main regulator of sympathetic nerve activity and is an important monitor of cardiovascular health problems. Microglia are neuroinflammatory cells within the central nervous system that regulate inflammatory events such as cardiovascular disease. This study examines the role of microglia during neuroinflammation.

Rats were divided into three groups: A control group, a chronically stressed group, and a chronically stressed group given minocycline, which is a microglia activation inhibitor. To produce the stress environment, rats were placed inside a cage containing floors that delivered randomized shocks and a buzzer to accompany the shocks.

The chronically stressed rats showed increased blood pressure, which may be caused by the elevated levels of norepinephrine (NE), glutamate, and decreased levels of GABA that were measured. Ionized calcium-binding adapter molecule -1 (Iba-1) is a protein that is expressed in microglia. Chronic stress was shown to significantly increase Iba-1 levels in SIH rats. About 10% of microglia were active in control rats, while 40-50% were active in stressed rats.  Pro-inflammatory cytokine (PIC) levels were also increased in chronically stressed rats when compared to non-stressed rats. Although autophagic vacuoles are found in small numbers within the RVLM of control rats, the concentration was shown to double within chronically stressed rats. Autophagic markers LC3 and p62 were  also shown to increase in chronically stressed rats. RVLM neurons typically show low levels of autophagic flux, which is a measure of autophagic degradation activity, but chronically stressed rats exhibited a significantly elevated autophagic flux.

A third group, that received minocycline, expressed much lower levels of PICs when compared to the untreated stressed group. What was interesting was that expression of TNF-alpha within the RVLM in stressed rats treated with minocycline was actually lower that the control group. Although the minocycline treated stressed group still showed elevated levels of NE and glutamate, it was still significantly lower than the untreated stressed groups. When treated with minocycline, the stressed rats showed lower levels of autophagic flux when compared to untreated stressed group, even though it was still significantly higher than the control group

They concluded that the microglia activate and release PICs, which causes the increases RVLM sympathetic nerve activity. Minocycline was also found to decrease the overall sympathetic output from the RVLM and increase the autophagic flux of stressed rats. I chose this article after finishing up my blog post about microglia from last week. I’m beginning to learn just how big of a role microglia play in the equilibrium and immune defense of the brain. Although we are not examining microglia in the lab currently, it would be fascinating to see the activation levels during the microinjection trials.

Paul M

Gender-Selective Interaction Between Aging, Blood Pressure, and Sympathetic Nerve Activity

By: Krzysztof Narkiewicz, Bradley G. Phillips, Masahiko Kato, Dagmara Hering, Leszek Bieniaszewski, and Virend K. Somers

Despite both sexes seeing an increase in risks for cardiovascular disease as they age, women typically see a sharper one, and the gap between men and woman continues to increase. Cholesterol and weight increase only account for 50% of the age-related increases in risk. Thus this study looked at the sympathetic nervous system and its role in the development of age-related cardiovascular disease. At the time of the study, little was known about the effects of sex-differences on age-related sympathetic outflow, and this seems to be the case today. However, this study provided a good foundation for researchers to continue to investigate the role of sympathetic outflow on age-related blood pressure changes and cardiovascular disease development.

The study looked at the sympathetic outflow, blood pressure, and heart rate in “normal” white subjects from ages 20 to 72. They examined the influence of age on the interaction between sympathetic outflow and blood pressure. They also looked at the influence of menopause on sympathetic outflow. They hypothesized that aging will have a more significant impact on sympathetic outflow in women versus men.

Intraneural measurements of muscle sympathetic nerve activity (MSNA) were taken from the peroneal nerve. Blood pressure and heart rate were also measured. MSNA was recorded as bursts per minute, and only those that achieved a minimum signal-to-noise ratio of 3:1 were counted. Mean blood pressure was taken every minute, and rate heart was measured by an electrocardiogram. Regression slopes were made to analyze the interaction between age and gender. Effects of age on MSNA were evaluated by 2-way analysis of variance with the decade of life each subject was in and gender as the groups.

