Role of the caudal pressor area in the regulation of sympathetic vasomotor tone

Full cite: Campos, R.R., Carillo, B.A., Oliveira-Sales, E.B., Silva, A.M., Silva, N.F., Futuro Neto, H.A., Bergamaschi, C.T. Role of the caudal pressor area in the regulation of sympathetic vasomotor tone  (2008) Brazilian Journal of Medical and Biological Research, 41 (7), pp. 557-562.

Role of the caudal pressor area in the regulation of sympathetic vasomotor tone

R.R. Campos¹, B.A. Carillo¹, E.B. Oliveira-Sales¹, A.M. Silva¹, N.F. Silva²,

H.A. Futuro Neto^3,4 and C.T. Bergamaschi⁵

1Departamento de Fisiologia, Disciplina de Fisiologia Cardiovascular e Respiratória, Escola Paulista de Medicina, Universidade Federal de São Paulo, São Paulo, SP, Brasil

2Laboratório de Neuromorfologia,

3Programa de Pós-Graduação em Ciências Fisiológicas, Universidade Federal do Espírito Santo, Vitória, ES, Brasil

4Escola Superior de Ciências, Santa Casa de Misericórdia de Vitória (EMESCAM), Vitória, ES, Brasil

5Departamento de Biociênicas, Universidade Federal de São Paulo, Campus Baixada Santista, Santos, SP, Brasil

This review examines how the Caudal Pressor Area (CPA) controls sympathetic outflow. The CPA is a relatively undefined region that is located at the caudal end of the Caudal Ventrolateral Medulla (CVLM). This region has shown to illicit sympathoexcitatory responses when it is directly stimulated, suggesting that the CPA either contains SPNs or innervates SPNs via a direct or indirect pathway. The CPA is not widely considered to be one of the main contributors to SPN innervation, however, there is quite a bit of ambiguity as to where the SPNs receive their excitatory drive from. One of the main statements of this paper is that the CPA provides a significant source of tonic excitatory drive to the RVLM, which I find that surprising considering this is one of the very few times I have read about the CPA, so there must be some complication to the CPA that I simply do not know yet. Apparently the effect of the CPA is mediated through the RVLM, suggesting some direct projection from CPA to the RVLM. According to this review, the CPA projects to the CVLM as well, and that inhibition of the CPA neurons has the greatest effect on the slow frequency neurons in the RVLM, not the fast ones. This response may suggest that the CPA has a selected enhancement of C₁ neurons. It would be interesting to see how the CPA relates to our model, especially if it does have as significant of an effect on RVLM neurons as this review suggests. It would also be interesting to see if the Caudal Pressor Area is stimulating the RVLM via glutamate or a different excitatory agonist. Also the CPA could be selectively innervating particular neuronal beds in the RVLM which could account of the differential control of nerve activity results that we have shown previously. It is certainly possible those different brain regions are responsible for stimulating different subsets of RVLM neurons and that these brain regions are stimulated by different phenomena throughout the body. That being said, I do not necessarily know of any evidence of preferential control of RVLM in regards to different brain regions, however, I believe that it is certainly possible given the fact that the RVLM has to manage MANY inputs and I do not believe that it is sophisticated enough to act as an integration center as well as a delivery center without some help from its neuronal circuitry.  - M.T.L.

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

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

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

The sympathetic control of blood pressure

Patrice G. Guyenet Nat Rev Neurosci. 2006 May; 7(5):335-46 I’ve been writing my introduction and figured this paper would be a great resource, so I read it *again*…………… The overall purpose of this review is to examine sympathoregulation and how altered sympathoexcitation can result in hypertension and other cardiovascular diseases. It is also important to mention that while SNA has been correlated to CVD, there are likely other mechanisms involved in the development of CVD, including hypertension. Apparently renal SNA is selectively regulated by volume receptors whereas the other sympathetic nerves are not. Also the rostral ventromedial medulla (RVMM) and medullary raphe are involved in cutaneous circulation, whereas the rostral ventrolateral medulla (RVLM) is not. I also did not know that the C1 neurons were only one of three clusters of adrenaline-synthesizing cells in the CNS, which is particularly interesting because we still don’t particularly know the role that adrenaline synthesis is even having in the RVLM, aside from being a good marker. Here are C1 cells that are non-barosensitive that control adrenaline-releasing chromaffin cells. Guyenet mentions that the baroreflex can be set (towards a higher level) in the case of different activities that require heightened sympathetic tone, exercise for example. I do not know the mechanism by which the baroreflex can be altered; I know that reflexes like the muscle metaboreflex exist and different types of hypoxia can raise blood pressure above baroreflex regulation. Something interesting, which is slightly unrelated (however mentioned in the review), is that obstructive sleep apnea (OSA) increases SNA throughout the waking period of the day, which I thought was surprising because I figured it would only have an effect during the night. Something that I found particularly interesting (as well as something that I don’t necessarily agree with) is the remark that “sympathetic efferents that innervate the kidneys are commonly presented as the only ones that are capable of influencing 24-h average BP.” I don’t think that renal controlling neurons are the only neurons involved in tonic cardiovascular regulation, I think that looking at only the neurons controlling the kidneys is totally short sighted. It would be interesting to look at a review with a similar approach as this, but considering all of the papers presented after 2006 as well. -MTL

