Differentiation of two cardiovascular regions within caudal ventrolateral medulla.

Cravo SL, Morrison SF, Reis DJ.
Am J Physiol. 1991 Oct;261(4 Pt 2):R985-94.

   Leading up to this paper, there had been some disagreement over the role of CVLM neurons in regulating SNA via inhibition of the RVLM. Some groups had seen barorecepter-dependent inhibition, while others had seen baroreceptor-INdependent inhibition. This paper examined the possiblity that there were 2 different subpopulations of CVLM neurons, rostral and caudal, that had different functions in regulating RVLM activity. They used kainic acid (KA) as an excitotoxic agent to inactivate subregions of the CVLM and examined what happened to splanchnic sympathetic nerve activity (SSNA) when they "activated the baroreceptor" by electrically stimulating the aortic depressor nerve (ADN).
   In control rats, baseline SSNA showed bursts tied to the pulse (frequency analysis of SSNA showed the most power at ~6Hz), and stimulation of the ADN caused inhibition of SSNA, as expected. However, after bilateral injections of KA into the rostral CVLM, they found that ADN stimulation no longer caused inhibition of SSNA and the power at 6Hz was almost completely eliminated. The arterial pressure and SSNA were both greatly increased following KA injection (after a brief decrease in both), consistent with excitotoxic lesioning of neurons that would normally inhibit the RVLM. This effect required bilateral lesioning of the rostral CVLM. Unilateral lesioning only abolished ADN-dependent inhibtion of SSNA when the ipsilateral ADN was stimulated.
   When the caudal CVLM was lesioned by KA, an increase in arterial pressure and SSNA were seen, similar to what happened when the rostral CVLM was lesioned.  However, with the caudal CVLM lesioned, stimulation of the ADN was still capable of causing an inhibition of SSNA and a drop in blood pressure. It also cause a huge increase in the relative power of the 6Hz SSNA frequency (or a huge decrease in all other frequencies, depending on how you look at it). The proposed reason for this is that with the baroreceptor-independent inhibition abolished, the increased arterial pressure caused a greater activation of baroreceptors, resulting in a greater coherence between SSNA and the frequency of the heart.  The final "trick" they did in this paper was to take the same rats that had caudal CVLM lesions, and then give them lesions in the rostral CVLM. This was then able to wipe out the effect of ADN stimulation on SSNA and arterial pressure, showing that the baroreceptor-dependent inhibition of SSNA is tied to the rostral but not the caudal CVLM. -dh

Reticulospinal vasomotor neurons in the RVL mediate the somatosympathetic reflex.

Morrison SF, Reis DJ.
Am J Physiol. 1989 May;256(5 Pt 2):R1084-97.

   In this paper, they wanted to see if the RVLM neurons responsible for basal sympathetic tone were also responsible for the somatosympathetic reflex. To do this, they stimulated the sciatic nerve to increase SNA while monitoring RVLM unit activity and efferent splanchnic SNA.
   When they examined the effects of individual sciatic stimuli, they found 2 separate SNA were evoked in response. By varying stimulus intensity, they determined that these were not due to two separate populations of sciatic fibers. However they did estimate afferent conduction velocity (at the lumbar dorsal root) and found 2 lightly myelated types and 1 unmyelinated type when they gave a strong enough stimulus. This suggests that the sensory afferents that drive a sympathetic response are the lightly myelinated ones and that the C-fibers play a much smaller role in this effect.
   They next used the classic technique of doing transections to find out which regions are necessary for the response, and found that the lower brainstem and below is sufficient. They then checked that the RVLM was involved in the reflex by microinjection of kainic acid, which caused a depolarization block that was able to reduce the SNA response to sciatic stimulation by ~90%.
   To examine how individual neurons played into the response, they found barosensitive neurons and determined that they were spinally projecting and verified (in post processing) that they were cardiovascular-related and probably functioned to stimulate SNA. They then found that most of these neurons would show early and late action potentials linked to sciatic stimuli, very similar to what was seen for splanchnic SNA. The neurons also increased their rate of firing for about 20 milliseconds after each of the evoked action potentials.
   Finally, in the discussion, they dedicate a lot of space to the argument (supported by addition of afferent and efferent conduction velocities) that the somatosympathetic effect occurs primarily by sensory neurons in the sciatic bundle synapsing onto two populations of neurons in the spinal cord, which have different conduction velocities but both directly excite the neurons in the RVLM that are responsible for stimulating the splanchnic nerve. - DH

In vivo axonal transport rates decrease in a mouse model of Alzheimer's disease.

