The blood-brain barrier is the boundary between blood and fluid in the brain formed by endothelial cells and astrocytes. Endothelial cells are found in blood vessels, and throughout the body, they keep blood from flowing out of the blood vessels. Astrocytes are specialized glial cells, which help maintain the chemical environment of a neuron. The blood-brain barrier keeps bacteria and other water-soluble substances from entering the brain. Hormones can pass through the barrier, however, along with oxygen, carbon dioxide and glucose.
So, the blood-brain barrier is useful when it comes to protecting the brain. However, in some cases, it is also an additional hurdle when it comes to curing a disease in the brain; medicines cannot cross this barrier either. Therefore, in very severe cases, drugs are injected directly into the brain (kind of like with an epidural). The problem of the blood-brain barrier becomes especially apparent in Parkinson's disease, where there is too little dopamine in the brain. At first, it may seem simple to treat this: a dose of dopamine in a pill and any problem should be solved. However, dopamine is a substance that is not passed through the blood-brain barrier, and that’s why it’s so difficult to treat this disease.
Sometimes harmful substances also pass through the blood-brain barrier simply because they are too small to stop. Examples are hard drugs, such as cocaine and heroin, as well as caffeine and alcohol. This is also the reason why the cognitive effects of these substances become noticeable to the user pretty quickly.
Author: Myrthe Princen (translated by Melanie Smekal)