A star-shaped brain cell called an astrocyte appears to help keep blood pressure and blood flow inside the brain on a healthy, even keel, a new study from researchers at Georgia Regents University shows. The team state that like a health-care worker keeping tabs on a patient’s blood pressure, the finger-like appendages of astrocytes, called endfeet, quite literally wrap around the countless, fragile blood vessels in the brain, constantly monitoring what’s going on inside and around them.
The researchers explain that the brain is one of the body’s most vascular organs. The subtly pulsing carotid arteries on either side of the neck are essentially where the brain’s vascular system starts. The carotids come together at the base of the brain to form the Circle of Willis, and branches off that structure give rise to smaller, muscular extracerebral blood vessels that eventually cover the surface of the brain. These muscular vessels, also called resistance cells, branch many times, shedding all but one smooth muscle cell layer as they dive into the brain tissue to become parenchymal arterioles. Astrocytes have connections that reach all the way back to the muscular extracerebral blood vessels.
The sturdy, extracerebral blood vessels also play an important role in pressure control and vascular tone inside the brain, state the team. As examples, when anger or terror cause a sharp increase in blood pressure in the body, these big vessels will constrict to prevent sudden changes in cerebral blood flow and prevent large pressure changes from reaching fragile parenchymal arterioles. Even routine changes in blood pressure, such as those that take place in the simple act of standing, require these adjustments. The group note that when a person stands up quickly, the reason they may feel dizzy is, in part, because astrocytes and smooth muscles didn’t have enough time to make their respective proper adjustments.
The data findings indicate that when astrocytes sense a change in blood pressure inside the fragile parenchymal arterioles, one of their many duties is releasing signals that help dilate or constrict the blood vessels, whichever it takes to maintain the healthy status quo. The team state that this is the first evidence of the astrocyte’s role in pressure-induced myogenic (muscle) tone. The current study also showed that astrocytes keep their fingers on the pulse of blood vessels and neurons simultaneously, apparently playing an important role in balancing their needs. The astrocytes were shown to be bridges between what is going on with neuronal activity and blood flow changes in the brain.
The results suggest that even if the neurons are not in need of an adjustment to accommodate a changing demand for energy and oxygen, astrocytes relentlessly monitor and respond to changes in pressure in the parenchymal arterioles to help keep the brain from getting too much blood. The team stress that the brain actually requires active astrocytes to maintain a constant tone.
Previous studies show that astrocyte activity is driven by changes in the cell’s internal calcium levels, usually an increased calcium level means increased activity. The current study used a model that enabled the researchers to perfuse the parenchymal arterioles in a brain slice. The results showed that when pressure was increased inside the blood vessels, they constricted as they should, which triggered an increase in the astrocytes’ intracellular calcium levels that enabled the cells to help the blood vessels maintain a healthy vascular tone. When the available calcium inside astrocytes was reduced via chelation the parenchymal arterioles still responded to changes in pressure, however, they could not hold a consistent tone without the support of the astrocyte endfeet.
The researchers explain the calling fragile parenchymal arterioles astrocyte-coated is no exaggeration as about 99 percent of their outside surface is literally covered by endfeet, which have the look of spider webs. They go on to add that chronic stress, such as high blood pressure, can actually damage multiple layers of protection, leaving fragile parenchymal arterioles rigid and unable to do their jobs and astrocytes without their supportive, responsive grip. Too much blood flow or too high pressure in the brain can result in swelling, hemorrhage and/or stroke and leave the brain more vulnerable to ischemia and infection. Too much calcium in astrocytes also has been associated with neurodegenerative maladies such as Alzheimer’s.
The team now plan to investigate what effect activating astrocytes has on neuronal activity with the lab now recording from neurons as they change pressure inside blood vessels.
Source: Georgia Regents University