Surprising findings on how salt affects blood flow in the brain

Date:
November 11, 2021
Source:
Georgia State University
Summary:
Researchers reveal surprising new information about the relationship
between neuron activity and blood flow deep in the brain, as well
as how the brain is affected by salt consumption.



FULL STORY ==========================================================================
A first-of-its-kind study led by researchers at Georgia State reveals surprising new information about the relationship between neuron activity
and blood flow deep in the brain, as well as how the brain is affected
by salt consumption.


==========================================================================
When neurons are activated, it typically produces a rapid increase of
blood flow to the area. This relationship is known as neurovascular
coupling, or functional hyperemia, and it occurs via dilation of blood
vessels in the brain called arterioles. Functional magnetic resource
imaging (fMRI) is based on the concept of neurovascular coupling:
experts look for areas of weak blood flow to diagnose brain disorders.

However, previous studies of neurovascular coupling have been limited
to superficial areas of the brain (such as the cerebral cortex) and
scientists have mostly examined how blood flow changes in response to
sensory stimuli coming from the environment (such as visual or auditory stimuli). Little is known about whether the same principles apply to
deeper brain regions attuned to stimuli produced by the body itself,
known as interoceptive signals.

To study this relationship in deep brain regions, an interdisciplinary
team of scientists led by Dr. Javier Stern, professor of neuroscience
at Georgia State and director of the university's Center for
Neuroinflammation and Cardiometabolic Diseases, developed a novel approach
that combines surgical techniques and state-of-the-art neuroimaging. The
team focused on the hypothalamus, a deep brain region involved in critical
body functions including drinking, eating, body temperature regulation and reproduction. The study, published in the journal Cell Reports, examined
how blood flow to the hypothalamus changed in response to salt intake.

"We chose salt because the body needs to control sodium levels very
precisely.

We even have specific cells that detect how much salt is in your blood,"
said Stern. "When you ingest salty food, the brain senses it and activates
a series of compensatory mechanisms to bring sodium levels back down."
The body does this in part by activating neurons that trigger the
release of vasopressin, an antidiuretic hormone that plays a key role in maintaining the proper concentration of salt. In contrast to previous
studies that have observed a positive link between neuron activity and increased blood flow, the researchers found a decrease in blood flow as
the neurons became activated in the hypothalamus.

"The findings took us by surprise because we saw vasoconstriction, which
is the opposite of what most people described in the cortex in response to
a sensory stimulus," said Stern. "Reduced blood flow is normally observed
in the cortex in the case of diseases like Alzheimer's or after a stroke
or ischemia." The team dubbed the phenomenon "inverse neurovascular
coupling," or a decrease in blood flow that produces hypoxia. They also observed other differences: In the cortex, vascular responses to stimuli
are very localized and the dilation occurs rapidly. In the hypothalamus,
the response was diffuse and took place slowly, over a long period
of time.

"When we eat a lot of salt, our sodium levels stay elevated for a
long time," said Stern. "We believe the hypoxia is a mechanism that
strengthens the neurons' ability to respond to the sustained salt
stimulation, allowing them to remain active for a prolonged period."
The findings raise interesting questions about how hypertension may affect
the brain. Between 50 and 60 percent of hypertension is believed to be
salt- dependent -- triggered by excess salt consumption. The research
team plans to study this inverse neurovascular coupling mechanism in
animal models to determine whether it contributes to the pathology of salt-dependent hypertension. In addition, they hope to use their approach
to study other brain regions and diseases, including depression, obesity
and neurodegenerative conditions.

"If you chronically ingest a lot of salt, you'll have hyperactivation of vasopressin neurons. This mechanism can then induce excessive hypoxia,
which could lead to tissue damage in the brain," said Stern. "If we can
better understand this process, we can devise novel targets to stop this hypoxia- dependent activation and perhaps improve the outcomes of people
with salt- dependent high blood pressure." The study authors include
Ranjan Roy and Ferdinand Althammer, postdoctoral researchers in the
Center for Neuroinflammation and Cardiometabolic Diseases, Jordan Hamm, assistant professor of neuroscience at Georgia State, and colleagues at
the University of Otago in New Zealand, Augusta University and Auburn University. The research was supported by the National Institute of Neurological Disorders and Stroke.

========================================================================== Story Source: Materials provided by Georgia_State_University. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ranjan K. Roy, Ferdinand Althammer, Alexander J. Seymour,
Wenting Du,
Vinicia C. Biancardi, Jordan P. Hamm, Jessica A. Filosa,
Colin H. Brown, Javier E. Stern. Inverse neurovascular coupling
contributes to positive feedback excitation of vasopressin neurons
during a systemic homeostatic challenge. Cell Reports, 2021; 37
(5): 109925 DOI: 10.1016/ j.celrep.2021.109925 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211111154256.htm

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