Phenotypic identification of rat rostroventrolateral medullary presympathetic vasomotor neurons inhibited by exogenous cholecystokinin.

J Comp Neurol. 2003 Oct 27;465(4):467-79.
Sartor DM, Verberne AJ.
I guess I'm having a "reading Sartor and Verberne week" in honor of the assistance and advice coming from their way. And their stuff is highly related to my stuff, so I like trying to steal tips where I can. This paper sort of addressed the idea of differential control by looking at how cholecystokinin (CCK), a peptide released by the gut, can inhibit the activity of presympathetic neurons that may control splanchnic nerve activity. They examined RVLM neurons, verified them to be spinally projecting by antidromic action potentials, verified them to be presympathetic by seeing them decrease firing while they raised blood pressure via aortic occlusion, re-verified them to be presympathetic by seeing them decrease firing during i.v. administration of phenylbiguanide (PBG) aka the bezold-jarisch reflex, and then tested them with i.v. CCK to see if they would be activated or inhibited. Neurons were either inhibited, activated, or not affected by CCK administration. Conduction velocities were slow and similar between not-affected and activated groups, so these two populations were pooled in to one. In general, neurons that had slow conduction rates also had a low firing frequency. On the other hand, they also found neurons with high firing frequencies had faster conduction rates. The neurons that were inhibited by CCK fell into this group.
So far, the experiments were pretty indirect and unable to say that any particular neuron controlled the splanchnic nerve. Directly testing this would be extremely difficult and a technical nightmare... so they did the next best thing, which was to electrically stimulate the region of the periaquaductal gray which is responsible for activating RVLM neurons to cause mesenteric vasoconstriction. They found that 79% of the neurons that were inhibited by CCK were activated by PAG stimulations. Only 50% of the neurons not inhibited by CCK were activated by PAG stimulation. The conclusion from this experiment was that stimulation of the PAG was more likely to activate neurons that get inhibited by CCK than it is to activate non-CCK sensitive neurons. They also did juxtacellular labeling but did not find any correlation between phenotype and response to PAG stimulus. However, they did find that the fast-firing, fast-conducting, CCK-sensitive cells were more likely to be non-C1 (but only at 59%) Cells not inhibited by CCK were more likely to be slow firing and conducting C1 cells (79%).
What I did find very interesting is that they said in this paper that "All medullospinal (presympathetic) C1- and non-C1 neurons are inhibited by elevation of arterial blood pressure..." which is in contrast to what was said in the other paper from them that I blogged this week, which was that some presympathetic neurons are NOT barosensitve, but are sensitive to neuroglucoprivation.  However, there's about 7 years in between these two papers, with this one being the early one - so I guess that just shows that I have to keep track of publication dates and learn the timeline of discoveries. -DH

Sympathoexcitation and pressor responses induced by ethanol in the central nucleus of amygdala involves activation of NMDA receptors in rats

Andrew D. Chapp , Le Gui , Michael J. Huber , Jinling Liu , Robert A. Larson , Jianhua Zhu , Jason R. Carter , Qing-Hui Chen. American Journal of Physiology - Heart and Circulatory Physiology Published 1 September 2014Vol. 307 no. 5, H701-H709DOI: 10.1152/ajpheart.00005.2014. Ingestion of alcohol leads to pressor and sympathoexicition. This purpose of this study was to elucidate the mechanism responsible for the activation of the sympathetic nervous system. By using microinjection technique they demonstrated that ethanol and acetate, a byproduct of ethanol caused pressor and increases in SSNA and LSNA when injected into the amygdala (CeA).  In order to determine whether this response is being mediated by glutamate, they injected kyn, an EAA receptor blocker, NMDAR blocker and a non NMDAR blocker mixed with ethanol. They found that Kyn and the NMDAR blocker both attenuated the pressor response and SSNA. Then in order to determine whether the RVLM was playing role in the response kyn was injected into the RVLM they found that this also attenuated the response. Finally they showed that anatomically there was an connection between RVLM and CeA and that NMDAR were present on CeA-RVLM neurons. These data show that alcohol and alcohol metabolites can lead to activation of RVLM neurons which may be the cause of increase sympathetic outflow in response to alcohol consumption.-MD

