Detection of Multifiber Neuronal Firings: A Mixture Separation Model Applied to Sympathetic Recordings

By: Can Ozan Tan, J. Andrew Taylor, Albert S. H. Ler, and Michael A. Cohen

IEEE Transactions on Biomedical Engineering, 2009

            Regional blood flow is primarily controlled by sympathetic nervous outflow to the vasculature, and when dysregulation occurs pathological conditions such as sleep apnea, hypertension, heart failure, and metabolic syndrome can arise. Therefore, it is important to cardiovascular research to quantify the sympathetic activity. Within traditional human sympathetic nerve recordings, there are three stages: bandpass filtering, full-wave rectification, and integration with a slow time constant. These are all used to purify the signal and obtain action potentials. Bandpass filtering removes low-frequency content of the signal that is possibly physiologically relevant. Full-wave rectification yields artificial amplification of the filtered recording, and integration averages the sympathetic activity over time. In past years, the signals were read “by hand” and experts had to pick out the specific action potentials themselves. To bypass potential problems with this, wavelet decomposition methods have been designed to denoise sympathetic activity recordings and improve quantification. The primary aim of this study was to derive a generic, fully automated technique for restoring raw, multifiber nerve signals that were buried in high levels of noise and other artifacts.

            To achieve this, researchers proposed an algorithm to minimize signal degradation and it was sensitive to different firing modes of multifiber signals. This algorithm followed the basic steps of 1. Removing line noise and movement artifacts from the recording, 2. Using the characteristic cardiac rhythmicity of sympathetic firing to identify action potential candidates and create a mean action potential template (helped identify noise spikes) and 3. Identify actual spikes and background noise. To validate the technique, the researchers produced 5 minute long 10-KHz artificial datasets with various signal-to-noise ratios (SNR). The two types of common artifacts within the raw multifiber recordings were 60-Hz line noise and movement artifacts. The 60-Hz line noise was removed by notch filtering and the movement artifacts were removed by clipping cutoff points in certain quartiles of the normal distribution to fit the time domain. In addition, they considered that bursts within the recording that exceeded 2.5 standard deviations of the whole neurogram were most likely the ones that contained the action potentials.

            The results of the study showed that the new technique produced performance improvement that was statistically significant. To further test the algorithm, they applied to it to renal sympathetic nerve recordings from freely moving rats under baseline conditions. High SNR ensured that there was successful detection of nerve activity, despite the fact that they had a low sampling rate. This showed that they could use the algorithm in species other than humans, and also apply it to multifiber recordings from different types of nerves.

            The researchers successfully described a new algorithm that is capable of restoring multifiber nerve signals buried within levels of background noise. They did consider a few limitations: 1. When the rate goes above 400 spikes per second, their methods tended to overestimate the number of spikes and 2. The method requires a relatively high sampling rate for recordings. Overall, the artificial datasets produced results that showed the algorithm providing an accurate measure of nerve signal, and it actually provided better quantification of the sympathetic signal compared to traditional methods. The major improvements with the new algorithm include that it is able to minimize signal degradation and retain more information about nerve firing.

This is important to our lab because the algorithm is capable of detecting nerve signals in recordings that are heavily contaminated with background noise. Sending our SSNA recordings to this lab will allow them to help us determine if glutamate activation of the RVLM causes more neurons to fire within the nerve, or if it causes the same neurons to fire faster.

-L. Matus

Brain natriuretic peptide-mediated changes in the extracellullar neurotransmitter turnover in the rostral ventrolateral medulla

By: B.R. Dev, M. Nandakumaran, L. Philip, S.J. John. Published in the journal Neuroscience. 

 

Brain natriuretic peptide (BNP) is a hormone secreted by the ventricles of the heart in response to excessive stretching. It is associated with regulation of CV functions, BP, and HR. Increased plasma BNP concentrations have been found in people with CV dysfunctions such as congestive heart failure, hypertension, acute myocardial infarction, etc. Interestingly, other studies have shown that when BP is injected into rats, it only affects HR and BP in hypertension-model rats and not in normotensive-model rats. 

 

The purpose of this study was to observe changes in CV functions (MAP and HR) and in extracellular concentrations of certain neurotransmitters in the RVLM following introduction of BNP. This was accomplished in two ways in this study. The first method involved the use of microinjections. BNP was injected directly into the RVLM at different concentrations of BNP – 2,4,20, & 40 pmol. Changes in HR and MAP were observed throughout the experiment. The study found an overall dose-dependent decrease in HR and MAP following injection of BNP. The microinjections were later confirmed with histology techniques similar to the ones our lab is using.  