MSNA correlated with age in both genders. However, MSNA had a far stronger correlation in women (r=0.73; P<0.0001) versus men (r= 0.31; P=0.003). Age explained 53% of MSNA variance in females but only 8% in men. As the subjects aged, there were increases in MSNA bursts for both sexes. However, women saw a greater increase for every decade aged compared to men (6.5 bursts/min versus 2.6 bursts/min; similar results when compared at bursts/100 heart beats).

MSNA was lower in females than males for younger subjects. As the subjects aged, the age-related increase in MSNA was greater in females than in males. The interaction of sex and age was significant, suggesting that the effect age has on MSNRA is sex-dependent. Premenopausal and postmenopausal women within the same decade of age had similar MSNA, indicating no relationship between menopause and MSNA.

Blood pressure and MSNA was not related in young subjects. However, there was a correlation in both male and female subjects above the age of 40. For these older individuals, every 10-burst/min increase in MSNA in women showed a rise of 6.1 mmHg in their blood pressure, versus a 2.7 mmHg in men.

The study was able to demonstrate a higher possible correlation between aging and an increase in sympathetic outflow in white women over white men. They also found that menopause does not explain the age-related increase in sympathetic outflow in women. Finally, they found a positive interaction between blood pressure and MSNA for people over 40 years of age. MSNA increased at a rate of 2.5-times higher in women than in men. Age contributed to the MSNA variance 6 times greater in women than in men. These results were consistent with previous studies done in Japanese subjects that showed MSNA was lower in younger women than in younger men and similar amongst the older subjects, suggesting that their results are not race-dependent (although, I disagree, because they only looked at individuals from a few, small populations located in particular parts of the world. They did not include other races aside from White Americans or Japanese individuals in their current or past studies, and thus their results cannot be said to be “race-independent”).

The significant correlation between MSNA and blood pressure provided support for the role of sympathetic activation in the link between aging and hypertension. This is more relevant in women than in men. However, the at the time of the study, the mechanisms that lead to the sex-differences between MSNA and blood pressure were unknown. They suggested that hypertension initially arises from an increased sympathetic output at the developing stages of the disease. As subjects age, they suggested that the increased sympathetic outflow leads to an age-related decrease in compliance as subjects age.

Despite their results suggesting that menopause does not contribute to age-related increases in MSNA, they were unable to show that female sex hormones do not contribute to the lower blood pressure and MSNA measured in younger subjects. However, the results from this study provided a solid foundation for other researchers to examine the role of female sex hormones in the alteration of sympathetic outflow, much like what we are starting to do in our lab.

I chose to look at a human study this week because I felt that it was necessary to see why we are studying sex-differences in RVLM activation. Although we study rats, we are not looking to solve the physiological problems in animal models. We hope that our results will translate to humans so that we can better understand the sex-differences that play a possible role in altering sympathetic activation and hypertension in our bodies. The study that I reviewed this week provided a substantial starting point for research in the hypertension field. The researchers examined a hypertensive, sex-dependent, external phenotype and were left wondering about the underlying mechanisms. Thus, the animal studies that followed are essential to solve those remaining questions. We cannot look at the neuronal components of this pathway in human subjects. Therefore, we look at rats. However, we still see results in animals similar to those in human subjects, such as changes in blood pressure and sympathetic outflow. Studies like the one reviewed today help to verify that the results we are producing in animal models are translatable back to human subjects.

-O. Flessland

Sex Differences in Angiotensin Signaling in Bulbospinal Neurons in the Rat Rostral Ventrolateral Medulla

Gang Wang, Teresa A. Milner, Robert C. Speth, Andrea C. Gore, Di Wu, Costantino Iadecola, and Joseph P. Pierce

The risks for hypertension have long been studied, but more recently the differences in risks between sexes have started to be studied as it has been found that there is a significant difference between men and women. Bulbospinal neurons in the rostral ventrolateral medulla (RVLM) control the regulation of blood pressure and are implicated in the development of hypertension. Specifically, angiotensin II and its receptor AT1 in the RVLM are implicated it the development of hypertension. In this study RVLM neurons were studied to evaluate the difference in AT1 receptors, subunit p47 of NAPDH oxidase and the effects of angiotensin II on reactive oxide species (ROS) production between male and female rats.