Olfactory exposure to males, including men, causes stress and related analgesia in rodents

Sorge, R.E., Martin, L.J., Isbester, K.A., Sotocinal, S.G., Rosen, S., Tuttle, A.H., Wieskopf, J.S., (...), Mogil, J.S. Olfactory exposure to males, including men, causes stress and related analgesia in rodents (2014) Nature Methods, 11 (6), pp. 629-632. doi: 10.1038/nmeth.2935 Dr. Berkowitz had mentioned this paper…………………… The goal of this paper is to look at the responses of male and female mice to different stimuli which are intended to gauge the stress or withdrawal response to the presence or perceived presence of a male or female researcher or animal. The first experiment consisted of an injection of zymosan, an inflammatory agent. The researchers studied the facial grimacing of mice in the presence or absence of an experimenter (male or female, seated at a distance of ~0.5m). All of the four male observers elicited significant changes in grimaces, while all four of the women did not. They conducted the same study, but instead of using the researcher, they placed a shirt worn the previous night by the male or female experimenters ~0.5 away from the mice. They found that they were able to see the same trend of responses. Interesting enough, the rats did not show the response if both the male and female shirts were placed together. Next, they placed the mice in different bedding materials from either unfamiliar male mice, nonpredator male guinea pigs, rats, cats, and dogs, and once again saw the same decreases in facial grimacing. Castrated male mice, cats, and dogs did not produce the same effect in mice, suggesting that this response is likely in large part due to androgens. A longitudinal study was performed to examine their previous experiments and look at the baseline and peak responses of mice, and found that mice tested by male experimenters displayed lower baseline pain sensitivity. This stress induced analgesia in the mice is shown only when in the presence of a male. The stress caused by male experimenters is short lasting, however, it is something to consider when conducting animal research, particularly when interested in different stress responses. -MTL

Role of rostral ventrolateral medulla centrally mediated pressor responses

Full cite: Kiely JM, Gordon FJ. Role of rostral ventrolateral medulla centrally mediated pressor responses. Am J Physiol Heart Circ Physiol 267:H1549–H1556, 1994. Role of rostral ventrolateral medulla centrally mediated pressor responses James M. Kiely and Frank J. Gordon Department of Pharmacology, Emory University of Medicine, Atlanta, Georgia 30322 This paper is one of the first to look at how the RVLM mediates different central pressor responses, in particular how EAA receptors play a role in the proliferation of these responses. In this particular experiment they used female Sprague-Dawley rats, recorded BP, and instrumented for sciatic nerve stimulation (SPR). Electrodes were positioned into the periventricular nucleus (*which I thought was pretty cool*). Then a head surgery was performed to expose the brainstem, similar to how we prepare for our microinjection experiments. Kyn was injected bilaterally and there was no BP response, similar to in our experiments. When Kyn was injected in the RVLM, the hypothalamus’s pressor responses were intact. Injection of kainic acid increased BP for a short period of time (= 10s), after that point BP fell around 35 mmHg (~5 min). SPR and PFH responses were attenuated, PVN and AH responses were intact. PFH and PVN responses were reduced following injection of muscimol; lidocaine into RVLM abolished SPR, and greatly reduced the PFH and PVN responses. This paper is likely one of the first to demonstrate that the PVN and PFH have spinal projections of their own and are able to function at least somewhat independently from RVLM. Also it seems that SPR are almost exclusively mediated through the RVLM. It would be interesting to see how the activity of PVN and PFH might be altered during an RVLM knockout or blockade. I also do not know whether or not PVN or PFH are barosensitive. I think that at some point we are either going to have to integrate PVN into our experiments or knock it out in some way, because it may very well be playing a role in our responses. MTL