Modulation of inducible nitric oxide synthase (iNOS) expression and cardiovascular responses during static exercise following iNOS antagonism within the ventrolateral medulla

 

Pasarapa Towiwat, Siripan Phattanarudee,Timothy J. Maher, Ahmmed Ally. Mol Cell Biochem DOI 10.1007/s11010-014-2218-9.  The exercise pressor reflex leads to an increase in BP in response to static exercise. This reflex mediates its control over the cardiovascular system  by modulating the activity of CVLM and RVLM neurons. As we already know, there are different neurotransmitters that modulate the activity neurons in the RVLM and CVLM. It has been demonstrated in the literature that microdialysis of L- arginine into the CVLM lead to enhanced pressor response during static exercise along with decreased GABA release.  However, when L-arginine was infused into the RVLM blocked the cardiovascular responses to static exercise.  Blockade of both nNOS and eNOS lead to enhanced GABA and decrease glu in CVLM. Blockade of both nNOS and eNOS in the RVLM lead to enhanced Glu and decrease GABA.  Since it has been shown that blockade of iNOS in SHR leads to attenuation of there hypertensive state, This study wanted to further investigate the role of iNOS in the exercise pressor response.  They found that blockade of iNOS by infusion into the RVLM,  lead to attenuated HR and blood increase in response to activation of the exercise pressor reflex. they also showed a decrease in iNOS protein expression in the RVLM following blockade of iNOS in the RVLM and no change in CVLM. However, when the antagonist for iNOS was infused into CVLM they observed the response to activation of the exercise pressor reflex was potentiated. The protein expression of iNOS was not different when compared to controls for RVLM and CVLM. These data demonstrate that iNOS may be lead playing a role in the exciting neurons in the RVLM possibly through enhancing Glu and/or decreasing GABA release.-MD

Rostroventrolateral medullary neurons modulate glucose homeostasis in the rat.

Verberne AJ, Sartor DM.

Am J Physiol Endocrinol Metab. 2010 Nov;299(5):E802-7.

I came across this paper by accident, but it is dealing with presympathetic RVLM neurons, so I read it. It’s not about the ones we normally study, but it very much could be about some of the neurons I’ve been finding in my recordings. In this paper they looked at spinally projecting unmyelinated RVLM neurons that were NOT barosensitive, but were sensitive to changes in glucose concentration. These neurons increased their activity during 2-deoxyglucose (2-DG) induced neuroglocoprivation, presumably to drive increased epinephrine release from the adrenal glands in order to compensate for hypoglycemia. When they injected bicuculline in to the region of the RVLM where these glucosensitive were found, not only did they see the expected increase in arterial pressure, but they also saw that the rat’s blood glucose levels went up. This did not occur in rats that had undergone bilateral adrenalectomy. Interestingly, the barosensitive neurons they found were either not affected by 2-DG, or were slightly inhibited – so this is another subpopulation of neurons in the same region, which may support the idea of differential control that we’ve been looking at. -DH

Peripheral nociception associated with surgical incision elicits remote nonischemic cardioprotection via neurogenic activation of protein kinase c signaling

Jones, W. Keith, et al. "Peripheral nociception associated with surgical incision elicits remote nonischemic cardioprotection via neurogenic activation of protein kinase C signaling." Circulation 120.11 suppl 1 (2009): S1-S9.