Sprague-Dawley rats were used in this study and all female rats received bilateral ovariectomies and then were given estrogen replacement, this was to take out the variability of all hormones that come with being a female and just examine the effect of estrogen. Female rats were then implanted with capsules containing estradiol or a 100% cholesterol vehicle. In this study antibodies were used to assess the level of expression of the parameters described above. After rats were sacrificed coronal sections of the brain were taken and antibody labeling of AT1 and the p47 subunit was done. A certain population of cells was also exposed to angiotensin II and then the level of intracellular reactive oxygen species was measured.

This study found that there is more AT1 in females than in the males. Similarly, it was found that the females that were ovariectomized and given estrogen had more AT1 than the females that were ovariectomized and not given estrogen. Females also contained lower levels of p47 subunits that the males. ROS production after administration of angiotensin II did not differ between male and female rats, although it was found that the L-type calcium currents induced by angiotensin II were significantly larger in the females compared to the males.

This study shows that estrogen, which is the major hormone in females, can modulate differences in properties of the RVLM and that these differences can contribute to molecular mechanisms that are linked to hypertension. This was interesting because it shows that there are many differences between the male and female brain and that there is a lot to explore along this line of thinking. It is already known that there is a major difference in the development of hypertension between men and women, the mechanisms behind this may be related to hormones, but there is still that is unknown.

Ben R

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla

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

Hypertension due to increased salt levels have been shown to be associated with increased levels of Na in cerebrospinal fluid (CSF) and increased sympathetic nerve activity (SNA). This study focuses more so on the effects of hypernatremia in CSF on the rostral ventrolateral medulla (RVLM) and how it leads to an increase in arterial blood pressure (ABP) and SNA. 

CSF with varying levels of Na concentrations were infused in male rats. During these infusions, recordings of the ABP and SNA were active. 

The results showed concentration-dependent increases in lumbar SNA, adrenal SNA, heart rate, and mean ABP. To observe the role of the RVLM in the sympathoexcititory response, the RVLM was inhibited following the infusions of the CSF NaCl. A GABA agonist was then given to observe the acute effects of the CSF NaCl. Lumbar, renal, adrenal and splanchnic SNA was reduced in addition to mean ABP and heart rate. 

The results led to the conclusions that acute infusions of different concentrations of NaCl increased lumbar SNA and adrenal SNA while decreasing renal SNA and not changing splanchnic SNA. In relation to the RVLM, sympathetic and ABP responses were reduced when the RVLM neurons were inhibited with a GABA agonist or glutamate receptor blockers. The sympathoexcitatory responses are deemed likely to be from end organs. When the RVLM neurons were inhibited, the sympathetic and pressor responses to the NaCl was reduced, explaining the role of the RVLM in ABP and SNA control. 

This study has relation to our lab due to the questions surrounding the end responses the RVLM has on ABP and SNA. It was clear in this study that the responses were attenuated when the RVLM neurons were inhibited. That tells us that the RVLM does in fact have role in responding to the changes in SNA and ABP. 

Questions:

1. In experiment 1, it says that adrenal SNA was increased, but then in their concluding statements, they state that adrenal SNA was decreased. Can you help explain this?

-Tsetse Fly

Estrogen Protects Against Increased Blood Pressure in Postpubertal Female Growth Restricted Offspring

By N.B. Ojeda, D. Grigore, E.B. Robertson, and B.T. Alexander

Upon the onset of menopause, women typically have higher rates of hypertension compared to men, and the risk continues to rise as women age. To better understand the sex-differences of female rats and hypertension, studies began to look at the effects of estrogen loss. Past research suggested that estrogen provides a protective role against heart disease like hypertension. In animal models, normotensive female rats became hypertensive after undergoing ovariectomies. Furthermore, rat offspring that undergo intrauterine growth-restriction (IUGR) are born hypertensive. However, at the onset of puberty, only female rats stabilize their blood pressure, further suggesting the importance of sex hormones in the stabilization of hypertension. There have also been studies that have suggested estrogen modulates the renin-angiotensin system (RAS) as a mechanism to maintain hypertension in IUGR rats. Thus, these results indicate that a loss of estrogen contributes to the development and maintenance of hypertension in female rats.

