Hot off the press!! Splanchnic sympathetic nerves in the development of mild DOCA-salt hypertension.

Hi everyone, sorry for the gap since my last post, but I hope I can make up for it with this exciting new paper from our friends down the road at Michigan State.

Recent work from Greg Fink's lab (last author) and others has highlighted the importance of examining splanchnic sympathetic activity in the development and/or maintanance of hypertension. For example, experimental hypertension induced in rats using exogenous angiotensin II and a high salt diet is characterized by differential sympathetic responses depending on which "sympathetic regions", including splanchnic, are examined. In that model, renal nerve activity is decreased and lumbar nerve activity remains unchanged as measured by chronic nerve recordings in conscious rats. If the splanchnic sympathetic bed is denervated by removing the celiac ganglia (CGx) in the same model, blood pressure falls dramatically, suggesting that sympatehtic activity to the splanchnic bed is increased.
In this new study, they examine the role of the splanchnic bed in another clinically relevant model of hypertension, the DOCA-salt model. Similar to the ANGII-salt model, it appears that the combination of high salt with overexposure to a blood pressure regulating hormone, in this case the aldosterone-like molecule deoxycorticosterone acetate (DOCA), produces significant hypertension.
The lead author implanted a long-term release DOCA pellet in rats at the same time as implanting a radiotelemeter to measure arterial pressure chronically. Some rats were subjected to  CGx and some underwent the same surgery except not denervated. Over a period of two weeks to a month, blood pressure significantly increased in both groups, but the increase was significantly attenuated in CGx rats. At the end of the experiment, tissue supplied via the spalnchnic circulation was harvested and analyzed for norepinephrine (NE) content. NE was markedly diminished in the sampled tissues, indicating that CGx was effective. Additionally, the lead author infused radiolabeled NE into the rats until it reached steady state concentrations, then simultaneously sampled arterial and venous splanchnic blood for radiolabeled and endogenous NE levels. A calculation of the differences between arterial and venous values of radiolabeled and endogenous NE indicates how much endogenous NE is has been used by sympathetic nerve terminals as a neurotransmitter and then "spilled over" into venous blood. This procedure is a way to measure NE handling in specific tissues, with increased NE spillover indicating increased sympathetic outflow to the tissue. They found that whole-body NE spillover and NE plasma levels were decreased in CGx during the control period, but not during DOCA-salt. Similar findings were observed with splanchnic NE spillover.
The data indicate that the splanchnic bed certainly contributes to hypertension in this model but is not wholly responsible for it. In addition, splanchnic SNA does not appear to be increased in this model when assessed by the NE spillover method. Thus, it is possible that, in this model, increased reactivity of the splanchnic vasculature decreased arterial and venous compliance and leads to increased central arterial pressure.

-Nick

Hot off the press! Patterning of somatosympathetic reflexes reveals non-uniform organization of presympathetic drive from C1 and non-C1 RVLM neurons

It is well known that activation of muscle afferents by contracting muscle causes an increase in blood pressure. This is known as the somatopressor reflex. The reflex is driven by the SNS, and neurons in the RVLM mediate it. Some differential patterning of responses between different sympathetic nerves has been observed before, but not thoroughly investigated. This study investigates differential sympathetic nerve responses to graded activation of the afferent arm of the sciatic nerve, a.k.a. the somatosympathetic reflex. Furthermore, the relative contributions of C1 to non-C1 neurons in this response were examined.
Briefly, anesthetized rats were instrumeted to record arterial pressure and sympathetic activity from four nerves, cervical, splanchnic, lumbar, and renal. Stimulating electrodes were implanted on the left or right sciatic nerve. They performed sciatic nerve stimulation (ScNS) at varying intensities and amplitudes in normal rats. An additional set of experiments was performed in C1 neuron-depleted rats in which only splanchnic SNA was recorded.
Under normal conditions, ScNS produced double-peak responses in all but the cervical nerve. The peaks in the lumbar and renal nerves had slightly longer latency than splanchnic and cervical. In addition, the first peak of the splanchnic nerve was of larger amplitude than the second, but the opposite was seen in renal and lumbar nerves. In rats which had about 60% depletion of C1 neurons, the second peak in splanchnic SNA was abolished, but the first peak was only slightly attenuated.
These results suggest that, indeed, different sympathetic nerves have specific roles in the SNS response to muscle contraction. Moreover, C1 and non-C1 neurons are both involved in the response, each producing distinct peaks in splanchnic SNA, possibly according to the conduction velocity of their axons (C1 fast, non-C1 slow). This study provides even more evidence that SNA to specific vascular beds may be differentially controlled. It also highlights the importance of recording from multiple versus a single sympathetic output(s) in a single animal when performing in vivo experiments.
-Nick

Effect of renal sympathetic denervation on glucose metabolism in patients with resistant hypertension: a pilot study.