Ischemia reperfusion injury associated with myocardial infarction is a major contributor to cardiovascular related death.  Unfortunately, the only treatment we have for cardiac ischemia, is reperfusion of oxygen to the ischemic tissue, which in itself can also lead to cell death.  Importantly, ischemic preconditioning (IPC) has been shown to limit the damaged induced by ischemia/reperfusion and act as a cardioprotectant.  However, IPC is somewhat impractical in a clinical setting in terms of feasibility.  This study was able to show that a preconditioned non-ischemic remote trauma was also able to induce cardioprotection through neurogenic mechanisms.  Specifically they were able to show that stimulation of pain receptors located in the skin activated an antidromic spinal reflex, which in turn activated cardiac sympathetic nerves via calcitonin gene-related peptide (CGRP).  CGRP then induced NE and Bradykinin release, stimulating beta-AR and BK2R.  Activation of these receptors then caused an upregulation of PKC-epsilon (mediator of cardioprotection) and a down regulation of PKC-delta (mediator of cell-death via necrosis).  They believe that one of the cardioprotective effects of PKC-epsilon is the activation of mitochondrial Katp channels, which when inhibited attenuated the cardioprotective effects.  Conclusively, this study may provide to be extremely important clinically due to the ease and efficiency of the remote preconditioning of trauma on reducing I/R injury following an MI.

~JI 

Methods of analysis and physiological relevance of rhythms in sympathetic nerve discharge.

Barman SM, Kenney MJ. Clin Exp Pharmacol Physiol. 2007 Apr;34(4):350-5.
As we all figured out at a recent meeting, I'm desperately in need a crash course in sympathetic nerve and RVLM unit coherence analysis. So I did the only thing I know how to do; I found a review paper from Dr. Barman and started reading. This review gives some background on frequency analysis and shows how all the burst we see in our splanchnic nerve recordings can be examined and linked to respiratory or cardiovascular rhythms, or both. Figure 1 (included) is very simplified, but it shows how a fast Fourier transformations can pull apart one complicated waveform and show the contribution of different sine waves, or rhythms. Baby steps.
It gets a bit more complicated from there, immediately going in to how different nerves can be affected by the same stimuli. It also starts to hint at how coherence analysis can show that the nerves are showing the same influence and rhythmicity, even though the averaged nerve recordings look to be completely different. This is what I was really hoping to learn about, but I guess it's important to learn to walk before trying to run that marathon, right? -DH

Role of ventrolateral medulla in generating the 10-Hz rhythm in sympathetic nerve discharge.

Barman SM, Gebber GL. Am J Physiol Regul Integr Comp Physiol. 2007 Jul;293(1):R223-33 I'm kind of new to the details of the 10Hz rhythm. It's seen in sympathetic nerves and when it becomes apparent, it is linked with an increase in arterial pressure. It seems to be always somewhat in effect, but it's most noticeable after barorecepter denervation. Generation of the 10Hz rhythm seems to originate in the brainstem, since decerebration doesn't stop it, but spinal transection does. The activity of individual brainstem neurons in RVLM, CVLM, and medullary raphe is correlated with 10hz. This is somewhat strange because CVLM neurons do not have spinal projections, but they're still correlated with this 10Hz activity. They wanted to look at how these different regions interact, so they used microinection of drugs in to these regions to figure out the microcircuitry involved. What they found is that inection of GABAergic antagonists in to the VLM (rostral OR caudal) could eliminate the 10Hz signal, even with only unilateral injections. Strangely, glutamatergic antagonists could reduce the 10Hz too. They think this comes down to a network of GABAergic interneurons in the CVLM and the RVLM communicating with the neurons that we generally think of when we think of those regions. This should make Max happy, since he's pretty excited about interneurons. -DH

GABAergic mechanism in the rostral ventrolateral medulla contribute to the hypotension of moxonidine

Wang et al. cardiovascular research (2011)89 473-481; doi:10.1093/cvr/cvq289. Moxonidine is a antihypertensive drug that is thought to act centrally. This study wanted to investigate whether GABA A receptor blockade would affect the effects of mononidine on the cardiovascular system, also whether GABA release and receptor expression was affected by moxonidine. What they found was that microinjection of moxonidine leads to decreases in BP, HR and RSNA.  When GABA A receptors and GABA B receptors were unilaterally blocked and they injected moxonidine into RVLM they were able to attenuate the BP, HR and RSNA decreases. They also did bilateral blockade of GABA A receptors in RVLM and gave moxonidine systemically and showed that bilateral blockade of GABA A receptors lead to attenuated decreases in BP, HR, and RSNA.  Intravenous administration of moxonidine leads to increase in the release of GABA. In response to a week of icv treatment with moxonidine, GABA ARα1 and GABA BR1 when compared to acsf control group. These data demonstrate that GABAergic neurotransmission in the RVLM are important for mediating the effects of moxonidine. -MD

Brain angiotensin- converting enzyme type 2 shedding contributes to the development of neurogenic hypertension

Huijing Xia, Srinivas sriramula, Kavaljit H. Chhabra, Eric Lazartigues.Circ Res.2013;113:1087-1096; doi:10.1161/circresaha.113.301811.

