The activation of neurons is usually associated with a sharp increase in the amount of blood flowing through a given region of the brain. This relationship is known as neurovascular coupling and it occurs by widening of blood vessels in the brain called arterioles. Using functional magnetic resonance imaging (fMRI) scientists are looking for areas of poorer blood flow to diagnose brain disorders.
Earlier studies of neurovascular coupling have been limited to the superficial regions of the brain (like the cerebral cortex), and scientists have studied how it changes blood flow in response to stimuli from the environment (e.g. visual or auditory). It was not known whether the same principles applied to deeper regions of the brain that are attuned to stimuli produced by the body itself.
To study this relationship in the deep regions of the brain, a team of scientists from Georgia State University, led by neurobiologist Prof. Javier Stern developed a novel approach combining surgical techniques and imaging methods. Scholars focused on hypothalamus, a region of the brain involved in critical body functions, incl. drinking, eating or regulating the temperature. Research has shown that blood flows to the hypothalamus was changing in response to salt intake.
– We chose salt because the body needs to very precisely control sodium levels. We even have specific cells that detect how much salt is in the blood. When you eat salty food, your brain senses it and triggers a series of compensatory mechanisms to restore your sodium levels, Prof. Stern.
The body does this in part by activating neurons that trigger release vasopressin – a hormone that plays a key role in maintaining the proper concentration of salt. Scientists found a decrease in blood flow as neurons in the hypothalamus were activated.
‘The findings surprised us because we observed a spasm of blood vessels, which is the opposite of what most people describe in the cerebral cortex in response to a sensory stimulus. Reduced blood flow is usually observed in the cerebral cortex in diseases such as Alzheimer’s or after stroke or ischemia, added Prof. Stern.
Scientists named this phenomenon reverse neurovascular coupling, which is a decrease in blood flow that causes hypoxia. Other differences were also observed – in the cerebral cortex, vascular responses to stimuli are localized, and vasodilation occurs rapidly. In the hypothalamus, the response was diffuse and slow.
When we eat a lot of salt, our sodium levels remain elevated for a long time. We believe that hypoxia is a mechanism that enhances the ability of neurons to respond to sustained salt stimulation, allowing them to remain active for longer periods, concluded Prof. Stern.
New discoveries could shed light on how hypertension can affect the brain. It is believed that 50-60 percent. High blood pressure is caused by excessive salt intake. Scientists intend to investigate this mechanism to determine how it contributes to disease phenomena.