As most of you probably know, a study came out a couple of years showing that renal sympathetic nerve denervation can dramatically lower blood pressure in a hypertensive patient.  While the reno-centric scientists out there said "See! I TOLD you it was the kidney!", the evidence as a whole suggested that a more global phenomenon was occurring, since whole body norepinephrine spillover and sympathetic activity to the muscle were reduced in these patients.  In addition to affecting blood pressure, symptahetic overactivity can negatively affect glucose metabolism.  When beta adrenergic receptors in the liver are stimulated by cuirculating epinephrine, liver cells increase glucose production and subsequently plasma glucose and inculin levels increase. In turn, insulin stimulates sympathetic nerve activity, forming a cycle of activation. The authors guessed that glucose metabolism would benefit from renal denervation as well.
They performed the procedure in 37 patients with resistant hypertension and assigned 13 to the control group. A catheter was inserted into the renal artery and radiofrequency pulses were used ot heat the tip of the catheter and ablate the artery at various spots. This procedure kills any nerves running in and around the artery and causes a scar to form that prevents reinnervation. Blood pressure and markers of glucose metabolism were measured before and 1 and 3 months after the study.
The authors found that renal denervation had a marked effect on glucose metabolism. It is important to note that the patients' glucose and insulin levels were only slightly above normal before the study. fasting gluicose decreased, baseline insulin levels decreased, and blood pressure decreased as expected. Interestingly, some members in the trestment group were 'cured' of diabetes or pre-diabetes, depending on how you look at it. Now, does this prove that denervation blunts central sympathetic outflow to the adrenal gland and thus epinephrine secretion to cause these effects? No. Is it enough evidence to be suggestive? To me, yes. But then again, the appearance of the moon may suggest that it is made of cheese.  In any case, this study reminds one to remember the BIG PICTURE. Sympathetic overactivity means much more than just blood pressure.

Link to article

-Nick

"Rewiring" the nervous system after a spinal cord injury: How WSU-SOM secretly produces really good scientists

I came across this article last week while browsing on Pubmed. Two things jumped out at me. One, it's a Nature  paper and, two, the first author used to be Harry Goshgarian's PhD. student. I gave it a look and here's what I learned.

Spinal cord injuries are terrible. In particular though, injuries which occur above the motor neurons innervating the diaphragm (phrenic motor neurons, PMNs in C3-C6) are imminently life-threatening and the prognosis is very poor. The first obvious problem is that PMNs no longer recieve input from higher centers. In addition, inflammation due to degeneration of afferent fibers leads to upregulation of extracellular matrix molecules which potentially inhibit re-innervation. The authors hypothesized that enzymatic digestion of these molecules would improve recovery from injury. They injected the enzyme chondroitinase ABC into the phrenic motor nucleus at the same time as recieving a hemisection at C2. With the enzyme alone, the treated animals showed improved recovery of breathing, but the authors took it one step further. In another group of animals, they grafted a section of the tibial nerve from the site of the lesion (C2) to C4. Remarkably, animals in this group showed near-normal breathing activity 12 weeks after the lesion. Immunohistochemistry showed that fiber regeneration was robust and extensive at the lesion site and at the distal graft site. If this procedure works with fiber tracts to other skeletal muscles, then WOW.

Posted by Nick

Chronic estradiol-17β exposure increases superoxide production in the rostral ventrolateral medulla and causes hypertension: reversal by resveratrol

This very well-written study investigates the effect of chronic exposure to estradiol on blood pressure regulation in female rats. It is known that hormone replacement therapy (HRT) in post-menopausal women causes a slight, but significant, increase in blood pressure and cardiovascular disease risk. In addition, a large number of young women in industrialized countries take forms of estrogen orally for contraceptive purposes. Since the rostral ventrolateral medulla plays such an important role in blood presure control, they focused on changes occurring in the RVLM associated with changed in blood pressure. Specifically, they were interested in superoxide production since many studies have shown increased oxidative stress in the RVLM of animal models of cardiovascular disease.
Female rats were implanted with slow release estradiol pellets, or not, for three months. During the third month they were implanted with radiotelemetry transmitters and their BP was monitored for about two weeks. At the end of the experimental period, animals were sacrificed. Brains and trunk blood were removed and frozen for later study. In a second experiement, estradiol implanted and sham animals were divided into two groups, one recieving the antioxidant resveratrol in their food and the other not. Animals were sacrificed at the end of the 2nd experiment as in the 1st.
The author's original hypothesis was right. They showed that animals recieving chronic estradiol developed higher blood pressures than those who did not. Also, treated animals were shown to have increased superoxide levels in the RVLM. Most interesting was that treatment with resveratrol reversed these deleterious effects. These data suggest that increases in blood pressure seen in women recieving HRT and young women on contraceptive therapy may be due to oxidative stress in the RVLM. This effect can be mitigated by simply adding resveratrol in the diet. "A glass of red wine for me keeps my RVLM happy"
Althought these data are interesting and certainly compelling, the study could have been greatly strengthened by adding an additional group of ovarectomized animals. This would make the study much more applicable to post-menopausal women recieving HRT. For shame authors, for shame. :-)