It has been shown that overactivity of the renin angiotensin system can lead to hypertension. Ace 2 is an enzyme that can convert angII to ang 1-7. Ang1-7 has an vasodilatory affect on vasculature.  Ace 2 is down regulated in hypertension further contributing to the dysfunction of the RAS. What is really interesting is that it has been shown that ectodomain of ACE2 is cleaved and can be found in CSF. Thus hypertension leads to ace 2 shedding. The question becomes what is responsible for the removal of ace 2 from the cell membrane. So the hypothesis was adam 17 is responsible for the cleavage of ace 2 and leading to the development of hypertension.  So they used transgenic mice in order to test there hypothesis. Using mice that had overexpression of Ace2 in neurons that were DOCA- high salt diet mice improved the spontaneous Baroreflex function. Ace2 overexpression also leads to decreased vasopressin release, norepinephrine, and ang II. Along with that vagal tone was restored and sympathetic drive to the heart and vasculature was brought back down to control levels. They knockdown adam 17 and showed that this prevented the development of hypertension and lead to a decrease in ace 2 shedding in the csf. These data demonstrate that hypertension that is due to a high salt diet may be leading to increased adam 17 expression and leading to decrease ace 2 expression on neurons and ultimately leading to hypertension. –MD

Attenuated baroreflex control of sympathetic nerve activity in obese Zucker rats by central mechanisms

Full cite: Huber DA & Schreihofer AM (2010). Attenuated baroreflex control of sympathetic nerve activity in obese Zucker rats by central mechanisms. J Physiol 588, 1515–1525.

Attenuated baroreflex control of sympathetic nerve activity in obese Zucker rats by central mechanisms

Domitila A. Huber and Ann M. Schreihofer

Department of Physiology, Medical College of Georgia, Augusta, GA, USA

This study is looking at how reflex control of the vasculature is affected by obesity; the main technique they are using in this paper is electrophysiology. It is widely known that obesity is a risk factor for the development of hypertension, in addition to this; obesity has been shown to be an independent contributor to the elevation of sympathetic nerve activity (SNA). The goal of this paper is to illuminate how obesity alters sympathoregulation, in an attempt to tie the condition to other disease states. The main component of sympathetic control examined is the sympathetic baroreflex responses, with an attempt to see if it is altered due to altered sensory or central mechanisms. It has been a long while since I’ve read a Zucker rat paper, adult obese Zucker rats (OZRs) vs. lean Zucker rats (LZRs) were the rats used in this study. The OZRs weighed about 600g and the LZRs weighed about 400g on average. The OZRs had significantly higher resting sympathetic nerve activities and blood press, interestingly enough they did not have significantly different heart rate. Stimulation of the aortic depressor nerve (ADN) elicited blunted responses in the OZRs, whereas the net responses in the LZRs were significantly greater. This result suggests that OZRs are less sensitive to stimulation of the ADN. In addition, stimulation of the vagal afferent nerve brought about significantly greater responses in the LZR when compared to the OZR. These results suggest that the vagal afferents have inherently less sensitive in the OZR rats. This reduction in baroreflex sensitivity in the OZRs may contribute to the likelihood of cardiovascular diseases in overweight individuals. -MTL

Intrinsic chemosensitivity of RVLM sympathetic premotor neurons in the in situ arterially perfused preparation of rats.

Koganezawa T, Paton JF. Exp Physiol. 2014 Jul 11.