Smith KD, Kallhoff V, Zheng H, Pautler RG.
Neuroimage. 2007 May 1;35(4):1401-8.

   Some in vitro models of alzehimers disease (AD) show a decreased rate of axonal transport linked with accumulations of tau and amyloid-B (AB) proteins. Adding AB to cultured neurons also inhibits transport, possibly through interactions with actin that change its level of polymerization. The aim of this study was to develop a new way to measure in vivo axonal transport. To study in vivo transport rates, the group used MEMRI and a strain of mouse which expressed an aggressive mutant form of amyloid precursor protein and accumulations of AB plaques. Mice were given Mn through nasal lavage and repeatedly imaged for an hour to see how transport to and uptake into the olfactory bulbs occurred. Post-Mn signal intensities were normalized to pre-Mn levels to find when Mn had reached regions of interest. As expected, MRI signals incrased over time in manganese-treated mice, but it did not increase in control animals. They then used decreased temperature and cholchicine (which prevents polymerization of actin microtubules) to show that by inhibiting axonal transport, they could prevent the the transport-dependent increase in signal intensity observed in control animals.
   To look at developing an assay for diagonsing AD, they used MEMRI and a model of mouse which expressed a mutant form of amyloid precursor protein and increasing accumulations of AB plaques with age. They found that after manganese treatment, young AD mice showed similar transport rates (as demonstrated by MEMRI signal increases) to regular mice, but the signal was reduced in 7-8 month old mice, and reduced even further in 11-14 month old mice. This is cool because there aren't currently good ways to diagnoze alzheimers disease while a person is still alive. Diagnosis is usually done post-mortem, after years of AD-like symptoms. Finding a way to diagnose it in the early stages might be a great way to prevent it from getting worse before it's too late to treat. -DH

Role of presympathetic C1 neurons in the sympatholytic and hypotensive effects of clonidine in rats

Schreihofer AM, Guyenet PG.
Am J Physiol Regul Integr Comp Physiol. 2000 Nov;279(5):R1753-62

   Clonidine is an a2-adrenergic receptor agonist that works in multiple places in the CNS. It is frequently used as an antihypertensive drug for its ability to decrease blood pressure. Microinjections of clonidine into the RVLM produce a drop in MAP, similar to IV administration, and the effect can be blocked by co-injection with an a2 antagonist. Earlier work had shown that slow firing (likely C1) neurons in the RVLM were affected by clonidine injection, but it wasn't clear if it was a direct effect on C1 neurons or if it was mediated by receptors on presynaptic cells. Also, the effect of clonidine on non-C1 presympathetic neurons was similarly unclear.
   To investigate these questions, RVLM neurons were recorded in rats that were treated with multiple IV doses of clonidine. Cells were also labeled with biotinamide for later reconstruction and examination of phenotype. They also used anti-DBH-saporin in some rats to lesion C1 cells to see what effect clonidine would have when the majority of C1 neurons had been eliminiated.
   They found that increasing doses of clonidine contributed to a gradually decreasing frequency of action potential frequency among all cell types - slow conducting (unmyelinated C1), medium conducting (mostly lightly myelinated C1) and fast conducting (mostly non-C1). They pointed out that the response was extremely variable within each group, and that overall SNA was more inhibited than any group was. So either something else which contributes to SNA was also inhibited, or that the nerve itself was inhibited somehow.
   After they reduced PNMT positive neurons in the rostral end of the C1 region by ~76% using anti-DBH-sapporin, they saw the same effect of clonidine as they did in untreated control and IgG-saporin control rats. So in this paper they showed that even though C1 neurons are inhibited by systemic clonidine and the inhibition could contribute to a decrease in SNA, C1 neurons aren't NECESSARY for this to happen, and that other non-C1 presympathetic neurons act the same way. - DH

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury

Rodriguez et al.
Journal of Neurotrauma. April 2016, 33(7): 662-671.