Somatic nerve stimulation evokes qualitatively different somatosympathetic responses in the cervical and splanchnic sympathetic nerves in the rat

Full cite: S. McMullan, et al. Somatic nerve stimulation evokes qualitatively different somatosympathetic responses in the cervical and splanchnic sympathetic nerves in the rat. Brain Res., 1217 (2008), pp. 139–147 Simon McMullan, Karrnan Pathmanandavel, Paul M. Pilowsky, Ann K. Goodchild Australian School of Advanced Medicine, Macquarie University, Australia University of Sydney This study is examining the reflex responses of two different sympathetic nerves in response to somatic pressor response– from the sciatic nerve. The two sympathetic nerves that they are looking at is the splanchnic and cervical nerve. The primary technique used is this study is electrophysiology. They stimulated from the aortic depressor nerve, using antidromic action potentials to confirm that the neurons that they were recording from in RVLM were barosensitive. They found that the splanchnic nerve had a primarily biphasic response to sciatic nerve stimulation, whereas the cervical nerve was primarily monophasic. This suggests qualitatively different responses between the sympathetic nerves given the same stimulus, suggesting a difference in the pathways controlling these nerves. It was determined that the first peak in both cervical and splanchnic were mediated throughout the same pathway, however, the second peak of the splanchnic –which was not present in the cervical– was due to an additional response in the splanchnic to the stimulus. This additional response to the splanchnic might be likely due to some input from unmyelinated afferents, that are selectively absent in the cervical control. The splanchnic receives both A- and C- fibers, whereas the cervical only receives A- fibers, suggesting a non-uniform organizational scheme when it comes to the control of RVLM. This paper provides a little bit of additional evidence into the complexity of RVLM’s control of the sympathetic nervous system, and that each sympathetic nerve has different properties to its control and function. There are likely different neurons controlling the different nerve activities, but also different afferents projecting to these nerves. My presentation and the study paired with it is another piece of evidence to RVLM’s sympathetic control, showing that adrenal and splanchnic control are mediated differentially by GABA following baroreflex unloading. -MTL

Lateralisation of projections from the rostral ventrolateral medulla to sympathetic preganglionic neurons in the rat

Lateralisation of projections from the rostral ventrolateral medulla to sympathetic preganglionic neurons in the rat Elizabeth A. Moon, Ann K. Goodchild, Paul M. Pilowsky Hypertension and Stroke Research Laboratories, Departments of Physiology and Neurosurgery, University of Sydney, Block 3 Ground Floor, Royal North Shore Hospital, St. Leonards, 2065 Sydney, NSW, Australia This study contains three different techniques: anterograde tract-tracing (using Phaseolus vulgaris leucoagglutinin, PHA-L), retrograde tract-tracing(using CTB) and electrophysiology. Spinally projecting neurons (SPNs) that project to the adrenal gland or to the superior cervical ganglia were labelled retrogradely. The retrograde labeling was paired with anterograde tract-tracing from the RVLM to see if lateralization in the bulbospinal projection to SPN innervating the adrenal gland or superior cervical ganglia could be identified. Unilateral injections of the spinal cord were made with a retrograde tracer in order to determine the amount of lateralization between the upper and lower thoracic spinal cord. The last experiment they did was stimulate the RVLM with glutamate and record from the left cervical sympathetic and left adrenal nerve to test for functional lateralization. They found that the SPNs controlling the SCG were primarily bilateral, meaning that both the ipsilateral and contralateral sides were nearly equally labelled following anterograde labeling. The SPNs controlling the adrenal medulla were almost exclusively labelled on the ipsilateral side, suggesting ipsilateral control. Next in the retrograde tracing studies, they showed that CTB injected unilaterally at T2 and T8 both showed about 66%-75% ipsilateral projections to 25%-33% contralateral projections. The electrophysiological studies showed that following glutamatergic stimulation of RVLM, there are no differences in the ratio of responses to either the cervical sympathetic truck or adrenal nerve, suggesting that while there are differences in anatomical connections, the functional contribution of each RVLM to the respective nerve activity may very well be the same under the given conditions. This paper will provide helpful anatomical knowledge for my presentation where I will present my differential control data. My results corroborate with their anatomy data for the adrenal nerve, demonstrating that under normotensive conditions, that the adrenal nerve responds the primarily ipsilateral to a given injection of glutamate. It would be interesting to do a similar tract tracing study to see if the nature of the ipsilateral/contralateral connections might be altered following physical (in)-activity. -MTL

Tonic glutamate-mediated control of rostral ventrolateral medulla and sympathetic vasomotor tone.