Time course of sympathoadrenal adaptation to endurance exercise in man

Today's blog post is on a paper published before we were born. Well, most of us anyway :-P

One of the first documented physiological effects of endurance exercise training was that trained subjects exhibited lower heart rates during exercise as compared to untrained. In this study, the authors wanted to determine the time course of development of this phenomenon as well as the relationship to whole body sympathetic nervous system activity. They took six normal dudes and put them on a seven week exercise training program involving both cycling and running. The intensity of training was increased at week four of the program. At one week intervals while subjects were training, they measured heart rate and plasma levels of catecholamines and lactate during isolated acute bouts of exercise. As expected, heart rate during acute bouts of exercise decreased over time, with further reduction when intensity was increased. Interestingly, plasma catecholamines decreased over time as well, but were not further decreased when training intensity was increased. The authors suggest that the decrease in heart rate during the first phase of training is mediated by decreased sympathetic action on the heart. The second phase of training however, produces effects on heart rate independent of the SNS, maybe through increased parasympathetic tone alone.

This and other exercise training studies performed around that time were among the first to establish the effect that chronic exercise has on autonomic control of the cardiovascular system. Without it, we would all be just running blots and doing PCR instead of cool rat experiments.

Addressing an ongoing controversy: Do C1 neurons contribute to resting blood pressure?

Control of sympathetic vasomotor tone by catecholaminergic C1 neurones of the rostral ventrolateral medulla oblongata.

Marina N, Abdala AP, Korsak A, Simms AE, Allen AM, Paton JF, Gourine AV.

This study was designed to investigate the role of rostral ventral medullary catecholaminergic neurons in the control of resting blood pressure. We all know that most of supraspinal resting vasomotor tone comes from some neurons that "live" in the rostral portion of the ventrolateral medulla (RVLM). We also know that many neurons in RVLM are "C1" neurons, that is, they can synthesize catecholamines. Finally, we know that both C1 and non-C1 neurons send axons to sympathetic preganglionic neurons in the spinal cord, with most being C1. At the time of these discoveries it was assumed that C1 neurons were a major source of resting and relfex RVLM-mediated sympathetic activity. However, studies done in the past ten years have been able to investigate the function of C1 neurons by specifically targeting them with antibodies and viruses. Schreihofer and Guyenet showed that when about 75% of C1 neurons were ablated, sympathoexcitatory responses to direct or reflex-mediated stimulation were markedly reduced, but resting tone of SNA and MAP were unaffected. Further work from this lab showed that specific direct activation of C1 neurons produced increases in SNA and MAP. Also, work from another lab showed that specific "re-expression" of angiotensin II receptors in C1 neurons of an angiotensin receptor knockout mouse model restored SNA responses to angiotensin II in the RVLM. Together, these data suggest a prominent role for C1 neurons in SNS responses but doesn't fully address the role of C1 neurons in the maintanance of resting tone.

The very innovative and unique aspect of this study was the system the authors designed to test direct acute inhibition of C1 neurons in vivo. This was accomplished using lentiviruses specific for C1 neurons, but instead of the virus encoding for something that kills the cell, it encodes an inhibitory G protein-coupled receptor that has a specific ligand (allatostatin) which is not present in mammals. After RVLM injection of the virus and a 5 to 6 week recovery period, rats were anesthetized and instrumented to record MAP and renal SNA. They showed convincingly that acute specific inhibition of C1 neurons in vivo causes a reversible fall in MAP and RSNA in animals that recieved the virus but not in sham rats. They then tested the role of C1 neurons in the SNS response to hypercapnia, or high levels of arterial CO2. Interestingly, if C1 neurons were inhibited before experimentally varying CO2 levels, the response was the same as control. If C1 neurons were inhibited during high CO2, the peak response was attenuated. Also, they suggest that blockade of ionotropic glutamate receptors did not affect this phenomenon. I engourage anyone reading this post to look at the results of this second-to-last experiment and decide for yourself what the results suggest. Finally, they used an isolated heart-brainstem preparation to show that C1 neurons generate much of the respiratory-related SNA bursts.

These data contrast with previous work described. However, the authors note an important distinction in the discussion. By using a toxin that killed C1 neurons over the course of days, other vasomotor centers had the opportunity to increase their activity in compensation. Importantly, they showed in this study that the inhibition was immediate and reversible. Additionally, it appears (to me) that glutamatergic inputs on C1 neurons do have some role in the sympathetic response to hypercapnia, although I admit that the results from this study don't clearly suggest whether they do or do not. Finally, C1 neurons clearly are responsible for respiratory-related bursts of SNA in the authors' in situ  heart-brainstem prep. They could confirm these results in vivo by performing experiments similar to those from Ann Schreihofer's lab of the past 5 years or so.