   In this paper, they used a form of manganese-enhanced MRI (MEMRI) to study the effects of blast traumatic brain injury, a severe form of TBI which sometimes leads to long term neuropsychiatric problems. They were looking for markers in mouse brains which might lead to markers to identify injury in humans. In order to do this, they worked with a specially designed chamber that used compressed helium to send blast waves of different strengths at anesthetized mice.  Mice were then given IP injections of 40mg/kg manganese. Mice brains were imaged using T2 scans to be sure there were no major structural changes or damage, and T1 weighted scans to examine manganese uptake at 6, 24, 48, 72hrs, and 14 and 28 days after the helium blast.
   Blast-exposed mice showed greater transmission of manganese across the blood-csf barrier at 6 hours post-injection compared to controls. By 24 hours, this increased uptake caused greater signal throughout the brain in blast mice. The differences lasted for at least a week in all regions, in some regions after two weeks, and disappeared after a month. They found that in a few tissues, signal intensity kept increasing for 72 hours before going back down.
   One very interesting thing in this study was that they tried to use a 0.01M Mn phantom in their scans as a control, but found that its image values were more variable than expected from scan to scan (presumably due to microscopic air bubbles and changes in position of the phantoms), but the brain ROIs they studied didn't show the same variability, so they resorted to using non-blast-exposed saline-vehicle-injected mice as image quality controls for normalizing T1 values.
   There was also one other finding that I have a hard time understanding. They found that if mice underwent the blast treatment while wearing little mousey body armor, the manganese enhancement in the brain was attenuated and happened at levels similar to those in a mouse not exposed to a blast. -DH

Lateral tegmental field neurons of cat medulla: a source of basal activity of ventrolateral medullospinal sympathoexcitatory neurons.

Barman SM, Gebber GL.
J Neurophysiol. 1987 May;57(5):1410-24.

   The RVLM does not exist all by itself, no matter how much I focus my attention on it, that is not going to change. This goal of this paper was to look at another region involved in control of SNA, the lateral tegmental field (LTF) of the cat medulla, and how it interacted with the RVLM. Both the RVLM and the LTF  contain neurons with activity correlated with cardiac SNA, slow or stop their rate of firing with increases in blood pressure, and glutamate or electrical stimulation of both areas increases SNA. The hypothesis was that the LTF drives activity in the RVLM, which then drives SNA.
   They located neurons in the LTF, were able to antidromically activated them from the ipsilateral or contralateral RVLM (conduction velocity ~0.5m/s), and check them for barosensitivity and cardiac related rhythmic activity. They also found a few neurons that had 2 different conduction velocities when the stimulating electrode was placed at different depths of the RVLM, suggesting multiple axon branches within the RVLM region from the same LTF neuron that either took a short path and a long path, or had very different properties between collaterals. The long/slow path was always more ventral than the short/fast path, with the ventral sites being located outside the RVLM.
   Then things got more complicated!  They found that if they recorded spinally projecting RVLM neurons and stimulated in the LTF, they could find cases where some RVLM neurons could be antidromically activated, some neurons could be synaptically activated (variable latency action potentials that could not follow at high frequencies), and some neurons showed both characteristics, suggesting reciprocal communication between the LTF and the RVLM. They also found a number of RVLM neurons which could be antidromically activated by stimulating either in the T2 IML or the LTF, which indicates the presence of collateralizing axons with both ascending and descending projections.
   There was a lot more to this paper that I did not cover in this blog entry, including where the axons of RVLM neurons likely branched, latencies of efferent signals from the different regions, discusson on potentially sympathoinhibitory cells in the LTF, and more. There's a lot of info in this paper and it's worth a couple of reads to try to get it all. -DH

Morphology of rostral medullary neurons with intrinsic pacemaker activity in the rat.