The purpose of this study was to determine whether or not estrogen has protective properties against hypertension in IUGR rats and to examine if estrogen acts through the RAS to complete its protective measurements. Ovariectomies (OVX) and sham surgeries were performed in 10-week old female IUGR and standard "control" animals. These procedures created four general groups of animals: control-intact (no OVX performed); IUGR-intact; control-OVX, and; IUGR-OVX. The ACE-inhibitor enalapril was then given to a subset of these animals: control-intact+enalapril; IUGR-intact+enalapril; control-OVX+enalapril, and; IUGR-OVX+enalapril. 

Another subgroup of OVX animals received estradiol (E2) supplements post-OVX: control-OVX+E2, and; IUGR-OVX+E2. OVX animals' E2 levels returned to normal ranges after the E2 supplements. The remaining animals who did not receive an additional drug were used as controls for each group. E2 levels were collected an measured while under anesthesia at ages 4, 6, 8, 10, 12, and 16 weeks of age. These measurements were used to determine the onset of adolescence while measuring the levels and stabilization of E2 in control and IUGR animals. 

The kidneys of the rats in each general group were removed and used to measure the amounts of ACE and ACE2 mRNA present in them. Lastly, plasma renin activity (PRA) and plasma renin substrate were collected from intact and OVX animals post-decapitation. The study analyzed the relationship between E2 and the RAS using these two measurements. Mean arterial pressure (MAP) was measured and averaged at the first three days of the week for each rat.

MAP did not differ between the IUGR and control adult females. However, OVX induced hypertension in IUGR animals and had no significant effect on the control adults. However, their normalized values were comparable. E2 replacement at 14 weeks of age reduced the blood pressure in both groups by 16 weeks of age. Starting at 14 weeks of age, animals received enalapril to inhibit ACE. This drug did not cause significant differences in MAP between intact-control+enalapril and OVX-IUGR+enalapril animals. Enalapril also removed the difference in MAP between the untreated OVX-control and OVX-IUGR animals initially measured. E2 levels remained similar between the IUGR and control animals throughout the entire study. At 8 weeks of age, the E2 levels increased. Ovariectomies reduced the E2 levels tremendously, and the E2 supplements returned the levels to normal. PRA and PRS levels remained the same between the groups, and ovariectomies did not affect them either. The only group that saw reductions in renal ACE2 mRNA were the OVX-IUGR animals. The OVX-controls did not see this reduction. 

The study's results suggest that estrogen provides protective measures against the development and maintenance of hypertension in post-adolescent IUGR rats. The only factor that led to increased MAP from baseline were the ovariectomies done to the IUGR animals. Taking into consideration that both groups usually have a similar MAP, this result suggests that E2 plays a role in normalizing blood pressure. E2 also brought MAP back to normal levels in these animals to further argue for the protective factors of E2. The study's results also argue that the RAS is a possible route of action for E2 to take to control blood pressure. Furthermore, there were ACE2 mRNA results that suggest E2 acts through ACE2 to maintain blood pressure. IUGR-OVX animals were the only group that showed reductions in their ACE2 mRNA expression, suggesting that there are effects to the system while in utero. However, despite the success of their study, based on their results and study design, this data is only applicable to IUGR rats that lose estrogen post-adolescence through ovariectomies. 

- O. Flessland

Gamma Frequency Entrainment Attenuates Amyloid Load and Modifies Microglia

Iaccarino H, Singer A, Martorell A, et al. Nature. 2016.

Alzheimer’s Disease (AD) is classified as a chronic neurodegenerative disease and is the most common form of dementia with 5 million cases in the US alone. Although it is not fully known what causes AD, researchers are beginning to find correlations between decreased brain oscillations and AD.