Full cite: Ito S, Sved AF. 1997. Tonic glutamate-mediated control of rostral ventrolateral medulla and sympathetic vasomotor tone. Am J Physiol 273:R487-R494. Department of Neuroscience, University of Pittsburgh, Pennsylvania 15260, USA. This paper was one of the first that demonstrated that EAA is likely driving tonic GABAergic input by some indirect connection. It is either that EAA are driving GABAergic input, or that EAA input is only revealed following the removal of GABAergic inhibition. In this particular paper, they injected muscimol into CVLM to illicit a positive pressor response, similar to how we inject bicuculline into RVLM. In addition to this, they injected kynurenic acid into RVLM to decrease BP. It is important to note that they did not look at SNA, only BP in this paper. It believed that the BP decrease following injection of Kyn is due to the fact that glutamate is playing some kind of excitatory role only after removal of GABA. Something interesting out of this paper is that Sved suggests that the RVLM is feeding back to CVLM via some kind of interneuron. In addition to this, he suggests that the CVLM has some excitatory projections to RVLM and that they are not EAA-related. Something important to note is that they were not able to see any differences in responses due to the anesthetic used in the experiments (urethane v. chloralose). Similar to Nick’s 2012 paper, once the GABAergic input into RVLM is blocked, pressor responses to glutamate are increased, suggesting that GABA is limiting the amount of glutamate excitability under normotensive conditions. This paper is one of (if not the first) that demonstrates that injection of kyn alone has no particular effect on BP, suggesting that kyn under baseline conditions either has no effect, or that there are competing glutamatergic mechanisms that makes it seem like kyn as no effect. This paper is the primary pieces of background for my study looking at tonic glutamatergic/GABAergic input into RVLM and is one of the first that suggests that there may be some kind of interneuron activity in RVLM, whose activity is undetermined. -MTL

Patterning of somatosympathetic reflexes reveals nonuniform organization of presympathetic drive from C1 and non-C1 RVLM neurons

Full cite: Burke, P. G. R., Neale, J. K. W. S., and McMullan, S. G. A. K. (2011). Patterning of somatosympathetic reflexes reveals nonuniform organization of presympathetic drive from C1 and non-C1 RVLM neurons. Am. J. Physiol. 301, R1112–R1122. doi: 10.1152/ajpregu.00131.2011 Peter G. R. Burke, Jemima Neale, Willian S. Korim, Simon McMullan, and Ann K. Goodchild Australian School of Advanced Medicine, Macquarie University, Sydney, Australia Submitted 16 March 2011; accepted in final form 20 July 2011 This paper is looking at the makeup/organization of C1 and non-C1 neurons within the RVLM. The method that was used to differentiate between these two subgroups was by recording sciatic nerve responses (SN) from several different afferent inputs. These inputs were activated at graded intensities by utilizing different intensities of SN they were able to excite different nerve fiber afferents specifically: A-fiber (low intensity), and A- and C- fiber (high intensity). In addition to this, they looked at low-intensity SN stimulation, they examined cervical somatosympathetic reflex (SSR) following RVLM microinjection of somatostatin. By using intraspinal injections of anti-dopamine-β-hydroxylase-saporin (anti-DβH-SAP) they were able to selectively lesion C1 neurons. As a result of the study, they determined that use of anti-DβH-SAP abolished the activity of RVLM neurons with slow conduction velocities and sparred RVLM neurons with fast velocities. They concluded that axons projecting from C1 neurons are likely unmylenated, whereas the axons from non-C1 neurons are likely mylenated, allowing for higher conduction velocities. If there was some means by which we could selectively lyse the non-C1 neurons in the RVLM, similar to how anti-DβH-SAP removes C1 neurons, by utilizing the fact that non-C1 neurons are myelinated, then we could prove whether or not non-C1 neurons are maintaining basal sympathetic outflow. In addition to this, I believe that looking at graded action potentials while pairing with immunohistochemistry may be the means by which we will be able to figure out where the excitation of RVLM neurons is coming from. If we are able to either expose some kind of excitatory projection to RVLM neurons that are not glutamatergic via immunohistochemistry, or characterize the neurons electrochemically to another similar known group of neurons, we might be able to determine the tonic excitatory drive. - MTL