We normally think of the RVLM as something that responds to incoming signals from other areas and adjusts its activity accordingly. However, it has been shown that if you block synaptic transmission, the neurons still fire on their own despite a lack of incoming signals. Other groups have looked at their “pacemaker” properties, but in this paper, they decided to look at the intrinsic chemosensitvity of presympathetic neurons – something we haven’t really looked at. They used a modified version of the working heart brainstem preparation so that they could record the activity of nerves and neurons while they switched the perfusion solution to ones that would induce periods of eucapnia, normoxia, hypercapnia, hypoxia, etc. First, they blocked fast synaptic transmission and looked at spontaneous activity under the different conditions and found that in all 26 presympathetic neurons, activity was not significantly affected. During hypercapnic-hypoxia, they found that all presympathetic neurons increased their firing rate by an average of 130%. This indicates that these neurons are themselves chemosensitive, independent of afferents from chemoreceptors. They also tested the combined effects of cyanide and hypercapnia on presympathetic neurons during synaptic blockade and found that 7 had their activity depressed by -82%, while 3 were slightly excited by it, though the effect was not significant due to a wide variability in response. Interestingly, and in terms of differential control, they found out that if they block a particular sodium current known to be involved in C1 neuron activity, they can block the preparation from “gasping” during asphyxia, but sympathetic chain activity remains unaffected. Personally, my take-home message from this paper is that I really need to watch my blood gas measurements and make sure that I’m not putting my rat into a state that could compromise my recordings. -DH

Leptin into the rostral ventral lateral medulla (RVLM) augments renal sympathetic nerve activity and blood pressure

Barnes and McDougal.Front. Neurosci.8:232.doi:10.3389/fnins.2014.0232.

Leptin is released from fat cells in order to promote satiety and increases energy expenditure. One mechanism by which leptin affects energy expenditure is by increasing sympathetic nerve  activity. Leptin signaling is mediated through the hypothalamus and has been shown to increase blood pressure and renal sympathetic nerve activity (RSNA).   we know that PVN sends projections to the RVLM and to the spinal cord to modulate sympathetic nerve activity. This study investigated if the leptin receptor is present in brainstem regions that control RSNA./For their anatomical studies they injected PRV into the  cortex of the kidney and did immunofluorescent staining for the leptin receptor (obrb). As for the functional studies they microinjected leptin and leptin receptor antagonist into the rvlm and recorded blood pressure and RSNA.  They found obrb positive cells in the C1/A1 region, VMM, caudal raphe and A5 area. Most of the PRV positive cells were found in the rostral portion of the RVLM (similar Nick’s paper) and found no difference between ipsilateral and contralateral RVLM.  They found that 65% of the C1/A1 region was double labeled.  As for the functional studies, they found that leptin lead to an increase in RSNA and BP. Injection of the antagonist prior to the leptin injection attenuated the BP and RSNA response.  Conclusion leptin release from fat cells is leading to increased energy expenditure via increasing sympathetic nervous system activity through activation of obrb receptors in the RVLM. MD

 

Cardiovascular effect of angiotensin-(1–12) in the caudal ventrolateral medullary depressor area of the rat

Tetsuya Kawabe, Kazumi Kawabe, and Hreday N. Sapru.Am J Physiol Heart Circ Physiol 306: H438–H449, 2014. First published November 27, 2013; doi:10.1152/ajpheart.00628. 2013. This study investigated whether ang 1-12 is leading to increased neuronal activity through AT1 Receptors in the cvlm. In Wistar rats, they recorded GSNA, BP, and HR  and used microinjection technique. They found that microinjections of ang 1-12 into cvlm lead to decreases in BP, HR and GSNA. Ang 1-12 mediates it response through AT1R. The activation of AT1R leads to the release of GABA from the terminals of CVLM neurons and acts on GABA receptors in the RVLM in order to cause sympathoinhibition.  In conclusion, Ang 1-12 maybe playing a role in the activation of neurons in the CVLM. I wonder if the level of Ang 1-12   is changing in response exercise…MD

Temporal Changes in the T1 and T2 Relaxation Rates (delta R1 and delta R2) in the Rat Brain Are Consistent With the Tissue-Clearance Rates of Elemental Manganese

Chuang, Kai‐Hsiang, Alan P. Koretsky, and Christopher H. Sotak. "Temporal changes in the T1 and T2 relaxation rates (ΔR1 and ΔR2) in the rat brain are consistent with the tissue‐clearance rates of elemental manganese." Magnetic Resonance in Medicine 61.6 (2009): 1528-1532.