Sun MK, Stornetta RL, Guyenet PG.
Brain Res. 1991 Aug 9;556(1):61-70.

   In slices, and in some in vivo preparations, some presympathetic RVLM neurons fire spontaneously, even when glutamate receptors are blocked. They had previously shown that spontaneously active spinally projecting RVLM neurons were non-C1 type cells, so this study followed up on that by looking at these "pacemaker" neurons and get phentotypic information about them, including cell type and morphology. To do this, they used rat medulla slices and intracellular recording with peroxidase- or lucifer yellow-filled electrodes. They located and recorded from spontaneously active cells before injecting the labeling compound.
   They found that the pacemaker cells had fusiform or triangular bodies that were significantly larger compared to 46 PNMT positive cells they measured, though they note that there may have been some bias toward recording larger cells. They found that the cells had fairly simple dendrites which mostly spread toward the ventral surface of the slice and then out to the ventromedial and dorsolateral edges, which didn't seem to be unique to the pacemaker cells (C1 cells may also fit this pattern). This made a lot of sense since previous experiments had been done by some groups simply by dripping drugs onto the ventral surface of the RVLM.
   They identified what they believed to be axons (though this was not confirmed by electron microscopy) through their thin, smoother appearance which was different from the light branching seen on most dendrites. The axons projected dorsally and medially, similar to C1 cells. This was in contrast to the lateral course seen by another group, though they suggest that might have been a different subpopulation of cells. They also did not find axonal branching (ascending and descending) seen by others in C1 neurons and say this may be more evidence of different cell types. I admit to being a little cofused about this interpretation since they say the branching has been shown by others to be ~2.6mm away from the body, and their slices were only 500um thick. They had to use cells towards the middle of their slices to limit cutting off dendrites, so that means they could only really observe ~250um. Maybe I'm just misinterpreting the text, because in the conclusions they state that their prior electrophysiological studies DO support axonal branching. Either way, it is nice to learn more about the types of cells I can expect to see in my reconstruction study. -DH

Identification of C1 presympathetic neurons in rat rostral ventrolateral medulla by juxtacellular labeling in vivo.

Schreihofer AM, Guyenet PG.
J Comp Neurol. 1997 Nov 3;387(4):524-36.

   Before this paper, it was known that some RVLM neurons were barosensitive, but showing the phenotypes of which ones were linked to SNA had been difficult. There were multiple lines of evidence stating that C1 neurons were involved, but the presympathetic non-C1 cells complicated the issue. In fact, experiments that measured conduction velocities of spinally projecting neurons suggested that C1 neurons might not be a major player, despite making up 50-70% of the cells projecting to the IML.

   In this study, they recorded 96 cells in 41 rats, and 87% of the cells were spinally projecting (tested via antidromic activation). Many times the blood pressure needed to be increase to stop spontaneous action potentials in order to induce an antidromic one. The distance between recording and stimulating electrodes was estimated at 35mm, which allowed them use constant antidromic latencies to estimate conduction velocity. They attemped to label 67 of the 87 cells and recovered 49 of the ones they attempted. They found that faster conducting (and more spontaneously active) cells were harder to label than slower ones (presumed to be C1). When they looked for TH immunoreactivity, they were able to recover 39 labeled, processed, spinally projecting cells.

   The cells broke roughly into thirds as non-TH-ir cells, slow TH-ir cells, and fast TH-ir cells. They applied a metric of TH immunoreactivity densitometry to find that slow TH-ir cells had much greater signal than fast TH-ir cells. All slow firing cells were TH-ir. They confirmed this by labeling analyzing another set of 5 labeled cells (3 slow, 2 fast) and staining for PNMT and got the same results - slow cells showed stain while fast cells didn't. They also reconstructed the neurons similar to how our lab has, but they didn't find any differences between the 3 types of neurons in terms of size or shape, which is something I would have been interested in seeing.

   The main findings here were that ~70% of the spinally projecting cells they found were indeed C1 cells, and that ALL of the slow firing spinally projecting cells were C1. This was also evidence that C1 are inhibited by increases in blood pressure, and their function was most likely sympathoexcitatory. It also helps explain why previous papers did not report C1 spinally projecting neurons - because they only recorded fast neurons, which selected against ~50% of the C1 neurons. -DH

A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies.