 

The second part of the study incorporated microdialysis. Microdialysis is a method by which a solution is slowly perfused through a tissue. As the solution enters the tissue, it may cause changes in the chemical concentrations inside and outside of the cell. These chemicals will diffuse across the membrane of the cell and will be collected in order to analyze. 

 

The BNP solution was perfused into the RVLM over an extended period of time (the full experiment, including use of a control perfusion solution, lasted 7h). The perfusate was collected and analyzed using a high-pressure liquid chromatography machine which separates chemicals in a column based on their weight. The study found a decrease in levels of norepinephrine and epinephrine, as well as an increase in 5-hydroxyindoleacetic acid (5-HIAA, a marker for serotonin). There was no significant change in concentrations of DOPAC (a marker for dopamine).  

 

Some questions I have that are inspired by this study: What are the mechanisms by which BNP causes a change in MAP and HR? Does it cause a change in receptor sites for GABA and glutamate, similar to what we may have noticed? Will a previously sedentary individual who begins to exercise see an increase or decrease in BNP?    

 

-- S. Kulkarni     

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

By: Maryetta D. Dombrowski and Patrick J. Mueller

Am J Physiol Regul Integr Comp Physiol 2017

Overactivity of the sympathetic nervous system has been found to be associated with a sedentary lifestyle, which in turn increases the risk factor for cardiovascular disease. The RVLM regulates the discharge of sympathetic nerves, and the RVLM itself is regulated by excitatory (glutamate) and inhibitory (GABA) neurotransmitters. In previous studies it has been reported that sedentary conditions increase GABAergic modulation of sympathoexcitation in the RVLM. The first purpose of this study was to determine if sedentary conditions enhance GABA sensitivity in the RVLM compared to physically active conditions. Other previous studies found that contralateral inputs involved in the baroreflex pathway may buffer responses when GABA is unilaterally injected into the RVLM. Therefore, the second purpose of this study was to examine those potential buffering differences between the right and left RVLM in both sedentary and physically active rats.

Male Sprague-Dawley rats were used and divided into physically active groups (provided with running wheels) and sedentary groups (no running wheels) who were housed under those conditions for 12-15 weeks. Afterwards, acute RVLM microinjections were performed with triple barrel pipettes containing the following GABA concentrations: 0.3, 3, 30, 300, and 600 mM. These microinjections were performed in the left RVLM and the MAP, HR, and SSNA activity were recorded. To test whether the buffering occurred, long acting GABA (muscimol) was injected into the right RVLM. Then, the GABA microinjections were repeated in the left side 15 minutes later followed by a muscimol injection into the left RVLM to inhibit neuronal activity and confirm that the right RVLM was previously inhibited. Lastly, bilateral sinoaortic denervations were performed in both physically active and sedentary groups.

The findings of this study were three-fold. First, they found that when the RVLM was directly inhibited with increasing doses of GABA, dose-dependent decreases in MAP and SSNA occurred in both the sedentary and physically active groups. However, there was no difference in response between the groups. Second, when the right RVLM was inhibited with muscimol and then microinjections of GABA proceeded in the left RVLM, there was enhanced sympathoexcitatory responses in the sedentary rats but not the physically active rats. Third, in both groups denervation of the arterial baroreceptor afferents showed no difference in sympathoinhibitory responses with the increasing GABA concentrations microinjected into the left RVLM.

The conclusion of this study was that in sedentary rats the contralateral RVLM compensates for decreases in sympathetic outflow when acute inhibition occurs in the opposing RVLM. This, however, does not occur in physically active rats. In addition, the neurons that are not inhibited in the RVLM (for example the ones in the left RVLM when the right is inhibited) become more sensitive to GABAergic inhibition in sedentary rats but not in physically active rats.

-L. Matus

Selective C1 Lesioning Slightly Decreases Angiotensin II Type I Receptor Expression in the Rat Rostral Ventrolateral Medulla (RVLM)

Erick A. Bourassa, Kristen A. Stedenfeld, Alan F. Sved, Robert C. Speth

Neurochemical Research Vol. 40, Issue 10 (October 2015)

The rostral ventrolateral medulla (RVLM) contains bulbospinal neurons that mediate sympathetic tone. These bulbospinal neurons are activated in two ways: baroreflex (drop in blood pressure) or by excitatory inputs from the paraventricular nucleus of the hypothalamus (PVN). The renin-angiotensin (RAS) system also plays a role in the regulation of peripheral blood pressure. Data suggests that elevated activity of the brain-specific renin-angiotensin system contributes to hypertension.

