Jan 062006

Astrocytes, also known as astroglia, are star-shaped cells in the brain whose function and importance has never been fully understood by neuroscientists. Once thought to be housekeeping cells under the control of neurons, LiveScience.com is reporting today that researchers have found that astrocytes can directly and independently perform the critical function of controlling blood flow in the brain. This discovery could influence how brain scans are interpreted and may lead to breakthroughs in understanding and treating brain injuries and neurological diseases.

Outnumbering neurons ten to one, astrocytes were originally thought to be “gap fillers,” providing physical structure to the brain. Next they were considered housekeeping cells providing nutrients like glucose to the brain and handling some extracellular clean up like transmitter re-uptake and ion concentration regulation. Most recently it was determined that they also act as a vasomodulation intermediary, connecting to blood vessels and regulating the flow of nutrients like oxygen to neurons. During increased activity, neurons direct the astrocytes to increase blood flow which they do by releasing calcium to dilate the blood vessels.

Experiments by Dr. Maiken Nedergaard, a professor in the Department of Neurological Surgery at the University of Rochester who has been studying astrocytes for many years, indicate that in addition to working in conjunction with neurons, astrocytes may also act on their own to increase or decrease blood flow without any external prompting.

This discovery could affect the way brain imaging scans like PET and functional MRI are read. Scientists have used these tools to map the functions of brain regions by doing brain scans of subjects performing different tasks. The core assumption has been that there is a direct relationship between blood flow and neural activity in a region. Nedergaard’s findings indicate that this may not always be the case since astrocytes seem to be able to increase blood flow on their own regardless of neural activity in some circumstances.

Suddenly scientists have a new place to look for an explanation for what happens in the brain during a head injury or neural disease. For example, after a brain trauma the blood vessels in the brain often constrict and restrict blood flow. Doctors and scientists never had a solid reason for this to occur, but now Nedergaard has a theory. She believes that during brain trauma, astrocytes may be damaged in such a way that they release chemicals to constrict the blood vessels instead of dilating them.

What may be more interesting are the implications Nedergaard’s work has for the diagnosis and treatment of neurological disorders like Alzheimer’s disease. The horrific symptoms of Alzheimer’s, dementia and memory loss over an extended period of time, are a result of neurons dying. Scientists knew through brain scans that blood flow was decreased to the areas of the brain affected by Alzheimer’s. They assumed it was because the neurons were dead or dying, and there was less demand for blood. According to Nedergaard, the decreased blood flow is not a result of the dying neurons, but just the opposite: the neurons were dying because of the lack of blood. Therefore, Alzheimer’s may be affecting the performance of astrocytes and not directly killing neurons as widely thought, an idea which could open new doors to understanding the disease.

Dr. Nedergaard’s work was published in the online version of the journal Nature Neuroscience.

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