Ricketts JH, Head GA.
Am J Physiol. 1999 Aug;277(2 Pt 2):R441-54.

   This paper started off with the first two pages as a review of different techniques which had previously been used to mathematically describe baroreflex curves measuring arterial pressure against nerve activity and heart rate. It then went on to describe a new method, a 5-parameter equation, that was predicted to be able to account for asymmetry sometimes seen in baroreflex curves. The argument in favor of this new method was that it would be able to fit a curve better when the curve showed asymmetry between upper and lower limits, or differences in slope on either side of the midpoint. If no such asymmetry existed, the 5th parameter was averaged out and the formula was able to approximate the curve the same way a traditional 4-parameter equation would.

   The paper then went on to analyze pre-existing baroreflex data of MAP vs heart rate and renal nerve sympathetic activity in rabbits and dogs via 4- and 5- parameter curves - in both cases, they tested by forcing the curves to fit to the resting points and tested again without forcing them. They found that when there was an asymmetry in the curves of some subjects, the 5-parameter equation could account for it, though for the most part the 5-parameter method did not report values that were significantly different than the 4-parameter method. The biggest changes seemed to be caused by forcing the curve through the resting point, and according to this paper, doing that seems like a questionable technique. The paper did make a good point that even though this technique may not result in significantly different results over the 4-parameter technique in most cases, in cases where asymmetry is present, it will yield a more accurate result - and if you need it for one parameter, you should use the same technique for all parameters within a study. I think that's a pretty fair argument. -DH

Physical Activity and Cardiorespiratory Fitness Are Beneficial for White Matter in Low-Fit Older Adults

Agnieszka Zofia Burzynska, Laura Chaddock-Heyman, Michelle W. Voss, Chelsea N. Wong, Neha P. Gothe, Erin A. Olson, Anya Knecht, Andrew Lewis, Jim M. Monti, Gillian E. Cooke, Thomas R. Wojcicki, Jason Fanning, Hyondo David Chung, Elisabeth Awick, Edward McAuley, and Arthur F. Kramer

PLoS One. 2014; 9(9): e107413.

Our lab studies effects of sedentary vs physical activity on the brain and have shown an active lifestyle to be overall beneficial to cardiovascular health. I found this paper exciting because it looks at not only how an active lifestyle and good cardiovascular health can have positive effects on white matter later in life but also the neural correlates for these relationships. Using 88 healthy low-fit adults they examined the relationship between cardiorespiratoy fitness (CRF) with three different levels of physical activity (PA) and how it affected the levels of white matter (WM). Adults used for this study were between the ages of 60-79 years old, had no history of stroke or neurological illness, and had similar scores thresholds many other test. Participants PA was monitored for a week with a accelerometer worn on the hip and measurements collected placed them in one of the three levels of PA: sedentary behavior, light PA, and moderate to vigorous PA (MV-PA). T2 weighted MRI images were used to examine the integrity of WM and presence of white matter hyperintensities (WMH); lesions that appear in WM during old age that damage WM integrity. Age-related degeneration of WM micro-structures can be captured as decreased fractional anisotropy (FA) measured with diffusion tensor imaging (DTI). The study concluded that higher levels of MV-PA were linked to lower WMH volume, PA and CRF are related but not equivalent in their relationships with WM health in aging, and PA is associated with WM health in aging in an intensity- and region-specific manner. 

It’s interesting to look at the additional effects an active lifestyle can have on the body past the cardiovascular level. This paper shows it’s never too late to start doing any level of PA and that its has many of its benefits are still unknown to us. 

Zachery

Nucleus ambiguus inhibits activity of cardiovascular units in RVLM.

McKitrick DJ, Calaresu FR.
Brain Res. 1996 Dec 2;742(1-2):203-10.