           

Within the RVLM there are two distinct types of neurons: C1 and non-C1. C1 Both C1 and non-C1 neurons are spinally projecting (bulbospinal), but they differ in that C1 neurons contain the enzymes required to synthesize epinephrine (tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT)), while non-C1 neurons do not. However, non-C1 neurons still express vGlu2, suggesting that they are glutamatergic like the C1 neurons. Studies have shown that both groups of neurons are involved in cardiovascular responses, but it is unknown whether the affects of angiotensin II (Ang II) in the RVLM are due to activation of the C1 neurons, the non-C1 neurons, or both. In this study, the AT1 receptor (Ang II receptor in the RVLM) is used to determine how active the C1s are versus the non-C1s in response to Ang II.

To test this, rats were either injected with DSAP (anti-dopamine-beta-hydroxylase-saporin) or MZAB (control) into one side of the RVLM. The other side of the RVLM was left alone to provide something to compare the experimental side of the RVLM to. The DSAP targets and destroys C1 neurons, while the MZAB has no effect. The rats were then decapitated 3-4 weeks post-injection and their brain was sectioned and stained for TH (indicative of C1 neurons). The number of C1 neurons in the RVLM of each rat was determined by examination of the processed brain tissue via light microscopy. The number of AT1 receptors was also determined, but by a different technique called quantitative angiotensin receptor autoradiography. It is important to note that two comparisons were made to determine results: comparison between right and left RVLM (contralateral) of the same rat to determine percent change, followed by comparison of the determined percent change between experimental RVLM and control RVLM of different rats.

DSAP produced a 57% depletion of C1 neurons, using the contralateral RVLM comparison, when compared to the contralateral comparison of the control MZAB injected RVLM. Also, the AT1 receptor binding was 10% lower in the experimental group when using the contralateral comparison, and 19% lower in the experimental group when comparing the binding between the DSAP and MZAB groups, not comparing contralaterally. If C1 neurons contained most of the AT1 receptors, as was previously thought, then a 57% decrease in C1 neurons would result in a similar decrease in the amount of AT1 receptors, but this was not the case. These results suggest that the non-C1 neurons in the RVLM contain most of the AT1 receptors.

The result of this study is significant to my work in the lab because it reminds us that while we focus on one role of the RVLM, it has other functions as well. It can be activated and inhibited by other pathways and it is important to remember that these other pathways may affect the results of our studies

- Ben Huber

Gender differences in autonomic functions associated with blood pressure regulation

By: Victor A. Convertino 

Journal of American Physiology 

 

Women have a decreased ability to regulate blood pressure. This could be due to many factors, including decreased baroreflex sensitivity, reduced blood volume, lower stroke volume (amount of blood pumped per beat) and cardiac output (amount of blood pumped per minute), and increased venous compliance in the lower extremities (causes blood flow to slow down so that any autonomic regulatory response would be slower). Because of this decreased regulatory ability, women are more prone to loss of consciousness and syncopal episodes than men. In this study, blood pressure regulation was compared specifically in orthostatic state (when standing).  

 

Seven women and ten men were age-matched and asked to complete a variety of orthostatic challenges. One test was performed to look at lower body negative pressure (LBNP). The pressure around the participant's lower body was gradually decreased until they began experiencing pre-syncopal symptoms, such as decreased systolic pressure, nausea, dizziness, and gray-outs. This procedure was carried out with the patient in supine position; as the pressure around the lower body decreases, the blood moves towards the head. The study found that women have LBNP threshold almost 35% lower than men. However, the HR at which pre-syncopal symptoms on-set was not significantly different between men and women (this would suggest that women have higher HR variability).  

 

Another test considered adrenoreceptor responsiveness. The study looked at two types of adrenoreceptors: α1- and β-.  Adrenoreceptor-agonists were administered intravenously to observe changes in blood pressure. The study found that adrenoreceptors were more responsive in women than men.  

 

In addition, MAP was compared using the Valsalva maneuver (forceful expiration against a closed nose and mouth – close mouth, pinch nose, exhale). This maneuver decreases blood pressure. At one stage of the maneuver, it was observed that the male HR increased at a higher rate than the females, in response to the decrease in blood pressure.  

