How can fat talk to the brain through the kidneys?

FOCUS ON ACTA PHYSIOLOGICA

Editorial from Acta Physiologica: Evans, R. G. “Your fat talks to your brain through your kidneys.” Acta Physiologica (2015).
Research paper from Acta Physiologica: Khan, S. A., et al. “Obesity depresses baroreflex control of renal sympathetic nerve activity and heart rate in Sprague Dawley rats: role of the renal innervation.” Acta Physiologica (2015).

Did you know that your kidneys chit-chat with your brain?

The kidneys are amazing! Not only do they ensure that the blood is kept clean from dangerous nitrogen-containing compounds originating from protein metabolism but they also maintain the body’s water/salt balance. Thus, the “hydration status” is closely monitored and communicated between the kidneys, the brain and small receptors (baroreflex receptors) that are located in the blood vessels.

The kidney! (image credit: "Illu kidney2")

The kidney! (image credit: “Illu kidney2“)

It is not only thirst that is communicated between the kidneys and the brain:

“Your fat talks to your brain through your kidneys”

the title of a recent editorial from Acta Physiologica.

A quick recap of the nervous system:

The hypothalamus controls the autonomic nervous system (ANS) which is turn controls the internal organs. The ANS is responsible for our “unconscious” actions like breathing and digestion. It is divided into two branches: the sympathetic nervous system and the parasympathetic nervous system. In very simple terms these two branches are complementary; the sympathetic branch represents the “flight and fight” responses and the parasympathetic the more relaxed “digest and rest” ones. In this article we will remain on the sympathetic branch.

Did you know that obesity can change how your nervous system works? Obesity-related changes to the sympathetic nervous system can, for example, result in high blood pressure (hypertension).

Connections of the Sympathetic Nervous System (image credit: OpenStax College)

Connections of the Sympathetic Nervous System (image credit: OpenStax College)

How obesity can disturb the kidney-brain communication

Baroflex receptors are small stretch receptors located in the blood vessels. If the blood pressure increases, the baroflex receptors become active and take action by sending signals to the brain stem that in turn ensures that the pressure remains within the normal range. This can be carried out by adjusting heart rate for example.

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These baroreflex receptors were found to be less active in obese rats than in rats with normal weight. This resulted in the obese rats showing an altered control of heart rate and also renal sympathetic nerve activity. This means that some reflexes are dampened during extreme overweight that are crucial for a healthy blood pressure to be maintained.

"The Principle Organs and Vascular and Urino-Genital Systems of a Woman" (Wikimedia public domain)

“The Principle Organs and Vascular and Urino-Genital Systems of a Woman” (Wikimedia public domain)

How can fat talk to the brain through your kidneys?

There are several hypothetical ways fat can communicate with the kidneys and the brain, but the actual mechanics behind this particular discovery are still unknown. R. G. Evans hypothesizes the following in the Acta Editorial:

First: due to a large mass of fat in the abdomen the internal pressure can increase and as a consequence the hydrostatic pressure in the kidneys increases. This may, in turn, activate neurons that send signals to the brain and may set blood pressure too high.

Second: fat within the renal pelvis (acts as a funnel for urine flowing to the ureter, see above diagram) obstructs urine flow and this builds up pressure which may activate renal (kidney) mechanoreceptors which send signals to the brain…

Third: the renal afferents may become activated during kidney disease or during the many conditions when the body experiences inadequate supply of oxygen (e.g. hypoxia or ischaemia). This could reduce the baroreflex control of the renal sympathetic nerve activity.

One of the characteristics of obesity is that levels of the hormones insulin and leptin change. Studies have shown that showed that leptin could activate the sympathetic nervous system but no studies so far have conducted tests on the renal nerves in particular, therefore it is still unknown if this can happen in the kidney.

So what does this mean for the common man?

The WHO estimates that 22 % of the world’s population (age 18 and over) suffers from high blood pressure. With the obesity epidemic spreading, this number is likely to increase every year. However this discovery opens possibilities for new treatments for hypertension in obese patients with a focus on the renal nerves (denervation).

Further reading:

In a recent editorial in Acta Physiologica R.G. Evans (HERE) explores the recent advances in the communication between the kidney and the brain and highlights in particular a paper from Acta Physiologica by Khan et al. 2015 (Open Access: HERE). This paper elegantly showed a novel way that the cardiovascular system can be affected by obesity through altered autonomic control.

This is the third in a series of article-highlights from the high ranking peer reviewed scientific journal Acta Physiologica (impact factor 4.38).

Khan, S. A., et al. “Obesity depresses baroreflex control of renal sympathetic nerve activity and heart rate in Sprague Dawley rats: role of the renal innervation.” Acta Physiologica (2015).

Evans, R. G. “Your fat talks to your brain through your kidneys.” Acta Physiologica (2015).

Dorland’s (2012). Dorland’s Illustrated Medical Dictionary (32nd ed.). Elsevier Saunders. p. 1862. ISBN 978-1-4160-6257-8.

Posted in Acta Physiologica, Biology, Medicine and tagged , , , , .

Dr. Anna

Dr. Anna Zakrisson is a multifaceted biologist with degrees from world-renowned institutions such as Cambridge University, U.K. and the Max-Planck Institute in Germany. She has published a range of high-ranking scientific papers and crossed oceans on research vessels. She runs The Imaginarium science blog and YouTube channel and speaks English, German and Swedish fluently.

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