One of the main points of recent MeMRI study was to determine the time course of influx and efflux of manganese in the RVLM following systemic injections of manganese.  As a trace mineral, manganese can be found in multiple forms in the body that effect both its rate of efflux from a cell and its effect on proton relaxation rates.  While we discuss signal intensity in our study as manganese uptake within a cell, signal intensity may also be dictated by the relative state in which the manganese is able to change the proton relaxation rates.  To determine the extent that changes in relaxivity (how effectively manganese is able to change relaxation rates) are contributing to changes in signal intensity MeMRI studies were done in combination with NMR, directly examining manganese concentrations in neuronal tissue.  Initial results showed that T1 and T2 relaxation times, as well as delta R1 and delta R2 peak 1 day after administrations of manganese, which corresponded to greatest concentrations of manganese in neuronal tissue.  In all cases manganese uptake was seen to be greater in the olfactory bulb than in the cortex.  Both relaxation rates and relaxivity levels return to baseline levels 4 and 5 weeks post injections, conclusive with absolutely manganese concentrations also returning to baseline at 4 weeks.  Conclusively, this study was able to show that T1 and T2 relaxation times were mainly influenced by transport of manganese in and out of the cell, compared to manganese taking different forms while remaining in the cell (relaxivity).  This is important, because it further validates manganese enhancement being measured following systemic injections of manganese is an appropriate representation of in vivo neuronal activity.

~JI

Tonic glutamatergic input in the rostral ventrolateral medulla is increased in rats with chronic heart failure

Full cite: Wang WZ, Gao L, Wang HJ, Zucker IH, Wang W. 2009. Tonic glutamatergic input in the rostral ventrolateral medulla is increased in rats with chronic heart failure. Hypertension 53:370–374.

Wang WZ, Gao L, Wang HJ, Zucker IH, Wang W.

Department of Cellular and Integrative Physiology (W.-Z.W., L.G., H.-J.W., I.H.Z., W.W.), University of Nebraska Medical Center, Omaha; and the Department of Physiology (W.-Z.W.), Second Military Medical University, Shanghai, China.

The headline of this paper essentially sums up the message of this paper; chronic heart failure animals have heightened glutamatergic tone. While the finding of this paper is pretty straightforward, what I thought was particularly cool was how they used triple barrel pipettes and paired it when electrophysiology. They used kynurenic acid, an NMDA/non-NMDA receptor antagonist, non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and NMDA antagonist D-2-amino-5-phosphonopentanoate (D-AP5) to block glutamate reception and recorded the responses via single-unit electrophysiology. In addition to the triple barrel pipette they used, they had a 5-barrel micropipette containing an electrode. Using a penta barrel pipette sounds like a huge technical difficulty, that being said, if we could get some way for it to work, we could easily test the responsiveness of neurons to various direct stimuli, something we’ve never done before. With a 5-barrel micropipette, it might actually be possible to record in one piece of the pipette, have neurobiotin in another, and maybe some kind of antagonists, say bicuculline, xanthurenic acid, and kynurenic acid. I could imagine that it would be really difficult to ensure that the pipette is successfully able to inject out of each portion, but the ability to sit on a neuron and inject a slew of drugs would be incredibly powerful. It would be cool to do a similar study in runners & seds in order to get some definitive proof onto the tonic input of glutamate in seds. I believe though, we would likely see a prevalence of glutamatergic tone in the sedentary animals; it just would not be as distinct as that of the heart failure animals. -MTL

Specific respiratory neuron types have increased excitability that drive presympathetic neurones in neurogenic hypertension.

Moraes DJ, Machado BH, Paton JF.