 

There was one interesting hypothesis presented in the abstract that was not directly addressed elsewhere: when men stand up, they respond with greater sympathetic stimulation to periphery; when women stand up, they respond with greater vagal withdrawal.  

 

-- S. Kulkarni 

Effect of endurance exercise on ovulation in the rat

By: Robert T. Chatterton, Jr., Linda Hrycyk and Robert C. Hickson

Official Journal of the American College of Sports Medicine (1995)

       This paper aimed to provide more information on the possible mechanism by which ovulation and fertility are adversely affected in females due to physical stress. In addition, the stage of the rat estrous cycle was previously determined through cytology with vaginal lavage, however former studies showed that the vaginal lavage was an indirect measure of potential ovulation. Therefore, this new study wanted to test the stage of ovulation based on histological measure of newly formed corpora lutea on the ovary, along with the presence of ova in the oviducts. The overall purpose of the study was to determine the effects of endurance exercise on ovarian progesterone secretion and ovulation, and to test the consistency with which effects occur in individual animals.

       To test this, female Sprague-Dawley rats were used and divided into two groups: exercise and sham-exercise. The exercise group underwent a rigorous 90 minute exercise period every day for a length of time equivalent to three of their estrous cycles (equated to around 21-22 days). The rats in the sham-exercise group were placed on stationary treadmills for the same amount of time each day and for the same number of days. Both groups were subject to a small electrical shock behind them to keep them on the treadmill for the required period of time. After the period of sham-exercise or exercise, the rats were sacrificed on the morning of the estrus stage of their cycles. If the rat had an irregular cycle or was acyclic, the rat was sacrificed after the prolonged diestrus stage. In addition, plasma progesterone levels were measured in the blood after each rat was sacrificed. This was done in order to determine the proper function of the ovary. Ovaries were then removed for histological examination to determine the presence of the corpora lutea, and ultimately to confirm ovulation.

       The results showed that the sham-exercising rats maintained a consistent 4-5 day cycle. However, 9 of the 17 exercising rats had cycles greater than 5 days. In addition, based on the histological study, 17 of the 19 sham-exercising rats ovulated, while only 7 of the 17 exercising rats ovulated. In terms of progesterone levels, there was no difference between the sham-exercise vs. the exercise rats despite there being a difference in the number of rats that ovulated based on the presence of corpora lutea. However, within the exercise group itself, the rats that did ovulate had significantly higher progesterone levels compared to those rats that were anovulatory.

       These findings show that a high percentage (almost half) of the female rats that ran daily for 90 minutes developed changes in their estrous cycles that included a significantly extended diestrus stage. Additionally, they found that studying the ovaries and oviducts for ova is a better indication for ovulation rather than the presence of cornified epithelial cells in vaginal cytology. In the rats that experienced prolonged diestrus stages, old corpora lutea were present, and this could be similar to the mechanism of pseudopregnancy in rats. The stress of running may have either decreased plasma LH (leutenizing hormone) concentrations or increased plasma prolactin levels. A decrease in plasma LH concentrations is usually observed in human females during endurance training. However, it is unknown if the maintained progesterone levels that occurred within the prolonged diestrus stage were due to secretion by the old corpora lutea or by the adrenal gland, since the adrenal gland is a significant source of progesterone in rats (but not in humans). Overall, reduced ovulation did occur in susceptible female rats that underwent endurance training.

-L. Matus

Disinhibition of the rostral ventral medulla increases blood pressure and Fos expression in bulbospinal neurons

J.B. Minson, I.j. Llewellyn-Smith, L.f. Arnolda, P.M. Pilowsky, J.R. Oliver, J.P. Chalmers
Brain Research 646 (1994)

The rostral ventral medulla (RVM) contains neurons that project to the spinal cord, and are the major source of drive to the sympathetic neurons of the spinal cord. These tonically active bulbospinal neurons are the major source of sympathetic control of arterial pressure. The activity of these neurons is regulated by inhibitory inputs that originate in the caudal ventrolateral medulla (CVLM) and utilize GABA as the neurotransmitter. This inhibitory pathway is likely to be the critical inhibitory link in the baroreceptor reflex control of efferent sympathetic nerve activity. This study tests whether inhibition of this GABA-mediated pathway will result in increased activation of the RVM.