Brain oscillations are broken down into different frequencies, including the gamma frequency which ranges from 35-100 Hertz (Hz). During AD, brain oscillations have been shown to change. The researchers in this study thought that if they could somehow artificially stimulate the gamma frequency in a patient with AD back to normal levels, this could slow or possibly reverse the effects of AD. They designed a study where mice had a fiber optic cable placed through the skull and into the brain.Each test subject received one of three stimulations: 8 Hz, 40 Hz, or a random stimulation. Researchers looked at amyloid beta levels in the hippocampus of the mice after the simulations. While 8 Hz and random stimulation did not produce significant results, 1 hour of stimulation at 40 Hz reduced two distinct amyloid beta levels by 53.22% and 44.62% and increased microglia activity. Researchers wanted to recreate similar outcomes, but in a less invasive study that could possibly be recreated on a human patient. The thought was that instead of introducing the stimulus directly into the brain, could they recreate the experiment by having the stimulus come through the eyes as a visual stimulus. The mice were then placed in a dark room and exposed to one of five different stimulations: dark, light, 20 Hz, 40 Hz, or 80 Hz. Although all other groups were not significantly different, one hour of 40 Hz flicker reduced distinct amyloid beta levels by 57.96% and 57.97%. Researchers also looked at microglia activity after 1 hour in a 40 Hz flicker room. Results showed that the microglia count had stayed constant, but microglia cell body diameter increased by 165.8%, which they concluded indicates boosted phagocytic activity. The final part of the study considered how light flicker effects amyloid plaques. Interestingly enough, after I hour of 40 Hz flicker, the plaques were reduced in size by 63.7% and decreased in number by 67.2%. Plaques began to return to normal size along with lowered microglia activity after the mice would not receive the flicker treatment for multiple days. 

The research concluded that it is possible to slow the effects of AD temporarily in mice, but the results from the flicker test show a non-invasive treatment that might be possible on a human patient and may lead to a new form of therapy for Alzheimer’s Disease in the future.

I came across this article after reading a few other papers that stated hypertensive rats showed increased levels of activated microglia in the rostral ventrolateral medulla (RVLM). I wanted to understand the role of microglia in the other parts of the brain before I compared it to their role in the RVLM.

Paul M

Physical Activity Correlates with Glutamate Receptor Gene Expression in Spinally-Projecting RVLM Neurons: A Laser Capture Microdissection Study

Madhan Subramanian, Avril G. Holt, Patrick J. Mueller

The economic cost of cardiovascular disease in the United States is enormous. Treatment for cardiovascular disease accounts for one sixth of all health care dollars spent in the United States, and in 2010 the economic cost of cardiovascular disease was almost $450 billion. Studies have shown that the rostral ventrolateral medulla is the main control of sympathetic outflow and that increased sympathetic outflow is correlated with cardiovascular diseases such as hypertension. The exact mechanisms of this process are yet to be elucidated.

The current study aimed to assess glutamate and GABA receptor gene expression in RVLM neurons in sedentary and active rats. Male Sprague-Dawley rats were used during this study. The active group was put in a cage with a running wheel and running parameters were tracked using a bike computer; the sedentary group was put in a cage with no running wheel. At the end of the running time rats were sacrificed to allow evaluation of gene expression in RVLM neurons. Prior to sacrificing the rats, neurons in the RVLM were retrogradely labeled using Flourogold so specific RVLM neurons could be identified. Laser capture microdissection was used to separate specific populations of spinally projecting RVLM neurons that have previously been shown to be involved in control of the cardiovascular system.

After laser capture microdissection was used Glutamate and GABA subunit gene expression was compared in the active and sedentary rats. The NR1 subunit of the NMDA receptor showed a trend towards upregulation in the active group of rats, while the other subunits of the NMDA receptor showed no change. None of the AMPA receptor subunits showed a change in gene expression in the active vs. sedentary groups. In evaluation of the GABA-A receptor subunits it was found that GABA-A2 subunit had a trend towards upregulation in the active group compared to the sedentary group. In further analysis it was found that two subunit’s gene expression, NR2C of the NMDA receptor, and GLUR3 of the AMPA receptor, both showed a positive correlation with the total running distance of the rats in the active group.

This study was successful in showing that laser capture microdissection is a good tool to use in identifying specific populations of cells and using them to assess gene expression using their RNA. The study did not find any significant changes in gene expression between Glutamate/GABA receptors in the sedentary vs. active rat groups. Further studies need to be conducted to evaluate the effect that physical activity has on receptor subunit gene expression.

- Ben R.