Selective enhancement of glutamate-mediated pressor responses after GABAA receptor blockade in the RVLM of sedentary versus spontaneous wheel running rats

Patrick J. Mueller* and Nicholas A. Mischel Department of Physiology, Wayne State University School of Medicine, Detroit, MI, USA I needed an excuse to read this paper again……. In this study we looked at the response of WRs and SEDs to exogenous glutamate following the removal of GABAergic tone via bicuculline. This paper is an attempt to build on Nick’s 2011 paper which looked at sympathoexcitation following a dose of exogenous glutamate and looked at the blood pressure and splanchnic sympathetic nervous activity response. This paper is examining lumbar sympathetic nervous activity and blood pressure response to different doses of glutamate, with the inclusion of pre-bic and post-bic protocols. Interestingly enough he was not able to duplicate the same dose response curve of the 2011 paper for lumbar nerve activity, suggesting that SEDS/WRs do not have different sympathoexcitatory responses to solely exogenous glutamate in regards to lumbar nerve. They were able to show sympathoexcitatory differences in the post-bic protocols 5’ and 15’ minutes after the injection of bic, suggesting that GABAergic transmission is hiding the sympathoexcitatory differences between WRs and SEDs in regards to lumbar and blood pressure. It is important to consider that bicuculline was injected unilaterally so there could have been some kind of compensation from the contralateral RVLM. I would like to see what the glutamate responses would be if the RVLMs were blocked bilaterally, that being said, the blood pressure might raise too high (190 mmHg) with a bilateral injection of bic. It would also be interesting to see if there were any differences in glutamate responses with the ipsilateral RVLM completely disinhibited with bic and the contralateral RVLM excitated with glutamate. Venturing to what I have been looking at with my endogenous input experiments, it would be important to see if there were differences in the nerve activity responses to bicuculline in regards to the lumbar nerve, or these nerves (splanchnic and lumbar) might be differentially inhibited by GABA. -M.T.L.

Vesicular glutamate transporter 2 is required for the respiratory and parasympathetic activation produced by optogenetic stimulation of catecholaminergic neurons in the rostral ventrolateral medulla of mice in vivo

Full cite: Abbott, S.B., Holloway, B.B., Viar, K.E. & Guyenet, P.G. (2013)Vesicular glutamate transporter 2 is required for the respiratory and parasympathetic activation produced by optogenetic stimulation of catecholaminergic neurons in the rostral ventrolateral medulla of mice in vivo. Eur. J. Neurosci., 39, 98-106. Stephen B. G. Abbott,1 Benjamin B. Holloway,1 Kenneth E. Viar1 and Patrice G. Guyenet1,2 1Department of Pharmacology, University of Virginia, Charlottesville, VA, USA 2University of Virginia Health System, P.O. Box 800735, 1300 Jefferson Park Avenue, Charlottesville, VA 22908-0735, USA Our lab and others have looked at the RVLM’s role in reflex responses to different homeostatic challenges such as hypotension, hypoxia, and hypoglycemia. The general belief is that the RVLM utilizes glutamate as the primary transmitter in response to these homeostatic challenges; however, this has never been proven in-vivo. Guyenet’s laboratory utilized a combination of optogenetics, immunohistochemistry, and multi-cell recordings in gene knockout mice in an attempt to confirm that these responses are glutamate-mediated. In particular, they were looking at the role of a glutamate transporter VGLUT2. They used DβHCre/0 mice which mark noradrenergic and adrenergic neurons with Cre-recombinase. In addition to this they paired the DβHCre/0 with rats that knock out the VGLUT-2 gene, called cKO mice. By utilizing DβHCre/0 and cKO mice using optogenetics, the team was able to determine the role of glutamate and VGLUT-2 in these responses. They found that VGLUT-2 deletion has no effect on the number or morphology of C1 neurons in RVLM, but completely eliminated several homeostatic responses. The acute respiratory and parasympathetic activation produced via optogenetic stimulation was abolished following removal of VGLUT-2. It would be interesting to do this same study, but incorporate the non-C1 neurons via some other technique. Most importantly though, this once again plays into Guyenet’s belief that C1 neurons function to maintain homeostasis following different physiologic challenges, that C1 neurons are the body’s EMTs and glutamate is the main means by which the RVLM affects the periphery. At some point we could look at adding optogenetics into our experiments, perhaps even with MEMRI. -M.T.L.