Ablation of brainstem C1 neurons improves cardiac function in volume overload heart failure

David C. Andrade, Camilo Toledo, Hugo S. Diaz, Claudia Lucero, Alexis Arce-Alvarez,
Luiz M. Oliveira, Ana C. Takakura, Thiago S. Moreira, Harold D. Schultz, Noah J. Marcus, Julio Alcayaga, Rodrigo Del Rio. Clinical Science (2019).

Heart failure (HF) is a major problem among the older generations and is characterized by increased sympathetic activity. One of the most important regions in the brain which is in charge of sympathetic control is the Rostral Ventrolateral Medulla (RVLM). Recent research has shown that in cases of HF, the neurons in the RVLM are very active compared to normal. C1 neurons, catecholaminergic neurons, in the RVLM are known to control sympathetic outflow. The goal of this study was to determine the role of C1 neurons in the RVLM of HF rats.

This study used 45 adult male Sprague Dawley rats that were kept on a light dark cycle and had access to food and water. Heart failure was induced by creating an opening between the vena cava and the aorta using a needle. This causes volume overload heart failure. Sham rats were also given the same anesthesia and surgeries, but no anastomosis was created. four weeks after the heart failure surgery, rats were placed under anesthesia and given anti-dopamine-beta-hydroxylase saporin (DBH-SAP) injections bilaterally into the RVLM to destroy C1 neurons. Control rats were injected with a saline solution. Sympathetic and parasympathetic activity was tested by giving injections of Propranolol, a beta blocker, and atropine, which is used to increase heart rate. 

The injections of DBH-SAP resulted in a 1.9 fold decrease in C1 neurons in the RVLM for both the control and HF rats. When rats were given Propranolol, HF rats that were given a control injection exhibited a larger decrease in heart rate when compared to the control rats given the control injection. This suggests that the HF rats had higher sympathetic activity. HF rats that were given the DBH-SAP injection exhibited a larger decrease in heart rate compared to HF rates given a control. No significant difference was shown in the control rats given the DBH-SAP injection. Atropine injections were then given to compare parasympathetic activity. HF rats given a control injection showed significantly larger decrease in heart rate than the control rats. The DBH-SAP injection did not exhibit any significant results in the HF or control rats. HR rats also showed a significant reduction in baroreflex sensitivity (BRS). After the injection of DBH-SAP, BRS was significantly improved in HF rats. HF rats given a control injection showed a decrease in both diastolic and systolic cardiac function, when compared to the control rats. Injection of DBH-SAP was shown to significantly improve cardiac function in the HF rats. HF rats exhibited more cardiac arrhythmias when compared to the control rats, but this number was significantly reduced after the injections of DBH-SAP was given. Although arrhythmias were reduced, DBH-SAP did not exhibit a change in the tissue damage found in the HF rats.

In conclusion, targeted ablation of C1 neurons in rats with HF exhibited reduced sympathetic activity and improved baroreceptor sensitivity. Frequency of cardiac arrhythmias also decreased after the ablation of C1 neurons, but this did not lead to a change in the damaged tissue. This study shows that the progression of HF is dependent on C1 neurons in the RVLM. This past Thursday, the department hosted Dr. Osborn to discuss renal hypertension and inflammation. He discussed in detail how neuron ablation can affect sympathetic and parasympathetic responses. I have to be honest and say I did not know this was a possible treatment option. I then wondering how this would affect neurons in the in RVLM region, which brought me to this research. The researchers in this study discuss the possibility of targeted ablation becoming a possible hypertension or heart failure treatment, but stress that much more research must be done to determine if it is possible in humans. Dr. Osborn further discussed how neuromodulation may be the future treatment option due to the fact that completely ablating sets of neurons can lead to many side effects. 

-Paul M

Effects of HCN Channels in the Rostral Ventrolateral Medulla Contribute to the Cardiovascular Effects of Propofol

Zhiqiang Hu, Zhilin Wu, Jie Gao, Qi Jia, Na Li, Yeling Ouyang, Shanglong Yao, Xiangdong Chen.

Molecular Pharmacology (2018)

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are intermembrane proteins that act as voltage gated channels expressed throughout the heart and central nervous system. HCN channels have been shown to be inhibited by the anesthetic Propofol. Propofol is a general anesthetic that has been shown to have side effects such as bradycardia and hypotension. Previous research has suggested that Propofol inhibits vasomotor pressor neurons in the rostral ventrolateral medulla (RVLM), which could lead to the side effects. In this study, researchers tested how HCN1 and HCN2 channels in the RVLM play a role in mediating the effects of Propofol.

Mice were kept under a 12 hour light/dark cycle and had free access to food and water. Mouse blood pressure was measured by a tail cuff while they were awake. All infusions of Propofol were given through the tail vein using a catheter. For microinjection trials, mice were anesthetized using sodium pentobarbital and warming pads were used to maintain body temperature. To access the RVLM, the two occipital bones were removed and then microinjections were given with multibarrelled micropipettes. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to test protein expression levels in different aged mice

While mice that were not under anesthetic, baseline blood pressure was not different between wild type and HCN1 KO mice, but heart rate in the HCN KO mice was significantly lower than the wild type. When Propofol was given continuously for 10 minutes, blood pressure exhibited a significantly larger decreased in wild type mice when compared to HCN KO mice. Heart rate was also shown to decrease more in the wild type mice during the Propofol infusions. Researchers then observed responses to RVLM microinjections of both Propofol and ZD-7288, the HCN channel blocker. Blood pressure and heart rate was shown to significantly decrease after the microinjections. When the RVLM was injected with ZD-7288 shortly before Propofol, the decrease in blood pressure and heart rate was not as severe as a solo Propofol injection. The inhibitory effects of HCN channels were then tested using voltage and current clamps. Propofol was shown to inhibit HCN channel current and cause an increase in resistance in the RVLM neurons, but had no effect on the resistance in the RVLM neurons after ZD-7288 eliminated the current. ZD-7288 was shown to decrease the firing rate of RVLM neurons. Researchers then used Real-time PCR and western blotting to see if aging has any effect on HCN1 and HCN2 protein channels in RVLM neurons. HCN1 expression was shown to significantly increase from 2-3 weeks of age to 8-10 weeks of age and then again from 8-10 weeks to 40-60 weeks of age. HCN2 expression was only shown to significantly increase from 2-3 weeks old age to 40-60 weeks.

In conclusion, blood pressure and heart rate were not different in wild type and HCN KO mice, but once Propofol was given, heart rate and blood pressure showed a larger decrease in wild type mice. ZD-7288 given before Propofol was shown to attenuate the decrease in blood pressure and heart rate. Propofol reduced the firing rate and caused hyperpolarization, but after ZD-7288 was given before Propofol, the firing rate was reduced and hyperpolarization did not occur. Lastly, HCN channel protein expression was shown to increase as age increases. The researchers discuss that these results could lead to the creation of new anesthetics that have fewer side effects on the cardiovascular system. Since most drugs have a long list of side effects, more research must be done to determine how these drugs are effecting the peripheral parts of the body and not just the singular location that the drug in designed to act on.

-Paul M