Hypertension. 2014 Jun;63(6):1309-18.

In this paper they were trying to find out what differences between spontaneously hypertensive (SH) rats and WKY control rats are responsible for the increases in nerve activity and blood pressure seen in the SH group. To investigate this they looked at a lot of nerves, a lot of neurons, and a lot of factors that our lab often don’t consider. Their primary focus was on seeing if the increased respiratory modulation RVLM neurons seen in hypertensive rats was due to changes in the RVLM neurons themselves, or if it was due to changes in synaptic input. To make a long paper short, they found differences in nerve activity and neuronal behavior between groups (SH rats showed increases in both categories) that pretty much disappeared when they blocked synaptic input to the RVLM. This suggested that the changes in neuron and nerve activity actually came from upstream sources, altering synaptic communication in the RVLM.

They then started looking at the pre-BotC to see if the differences might be there, because respiratory modulation of RVLM neurons seems to be sourced from a place full of respiratory neurons.  What they found was that pre-inspiratory neurons in the pre-BotC were more excitable in SH rats, and it seemed to be due to reductions in the leak-K current compared to WKY rats (which should make Dr. Holt happy to learn). They also looked at post-inspiratory neurons and found these to be more excitable in SH rats as well. However, in their case, it appeared that it was due to reductions in the big calcium-activated potassium current. In fact, when they blocked the BKca channels in WKY rats, the recordings from WKY rats started to look like those from SH rats. There was no effect of the drug on SH rats, indicating that this current was pretty much non-functional, so they couldn’t drug it out.

This was a difficult paper to read and it had a lot of detail, but it really does take a good look at some of the inputs to the RVLM, and makes me remember that the RVLM isn’t an island, and it’s not the only thing that changes in hypertensive rats. -DH

Regulation of visceral sympathetic tone by A5 noradrenergic neurons in rodents.

Kanbar R, Depuy SD, West GH, Stornetta RL, Guyenet PG.

J Physiol. 2011 Feb 15;589(Pt 4):903-17

We’re pretty focused on the RVLM, but we know it’s not the only region of the brain that controls the same things we study. In this paper they looked at the A5 region, which also has spinal projections that go to IML. It has long been implicated in regulation of the autonomic system and respiration In fact, this function of the A5 has been suspected  since 1982. The cells here have been shown to have slow regular resting discharge, activated by nociceptive stimuli, inhibited by A2 agonists, activated by hypoxia and show respiratory modulation, but seem to be unaffected by changes in BP. The problem with pinpointing these effects was that there aren’t many A5 cells and they’re scattered around a small area that is surrounded by other areas that would affect nerve activity – this means microinjection studies were very hard to conduct, and A5’s effect on the control of SNA and blood pressure was difficult to establish. In this paper, they used an optogenetic approach (PRSX8-ChR2 again), combined with electrophysiology (including antidromic action potentials in the spinal cord) and juxtacellular labeling (along with immunology/TH staining), to isolate the cells of the A5 and see how they contribute to SNA.

They found 2 main types of neurons in the A5 region – the first group never showed antidromic action potential and had a huge response to increases in CO2. These cells were the most caudal and ventral ones, indicating that they were actually RTN cells living at the A5/RTN border. The other type were slow firing (<1Hz) bulbospinal ones that slightly increased activity with increasing CO2, or had no response. They were strongly activated by chemoreceptor activation (with cyanide). 4 out of 10 that they recorded were pulse modulated. Through juxtacellular labeling and immuno, these cells were shown to be TH positive, and optogenetic activation of these cells caused an increase in renal SNA (but had a very small effect on lumbar SNA). The cool thing about this paper was that they were able to combine old and new techniques to get a lot of information that would not have been possible through any of the individual techniques alone. -DH

Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm

Full cite: Orer HS, Gebber GL, Barman SM. Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm. Am J Physiol ReguI Integr Comp Physiol 2008;294:R1435-44.