Exercise Training Attenuates Proinflammatory Cytokines, Oxidative Stress and Modulates Neurotransmitters in the Rostral Ventrolateral Medulla of Salt-Induced Hypertensive Rats

It is known that the RVLM is a key area that controls blood pressure and sympathetic activity. This study focused on a few of the mechanisms that are taking place within RVLM during exercise and how they could affect adjacent pathways.

The authors suggest that pro-inflammatory cytokines such as TNF-alpha, IL-1b and IL-6 are involved in high blood pressure. To study the neural activity in the RVLM, the researchers focus on two mechanisms. GAD67 is an enzyme that regulates the production of GABA, which is known to inhibit the RVLM sympathetic activity. Fra-LI is a gene that, when expressed, demonstrates overexcitation. To assess these mechanisms, rats were fed high salt diets to induce high blood pressure.

One group of rats received a normal salt diet (NS), while another group was given a high salt diet (HS). For a period of twelve weeks, the rats were selected to receive exercise or sedentary conditions. Therefore, four different groups were formed: HS exercise, HS Sed, NS exercise, and NS Sed.

Beginning at week eight, blood pressure had significantly increased in the rats with high salt diets compared to the rats with normal salt diets. Within the group of rats fed a HS diet, the sedentary group continued to significantly increase whereas the exercise rats began to plateau. Following a similar pattern as blood pressure, higher RSNA was observed in the HS Sed rats than in the HS exercise rats. Although the HS exercise rats had lower RSNA, it was significantly higher than either the NS exercise or Sed rats. Pro-inflammatory cytokines were also detected in elevated levels in the RVLM and plasma of HS rats, but was shown to be less in exercise rats. The difference in PICs between HS Sed rats and HS exercise rats was very dramatic, yet the levels of TNF-alpha in NS Sed rats was less than half of the levels in HS exercise rats. 

In regards to the neural activity in the RVLM, the number of neurons containing GAD67 were much higher in NS exercise and Sed rats. HS exercise and Sed rats showed a higher level on neurons containing Fra-LI, which makes sense because it has the opposite effect of the inhibitory pathway of GAD67.

This study suggests that exercise can reduce neural activity of the RVLM and reduce blood pressure. What I found interesting about this study was the research into the imbalance between excitatory and inhibitory neurotransmitters. I'm interested in learning more about how this imbalance could possibly lead to many other pathways being affected which could bring about other damage.

Paul M

Blockade of Rostral Ventrolateral Medulla (RVLM) Bombesin Receptor Type 1 Decreases Blood Pressure and Sympathetic Activity in Anesthetized Spontaneously Hypertensive Rats

Bombesin is a peptide studied in this experiment which can activate G protein coupled receptors (Bombesin receptor 1,2, and 3) which ultimately can increase blood pressure and sympathetic nerve activity. The RVLM is a brain region in which the neuronal activity can directly relate to the sympathetic nerve activity of the individual. Within this study, the laboratory injected bombesin into the RVLM in order to test how it affects sympathetic nerve activity in normotensive and hypertensive rats. More specifically, the laboratory tested how mean arterial pressure, diaphragm motor activity and renal sympathetic nerve activity changed in the rat models. The results of this study showed that bombesin being injected into the RVLM caused an increase in MAP, and RSNA in both normotensive rats and hypertensive rats. The results also showed that blockade of bombesin 1 receptors in the RVLM caused a decrease in MAP and RSNA in hypertensive rats but not normotensive rats. These observations and future studies can greatly contribute to the maintenance of high blood pressure in hypertensive individuals.

Hash

Sedentary conditions and enhanced responses to GABA in the RVLM: role of the contralateral RVLM.

By Dombroski, MD and Mueller, PJ


Research has shown that the development of cardiovascular diseases, such as hypertension, are linked to a sedentary lifestyle. Overactivity of the sympathetic nervous system has been shown to contribute to hypertension through its regulation of resting blood pressure through tonic activation of the RVLM. The RVLM is regulated through inhibitory signals by the CVLM and its GABAergic projections. The GABA projections are altered under different disease states, thus changing the ability of the RVLM to maintain a “normal” sympathetic outflow to control resting blood pressure. However, previous studies have shown that brain regions that exist on both sides of the brain, like the RVLM, contribute to a compensatory buffering input in the contralateral side. Thus, the study hypothesized that the contralateral RVLM would help to compensate for decreased sympathoinhibition at this level, thus helping to maintain blood pressure. The primary focus of the research was to determine whether sedentary conditions enhanced the sensitivity of GABA when compared to active conditions, while simultaneously establishing if there is a buffering response from the contralateral RVLM.