   In our lab we tend to think mostly of the activity in the RVLM as increasing sympathetic nerve activity because of signals coming from the baroreceptors through the NTS and the CVLM. This paper examines the interaction between the sympathetic and parasympathetic nerve activity (PSNA) by injecting glutamate into the nucleus ambiguus (NA, excited by the NTS to activate PSNA) and measuring the effect on the activity of neurons in the RVLM of urethane-anesthetized rats. Because activating the NA would cause a direct drop in cardiac output which would also activate the RVLM, they used atropine to block the inhibitory effect of the NA on the heart.
   The cells they examined fired at about 7Hz at resting. They were not examined for latency or conduction velocity via antidromic activation because they considered the cardiac rhythmicity and barosensitivity to be indicitive of presympathetic neurons, but they mentioned that the possibility exists that some of them may not have been. Of the neurons they tested, 24 of 36 decreased activity after microinjection of glutamate into the nucleus ambiguus (NA), hinting at an inhibitory connection between the two structures. Injection of the inhibitory amino acid, glycine, into the NA, did not cause an increse in firing frequency, but did enhance the cardiac rhythmicity. They discuss that the data suggests that the NA might have a a direct effect on the RVLM, but they suggest that there might be other pathways in play, such as a direct connection from the NTS to the RVLM, that they did not eliminate in their preparation. So they can't conclude in this paper that that the NA directly connects to the RVLM, but it hints at a possible mechanism for a cool effect. -DJH

Sympathetic activity and blood pressure are tightly coupled at 0.4 Hz in conscious rats

David R. Brown, Laura V. Brown, Abhijit Patwardham, and David C. Randall

Am J Physiol. 1994 Nov;267(5 Pt 2):R1378-84

This study looked at the interactions of sympathetic nerve activity (SNA), blood pressure (BP), and heart rate (HR) at the 0.4Hz frequency. As where the studies in our lab are looking at the changes of signal coherence of active and sedentary conditions is study look at the coherence in rats while they rested in a cloth sock, roamed freely in there home cage, and after anesthesia. The data was obtained in a similar method that our lab uses. Electrodes were attached to the renal sympathetic nerve to measure SNA, while catheters were placed in the left jugular and aorta to administer drugs. All the data for each rat was collected over a 9.56 min period. The data showed in conscious rats that SNA frequency peaked at 0.4Hz while the BP and HR were at lower frequency's. Although they did note that the BP did display a some higher 0.4Hz frequency not seen in HR. The coherence of the SNA - BP was at its highest at the 0.4Hz and declined as the frequency got lower. Anesthesia did not change the coherence of SNA - BP. This study shows that SNA and BP are coupled at the 0.4Hz frequency and suggest that maybe BP reflects SNA. I agree with these results because SNA results from output of rostral ventral lateral medulla (RVLM) unit neurons which are influenced by BP.

-Zachery

Ventrolateral medulla in spontaneously hypertensive rats and the role of angiotensin 2

In this week’s blog, I reviewed a paper that studied the role of Angiotensin II in the ventrolateral medulla (VLM) in spontaneously hypertensive rats (SHR) and its contributions to cardiovascular regulation. Initially, Muratani et al, examined the VLM by microinjection of Angiotensin II antagonist into the rostral or ventral portion of the VLM. The antagonist caused a depressive effects and bradycardia in the RVLM, conversely, causing stimulating effects and tachycardia in the CVLM. Ultimately, blood pressure increased in SHR compared to the control group of normotensive rats. A group of 32 male rats between 14-16 week old were used in this study. Animals were anesthetized using urethan before injection. The results of this experiment indicated microinjections of Angiotensin II antagonist into the CVLM increased blood pressure and heart rate in SHR compared to normotensive rats. On the contrary, Angiontensin II antagonist injection into the RVLM produced a depressive response ultimately decreasing mean arterial pressure and heart rate. Therefore, angiontensin II receptor blockage in the VLM imply that angiotensin II has a tonic effect on the neural activity of the RVLM and CVLM of SHR. Furthermore, the results of this study show that brain Angiotensin II contribute to the continual regulation neural activity of the VLM and its regulation of the blood pressure.