 In this study, the spinal cords of 10 male rats were injected with Cholera toxin B (CTB), which is a tracer that travels retrogradely from the synapse at the preganglionic injection site to the cell body in the brain that innervates the preganglionic neuron. After recovering from the injection for 3 days, the rats underwent injection of either vehicle (control) or muscimol into the CVLM. Muscimol (GABA_A receptor agonist) was injected to inhibit neuronal activity at the CVLM, thus disinhibiting the RVLM. Blood pressure and heart rate were recorded for 120 minutes post-injection from each rat. Following these recordings, rats were perfused and their tissue was immunohistochemically processed for Fos and CTB. Fos is a marker for neuronal activation and is used in this study to determine the activity of the rat RVLM post-Muscimol injection. Essentially, if a neuron became activated enough, then it would express Fos. Finally, Fos- and CTB-positive neurons were counted via light microscopy.

 Both the heart rate and the blood pressure increased in response to muscimol injection (256+17 BPM to 428+19 BPM, 103+2 mmHg to 178+6 mmHg, respectively). More neurons positive for both Fos and CTB were observed in the RVM of the muscimol-injected rats than those of the vehicle-injected rats. Muscimol injected rats demonstrated a 22+1.9% increase in Fos+CTB neurons from the average number of Fos+CTB neurons throughout both groups of animals prior to injection, 154+28, while vehicle injection yielded a 5+0.8% increase from the baseline. This suggests that by inhibiting the CVLM, RVM activation will increase. This finding helps us to better understand the baroreceptor reflex control pathway and how it is mediated.

This study has proven to be very useful for our work at the Mueller lab. We used the Fos+CTB staining technique in the current study that I am working on. Also, this study provided us with a better understanding of the overall baroreceptor reflex control of efferent sympathetic nerve activity.

- Ben Huber

Dystrophic heart failure blocked by membrane sealant poloxamer

Within this article, Duchenne muscular dystrophy (DMD) is discussed in its affect towards heart failure. It is explained how dystrophin, a protein that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane, deficiency can cause DMD. It was seen that a deficiency in dystrophin can cause less stability in the myocyte membranes and lead to stretch mediated calcium overload. In attempt to learn more about counteracting this, the lab had one control group of rats and monitored the ability of the myocytes to stretch and relax. The control group was compared to a group of dystrophin deficient mice (mdx). The lab would treat mdx rats with poloxamer 188 (p188) and it showed to significantly reduce tension compared to mdx rats not treated with it.

Hashbrown

Estrogen enhances baroreflex control of heart rate in conscious ovariectomized rats

By: Mahmoud M. El-Mas & Abdel A. Abdel-Rahman

The ability to autonomically regulate changes in BP is an important indicator of cardiovascular health. BP is autonomically regulated by baroreflex mediated HR responses. Previous studies have shown that baroreflex control of HR is significantly lower in females than in males. This study explores whether estrogen has any effect on baroreflex sensitivity (BRS).  

 

In this study, BRS was measured by administrating phenylephrine, a vasoactive drug that increases BP. MAP was plotted against HR – the slope of the line represents the BRS. There was a significant difference in BRS of almost 30% between males and females. This difference was even greater in ovariectomized rats. However, ovariectomized rats that were treated with estradiol expressed no significant difference in BRS as compared to the males.  

 

Age-matched (11-12 week old) male and female Sprague-Dawley rats were used in this study. At 9-10 weeks, in one group of females, the ovaries were tied off and isolated. In the other female group, a sham operation was performed which exposed the ovaries but did not isolate them. Both groups were given two weeks to recover. The ovariectomized rats were further divided into two groups: those injected with estradiol and those injected with a "vehicle" drug (that could be used as a control).  

According to the authors, one important point to consider is age-related differences in BRS. Studies have shown that BRS decreases as age increases. Thus, the results of this study may not be replicable in rats of different ages.  

The following excerpt from the study provides some indication of how these finding relate to neurons found in regions of the brainstem: 

"First, estrogen exerts facilitatory and inhibitory roles on central glutamatergic (Wong and Moss 1992) and GABAergic (Kelly et al. 1992) neurotransmission, respectively. Both types of neurotransmissions are essential for central baroreflex function (Lawrence and Jarrott 1996). Furthermore, estrogen receptor mRNA containing neurons have been identified in nucleus tractus solitarius and caudal ventrolateral medulla (Simerly et al. 1990), brainstem areas that are involved in the central processing of baroreceptor information (Chalmers and Pilowsky 1991; Lawrence and Jarrott 1996). Further studies are needed, however, to determine the exact mechanism of baroreflex enhancement by estrogen." 

 

Sex differences in angiotensin signaling in bulbospinal neurons in the rat rostral ventrolateral medulla

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.

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) 

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