Hakan S. Orer, Gerard L. Gebber, and Susan M. Barman

Department of Pharmacology and Toxicology, Michigan State University, East Lansing, Michigan; and

Department of Pharmacology, Faculty of Medicine, Hacettepe University, Ankara, Turkey

Submitted 7 January 2008; accepted in final form 10 March 2008

A majority of Dr. Barman’s research seems to involve the study of the 10 Hz rhythm in sympathetic neurons. In this particular study she is looking at the role of serotonin in the maintenance of this rhythm. The 10 Hz rhythm is correlated to sympathetic nerve discharge (SND) which raises blood pressure and is one of the main properties examined in sympathetic tone. Dr. Barman demonstrated in a previous study that the 10 Hz rhythm was GABA-mediated, as such; the rhythm was abolished with injection of a GABA antagonist into the ventrolateral medulla. Interestingly enough, it seemed that both the rostral and caudal portions of the VLM were contributing to the 10 Hz rhythm, which seemed weird to me because the rostral and caudal VLMs have entirely different functional roles – based on what I have gathered from prior reading. The caudal medullary raphe seems to play a modulatory role in the 10 Hz rhythm presumably by serotonergic neurons to the VLM. By injecting serotonin antagonists in to VLM, they found that the 10 Hz rhythm was significantly reduced, while the lower frequency bursts were maintained. It was also demonstrated that the decrease in 10 Hz was paired with a significant fall in MAP. By stimulating the caudal raphe, they were able to demonstrate an increase in the 10 Hz rhythm and blood pressure. It would be interesting to look at the input of serotonin within our model (presuming we haven’t done it before). In addition it would be interesting to look at 5-HT2 receptors and see if they are changing following physical (in)activity. It is strange—in my opinion— that blocking serotonin in both RVLM and CVLM decreases blood pressure. Of course, this suggests that the relationship between RVLM and CVLM are EVEN more complicated that what we had originally thought, much less the implications on physical activity dependent changes. The RVLM has such complex relationships with its inputs that it seems nearly impossible to illuminate the web of communication between RVLM and other brain regions based off of the current methods at our disposal, adding any physiological state such as heart failure, hypertension, high-salt, obesity, (in)activity, makes it seemingly impossible for us to come up with any conclusive and overarching understandings about any of the more intricate functions of RVLM. - MTL

Voltage-dependent calcium currents in bulbospinal neurons of neonatal rat rostral ventrolateral medulla: modulation by alpha-2-adrenergic receptors

Li, Yu-Wen, Patrice G. Guyenet, and Douglas A. Bayliss. "Voltage-dependent calcium currents in bulbospinal neurons of neonatal rat rostral ventrolateral medulla: modulation by α2-adrenergic receptors." Journal of neurophysiology79.2 (1998): 583-594.

Previous research has shown that both high voltage activated (HVA) and low voltage activated (LVA) calcium channels are present in the brain and may play different physiological roles.  The HVA calcium channels consist mostly of N type, P/Q-type, and to a lesser extent L-type components which also contribute to different physiological processes such as neurotransmission, and calcium mediated gene expression.  Until this study, little was known about calcium channel properties in the RVLM.  However, it was known that C1 neurons in the RVLM expressed alpha 2A-adrenergic receptors that had been shown to inhibit calcium channels in other neurons via activation of an inwardly rectifying potassium conductance.  Through the combination of using 2A-adrenergic receptor agonists and antagonists with electrophysiology, this study was able to characterize calcium channels and their regulation via norepinephrine for the first time.  Results showed that the HVA current consisted mainly of N-type calcium channels, with a significant but lesser contribution from P/Q-type. They also found small, but present L-type calcium channel activity.  As for LVA current, they found it was present in the RVLM and was activated with a much lower (~-50mV compared to ~0mv) as expected.  Conclusively, the study revealed that norepinephrine mediated activation of alpha 2-adrenergic receptors is able to inhibit HVA currents, specifically N-type and P/Q-type, but not LVA currents.  This study was relevant to MeMRI studies because manganese enters neurons through active L-type calcium channels.  If little L-type calcium channel current is present, it would need to be addressed for future studies using MeMRI to examine neuroplasticity in the RVLM.

~JI