The experiment was completed through unilateral microinjections of GABA within the left RVLM at 0.3, 3, 30, 300, or 600mM concentrations. Splanchnic sympathetic nerve activity (sSNA) was measured at baseline and after the GABA injection, as well as mean arterial blood pressure (MAP) and heart rate (HR). Initially, Both sides of the RVLM remained intact while performing microinjections to determine the influence that sedentary conditions have on the sensitivity of the RVLM. Later, injections of the long-acting GABA agonist muscimol into the right RVLM inhibited the area. The left side continued to receive received microinjections following the same dose-response protocol as before. Lastly, both active and sedentary groups received bilateral sinoaortic denervations (SAD), followed by unilateral GABA microinjections into the left RVLM.

During the intact studies, a dose-dependent response for the MAP and HR were measured following the GABA injections in both active and sedentary animal. However, there was no significant difference between the groups. Upon the inhibition of the right RVLM, there was enhanced sympathoexcitation in sedentary versus active rats. Lastly, there was no difference between the groups in sympathoexcitation when conducted with the SAD animals.

The research successfully demonstrated that there is enhanced GABA sensitivity in the RVLM neurons of sedentary versus active rats. This sensitivity could not be measured, however, until the contralateral RVLM was inhibited and thus prevented from hindering the response from the neurons in the left RVLM. Through these studies, the research helped to support the notion that there is a buffering response between the contralateral RVLM when there are decreases in sympathetic outflow in sedentary rats. With the SAD animals, the lack of a difference between the sedentary and active animals demonstrated that this buffering is not originating from the arterial baroreceptors themselves. Thus, the results suggest that the change is occurring within the central versus peripheral nervous system in sedentary rats.


-O. Flessland

Efficacy of clonidine in patients with essential hypertension with neurovascular contact of the rostral ventrolateral medulla

By Sakuma et al

The goal of this paper is to study the effects of neurovascular compression. This is done by looking at the regions of the brain that experience this condition. One region that was focused on was the rostral ventro lateral medullam (RVLM). It has been shown to be vital in regulating sympathetic and cardiovascular activities. One association that was looked at was between the RVLM and neurovascular contact (NVC), meaning an arterial vessel or vascular loop made contact with the RVLM. It was explained that when NVC was associated with RVLM, hypertension was observed. A technique that could help this condition is microvascuar decompression (MVD). It was shown to decrease BP in patients with NVC of the RVLM. The purpose of the study was to study a different method to treat hypertension in patients with NVC of RVLM because MVD was too risky being it is an invasive procedure. To accomplish this they treated patients with clonidine, which is an alpha 2 adrenergic agonist, and observed whether it lowered BP.

26 patients with NVC of RVLM and 28 patients without NVC of RVLM were studied. All patients were hypertensive. Both groups were treated with 225 ug of clonidine per day for 4 weeks. Both groups were also administered the same antihypertensive drugs in addition to clonidine.

After the conclusion of the study, the results showed that there was a significant decrease in systolic BP in NVC- patients. In NVC+ patients, there was a significant decrease in systolic and diastolic BP. Both decreases were greater in NVC+ patients compared to NVC-. There was no sig. change in HR. NE was significantly decreased in both groups, but more significant in NVC+.

Concluding these results they found out that clonidine did in fact show sig greater reductions in BP and NE levels in patients with NVC compared to without. This indicates that clonidine has effective antihypertensive properties, especially in NVC+ patients. It is key to understand that though after administration of clonidine to the patients showed a decrease in BP, we must also consider the fact that they did gave other antihypertensives with it as well. A suggestion was made that clonidine could be used as a secondary line of treatment. This study also only lasted 4 weeks so a longer study would show if clonidine's affects are sustainable long term.

-Tsetse Fly