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

Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation

Foley CM, Mueller PJ, Hasser EM, Heesch CM.
Am J Physiol Regul Integr Comp Physiol. 2005 Nov;289(5):R1440-7.

In this paper, they looked at the effect of simulated microgravity on renal sympathetic nerve activity (RSNA) and heart rate (HR) and the differences between male and female rats. It was already known that after either circumstance, people were generally less physiologically able to respond to the demands of even simple tasks, like standing up (i.e. they demonstrated orthostatic intolerance) and that women were more likely to have problems than men. This study tried to examine some of the causes of that phenomenon since the effects had been seen, but the causes weren't well demonstrated. They did this by partially suspending rats so that their back legs couldn't touch the ground - in essence, putting the rats through this hindlimb unloading (HU) made them "weightless" for 2 weeks.
Aside from significant reductions in the mass of muscles associated with running, the HU rats had higher resting mean arterial pressure (MAP) and HR compared to controls after they were returned to the ground. They also had a greater maximum HR on the MAP vs HR curve (tested via SNP and PE infusion). When they looked at the baroreflex curves of RSNA, they found that there were reductions in the maximum RSNA during decreases in MAP as a result of both being female and HU, with HU females having the least responsiveness to demand for RSNA. The midpoint of the curves also slightly shifted right (p=0.06), a result of increased in function when there may be no real need for it at higher MAPs.
The takehome message of this paper is a very translational approach to the kind of studies we do - life under different conditions can radically alter the function of the sympathetic nervous system and your ability to respond appropriately to physiological demand for more or less SNA, and these changes may be affect one sex more than the other. -DJH