This was an older paper that I read on angiotensin II and though it doesn’t directly relate to what I am currently working on, it did focus on the VLM and how different aspects can effect its regulation of blood pressure. Furthermore, since CV is the leading cause of death, it shows that continual research still must be conducted on this brain region to figure alternative ways to regulate on blood pressure and ultimately reduce the amount of deaths from CV.

Dean

Blunted sympathoinhibitory responses in obesity-related hypertension are due to aberrant central but not peripheral signalling mechanisms

Jackie M. Y. How, Suhail A. Wardak, Shaik I. Ameer, Rachel A. Davey, Daniela M. Sartor
J Physiol. 2014 Apr 1;592(7):1705-20.


While our lab studies the effect of sedentary conditions, other labs look at other aspects of an unhealthy lifestyle. In this study, they examined the effect of a medium high fat diet vs a low fat diet. The rats on the medium high fat diet that gained the most weight were dubbed obesity prone (OP) while the ones that didn't gain as much were dubbed obesity resistant (OR). Their goal was to look for changes in central mechanisms and/or peripheral mechanisms of controlling sympathetic nerve activity. To do this, they looked at the activity of individual RVLM neurons, in vivo, and tested two sympathoinhibitory stimuli - the baroreflex (via aortic occlusion) and infusion of cholecystokinin (CCK, acts at vagal afferents) upstream from the coeliac artery.
RVLM neurons in OP rats showed a poor synchronicity with the pulse pressure, and their CCK-induced inhibition was pretty much abolished compared to OR  and control rats fed low fat diets - so much so that some neurons were actually slightly excited by it. The also showed a decreased inhibition caused by increases in blood pressure. The pressure needed to attain 20% reduction in firing frequency was increase both in terms of percent change and absolute pressure. Only 2 of 8 neurons from OP rats could even be inhibited beyond 20%, compared to 10 of 11 neurons in the other groups being inhibited 20-100%. Despite these changes in response to stimuli, there was no significant difference in baseline firing frequency, though OP rats were slightly higher.
They also looked at FOS immunoreactivity in the NTS and CVLM, finding that the OP rats had fewer FOS-IR in the NTS after CCK administration compared to saline controls, but in the CVLM, OP rats had fewer FOS-IR cells compared to both OR and controls.
To show that the diet-induced changes were central rather then peripheral, they looked at subdiaphragmatic nerve discharge during CCK infusion and found no difference between groups.
I liked this paper because it adds to the growing body of knowledge that says the brain does stuff. Many people still think that changes in blood pressure are due to peripheral effects rather than central, but this study shows there are strong changes in the RVLM due to differences in something simple as your diet. I guess the only other question is that if the presympathetic nerves are changing, what good does that do if it's not affecting sympathetic nerve activity (in that particular nerve)? -DJH

Attenuated baroreflex control of sympathetic nerve activity after cardiovascular deconditioning in rats

J. A. Moffitt, C. M. Foley, J. C. Schadt, M. H. Laughlin, E. M. Hasser
American Journal of Physiology - Regulatory, Integrative and Comparative Physiology Published 1 May 1998 Vol. 274 no. 5, R1397-R1405

  This study used male rats to measure the effect of 14 days of hindlimb unloading (HU) on mean arterial pressure (MAP) vs heart rate(HR) , renal sympathetic nerve activity (RSNA), and lumbar sympathetic nerve activity (LSNA - controls hindlimb skeletal muscle) by intravenously infusing PE and SNP. This gave them the ability to construct baroreflex curves covering roughly 45-170mmHg MAP for each animal. The curve parameters for each animal were then averaged to make group curves and compared between HU and control rats.
  Similar to the other study on HU I blogged (which was done by some of the same people, after this study), HU rats had decreased body weight and soleus muscle mass. They found that HU also caused a resting tachychardia. However,  there were no significant differences in resting MAP, or maximum/minimum HR.
  LSNA was greatly attenuated in the HU group. Where control rats showed a LSNA increase to 350% of baseline at MAP of ~40mmHg, HU rats were only able to increase LSNA to 200%. the midpoint MAP was not different between groups, and the minimum LSNA at high MAP for both groups was at ~0%. This produced a significant reduction in gain as well. When viewed as a percent of maximal nerve activity, HU rats showed a significantly higher resting LSNA than controls, suggesting a reduction in response range rather than a reduction in absolute nerve activity.
  The results of looking at the RSNA were pretty similar to those of the LSNA; reduced maximum as a % of baseline, reduced gain, and a higher resting RSNA (as a percentage of the maximum). However, minimum RSNA was significantly lower in HU rats, where it wasn't in terms of LSNA.
  The take-home message here is that lack of regular exercise, or deconditioning, causes increases in resting SNA at the level of the kidney and skeletal muscle, and that leads to a reduction in the reserve SNA needed to meet the demands of decreases in MAP. -DJH

Daily exercise and gender influence arterial baroreflex regulation of heart rate and nerve activity

C. Y. Chen, S. E. DiCarlo
American Journal of Physiology - Heart and Circulatory Physiology Published 1 November 1996 Vol. 271 no. 5, H1840-H1848

  Before this study, groups had found that exercise training lead to differences in renal sympathetic nerve activity and heart rate, but information was lacking in other nerves. The previous studies had also used either all male or mixed males and female rats, but they hadn't looked specifically at males vs females despite the fact that there were some known differences between the sexes in terms of heart rate during changes in posture.
   Rats were allowed 8-9 weeks of sedentary conditions vs daily spontaneous running (DSR, just like our wheel runners) before being fitted with chronic catheters to allow drug infusion and monitor blood pressure and a chronic electrode to record lumbar sympathetic nerve activity (LSNA). Two days after surgery, the venous catheters were used to infuse PE or nitroglycerin to raise or lower blood pressure, respectively, by 25mmHg. Curves were constructed by comparing the drug-induced changes in blood pressure vs heart rate (HR) and LSNA, compared to resting values.
  DSR increased heart to body weight in both males and females, and caused a resting bradycardia in males but not females. Exercise reduced the maximum HR and its range (maximum minus minimum HR) in females, and the maximum and minimum HR, but not the range.  This corresponded with a DSR-induced reduction in maximum HR gain in females but not males. For LSNA, both males and female had an attenuation in maximum nerve activity and its range, and a corresponding decrease in maximum gain.
  I guess this all means that exercise training lets your body meet physiological demand without doing as much "work" in terms of HR or LSNA, which makes sense. There seems to be a disconnect between nerve activity and heart rate between the sexes though, because there were differences between male and female rats in HR but not LSNA. But trying to get at what those differences might be is an entirely different field with entirely different models. - DJH

The paper I read for this week’s blog directly related to our work using MEMRI and the mapping of brain regions in a rat experimental design of runners versus sedentary animals. MEMRI is commonly used to map the functional and structural organization of the brain. By using manganese as a retrograde tracer, it is able to give a visualization of tracks in the brain. This paper, Eschenko et al, followed closely to the experimental design we are currently studying in the lab. The first experiment utilized 12 rats that received a subcutaneous injection of manganese chloride and were scanned over a period of 16 days. The second experiment utilized 6 rats and were injected with manganese chloride with the intraperitoneal technique. They were tested for a total of one week. Finally, in the third experiment, manganese was enhanced using osmotic pumps. The osmotic pumps were surgically implanted and were used to slowly release manganese over time to eliminate any toxicity that may occur in have adverse effects on the data or even may cause death. The pump was placed into the intraperitoneal space. The most interesting aspect of this paper was the use of the osmotic pump to slowly release manganese into the animal and monitor them over time. This contrasts our experiment in such that we administered an intraperitoneal injection once and scanned the animal over the course of 3 weeks. There results indicated that the osmotic pumps having a slow release of manganese is more manageable by the bodies homeostatic mechanisms and will not produce and toxic effects. Secondly, the method was explored the ability of functional brain mapping in naturally behaving rats.

This is an interesting technique used in the lab and directly relates to our experiment. Depending on the results of our current research, I would be curious to see if we could use an osmotic pump in our rats and compare the results of IP vs Osmotic pump.

Dean